Photochemistry in Zeolite Nanocavities 1588830640

Citation preview

Encyclopedia of Nanoscience and Nanotechnology

www.aspbs.com/enn

Photochemistry in Zeolite Nanocavities Masanobu Kojima Shinshu University, Kami-ina, Nagano, Japan

CONTENTS 1. Introduction 2. Zeolite Nanocavities as Photochemical Microreactors 3. Influence of Brønsted and Lewis Acid Sites in Zeolite Nanocavities 4. Characteristics of Zeolite Nanocavities Affecting Photochemical Reactions 5. Photochemical Behavior of Alkenes in Zeolite Nanocavities 6. Concluding Remarks Glossary References

1. INTRODUCTION The spectroscopic and chemical behavior of ion radicals generated through photo-induced electron transfer reaction has attracted much attention in the last several decades. The invention of flash photolysis techniques and the improvement of laser light pulse-width made it possible for us to directly observe ion radicals with nano- and pico-second order lifetimes and to make a significant contribution to our understanding of the reaction mechanisms involved in organic photochemistry. However, now that the behavior of ion radicals has been well elucidated, the interest of researchers in photochemistry is shifting from small molecules to supramolecules. There has been much interest in the discovery that the spectroscopic and photochemical behavior of organic small molecules included in zeolite nanocavities (zeolite supramolecules) is very different from that in solution and also that short-lived species such as ion radicals and carbocations generated spontaneously in the cavities are surprisingly stable [1–13]. These findings suggest that zeolite nanocavities have considerable potential as novel reaction vessels which can regulate the photochemical reaction of organic guest molecules. The characteristics of zeolite nanocavities affecting spectroscopic and photochemical behavior are well ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

understood, and as a result, it is to be expected that the cavities will be used increasingly as photoreaction vessels. This article provides a brief summary of the spectroscopic and photochemical behavior of organic guest molecules in the cavities, mainly using aliphatic and aromatic alkenes: 1. direct (nonsensitized) photo-oxygenation by excitation of contact charge-transfer (CCT) complexes between alkenes and oxygen molecules, 2. oxygenation by singlet oxygen, 3. cis-trans photoisomerization, 4. photodimerization, 5. photorearrangement, 6. spectroscopy and photoreaction of short-lived species, 7. photochemical asymmetric induction. Particular emphasis will be given to the differences between the reaction behavior in the zeolite nanocavities and that in solution. In addition, the factors influencing the excited states of guest molecules will be concisely summarized.

2. ZEOLITE NANOCAVITIES AS PHOTOCHEMICAL MICROREACTORS Zeolites are crystalline aluminosilicates that have been widely utilized as molecular sieves and in thermal catalysis. 5− They comprise corner-sharing SiO4− 4 and AlO4 tetrahedral, and the framework contains a large number of pores, channels, and cages of various dimensions that can accommodate organic molecules of the right size. The supercage structure of X- and Y-type synthetic zeolites, and the cation location (I, II, and III) within the cages are illustrated in Figure 1 [1]. The two zeolites have the following typical unit cell compositions: X type

M86 (AlO2
86 (SiO2
106  264H2 O

Y type

M56 (AlO2
56 (SiO2
136  253H2 O

where M is a monovalent cation. These zeolites can adsorb most small organic molecules including aromatic compounds in the cavities, which have supercages (diameter, ca. 13 Å) and pore windows (ca. 7.4 Å) of sufficient size; accordingly,

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (549–567)

Photochemistry in Zeolite Nanocavities

550

3. INFLUENCE OF BRØNSTED AND LEWIS ACID SITES IN ZEOLITE NANOCAVITIES Brønsted and Lewis acid sites in zeolite nanocavities may spontaneously generate the carbocations and ion radicals of alkenes adsorbed in the cavities [17–23]. Thus, these acid sites sometimes impede the study of the photoreaction of the guest molecules. It is very common for the acidity of Brønsted acid sites on solid surfaces to be determined by using Hammett indicators. Recently, it was found that the fluorescence and phosphorescence characteristics of organic guest molecules, such as pyrene and acetophenone, were also useful in determining the acidity in the cavities [24–27]. Kojima et al. [28] studied the influence of Brønsted and Lewis acid sites on the fluorescence lifetime of 2-quinolone (1) in order to estimate the acidity. The lifetimes () of 1– Brønsted acid (1.2 ns) and 1–Lewis acid complexes (1.6 ns) have been measured in dichloromethane [29]. It was found that the acidity of the sites in zeolite nanocavities significantly affected the lifetimes of the complexes, as can be seen in Table 2. On the basis of the lifetimes, the acidity can be estimated more precisely than by using Hammett indicators. Therefore, the data provided the criteria necessary to select zeolites suitable for use in studying the photochemistry of guest molecules. Corrent et al. [30] found that the dye coumarin 6 (2) was extremely sensitive to the presence of Lewis and Brønsted sites and that it could be employed to determine acidity in NaY, which was usually considered to be nonacidic, as can be seen in Figure 2 and Scheme 1. Due to the large shifts seen for the dye molecule in its neutral 2, monocation 3, and dication forms, both the absorption and fluorescence spectra resulted in detection of the dye species in zeolites with intermediate acidity. The ability of 2 to sense small amounts of Lewis and Brønsted sites may prove useful in studying other systems.

Figure 1. Supercage structure, cation location (I, II, III) within X and Y type zeolites. Bottom portion shows the reduction in available space (relative) within the supercage as the cation size increases. Reprinted with permission from [45], V. Ramamurthy and N. J. Turro, J. Incl. Phenom. Mol. Recog. Chem. 21, 239 (1995). © 1995, Kluwer Academic Publishers.

they have often been used in the photoreactions described in this article. The average number S of guest molecules adsorbed in a supercage per 1 g of dry NaY zeolite can be estimated as follows [14, 15]. Because the weight of a unit cell with eight supercages is M56 (AlO2
56 (SiO2
136 = 12742 g, the weight of a supercage is 12742 g/8 = 1594 g. Accordingly, the number of supercages per 1 g of dry NaY is estimated to be the Avogadro number × (1 /1594) = ca. 3.8 × 1020 . The supercage capacity per 1 g of dry NaY has been determined experimentally to be 0.378 cm3 [16]. When this value is divided by the volume (827 Å3 ) of a supercage for NaY, the number of supercages is calculated to be ca. 4.6 × 1020 . This is in close agreement with the above calculated value, ca. 3.8 × 1020 . Reference [1] should be referred to for other zeolites such as ZSM-5 and Mordenite. The physical parameters of M+ Y zeolite are summarized in Table 1. As zeolites are photo-inert, the confined environment of their framework can be successfully utilized to attain remarkable selectivity in photochemical reactions [1–13].

S

N

N

S

H

N

O O

H

Scheme 1

Table 1. Dependence of physical parameters of MY zeolites on the cation.

Cation (M+ ) Li Na K Rb Cs

Electrostatic field (V/Å) within the case

Electrostatic potential (e/r) of the cation

spin-orbit coupling

076 102 138 152 167

21 13 10 08 06

167 105 075 067 059

27 87 360 840

O O

3

2

Ionic radius of the cation (Å)

N

Vacant space within the supercage (Å3 ) Y Zeolite 834 827 807 796 781

X Zeolite 873 852 800 770 732

Source: Reprinted with permission from [2], V. Ramamurthy et al., Acc. Chem. Res. 25, 299 (1992). © 1992, American Chemical Society.

Photochemistry in Zeolite Nanocavities

551

Table 2. Fluorescence decay lifetimes of 2-quinolone (1) in solution and in zeolite.a Lifetime (i )/nsb Condition CH2 Cl2d EtOHd HCl/CH2 Cl2e BF3 /CH2 Cl2f NaY KL HY H-mordenite H-ZSM-5

1

2

016 018
g 016 016 016 030 033 033 032 063

042 12 16 16
g 10 11 18 23 26

Preexponential factor (ai ) 3

a1

a2

78

10 069 093 079 088 094 088 088 082

031 007 021 012 006 012 012 017

a3

Hamett acidityc

001

+33∼+15 +40∼+33 −30∼−56 −30∼−56 −56∼−82

a

The fluorescence decay lifetimes were determined by using a single photon counting apparatus, as reported in [149], H. Satozono, S. Suzuki, N. Tokou, H. Takehara, and Y. Uno, J. Chem. Soc. Jpn., Chem. Indus. Chem. 115 (1994). The decay curves were analyzed by using the equation, I t
= ai exp −t/i ); the excitation wavelengths were 320 nm for solution and 330 nm for zeolite; the emission wavelength was 373 nm. b Measured under air. c Determined by using Hammett indicators. d Data for 1 = 10 × 10−4 M in the absence of HCl and BF3 . e Data for 1 = 10 × 10−4 M in the presence of 2 equiv. of HCl-OEt2 . f Data for 1 = 10 × 10−4 M in the presence of 2 equiv. of BF3 -OEt2 . g Lifetime reported in [29], F. D. Lewis, G. D. Reddy, J. E. Elbert, B. E. Tillberg, J. A. Meltzer, and M. Kojima, J. Org. Chem. 56, 5311 (1991). Source: Reprinted with permission from [28], M. Kojima et al., Chem. Lett. 675 (1999). © 1999, The Chemical Society of Japan.

4. CHARACTERISTICS OF ZEOLITE NANOCAVITIES AFFECTING PHOTOCHEMICAL REACTIONS 4.1. Adsorption Sites The distribution of guest molecules in zeolites is more complex than in homogeneous solution. The adsorption sites for guest molecules in zeolites significantly influence the photochemical behavior of the molecules. It is thought that there are two important adsorption sites in zeolites, one of which is a pore window and the other a metal cation in the cavities. Analysis of fluorescence decay from aromatic hydrocarbons such as pyrene, naphthalene, and phenanthrene provided an insight into distribution of the sites in zeolites [14]. It was quite clear, as can be seen in Figure 3, that the distribution of lifetime measured in NaY reflected the presence of two main adsorption sites, which are heterogeneous in contrast to the homogeneous environment in solution. The width of

distribution of lifetime was affected by the structure of the guest molecules and co-adsorbents in NaY. The effect of the co-adsorbents was demonstrated by the results obtained in the case of pyrene with water and other solvents, as shown in Figure 4.

