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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Excited-State Dynamics of Nitrated Push-Pull Molecules: The Importance of the Relative Energy of the Singlet and Triplet Manifolds
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Excited-State Dynamics of Nitrated Push-Pull Molecules: The Importance of the Relative Energy of the Singlet and Triplet Manifolds

机译：硝化推挽分子的激发态动力学：单重态和三重态歧管的相对能量的重要性

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We present a study of the dynamics following photoexcitation in the first electronic band of NO2-parasubstituted nitronaphthalenes. Our main goal was to determine the interplay between the nitro group, electrondonating substituents, and the solvent in defining the relative excited-state energies and their photoinduced pathways. We studied 4-nitro-1-naphthylamine and 1-methoxy-4-nitronaphthalene in solution samples through femtosecond fluorescence up-conversion and transient absorption techniques. In all solvents, both compounds have ultrafast fluorescence decays, showing that, similarly to the parent compound 1-nitronaphthalene, these molecules have highly efficient S1 decay channels. The evolution of the transient absorption signals in the visible region reveals that for the methoxy-substituted compound, independently of solvent polarity, the photophysical pathways are the same as in 1-nitronaphthalene, namely, ultrafast intersystem crossing to an upper triplet state (receiver Tn state) followed by relaxation into the lowest energy phosphorescent triplet T1. In contrast, for the amino-substituted nitronaphthalene, the excited-state evolution shows a strong solvent dependence: In nonpolar solvents, the same type of intersystem crossing through an upper receiver triplet state dictates the photochemistry. However, in methanol, where the first singlet excited state shows an important solvent-induced stabilization, we observed typical signals of the repopulation of the electronic ground state in the time scale of less than 1 ps followed by vibrational cooling within S0. Excited-state calculations at the time-dependent density functional level with the PBE0 functional give an approximate characterization of the states involved and appear to correlate well with the experimental results as they show that the S1 state of the amino compound is stabilized with respect to upper triplet states only in the polar solvent. These findings sustain and illustrate the recent view that the intersystem crossing channel so prevalent in nitroaromatic compounds is related to an energy coincidence between the π-π* first singlet excited state and upper triplet states with n-π* character. Our results indicate through direct observations that if the S1 state is sufficiently stabilized, other rapid decay channels like internal conversion to the ground state will minimize the transfer of population to the triplet manifold.
机译：我们介绍了在NO2-对位取代的硝基萘的第一个电子带中进行光激发后的动力学研究。我们的主要目标是确定硝基，给电子取代基和溶剂之间的相互作用，以定义相对激发态能量及其光诱导途径。我们通过飞秒荧光上转换和瞬态吸收技术研究了溶液样品中的4-硝基-1-萘胺和1-甲氧基-4-硝基萘。在所有溶剂中，两种化合物均具有超快的荧光衰减，表明与母体化合物1-硝基萘相似，这些分子具有高效的S1衰减通道。可见光区域中瞬态吸收信号的演变表明，对于甲氧基取代的化合物，与溶剂极性无关，其光物理路径与1-硝基萘中的相同，即超快的系统间相交到较高的三重态（受体Tn状态），然后弛豫到最低能量的磷光三重态T1中。相比之下，对于氨基取代的硝基萘，激发态的演化显示出强烈的溶剂依赖性：在非极性溶剂中，穿过上层受体三重态的相同类型的系统间相互作用决定了光化学性质。但是，在甲醇中，第一个单重态激发态显示出重要的溶剂诱导的稳定作用，我们观察到在小于1 ps的时间范围内电子基态再填充的典型信号，然后在S0内进行振动冷却。具有PBE0官能团的随时间变化的密度官能团级别的激发态计算给出了所涉及状态的近似表征，并且似乎与实验结果很好地相关，因为它们表明氨基化合物的S1态相对于上位稳定了。三重态仅在极性溶剂中存在。这些发现支持并说明了最近的观点，即硝基芳族化合物中如此普遍的系统间交叉通道与π-π*第一单重态激发态和具有n-π*特征的上三重态之间的能量符合有关。我们的结果通过直接观察表明，如果S1状态足够稳定，则其他快速衰减通道（如内部转换为基态）将使种群向三重态歧管的转移最小化。

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