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

二硝基苯并吡啶(DNBP)是光致变色化合物的一种，它伴随着光诱导的分子内质子转移(PIPT)而发生结构变化。由于即使在固体中，PIPT反应也可以发生多个循环，DNBP可能是新的固态光存储器件的很好候选者。本项目的目标是阐明PIPT反应的机理，控制PIPT反应，并为利用PIPT反应的光学器件提供新的思路。测量了DNBP分子的飞秒和非秒的瞬时吸收光谱，以阐明该材料的PIPT和热反应时间。研究了PIPT距离不同的两种不同DNBP异构体(α-DnBP和γ-DnBP)的光诱导吸收光谱及其热漂白，以阐明PIPT反应机理。测量了DNBP晶体的偏振反射光谱和吸收光谱，并用分子轨道计算方法计算了晶体的电子光谱。上述实验和计算结果表明：1)从无色形式(CH2形式)到深蓝色形式的PIPT反应发生在-300fsec，低频扭转模可能是PIPT反应的原因。2)深蓝色形式由两种不同的光产物组成。4)PIPT反应的效率取决于PIPT距离，并且可以通过改变不同PIPT距离的DnBP衍生物的种类来控制。5)DnBP晶体的PIPT反应不同于DnBP分子的PIPT反应，这意味着DNBP晶体中发生了分子间的质子转移。

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.