Symmetry-Dependent Intramolecular Charge Transfer Dynamics of Pyrene Derivatives Investigated by Two-Photon Excitation.

Intramolecular charge transfer (ICT) processes in two-photon (TP) allowed states were investigated using three pyrene derivatives N1, N2C, and N2T, which have different molecular symmetry depending on the number and position of N,N-dimethylaniline donating substituents. On the basis of steady-state and nondegenerate two-photon absorption measurements, we investigated femtosecond transient absorption (TA) spectra by one-photon (OPE) and two-photon excitation (TPE). In the analysis of TA spectra, we discovered that the transfer rate from locally excited state to the ICT state by TPE is slower than that by OPE, indicating that the energy barrier between the TP and ICT states is higher than that between the one-photon (OP) allowed and ICT states. Furthermore, we demonstrated that ICT dynamics in a TP state are affected by molecular symmetry through the disappearance of stimulated emission from the locally excited state in the TA spectra of N2T obtained by TPE. We believe that our findings will provide fundamental information for a better understanding of excited-state ICT dynamics.