4.2. Effect of Co-Adsorbed Water Co-adsorbed water in zeolites can impede the interaction of guest molecules with adsorption sites. For example, the formation of dimers of cationic dyes such as methylene blue (4) and thionin (5) was accelerated in NaY containing coadsorbed water, while in dry NaY the monomers exist as a main species as shown in Figure 5 and Scheme 2 [31, 32]. The decay rate constants for fluorescence [33] and triplet excited state [16] were also greatly affected, as was the distribution of fluorescence lifetimes of aromatic hydrocarbons by co-adsorbed water as described above. Therefore, careful attention should be paid to the influence of moisture under irradiation conditions [34]. Cl H2N H2N

Cl S

NH2

S N

NH2

N

Cl

dry

H2N

S

NH 2

wet N

5 Scheme 2

Figure 2. Absorption (A, left scale) and fluorescence (F, right scale) spectra of 4 × 10−6 M coumarin 6 in CH2 Cl2 for the neutral species (solid line) and for the monocation formed upon the small addition of acid (broken line). Reprinted with permission from [30], S. Corrent et al., J. Phys. Chem. B 102, 5852 (1998). © 1998, American Chemical Society.

4.3. Acceleration of Intersystem Crossing by Metal Cations Intersystem crossing from the excited singlet state of guest molecules to the excited triplet state is accelerated in zeolite nanocavities due to electrostatic interaction with the metal

Photochemistry in Zeolite Nanocavities

552

Figure 4. Distribution of lifetime of the excited monomer of pyrene within NaY co-included with organic solvent and water. Compare Figure 3a and note the change in distribution width. Reprinted with permission from [14], V. Ramamurthy, Mol. Cryst. Liq. Cryst. 240, 53 (1994). © 1994, Taylor & Francis Ltd.

Figure 3. Distribution of S1 lifetime of (a) pyrene, (b) naphthalene, and (c) phenanthrene included within “dry” NaY at room temperature. Reprinted with permission from [14], V. Ramamurthy, Mol. Cryst. Liq. Cryst. 240, 53 (1994). © 1994, Taylor & Francis Ltd.

Figure 1; therefore, S decreases and also steric hindrance on bond formation increases. In addition, it is important to note that the strength of the electrostatic field depends on the metal cations, as shown in Table 1. These factors may affect the spectroscopic and photochemical behavior of guest molecules. Sadeghpoor et al. [36] and Pitchumani et al. [37] found that photolysis of ,-unsaturated ketone 7 and 8 included in MY zeolites, as shown in Scheme 3, gave the oxa-di-methane products 9 and 10, which were believed to originate from the triplet state caused by heavy atom cation present in the supercage. Warrier et al. [38] observed a difference in product selectivity between dibenzyl ketones 11 and naphthyl esters in zeolites and concluded that this was due to the difference in spin of the radical pairs formed from these precursors (Scheme 4). Heavy cations present in zeolites can enhance intersystem crossing between triplet and singlet

O

cations [35]. Measurements of the fluorescence and phosphorescence intensity of aromatic hydrocarbons like naphthalene (6) indicated that for light metal cations, such as Li+ and Na+ , the intensity was relatively unaffected by the interaction. In contrast, for heavy metal cations such as Rb+ , Cs+ , and Tl+ , it was found that the rate of intersystem crossing increased remarkably to enhance the phosphorescence intensity, as can be seen in Figure 6. This indicates that metal cations can be used for controlling the excited state of guest molecules. On the other hand, it should be noted that the enlargement of the ion radius of a metal cation causes a decrease in the vacant space in zeolite cavities, as shown in

hν / M Y

O

M = Tl, C s 7

9

O

hν / M Y O M = Tl, C s 10

8 Scheme 3

Photochemistry in Zeolite Nanocavities O

553

O hν O

11 % of Products LiY NaY KY RbY CsY TlY PbY

95 90 76 79 33 21 11

4 8 17 18 54 22 43

1 2 7 3 13 57 46

Scheme 4

geminate radical pairs. Furthermore, ab initio molecular orbital calculations and product analysis have shown that the nature of the lowest triplet state of enones 12 is altered by the cations present in Y zeolites (Scheme 5). Excited state energy, estimated at the CIS(D)/6-31+G* level, indicated that the lowest triplet was n- ∗ in character for the enones, but switched to - ∗ on coordination with alkali metal ions, O hν

O

Figure 6. Emission spectra at 77 K of naphthalene included in LiX, RbX, and CsX (excitation  285 nm). Note that the ratio of phosphorescence to fluorescence changes with the cation, but the ratio change is independent of the excitation wavelength. Reprinted with permission from [2], V. Ramamurthy et al., Acc. Chem. Res. 25, 299 (1992). © 1992, American Chemical Society.

O 12

(nπ*)

(ππ*)

such as Li+ , due to interaction with the carbonyl unit of the enones. The observed product distribution in zeolite was consistent with this theoretical prediction [39].

4.4. Electrostatic Interaction of Metal Cations with Guest Molecules

% of Products

Medium hexane

100%

0%

MeOH

78

22

MeCN

76

24

KX/hexane

25

75

KY/hexane

26

74

Scheme 5

Aromatic carbonyl compounds rarely undergo dark reaction due to the acid sites in zeolite cavities. Therefore, the differences between the spectroscopic and photochemical behavior in zeolite cavities and that in solution can be clearly distinguished. The data obtained from the carbonyl compounds are also useful for predicting the photochemical reaction of alkenes in zeolites. It was found for carbonyl compounds in the excited state that the selectivity of -bond fission (Norrish type-I reaction) and -hydrogen abstraction, followed by -bond fission from the 1,4-biradical intermediates generated (Norrish type-II reaction), was regulated by the electrostatic interaction of the carbonyl oxygen with metal cations in the zeolite nanocavities, as illustrated in Scheme 6 [40–45]. This interaction with metal cations also influences the photochemical behavior of alkenes as described below; thus, this is one of the most important factors for regulating the photoreaction of guest molecules.

H M+ Figure 5. Diffuse reflectance spectra of thionin include within KL and NaY. ( ) NaY, hydrated; (—-) NaY, anhydrous; and (- - -) KL, hydrated or anhydrous. Reprinted with permission from [31], V. Ramamurthy et al., J. Am. Chem. Soc. 115, 10438 (1993). © 1993, American Chemical Society.

R

M+

O

O

H Scheme 6

R

Photochemistry in Zeolite Nanocavities

554

5. PHOTOCHEMICAL BEHAVIOR OF ALKENES IN ZEOLITE NANOCAVITIES Although it seems likely that alkenes, which are more electron-donating compounds than ketones, are sensitive to Brønsted and Lewis acid sites in zeolite nanocavities [17–23], their photochemical behavior can be easily observed if the dark reaction is suppressed by choosing a combination of guest molecules and zeolites. For Na+ - and K+ -exchanged zeolites, in general, due to weak acidity in the cavities, the dark reaction of alkenes with the acid sites does not occur. However, alkenes with an electron-donating group like a methoxy group readily react with the acid sites [46–48]. Particularly, H+ -exchanged zeolites often generate the carbocations and cation radicals of the alkenes.

5.1. Direct Photo-oxygenation In the 1950’s, Evans [49] discovered the formation of CCT complexes between aromatic substrates and oxygen molecules (O2 ). However, the quantity of literature related to the photochemical reaction of CCT complexes is relatively small [50]. Kojima and Onodera et al. [51–56] studied the photoreaction of the CCT complexes for 4-substituted styrenes, 1-arylcyclohexenes, and 1,1-diarylethenes and found that excitation of the CCT band caused photo-induced electron transfer reaction from the alkenes to O2 . For aliphatic alkenes, however, there is no report of photoreaction of the CCT complexes in solution. Interestingly, Blatter and Sun et al. [57–68] found that hydrocarbons such as 2,3-dimethyl-2-butene (13), propane, cyclohexane, and toluene, formed CCT complexes with O2 in zeolite NaY, the absorption band of which was observed up to the visible wavelength region, as can be seen in Figure 7. The stabilization of the CCT complexes was explained as being due to the strong electrostatic field in the zeolite nanocavities. When the CCT absorption band in the visible wavelength region was excited by using a laser light, it was suggested on the basis of infrared (IR) absorption spectra that the alkene cation radicals and superoxide anion radical (O− 2 ) were generated through photo-induced electron transfer (Scheme 7). Myli, Xiang, Panov, and Larsen et al. [69–72] further studied the photo-oxygenation of hydrocarbons in zeolite nanocavities to find a close correlation δ

O2

hνcct

+ O2

δ

13

O O HO2

+

O OOH Scheme 7

+

O

Figure 7. Alkene/O2 /NaY diffuse reflectance spectra ratioed against alkene/N2 /NaY reflectance spectra. (a) 2,3-Dimethyl-2-butene (13) • O2 , (b) 2-methyl-2-butene • O2 , and (c) trans-2-butene • O2 . Reprinted with permission from [60], F. Blatter et al., J. Phys. Chem. 98, 13403 (1994). © 1994, American Chemical Society.

between the strength of the electrostatic field in the cavities and the distribution and yield of photo-oxygenation products. Takeya, Kojima, and Matsubara et al. [73–76] investigated the photoreaction of CCT complexes comprising aromatic alkenes and O2 formed in zeolites and compared the distribution and yield of products with those in solution. Although styrene (14), 1,1-diphenylethene (15), and cis- and trans-stilbenes (t-16), but not triphenylethene (17), formed their CCT complexes with O2 in solutions, the CCT absorption band was observed in NaY only for 14 and 16, as can be seen in Figure 8. Irradiation of alkenes 14–17 included in the zeolite nanocavities under O2 -produced benzaldehyde (18) and benzophenone (19) as the major oxygenation products. In particular, the photo-oxygenation for 14 proceeded at the expense of the dimer formation and that for 16 and 17 competed with a photoelectrocyclic reaction, which subsequently yielded phenanthrenes (20) as the

