Article pubs.acs.org/JPCA
Photoisomerization of cis-1-(3-Methyl-2-naphthyl)-2-phenylethene in Glassy Methylcyclohexane at 77 K Jack Saltiel,*,† Stuart R. Hutchinson,† Katelyn Chitwood,† and Olga Dmitrenko‡ †
Departments of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States University of Delaware, Newark, Delaware 19716, United States
‡
S Supporting Information *
ABSTRACT: The cis−trans photoisomerizations of cis-1-(3-methyl-2-naphthyl)-2-phenylethene (c-3-MPE) was studied in methylcyclohexane (MCH) glass at 77 K. The fluorescence spectra of c- and t-3-MPE are excitation wavelength (λexc) independent because the steric requirement of the methyl group restricts the conformational space of each isomer to a single conformer. Photocyclization, the dominant reaction pathway of c-3MPE in solution, is entirely suppressed in MCH glass at 77 K. The only reaction on 313 nm irradiation of c-3-MPE in MCH glass is cis−trans isomerization. As the reaction progresses, the structureless fluorescence of c-3-MPE is replaced by the vibronically resolved fluorescence of the stable conformer of the trans isomer. The results are consistent with photoisomerization by the conventional one bond twist (OBT) pathway. Previously reported results on the photoisomerization of cis-1-(2-naphthyl)-2-(o-tolyl)ethene (c-NTE) are reinterpreted. Calculated geometries and energy differences for c- and t-3-MPE and c- and t-NTE [DFT using B3LYP/6311+G(d,p)] are consistent with the interpretation of the experimental results.
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conformers (t-NPEA and t-NPEB) in solution20 and in glassy media,21 and cis−trans photoisomerization in solution involves mainly c-NPE B , the more extended conformer, by a combination of adiabatic and diabatic OBT pathways.22,23 On the basis of the structure of, by far, the major benzophenanthrene product formed in solution, the more compact conformer, c-NPEA, is known to undergo mainly photocyclization.24−26 Direct formation of t-NPEB, the lower energy conformer of the trans product, on irradiation of c-NPE in methylcyclohexane (MCH) glass at 77 K led us to conclude that the reaction occurs by the OBT mechanism as in solution, Chart 1a.27 That conclusion was disputed because HT photoisomerization about the benzylic CH would lead to the same result.16 Formation of an unstable trans conformer starting from the o-tolyl analogue of c-NPE (c-NTE) in EPA glass at 77 K was presented as confirmation of the benzylic HT pathway, Chart 1b.28 However, comparison of the absorption spectrum of the initial t-NTE photoproduct and the spectrum of the thermally stable conformer to which it relaxes with the resolved t-NPE conformer absorption spectra in solution20 requires reassignment of the two spectra to structures t-NTEA and t-NTEB, respectively, Chart 1c. It follows that, depending on the correct structural assignment to the c-NTE conformer that exists in EPA glass, either the HT or the OBT mechanism will account for the results. However, if c-NTE were to undergo HT photoisomerization, it would have to be about the naphthylenic vinyl CH, the process that was ruled out in the
INTRODUCTION Viscous media inhibit one bond twist (OBT) cis−trans photoisomerization of olefins1−3 by enhancing torsional barriers.4−6 Two mechanisms have been proposed to account for photoisomerization in volume confining media. Initially postulated to explain retinyl photoisomerization within the protein environment in rhodopsin and bacteriorhodopsin, each involves concerted rotation about two bonds in S1. The bicyclepedal mechanism (BP) involves simultaneous rotation about two polyene double bonds7 and the Hula-twist mechanism (HT) involves simultaneous rotation about a double bond and an adjacent essential single bond (envisioned as equivalent to a 180° translocation of one CH unit).8,9 These mechanisms are assumed to reduce free volume requirements by confining motion to the vicinity of the isomerizing double bonds while minimizing the motion of bulky substituents. The claim that the photoisomerization of previtamin D to tachysterol at 92 K in an EPA glass (ether/isopentane/ethyl alcohol in 5:5:2 ratio by volume) gives HT products,10 led to a flurry of investigations seeking evidence for the HT mechanism.11−17 Formation of trans photoproduct conformer mixtures in organic glasses at low T on irradiation of cis isomers of naphthyl substituted ethenes, different from their equilibrium compositions, were interpreted to reveal formation of HT products.11−17 HT was favored11−17 over the earlier interpretation, advanced by the Alfimov18 and Fischer19 groups, that such results reflect different equilibrium conformer compositions for the two isomers. For our initial test of the HT mechanism, we chose cis-1-(2naphthyl)-2-phenylethene (c-NPEA and c-NPEB) because structures had been assigned to the spectra of the trans © 2012 American Chemical Society
Received: February 21, 2012 Revised: May 4, 2012 Published: May 10, 2012 5293
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Chart 1
Chart 2a
a
Numbers are relative E + ZPVE and free energies (G) in parentheses, both in kcal/mol; d1 and d2 are phenyl/vinyl and naphthyl/vinyl dihedral angles according to the Klyne−Prelog convention.
