Department of Chemistry, University of Washington, Seattle, Washington 98195, USA.
J Phys Chem B. 2010 Dec 9;114(48):15808-17. doi: 10.1021/jp109624d. Epub 2010 Nov 15.
The nature of one and two-photon absorption enhancement in a series of oligothiophene dendrimers, recently proposed for applications in entangled photon sensors and solar cells, has been analyzed using both theory (time dependent density functional theory calculations) and experiment (fluorescence upconversion measurements). The linear absorption spectra exhibit a red shift of the absorption maxima and broadening as a function of dendrimer generations. The two-photon absorption cross sections increase sharply with the number of thiophene units in the dendrimer. The cooperative enhancement in absorption two-photon cross sections is explained by (i) an increase in the excited-state density for larger molecules and (ii) delocalization of the low-lying excited states over extended thiophene chains. Fluorescence anisotropy measurements and examination of the calculated excited-state properties reveal that this delocalization is accompanied by a size-dependent decrease in excited-state symmetries. A substantial red shift of the emission maxima for larger dendrimers is explained through the vibronic planarization of the longest linear α-thiophene chain for the emitting excited state. For higher generations, the fluorescence quantum yield decreases due to increased nonradiative decay efficiency (e.g., intersystem crossing). The detailed information about the dendrimer 3D structure and excitations provides guidance for further optimizations of dendritic structures for nonlinear optical and opto-electronic applications.
一系列寡聚噻吩树枝状大分子的单光子和双光子吸收增强的性质，最近被提议应用于纠缠光子传感器和太阳能电池，已通过理论（含时密度泛函理论计算）和实验（荧光上转换测量）进行了分析。线性吸收光谱表现出吸收最大值的红移和随树枝状大分子代际数的展宽。双光子吸收截面随树枝状大分子中噻吩单元数的增加而急剧增加。吸收双光子截面的协同增强可归因于（i）较大分子中激发态密度的增加和（ii）低能激发态在扩展噻吩链上的离域。荧光各向异性测量和计算的激发态性质的检验表明，这种离域伴随着激发态对称性随尺寸的减小。对于较大的树枝状大分子，发射最大值的显著红移可以通过发射激发态中最长线性α-噻吩链的振动态平面化来解释。对于更高的代际，荧光量子产率由于非辐射衰减效率（例如，系间穿越）的增加而降低。关于树枝状大分子 3D 结构和激发态的详细信息为进一步优化非线性光学和光电应用的树枝状结构提供了指导。
