Photoisomerization and Photooxygenation of 1,4-Diaryl-1,3-dienes in

Article pubs.acs.org/JPCA

Photoisomerization and Photooxygenation of 1,4-Diaryl-1,3-dienes in a Confined Space Shampa R. Samanta, Rajib Choudhury, and V. Ramamurthy* Department of Chemistry, University of Miami, Room No. 315 Cox Science Building, Coral Gables, Florida 33146, United States S Supporting Information *

ABSTRACT: Geometric isomerization of light-activated olefins plays a significant role in biological events as well as in modern materials science applications. In these systems, the isomerization occurs in highly confined spaces, and concepts derived from solution investigations are only partially applicable. This study makes contributions in understanding the excited-state behavior of olefins in confined spaces by investigating the excited-state behavior of 1,4-diphneyl-13-butadiene (DPB) and 1,4-ditolyl-1,3-butadiene (DTB) encapsulated in a well-defined organic capsule made up of the octa acid (OA) host. Both of these dienes that exist in three isomeric forms (trans,trans; trans,cis; and cis,cis) formed 1:2 guest−host complexes with OA in aqueous borate buffer. Competition experiments monitored by 1H NMR signals revealed that among the three isomers the cis,cis isomer of DPB and DTB formed the most stable complex with OA. Molecular modeling studies suggested that all six isomers of DPB and DTB preferred the cisoid conformation within the OA capsule. Irradiation (>280 nm) of the diene−OA complex (diene@OA2) resulted in geometric isomerization, and the photostationary state consisted of cis,trans isomer as major and cis,cis as minor products. The photostationary state could be enriched with the cis,cis isomer in yields close to 70% with proper cutoff filters because the cis,cis isomer absorbs at shorter wavelength than the other two isomers. Consistent with the MD simulation prediction that trans,transDPB and trans,trans-DTB existed in cisoid conformation within OA capsule, the generation of singlet oxygen in the presence of OA encapsulated DPB or DTB resulted in facile [4 + 2] addition between the diene and the singlet oxygen.

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Continuing our longstanding interest17−20 in the excitedstate behavior of organic molecules in confined media, we chose 1,4-diaryl-1,3-butadienes included within a closed container made up of two molecules of host octa acid (OA)21 for this study. Details of the molecular structure, cavity size, and shape of OA are provided in Figure 1. Our recent studies led to the conclusion that the photochemistry of dimethyl stilbenes was altered by OA and the location of the methyl group on the phenyl rings of stilbenes controlled their photostationary composition.22−24 We attributed these changes to the supramolecular steric effect and to the extent and location of free space within the reaction cavity of the host− guest complex. These unexpected observations stimulated our interest in the excited-state behavior of 1,4-diaryl-1,3butadienes (for structures of 1−6 see Figure 1) included within OA capsule. Prior to the current study, the photochemistry of 1,4diphenyl-1,3-butadiene (DPB) has been investigated in organic solvents by a number of groups. Pioneering studies by Zechmeister’s group demonstrated trans,trans-DPB (3) and cis,cis-DPB (1) upon irradiation converted to the corresponding cis,trans-DPB (2).4,5 Approximately 25 years later, Whitten’s

INTRODUCTION Photoinduced geometric isomerization plays an important role in a number of biological events1,2 such as vision (rhodopsin), energy capture by bacteria (bacteriorhodopsin), and light sensing by plants (phytochromes or xanthopsins). This process also helps treat conditions such as neonatal jaundice (bilirubin) and helps in the industrial and biological synthesis of vitamin D.3 Because of its importance in life processes such as those previously mentioned, the excited-state chemistry of model compounds such as stilbene and 1,n-diphenylpolyenes has attracted considerable attention for over seven decades.4,5 Extending the knowledge of excited-state behavior of small organic molecules from studies of model compounds in solution to those in biological systems oftentimes is only partially successful. Predicting the reactivity of organic/ inorganic molecules incorporated into biological molecules like proteins, DNA, and lipids where reactions generally tend to be selective requires consideration of the confined environment and weak interactions between the reactant molecule and the reaction cavity.6 In this context, photoinduced geometric isomerization of olefins has been investigated in a variety of molecular containers such as organic and inorganic host cavitands, bilayers, crystals, organic glasses, zeolites, and antibodies that mimic the confined, biological environment.7−16 16 Differences between isotropic solution and these containers have been identified in terms of the mechanism of geometric isomerization and selectivity in behavior. © XXXX American Chemical Society

