Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7

g-Cyclodextrin mediated photo-heterodimerization between cinnamicacids and coumarins$

Aspen Rae Clements, Mahesh Pattabiraman *University of Nebraska Kearney, Department of Chemistry, 905 west 25th St., Kearney, NE 68849, USA

A R T I C L E I N F O

Article history:Received 6 September 2014Received in revised form 1 October 2014Accepted 3 October 2014Available online 7 October 2014

Keywords:PhotochemistryHost–guestSupramolecularPhotodimerizationCyclodextrinHeterodimerizationWeak interactions

A B S T R A C T

The ability of g-cyclodextrin to form ternary inclusion complexes has been utilized to mediate photo-heterodimerization between cinnamic acids and coumarins in the solid-state. Stabilizing steric andelectronic interactions between the alkenes in the inclusion complex appear to be primarily responsiblefor directing the complexation toward the 1:1:1 hetero-guest pair complex. Photoexcitation of thecomplex resulted in formation of the heterodimers in proportions significantly higher than the combinedyields of the homodimers for alkene pairs in which complementary electronic and/or stericcharacteristics were present.

ã 2014 Elsevier B.V. All rights reserved.

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Journal of Photochemistry and Photobiology A:Chemistry

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1. Introduction

Alkene [2 + 2] photodimerization is one of the well-studied andhighly useful organic photochemical reactions [1]. As a one-stepchemical transformation that produces highly strained cyclo-butane ring systems, the reaction finds application in organicsynthesis [2], green chemistry [3], and polymer [4] and materialsciences [5]. However, unimolecular photoisomerization oftenprecludes the bimolecular photodimerization resulting in poordimer yields. Bimolecular nature of photodimerization reactionalso makes it difficult to achieve stereoselectivity in the reaction.Strategies were developed in the past to improve the efficiency ofphotodimerization [6], thereby improving the applicability of thisreaction. While photodimerization between two identical alkenes(homodimerization) is very well explored, the reaction betweennon-identical alkenes (heterodimerization) has been studied tomuch lesser extent. Given the potential of photodimerizationreactions in fields of applied chemistry, developing new means foreffecting photo-heterodimerization will expand its utility.

Two frequently employed methods used to achieve stereo-selective photodimerization are the solid-state templation [7], and

$ The enantiochemical aspect of this chemistry has been consciously neglected.We believe that lack of focus on this particular aspect does not limit ourunderstanding of supramolecular principles governing cavity mediated hetero-dimerization or diminish its significance. Any and all mentions of heterodimersmade herein refer to both the optical antipodes.* Corresponding author. Tel.: +1308 865 8385.E-mail address: pattabiramm2@unk.edu (M. Pattabiraman).

http://dx.doi.org/10.1016/j.jphotochem.2014.10.0011010-6030/ã 2014 Elsevier B.V. All rights reserved.

the host–guest inclusion complex methods [6b,8]. The formertakes advantage of the molecular templates, which control relativemolecular orientations to pre-dispose the alkenes in the solid-statethrough intermolecular forces. Photoirradiation of these solid-state molecular assemblies often result in high yields of photo-dimers with remarkable stereoselectivity. However, this method isless general due to the unpredictable nature of molecular packingin the solid-state [7b,9]. On the other hand, the host–guestcomplexation method has been shown to be simple and efficient inmediating the photodimerization of a number of alkenes. In thismethod, two alkenes are encapsulated within a macromolecularhost to form 1:2H:G inclusion complex [6b,8a,b,10]. The juxtapo-sition of alkene bonds achieved through inclusion is conducive forphotodimerization. In addition, owing to the limited cavity volumethe alkenes experience restricted intermolecular and conforma-tional freedoms. Photodimerization is facilitated within the cavityas it requires less structural reorganization than the unimolecuarisomerization process.

In 2003, Yoshizawa et al. successfully reported the macromolec-ular host directed photo-heterodimerization between acenaphthy-lenes and maleimides within the cavity of synthetic palladium-nanocage (coordination cage) in aqueous medium [11]. Theirstrategy involved directing the inclusion process toward the1:1:1 complex by taking advantage of complementarity in sizebetween a large guest (acenaphthylene) and small guest (malei-mide), as the cavity of the nanocage is not large enough toaccommodate two large guests (acenaphthylene) simultaneously.Photoirradiation resulted in near quantitative conversions withabsolute stereoselectivity. Interestingly, there have been no

2 A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7

significant research activity exploring the macromolecular hostmediatedphoto-heterodimerizationsince then. Considering the lackof research activity in this area despite the need for developing newstrategies for photodimerization, and inspired by the report byYoshizawa et al., we endeavored to study heterodimerization withina readily available and environmentally benign macromolecularhost–g-cyclodextrin.

