Solvation Shell Helps Track Charge Transfer

Carlos R. Baiz and Kevin J. Kubarych of the University of Michigan have tracked ultrafast charge transfer in a dye molecule by monitoring changes in the infrared spectrum of its solvent shell (J. Am. Chem. Soc., DOI: 10.1021/ja1043853). The researchers worked with a system in which the dye, betaine-30, was immediately surrounded by a solvation shell of 10 molecules of NaSCN solvent in a larger solution of ethyl acetate. The NaSCN molecules partially aligned with the dipole moment of the dye in its ground state. The researchers excited the dye molecules using laser pulses to induce charge transfer from an orbital that localized electron charge on the dye’s oxygen atom to one that localized charge on the nitrogen atom. They then monitored the shift in the NaSCN vibrations in response to the electron redistribution and used those IR shifts to map the electric field change induced by the charge transfer. That map, in turn, can be combined with dynamics simulations of the charge-transfer reaction, providing a new way to study dynamics on subpicosecond timescales, the authors say.