Special Issue: Current Topics in Photochemistry Received: May 27, 2014 Revised: July 25, 2014

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dx.doi.org/10.1021/jp505196v | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Chemical structure of octa acid molecule and its cartoon representation along with dimensions (top). Chemical structure of 1,4-diaryl-1,3butadienes 1−6 used as reactant guests (bottom).

group confirmed these results and by measuring quantum yields of isomerization suggested an excited singlet state decay of 3 consisting of isomerization, fluorescence, and possibly a radiationless process. They also pointed out for the first time that cis,cis-DPB (1) may also be formed during light-initiated isomerization and that the process could be solvent-dependent.25−27 Following this, Görner,28 Yee,29,30 and Saltiel30 established that: (a) geometric isomerization upon direct excitation did not involve triplet state, (b) the photoisomerization quantum yield was solvent-dependent, (c) cis,cis-DPB (1) was formed as one of the products at least in hexane, (d) the radiationless decay not involving isomerization contributed to the excited-state decay, and (e) the photoisomerization in organic solvents proceeded mostly by one bond isomerization. Recently, the formation of cis,cis isomer directly from trans,trans-DPB in hexane has been reported; however, the exact mechanism remains unexplored. Directly relevant to the current study are prior reports on 1,4-diaryl-1,3-butadienes as crystals, dissolved in organic glasses at low temperature, included in zeolites (ZSM-5, -8, and -11) and aligned as part of lipid bilayers.9,14,15,31−36 In ZSM-5, -8, and -11 zeolites, no isomerization occurred and DPB had a long S1 lifetime (>12 ns). In organic glasses and crystals at low temperatures, 1 and cis,cis-ditolyl-1,3-butadiene (DTB; 4) isomerized to respective trans,trans isomer by simultaneous two-bond isomerization. The studies in crystals and organic glasses conducted by Saltiel’s and Liu’s groups have led to an interesting and heated debate on the importance of bicycle pedal37 and hula-twist38 mechanisms in the excited-state isomerization process. In addition to isomerization, 1,4-diaryl1,3-butadienes are known to undergo specific reactions from the cisoid conformers (Scheme 1) such as (i) the [4 + 2] addition to singlet oxygen to yield endoperoxides (Scheme 2 (a))39 and (ii) the 6e cyclization (Scheme 2 (b)).40 DPBs believed to be included as cisoid conformers within the narrow channels of ZSM-5 zeolites reacted with singlet oxygen more

Scheme 1. Transoid and Cisoid Conformers of 1,4-Diaryl1,3-butadiene

Scheme 2. (a) Oxygenation of 3 and 6 to Yield Endoperoxide [4 + 2] and Oxetane ([2 + 2] Additions and (b) Excited State 6e Cyclization Followed by Dehydrogenation.

efficiently than in solution.31 The current study is devoted to understanding the singlet excited-state behavior of DPB and DTB included within a well-defined organic capsule made up of two molecules of OA. The goals of the current undertaking included finding answers to the following questions: (a) Are there any preferences between the isomers (trans,trans; cis,trans; and cis,cis; see Figure 1 for structures) for the OA B



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Figure 2. 1H NMR spectra (500 MHz, 298 K) of (i) OA (1 mM), (ii) 1@OA2, (iii) 2@OA2,(iv) 3@OA2, (v) 4@OA2, (vi) 5@OA2, and (vii) 6@ OA2. All spectra were recorded in borate buffered solution of D2O. Host and guest signals were assigned by recording 2D COSY 1H NMR spectra. Guest protons for 2@OA2 and 5@OA2 are assigned by “*”, and the signal for residual water is assigned by “•”.

