Femtosecond probing of the excited state absorption and structural relaxation in biphenyl derivatives

The excited state absorption spectra of biphenyl and 2,5-dimethyl biphenyl have been investigated by femtosecond broad band pump–probe spectroscopy in organic solvents. The results show vastly different intensities that were analyzed by calculating transition density surfaces for characteristic S1→Sn absorptions. The analysis revealed strong symmetry-lowering substituent effects in 2,5-dimethyl biphenyl which directly affect the transition dipole moments for excited state absorption. By measuring the temporal evolution of the transient spectra of the biphenyl derivatives we observed a biphasic relaxation mechanism in the excited state characterized by two distinct time scales of 400 fs and 12 ps.