Characterization of a Conical Intersection in a Charge-Transfer Dimer with Two-Dimensional Time-Resolved Stimulated Raman Spectroscopy

Photochemical reactions are mediated by conical intersections (CI), which are difficult to directly probe and characterize. To gain insight into CIs, two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS) is used to examine a model excited-state charge-transfer (CT) complex consisting of an electron donor, tetramethylbenzene (TMB), and an acceptor, tetracyanoquinodimethane (TCNQ). Following impulsive excitation, the excited-state transient absorption reveals large-amplitude excited-state wave packet motion along low-frequency modes, in particular TCNQs totally symmetric 323 cm(-1) CCN bend, which persist for similar to 5 ps. These low-frequency coherences modulate the intensity and peak frequencies of the excited-state FSRS vibrational spectra. In particular, large-magnitude oscillations at 323 cm(-1) are observed in the peak frequency (Delta omega = 2 cm(-1)) and intensity (Delta OD = 1.5 mOD) of the nontotally symmetric 1271 cm(-1) C=C rocking mode. The magnitude of these oscillations is analyzed to determine the first-order anharmonic coupling between the high- and low-frequency degrees of freedom in the excited state. The anharmonic coupling between the totally symmetric 323 cm(-1) and the nontotally symmetric 1271 cm(-1) modes is estimated to be in excess of 50 cm(-1), strongly suggesting that they are the tuning and coupling modes, respectively, for the CI that connects the CT excited state to the neutral ground state and controls charge recombination internal conversion.