Stephansen Anne B, Sølling Theis I
Department of Chemistry, University of Copenhagen , Universitetsparken 5, DK-2100 København Ø, Denmark.
Struct Dyn. 2017 Feb 28;4(4):044008. doi: 10.1063/1.4977735. eCollection 2017 Jul.
The competition between ultrafast intersystem crossing and internal conversion in benzene, toluene, and p-xylene is investigated with time-resolved photoelectron spectroscopy and quantum chemical calculations. By exciting to S out-of-plane symmetry breaking, distortions are activated at early times whereupon spin-forbidden intersystem crossing becomes (partly) allowed. Natural bond orbital analysis suggests that the pinnacle carbon atoms distorting from the aromatic plane change hybridization between the planar Franck-Condon geometry and the deformed (boat-shaped) S equilibrium geometry. The effect is observed to increase in the presence of methyl-groups on the pinnacle carbon-atoms, where largest extents of σ and π orbital-mixing are observed. This is fully consistent with the time-resolved spectroscopy data: Toluene and xylene show evidence for ultrafast triplet formation competing with internal conversion, while benzene appears to only decay via internal conversion within the singlet manifold. For toluene and -xylene, internal conversion to S and intersystem crossing to T occur within the time-resolution of our instrument. The receiver triplet state (T) is found to undergo internal conversion in the triplet manifold within ≈100-150 fs (toluene) or ≈180-200 fs (p-xylene) as demonstrated by matching rise and decay components of upper and lower triplet states. Overall, the effect of methylation is found to both increase the intersystem crossing probability and direct the molecular axis of the excited state dynamics.
利用时间分辨光电子能谱和量子化学计算研究了苯、甲苯和对二甲苯中超快系间窜越和内转换之间的竞争。通过激发到S面外对称性破缺，在早期激活了畸变，于是自旋禁阻的系间窜越变得（部分）允许。自然键轨道分析表明，从芳香平面扭曲的尖顶碳原子在平面弗兰克-康登几何结构和变形（船形）S平衡几何结构之间改变杂化。观察到在尖顶碳原子上存在甲基的情况下，这种效应会增强，在那里观察到最大程度的σ和π轨道混合。这与时间分辨光谱数据完全一致：甲苯和二甲苯显示出超快三重态形成与内转换竞争的证据，而苯似乎仅通过单重态流形内的内转换衰减。对于甲苯和二甲苯，向S的内转换和向T的系间窜越发生在我们仪器的时间分辨率范围内。通过匹配上、下三重态的上升和衰减分量表明，接收三重态（T）在≈100 - 150飞秒（甲苯）或≈180 - 200飞秒（对二甲苯）内的三重态流形中发生内转换。总体而言，发现甲基化的作用既增加了系间窜越概率，又引导了激发态动力学的分子轴。
