Gan Yanzhen, Yue Ling, Guo Xugeng, Zhu Chaoyuan, Cao Zexing
State Key Laboratory of Physical Chemistry of Solid Surfaces and Fujian Province Key Lab of Theoretical and Computational Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, P. R. China.
Phys Chem Chem Phys. 2017 May 17;19(19):12094-12106. doi: 10.1039/c6cp08929a.
An on-the-fly trajectory surface hopping dynamic simulation has been performed for revealing the multi-state nonadiabatic deactivation mechanism of coumarin. The mechanism involves three adiabatic excited states, S(ππL), S(nπ, ππL) and S(ππL, nπ*), and the ground state S at the four state-averaged complete active space self-consistent field, SA4-CASSCF(12,10)/6-31G* level of theory. Upon photoexcitation to the third excited state S(ππL) in the Franck-Condon region, 80% sampling trajectories decay to the dark S(nπ) state within an average of 5 fs via the conical intersection S(ππL)/S(nπ), while 20% decay to the S(ππL) state within an average of 11 fs via the conical intersection S(ππL)/S(ππL). Then, sampling trajectories via S(nπ)/S(ππL) continue with ultrafast decay processes to give a final distribution of quantum yields as follows: 42% stay on the dark S(nπ) state, 43.3% go back to the ground S state, 12% undergo a ring-opening reaction to the Z-form S(Z) state, and 2.7% go to the E-form S(E) state. The lifetimes of the excited states are estimated as follows: the S state is about 12 fs on average, the S state is about 80 fs, and the S state has a fast component of about 160 fs and a slow component of 15 ps. The simulated ultrafast radiationless deactivation pathways of photoexcited coumarin immediately interpret the experimentally observed weak fluorescence emission.
为揭示香豆素的多态非绝热失活机制，进行了实时轨迹表面跳跃动力学模拟。该机制涉及三个绝热激发态，即S(ππL)、S(nπ, ππL)和S(ππL, nπ*)，以及基态S，计算采用四态平均完全活性空间自洽场SA4-CASSCF(12,10)/6-31G理论水平。在弗兰克-康登区域光激发至第三激发态S(ππL)后，80%的采样轨迹通过锥形交叉点S(ππL)/S(nπ)在平均5 fs内衰减至暗态S(nπ*)，而20%的轨迹通过锥形交叉点S(ππL)/S(ππL)在平均11 fs内衰减至S(ππL)态。然后，通过S(nπ)/S(ππL)的采样轨迹继续超快衰减过程，得到如下量子产率的最终分布：42%停留在暗态S(nπ)，43.3%回到基态S，12%发生开环反应至Z型S(Z)态，2.7%变为E型S(E)态。激发态的寿命估计如下：S态平均约为12 fs，S态约为80 fs，S态有一个约160 fs的快速成分和一个15 ps的慢速成分。光激发香豆素的模拟超快无辐射失活途径立即解释了实验观察到的弱荧光发射。