Cyclodextrins are a family of macrocyclic cavitands known fortheir versatile host–guest chemistry [12]. The ability of g-cyclo-dextrin (g-CD) to encapsulate two alkenes simultaneously to form1:2 inclusion complexes has been employed to direct stereo-selective photodimerization in solution phase and in the solid-state [6b,8b,10a–d]. However, photo-heterodimerization withing-CD has not been demonstrated. In addition, g-CD possessesadvantages over the nanocage due to its low cost, readyavailability, and lack of aromatic chromophores. In this communi-cation, we report the successful selective photo-heterodimeriza-tion between substituted cinnamic acids and coumarins directedby g-CD, in which the physicochemical complementarity betweennon-identical alkenes has been utilized to direct selectivity.Intrigued by the strategy employed by Yoshizawa et al., wereasoned that spatial and electronic complementarity betweenalkenes could be realized when alkenes are confined to the limitedcavity volume of g-CD through inclusion complex formation. Thisidea is summarized in Scheme 1 wherein two large guestmolecules are unable to be fit within a cavity. This drives thecomplexation toward the 1:1:1 complex. Thus, the hetero-guestpair complex, which is more favorable, is expected to be present inhigher proportion(s) than the homo-guest pair complexes yieldingpredominantly heterodimers upon irradiation (Scheme 1). Wechose to study the cross-dimerization between cinnamic acids [13]and coumarins [14] as their individual photochemistries are welldocumented in literature. Given the practical significance of thetwo alkene systems, the cross-photodimerization between them isexpected to be of importance.

2. Results and discussion

We initiated our investigation with the 6-methyl coumarin–cinnamic acid pair. The choice of alkenes was based on the possiblesteric complementarity between 6-methyl coumarin (6-MeCU)with a bulky methyl group, compared to than the unsubstitutedcinnamic acid (CA) (Scheme 2). Sonication of an equimolar mixtureof 6-methyl coumarin, cinnamic acid, and g-CD in water yielded awhite precipitate which was filtered, washed with water andorganic solvents, dried, and irradiated between glass plates using amedium pressure mercury vapor lamp.

1H NMR analysis of the photoproduct mixture isolated usingbiphasic extraction showed the presence of four distinct pseudo-triplet signals of equal intensity (Fig. 1, bottom spectrum) in thetypical cyclobutane region (3–5 ppm); the signals were presentalongside the syn H–H homodimers of cinnamic acid and 6-methylcoumarins which are known to be formed within g-CD [8b,10c].The pseudo-triplets corresponded to the multiplicity pattern ofcyclobutane skeleton with non-equivalent protons, as is expectedin a heterodimer. The compound corresponding to the signal was

Scheme 1. Depiction of the strategy employed for directin

isolated through flash column chromatography and characterizedfurther using COSY (Fig. 2) in which the cross-peaks between thepseudo triplets further confirmed that the isolated product is infact a heterodimer.

Based on the 1H NMR profile, peak integrations, and correlationbetween the cyclobutane signals in COSY analysis, the compound isascertained to be the 6-methyl coumarin–cinnamic acid hetero-dimer. We are not able to identify the stereochemistry of theheterodimer through spectroscopy alone. However, we willattempt to deduce it based on the known host–guest chemistryof cyclodextrin complexes and computational analysis of theinclusion complexes (vide infra). Irradiation of mechanicallyground mixture of the two alkenes without subjecting it to thecomplexation process, used as the control, did not yield the hetero-dimer in any significant quantity (Fig. 1, top spectrum). The homo-dimers were the predominant products which presumablyresulted from the topochemical reactivity of the respective alkenesin solid-state; the neat solids of cinnamic acids and coumarins areknown to yield various photodimers depending on the packing insolid-state.

