Verma Pragya, Tasior Mariusz, Roy Palas, Meech Stephen R, Gryko Daniel T, Vauthey Eric
Department of Physical Chemistry, University of Geneva, 30 Quai Ernest-Ansermet, CH-1211, Geneva 4, Switzerland.
Institute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland.
Phys Chem Chem Phys. 2023 Aug 30;25(34):22689-22699. doi: 10.1039/d3cp02810k.
A significant number of quadrupolar dyes behave as their dipolar analogues when photoexcited in polar environments. This is due to the occurrence of excited-state symmetry breaking (ES-SB), upon which the electronic excitation, initially distributed over the whole molecule, localises preferentially on one side. Here, we investigate the ES-SB properties of two A-D-A dyes, consisting of a pyrrolo-pyrrole donor (D) and either cyanophenyl or dicyanovinyl acceptors (A). For this, we use time-resolved vibrational spectroscopy, comparing IR absorption and femtosecond stimulated Raman spectroscopies. Although dicyanovinyl is a stronger electron-withdrawing group, ES-SB is not observed with the dicyanovinyl-based dye even in highly polar media, whereas it already takes place in weakly polar solvents with dyes containing cyanophenyl accepting groups. This difference is attributed to the large electronic coupling between the D-A branches in the former dye, whose loss upon symmetry breaking cannot be counterbalanced by a gain in solvation energy. Comparison with analogues of the cyanophenyl-based dye containing different spacers reveals that interbranch coupling does not so much depend on the distance between the D-A subunits than on the nature of the spacer. We show that transient Raman spectra probe different modes of these centrosymmetric molecules but are consistent with the transient IR data. However, lifetime broadening of the Raman bands, probably due to the resonance enhancement, may limit the application of this technique for monitoring ES-SB.
相当数量的四极染料在极性环境中光激发时表现得如同它们的偶极类似物。这是由于激发态对称性破缺（ES-SB）的发生，在此过程中，最初分布在整个分子上的电子激发优先定域在一侧。在此，我们研究了两种A-D-A染料的ES-SB性质，这两种染料由吡咯并吡咯供体（D）和氰基苯基或二氰基乙烯基受体（A）组成。为此，我们使用时间分辨振动光谱，比较红外吸收光谱和飞秒受激拉曼光谱。尽管二氰基乙烯基是更强的吸电子基团，但即使在高极性介质中，基于二氰基乙烯基的染料也未观察到ES-SB，而对于含有氰基苯基接受基团的染料，在弱极性溶剂中就已经发生了ES-SB。这种差异归因于前一种染料中D-A支链之间的大电子耦合，对称性破缺时这种耦合的损失无法通过溶剂化能的增加来抵消。与含有不同间隔基团的氰基苯基染料类似物的比较表明，支链间耦合与其说取决于D-A亚基之间的距离，不如说取决于间隔基团的性质。我们表明，瞬态拉曼光谱探测这些中心对称分子的不同模式，但与瞬态红外数据一致。然而，拉曼带的寿命展宽可能是由于共振增强，这可能会限制该技术在监测ES-SB方面的应用。
