Electronic Absorption and Fluorescence Properties of 2,5-Diarylidene-Cyclopentanones

Spectroscopic properties for a series of 2,5-diarylidene-cyclopentanones are reported. Electronic absorption and fluorescence spectra have been measured for the all- E configurations of 2,5-dibenzylidene-cyclopentanone ( 1 ), 2,5-bis-(3-phenyl-allylidene)-cyclopentanone ( 2 ), and 2,5-bis-(5-phenyl-penta-2,4-dienylidene)-cyclopentanone ( 3 ). The absorption spectra have been assigned with the aid of INDO/S calculations. Molecular structures used for the INDO/S calculations were computed with the PM3 Hamiltonian. Agreement between absorption spectra obtained in cyclohexane at room temperature and the theoretical predictions is good. For 1, 2, and 3 the general features of the spectra are similar. The transition to S 1 (weak) is assigned as n → π* (A 2 ← A 1 ), to S 2 (strong) as π → π* (B 2 ← A 1 ), and to S 3 (moderate) as π → π* (A 1 ← A 1 ). The energy gap between S 1 and S 2 is seen to decrease as the length of the polyene chain increases in going from 1 to 3 . Fluorescence is not observed for 1 in any of the solvents studied (protic and aprotic). Fluorescence is observed for 2 in protic solvents only. For 3, fluorescence is observed in a number of protic and aprotic solvents. Solvents which are able to induce fluorescence are believed to do so by inverting the order of 1 ( n π*) and 1 (ππ*) states. The influence of hydrogen bonding on the excitation spectra of 2 and 3 is discussed. Solvent-induced shifts in the absorption and fluorescence spectra of 3 in combination with the PM3 calculated ground-state dipole moment (2.8 D) are used to determine the excited-state dipole moment of 3 (6.4 D/protic solvents; 6.6 D/aprotic solvents). Fluorescence quantum yields in different solvents for 3 vary as the fluorescence maxima shift in these solvents, going through a maximum in the mid-frequency range. The variation in quantum yields with different solvents is primarily attributed to changes in the nonradiative rate of decay from S 1 . Excitation, polarized excitation, and fluorescence spectra have been measured for 2 and 3 at 77 K in ethanol/methanol glass. Vibronic features not observed in the broad spectra obtained in alcohols at room temperature become clearly resolved at 77 K. Evidence is provided that indicates that 2 and 3 undergo excited-state proton-transfer reactions in acetic acid at room temperature.