The heterodimer was the major product in the reaction (60%)followed by significantly lesser amount of the 6-methyl coumarinhomodimer. The homodimer of cinnamic acid and cis-cinnamicacid were minor products. Mass balance for the reaction at around85% conversion was greater than 93%. This indicated that theobserved selective formation of heterodimer as the major productis due to the overall reactivity of the alkenes in the bulk of thesample, and not due to uneven reactivity from multiple alkeneorientations or selective extraction of the heterodimer from theproduct mixture.

As both alkenes are photoactive, a total of twelve homo- andheterodimers are possible in the reaction, in addition to theisomerization product; it is remarkable to observe that the 1H NMRspectrum of the reaction mixture is much cleaner signifying theextent of reaction control achieved through the cavity directedstrategy. In order to better understand the reaction outcome anddeduce the heterodimer structure, we decided to study thestructure of heterodimer included within the cyclodextrin cavitycomputationally. Geometry optimized structures of inclusioncomplexes of the four possible coumarin–cinnamic acid hetero-dimers included within g-CD performed using the semi-empiricalPM3 method built in Gaussian1 09 program [15] are presented inFig. 3. Optimizations were performed on initial structures in whichboth the aromatic groups were included within the cavity, withoutregard to the van der Waals radii. Among the optimized fourinclusion complexes, the syn H–H dimer was the only dimercomplex in which both the hydrophobic aromatic rings wereincluded well within the cavity, while the hydrophilic carboxygroups of cinnamic acid and coumarin protruded outward. In allother cases, despite lack of any bulky substituents in the aromaticring, only one of the aromatic rings remained included within thecavity, while the other was expelled. This is presumably so becausethe syn H–H dimer was compact enough to fit both the aromaticrings within the cavity, while the other three heterodimers consistof aromatic rings that are spaced much farther away to besimultaneously included within the cavity. As the inclusion

g heterodimerization based on size complementarity.

xelpmoc2:1xelpmoc1:1:1 1: 2 complex

Heterodime rization Homod imerization

hυ hυ hυ

6-methyl coumarin

Cinnamicacid

O

H HH

O

O

H HH

O

O

H HH

O

COOHO O

COO HCOOH

Scheme 2. Photo-heterodimerization between 6-methyl coumarin and cinnamic acid mediated by the cavity of g-cyclodextrin (g-CD). Complex structures are depictedbased on previously reported studies which indicate that the hydrophobic aromatic moieties remain included within the cavity, while the hydrophilic groups protrude out.

2.53.03.54.04.55.05.56.06.57.07.5 ppm

2.53.03.54.04.55.05.56.06.57.07.5 ppm

* *• •

#^^^^

#

HOOC COOHO O

O O

HOOCO

O

syn H- H cinnamic acid dimer syn H-H 6- methyl

coumarin dimer

*

syn H- H heterod imer#

Fig. 1. (Top) Irradiation of mechanically ground mixture of cinnamic acid and 6-methyl coumarin in absence of g-CD. The cyclobutane proton signals of syn H–H photodimersof cinnamic acid (*) and 6-methyl coumarin (�) could be seen. (Bottom) 1H NMR of reaction mixture from irradiation of g-CD mediated heterodimerization. The cyclobutanesignals of heterodimer appear as pseudo-triplets (^).

Fig. 2. Partial COSY spectrum of the 6-MeCU–CA photo-heterodimer purified from the reaction mixture with column chromatography.

A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7 3

4 A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7

complex formation is based on the ability of the hydrophobicmoieties to avoid aqueous environment, the complex in which thehydrophobic groups are included within the cavity and thehydrophilic groups are facing out is expected to be the actualstructure. The inability of the cavity to include the non-syn H–Hheterodimers is expected be even greater when a methyl group ispresent in the aromatic ring. Therefore, we deduce that theheterodimer observed to be the major product in the 6-methylcoumarin–cinnamic acid reaction is the most likely the syn H–Hisomer. This finding is also consistent with the stereochemistry ofthe structures previously reported for the homodimerization ofcinnamic acids, and coumarins within g-CD cavity in which themost compact syn H–H was formed almost exclusively [8b,10c].

Unlike the inclusion complexes in solution phase, host andguest in the solid-state complexes do not remain in dynamicequilibrium through the bulk. Moreover, energy required forstructural reorganization in solid-state is quite high. Therefore, it isreasonable to assume that the structure of the photodimer isstructurally similar to the arrangement of the alkenes within thecavity. Thus, the presence of �60% heterodimer in the productdistribution suggests that the predominant complex stoichiometryis 1:1:1 (g-CD:CA:6MeCU), with the remaining complex stoichi-ometry being 1:2 of homo-complexes (though predominantly the6-methyl coumarin homo-complex).