on alumina yielded pure c-3-MPE.30 The alumina stationary phase was packed in a solution of hexane (Sigma-Aldrich HPLC grade), which served as the eluent. TLC plates were used to identify fractions containing each isomer. Methylcyclohexane (MCH, Sigma-Aldrich, spectrograde) was used as received. Irradiation Procedures. Irradiations of dilute MCH solutions of c-3-MPE were carried out at 77 K in cylindrical (2 mm inside diameter) quartz tubes immersed in liquid nitrogen in the Dewar of the phosphorescence accessory of a Hitachi F-4500 fluorimeter, and the progress of the reaction was followed in situ by fluorescence spectroscopy. Excitation was either within the sample chamber of the fluorimeter using its monochromator and 150 W Xe lamp or externally using Hanovia 450 or 550 W medium pressure Hg lamps (Ace Glass, Inc.). Solute absorbances, measured on a Varian Cary 300B UV−vis spectrophotometer in 1 cm standard quartz cuvettes, were kept in the 0.1−0.3 range to ensure that concentrations were sufficiently low to minimize distortion of fluorescence spectra by self-absorption. The 313 nm Hg line was isolated using a potassium perchromate/potassium carbonate filter solution in parallel with Corning CS 7-54 filter plates.
parent c-NPE. It follows that the c-NTE results do not invalidate the OBT mechanism for c-NPE photoisomerization. The potential OBT process in Chart 1c involves photoisomerization of c-NTEA, which is analogous to c-NPEA, whose excitation in solution leads primarily to cyclization.24−26 The possibility that the glassy medium and low T divert the photoreaction from cyclization to cis−trans isomerization is tested here by the study of the photoisomerization of cis-1-(3methyl-2-naphthyl)-2-phenylethene, c-3-MPE, which, due to the steric requirements of the methyl group at the 3 position of the naphthalene, is expected to exist primarily as the compact c3-MPEA conformer.29 We show below that, consistent with the OBT process, c-3-MPEA readily photoisomerizes to t-3-MPAA in glassy MCH at 77 K.
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EXPERIMENTAL SECTION Materials. The synthesis of trans-1-(3-methyl-2-naphthyl)2-phenylethene, t-3-MPE, was previously described.29 The fluorenone-sensitized photoisomerizations of t-3-MPE in N2outgassed benzene (Sigma-Aldrich spectrophotometric grade), as previously described for NPE,30 gave a photostationary state that favors the cis isomer (∼70% c-3-MPE). Chromatography 5294
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Figure 1. (a) Fluorescence spectra of t-3-MPE in MCH at 77 K; the blue, green, and red curves are for λexc = 275, 313, and 330 nm, respectively. (b) Spectral evolution on irradiation of c-3-MPE at 288 nm in MCH at 77 K (normalized spectra).
Figure 2. (a) Principal eigenvectors for the 3-MPE spectral matrix in Figure 1. (b) Corresponding normalization line.
