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 ∼75 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.