In order to understand that the extent to which steric effect isresponsible for the observed selectivity more cinnamic acid–coumarin combinations were tested. List of alkene combinationsand product distributions obtained in the reactions are provided inTable 1. The proportions of the photoproducts were calculatedbased on intensities of unambiguous and characteristic signals ofheterodimer, homo-dimer, and cis cinnamic acids in 1H NMRspectrum of the product mixtures.

The cinnamic acid (CA) and coumarin (CU) pair was explorednext. In this pair no bulky methyl groups are present, and hence nosignificant size complementarity. This combination still yieldedheterodimer as the major product, but the proportion of theheterodimer was lesser than that of the 6-MeCU–CA pair. Wespeculate that the lower proportion of the heterodimer in the CA–CU combination was due to the lack of steric complementarity, aswas present in the 6-MeCU–CA pair. The fact that the heterodimeris still the major dimer despite lack of bulky methyl group suggeststhat electronic interactions might also be in play between the twoalkenes. Possibly weak electronic interactions such as pi–pi and/ordonor–acceptor type of interactions might be responsible for the

Fig. 3. Geometry optimized semi-empirical structures of four different heterodimerimplemented in Gaussian1 09.

higher proportion of the heterodimer. However, more experimen-tal and computational studies need to be performed to categori-cally state the effect of such influence. We will be studying this ingreater depth in the future.

Similar combinations (3-MeCA–CU, and 3-MeOCA–CU), inwhich only one alkene possessed a bulky group, also yieldedproduct distribution in which the heterodimer is present inproportions much higher than both the homodimers combined.We also explored one pair in which both the alkenes possessed abulky substituent: 6-methyl coumarin and 3-methoxy cinnamicacid. In this combination, the proportion of the heterodimer wassignificantly lower than that of the homodimers. The results werenot surprising, as there is no complementarity in this alkenecombination, and hence, no significant driving force for selectiveformation 1:1:1 complex. These observations appear to validateour original idea that the spatial complementarity could have astrong influence in governing the proportion of homo- and hetero-guest pair complexes, and thus, utilized in mediating photo-heterodimerization. The 4-MeCA–CU pair, despite the sizecomplementarity, did not yield heterodimer as the major product.While we are unable to pin-point the reason behind thisobservation, we believe that the placement of the methyl groupin the para position allows it to remain outside the cavity. Thiswould result in a 1:2 homo-complex with lower steric conflictbetween the methyl groups as they need not be confined withinthe cavity.

Alkene pairs which differed in their electronic properties werealso investigated. The second set of alkenes listed in Table 1consists of halogenated cinnamic acid (electron deficient aromaticring) paired up with coumarin/6-methyl coumarin, in whichcomplementary electronic interactions could potentially occur.The photodimerization between 2-chlorocinnamic acid (2-ClCA)and coumarin (CU) yielded 51% heterodimer and 40% ofhomodimers. The reaction of the same halogenated compoundwith 6-MeCU yielded 12% higher amount of heterodimer. This isperhaps due to the presence of the electron withdrawing chlorineatom (negative inductive effect) in CAs which enables them toengage in a stabilizing weak interaction with CU and 6-MeCU. Incase of the bromocinnamic acids, the corresponding heterodimerswere still the major products though their proportions were not ashigh as it was in the case of the chlorocinnamic acids. Moreover,there was no significant difference in heterodimer proportionbetween the 3-Br CA–CU and 3-Br CA–6-MeCU (4% increase).While we cannot ignore the size effect of the halogen substituents

s@g-CD inclusion complexes generated using the PM3 semi-empirical method

Table 1Summary of photoproduct distribution resulting from the g-CD directed photo-heterodimerization of cinnamic acids and coumarins.