cavitand OA hydrogens are marked as a, b, and g. The two cavitand OA molecules of the capsule in the presence of a symmetrical guest, as in the case of trans,trans and cis,cis, experience the same magnetic environment to result in the observed single set of signals for both OA hydrogens. However, when the guest is unsymmetrical, the two cavitands that make up the capsule experience different magnetic environments; therefore, chemically equivalent hydrogens may not be magnetically equivalent. Therefore, two sets of signals for the two OA molecules are possible. This occurs in the case of cis,trans isomer, and these are marked as a, a′, b, and b′ in Figure 2iii,vi. Furthermore, the signals due to OA were disturbed upon the addition of guests (Figure 2i vs ii−vi). These changes are indicative of guest inclusion within OA capsule. 1H NMR titration data as well as the diffusion constants measured by DOSY (diffusion-ordered spectroscopy) experiments (Table S1 in SI)42 suggested the formation of 2:1 host−guest complexes (capsuleplexes).44 In the 1H NMR spectra of the complexes, the absence of signals due to free dienes as well as invariable chemical shift for the included guests independent of their concentration or OA in solution suggested that the complexes of 1−6 and OA were stable under the experimental conditions with no free diene molecules in aqueous solution. The capsule’s partial open-close on the time scale >5 μs45 and full break up on the time scale of ∼3 s46 has been previously established. Given that the lifetimes of S1 states of dienes within OA capsule are expected to be in the nanosecond range, the excited molecules present within OA would not be able to escape to the aqueous solution during their lifetimes. On the basis of these data, we believe the photochemistry reported here to be that of the dienes included within OA and not those in free solution. Competition Studies. Competition experiments between the three isomers of DPB (1−3) and DTB (4−6) for OA were carried out to probe the order of stabilities of the OA

capsule? (b) Does the capsule influence the photostationary state composition of geometric isomers? (c) What is the mechanism of isomerization within the OA capsule? (d) Is there any preference for the dienes to exist in cisoid or transoid conformations within the OA capsule? Structures of transoid and cisoid conformers of 1,4-diaryl-1,3-butadiene are presented in Scheme 1. Unlike cis and trans isomers, these two conformers are readily interconvertible at room temperature by rotation of C2C3 single bond. (e) Would the entrapped 1,4-diaryl-1,3-butadienes react with another molecule such as singlet oxygen? To address these questions, we have carried out extensive 1-D and 2-D 1H NMR, MD simulation, and photochemical studies. Results of these studies are presented and discussed later.

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RESULTS AND DISCUSSION Complexation Studies. Photochemistry of six 1,4-diaryl1,3-butadienes 1−6 (Figure 1) included within OA capsule was investigated and compared with that in hexane solvent. These hydrophobic olefins were solubilized in water with the help of hydrophilic host OA. It is known that two molecules of OA self-assemble in borate buffer (pH ∼9) in the presence of hydrophobic guest molecule(s).41 Capsular host−guest complexes (2:1) of OA and 1−6 were prepared by stirring an aliquot of DMSO stock solution of the guests with OA in 2:1 (host-to-guest) ratio in borate buffer solution (0.6 mL; pH ∼9). 1 H NMR spectra of OA and 1−6 dissolved in D2O in 2:1 ratio are presented in Figure 2. The signal assignments made with the help of COSY experiments42 are included in Figure 2. (For COSY spectra, see Figures S2−S7 in the Supporting Information section). As seen in Figure 2, all methyl, olefinic, and aromatic hydrogen signals of the guests were upfield shifted relative to those in CDCl3.43,44 The spectral data for the guests DPB and DTB in CDCl3 available in the literature were used for comparison.14,35 In the spectra, the signals due to the C



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complexes of the three isomers (trans,trans; cis,trans; and cis,cis), .23,24 For such studies, 2:1 host−guest complex of one of the three isomers of DPB (or DTB) was taken in a NMR tube, and the 1H NMR spectra were recorded upon gradual addition of aliquots of DMSO-d6 solution of the other isomer. The spectra obtained for DTB are provided in Figures 3−5, and

Figure 5. 1H NMR spectra of competition experiments of 5 and 6. (i) 6@OA2 (guest/host 1:2), (ii) after addition of 0.5 equiv of 5 to (i) solution, and (iii) after addition of 1 equiv of 5 to (i) solution. Proton signal for 6 is assigned by “⧫” and for 5 is assigned by ‘*’.

