Four-Component Fluorescence of trans-1,2-Di(1-methyl-2-naphthyl)ethene at 77 K in Glassy Media. Conformational Subtleties Revealed

Abstract

The vibronic structure of the fluorescence spectrum of trans-1,2-di(1-methyl-2-naphthyl)ethene (t-1,1) in methylcyclohexane (MCH) solution at room temperature was expected to become better defined upon cooling of the solution to 77 K. Instead, a broad, λexc-dependent fluorescence spectrum was observed in the glassy medium. Vibronically structured t-1,1 fluorescence spectra were obtained in the MCH glass only upon irradiation at the long-λ onset of the absorption spectrum. The application of singular value decomposition with self-modeling on the fluorescence spectral matrices of t-1,1 allowed their resolution into major and minor pairs of vibronically structured spectra that are assigned to two structural modifications of each of two relative orientations of the 1-methyl-2-naphthyl moieties. The difference between the two structures in each pair lies in the direction of rotation of each naphthyl group away from the plane of the olefinic bond. A complex but different conformer distribution is also responsible for the fluorescence spectra of t-1,1 in 5:5:2 (v/v/v) diethyl ether/isopentane/ethyl alcohol (EPA) glass at 77 K. The conformer distributions are also sensitive to the rate of cooling used in glass formation. Conformer distributions based on predicted small energy differences from gas-phase theoretical calculations are of little value when applied to volume-constraining media. The photophysical and photochemical properties of the analogues of the other two conformers of trans-1,2-di(2-naphthyl)ethene, trans-1-(1-methyl-2-naphthyl)-2-(3-methyl-2-naphthyl)ethene (t-1,3) and trans-1,2-di(3-methyl-2-naphthyl)ethene (t-3,3), were determined in solution. However, it is the calculated geometries and energy differences of the t-1,1 conformers [DFT using B3LYP/6-311+G(d,p)] that are essential guides to the interpretation of the experimental results.

Photoisomerization of cis‐1,2‐di(1‐Methyl‐2‐naphthyl)ethene at 77 K in Glassy Media

“…Paradoxically, it is the trajectories formed by the combination coefficient points () for intermediate cooling that shift toward the more stable t ‐ 1,1 A corners. As we observed in the case of t ‐ 1,1 , anomalous behavior would not be surprising in MCH because the crystalline environment expected on slow cooling may present to the c ‐ 1,1 guest cavities in the MCH crystal lattice that differ significantly from those in glassy MCH. Confining the comparison to the slow and intermediate cooling rates in MCH indicates that slower glass formation favors the thermodynamically more stable t ‐ 1,1 A product by shifting the c ‐ 1,1 conformer equilibrium toward the more stable c ‐ 1,1 A conformer.…”

“…c to the c ‐ 1,1 A and c ‐ 1,1 B conformers, respectively. Previously, we had reached the conclusion that the spectra of the t ‐ 1,1 conformers shift in the opposite direction than the spectra of the t ‐DNE conformers . As one and then two s‐cis ‐diene moieties are introduced in c ‐ 1,1 the absorbance ratio of the first to the second absorption band in the calculated spectra decreases, Fig.…”

“…Excitation of the same samples, first at 355 nm until no more conversion was observed, and then at 313 nm ensured that conversion progress was for the same reaction volumes. In both MCH and EPA, red edge excitation of c ‐ 1,1 leads to very low conversions and favors formation of t ‐ 1,1 A , the 1,1 conformer that emits at longer wavelengths . Assuming that, on the wavelength scale, the relative position of the absorption spectra of the A and B conformers of the two isomers is the same, we can conclude that the onset of the absorption spectrum of the lowest energy c ‐ 1,1 A conformer extends slightly to the red of the c ‐ 1,1 B absorption spectrum.…”

“…Rotation about the naphthyl‐vinyl single bonds in either isomer leads to three main relative orientations of the two naphthyl groups. In the case of t ‐ 1,1 , the calculations reveal two conformer structures for each of those relative naphthyl group orientations . The conformers of the trans isomer differ also in the sense of the rotation of the naphthyl groups (designated as + for clockwise and – for counterclockwise in Chart ) away from the ethylene plane .…”

“…The spectra are almost identical. It is the similarity of the c ‐ 1,1 A and c ‐ 1,1 B fluorescence spectra that accounts for our ability to treat spectral matrices, augmented by the introduction of the t ‐ 1,1 A and t ‐ 1,1 B fluorescence spectra , as three component systems. SVD‐SM treatment of the spectra in Fig.…”