Ultrafast dynamics of a charge-transfer dimer as a model for the photoinduced phase transition of charge-transfer compounds

By applying ultrafast pump-probe spectroscopy with 15 fs temporal resolution to (TMTTF+)(2) dimers we provide a full picture of the structural relaxation following photoexcitation of their CT transition. Both population and coherent phonon dynamics allow tracking wave packet motion onto the multidimensional excited state potential energy surface, as obtained by density functional theory calculations. We show that the vibrations that are strongly coupled to the charge-transfer transition of the dimer correspond to those driving the photoinduced phase transition occurring in charge-transfer crystalline solids.