Highly efficient photoinduced phase transition in an organic radical crystal via two-photon absorption process

We have studied excitation energy dependence of the conversion yield of two-photon-mediated phase transition in organic radical 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) crystal. Even under photoexcitation far below the lowest charge-transfer absorption edge, the photoinduced phase transition takes place from diamagnetic low-temperature phase to paramagnetic high-temperature phase with threshold excitation density. By comparing the excitation energy dependence of the conversion yield with the two-photon absorption spectrum, we found that the conversion is resonantly enhanced by two-photon absorption process. Because of the strong three-dimensional interaction between the excited states of TTTA molecules, the homogeneous two-photon excitation can induce the phase transition with high efficiency.