CNR-Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organo Metallici (ICCOM-CNR), SS di Pisa, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.
Phys Chem Chem Phys. 2020 Nov 25;22(45):26525-26535. doi: 10.1039/d0cp04123h.
We study the ultrafast dynamics of 1,5-dimethyl-cytosine, a model for 5-methyl-cytidine, after photoexcitation to the first two bright ππ* states, focusing on the possible population transfer to dark nπ* states. To that end we propagate the initial wave packets on the coupled potential energy surfaces of the seven lowest energy excited states modelled with a diabatic linear vibronic coupling (LVC) model, considering all the vibrational coordinates. Time-evolution is computed by the multilayer version of the multiconfigurational time dependent Hartree (ML-MCTDH) method. The LVC Hamiltonian is parametrized with time-dependent density functional theory (TD-DFT) calculations adopting PBE0 and CAM-B3LYP functionals, which provide a different energy gap between the lowest energy nπ* states and the spectroscopic ππ* state. Population of the lowest ππ* flows to a dark nπ* state which involves a lone pair (LP) of the carbonyl oxygen (nOπ*), but the extent of such transfer is much larger according to PBE0 than to CAM-B3LYP. Photoexcitation to the second bright state gives rise to much richer dynamics with an ultrafast (50 fs) complete decay to the lowest ππ*, to nOπ* and to another nπ* in which the excited electron comes from the LP of the ring nitrogen. We perform a detailed analysis of the vibronic dynamics both in terms of normal modes and valence coordinates (bond lengths and angles). The comparison with the analogous dynamics in 1-methyl-cytosine, a model for cytidine, provides insights into the effect of methylation at carbon 5 on the electronic and nuclear dynamics.
我们研究了 1,5-二甲基胞嘧啶（5-甲基胞嘧啶的模型）在光激发到前两个亮ππ态后的超快动力学，重点关注可能向暗 nπ态的种群转移。为此，我们在由绝热线性振子耦合（LVC）模型建模的七个最低能量激发态的耦合势能面上传播初始波包，考虑所有振动坐标。时间演化通过多层多组态含时哈特里（ML-MCTDH）方法的版本计算。LVC 哈密顿量采用时变密度泛函理论（TD-DFT）计算进行参数化，采用 PBE0 和 CAM-B3LYP 泛函，它们提供了最低能量 nπ态和光谱 ππ态之间不同的能隙。最低 ππ的种群流向涉及羰基氧（nOπ）孤对电子（LP）的暗 nπ态，但根据 PBE0 比根据 CAM-B3LYP 转移的程度要大得多。光激发到第二亮态会引起更丰富的动力学，超快（50 fs）完全衰减到最低的 ππ、nOπ和另一个 nπ，其中激发电子来自环氮的 LP。我们根据正常模式和价坐标（键长和角度）对振子动力学进行了详细分析。与 1-甲基胞嘧啶（胞嘧啶的模型）的类似动力学的比较提供了对 5 位甲基化对电子和核动力学的影响的深入了解。
