Bichromophoric interactions and time-dependent excited state mixing in pyrene derivatives. A femtosecond broad-band pump-probe study

Femtosecond broad-band pump-probe spectroscopy has been used to study intramolecular bichromophoric coupling and structural relaxation in pyrene and aryl pyrene derivatives in solution. The influence of aryl substituents on the S-2 --> S-1 internal conversion process, which occurs with a time constant of similar to75 fs in pyrene, has been investigated. While in 1-phenylpyrene the internal conversion is faster than 50 fs, it is slower in 1-biphenyl-4-yl-pyrene (105 fs). The temporal evolution of the transient absorption spectrum indicates strong mixing of several "zero-order" electronic configurations-which evolve separately with time-in the S-1 and the S-2 states. The time-resolved spectra are interpreted within the framework of an adiabatic state model which includes interchromophoric electronic coupling. In this paper we give a full description of the experimental setup, the data acquisition procedure, and several experimental details about the characterization of the broad-band femtosecond white light source.