Ultrafast excited state proton transfer dynamics of 1,2-dihydroquinolines in methanol solution

Abstract

Femtosecond and picosecond dynamics of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) and 1,2,2,4,6-pentamethyl-1,2-dihydroquinoline (2) were studied in MeOH, MeOD, and PrⁱOH to probe the early events of the photoinduced proton transfer (PT) between 1,2-dihydroquinolines (DHQ) and a solvent. From studies in the two solvents MeOH and PrⁱOH and by examining the effect of deuterium replacement of proton, it has been established that PT takes 150–200 fs in MeOH, but does not occur in PrⁱOH. The formation of PT products in the ground state proceeds concurrently to the relaxation of the higher vibrational excited singlet state to the thermally equilibrated state S₁ of DHQ. The absorption spectrum of the S₁ state was registered, and the time constant of its decay in MeOH (ca. 1 ns) agrees well with the lifetime of fluorescence measured recently by single photon counting.