cis,trans- and trans,trans-DTB suggested that there was no preference between the two isomers for the OA capsule. On the basis of the previous competition studies, we concluded that (a) the cis,cis isomer formed the most stable complex with OA and (b) the OA complexes of cis,trans is slightly more stable than trans,trans complexes. The same conclusions were drawn for DPB based on similar experiments. (For 1H NMR spectra, see Figures S8−S10 in SI.) Our desire to seek further evidence to support this conclusion by estimating binding constants through isothermal titration calorimetric (ITC) experiments was hampered by the insolubility of dienes in aqueous buffer solution. We believe that the compact size of the cis,cis isomer with respect to the other two isomers may be responsible for the preference of this isomer by OA capsule. It is quite possible that the compact structure offers better van der Waals contact between the guest and the interior of OA capsule. Structure of Guests within the OA Capsule. Having concluded that of the three isomers the cis,cis isomer formed the most stable complex with OA, we were interested in knowing whether the dienes 1−6 within the OA capsule preferred one of the two possible conformers (transoid or cisoid, Scheme 1) in each case. In solution the dienes exist as an equilibrium mixture of transoid and cisoid conformers (Scheme 1), with the former in larger amounts. Upon reaction with singlet oxygen the transoid conformer yields oxetanes via [2 + 2] addition, while the cisoid conformer gives endoperoxide via [4 + 2] addition (Scheme 2 (a)).39 We believed that by probing the reaction of singlet oxygen with the dienes we would be able to gain information regarding the preferred conformation of the dienes 1−6 within OA capsule. Before carrying out oxidation studies we performed MD simulations to get an insight into the conformational preference within the confined space of OA capsule. MD simulations were performed as follows; In the first step, a 3D structure of OA was constructed and optimized using the Merck Molecular Force Field47 included in the SPARTAN 04 program. This structure was further equilibrated through 40 ns all-atom MD simulations in an explicit aqueous solution utilizing the OPLS-AA force field48,49 and GROMACS program.50,51 The structures of the guest molecules were constructed and optimized using the Chembio3D program and MM2 force field.52 In the next step, all guest molecules were

Figure 3. 1H NMR spectra of competition experiments of 6 and 4. (i) 6@OA2 (guest:host =1:2), (ii) after addition 0.5 equiv of 4 to (i) solution, and (iii) after addition of 1 equiv of 4 to (i) solution. Proton signal for 6 is assigned by “⧫” and for 4 is assigned by “▲”.

Figure 4. 1H NMR spectra of competition experiments of 5 and 4. (i) 5@OA2 (guest/host 1:2), (ii) after addition of 0.5 equiv of 4 to (i) solution, and (iii) after addition of 1 equiv of 4 to (i) solution. Proton signal for 4 is assigned by “▲” and for 4 is assigned by ‘*’.

those for DPB are provided in the SI (Figures S8−S10). In this presentation, we represent the inclusion of guest within OA capsule as guest@OA2 (e.g., DPB@OA2). Figure 3 presents 1H NMR spectra of competition studies between cis,cis-DTB and trans,trans-DTB complex. The addition of one equivalent of cis,cis-DTB to trans,trans-DTB@OA2 complex resulted in quantitative displacement of trans,trans-DTB from the OA capsule, resulting in cis,cis-DTB@OA2 complex. Thus, a preference for cis,cis-DTB over trans,trans-DTB for OA capsule was evident. We interpreted this to mean that cis,cis-DTB formed a stronger complex than trans,trans-DTB with OA. Figure 4 presenting 1H NMR spectra from similar competition experiments between cis,trans-DTB and cis,cis-DTB established that the latter isomer formed a stronger complex. In contrast with the previous results, addition of cis,trans-DTB to the trans,trans-DTB@OA2 led to only partial displacement of the trans,trans isomer. As illustrated in Figure 5, the formation of nearly equal amounts of trans,trans-DTB@OA2 and cis,transDTB@OA2 from competition between equal concentrations of D



