Sanches de Araújo Adalberto Vasconcelos, Borin Antonio Carlos
Department of Fundamental Chemistry , Institute of Chemistry, University of São Paulo , Av. Prof. Lineu Prestes 748 , 05508-000 São Paulo , SP , Brazil.
J Phys Chem A. 2019 Apr 11;123(14):3109-3120. doi: 10.1021/acs.jpca.9b01397. Epub 2019 Apr 1.
The photochemical reaction path approach, the MS-CASPT2 quantum-chemical method, and double-ζ basis sets (cc-pVDZ) were used to study 9 H-8-azaguanine and 8 H-8-azaguanine relaxation pathways. Several potential energy hypersurfaces were characterized by means of equilibrium geometries, surface crossings (conical intersections and singlet-triplet intersystem crossings), minimum energy paths, and linear interpolation in internal coordinates. The 9 H-8-azaguanine main photochemical event begins with the direct population of the (ππ* L) state, which evolves toward a conical intersection with the ground state after surmounting a small energy barrier, explaining why it is nonfluorescent. For 8 H-8-azaguanine, two relaxation mechanisms are possible, depending on the excitation energy. If the S (ππ*) state is initially populated (lower-energy region), the system evolves barrierless to the S (ππ*) region, from where the excess energy is released. If the (ππ* L) state is populated (higher-energy radiation), the system will encounter conical intersections with the S (nπ*) and S (ππ*) states before evolving to the (ππ* L) region, from where a conical intersection with the ground state is accessible, favoring radiationless deactivation to the ground state. However, because a fraction of the population can be transferred from (ππ* L) to the S (ππ*) state, emission from the S (ππ*) region is also expected, although weaker than it would be if the S (ππ*) state were populated directly. Irrespective of the excitation energy, the emissive state is the same and a single fluorescence band is observed, with the strongest emission occurring upon excitation in the lower-energy region, as observed experimentally. Therefore, our computational study corroborates experimental results, attributing the emission of the neutral form of 8-azaguanine in solution to the presence of the minor 8 H-8-azaguanine tautomer, while the 9 H-8-azaguanine major tautomer is nonfluorescent.
采用光化学反应路径方法、MS-CASPT2量子化学方法和双ζ基组（cc-pVDZ）研究了9H-8-氮杂鸟嘌呤和8H-8-氮杂鸟嘌呤的弛豫路径。通过平衡几何结构、表面交叉（锥形交叉和单重态-三重态系间交叉）、最小能量路径以及内部坐标中的线性插值等手段对多个势能超曲面进行了表征。9H-8-氮杂鸟嘌呤的主要光化学事件始于（ππL）态的直接布居，该态在跨越一个小的能垒后向与基态的锥形交叉演化，这解释了其为何不发荧光。对于8H-8-氮杂鸟嘌呤，根据激发能量的不同，可能存在两种弛豫机制。如果最初布居的是S（ππ）态（低能区），系统无势垒地演化至S（ππ*）区域，多余的能量从该区域释放。如果布居的是（ππL）态（高能辐射），系统在演化至（ππL）区域之前会与S（nπ*）态和S（ππ*）态发生锥形交叉，从该区域可与基态发生锥形交叉，有利于无辐射失活至基态。然而，由于一部分布居可以从（ππL）态转移至S（ππ）态，因此预计也会有来自S（ππ*）区域的发射，尽管比直接布居S（ππ*）态时要弱。无论激发能量如何，发射态都是相同的，并且观察到单一荧光带，在低能区激发时发射最强，这与实验观察结果一致。因此，我们的计算研究证实了实验结果，将溶液中8-氮杂鸟嘌呤中性形式的发射归因于次要的8H-8-氮杂鸟嘌呤互变异构体的存在，而主要的9H-8-氮杂鸟嘌呤互变异构体不发荧光。
