Temperature dependence of photoisomerization. Part 9. Electronic spectra and excited-state behaviour of helicene-forming diarylethylenes and their photocyclization products, and properties of the latter

Abstract

trans ⇄ cis Photointerconversion, photocyclization, cis→ DHP (= 4a,4b-dihydrophenanthrene derivatives), photoinduced ring opening DHP →cis, and photocyclodehydrogenation cis→ P of solutions of the title compounds were investigated in the temperature range +25 to –180° with static and flash photolytic methods, and quantum yields were determined. Absorption and emission spectra of both isomers of all compounds, and absorption spectra of most cyclization products were measured over the same temperature range. A pronounced variation of the fluorescence spectra of several trans-compounds with the temperature and the excitation wavelength was observed, and may be due to conformer equilibria. The cis isomers exist as equilibrium mixtures of two to four possible conformers, A, B, B′, and C. Of these, short-lived cyclization products of the A and B conformors, DHP_A, and DHP_B, were observed at room temperature in most cases. At reduced temperatures DHP_B is the predominant product; it was also observed for three compounds with static methods. Quantum yields of all photoreactions are attenuated on cooling, while fluorescence yields increase concurrently, the calculated corresponding critical increments amounting to 1–4 kcal mol^–1. This indicates the existence of potential barriers on the path from excited starting molecules to final products. The rate of the thermal reversion DHP →cis increases on heating, with calculated critical increments of 8–18 kcal mol^–1.