Cinnamic acid (CA) Coumarin (CU) Heterodimer CA homodimer CU Homodimers cis CA

CA 6-MeCU 60% 3% 29% 8%CA CU 42% 18% 31% 9%3-Me CA CU 57% 14% 13% 16%3-MeO CA CU 45% 18% 15% 22%3-MeO CA 6-MeCA 24% 22% 14% 40%4-Me CA CU 32% 28% 25% 15%2-Cl CA CU 51% 15% 25% 9%2-Cl CA 6-MeCU 63% 18% 8% 11%3-Br CA CU 44% 11% 29% 16%3-Br CA 6-MeCU 48% 27% 11% 14%4-HO CA 6-MeCU 2% 45% 47% 6%4-CF3 CA 6-MeCU 12% 37% 48% 3%

Numbers reported are an average of three independent experiments. The names of the alkenes are abbreviated. The numbering system used for naming the coumarins (CU)and cinnamic acids (CA) could be found in Fig. 4. Proportions of photoproducts are reported for at least 85% conversion of CA or CU. Mass balances of the reactions were above90%.

Fig. 4. Numbering convention and head/tail reference used in naming coumarinand cinnamic acid derivatives.

A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7 5

and its influence (especially in case of bromine), it is clear that thepairs are electronically complementary. Thus, the higher percent ofheterodimers in the product mixtures could be attributed to theelectronic factor. The heterodimers in the all the halogensubstituted cinnamic acid pairs were formed in significantlyhigher quantities than the combined proportion of homodimers.This indicates that the weak electronic interactions between thearomatic rings of the alkene pairs could in fact be strong enough togovern the inclusion complex arrangement to favor the formationof hetero-guest pair complex.

Finally, we report two reactions in which the homodimerswere formed in high proportions while heterodimers wereformed in negligible amounts (Table 1, set 3). This means thatthe interaction between identical alkenes is more favorable thanthe interaction between non-identical alkenes. The reactionbetween 4-HO CA and 6-MeCU did not yield any significant ofheterodimer. This may be due to the potential hydrogen bondinginteraction that may occur between phenolic groups in the1:2 hydroxy cinnamic acid homo-guest pair inclusion complex.However, this effect is not expected to be so strong to completelysuppress heterodimerization because the phenolic groups incinnamic acid could also engage in hydrogen bonding withhydroxyl groups in portals of the host which is viable in bothhetero- and homo-complexes. The second alkene pair thatyielded predominantly homodimers is the reaction between 4-trifluoromethyl cinnamic acid (4-TFM CA) and 6-MeCU whereinthe aromatic rings possess electron depleting and enrichingsubstituents respectively. Despite the electronic complementari-ty, the homodimers were the predominant products. We areunable to explain this observation as well, and hope that future

5 10 15 20 Scat ter ing angle

Inte

nsity

Fig. 5. Powder X-ray diffraction patterns of the host,

investigations will reveal the reason behind the counterintuitiveoutcomes.

The complexes were also studied to understand their solid-state characteristics. Powder X-ray diffraction patterns of theindividual components in Fig. 5 show that the individualcomponents are present in their crystalline forms. The X-raydiffraction pattern of the complex is also crystalline and clearlydifferent from that of the individual components. In fact, the lack ofany distinct signals of the pure components suggests a high level ofhomogeneity and chemical purity in sample. 1H NMR spectra of thedried complexes prepared after prolonged sonication followed bythorough washing with methanol and water contained g-CD, 6-methyl coumarin, and cinnamic were present in roughly equalproportions suggesting a 1:1:1 stoichiometry qualitatively. Thesefindings further validate our inference that photoproductsobserved are resulting from the alkenes reacting from withinthe cavity, and that the proportion and stereochemistry ofheterodimer is an indirect indicator of the relative arrangementwithin the cavity.

25 30 35 2θ (degrees)

1:1: 1 compl ex

6-me thyl coumarin

cinnamic acid

γ-CD

and guests individually, and that of the complex.

6 A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7

3. Conclusions

To conclude, we have demonstrated that the readily availableand environmentally benign macrocyclic cavitand g-CD could beused to effect selective photo-heterodimerization between non-identical alkenes. We originally hypothesized that complementarysteric and electronic influences between alkenes could beemployed to steer the complexation event toward the hetero-guest pair complex, which upon irradiation would yield theheterodimer. Alkene pairs in which only one of the alkenespossessed a sterically demanding group in the aromatic moietyyielded predominantly the heterodimer. Similarly, alkene pairs inwhich one of the alkenes possessed an electronegative substituentin the aromatic ring also yielded the heterodimer as the majorproduct. Both these observations support our original hypothesis,and demonstrate the effectiveness of g-CD cavity mediationstrategy for photo-heterodimerization. Computational studieswere performed to identify the supramolecular factors that couldhave been responsible for the observed selectivity, and deduce thestructure of the heterodimer. Considering the importance ofphotodimerization reaction in areas of applied chemistry, webelieve that the findings reported herein will expand the utility ofthe reaction.

