Excited-state energy levels and photophysics of a short polyene 2-(4-phenyl-1,3-butadien-1-yl)thiophene

• Emission spectra of 2-(4-phenyl-1,3-butadien-1-yl)thiophene (PBT) were measured under different conditions.
• The existence of a forbidden singlet excited state at energies below the strongly absorbing state is indicated.
• The emission of PBT consists of dual fluorescence from S2 and S1 in low polarizable solvents.
• The emission of PBT consists mostly of the S1 florescence in the vapor phase.
• Analysis of the temperature dependence of the fluorescence provided the excited-state energies and various parameters.

Emission, excitation and absorption spectra of a new and short polyene, 2-(4-phenyl-1,3-butadien-1-yl)thiophene (PBT), have been measured under different conditions by varying temperature and solvent and in the vapor phase, along with those of 2-(2-phenylethenyl)-thiophene (PET). The presence of a forbidden excited singlet state, located at energies slightly below the strongly absorbing state, is indicated for PBT, although the forbidden state was not identified for PET. The emission of PBT consists of fluorescence from the allowed S2 and forbidden S1 states in low polarizable solvents, while it consists of florescence mainly from forbidden S1 in the vapor phase and that from allowed S1 state in high polarizable solvents. Quantitative analysis of the temperature dependence of the S1 and S2 fluorescence spectra provides the S1 and S2 state energies and physical parameters that characterize the excited states of PBT.