Photoinduced cooperative charge transfer in low-dimensional organic crystals

We demonstrate that the cooperative charge transfer among constructive mole cules in organic crystals of tetrathiafulvalene-chloranil, i.e., the bidire ctional transition between the neutral and ionic phases, can certainly be i nduced by irradiation of a laser pulse with an 80-fs width. We also report the dynamics of the photoinduced neutral-ionic phase transition by means of the time-resolved spectroscopic technique. Studies using various excitatio n intensities have shown that the photoconverted fraction (Phi) depends non linearly on the excitation intensity. In addition, it has been found that t he growth dynamics of the macroscopic domain of the metastable phase strong ly depends on the excitation intensity as well. The role of the cooperative charge-transfer interaction in the driving process of this unusual photoef fect is discussed from both the experimental and the theoretical viewpoints .