The Spectral Fingerprint of Excited-State Energy Transfer in Dendrimers through Polarization-Sensitive Transient-Absorption Pump-Probe Signals: On-the-Fly Nonadiabatic Dynamics Simulations

The time-resolved polarization-sensitive transient-absorption (TA) pump-probe (PP) spectra are simulated using on-the-fly surface-hopping nonadiabatic dynamics and the doorway-window (DW) representation of nonlinear spectroscopy. A typical dendrimer model system composed of two linear phenylene ethynylene units (2-ring, 3-ring) is taken as an example. The fewest switches trajectory surface hopping algorithm along with the TDDFT method is adopted in the nonadiabatic dynamics simulations. The ground-state bleach (GSB), stimulated emission (SE), excited-state absorption (ESA) contributions as well as the total TA PP signals are obtained and carefully analyzed. The correlations between these signals and the coupled nuclear-electronic dynamics are established. It is shown that intramolecular excited-state energy transfer from the 2-ring unit to the 3-ring unit can be conveniently monitored and accurately identified by employing pump and probe pulses with different polarizations. Our on-the-fly nonadiabatic simulation results demonstrate that time-resolved polarization-sensitive TA PP signals provide a powerful tool for the elucidation of excited-state energy transfer pathways, notably in molecular systems possessing several optically-bright nonadiabatically-coupled electronic states with different orientations of transition dipole moments.