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 --> pi* (A(2) <-- A(1)), to S-2 (strong) as pi --> pi* (B-2 <-- A(1)), and to S-3 (moderate) as pi - pi* (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)(npi*) and (1)(pipi*) 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 S1. 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.