Ground and singlet excited state configurations and dynamics of the oligothiophenes. Theoretical and experimental studies

The absorption and fluorescence properties of oligothiophenes in various solvents are reported at various temperatures. Calculations at the CIPSI level for the ground and first two singlet excited states of thiophene and alpha-bithiophene (alphaBT) in geometries optimized in the ground state and for alphaBT at various values of the dihedral angle are performed. Allowing the excited molecule to relax, the minimum energy geometries are determined via the analytical gradient technique. In the first singlet excited state, it is planar in the s-cis and s-trans geometries. All results indicate for the lower energy band a pipi* transition with the transition dipole parallel to the long axis, and suggest that the oligothiophenes are related to the polyenes. The cisoid and transoid conformations have similar calculated oscillatory strengths and transition energies so that their spectral properties cannot be easily distinguished one from the other.