3.1. Experimental procedure

The host (g-CD) was purchased from CDT, Inc. and used asreceived. The guests, solvents, and drying agents were purchasedfrom commercial sources (Sigma–Aldrich, Fischer Scientific, andMP Biomedicals LLC) and used as received. Host–guest complexesare prepared by typically subjecting 50 mg of g-CD, and oneequivalent of each guest in 2 mL of water to sonication for 5 h,followed by 5 h of stirring. Resulting insoluble white precipitate(inclusion complex) was isolated by filtration and washed withwater, cold methanol and ether to remove uncomplexed compo-nents. The precipitate was dried under vacuum and used for futureexperiments. The powder prepared as mentioned above was placedbetween two Pyrex glass plates and irradiated for at least 24 h.Irradiations were performed with a medium pressure mercuryvapor lamp immersed in a water-cooled Pyrex jacket. Followingirradiation, the inclusion complexes are dissociated throughbiphasic extraction with water and ethyl acetate. The organic layerwas separated; residual water was removed using anhydrousNa2SO4, the solvent rotary evaporated, and the final sample driedunder high-vacuum. The dried samples were then subject to 1HNMR (300 MHz) analysis in CDCl3. The heterodimer in cases ofcinnamic acid–6-methyl coumarin, and cinnamic acid–coumarinwere isolated through flash chromatography over silica gel columnusing ethyl acetate dichloromethane (5:95) solvent mixture.

Acknowledgements

MP thanks NE-EPSCoR (EPS-1004094) for the grant moneyprovided to perform the work presented in this manuscript. MPthanks Department of Chemistry for the research infrastructureand the Office for Undergraduate Research at UNK for supportthrough SSRP and URF programs.

References

[1] (a) K.J. Crowley, P.H. Mazzocchi, in: J. Zabicky (Ed.), In Photochemistry ofOlefins, John Wiley & Sons, Ltd., Chichester, UK, 1970, pp. 267–325;

(b) W.M. Horspool, Photochemistry of alkenes, alkynes and related com-pounds, Photochemistry, (2001) , pp. 117–148;(c) K. Mizuno, Photochemistry of aromatic compounds, Photochemistry,(2009) , pp. 175–212;

(d) J. Vansant, Photochemical dimerization of arylethylenes, Chem. Mag.,(1978) , pp. 30–31.

[2] (b) L.R. MacGillivray, Organic synthesis in the solid state via hydrogen-bond-driven self-assembly, J. Org. Chem. 73 (9) (2008) 3311–3317S. Ichikawa,Photochemical preparation of cyclobutanetetracarboxylic dianhydrides.JP2006328027A, 2006.

[3] (a) L.R. MacGillivray, T.D. Hamilton, G.S. Papaefstathiou, D. Bucar, Q. Chu, Ingreen synthesis of organic ligands: applications in coordination-driven self-assembly, Am. Chem. Soc. (2010) INOR–1677;

(b) F. Toda, Solvent-free organic synthesis, Shokubai 43 (1) (2001) 29–33.[4] (a) M. Hasegawa, Topochemical [2 + 2] photocyclodimerization and -poly-

merization, Polym. Mater. Sci. Eng. 54 (1986) 211–217;(b) M. Hasegawa, Topochemical photopolymerization of diolefin crystals, PureAppl. Chem. 58 (9) (1986) 1179–1188;(c) M. Hasegawa, Photodimerization and photopolymerization of diolefincrystals, Adv. Phys. Org. Chem. 30 (1995) 117–171.

