Using polarization anisotropy to recover the directions of transition dipole moments from transient absorption spectra with overlapping bands.

Determining the directions and magnitudes of transition dipole moments for excited states provides valuable information about the direction and degree of intramolecular charge separation. Furthermore, state-selective excitation via laser pulses opens new possibilities for controlling charge separation dynamics, which is important for the design of new molecular-sized electronic devices, particularly optical switches. The transient absorption (TA) spectra of xanthione (9H-Xanthen-9-thione) and dinitroxanthione (2,7-dinitro-9H-xanthene-9-thione) in acetonitrile were measured with parallel and perpendicular polarization orientations of the pump and probe pulses. Quantum-chemical calculations revealed that the transition dipole moments for S1,2→Sn>2 and T1→Tn>1 transitions are orthogonal to the excitation transition S0→S2. A scheme for direct fitting of TA signals was developed and adapted to account for varying transition dipole moment directions at the pump and probe stages. Multiparameter fitting across a broad UV-visible spectral range was performed for both parallel and perpendicular pump-probe polarization orientations. The timescales of intersystem crossing (ISC) from the S1 state and internal conversion (IC) from the S2 state, and rotational relaxation were estimated for xanthione (τISC=1.2 ps, τIC=14.9 ps, τr=29.9 ps) and dinitroxanthione (τISC=7.3 ps, τIC=23.9 ps, τr=33.1 ps). It is shown that the initial polarization anisotropy and the orientation of the transition dipole moments have a strong influence on the accuracy of determining the kinetic parameters. These findings establish a close relationship between ultrafast spectroscopic observables and differences in the directions of the transition dipole moments during the pump and probe stages.