Department of Physical Chemistry, University of Geneva, 30, Quai Ernest-Ansermet, 1211, Geneva 4, Switzerland.
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224, Warsaw, Poland.
Angew Chem Int Ed Engl. 2016 Dec 12;55(50):15624-15628. doi: 10.1002/anie.201608567. Epub 2016 Nov 15.
Symmetry-breaking charge transfer upon photoexcitation of a linear A-π-D-π-A molecule (D and A being electron donating and accepting groups) could be visualized using ultrafast time-resolved infrared spectroscopy by monitoring the CN stretching modes on the A units. Whereas in apolar solvents, the S state remains symmetric and quadrupolar, symmetry breaking occurs within ca. 100 fs in polar solvents as shown by the presence of two CN bands, instead of one in apolar solvents, with a splitting that increases with polarity. In protic solvents, symmetry breaking is significantly amplified by H-bonding interactions, which are the strongest at the CN group with the highest basicity. In strongly protic solvents, the two CN bands transform in about 20 ps into new bands with a larger splitting, and the lifetime of the S state is substantially reduced. This is attributed to the formation of an excited asymmetric tight H-bond complex.
在对线性 A-π-D-π-A 分子（D 和 A 分别为电子供体和受体基团）进行光激发时，通过监测 A 单元上的 CN 伸缩模式，可以使用超快时间分辨红外光谱来可视化对称破缺的电荷转移。在非极性溶剂中，S 态保持对称和四极，而在极性溶剂中，约 100fs 内就会发生对称破缺，如在非极性溶剂中只有一个 CN 带，而在极性溶剂中则有两个 CN 带，分裂随着极性的增加而增加。在质子溶剂中，氢键相互作用会显著放大对称破缺，而在 CN 基团中，氢键相互作用最强，碱性最高。在强质子溶剂中，两个 CN 带在大约 20ps 内转变为具有更大分裂的新带，S 态的寿命大大缩短。这归因于形成了一个激发的不对称紧密氢键配合物。