Photochemistry in Zeolite Nanocavities

555 Na+

(a)

NaY

H

hν

Ph

Ph

0.8

R

Kubelka-Munk

1 atm O2

R

O2

H

-H2

R

R

16 (17)

20

0.4

0.6

0.1 0.4

Ph Ph

Ph

hν / O2

0.025 vacuum

R

Ph

R

+ O2

O O

Ph

O + PhCHO

R

18 (19)

18

0.2

Scheme 9 0 250

300

350

400

Itoh et al. [77] found that the mesoporous silica FSM16 is a recyclable promoter for the photo-oxidative cleavage of double bonds conjugated with an aromatic nucleus, affording the corresponding carboxylic acid in the presence of catalytic iodine in aerobic isopropyl ether. When 14 was irradiated under the same conditions, 68% benzoic acid was produced. However, 15 and 16, which possess di-substituted double bonds, behaved differently: the former gave 19 in low yield (12%), and in the case of the latter, the starting

450

Wavelength/nm (b)

Kubelka-Munk

0.8

1 atm O2 vacuum

0.6

0.4

(a) 20 100 ns 10 s 100 s 1 ms

0.2 15 300

350

400

450

Wavelength/nm Figure 8. Diffuse reflectance absorption spectra for 14 (a: S = 015) and t-16 (b: S = 019) in NaY observed under O2 (0.025–1 atm). Reprinted with permission from [74], M. Kojima et al., J. Chem. Soc. Perkin Trans. 2, 1894 (2002). © 2002, The Royal Society of Chemistry.

exclusive photoproducts under O2 in solution, as shown in Schemes 8 and 9, respectively. It is likely that the oxygenation products were produced through the alkene cation radicals and O− 2 generated by excitation of the CCT complexes and/or photo-induced electron transfer from the excited alkenes to O2 . Kojima et al. [74] also reported that YAG laser (266 nm) excitation of 14 included in the zeolite cavities under vacuum produced the alkene cation radical and the trapped electron, Na3+ 4 , both of which were quenched by O2 (Figure 9). On the basis of semi-empirical molecular orbital calculations, it was suggested that the photooxygenation reaction was regulated by the electrostatic interaction between the guest molecules and alkali-metal cations in the nanocavities.

10

5

0 300

350

400

450

500

550

600

Wavelength/nm (b) 10

100 ns 100 s 1 ms

8

Absorption/%

250

Absorption/%

0

6

4

2 Ph R

PhRCHCHO

hν R = Ph

O

Ph2CH2

0

O2 Ph

O2 / NaY

R

14 (15)

Ph δ

δ

O2

R CCT Complex

hν

Ph R

Ph + O2

R O–O

300 Ph R

O + CH2O

18 (19)

h ν / O2 / NaY R = Ph

Scheme 8

350

400

450

500

550

600

Wavelength/nm Figure 9. Transient absorption spectra observed on 266 nm laser excitation of 14 (S = 046 and 0.61) in NaY: (a) under vacuum and (b) at 0.2 atm O2 . Reprinted with permission from [74], M. Kojima et al., J. Chem. Soc. Perkin Trans. 2, 1894 (2002). © 2002, The Royal Society of Chemistry.

Photochemistry in Zeolite Nanocavities

556 material was recovered intact. Scheme 10 shows a possible path for the oxidative cleavage of double bonds with FSM-16.

Ar Vacuum/hν

Ar

Excimer

Ar Ar

hν

I2

2I

21

MeO

21

I

I

O2

21δ +

O2δ -

O2

O O O2 Ar O

O

O

O OH I H

I

Ar

+ O2

21

CCT Complex

R

R

hν

+

ArCHO

24

Ar = 4-MeOC6H4

Scheme 12

R

R O

chain alkenes and 1-alkylcycloalkenes by singlet oxygen (1 O2 ) [32, 79, 80]. It was found that the minor product in solution became the major product in the zeolites. The following two reasons were adduced to explain the high regioselectivity observed in the products: (i) because a metal cation interacted with  orbital of the carbon-carbon double (C C) bond, 1 O2 reacted with the alkenes from a

CO2H

I R

R

R= H, Me, t-Bu, MeO, NO2 Scheme 10

(a) 100

21/MeCN

80

Ar

Transmittance/%

For 4-methoxystyrene (21), about 30% of the alkene was consumed by dark reaction with Brønsted acid sites in NaY to produce chain dimers 22 and alcohol 23 through the alkene carbocation (Scheme 11) [46–48]. The photoreaction of the 21/NaY sample was compared to the dark reaction. As in the case of styrene (14) [74], the formation of dimers was a minor path in the zeolite, while the product selectivity that gave 4-methoxybenzaldehyde (24) increased significantly, as illustrated in Scheme 12 [75, 76]. This result was also understood in relation both to the formation of the CCT complex with O2 , as can be seen in Figure 10, and to the electrostatic interaction with Na+ in the nanocavities (Figure 11). Lakshminarasimhan et al. also observed the formation of CCT complex in NaY between 1,1-bis(4-methoxyphenyl)ethene (25) and O2 [78]. The intensity of the CCT band was much weaker than that for 21 [76]. The product distribution for 15 and 25 in NaY as hexane slurries were significantly affected by excitation wavelength and changed in comparison with that under dry conditions, reported in [74].

O2

60

40

20

0 300

350

400

450

500

Wavelength/nm (b) 100 21/NaY

80

5.2. Oxygenation by Singlet Oxygen Ramamurthy and Shailaja et al. prepared the cationic dyeexchanged zeolites X and Y using methylene blue and thionin [31, 32] and studied oxygenation of trisubstituted H+

Ar

21

+

-H+

+ Ar

Ar

MeO 21

H2O ArCHMe

Ar = 4-MeOC6H4

OH

Scheme 11

O2

60

40

20

Ar Ar trans-22 0

21 Polymers

-H+

Ar Ar cis-22

23

Reflectance/%

vacuum

250

300

350

400

450

500

Wavelength/nm Figure 10. Absorption spectra for the CCT complexes of 21 with O2 : (a) in MeCN and (b) in NaY. Reprinted with permission from [76], C. Matsubara and M. Kojima, Res. Chem. Intermed. 27, 975 (2001). © 2001, VSP.

Photochemistry in Zeolite Nanocavities

557

C(3)

C(8)

C(2)

C(4)

O(7)

C(1) C(5)

δ O

δ

C(10)

O

C(9)

D

C(6)

D2C

OOD

H3C

R

CH3 (a) 21: C10-C9-C1-C2 = 17.6o, C8-O7-C4-C5 = 0.0o, O7-C8 = 1.423 Å, C4-O7 = 1.381 Å.

M+

R

Scheme 14 C(8)

C(4)

O(7)

C(3)

C(2)

C(5)

C(6)

C(10) C(1)

C(9)

Na

+

(b) 21/Na complex: C10-C9-C1-C2 = 21.3o, C8-O7-C4-C5 = 91.2o, O7-C8 = 1.433 Å, C4-O7 = 1.396 Å, O7, Na = 3.266 Å.

Figure 11. Optimum structures for 21 (a) and 21–Na+ complex (b) obtained by a semi-empirical molecular orbital calculation (AM1 method). Reprinted with permission from [76], C. Matsubara and M. Kojima, Res. Chem. Intermed. 27, 975 (2001). © 2001, VSP.

sterically less-hindered site, where metal cation did not exist; (ii) because the electron density on the C C bond decreased due to interaction of the C C bond with a metal cation, electrophilic 1 O2 reacted selectively with a disubstituted carbon with relatively high electron density, as illustrated in Scheme 13.

R

t-16

R

1

O2 O R

CH3 CH3

reaction with alkenes that occurred in the nanocavities has led to an increased interest in zeolites. As illustrated in Scheme 15, dye-sensitized photooxygenation of stilbenes (16) in solution produced endoperoxide 26, while benzaldehyde was the sole product in zeolite nanocavities [86]. The oxygenation proceeded more efficiently in Li+ - and Na+ -exchanged zeolites which, compared to Rb+ - and Cs+ -exchanged zeolites, do not have the effect of accelerating the intersystem crossing process of the guest molecules. Therefore, it is likely that this sensitized oxygenation occurred through the singlet excited state of the dyes. The oxidation in zeolites was quenched by 1,4diazabicyclo[2.2.2]octane and N,N-dimethylaniline, but not by aliphatic alkene 13 and -carotene. This supported the conclusion that the oxidizing species generated in zeolites was not 1 O2 and that the oxygenation was initiated by photoinduced electron transfer between the dyes and the alkenes.

H H

H M+

δ

O

CH3

R H

hν / Dye / O2

hν / Dye / O2

δ

H2C

OOH

H3C

R

H

Scheme 13

Solution

Zeolite

R R

CHO O

O

O

26

Clennan and Zhou et al. [81–84] and Stratakis and Froudakis [85] independently studied the high regioselective oxygenation by 1 O2 in zeolites. On the basis of the correlation between distribution of products and the bulkiness of the alkyl groups on the C C bond, they first proposed another oxygenation mechanism by 1 O2 , whereby the high regioselectivity found in the oxygenation of trisubstituted alkenes was caused by complexation of a metal cation to the pendant oxygen in the perepoxide intermediate [81, 82, 85], as depicted in Scheme 14. However, Clennan and Sram et al. [83, 84] later revised this mechanism and explained that the novel regiochemistry of the reactions can be rationalized by invoking both cation complexation with the alkenes and electrostatic interaction between the cation and the pedant oxygen. Although further study is required in order to confirm the mechanism, the high regioselectivity in the 1 O2

Scheme 15

5.3. Cis-Trans Photoisomerization There have been many detailed studies of cis-trans photoisomerization in solution for C C and nitrogen-nitrogen double (N N) bond systems, which is one of the most important photoreactions so far discovered [87, 88]. Kuriyama et al. studied the direct (nonsensitized) photoisomerization of stilbene (16) adsorbed in NaY and KY zeolites [89]. Compared to the cis-trans isomer ratio in the photostationary state (PSS) obtained in solution using 254 nm light [(c/t)PSS = 76/24], the ratio of trans isomer in PSS in the zeolites was remarkably high [(c/t)PSS = 5/95].

