ChemInform Abstract: Fluorescence, Fluorescence‐Excitation, and UV Absorption Spectra of trans‐1‐(2‐Naphthyl)‐2‐phenylethene Conformers.

Abstract

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

“…We had based structure assignments to the resolved fluorescence spectra of the t -DNE conformers in MCH solution at ambient temperature on a comparison with the fluorescence spectra of the 3-methyl and 3,3′-dimethyl derivatives. , Supported by the findings of Bartocci and Spalletti, those assignments were consistent with low- T fluorescence spectra under conditions that favor the presence of only the thermodynamically most stable t -DNE A conformer − and with our present fluorescence results in MCH glass, which in addition to t -DNE A reveal a very small contribution from the higher energy t -DNE B conformer (Figure a). The MCH solution spectra of the 1,1′-dimethyl and 1,3′-dimethyl derivatives ( t - 1,1 and t - 1,3 , respectively) provide the missing links in this comparison (Figure and Table ).…”

“…The contributions of the minor conformers increase upon excitation at the red edge of the absorption spectrum. The fluorescence of t -DNE in a 1:1 methylcyclohexane/isohexane (MCH/IH) glass at 88 K gives mainly a spectrum with well-resolved vibronic bands (λ exc =337 nm, peak maxima at 360, 382, and 403 nm) that was assigned to the s-t , s-t conformer, t -DNE A . − A second spectrum with broader vibronic bands (peak maxima at 373, 395, and 420 nm) is observed upon excitation at 368 or 370 nm, the red edge of the absorption spectrum. This second spectrum is probably due to the s-t , s-c conformer, t -DNE B .…”

“…The α,β eigenvector combination coefficient normalization line and the resolved conformer fluorescence spectra are shown in Figure a,b, respectively. The vibronic band maxima of the fluorescence spectra of the two conformers are in excellent agreement with those in Fischer’s spectra in MCH/IH. − Discounting differences in the hydrocarbon glass compositions, it may be the case that the larger contribution of t -DNE B fluorescence in the earlier work reflects the larger contribution of the DNE B conformer in c -DNE than in t -DNE. As previously pointed out, the nonplanarity of cis -diarylethenes due to ortho -H van der Waals repulsion has a leveling effect on the energy difference between their conformers.…”

“…A Wittig reaction was used to prepare the diarylethene. To a solution of sodium ethoxide prepared from 0.55 g (0.024 mol) of sodium metal in 50 mL of absolute ethanol was added in one portion under nitrogen 9.92 g (0.020 mol) of the triphenylphosphonium salt of 2-bromomethyl-3-methylnaphthalene . After 30 min of stirring at room temperature, a solution of 3.05 g (0.018 mol) of 1-methyl-2-naphthaldehyde in 40 mL of absolute ethanol was added dropwise, and the reaction mixture was allowed to stir overnight.…”

“…As a result of steric hindrance, methyl substitution at the 3 positions of the naphthyl groups is expected to simplify the conformational landscape in predictable ways. Consequently, the spectra of 3-methylnaphthyl analogues were employed in the assignment of conformer structures to the resolved fluorescence spectra of t -DNE ,, and trans -1-(2-naphthyl)-2-phenylethene ( t -NPE) . Similarly, steric effects upon methyl substitution at the 1 positions of the naphthyl groups of DNE account for observations indicating that trans -1,2-di(1-methyl-2-naphthyl)ethene ( t - 1,1 ) and its cis isomer ( c - 1,1 ) behave as single conformers in solution …”