Photoisomerization of cis-1-(2-Naphthyl)-2-phenylethene in Methylcyclohexane at 77 K:  No Hula-Twist

Abstract

The cis-trans photoisomerization of cis-1-(2-naphthyl)-2-phenylethene (c-NPEA and c-NPEB) was studied in methylcyclohexane glass at 77 K. The progress of the reaction was followed by fluorescence spectroscopy. Formation of the extended trans conformer (t-NPEB) is revealed by the growth of sharp vibronic bands on the broad structureless fluorescence spectrum of c-NPE. Principal component analysis with self-modeling of a matrix consisting of spectra generated by irradiation of c-NPE at different excitation wavelengths and spectra of t-NPE measured under the same conditions as a function of excitation wavelength reveals that the diabatic photoisomerization is a completely conformer-specific one-bond twist process: c-NPEB --> t-NPEB. These observations are consistent with observations in solution at ambient temperature. They demonstrate unequivocally that free volume restrictions imposed by the amorphous glassy environment do not open a hula-twist process.

Mechanism of Photoisomerization of 1‐Naphthyl‐2‐Phenylethylenes in Organic Glasses

“…2b) (only minor changes of relative heights of some of the fine vibrational structures). The obvious conclusion of formation of stable conformer(s) of the trans isomer (based on UV absorption spectra) is similar to that reported earlier (based on fluorescence studies) (14). However, formation of such a stable product(s) is consistent with any one of the three reaction pathways: OBF around the double bond, HT of the benzylic C–H unit or HT of the C–H unit near the naphthyl group if a conformer mixture similar to that of the trans isomer is obtained.…”

“…Recently, in a report on low temperature photoisomerization of stilbene derivatives (15), the need to choose and design systems carefully for such mechanistic studies was emphasized. It was pointed out that the only chosen system in the said report (14) is incapable of yielding information suitable for establishing or eliminating any of the above‐mentioned reaction mechanisms. In this paper, we report results of our investigation on low temperature photochemical reactions of isomers of 1 and related systems ( 2 and 3 ).…”

A Fluorogenic Aldehyde Bearing a 1,2,3-Triazole Moiety for Monitoring the Progress of Aldol Reactions

“…If cis olefin is used instead of the triazole ring, the cis olefin may be isomerized to trans olefin under the UV required for the fluorescence measurements 16. In the resulting trans form, fluorescence may be different from that of the cis form,16 thus the cis – trans isomerization may complicate analysis of the fluorescence data. With the 1,2,3-triazole group, there is no opportunity of such isomerization.…”

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

“…Therefore, 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. But, for c ‐NTE to undergo HT photoisomerization it would have to do so about the naphthylenic CH, the process that we ruled out in the parent c ‐NPE . More explicitly, for the OBT photoisomerization mechanism to be correct, the B conformer of c ‐NPE must be dominant in MCH, but the A conformer of c ‐NTE must be dominant in EPA.…”

“…We showed that at 77 K in glassy MCH the photoisomerizations of cis ‐1‐(2‐naphthyl)‐2‐phenylethene ( c ‐NPE, Scheme , that exists mainly as the B conformer and cis ‐1‐(3‐methyl‐2‐naphthyl)‐2‐phenylethene ( c ‐3‐MPE) that should exist as the A conformer (Scheme , bottom left, proceed as shown by the OBT labeled pathways c ‐NPE B → t ‐NPE B and c ‐3‐MPE A → t ‐MPE A in Scheme . However, our conclusion that no HT is involved in those reactions was dismissed because HT photoisomerization about the benzylic CH would lead to the same result .…”