[5] (a) J. Fang, C. Whitaker, B. Weslowski, M.-S. Chen, J. Naciri, R. Shashidhar,Synthesis and photodimerization in self-assembled monolayers of 7-(8-trimethoxysilyloctyloxy) coumarin, J. Mater. Chem. 11 (12) (2001)2992–2995;

(b) M. Fujiwara, K. Shiokawa, N. Kawasaki, Y. Tanaka, Photodimerization ofcoumarin-derived pentacyclo[95,113,915,1517,13]octasiloxane to fabricate athree-dimensional organic–inorganic hybrid material, Adv. Funct. Mater. 13(5) (2003) 371–376;(c) H. Kihara, M. Yoshida, Reversible bulk-phase change of anthroylcompounds for photopatterning based on photodimerization in the moltenstate and thermal back reaction, ACS Appl. Mater. Interfaces 5 (7) (2013)2650–2657;(d) X. Yuan, K. Fischer, W. Schaertl, Reversible cluster formation of colloidalnanospheres by interparticle photodimerization, Adv. Funct. Mater. 14 (5)(2004) 457–463.

[6] (a) S. Devanathan, M.S. Syamala, V. Ramamurthy, Photoreactions inhydrophobic pockets, Proc. Indian Acad. Sci. Chem. Sci. 6 (1987) 391–407;

(b) V. Ramamurthy, A. Parthasarathy, Chemistry in restricted spaces: selectphotodimerizations in cages, cavities, and capsules, Isr. J. Chem. 51 (7) (2011)817–829;(c) A. Lalitha, K. Pitchumani, C. Srinivasan, Photodimerization of trans-2-styrylpyridine in zeolite cages, J. Photochem. Photobio. A 134 (3) (2000)193–197;(d) G. Lem, N.A. Kaprinidis, D.I. Schuster, N.D. Ghatlia, N.J. Turro, Regioselectivephotodimerization of enones in zeolites, J. Am. Chem. Soc. 115 (15) (1993)7009–7010;(e) D. Madhavan, K. Pitchumani, Photodimerization of enones in a claymicroenvironment, Photochem. Photobiol. Sci. 1 (12) (2002) 991–995;(f) J. Yang, M.B. Dewal, L.S. Shimizu, Self-assembling bisurea macrocycles usedas an organic zeolite for a highly stereoselective photodimerization of2-cyclohexenone, J. Am. Chem. Soc. 128 (25) (2006) 8122–8123;(g) J. Yang, M.B. Dewal, L.S. Shimizu, Self-assembling bisurea macrocycles usedas an organic zeolite for a highly stereoselective photodimerization of2-cyclohexenone, J. Am. Chem. Soc. 128 (25) (2006) 8122–8123;(h) S. Arumugam, L.S. Kaanumalle, V. Ramamurthy, Alkali ion exchangednafion as a confining medium for photochemical reactions, Photochem.Photobiol. 82 (1) (2006) 139–145.

[7] (a) E. Elacqua, R.C. Laird, L.R. MacGillivray, In Templated [2 + 2]Photo-dimerizations in the Solid State, John Wiley & Sons Ltd., Chichester, UK, 2012,pp. 3153–3165;

(b) D.-Y. Ma, K. Waernmark, Mechanoassisted supramolecular catalysis in solidstate synthesis, ChemCatChem, (2010) , pp. 1059–1060;(c) A. Chanthapally, W.T. Oh, J.J. Vittal, 2 + 2] Cycloaddition reaction as a tool tomonitor the formation of thermodynamically stable ladder coordinationpolymers, CrystEngComm, (2013) , pp. 9324–9327;(d) K.M. Hutchins, J.C. Sumrak, L.R. MacGillivray, Resorcinol-templated head-to-head photodimerization of a thiophene in the solid state and unusual edge-to-face stacking in a discrete hydrogen-bonded assembly, Org. Lett., (2014) ,pp. 1052–1055;(e) A.A. Parent, D.H. Ess, J.A. Katzenellenbogen, p–p Interaction energies asdeterminants of the photodimerization of mono-, di-, and triazastilbenes, J.Org. Chem., (2014) , pp. 5448–5462.

[8] (a) M. Pattabiraman, A. Natarajan, R. Kaliappan, J.T. Mague, V. Ramamurthy,Template directed photodimerization of trans-1,2-bis(n-pyridyl) ethylenesand stilbazoles in water, Chem. Commun. 36 (2005) 4542–4544;

(b) H.M. Wang, G. Wenz, Topochemical control of the photodimerization ofaromatic compounds by g-cyclodextrin thioethers in aqueous solution,Beilstein J. Org. Chem. 9 (2013) 1858–1866;(c) B.C. Pemberton, S. Jayaraman, In supramolecular photocatalysis: mecha-nism studies of cucurbit[8]uril facilitating the [2 + 2] photodimerization of6-methylcoumarin, Am. Chem. Soc. (2011) ORGN–574;(d) N. Barooah, B.C. Pemberton, J. Sivaguru, Manipulating photochemicalreactivity of coumarins within cucurbituril nanocavities, Org. Lett. 10 (15)(2008) 3339–3342;(e) N. Barooah, B.C. Pemberton, A.C. ohnson, J. Sivaguru, Photodimerizationand complexation dynamics of coumarins in the presence of cucurbit[8]urils,Photochem. Photobiol. Sci. 7 (12) (2008) 1473–1479.

