A series of 2,5-diarylidene-cyclopentanones (ndbcp), their protonated cations (ndbcp-H+), and a substituted compound, 2,5-bis-[3-(4-dimethylamino-phenyl)-allylidene-cyclopentanone (2dbma) have been synthesized. Their electronic absorption and fluorescence spectra have been measured. 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. Polarized excitation spectra have been measured for 2dbcp and 3dbcp at 77 K in ethanol/methanol glass and used as an aid for the assignments of electronic transitions. Absorption and fluorescence spectra have been measured in solvents of varying polarity for all compounds synthesized. The influence of hydrogen bonding on the excitation spectra of compounds has been investigated. Solvent induced shifts in the absorption and fluorescence spectra of 3dbcp and 2dbma in combination with the PM3 calculated ground state dipole moment have been used to determine the excited state dipole moment of these compounds. Fluorescence quantum yields have been obtained to analyze the changes in the nonradiative rate of decay from S1. The protonated cations have been prepared in acids of different strength. The influence of acid strength on the excitation and emission spectra has been analyzed by gradually diluting acid solution. Evidence for excited state proton transfer in weak acids has been obtained for 2dbcp and 3dbcp. Brief photochemical studies of 1dbcp and 1dbcp-H+ have been carried out and analyzed by HPLC.