Research Computer Center, Moscow State University, Leninskiye Gory 1, Moscow, Russia.
Phys Chem Chem Phys. 2011 May 28;13(20):9518-24. doi: 10.1039/c0cp02880k. Epub 2011 Apr 12.
Quantum-chemical calculations of ground and excited states for membrane fluorescent probe 4-dimethylaminochalcone (DMAC) in vacuum were performed. Optimized geometries and dipole moments for lowest-lying singlet and triplet states were obtained. The nature of these electronic transitions and the relaxation path in the excited states were determined; changes in geometry and charge distribution were assessed. It was shown that in vacuum the lowest existed level is of (n, π*) nature, and the closest to it is the level of (π, π*) nature; the energy gap between them is narrow. This led to an effective (1)(π, π*) →(1)(n, π*) relaxation. After photoexcitation the molecule undergoes significant transformations, including changes in bond orders, pyramidalization angle of the dimethylamino group, and planarity of the molecule. Its dipole moment rises from 5.5 Debye in the ground state to 17.1 Debye in the (1)(π, π*) state, and then falls to 2 Debye in the (1)(n, π*) state. The excited (1)(n, π*) state is a short living state; it has a high probability of intersystem crossing into the (3)(π, π*) triplet state. This relaxation path explains the low quantum yield of DMAC fluorescence in non-polar media. It is possible that (3)(π, π*) is responsible for observed DMAC phosphorescence.
在真空中对膜荧光探针 4-二甲基氨基查耳酮（DMAC）的基态和激发态进行了量子化学计算。获得了最低能单重态和三重态的优化几何形状和偶极矩。确定了这些电子跃迁的性质和激发态中的弛豫途径；评估了几何形状和电荷分布的变化。结果表明，在真空中，最低存在的能级为(n,π*)性质，最接近的能级为(π,π*)性质；它们之间的能隙很窄。这导致有效的(1)(π,π*)→(1)(n,π*)弛豫。光激发后，分子经历了显著的转变，包括键序、二甲氨基的锥形化角度和分子的平面性的变化。其偶极矩从基态的 5.5 德拜增加到(1)(π,π*)态的 17.1 德拜，然后在(1)(n,π*)态下降到 2 德拜。激发的(1)(n,π*)态是一个短寿命态；它有很高的概率通过系间窜跃进入(3)(π,π*)三重态。这种弛豫途径解释了 DMAC 在非极性介质中荧光量子产率低的原因。可能是(3)(π,π*)负责观察到的 DMAC 磷光。
