The State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
J Phys Chem A. 2010 Jul 15;114(27):7345-52. doi: 10.1021/jp1032355.
The nature of optical excitation and the degree of intramolecular charge transfer (ICT) as well as the dynamics of excited ICT states of two new tribranched donor-pi-acceptor molecules with acceptor-terminated (DA(3)) and acceptor-centered (AD(3)) geometries have been investigated by steady-state and femtosecond time-resolved stimulated emission fluorescence depletion (FS TR-SEP FD) measurements in different polar solvents. The interpretation of the experimental results is based on the comparative investigation of the two D-pi-A compounds with respect to the model monomer counterpart (DA). The larger solvatochromic effects and stronger solvent dependence of spectral properties of DA(3) than that of AD(3) indicate that the excited ICT state of DA(3) possesses higher polarity and larger dipole moments compared to those of AD(3). The similarity of absorption and strong solvent-dependent fluorescence spectra of DA(3) and DA reveals that the excited-state properties of DA(3) are identical to that of the model DA, which localized on one of the branches in DA(3). In contrast to DA(3), the large red shift in the absorption and the small Stokes shift of AD(3) suggest the formation of a delocalized ICT state to a certain extent in the excited state of AD(3). The dynamic behavior of excited ICT states for all three compounds are also investigated by femtosecond time-resolved stimulated emission depletion (FS TR-SEP FD) measurements, where the excited-state relaxations are highly dependent on both solvent polarity and the polar degree of the excited ICT states. Furthermore, the steady-state fluorescence excitation anisotropy shows that the intramolecular excitation transfer among the three disorder-induced localized ICT states with nondegenerate transition dipole moments is involved within DA(3). Compared to DA(3), a substantial red shift in the absorption of AD(3) results from the formation of a delocalized ICT state, where the specific excitation anisotropy spectrum shows that the excitation energy is mainly redistributed between the localized ICT state and the delocalized ICT state.
已通过稳态和飞秒时间分辨受激发射荧光损耗（FS TR-SEP FD）测量，在不同极性溶剂中研究了两个具有受体端（DA(3)）和受体中心（AD(3)）几何形状的新型三分支给体-π-受体分子的光激发性质和分子内电荷转移（ICT）程度以及激发 ICT 态的动力学。实验结果的解释基于对两种 D-π-A 化合物与模型单体对应物（DA）的比较研究。DA(3) 的较大溶剂化效应和光谱性质对溶剂的更强依赖性表明，与 AD(3) 相比，DA(3) 的激发 ICT 态具有更高的极性和更大的偶极矩。DA(3) 的吸收和强溶剂依赖性荧光光谱与 DA 的相似表明，DA(3) 的激发态性质与模型 DA 相同，DA 位于 DA(3) 的一个分支上。与 DA(3) 相反，AD(3) 的吸收较大红移和斯托克斯位移较小表明，AD(3) 的激发态在一定程度上形成了非定域 ICT 态。通过飞秒时间分辨受激发射荧光损耗（FS TR-SEP FD）测量还研究了所有三种化合物的激发 ICT 态的动态行为，其中激发态弛豫高度依赖于溶剂极性和激发 ICT 态的极性程度。此外，稳态荧光激发各向异性表明，在 DA(3) 中，三个非定域 ICT 态之间的分子内激发转移涉及具有非简并跃迁偶极矩的无序诱导局部 ICT 态。与 DA(3) 相比，AD(3) 的吸收的明显红移来自于非定域 ICT 态的形成，其中特定的激发各向异性谱表明激发能量主要在局部 ICT 态和非定域 ICT 态之间重新分配。
