PHOTOINDUCED PHASE TRANSITION IN QUASI ONE-DIMENSIONAL ORGANIC CRYSTALS BY MEANS OF THE-RESOLVED RAMAN SCATTERING SPECTROSCOPY

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The purpose of this research project was to establish general understanding of the mechanism of photoinduced structural transformation in quasi one-dimensional organic crystals, based on the dynamics in ultra short temporal domain and on the direct information on structural properties. For this, femtosecond time-resolved optical methods and time-resolved Raman spectroscopic methods were extensively used.For a charge-transfer organic crystal of tetrathiafulvalen-p-chloranil (TTF-CA), we could resolve completely the three consecutive primary steps involved in the transition ; the precursor-forming step, local proliferation, and N-phase order formation. The microscopic understanding in each step has been obtained by the results of multi-wavelength reflection measurements. For a process of forming a quasi one-dimensional conjugated it-bond order in diacetylene crystals, dynamics in forming the key entity of di-radical intermediatd in the polymerization reactions has been elucidated in 5,7-dodecadiyne-1,12-diol bis[phenyl carbamate] (TCDU) crystals : triplet-exciton fusion generates dimer diradicals. By applying Raman spectroscopy, we could obtain precise structural knowledge on each step of polymerization from diradicals to dicarben, origornaers and finally it-bond chains. The polydiacetylene crystals thus formed show both A and B phases, and relaxation dynamics in each phase have been studied by ferntosecond time-resolved reflection spectroscopy. Although photoinduced phase transition has not been induced in this poly TCDU, important process of forming intrinsic meta-stable states has been discovered.The results obtained in the present project have provided the important basis on which we can construct the mechanism of photoinudced phase transition based on microscopic understanding of structural knowledge in each step involved in the process.