On the mechanism of the cis-trans isomerization in the lowest electronic states of azobenzene:: S0, S1, and T1
On the mechanism of the cis-trans isomerization in the lowest electronic states of azobenzene:: S0, S1, and T1
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DOI：
10.1021/ja038327y
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发表时间：
2004-03-17
影响因子：
15
通讯作者：
Orlandi, G
中科院分区：
文献类型：
--
作者：
Cembran, A;Bernardi, F;Orlandi, G
在本文中，我们确定了最有效的衰减和异构化路线的S-1，T-1，和S-0状态的偶氮苯。利用量子化学方法，我们在S-1势能面上寻找过渡态（TS），并在S-1势能面上寻找更接近最小能量路径的S-0/S-1锥形交叉点（CIS）。我们发现只有一个TS，在60度的CNNC扭转从E异构体，这需要的活化能只有2千卡/摩尔。在CNNC角为95- 90 °的扭转路径上发现最低能量的CIS，也比S_1最小值高2千卡/摩尔。沿反转路径的最低Cl沿着被发现约。25 kcal/mol，比S，最小值高，其特征在于具有高度不对称的分子结构，其中一个NNC角为174度。这些结果表明，S-1状态的衰变主要涉及扭转路线和反转机制可能发挥作用，只有当分子被激发与至少25千卡/摩尔的过剩能量相对于S，最小的E异构体。我们计算了在沿着CNNC挠率坐标的几种几何构型下So和T之间的自旋轨道耦合。这些自旋-轨道耦合对于所考虑的所有几何形状都约为20-30 cm（-1）。由于S-0和T-1的势能曲线在扭曲的CNNC角区域交叉，这些耦合足够大，以确保T-1寿命非常短（近似于10 ps），并且热异构化可以通过涉及S-0-T-1-S-0交叉的非绝热扭曲路线进行，其指前因子和活化能与动力学测量的值一致。
In this paper, we identify the most efficient decay and isomerization route of the S-1, T-1, and S-0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S-1 potential energy surface and for the S-0/S-1 conical intersections (CIS) that are closer to the minimum energy path on the S-1. We found only one TS, at 60degrees of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIS, lying also 2 kcal/mol above the S, minimum, were found on the torsion pathway for CNNC angles in the range 95-90degrees. The lowest Cl along the inversion path was found ca. 25 kcal/mol higher than the S, minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174degrees. These results indicate that the S-1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S, minimum of the E isomer. We have calculated the spin-orbit couplings between So and T, at several geometries along the CNNC torsion coordinate. These spin-orbit couplings were about 20-30 cm(-1) for all the geometries considered. Since the potential energy curves of S-0 and T-1 cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T-1 lifetime is very short (similar to10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S-0-T-1-S-0 crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.