Petrone Alessio, Cerezo Javier, Ferrer Francisco J Avila, Donati Greta, Improta Roberto, Rega Nadia, Santoro Fabrizio
†Dipartimento di Scienze Chimiche, Università di Napoli 'Federico II', Complesso Universitario di M.S. Angelo, via Cintia, I-80126 Napoli, Italy.
‡CNR-Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici (ICCOM-CNR), UOS di Pisa, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy.
J Phys Chem A. 2015 May 28;119(21):5426-38. doi: 10.1021/jp510838m. Epub 2015 Mar 10.
We study the absorption and emission electronic spectra in an aqueous solution of N-methyl-6-oxyquinolinium betaine (MQ), an interesting dye characterized by a large change of polarity and H-bond ability between the ground (S0) and the excited (S1) states. To that end we compare alternative approaches based either on explicit solvent models and density functional theory (DFT)/molecular-mechanics (MM) calculations or on DFT calculations on clusters models embedded in a polarizable continuum (PCM). In the first approach (ClMD), the spectrum is computed according to the classical Franck-Condon principle, from the dispersion of the time-dependent (TD)-DFT vertical transitions at selected snapshots of molecular dynamics (MD) on the initial state. In the cluster model (Qst) the spectrum is simulated by computing the quantum vibronic structure, estimating the inhomogeneous broadening from state-specific TD-DFT/PCM solvent reorganization energies. While both approaches provide absorption and emission spectral shapes in nice agreement with experiment, the Stokes shift is perfectly reproduced by Qst calculations if S0 and S1 clusters are selected on the grounds of the MD trajectory. Furthermore, Qst spectra better fit the experimental line shape, mostly in absorption. Comparison of the predictions of the two approaches is very instructive: the positions of Qst and ClMD spectra are shifted due to the different solvent models and the ClMD spectra are narrower than the Qst ones, because MD underestimates the width of the vibrational density of states of the high-frequency modes coupled to the electronic transition. On the other hand, both Qst and ClMD approaches highlight that the solvent has multiple and potentially opposite effects on the spectral width, so that the broadening due to solute-solvent vibrations and electrostatic interaction with bulk solvent is (partially) counterbalanced by a narrowing of the contribution due to the solute vibrational modes. Qst analysis evidences a pure quantum broadening effect of the spectra in water due to vibronic progressions along the solute/solvent H-bonds.
我们研究了N-甲基-6-氧基喹啉甜菜碱（MQ）水溶液中的吸收和发射电子光谱，MQ是一种有趣的染料，其基态（S0）和激发态（S1）之间的极性和氢键能力有很大变化。为此，我们比较了基于显式溶剂模型和密度泛函理论（DFT）/分子力学（MM）计算的替代方法，以及基于嵌入可极化连续介质（PCM）中的团簇模型的DFT计算方法。在第一种方法（ClMD）中，根据经典的弗兰克-康登原理，从初始态分子动力学（MD）选定快照下的含时（TD）-DFT垂直跃迁的色散计算光谱。在团簇模型（Qst）中，通过计算量子振转结构来模拟光谱，从特定态的TD-DFT/PCM溶剂重组能估计非均匀展宽。虽然两种方法都能提供与实验结果吻合良好的吸收和发射光谱形状，但如果根据MD轨迹选择S0和S1团簇，则Qst计算能完美再现斯托克斯位移。此外，Qst光谱更能拟合实验线形，在吸收方面尤为明显。两种方法预测结果的比较很有启发性：由于溶剂模型不同，Qst和ClMD光谱的位置发生了偏移，且ClMD光谱比Qst光谱更窄，因为MD低估了与电子跃迁耦合的高频模式的振动态密度宽度。另一方面，Qst和ClMD方法都强调溶剂对光谱宽度有多种且可能相反的影响，因此溶质-溶剂振动和与本体溶剂的静电相互作用导致的展宽（部分）被溶质振动模式贡献的变窄所抵消。Qst分析表明，由于沿着溶质/溶剂氢键的振转跃迁，水中光谱存在纯量子展宽效应。
