Ramakrishna Guda, Goodson Theodore
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, USA.
J Phys Chem A. 2007 Feb 15;111(6):993-1000. doi: 10.1021/jp064004n. Epub 2007 Jan 24.
Branched macromolecular structures are now an important area of research for enhanced two-photon absorption (TPA) cross sections. The mechanism of this enhancement has been suggested as a complex interplay between intramolecular interactions and the extent of charge-transfer character in the branches. In order to probe these processes more clearly, excited-state dynamics of multibranched chromophores by means of femtosecond transient absorption spectroscopy are reported. Investigations have been carried out on the PRL dye series (PRL-101, PRL-501, PRL-701), which have shown cooperative enhancement of the TPA cross section. Upon photoexcitation, transient absorption measurements have shown the presence of a localized charge-transfer (intramolecular charge transfer, ICT) state independent of branching. The results point to ultrafast localization of charge in this particular system of chromophores. Pump-probe measurements in highly polar solvents have shown the presence of a nonemissive charge-transfer state which is a solvent stabilized and conformationally relaxed state. The population of this nonemissive state increases from monomer to trimer, and thus, it has been used as indicator of the polar nature of the Franck-Condon state. These results have shown an increase of charge-transfer character of the excited state with an increase in branching, and this explains the relative increase in the two-photon cross section of the PRL series.
支化大分子结构目前是增强双光子吸收（TPA）截面研究的一个重要领域。这种增强机制被认为是分子内相互作用与支链中电荷转移特性程度之间的复杂相互作用。为了更清楚地探究这些过程，本文报道了通过飞秒瞬态吸收光谱对多支化发色团的激发态动力学进行的研究。对PRL染料系列（PRL - 101、PRL - 501、PRL - 701）进行了研究，该系列染料已显示出TPA截面的协同增强。光激发后，瞬态吸收测量表明存在与支化无关的局域电荷转移（分子内电荷转移，ICT）态。结果表明在这个特定的发色团体系中电荷的超快局域化。在高极性溶剂中的泵浦 - 探测测量表明存在一种非发射性电荷转移态，它是一种溶剂稳定且构象弛豫的状态。这种非发射态的布居数从单体到三聚体增加，因此，它已被用作弗兰克 - 康登态极性的指标。这些结果表明，随着支化增加，激发态的电荷转移特性增强，这解释了PRL系列双光子截面的相对增加。
