Temporal and space resolved detection of laser-induced crystallization process

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With a view to elucidating molecular dynamics in the nucleation process of organic crystals in solids, we have studied model systems, pyrene molecules in low temperature glassy matrices of mixture solvent consisting of isopentane and methylcyclohexane. In the series of experiments we demonstrated that irradiating several thousands of nanosecond laser pulses at 355 nm to the model system induced the efficient aggregation of the solute in the solid matrices even at 77 K. During the crystallization induced by successive UV irradiation, an emission peak that could be attributed to the excimer of pyrene increased in intensity with an increase in the shot-number of the UV pulse, while the excimer emission was not detected without laser irradiation. The excimer emission appeared even by single shot irradition of the UV pulse, which indicates that the single shot of the laser pulse enabled pyrene molecules to stick together by translational/rotational diffusion. This can be explained by transient melting of the glassy matrices induced by the laser irradiation. Time-resolved fluorescence measurements from ps to ns region revealed that the solute molecules moved and formed excimer within the time-width of the UV pulse; the excimers were frozen before dissociation because of short duration of the transient melting almost same as that of the UV pulse and longer lifetime of pyrene excimer (〜 100 ns order). This different time-scale between aggregation and dissociation can explain the efficient aggregation of the solute by UV irradiation in this system.