Photochemistry in Zeolite Nanocavities

558

Na

Figure 12. Optimum structures for t-16–Na+ complex calculated using the AM1 method.

1

1

t*

p*

1

c*

80 Energy / kcal mol –1

On the basis of the optimized structure of Na+ /t-16 complexes calculated using semi-empirical molecular orbital calculations, it was suggested that the trans isomer was better stabilized than the cis isomer (c-16) due to the electrostatic interaction of -electrons on the C C bond with a metal cation, as depicted in Figure 12. Therefore, it was proposed that the interaction increased the activation energy from the excited singlet and triplet states to the ground states, as shown in Figure 13. Because of this, it was argued that deactivation from the excited states to the ground state would occur more efficiently in the zeolite cavities. For 4-aminostilbene (27), on the other hand, (c/t
PSS obtained in NaY using 313 nm light was in close agreement with that in solution: (c/t
PSS = 85/15 in solution and 75/25 in NaY [90]. The result of molecular orbital calculations suggested that the metal cation interacted electrostatically with a lone pair of the nitrogen atom, but not with the -electrons on the C C bond. Therefore, it was concluded that the activation energy from the excited state of the trans isomer to the twisted state was unaffected. Ramamurthy et al. [91] reported that the excited singlet state lifetime of t-16 adsorbed in NaY was 200 ps, which was determined by a single photon counting apparatus. Compared to the lifetime in methylcyclohexane (110 ps), the lifetime measured in NaY seemed to indicate the stabilization of the excited state of t-16 in the zeolite, as suggested by Kuriyama et al. [89]. However, Ellison and Thomas [92] also measured the fluorescence lifetime of t-16 in NaY, which was 52 ps compared to 66 ps in hexane and 32 ps in methanol. Evidently, there is a need for further study of the relation between the photoisomerization and lifetime. Recently, Kojima et al. [93] also investigated the photoisomerization of azobenzene (28) adsorbed in solution. On the basis of the observation that trans-to-cis photoisomerization was very inefficient in higher concentrations and that 4-methoxyazobenzene (29) gave unknown products, which may be dimers in higher concentration, a novel photoisomerization mechanism through excited complexes was proposed. In zeolites (c/t
PSS changed depending on the pore size as summarized in Table 3 [94]. In contrast to the photoisomerization in solution, regardless of the amount of 28 adsorbed, the cis isomer was produced efficiently in NaY (pore size, 7.4 Å) by irradiation using a high-pressure mercury lamp through a HOYA U-340 glass filter [(c/t
PSS = ca. 80/20]. However, (c/t
PSS in Na-mordenite (7.0 × 6.5 Å) was ca. 50/50 and no photoisomerization occurred in NaZSM-5 (5.3 × 5.4 and 5.1 × 5.5 Å). On the basis of molecular orbital calculations it was suggested that the metal cation interacts

60

3 3

t*

3

p*

c*

40 20 0

t-St

c-St

180 90 0 Angle of Twist / Degree Figure 13. Potential energy surface for photoisomerization of stilbene in zeolite cage. Reprinted with permission from [89], Y. Kuriyama et al., Chem. Lett. 843 (1998). © 1998, The Chemical Society of Japan.

with a lone pair of one of the two nitrogen atoms, as shown in Figure 14. These results indicate that photoisomerization can be regulated by both the cavity size and the adsorption sites in guest molecules. Hoffmann and Marlow et al. [95–97] found that cis-trans photoisomerization of 28 in molecular sieve hosts such as ALPO4 -5 and ZSM-5 caused large and reversible changes in the refractive index of the composite systems. This process depended heavily on both the irradiation wavelength and the molecular sieve hosts. Gessner et al. and Baldovi et al. achieved cis-to-trans [98] and trans-to-cis one-way photoisomerization of 16 [99], respectively, using cavity size and the shape of the zeolites. 3,4-Dimethylbenzophenone (30) used as a sensitizer and c-16 have molecular structures larger than the cavity size of silicalite S-115. Because of this, only the trans isomer can be adsorbed in the cavities. Thus, when an isooctane solution containing c-16 and 30 was irradiated with the silicalite, the trans isomer produced was selectively adsorbed in the nanocavities. Due to the constrained space in the cavities, the trans-to-cis photoisomerization did not occur in the cavities. When solvent slurry containing the trans isomer and 4-aminoazobenzophenone adsorbed in ZSM-5 was irradiated, the trans isomer was sensitized by the ketone in the cavities and finally isomerized to the cis isomer in the solution, as illustrated in Scheme 16. The cis isomer was stable because the isomer did not encounter the sensitizer within the cavities due to the fact that its molecular size is larger than the cavity size. This method has considerable potential for the synthesis of cis isomers. Lalitha et al. [100] described the influence of metal cations in Y zeolites in cis-trans isomerization of 4-bromophenyl styryl sulfone (31). The predominance of the cis isomer in zeolites with larger cations was attributed to the smaller cage size. When anils (32) with a C N bond was incorporated in NaY, Raman spectra analysis showed that 32 existed in the zeolite nanocavities predominantly in a zwitterionic form 33 [101] (Scheme 17). Steady state irradiation at 400 nm led to persistent changes in the diffuse reflectance UV-visible spectra with decrease of the reflectance of the band at 400 nm,

Photochemistry in Zeolite Nanocavities

559

Table 3. Cis/trans isomer ratio of azobenzene (28) in photostationary state (PSS) in cyclohexane and NaY and Na-MOR zeolites. cis/trans Isomer ratio in PSS Amountb

C6 H12d

2 × 10−4 M 1 × 10−2 M 2 × 10−4 M 1 × 10−2 M 69 × 10−5 mol/g 34 × 10−5 mol/g 69 × 10−5 mol/g 14 × 10−4 mol/g saturatedc saturatedc

10/90 15/85 80/20 55/45

Wavelengtha /nm 254 313 254 313

254 313

NaY (7.4 Å)e

Na-MOR (70 × 65 Å
e

20/80 75/25 80/20 80/20 20/80 50/50

a

Effective excitation wavelength. Concentration of c- and t-28 in cyclohexane and amount of t-28 loaded in 1 g zeolite. c Cavities of Na-MOR were saturated by t-28. d In cyclohexane. e Pore size. Source: Reprinted with permission from [94], M. Kojima et al., Mol. Cryst. Liq. Cryst. 344, 179 (2000). © 2000, Taylor & Francis Ltd. b

t-16*

t-16

c-16 N OH

hν

32

33

phenolic form

zeolite stabilized zwitterionic form

O ZSM-5

O

N H

NH2 Scheme 17

c-16

c-16* Scheme 16

as well as the appearance of a new absorption band at longer wavelengths. No photochromism was observed and this was attributed to the isomerization of the zwitterionic form to other stereoisomers, possibly by rotation about C C or C N bonds with partial double bond character. Photo-induced electron transfer isomerization of c-16 in Y zeolite cavities using 2,4,6-triphenylpyrylium cation (TP+ ), an electron accepting sensitizer, was studied by Corma et al. [102]. Isomerization of the cis- to trans-isomer occurred

via the corresponding cation radicals. The reaction was not affected by the presence of oxygen. This contrasted with the expensive photo-oxygenation by 2,4,6-triphenylpyrylium tetrafluoroborate under homogeneous conditions. A higher contribution of the in-cage isomerization, associated with retardation of back electron transfer in the ion radical pairs, appeared to be the most noticeable characteristic of the intrazeolite process (Scheme 18). When the novel aluminosilicate MCM-41 with 20 Å monodirectional channels hexagonally arranged was used as a host, the cis-totrans photo-induced electron transfer isomerization of 16 occurred more efficiently than in Y zeolite [103]. Lakshminarasimhan et al. [104] also studied photoinduced electron transfer isomerization of 16 with N-methylacridinium as the sensitizer and found that isomerization of c-16 cation radical to t-16 occurred. Their

Na

N

Ph

N

c-16 Ph

O

Ph

TP / c-16

Zeolite

TP / t-16

In-Cage

TP+

Figure 14. Optimized structure for trans-azobenzene (t-28)–Na+ complex calculated using the AM1 method. Reprinted with permission from [94], M. Kojima et al., Mol. Cryst. Liq. Cryst. 344, 179 (2000). © 2000, Taylor & Francis Ltd.

c-16

Scheme 18

t-16 Free

Photochemistry in Zeolite Nanocavities

560 proposal that a recombination of ion radicals resulted in excited triplet state 16 was based on a cis/trans isomer ratio in the photostationary state similar to that obtained using 2-acetonaphtone as a triplet sensitizer. Diffuse reflectance flash photolysis studies showed that independent of the isomer being sensitized only t-16 cation radical is formed. However, the T-T absorption spectrum for the triplet 16, believed to have been generated, was not observed by this time resolved spectroscopy.

in the NaY supercage probably played a role in controlling regiochemistry.

O

O

O HT

+

35

HH

Scheme 20

On the basis of fluorescence spectra, it was estimated that the number of pyrene molecules per unit cell needed to form an excimer was 0.14–0.30 and 0.02–0.08 in X and Y zeolites, respectively [105, 106]. The excimer emission was observed at a value of 0.00034 in zeolite L, while only the monomer emission was seen at the same value in Mordenite [107]. It has been found that the diffusion rate of guest molecules between the cages slows considerably [108]; therefore, it seems likely that the excimer emission observed coincided with the formation of the ground state complexes. These results indicate that guest molecules do not exist homogeneously in the cavities even in the smallest quantity of adsorbed molecules. The intermolecular interaction of guest molecules is probably enhanced in zeolite nanocavities as described above. But, although numerous investigations of the photodimerization of alkenes in solution have been reported, there are few studies of this process in zeolites. Ramamurthy et al. studied [2 + 2] photodimerization of acenaphthylene (34) in metal-cation exchanged Y zeolites (Scheme 19) [109]. It was found that the ratio of the cis- (35) and trans-cyclobutane dimers (36) changed depending on the intersystem crossing rate enhanced by the metal cations.

