Temperature dependence of photoisomerization. Part VIII. Excited-state behaviour of 1-naphthyl-2-phenyl- and 1,2-dinaphthyl-ethylenes and their photocyclisation products, and properties of the latter

Abstract

The main deactivation pathways of excited trans-isomers of the title compounds in dilute solutions are fluorescence and trans→cis conversion, those of excited cis-isomers are cis→trans and cis→ DHP conversion (DHP = 4a,4b-dihydrophenanthrene derivative, the photocyclisation product) and fluorescence, while those of excited DHP are DHP →cis conversion and, in some cases, fluorescence. The quantum yields of fluorescence and of the photoconversions were measured as a function of temperature, in alicyclic and aliphatic hydrocarbon solvents. The relative contributions of these deactivation paths vary sharply with the temperature and the compound, in the above series. In all compounds trans→cis, cis→trans(with one exception), and cis→ DHP photoconversions are slowed down sharply at low temperatures, while DHP →cis is attenuated on cooling only in the 2-naphthyl derivatives. Only in these three compounds was DHP fluorescence observed. The critical increments calculated by an ‘Arrhenius treatment’ of the variation of these quantum yields with temperature amount to 2–10 kcal mol^–1, and may indicate the existence of potential barriers on the way from the excited starting molecules to the final product. In the three 2-naphthyl derivatives, the ratio [trans]/[DHP] resulting from the reactions transâ†�cis*→ DHP was found to vary sharply with the wavelength of the exciting light (313,334, and 366 nm). This effect is explained as being due to selective excitation of the various more or less isoenergetic conformers of these compounds which exist in solution. Some of these conformers were observed to give rise to hitherto unknown types of DHPs and their oxidation products, again as a function of the wavelength of the exciting light. The structure of all DHPs formed via photocyclisation of the cis-isomers was proved, and their complete u.v. and visible absorption spectra determined, by dehydrogenation with atomic iodine to the corresponding benzo- or dibenzo-phenanthrenes. The proton n.m.r. spectrum of the DHPs derived from two dinaphthyl derivatives was determined, and signals were assigned to 4a-and 4b-H. Photodehydrogenation of DHPs involving excited DHP and ground-state oxygen or iodine was also observed. The thermal ring opening reaction DHP →cis was measured over a wide temperature range. The critical increments calculated therefrom were in the range 13–27 kcal mol^–1.