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conformer could be due to its compactness and better van der Waals contact between the guest and the interior of OA capsule in the case of cisoid than in transoid conformer. Further work is needed to confirm this suggestion. Geometric Isomerization upon Direct Excitation. We then embarked on finding whether the stability of the overall supramolecular complex would be reflected in the excited-state decay of the twisted isomers; that is, would the most stable cis,cis isomer be enriched at the photostationary state within the OA capsule. Keeping in mind that among the solvents investigated (cyclohexane, perfluorohexane, benzene, methylcyclohexane, and hexane) only in hexane was the cis,cis isomer formed in small amounts from the trans,trans isomer,30 we irradiated 1−6@OA2 in borate buffer. For comparison, 1−6 were irradiated alone in hexane. All irradiations (>280 nm) were conducted in Pyrex NMR tubes with 450 W medium pressure mercury lamp. The progress of the reaction in hexane and in OA/borate buffer was followed by HPLC and 1H NMR, respectively. Upon reaching the photostationary state the products obtained in borate buffer were extracted in hexane and analyzed by HPLC (equipped with UV−vis detector; monitored at 315 nm; product yields were corrected for absorption differences between isomers). All three isomers were thermally stable, and control experiments established that they did not undergo thermal isomerization within OA capsule. Irradiation was essential for the interconversion of the geometric isomers. The photostationary state was reached by independently irradiating all three isomers of DPB and DTB. At the photostationary state, as anticipated independent of the initial isomer, nearly the same percentages of the isomers were obtained (see Table 1). In the case of DPB and DTB, the cis,cis isomer was present in 280 nmb selective irradiationc

1−3 4−6 2−3 5−6

octa acid

cis,cis

cis,trans

trans,trans

cis,cis

cis,trans

trans,trans

3 6 23 50

71 73 58 47

26 20 19 3

6 7 43 77

80 74 44 21

14 17 12 2

a All reported numbers are the average of three independent runs and have an error limit of ±5%. Product yields were measured by HPLC. bThe solutions were irradiated for 5 to 6 h using 450 W medium pressure Hg lamp with pyrex filter. Guest concentration was 0.25 mM (guest/host 1:4). c For 2−3, in hexane hv > 320 nm and in OA hv > 340 nm. For 5−6 in both hexane and OA hv > 340 nm.

E



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Figure 7. Composition versus time of irradiation of (a) 3@OA2 at (>340 nm, [guest] = 2.5 × 10−5 M, guest/host = 1:2), (b) 2@OA2 (>320 nm, [guest] = 5 × 10−5 M, guest/host = 1:2), and (c) 1@OA2, (>315 nm, [guest] = 2.5 × 10−5 M, guest/host = 1:2). The products were analyzed upon extraction in hexane, followed by HPLC monitoring acquisition at 315 nm.

below 15 min in the case of 1@OA2 and 3@OA2 and 120 min in the case of 2@OA2. Because the photostationary state (Table 1) was rich in cis,trans isomer, in the case of cis,trans-2@OA2 photoisomerization to the other two isomers was slow. In the excited state, the 2 preferred to decay to 2 than to 1 and 3. Examination of the plot of irradiation time versus yield for 3@ OA2 shown in Figure 7a indicated that this isomer (trans,trans) isomerized exclusively to the cis,trans in the early stages of irradiation. Similarly, the irradiation time versus yield plot for 2@OA2 (cis,trans-DPB) shown in Figure 7b suggested that this isomer transformed to trans,trans and cis,cis isomers within the OA capsule. These two isomerizations are consistent with the postulate that one-bond isomerization is predominant in the excited states of 2 and 3 within OA. In contrast with this, irradiation of 1@OA2 resulted in trans,trans and cis,trans isomers in the early stages of irradiation (Figure 7c). We believe that the trans,trans isomer is formed by simultaneous two bond rotation. Such two-bond mechanism has previously been proposed during the conversion of cis,cis-DPB to trans,transDPB in crystalline state as well as in isopentane glass.15,32,33 On the basis of the behavior within OA, we concluded that the twobond rotation in the case of cis,cis isomers within confined spaces may be general. Photooxygenation of Encapsulated Dienes by Singlet Oxygen. According to MD simulation, isomers of DPB and DTB are present in their respective cisoid conformations within the OA capsule (Figure 6). It is well known that singlet oxygen reacts with cisoid DPB to yield an endoperoxide via [4 + 2] addition (Scheme 2a).39 In solution, trans,trans-DPB exists as an equilibrium mixture of transoid (99%) and cisoid conformers (1%) (Scheme 1). Despite the low amounts of the cisoid conformer, the kinetic equilibrium between the transoid and cisoid conformers allows quantitative conversion of trans,trans-DPB to the corresponding endoperoxides 7 and 8