34

+

37

5.4. Photodimerization

hν

O

O

hν

36

Scheme 19

Lem et al. [110] studied regioselectivity and stereoselectivity in [2 + 2] photodimerization of cyclopentenone (37) and cyclohexenone (38) in X and Y zeolites. Both enones formed a head-to-tail [2 + 2] dimer (HT) as the major product in solution, while the major dimer produced in zeolites was a head-to-head [2 + 2] dimer (HH), as shown in Scheme 20. The ratio of HT/HH was lowest in CsX, which also had the smallest supercage volume and the weakest electrostatic interaction of the ion. However, a similar result was obtained for 37 and 38 in NaX and CsX, the relative free volumes of which were quite different. Thus, it was suggested that the complexing effect of the charge-compensating cation and the size constriction factor

Lalitha et al. [111] investigated the photodimerization of trans-2-styrylpyridine (t-39) in various cation-exchanged Y zeolite. At a lower loading level, trans-cis isomerization was the only process observed. When the loading level was increased, in addition to isomerization, significant amounts of dimerization (40) and cyclization products (41) were also observed with the product distribution, depending on the free volume available inside the cages (Scheme 21). Acidic zeolites such as HY and MgY were found to catalyze the thermal reaction of t-39.

hν

N

zeolite

N N

N

t-39

N

c-39

40

41

Scheme 21

Brancaleon et al. [112] studied photo-induced electron transfer reactions between excited singlet acceptors and arylalkenes included in NaX zeolites. Diffuse reflectance flash photolysis studies indicated that quenching of singlet cyanoaromatic sensitizers such as 2,3-dicyanonaphthalene and 1-cyanonaphthalene by trans-anethole (42) and 21 occurred through electron transfer and yielded relatively long-lived radical cations, which finally yielded the dimeric cyclobutane products as in solution (Scheme 22 and Figure 15). However, the dimer ratios were substantially different with the cis/syn cyclobutanes formed preferentially in the zeolite reactions, presumably as a result of the constraints imposed by the restricted space of the zeolite supercage.

Ar

Ar Ar CN

MeO

Ar 42 Scheme 22

Photochemistry in Zeolite Nanocavities

561

Figure 15. Transient spectra measured after 355-nm excitation of 2,3dicyanonaphthalene (10 mol/g) plus trans-anethole (42: 52 mol/g) in NaX under nitrogen (•) and oxygen (
). The insert shows the spectrum obtained by direct 266-nm excitation of an oxygen-saturated sample of 10 mol 42 in NaX. Reprinted with permission from [112], L. Brancaleon et al., J. Am. Chem. Soc. 120, 4926 (1998). © 1998, American Chemical Society.

5.5. Photo-Fries and Photo-Claisen Rearrangements Pitchumani and Gu et al. demonstrated by investigating photo-Fries rearrangements of phenyl acetate (35), phenyl benzoate (43) [113], and 1-naphthyl esters (44) [114], in addition to photo-Claisen rearrangements of allyl phenyl ether (45) [113] and aryl benzyl ethers (46) [115], that sharp selectivity consisted in a subtle matching of size and shape of reactants and/or products with the size and shape of pores, cages, and pore volumes of the intracrystalline zeolite phase. In the photorearrangements, a nearly 1  1 mixture of ortho (47 and 48) and para isomers (49 and 50) was normally produced in solution (Scheme 23). However, the photoreactions in zeolites were controlled selectively toward either the ortho or the para products by conducting the reactions either in faujasites X and Y or pentasils ZSM-5 and ZSM-11, respectively. For the photo-Fries reactions of phenyl acetate and phenyl benzoate in X and Y zeolites, the formation of the ortho isomer was highly selective (76–98%). This selectivity was not the result of shape exclusion since both the ortho and para isomers fit well within the supercage. Therefore, it was suggested that the selectivity resulted from the restrictions imposed on the mobility of the phenoxy and the acyl O

R

R

O O

43 O

47

48 Scheme 23

O R

49 OH

OH

OH

45

OH

OH

OH

50

(benzoyl) fragments by the supercage framework and also by the interaction with the cations. Vasenkov and Frei monitored a transient at 2125 cm−1 assigned to acetyl radical generated in NaY by means of step-scan FT-infrared spectroscopy of 1-naphthyl acetate and gained a better mechanistic understanding of the photo-Fries reaction [116]. The kinetic result was interpreted in terms of a complete separation of the photogenerated pairs from the parent supercage, followed by random walks in the subspaces of the zeolite lattice imposed by the much less mobile precursor molecules. It is likely that these forced the geminate radicals to react and thereby contributed to the high selectivity of the photorearrangement. For pentasils ZSM-5 and ZSM-11 only, the para isomers fit the shape and size of the zeolites. Indeed, when photolysis was conducted in water and 2,2,4-trimethylpentane, a clear preference for the para isomer was observed.

5.6. Spectroscopy and Photoreaction of Short-Lived Species H+ -exchanged zeolites such as HY, H-mordenite, and H-ZSM-5 have strong Brønsted acid sites. Using the strong acid sites and an electrostatic field in the zeolites, it has been possible to generate very stable carbocations in the cavities [17–23, 46–48, 117]. Garcia et al. [118] investigated the photoisomerization of 1,3-diphenylpropenylium (51) and 1,5-diphenylpentadienylium (52) ion generated within acid ZSM-5 and Mordenite zeolites as a persistent species by adsorption of ,-disubstituted ,diphenylalkanes (Scheme 24). Irradiation of these allylic cations produced in zeolites led to cis-trans isomerization as the only observable process. X

X

Ph

H+Zeolite

Ph

Ph

51

X= OAc, Cl, OH

H

H

Ph

H Ph

H E,E-51

Ph

Ph

Ph

H

Ph

Ph

H

H

H

H

E,Z-51

Z,Z-51

Scheme 24

O’Neill et al. studied the reactivity of 4-methoxycumyl cation (53) [119] and 9-fluorenyl cation (54) [120] generated in nonacidic zeolites using nanosecond laser flash photolysis (Schemes 25 and 26, respectively). In dry zeolites, the absolute reactivity of 53 was found to be heavily dependent on the nature of the alkali counterion, the Si/Al ratio, and the framework morphology, with the lifetime of the carbocation in Na being almost 10,000-fold longer than in CsY. The observation of the highly reactive and highly unstable 54 under these conditions afforded a remarkable example of the extraordinary ability of nonproton exchanged zeolites to provide kinetic stabilization for electrophilic guests. The

Photochemistry in Zeolite Nanocavities

562

Ar

hν

Ar

Ar

fast

Ar

-e

> 108 s-1 OMe

OMe

53 λ max = 360 nm

λ max = 290 nm

Scheme 25 R

54 R 1* R OH hν

R

OH R

+

M Zeolite

OH

R = H, Me, Et, i-Pr R

OH

Scheme 26

reactivity of 54 was found to be highly influenced by the nature of the alkali metal counterion, as well as by the inclusion of cosolvents within the zeolite matrix. Turro et al. [121] investigated supramolecular effects on the dynamics of radicals generated in zeolites. Isomeric pairs of p p′ -dialkyl-substituted phenyl benzyl ketones (55) adsorbed on MFI zeolites (silicalite and ZSM-5) were photolyzed and investigated by electron paramagnetic resonance (EPR) spectroscopy. The photolysis produced persistent “benzoyl-type” and “benzyl-type” radicals, which depended on the length and position of the p-alkyl chain. The photolysis of dibenzyl ketones (56) adsorbed on MZSM-5 (M = Li, Na, K, Rb, Cs) zeolites also produced persistent carboncentered radicals that were easily observed by conventional steady-state EPR spectroscopy (Scheme 27) [122, 123]. The lifetimes of the radicals increased as the group X attached to the carbon atom at the radical center increased from X = H (t1/2 ca. 2 min) to X = (CH2
4 CH3 (t1/2 > 200 min), as can be seen in Figure 16. In addition, it was shown that the radical lifetimes depended heavily on the size of the metal cation (t1/2 ca. 10 min for Li and t1/2 > 200 min for Cs), as can be seen in Figure 17. Cozens et al. directly observed a single “free” benzyl radical by means of a nanosecond diffuse reflectance laser photolysis of phenylacetic acid (57) incorporated into cation-exchanged Y zeolites [124] (Figure 18). The spectra showed a strong absorption band centered at 315 nm and a shoulder at 305 nm that coincided with the known O CRH C CRH

hν

CRH

56 1R (R = H); 2R (R = Me) 3R (R = Et); 4R [R= (CH2)4CH3]

Scheme 27

absorption spectrum for the benzyl radical in solution. The mechanism proposed for the generation of the benzyl radical is shown in Scheme 28. A second weaker absorption band at 350 nm was assigned to a benzyl anion on the basis of a quenching experiment using oxygen and product analysis. On laser photolysis of xanthene-9carboxylate (58) incorporated in NaY in the absence of oxygen, prompt formation of the xanthyl radical and the xanthylium cation was observed [125]. The xanthyl radical O O

O

O

O

HO

hν

MY H

-CO2

-e

266 nm -CO2

CH2

fast CH2

57 Scheme 28

CRH

-CO

R= H, Me, Et, (CH2)4CH3

Figure 16. CW-EPR spectra (right) of radicals 1R–4R@NaZSM-5 produced by photolysis of 56@NaZSM-5 (irr > 280 nm) and corresponding decay kinetics of the EPR signal (left) after short irradiation (10 s, irr > 280 nm). Reprinted with permission from [123], N. J. Turro et al., J. Org. Chem. 67, 5779 (2002). © 2002, American Chemical Society.

Figure 17. CW-EPR spectra (right) of the radical 2R on cationexchanged zeolites (2R@MZSM-5) produced by photolysis of 56@MZSM-5 (irr > 280 nm) and corresponding decay kinetics of the EPR signal (left) after short irradiation (10 s, irr > 280 nm). Reprinted with permission from [123], N. J. Turro et al., J. Org. Chem. 67, 5779 (2002). © 2002, American Chemical Society.

