Mapping Vibronic Couplings in a Solar Cell Dye with Polarization-Selective Two-Dimensional Electronic-Vibrational Spectroscopy

This study uses polarization-selective two-dimensional electronic–vibrational (2D EV) spectroscopy to map intramolecular charge transfer in the well-known solar cell dye, [Ru(dcbpy)2(NCS)2]4– (N34–), dissolved in water. A static snapshot of the vibronic couplings present in aqueous N34– is reported. At least three different initially excited singlet metal-to-ligand charge-transfer (MLCT) states are observed to be coupled to vibrational modes probed in the lowest energy triplet MLCT state, emphasizing the role of vibronic coupling in intersystem crossing. Angles between electronic and vibrational transition dipole moments are extracted from spectrally isolated 2D EV peaks and compared with calculations to develop a microscopic description for how vibrations participate with 1MLCT states in charge transfer and intersystem crossing. These results suggest that 1MLCT states with significant electron density in the electron-donating plane formed by the Ru-(NCS)2 will participate strongly in charge transfer through these vibronically coupled degrees of freedom.