equilibrium mixture in the absence of solvent (0.48 kcal/mol free energy difference in favor of t-NTEB2). For the OBT photoisomerization mechanism to be correct, the B conformer of c-NPE must be dominant in MCH, but the A conformer of cNTE must be dominant in EPA. The reverse would have to apply if the HT mechanism were correct. The obligatory conclusion is that, in either case, the medium plays a critical role in controlling conformer distribution. Dramatic medium control of conformer distribution was first demonstrated by Havinga and co-workers who observed strikingly different CD spectra for previtamin D in EPA and in a hydrocarbon (3:1 isopentane/MCH) glass at 92 K.38,39 An especially relevant example concerns the control of t-NPE conformer distribution in solid n-alkanes at low T. t-NPEA and t-NPEB are present in similar concentration in n-hexane, whereas only the more extended t-NPEB conformer is present in n-decane, both at 5 K.40 Also relevant is that, following our lead,20,41 assignment of the spectrum of t-NPEA was based on its similarity to the spectrum of t-3-MPE, which exists exclusively as conformer A in n-hexane at 5 K.40 We recently presented evidence showing that the conformer distribution of cis,cis- and cis,trans-1,4-di(o-tolyl)-1,3-butadiene (cc- and ctDTB) is medium dependent. In contrast to EPA glass, the photoisomerization of cc-DTB in isopentane (IP) glass at 77 K proceeds by BP and OBT mechanisms without the intervention of unstable photoproduct conformers as required by HT photoisomerization.42 The conclusion that those observations rule out the HT photoisomerization process was criticized because of the low relative viscosity of the IP glass, which might soften further on electronic energy dissipation and be insufficiently rigid to trap HT products.43 However, glass softening would not explain why the IP glass at 77 K opens the BP photoisomerization channel in cis,cis-1,4-diaryl-1,3-butadienes,42,44 nor would it explain why the extent of BP
Condensation of atmospheric moisture on the Dewar in the course of fluorescence measurements was prevented by continuous purging of the sample chamber with argon. Computational Details. Quantum mechanical calculations were carried out using the Gaussian09 system31 utilizing gradient geometry optimization.32,33 All geometries were fully optimized using B3LYP/6-311+G(d,p).34−37
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RESULTS AND DISCUSSION Photoisomerization of c-NTE in EPA at 77 K. We sought guidance from theory in evaluating the effect of the o-methyl group on the conformer geometries and energetics of the two NTE isomers. The results of the DFT gas phase calculations are given in Chart 2. The inherent nonplanarity of the c-NPE conformers due to o-H steric interactions has a leveling effect on the influence of the o-methyl substituent. The calculated energy difference between the A and B conformers of c-NPE does not change upon o-methyl substitution (0.25 vs 0.26 kcal/ mol free energy difference in favor of the B conformer in NTE and NPE, respectively). The two cis conformers in Chart 1c are predicted to be almost isoenergetic, with the more extended conformer, c-NTEB2, only 0.2 kcal/mol lower in free energy than conformer c-NTEA1. It follows that theory, especially in the absence of explicit consideration of the solvent, does not provide a basis for preference of either the HT or the OBT photoisomerization pathway in Chart 1c. The importance of the solvent in controlling conformer distribution is also evident in the observed thermal isomerization on thawing and recooling the t-NTE photoproduct in EPA.28 The excellent agreement between the initial and final t-NTE absorption spectra at 77 K with the resolved t-NPEA and t-NPEB spectra in solution allows assignment of that conformational change to the t-NTEA1 → t-NTEB2 process.20 The spectral change indicates that the t-NTEA1 → t-NTEB2 reaction goes to completion and not to the predicted 30/70 room temperature 5295
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Chart 3a
a
See footnote in Chart2.
photoisomerization of cis,cis-1,4-diphenyl-1,3-butadiene depends on the rate of cooling of IP to 77 K.44 Photoisomerization of c-3-MPE in MCH at 77 K. The fluorescence spectra of the two isomers of 3-MPE in glassy MCH at 77 K are similar to those of NPE.27 However, in contrast to t-NPE, the fluorescence spectrum of t-3-MPE shows negligible dependence on λexc and exhibits a well-defined vibronic progression, Figure 1a, as is evident in the fluorescence spectra of both t-NPE conformers.27 The fluorescence spectrum of c-3-MPE is structureless, as is the case for c-NPE. On 288 nm excitation of c-3-MPE in MCH at 77 K in the fluorimeter, or at 313 nm externally, the structured fluorescence of the trans isomer is observed to progressively increase with irradiation time, Figure 1b. Principal component analysis (PCA)20 of the spectra in Figure 1b reveals a robust two component system with the product spectrum identical to that of t-3-MPE. The two eigenvectors and the adherence of the spectral combination coefficients to the normalization line that they define are shown in Figure 2. The observation that the fluorescence spectrum of the trans product does not change on thawing and recooling the solution, being identical to the fluorescence spectrum of the authentic sample of t-3-MPE, Figure 1a, confirms that the photoreaction yields the lowest energy conformer of t-3-MPE. The calculated structures and relative energies of the conformers of c- and t-3-MPE are shown in Chart 3. In both isomers, the steric requirement of the methyl group reverses the stability of the conformers relative to NPE. The compact A conformers are predicted to be lower in free energy than the extended B conformers by 1.1 and 2.2 kcal/mol in c- and t-3MPE, respectively. The calculations substantiate the earlier assignment of the lowest energy structure to t-3-MPEA.20,29,40 As with o-methyl