Direct Visualization of Excited-State Symmetry Breaking Using Ultrafast Time-Resolved Infrared Spectroscopy

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• 作者：Bogdan Dereka ; Arnulf Rosspeintner ; Zhiquan Li ; Robert Liska ; Eric Vauthey
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：April 6, 2016
• 年：2016
• 卷：138
• 期：13
• 页码：4643-4649
• 全文大小：547K
• ISSN：1520-5126

文摘

Most symmetric quadrupolar molecules designed for two-photon absorption behave as dipolar molecules in the S₁ electronic excited state. This is usually explained by a breakup of the symmetry in the excited state. However, the origin of this process and its dynamics are still not fully understood. Here, excited-state symmetry breaking in a quadrupolar molecule with a D-π-A-π-D motif, where D and A are electron donating and accepting units, is observed in real time using ultrafast transient infrared absorption spectroscopy. The nature of the relaxed S₁ state was found to strongly depend on the solvent polarity: (1) in nonpolar solvents, it is symmetric and quadrupolar; (2) in weakly polar media, the quadrupolar state observed directly after excitation transforms to a symmetry broken S₁ state with one arm bearing more excitation than the other; and (3) in highly polar solvents, the excited state evolves further to a purely dipolar S₁ state with the excitation localized entirely on one arm. The time scales associated with the transitions between these states coincide with those of solvation dynamics, indicating that symmetry breaking is governed by solvent fluctuations.