state. To test this possibility, we irradiated (450 W medium pressure mercury lamp with 320 nm cutoff filter for DPB and 340 nm cutoff filter for DTB; see Figures S13−S16 in the SI) a hexane solution (50 μM) of trans,trans and cis,trans isomers of DPB and DTB. A photostationary state was reached in ∼48 h. As expected, the cis,cis isomer was formed in much larger amounts compared with irradiation with Pyrex filter. The percentages of the isomers are listed in Table 1. Similar irradiation (450 W medium pressure mercury lamp with 340 nm cutoff filter) of trans,trans and cis,trans isomers of DPB and DTB included within OA in borate buffer (diene: 25 μM and OA: 50 μM) was conducted for ∼48 h. The cis,cis isomer was formed in ∼44% in the case of DPB and 77% in the case of DTB (Table 1). To our knowledge, such high yields of cis,cis isomers have not been previously realized upon direct excitation of 1,4-diarylbutadienes. This led us to investigate the mechanism of formation of cis,cis isomers from excited states of DPB and DTB. Although experiments were conducted with both DPB (1− 3) and DTB (4−6), we limit our discussion to DPB. OA complexes of individual isomers taken in borate buffer were irradiated in Pyrex tubes with appropriate filters (see Figure 7 for details) for different time periods and the products analyzed by HPLC (after extraction with hexane). A plot of composition of reaction mixture with respect to time of irradiation starting from 1@OA2, 2@OA2, and 3@OA2 is shown in Figure 7. Analysis of the progress of isomerization in the early stages provided insight into the nature of isomers formed directly from the excited state of the irradiated diene. It is important to note different irradiation wavelengths for each isomer provided in the caption of Figure 7. With the goal to identify the site(s) of geometric isomerization, secondary irradiation of the formed product was kept to a minimum by choosing appropriate irradiation wavelengths and by keeping the time of irradiation F



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(Scheme 2a), albeit slowly. To test the feasibility of such an addition within OA capsule, we performed singlet oxygen oxidation of dienes 1−6 included within OA capsule using Rose Bengal as the sensitizer. On the basis of MD simulation results, we predicted that [4 + 2] addition of diene to singlet oxygen would be more efficient than in solution. The established feasibility of selective singlet oxygen-mediated oxidation of OA encapsulated olefins55 is believed to be facilitated by the partial opening of the capsule (∼5 μs).45 In both CD3CN and aqueous solution containing OA, 1 mM of guest and 2 mM of Rose Bengal were used, and oxygen gas was bubbled through the solution prior to (for 30 min) and during photolysis. The photolysis was carried out with tungsten lamp (λ > 400 nm) to avoid direct absorption by DPB and DTB. Of the six isomers (1−6), only trans,trans-DPB and trans,trans-DTB reacted with singlet oxygen in both acetonitrile solution and within OA capsule. The products were characterized by 1H NMR and mass spectra and by comparison with authentic samples. The results obtained thus are consistent with those reported from photooxidation of trans,trans-DPB and trans,trans-DTB in organic solvents.31,39 Literature reports indicates that trans,cis and cis,cis isomers of 1,4-diaryl-1,3-dienes do not react with singlet oxygen.56−59 Thus, the reactivity of trans,trans and inertness of cis,trans and cis,cis isomers with singlet oxygen are consistent their reported behavior in solution. Comparison of the 1H NMR spectra of the irradiated sample and the authentic sample of the endoperoxide included within OA (Figure 8) confirmed that the endoperoxides 7 and 8 were

3@OA2 and the other is due to (the product complex) 7@OA2. The 1H NMR spectrum of the OA complex of authentic 7 is also included in Figure 8. It is important to note the similarity in the signals in the upfield region in the two spectra. This suggests that the product 7 formed by oxidation of 3 remained within the OA capsule. No products of reaction between the olefin and OA or singlet oxygen and OA were isolated upon oxidation of 3@OA2. Extraction and analysis of the photolyzed sample showed the presence of only three molecules, host OA, guest 3,and product 7. To probe the relative rate and the extent of oxidation within OA capsule, we carried out the oxidation was in OA and CD3CN under identical conditions. The conversion within OA was much faster than that in acetonitrile solution despite the 440 μs lifetime of singlet oxygen (versus 58 μs in water) and seven times greater solubility in acetonitrile. For example, after an identical 10 min of irradiation, 80% of endoperoxide 7 from DPB was formed in OA capsule, while only 18% conversion was achieved in acetonitrile (Table 2). DTB, although less Table 2. Relative Rate of Conversion to Endoperoxide upon Photooxygenation of 3 and 6 in OA and in CD3CNa % of conversion 3 to 7

