Photoinduced Structure Change of Photochromic Compounds Accompanied with a Proton Transfer Studied by Time-Resolved Spectroscopy

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Dinitrobenzylpyridine (DNBP) is one of the photochromic compounds, which shows a structure change accompanied with a photoinduced intramolecular proton transfer (PIPT). Because the PIPT reaction can take place for many cycles even in the solid, DNBP might serve as a good candidate for new solid state optical storage devices. The goals of this project are to elucidate the mechanism of the PIPT reaction, control the PIPT reaction, and propose a new idea for the optical devises using the PIPT reaction.Femtosecond and nonosecond transient absorption spectra of DNBP molecules were measured in order to elucidate the PIPT and thermal reaction times of this material. Photoinduced absorption spectra and their thermal bleaching of the two different DNBP isomers (alpha-DNBP and gamma-DNBP), whose PIPT distance is different, were also investigated to clarify the PIPT reaction mechanism. Polarized reflection and absorption spectra of DNBP crystal were measured and the electronic spectra were calculated by the molecular orbital calculation.The above experiments and calculation have given us the following results :1) The PIPT reaction from the colorless form (CH_2 form) to the dark blue form occurs at -300 fsec and the low frequency torsional modes might be responsible for the PIPT reaction.2) The dark blue form consists of two different kinds of the photoproducts, NH and OH forms.3) The OH form thermally returns to the NH form and both the NH and OH forms optically return back to the CH_2 form on the photolysis of visible light.4) The efficiency of the PIPT reaction depends on the PIPT distance and can be controlled by changing the kinds of the DNBP derivatives which have the different PIPT distance.5) The PIPT reaction of DNBP crystal is different from that of DNBP molecule, implying that the intermolecular proton transfer occurs in DNBP crystal.