Photochemistry in Zeolite Nanocavities

563 were much higher than in isotropic solvents. The ratios for the X zeolites were found to decrease as the cation size increased from Li+ to Cs+ , but this was not the case with the Y zeolites. The diastereoselectivity observed in the zeolites was interpreted using the conformational restriction imposed by the cavity size. CH3

CH3

h ν / Pyrex

O Ph

was formed by photoionization of 58 to the corresponding acyloxy radical, which then rapidly decarboxylates, as illustrated in Scheme 29. The xanthylium cation was produced by photoionization of the xanthyl radical. In addition, in the presence of oxygen, the formation of the xanthylium cation was also heavily dependent on the zeolite counterion.

Ph

Ph

OH

E-60

59

Figure 18. Transient diffuse reflectance spectrum generated upon 266nm laser photolysis of phenylacetic acid in evacuated (10−3 Torr) NaY under dry conditions. The inset shows the transient spectrum after the sample was exposed to the atmosphere for a period of 10 s and then reevacuated (10−3 Torr). Spectra were recorded at () 0.20, (△) 0.74, () 1.76, and (•) 6.36 s after the laser pulse. Reprinted with permission from [124], F. L. Cozens et al., J. Am. Chem. Soc. 120, 13543 (1998). © 1998, American Chemical Society.

OH

Z-60

Scheme 30

Lalitha et al. [134] found that irradiation of 2-phenylpropionic acid (61) in various cation-exchanged Y zeolites led to the predominant formation of dl-2,3-diphenylbutane (62) over the meso-isomer, in marked contrast to photolysis in isotropic solvents (Scheme 31). This zeolite-induced diasteroselectivity was attributed to steric and electronic factors.

COOH

α,α-coupling

hν -CO2

H

61

H

dl-62 and meso-62

Scheme 31 H COOH

O

H COO

H COO

h ν / NaY -e

O

O

58 H

h ν / NaY

O

-e

H

O

Scheme 29

5.7. Photochemical Asymmetric Induction

The ene reaction of 1 O2 with the chiral alkene 2-methyl-5phenyl-2-hexene (63) in solution was not regioselective and exhibited very poor diasteroselectivity (∼10% de), as shown in Scheme 32. In contrast, in thionin-supported NaY, significant enhancement of product regioselectivity (94%) and diastereoselectivity (44% de) was obtained [135].

5.7.2. With a Chiral Inductor Irradiation of achiral tropolone methyl ether (64) [130, 136], tropolone ethyl phenyl ether (65) [130, 137], and (S)tropolone 2-methyl butyl ether (66) [130, 138] in solution undergoes a four -electron disrotatory ring closure to yield the chiral bicycle[3.2.0] product 67. For 64 and 65, a racemic

The chiral induction of chemical reactions has been one of the main concerns of chemists for the past few decades. While great advances have been made in thermal asymmetric synthesis [126, 127], asymmetric photochemical reactions have not enjoyed the same level of success [128, 129]. However, several photochemists are exploring the possibility of employing zeolites as media for conducting chiral induction during photoreactions [130–132].

CH3 H3C Ph

1

O2

H3C Ph

5.7.1. With No Chiral Inductor Noh et al. [133] studied the diastereoselectivity in the photocyclization of -(o-ethylphenyl)acetophenone (59) to 1-methyl-2-phenyl-2-indanol (60) in X and Y zeolites (Scheme 30) and made comparisons with the diasteroselectivity in isotropic solvents. The E/Z ratios in the zeolites

CH3

63

CH3 OOH CH3

HOO H3C Ph

CH2 CH3

HOO H3C Ph

In Solution

53%

47% (de 10%)

In Zeolite

6%

94% (de 44%)

Scheme 32

CH2 CH3

Photochemistry in Zeolite Nanocavities

564 mixture of 67 was produced as the result of an equal probability of “in” and “out” rotation, as illustrated in Scheme 33. However, irradiation of 64 with (−)-norphedrine, a chiral inductor, in NaY at −20  C produced 50% ee. For 65, it was found that the nature of the favored enantiomer was reversed between wet and dry zeolites and between NaY and NaX. The extent of ee depended on water content, the nature of the cation (Li+ , Na+ , K+ , Rb+ ), and the number of cations (NaY versus NaX). For 66, it was clearly shown that the cation controlled both the extent and the direction of diastereoselectivity.

R*

O

Ph

R*

Ph

1

Ph

Ph

2

3

O

Ph

69

C1-C3 Rotation

R*

Ph H N

Ph

R*

Ph

Me H ;

R* =

H ;

H

Ph

Me

67

hν

;

etc.

H

Me

COOMe

Scheme 35 OMe

'Out'

NH2 (-)-norephedrine

Me

64

H N

H N

OH

O O

H N

hν

OMe

'In' O

O

Ph

O

O

C1-C2 Rotation

67 Scheme 33

Wada et al. [139] reported that enantio-differentiating photoisomerization of (Z)-cyclooctene (Z-68), sensitized by (R)- or (S)-1-methylheptyl benzoate immobilized in zeolites, afforded the respective enantiomer pair, (−)- and (+)-(E)isomer (E-68) in 5% ee (Scheme 34), while racemic E-68 was obtained on photosensitization with the sensitizer pair in an isotropic solvent.

of 2,2-dimethylnaphthalenone (70) [142] and 2,4cyclohexadienones (71) [143], and the Norrish–Yang reaction of 2-benzoyladamantane-2-carboxylic acid derivative (72) [142] with covalent chiral auxiliaries (Schemes 36 and 37). They also investigated the photochemical conversion of -oxoamides (73) into -lactam derivatives (74) (Scheme 38) [144]. O

O

O hν

COR

ROC

ROC

70

h ν (254 nm) pentane slurries at 25 oC

Medium

de/%

CH2Cl2

0

KY

81

CH2Cl2

9

KY

60

HN Sens immobilized NaY

Me

R= Z-68

(R)-(-)-E-68

(S)-(+)-E-68

Scheme 34

R= O

5.7.3. With a Chiral Auxiliary It was demonstrated by Chong and Sivaguru et al. [140, 141] that alkali metal cation-exchanged Y zeolites significantly enhanced asymmetric induction in the photoisomerization of a number of cis-1,2-diphenylcyclopropane derivatives (69) containing a chiral auxiliary (Scheme 35). The same chiral auxiliary failed to effect asymmetric induction during irradiation in solution. This fact suggested that the confined space of the zeolite was essential to force a chirally significant interaction between the auxiliary and the site of reaction on the three-membered ring. In addition, it was found that the cations present in the zeolite not only controlled the extent of diastereoselectivity but also the isomer that was being enhanced. Jayaraman et al. further confirmed the enhancement of enatio- and diastereo-selectivities via confinement in zeolites by exploring both the photorearrangement

Scheme 36

O

HO

Ph

HO

Ph

Ph

O

Ph

Ph

O

hν

COR

COR

72

R= O

Me R= O

Scheme 37

Medium

de/%

MeCN

22

LiY

79

MeCN Me

NaY

O COR

COR R

4 41

Photochemistry in Zeolite Nanocavities

O

OH

X hν

N X

565

O

74

73

O

Ph

X = p C NH C

H Me

O p C NH C

N

O

O Me ;; p C NH C H

O

Ph

m C NH C

Ph

CH2Ph

O

H COOMe

;

m C NH C

H Me

CH2Ph H COOMe

etc.

Scheme 38

A combination of a chiral inductor and a chiral auxiliary succeeded in boosting the photoproduct de for 66 [138, 145], but failed to do the same for 2,3-diphenylcyclopropane1-carboxylic acid (75) [146]. Although the uniqueness of zeolites as media-inducing enatio- and diastereo-selectivities has been established in the studies described above, the factors that control the high chiral induction in zeolite with respect to solution are yet to be established.

6. CONCLUDING REMARKS Zeolite nanocavities continue to hold considerable attraction for photochemists because of the unique characteristics regulating the spectroscopic and photochemical behavior of included organic substrates. However, it should be noted that the acidity of Brønsted and Lewis acid sites in zeolites may change during the preparation and/or pretreatment for adsorption of organic guest molecules even if the same type of zeolite is used. This problem sometimes makes it difficult to reproduce the results obtained in another laboratory. In addition, co-adsorbents such as water and organic solvents might influence the reactivity of organic guest molecules in the ground and excited states. Therefore, experimental conditions must be carefully arranged. However, there is no doubt that zeolites have considerable potential to act as novel microreactors, which control the spectroscopic and photochemical behavior of organic guest molecules. It is also likely that the use both of transparent poly(dimethylsiloxane) or polyimide films of zeolites incorporating organic guests [147] and also zeolite-coated quartz fibers as media for photochemical and photophysical study [148] will lead to practical applications in industry and to further developments in nanotechnology.

GLOSSARY Contact charge-transfer (CCT) Electronic interaction in which the highest occupied molecular orbital of an electron donor makes contact with the lowest unoccupied molecular orbital of an electron acceptor.

Excited singlet state Electronically excited state in which two electrons spin in opposed directions. Excited triplet state Electronically excited state in which the spin of two electrons is parallel. Fluorescence The emission of light by a molecule in the excited singlet state. Intersystem crossing The process of crossing from the excited singlet state to the excited triplet state. Laser photolysis Photochemical reaction caused by laser light in order to measure time-resolved absorption spectra for short-lived species. Photophorescence The emission of light by a molecule in the excited triplet state. Photostationary state (PSS) The state of equilibrium reached photochemically in which the ratio between starting materials and products remains unchanged on further irradiation. Singlet oxygen A reactive oxygen molecule in the excited singlet state. Zeolite supramolecule Molecules adsorbed in zeolite cavities.

ACKNOWLEDGMENTS The author is grateful to Professor M. Anpo of Osaka Prefecture University for his kind encouragement in studying the photochemistry of zeolite supramolecules and also thanks Ms. M. Nakajoh and Ms. S. Nebashi for their assistance in preparing this article.

