光学非线性LB膜的时间稳定性与光致漂白效应的关系

Correlations Between Photo Induced Bleaching and Temporal Stability on Optical Nonlinear Langmuir Blodgett Films

用光学二次谐波产生（ＳＨＧ）、拉曼散射和紫外－可见吸收光谱等方法研究了偶氮、苯腙及半花菁三种有机染料Ｚ型Ｌａｎｇｍｕｉｒ－Ｂｌｏｄｇｅｔ（ＬＢ）多层膜的时间稳定性问题。发现：随着放置时间的推延，其ＬＢ膜的二次谐波产生信号强度下降，然后逐渐达到稳定；在高压汞灯光照射下，粉末样品中Ｎ＝Ｎ（偶氮）、Ｃ＝Ｎ（苯腙）、Ｃ＝Ｃ（半花菁）键伸缩振动所对应的拉曼峰强度以及这三类染料在ＬＢ膜样品中所对应的生色团的吸收均呈现逐渐变小的现象。三个实验所确定的三种分子稳定性排列次序是完全一致的：Ｃ＝Ｃ键的结构最稳定，Ｃ＝Ｎ次之，Ｎ＝Ｎ最差。这三类有机染料中的“光漂白”现象是引起ＬＢ膜二次谐波产生信号下降的主要原因之一。

We investigated the stability of LB multilayers of three organic dyes, diazostilbenes (HPNA), phenylhydrazone (PD) and hemicyanine (DAEP), which all have large hyperpolarizability β but different double bonds in their conjugated π electron systems (N=N in HPNA, C=N in PD and C=C in DAEP). It was found that the second harmonic generation (SHG) intensities from those samples reduced significantly within a few tens of days. On the other hand, the Raman intensities from the powder samples of the three dyes due to the stretching modes of the double bonds and the UV vis absorption band from the LB multilayers of the three dyes corresponding to the original chromophores also decreased with increasing time of UV irradiation, confirming the breakdown (cleavage) of the double bonds (photo induced bleaching) in all the three dyes. The SHG, UV vis absorption and Raman experiments deduced the same sequence of stability: N=N the worst and C=C the best, suggesting that photo induced bleaching effect, instead of deterioration of the film frameworks, could possibly be responcible for the instability of the optical nonlinear LB films.