多枝结构分子中激发态分子内电荷转移及分子构型变化对双光子荧光性能影响规律的超快光谱研究

Multi-branched organic molecule is a class of potential two-photon fluorescence (TPF) materials, due to their strong cooperative enhancement of two-photon absorption (TPA) between branches, and their high fluorescence quantum yield. However, when the branches increased in order to further improve the TPF property, several problems occurred, such as absence of cooperative enhancement effect or decreasing of fluorescence quantum yield. The occurrence of these problems should be related to the ultrafast process of intramolecular charge transfer (ICT) and molecular structural evolution in excited states. In order to make clear the mechanism of the effect of molecular structure on the TPF property, a series of multi-branched molecules will be investigated by using a new ultrafast technique that could directly observe the molecular structural change in excited state―the femtosecond time-resolved transient grating scattering spectroscopy, together with time-resolved transient absorption spectroscopy and quantum chemical calculation. The intramolecular charge transfer and molecular structural evolution in excited states will be investigated, and the effect of them on the TPF property will be analyzed, in order to provide the theoretical bases of designing new multi-branched molecule TPF materials. We will also develop this research method for the effective investigation of the molecular structural evolution in excited states, in order to overcome the difficulties in directly observation of molecular structural in excited states.

多枝结构有机分子由于存在双光子吸收的分枝间联合增强效应，以及较高的荧光量子效率，是具有重要应用前景的双光子荧光材料。然而，在增加分枝数以进一步提高性能时，出现了如双光子吸收联合增强缺失，荧光量子效率降低等问题，这与激发态分子内电荷转移及分子构型变化等超快过程密切相关。为了阐明此类材料分子结构对双光子荧光性能的影响机理，本项目以基于苯撑乙烯的一类多枝结构分子为研究对象，拟采用一种新型的能直接探测激发态分子构型演化过程的超快时间分辨光谱技术- - 飞秒时间分辨瞬态光栅衍射光谱技术，并结合瞬态吸收光谱技术，及量子化学计算手段，对激发态分子内电荷转移性质及分子构型变化的超快过程进行研究，分析他们对双光子荧光性质的影响规律，为多枝结构双光子荧光材料分子结构的设计提供理论依据，并由此建立一套简单而有效的对于激发态分子构型超快演化过程的超快光谱研究手段，以解决目前存在的激发态分子构型实验探测困难的问题。

激发态上电子结构与分子结构的超快变化过程的探测一直都是物理化学研究的研究重点。本项目采用了多种超快激光光谱学的方法，包括瞬态吸收光谱，瞬态光栅光谱，以及相干反斯托克斯拉曼光谱等，研究了多种有机分子的激发态上电子结构与分子结构的超快变化过程，包括双光子荧光材料Ph-TPAx的双光子增强机理，有机染料分子Rh101和Rh6G与溶剂DEA之间的光致分子间电荷转移过程，以及含能材料硝基甲烷在强激光作用下的超快结构变化过程。研究结果表明，本项目采用的超快激光光谱技术具有飞秒级的时间分辨率，是研究分子结构超快过程的有效实验手段。