[9] G.R. Desiraju, Crystal engineering: a holistic view, Angew. Chem. Int. Ed. 46(44) (2007) 8342–8356.

A.R. Clements, M. Pattabiraman / Journal of Photochemistry and Photobiology A: Chemistry 297 (2014) 1–7 7

[10] (a) R. Kaliappan, M.V.S.N. Maddipatla, L.S. Kaanumalle, V. Ramamurthy,Crystal engineering principles applied to solution photochemistry: control-ling the photodimerization of stilbazolium salts within g-cyclodextrin andcucurbit[8] uril in water, Photochem. Photobiol. Sci. 6 (7) (2007) 737–740;

(b) M. Okada, Y. Takashima, A. Harada, One-pot synthesis of g-cyclodextrinpolyrotaxane: trap of g-cyclodextrin by photodimerization of anthracene-capped pseudo-polyrotaxane, Macromolecules 37 (19) (2004) 7075–7077;(c) M. Pattabiraman, A. Natarajan, L.S. Kaanumalle, V. Ramamurthy, Templat-ing photodimerization of trans-cinnamic acids with cucurbit[8]uril andg-cyclodextrin, Org. Lett. 7 (4) (2005) 529–532;(d) H.-X. Xu, S.-F. Cheng, X.-J. Yang, B. Chen, Y. Chen, L.-P. Zhang, L.-Z. Wu, W.Fang, C.-H. Tung, R.G. Weiss, Enhancement of diastereoselectivity in photo-dimerization of alkyl 2-naphthoates with chiral auxiliaries via inclusionwithin g-cyclodextrin cavities, J. Org. Chem. 77 (4) (2012) 1685–1692;(e) R. Wang, L. Yuan, D.H. Macartney, Cucurbit[7]uril mediates the stereo-selective [4 + 4] photodimerization of 2-aminopyridine hydrochloride inaqueous solution, J. Org. Chem. 71 (3) (2006) 1237–1239.

[11] M. Yoshizawa, Y. Takeyama, T. Okano, M. Fujita, Cavity-directed synthesiswithin a self-assembled coordination cage: highly selective [2 + 2] cross-photodimerization of olefins, J. Am. Chem. Soc. 125 (11) (2003) 3243–3247.

[12] (a) J. Szejtli, Cyclodextrins and Their Inclusion Complexes, Akad. Kiado,Budapest, Hungary, 1982, pp. 296;

(b) H. Dodziuk, Cyclodextrins and Their Complexes, Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim, Germany, 2008, pp. 489.

[13] (a) D.M. Bassani, In the Dimerization of Cinnamic Acid Derivatives, CRC PressLLC, Boca Raton, Fla, 2004, pp. 20/1–20/20;

(b) B.P. Emmer, Physical and photochemical properties of solid cinnamic acid,Baskerville Chem. J. City Coll. N. Y., (1966) , pp. 15–18;(c) G. Kaupp, Solid-state photochemistry: new approaches based on newmechanistic insights, Int. J. Photoenergy, (2001) , pp. 55–62.

[14] (a) K. Tanaka, Supramolecular photodimerization of coumarins, Molecules 17(2012) 1408–1418;

(b) M.T. Crimmins, T.L. Reinhold, Enone olefin [2 + 2] photochemical cyclo-additions, Org. React. (Hoboken, NJ, U. S. A.) 44 (1993) 297–588.

[15] (a) J.J.P. Stewart, Optimization of parameters for semiempirical methods II.Applications, J. Comput. Chem. 10 (2) (1989) 221–264;

(b) J.J.P. Stewart, Optimization of parameters for semiempirical methods I.Method, J. Comput. Chem. 10 (2) (1989) 209–220;(c) M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, Gaussian 03, revision C.02, Gaussian, Inc., Pittsburgh PA, 2003.