REFERENCES 1. V. Ramamurthy, “Photochemistry in Organized and Constrained Media,” VCH, New York, 1991. 2. V. Ramamurthy, D. F. Eaton, and J. V. Casper, Acc. Chem. Res. 25, 299 (1992). 3. V. Ramamurthy, Chimia 46, 359 (1992). 4. J. K. Thomas, Chem. Rev. 93, 301 (1993). 5. K. B. Yoon, Chem. Rev. 93, 321 (1993). 6. M. Anpo, “Surface Photochemistry,” Wiley, Chichester, 1996, Vol. 1. 7. V. Ramamurthy, R. J. Robbins, K. J. Thomas, and P. H. Lakshminarasimhan, in “Organized Molecular Assemblies in the Solid State” (J. K. Whitesell, ed.). John Wiley & Sons, Chichester, 1999, Vol. 2, p. 63. 8. C. Tung, L. Wu, Z. Yuan, J. Guan, Y. Ying, H. Wang, and X. Xu, Materials Science & Engineering C 10, 75 (1999). 9. J. C. Scaiano and H. García, Acc. Chem. Res. 32, 783 (1999). 10. C. Tung, L. Wu, L. Zhang, H. Li, X. Yi, K. Song, M. Xu, Z. Yuan, J. Guan, H. Wang, Y. Ying, and X. Xu, Pure Appl. Chem. 72, 2289 (2000). 11. H. Yamashita and M. Anpo, in “Photofunctional Zeolites” (M. Anpo, ed.). Nova Science Publishers, Inc., Huntington, NY, 2000, p. 99. 12. M. Kojima, Kokagaku 31, 173 (2000). 13. M. Kojima, Kagaku Kogyo 53, 666 (2002). 14. V. Ramamurthy, Mol. Cryst. Liq. Cryst. 240, 53 (1994). 15. W. Zhou and E. Clennan, J. Am. Chem. Soc. 121, 2915 (1999). 16. S. Hashimoto, T. Miyashita, and M. Hagiri, J. Phys. Chem. B. 103, 9149 (1999). 17. V. Ramamurthy, J. V. Casper, and D. R. Corbin, J. Am. Chem. Soc. 113, 594 (1991).

566 18. X. Liu, K.-K. Iu, J. K. Thomas, H. He, and J. Klinowski, J. Am. Chem. Soc. 116, 11811 (1994). 19. J. F. Haw, J. B. Nicholas, T. Xu, L. W. Beck, and D. B. Ferguson, Acc. Chem. Res. 29, 259 (1996). 20. V. Ramamurthy, P. Lakshminarasimhan, C. P. Grey, and L. J. Johnston, J. Chem. Soc. Chem. Commun. 2411 (1998). 21. H.-M. Kao, C. P. Grey, K. Pitchumani, P. H. Lakshminarasimhan, and V. Ramamurthy, J. Phys. Chem. A 102, 5627 (1998). 22. P. Lakshminarasimhan, K. J. Thomas, L. Brancaleon, P. D. Wood, L. J. Johnston, and V. Ramamurthy, J. Phys. Chem. B, 103, 9247 (1999). 23. T. Herbertz, P. S. Lakkaraju, F. Blume, M. Blume, and H. D. Roth, Eur. J. Org. Chem. 467 (2000). 24. W. L. Hsu and C. T. Lin, J. Phys. Chem. 94, 3780 (1990). 25. S. Pankasem and J. K. Thomas, J. Phys. Chem. 95, 7385 (1991). 26. P. Szedlacsek, S. L. Suib, M. Deeba, and G. S. Koermer, J. Chem. Soc. Chem. Commun. 1531 (1990). 27. X. Liu and J. K. Thomas, Langmuir 9, 727 (1993). 28. M. Kojima, A. Odaka, T. Nakajima, and Y. Kuriyama, Chem. Lett. 675 (1999). 29. F. D. Lewis, G. D. Reddy, J. E. Elbert, B. E. Tillberg, J. A. Meltzer, and M. Kojima, J. Org. Chem. 56, 5311 (1991). 30. S. Corrent, P. Hahn, G. Pohlers, T. J. Connolly, J. C. Scaiano, V. Fornes, and H. Garcia, J. Phys. Chem. B 102, 5852 (1998). 31. V. Ramamurthy, D. R. Sanderson, and D. F. Eaton, J. Am. Chem. Soc. 115, 10438 (1993). 32. J. Shailaja, J. Siveguru, R. J. Robbins, V. Ramamurthy, R. B. Sunoj, and J. Chandrasekhar, Tetrahedron 56, 6927 (2000). 33. K. Iu and J. K. Thomas, Langmuir 6, 471 (1990). 34. C.-H. Tung, H. Wang, and Y.-M Ying, J. Am. Chem. Soc. 120, 5179 (1998). 35. V. Ramamurthy, J. V. Caspar, D. F. Eaton, E. W. Kuo, and D. R. Corbin, J. Am. Chem. Soc. 114, 3882 (1992). 36. R. Sadeghpoor, M. Ghandi, H. M. Najafi, and F. Farzaneh, J. Chem. Soc. Chem. Commun. 329 (1998). 37. K. Pitchumani, M. Warrier, J. R. Scheffer, and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 1197 (1998). 38. M. Warrier, N. J. Turro, and V. Ramamurthy, Tetrahedron Lett. 41, 7163 (2000). 39. S. Uppili, S. Takagi, R. B. Sunoj, P. Lakshminarasimhan, J. Chandrasekhar, and V. Ramamurthy, Tetrahedron Lett. 42, 2079 (2001). 40. D. R. Corbin, D. F. Eaton, and V. Ramamurthy, J. Am. Chem. Soc. 110, 4848 (1988). 41. V. Ramamurthy, D. R. Corbin, N. J. Turro, and Y. Sato, Tetrahedron Lett. 30, 5829 (1989). 42. V. Ramamurthy, D. R. Corbin, and D. F. Eaton, J. Org. Chem. 55, 5269 (1990). 43. V. Ramamurthy, X.-G. Lei, N. J. Turro, T. J. Lewis, and J. R. Scheffer, Tetrahedron Lett. 32, 7675 (1991). 44. V. Ramamurthy and D. R. Sanderson, Tetrahedron Lett. 33, 2757 (1992). 45. V. Ramamurthy and N. J. Turro, J. Incl. Phenom. Mol. Recog. Chem. 21, 239 (1995). 46. V. J. Rao, N. Prevost, V. Ramamurthy, M. Kojima, and L. J. Johnston, J. Chem. Soc. Chem. Commun. 2209 (1997). 47. F. L. Cozens, R. Bogdanova, M. Régimbald, H. García, V. Martí, and J. C. Scaiano, J. Phys. Chem. B 101, 6921 (1997). 48. V. Fornés, H. García, V. Martí, and L. Fernández, Tetrahedron 54, 3827 (1998). 49. D. F. Evans, J. Chem. Soc. 345 (1953). 50. M. Kojima, in “Handbook of Photochemistry and Photobiology” (H. S. Nalwa, ed.). American Scientific Publishers, Stevenson Ranch, CA 2003, Vol. 2, Chap. 11, p. 501. 51. M. Kojima, H. Sakuragi, and K. Tokumaru, Tetrahedron Lett. 22, 2889 (1981).

Photochemistry in Zeolite Nanocavities 52. M. Kojima, H. Sakuragi, and K. Tokumaru, Bull. Chem. Soc. Jpn. 62, 3863 (1989). 53. K. Onodera, G. Furusawa, M. Kojima, M. Tsuchiya, S. Aihara, R. Akaba, H. Sakuragi, and K. Tokumaru, Tetrahedron 41, 2215 (1985). 54. M. Kojima, H. Sakuragi, and K. Tokumaru, Bull. Chem. Soc. Jpn. 60, 3331 (1987). 55. M. Kojima, A. Ishida, and S. Takamuku, Chem. Lett. 979 (1993). 56. M. Kojima, A. Ishida, and S. Takamuku, Bull. Chem. Soc. Jpn. 71, 2211 (1998). 57. F. Blatter and H. Frei, J. Am. Chem. Soc. 115, 7501 (1993). 58. F. Blatter and H. Frei, J. Am. Chem. Soc. 116, 1812 (1994). 59. H. Sun, F. Blatter and H. Frei, J. Am. Chem. Soc. 116, 7951 (1994). 60. F. Blatter, F. Moreau, and H. Frei, J. Phys. Chem. 98, 13403 (1994). 61. F. Blatter, H. Sun, and H. Frei, Catal. Lett. 35, 1 (1995). 62. F. Blatter, H. Sun, and H. Frei, Chem. Eur. J. 2, 385 (1996). 63. H. Sun, F. Blatter, and H. Frei, J. Am. Chem. Soc. 118, 6873 (1996). 64. H. Frei, F. Blatter, and H. Sun, Chemtech, 26, 24 (1996). 65. H. Sun, F. Blatter, and H. Frei, Catal. Lett. 44, 247 (1997). 66. S. Vasenkov and H. Frei, J. Phys. Chem. B 101, 4539 (1997). 67. F. Blatter, H. Sun, S. Vasenkov, and H. Frei, Catal. Today, 41, 297 (1998). 68. S. Vasenkov and H. Frei, Mol. Supramol. Photochem. 5, 295 (2000). 69. K. B. Myli, S. C. Larsen, and V. H. Grassian, Catal. Lett. 48, 199 (1997). 70. Y. Xiang, S. C. Larsen, and V. H. Grassian, J. Am. Chem. Soc. 121, 5063 (1999). 71. A. G. Panov, R. G. Larsen, N. I. Totah, S. C. Larsen, and V. H. Grassian, J. Phys. Chem. B 104, 5706 (2000). 72. R. G. Larsen, A. C. Saladino, T. A. Hunt, J. E. Mann, M. Xu, V. H. Grassian, and S. C. Larsen, J. Catal. 204, 440 (2001). 73. H. Takeya, Y. Kuriyama, and M. Kojima, Tetrahedron Lett. 39, 5967 (1998). 74. M. Kojima, M. Nakajoh, C. Matsubara, and S. Hashimoto, J. Chem. Soc. Perkin Trans. 2, 1894 (2002). 75. C. Matsubara and M. Kojima, Tetrahedron Lett. 40, 3439 (1999). 76. C. Matsubara and M. Kojima, Res. Chem. Intermed. 27, 975 (2001). 77. A. Itoh, T. Kodama, Y. Masaki, and S. Inagaki, Synlett 522 (2002). 78. P. Lakshminarasimhan, K. J. Thomas, L. J. Johnston, and V. Ramamurthy, Langmuir 16, 9360 (2000). 79. X. Li and V. Ramamurthy, J. Am. Chem. Soc. 118, 10666 (1996). 80. R. J. Robbins and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 1071 (1997). 81. E. L. Clennan and J. P. Sram, Tetrahedron Lett. 40, 5275 (1999). 82. W. Zhou and E. L. Clennan, Org. Lett. 2, 437 (2000). 83. E. L. Clennan and J. P. Sram, Tetrahedron 56, 6945 (2000). 84. E. L. Clennan, J. P. Sram, A. Pace, K. Vincer, and S. White, J. Org. Chem. 67, 3975 (2002). 85. M. Stratakis and G. Froudakis, Org. Lett. 2, 1369 (2000). 86. X. Li and V. Ramamurthy, Tetrahedron Lett. 37, 5235 (1996). 87. T. Arai and K. Tokumaru, Chem. Rev. 93, 23 (1993). 88. H. Rau, in “Photochromism: Molecules and Systems,” (H. Dürr and H. Bouas-Laurent, eds.), p. 165, Elsevier, Amsterdam, 1990. 89. Y. Kuriyama, H. Takeya, S. Oishi, and M. Kojima, Chem. Lett. 843 (1998). 90. M. Kojima and C. Matsubara, “The Conference of Japanese Photochemical Association 2000, The Book of Abstracts,” p. 35. 91. V. Ramamurthy, J. V. Casper, D. R. Corbin, D. F. Eaton and C. Dybowski, J. Photochem. Photobiol. A: Chem. 51, 259 (1990). 92. E. H. Ellison and J. K. Thomas, J. Phys. Chem. B 105, 2757 (2001). 93. M. Kojima, T. Takagi, and T. Karatsu, Chem. Lett. 686 (2000). 94. M. Kojima, T. Takagi, and T. Goshima, Mol. Cryst. Liq. Cryst. 344, 179 (2000). 95. K. Hoffmann, F. Marlow, and J. Caro, Adv. Mater. 9, 567 (1997). 96. F. Marlow and K. Hoffmann, Ber. Bunsenges. Phys. Chem. 101, 1731 (1997).

