In solution, particular combinations of donor and acceptor molecules can form ground-state pairs that exhibit a new, red-shifted, electronic absorption band not present in the individual donor or acceptor spectra. This new absorption is termed a ‘charge-transfer’ (CT) band and corresponds to excitation from the HOMO of the donor to the LUMO of the acceptor, effecting nearly instantaneous transfer of one electron from donor to acceptor. Charge-recombination (CR) then occurs in concert with solvent and vibrational relaxation and structural dyamics of the ion pair (IP). Due to the large distribution of ground-state complex structures, the excited state of the system is a composite of many different species. Although the thermodynamics and CR processes of such complexes have been the subject of intense study, the structural dynamics of the pairs and their relation to CR are still poorly understood.

We will present results of ultrafast visible (visTA) and infrared (TRIR) transient absorption experiments on the benzene/tetracyanoethylene (Bz/TCNE) pair in addition to a mixed quantum/classical computational study of the system. Population dynamics from the visTA and TRIR experiments reveal complex sub-10 ps dynamics followed by CR on a 55-60 ps timescale. Polarized TRIR anisotropy measurements reveal rich structural dynamics involving large-scale reorganization of Bz/TCNE radical ion pairs following excitation. A detailed computational study of the system combining quantum chemical calculations and classical molecular dynamics simulations was able to reproduce the experimental electronic absorption lineshape and TRIR anisotropy dynamics, allowing for a more detailed interpretation of the structural dynamics. We find that neither the ground nor excited state of the Bz/TCNE pair can be described using a single well-defined structure, and that the IPs convert from predominately face-to-face π stacks to edge-to-face T-shaped structures. Given the sensitivity of charge-transfer processes to donor/acceptor orientation, theoretical descriptions of the recombination dynamics must take into account the structural diversity and evolution of the excited state.