substitution at the phenyl group, 3-methyl substitution at the naphthyl group of NPE perturbs the inherently nonplanar c-NPE conformers less than the t-NPE conformers, which are planar without the methyl substituent. Photocyclization, the dominant reaction pathway expected24−26 and observed45 for c-3-MPEA in solution, is entirely suppressed in MCH at 77 K. The propensity of the c-NPEA conformer toward photocyclization in solution is illustrated by the very pretty example in Chart 4.46 Here, 1-methyl substitution on the naphthyl group fails to bias photocyclization in favor of the less sterically hindered c-NPEB conformer and leads to loss of the methyl group instead. Suppression of photocyclization in c-3-MPE at 77 K is consistent with observations on c-NPE for which systematically lowering T to −180 °C in MCH/isohexane (2:1) glass attenuates the photocyclization quantum yield much more steeply than the cis−trans photoisomerization quantum yield.47 Our photo-
Chart 4
chemical observations show unequivocally that the photoreaction involves interconversion of stable conformers only: c3-MPEA → t-3-MPEA. They rule out HT photoisomerization about the olefinic CH with the naphthyl substituent (red in Chart 5) but do not exclude HT photoisomerization about the benzylic CH (green in Chart 5). Chart 5
Following the initial Liu and Hammond designation,11 there is a tendency to refer to the classical photoisomerization mechanism that begins with torsional displacement about the olefinic bond and leads to the ground state isomeric product1−6 as the one-bond flip (OBF) mechanism. This description conjures up an extremely free volume demanding overall rotation about the pertinent olefinic bond of 180°. On the other hand, the HT mechanism is defined as requiring a 180° translocation of only one of the CH moieties of the olefinic bond followed by in-plane sliding of the bulky substituents (in the instant case, naphthyl and phenyl) to achieve the geometry of the isomeric product. Both of these descriptions neglect the 5296
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exclusively about the naphthylenic vinyl proton, the process that we ruled out for c-NPE.27 Furthermore, on the basis of the calculated structures in Chart 6 and as pointed out above, the OBT motions required to convert the structures in Chart 6 to excited perpendicular intermediates cannot be considered as more free volume demanding than those proposed for the HT process. Our observations on c-3-MPE demonstrate the viability of the OBT photoisomerization mechanism for cNTE, Chart 1c.
role of the inherent nonplanarity of the cis isomers and that medium interference with the photochemical reaction applies primarily to motion on the S1 potential energy surface. It is instructive, therefore, to examine the highly nonplanar calculated structures of c-NTEA1 and c-3-MPEA side by side, Chart 6. Almost identical dihedral angles between naphthyl and Chart 6
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ASSOCIATED CONTENT
S Supporting Information *
Cartesian coordinates, drawings, and total energies of optimized structures (minima on S0) using B3LYP/6-311+G(d,p) for the methyl substituted 1-(2-naphthyl)-2-ethenes. 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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phenyl planes of 75.7 and 75.8° are predicted for c-NTEA1 and c-3-MPEA, respectively. It follows that relaxation of the excited singlet states from their Franck−Condon geometries to the roughly perpendicular intermediates that are envisioned in the classical photoisomerization mechanism would only require a mutual 15° displacement of the aryl groups and a much less volume demanding minor adjustment of the CHs in the olefinic bond. It is this structural change that is inhibited by the medium as is evidenced by dramatically enhanced fluorescence quantum yields and diminished photoisomerization quantum yields of the cis isomers. It is for this reason that we consider OBT and not OBF the proper designation of this mechanism.
ACKNOWLEDGMENTS This research was supported by the National Science Foundation, most recently by Grant No. CHE-0846636. GridChem49 is acknowledged for computational resources (www.gridchem.org).
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REFERENCES
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CONCLUSIONS On the basis of the known t-NPEA and t-NPEB absorption spectra,20 we assigned the thermal relaxation of the photoproduct of c-NTE in EPA glass at 77 K28 to the t-NTEA → tNTEB process. We tested the viability of c-NTEA → t-NTEB, the OBT process that would produce the initial trans product, by studying the photoisomerization of c-3-MPE, which is expected to exist as the A conformer. Electronic excitation of c3-MPEA in glassy MCH at 77 K gives t-3-MPEA exclusively, in contrast to its photoisomerization in solution that favors photocyclization to the methyl substituted benzodihydrophenanthrene.45 This cis−trans photoisomerization study on 3MPE and the earlier study on NPE27 were carried out in MCH glass at 77 K. The estimated viscosities of the IP and MCH glasses at 77 K, 106 and 1017−1023 P, respectively, differ by at least 11 powers of 10.48 Certainly, the soft glass argument43 that was advanced in questioning the interpretation of our results in the study of the photoisomerization of cc-DTB in IP42 cannot be applied to MCH. No unstable conformers intervene in the cis−trans photoisomerizations of c-NPE and c-3-MPE. In both cases, the thermodynamically stable conformer of the cis isomer gives the thermodynamically stable conformer of the trans isomer as expected for the OBT mechanism. HT about the benzylic vinyl proton would give the same result. We use Occam’s razor as our basis for preferring the OBT mechanism and further emphasize that, for the HT mechanism to be valid in the photoisomerization of c-NTE, it would have to occur 5297
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