Photochemistry in Zeolite Nanocavities 97. K. Hoffmann, U. Resch-Genger, and F. Marlow, Microporous and Mesoporous Mater. 41, 99 (2000). 98. F. Gessner, A. Olea, J. H. Lobaugh, L. J. Johnston, and J. C. Scaiano, J. Org. Chem. 54, 259 (1989). 99. M. V. Baldoví, A. Corma, H. García, and V. Martí, Tetrahedron Lett. 35, 9447 (1994). 100. A. Lalitha, K. Pitchumani, P. Kannan, and C. Srinivasan, Tetrahedron 54, 15667 (1998). 101. I. Casades, M. Álvaro, H. García, and M. N. Pillai, Eur. J. Org. Chem. 2074 (2002). 102. A. Corma, V. Fornés, H. García, M. A. Miranda, J. Primo, and M. J. Sabater, J. Am. Chem. Soc. 116, 2276 (1994). 103. A. Corma, V. Fornés, H. García, M. A. Miranda, and M. J. Sabater, J. Am. Chem. Soc. 116, 9767 (1994). 104. P. H. Lakshminarasimhan, R. B. Sunoj, S. Karthikeyan, J. Chandrasekhar, L. J. Johnston, and V. Ramamurthy, J. Photochem. Photobiol. A: Chem. 153, 41 (2002). 105. S. L. Suib and A. Kostapapas, J. Am. Chem. Soc. 106, 7705 (1984). 106. X. Liu, K.-K. Iu, and J. K. Thomas, J. Phys. Chem. 93, 4120 (1989). 107. X. Liu and J. K. Thomas, Chem. Mater. 6, 2303 (1994). 108. A. V. Barzkin and S. Hashimoto, J. Chem. Phys. 113, 2841 (2000). 109. V. Ramamurthy, D. R. Corbin, C. V. Kumar, and N. J. Turro, Tetrahedron Lett. 31, 47 (1990). 110. G. Lem, N. A. Kaprinidis, D. I. Schuster, N. D. Ghatlia, and N. J. Turro, J. Am. Chem. Soc. 115, 7009 (1993). 111. A. Lalitha, K. Pitchumani, and C. Srinivasan, J. Photochem. Photobiol. A: Chem. 134, 193 (2000). 112. L. Brancaleon, D. Brousmiche, V. J. Rao, L. J. Johnston, and V. Ramamurthy, J. Am. Chem. Soc. 120, 4926 (1998). 113. K. Pitchumani, M. Warrier, and V. Ramamurthy, J. Am. Chem. Soc. 118, 9428 (1996). 114. W. Gu, M. Warrier, V. Ramamurthy, and R. G. Weiss, J. Am. Chem. Soc. 121, 9467 (1999). 115. W. Gu, M. Warrier, B. Schoon, V. Ramamurthy, and R. G. Weiss, Langmuir 16, 6977 (2000). 116. S. Vasenkov and H. Frei, J. Phys. Chem. A, 104, 4327 (2000). 117. K. Pitchumani and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 2763 (1996). 118. H. García, S. García, J. Pérez-Prieto, and J. C. Scaiano, J. Phys. Chem. 100, 18158 (1996). 119. M. A. O’Neill, F. L. Cozens, and N. P. Schepp, J. Am. Chem. Soc. 122, 6017 (2000). 120. M. A. O’Neill, F. L. Cozens, and N. P. Schepp, Tetrahedron 56, 6969 (2000). 121. N. J. Turro, X. Lei, S. Niu, Z. Liu, S. Jocksch, and M. F. Ottaviani, Org. Lett. 2, 3991 (2000). 122. N. J. Turro, X. Lei, S. Jockusch, W. Li, Z. Liu, L. Abrams, and M. F. Ottaviani, J. Org. Chem. 67, 2606 (2002). 123. N. J. Turro, S. Jockusch, and X. Lei, J. Org. Chem. 67, 5779 (2002).

567 124. F. L. Cozens, W. Ortiz, and N. P. Schepp, J. Am. Chem. Soc. 120, 13543 (1998). 125. F. L. Cozens, M. L. Cano, H. Garcia, and N. P. Schepp, J. Am. Chem. Soc. 120, 5667 (1998). 126. M. Avalos, R. Babiano, P. Cintas, J. L. Jimenez, and J. C. Palacios, Chem. Rev. 98, 2391 (1998). 127. A. Richards and R. McCague, Chem. Ind. 422 (1997). 128. Y. Inoue, Chem. Rev. 92, 741 (1992). 129. J. P. Pete, Adv. Photochem. 21, 135 (1996). 130. A. Joy and V. Ramamurthy, Chem. Eur. J. 6, 1287 (2000). 131. J. Sivaguru, J. Shailaja, S. Uppili, K. Ponchot, A. Joy, N. Arunkumar, and V. Ramamurthy, in “Organic Solid-State Reactions (F. Toda, ed.), Kluwer Academic Publishers, Dordrecht, 2002, p. 159. 132. E. Brunet, Chirality 14, 135 (2002). 133. T. Noh, K. Choi, H. Kwon, D. J. Chang, and B. S. Park, Bull. Korean Chem. Soc. 20, 539 (1999). 134. A. Lalitha, K. Pitchumani, and C. Srinivasan, Tetrahedron 57, 4455 (2001). 135. M. Stratakis and G. Kosmas, Tetrahedron Lett. 42, 6007 (2001). 136. A. Joy, A. Joy, J. R. Scheffer, D. R. Corbin, and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 1379 (1998). 137. A. Joy, J. R. Scheffer, and V. Ramamurthy, Org. Lett. 2, 119 (2000). 138. A. Joy, S. Uppili, M. R. Netherton, J. R. Scheffer, and V. Ramamurthy, J. Am. Chem. Soc. 122, 728 (2000). 139. T. Wada, M. Shikimi, Y. Inoue, G. Lem, and N. J. Turro, J. Chem. Soc. Chem. Commun. 1864 (2001). 140. K. C. W. Chong, J. Sivaguru, T. Shichi, Y. Yoshimi, V. Ramamurthy, J. R. Scheffer, J. Am. Chem. Soc. 124, 2858 (2002). 141. J. Sivaguru, J. R. Scheffer, J. Chandarasekhar, and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 830 (2002). 142. S. Jayaraman, S. Uppili, A. Natarajan, A. Joy, K. C. W. Chong, M. R. Netherton, A. Zenova, J. R. Scheffer, and V. Ramamurthy, Tetrahedron Lett. 41, 8231 (2000). 143. S. Uppili and V. Ramamurthy, Org. Lett. 4, 87 (2002). 144. A. Natarajan, K. Wang, V. Ramamurthy, J. R. Scheffer, and B. Patrick, Org. Lett. 4, 1443 (2002). 145. J. Shailaja, J. Sivaguru, S. Uppili, A. Joy, and V. Ramamurthy, Microporous and Mesoporous Mater. 48, 319 (2001). 146. E. Cheung, K. C. W. Chong, S. Jayaraman, V. Ramamurthy, J. R. Scheffer, and J. Trotter, Org. Lett. 2, 2801 (2000). 147. M. Alvaro, H. Garcia, S. Corrent, and J. C. Scaiano, J. Phys. Chem. B 102, 7530 (1998). 148. A. R. Pradhan, S. Uppili, J. Shailaja, J. Sivaguru, and V. Ramamurthy, J. Chem. Soc. Chem. Commun. 596 (2002). 149. H. Satozono, S. Suzuki, N. Tokou, H. Takehara, and Y. Uno, J. Chem. Soc. Jpn., Chem. Indus. Chem. 115 (1994).