University Heidelberg, Theoretical Chemistry, Institute of Physical Chemistry, INF 229, 69120 Heidelberg, Germany.
Phys Chem Chem Phys. 2013 Nov 7;15(41):17846-61. doi: 10.1039/c3cp52719k.
In the present work we have explored the ultrafast relaxation network of coumarin and umbelliferone (7-hydroxy-coumarin) using time-resolved femtosecond spectroscopy and quantum chemical calculations. Despite the importance of the photophysical properties of coumarin derivatives for applications in biomedicine, the low fluorescence quantum yield of coumarin itself has not been fully understood so far. On the basis of our combined experimental and theoretical results we suggest a model for the ultrafast decay after photoexcitation incorporating two parallel radiationless relaxation pathways: one within the initially excited state via ring opening and the other one by transition into a dark state along the carbonyl stretching mode. The fluorescence quantum yield is determined by the position of the branching point relative to the Franck-Condon region which is strongly influenced by interactions with the environment and the substitution pattern. This model is finally capable of giving a comprehensive account of the striking differences observed in the photophysical behavior of coumarin as opposed to umbelliferone.
在本工作中，我们使用时间分辨飞秒光谱学和量子化学计算研究了香豆素和伞形酮（7-羟基香豆素）的超快弛豫网络。尽管香豆素衍生物的光物理性质对于生物医学应用很重要，但香豆素本身的低荧光量子产率迄今尚未得到充分理解。基于我们的实验和理论结果，我们提出了一个光激发后超快衰减的模型，其中包含两个平行的无辐射弛豫途径：一个是通过环开经由初始激发态，另一个是通过沿羰基伸缩模式跃迁到暗态。荧光量子产率由分支点相对于 Franck-Condon 区域的位置决定，该位置受与环境和取代模式的相互作用的强烈影响。该模型最终能够全面解释在香豆素的光物理行为中观察到的与伞形酮显著差异。