6 to 8

duration of irradiation (min)

OA

CD3CN

5 10 15 30

53 79

7 18

OA

CD3CN

40 57

16 30

a

In all cases, the reaction condition was similar and oxygen was purged through the solution for 30 min prior and during the photolysis.

reactive than DPB, was still more reactive within OA capsule than in acetonitrile yielding after 15 min of irradiation 40 and 16% product 8, respectively. Thus, the OA encapsulated trans,trans dienes were reactive with singlet oxygen and more so than in acetonitrile. We speculate this variance to the dienes present in cisoid conformation within the OA capsule. Close to 80% conversion is suggestive of either most molecules being in cisoid conformation within OA capsule or, like in solution, a facile interconversion between transoid and cisoid conformers.

Figure 8. 1H NMR spectra (500 MHz, 298 K, buffered D2O) of (i) 7@OA2 (host/guest 2:1) and (ii) resultant solution after 5 min of photo-oxygenation of 3@OA2. Here ‘*’ represents guest signal of 7@ OA2 and “•” represents resonances due to unreacted 3@OA2.

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SUMMARY We have demonstrated that among the three isomers of DPB and DTB, in each case cis,cis isomers were preferentially included within the restricted space of OA capsule. However, this preference did not influence the decay of DPB and DTB in the excited state within OA capsule. MD simulations suggested preference for cisoid conformer within OA capsule, and this preference resulted in efficient oxidation of dienes by singlet oxygen to yield the endoperoxides 7 and 8. By selective excitation of the trans,trans-dienes, we found conditions under which cis,cis-dienes could be accumulated up to 75%. The absence of 6e cyclization (Scheme 1) product from the cisoid conformer of cis,trans-DPB in our studies is quite possibly due to the alignment of their orbitals within the confined space being not conducive to 6e disrotatory ring closure. The current studies have brought out the occurrence of simultaneous twobond rotation, possibly via the bicycle pedal mechanism during the conversion of cis,cis to trans,trans isomer. The previously described studies have established that the OA capsule provides

indeed the products of oxidation. The endoperoxides from DPB and DTB formed within OA were extracted with chloroform and compared with authentic samples prepared independently by known procedure.39 The 1H NMR spectra of the oxidation products from trans,trans-DPB and trans,transDTB were identical to the synthetic samples, confirming the formation of endoperoxides. Upon inclusion within OA, as expected the chemical shifts of the various protons of the product 7 were upfield shifted (Figure 8). To examine whether the oxidation product 7 remained within OA following reaction with singlet oxygen, the 1H NMR spectra were recorded in various stages of irradiation. The 1H NMR spectrum at ∼60% reaction is provided in Figure 8. Signals in the region δ 3.2 to 4 indicate the formation of new product(s). The complex signals due to OA in the region δ 5−8 is because the two sets of OA signals are present when the reaction was stopped at 60% conversion; one set of signals is due to (the reactant complex) G



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a well-defined hydrophobic confined environment, possibly better than cyclodextrins, to an organic guest molecule. Examination of the excited-state dynamics of capsule-confined 1,4-diaryldienes on a subnanosecond time scale is likely to yield fundamental information on the isomerization process.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, synthetic procedures, details of molecular dynamics simulation, and additional 1D and 2D NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS V.R. is grateful to the National Science Foundation (CHE0848017 and CHE-1411458) USA for continued generous financial support. We thank Professors J. Saltiel (Florida State University) and R. S. H. Liu (University of Hawaii) for providing encouragement and useful suggestions and K. E. O’Shea (Florida International University) for interest in the work.

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REFERENCES

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Photoisomerization and Photooxygenation of 1,4-Diaryl-1,3-dienes in

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