Silva Daniel L, Murugan N Arul, Kongsted Jacob, Ågren Hans, Canuto Sylvio
Instituto de Física, Universidade de São Paulo , Caixa Postal 66318, 05314-970 São Paulo, SP, Brazil.
J Phys Chem B. 2014 Feb 20;118(7):1715-25. doi: 10.1021/jp411178h. Epub 2014 Feb 6.
Dipolar aggregation is in many cases detrimental for the functioning of optical materials. In this study we investigate self-aggregation and optical absorption of stilbazolium merocyanine (SM) in chloroform solution by performing classical Molecular Dynamics (MD) simulations under ambient conditions. The reversal solvatochromic shift, the large bathochromic shift, and the structured absorption band presented by SM in chloroform solution are all aspects of its optical absorption behavior for which the existence of self-aggregation is yet not completely understood. Moreover, the spectroscopic properties of SM oligomers and their occurrence in solvent of low polarity remain a relevant topic that deserves to be investigated. Our analysis of the aggregation behavior of SM in chloroform verified that the majority of the chromophores are involved in the formation of oligomers in solution, where the whole dimer and part of the trimer populations present a stable π-stacking structure. The optical properties of the monomers and oligomers in solution were evaluated by means of a discrete polarizable embedding quantum mechanical/molecular mechanical (PE-QM/MM) response scheme where the quantum part is described at the level of density functional theory. The visible absorption spectrum of SM in chloroform is simulated using time average values obtained for the monomeric and oligomeric forms of SM from the PE-QM/MM calculations performed on uncorrelated configurations extracted from the classical MD simulations. This study shows that the self-aggregation of SM in chloroform may exist, but it is not essential for reproducing the reversal solvatochromic shift in chloroform and that the process does not contribute to enhance the bathochromic shift nor explain the structure observed in its absorption band. Moreover, it is verified that since the electronic transitions of the monomer and oligomers are close together, changes in the interplane separation between the monomeric units of the stacked oligomers substantially affect the spectral resolution of their contribution to the optical absorption spectrum.
在许多情况下，偶极聚集对光学材料的功能是有害的。在本研究中，我们通过在环境条件下进行经典分子动力学（MD）模拟，研究了氯仿溶液中苯乙烯基苯并噻唑啉甲川菁（SM）的自聚集和光吸收。SM在氯仿溶液中呈现的反向溶剂化变色位移、大的红移以及结构化吸收带，都是其光吸收行为的方面，而对于这些方面，自聚集的存在尚未完全理解。此外，SM低聚物的光谱性质及其在低极性溶剂中的出现仍然是一个值得研究的相关课题。我们对SM在氯仿中的聚集行为的分析证实，大多数发色团参与了溶液中低聚物的形成，其中整个二聚体和部分三聚体群体呈现出稳定的π堆积结构。溶液中单体和低聚物的光学性质通过离散极化嵌入量子力学/分子力学（PE-QM/MM）响应方案进行评估，其中量子部分在密度泛函理论水平上进行描述。使用从经典MD模拟中提取的不相关构型上进行的PE-QM/MM计算得到的单体和低聚体形式的SM的时间平均值，模拟了SM在氯仿中的可见吸收光谱。这项研究表明，SM在氯仿中的自聚集可能存在，但对于重现氯仿中的反向溶剂化变色位移并非必不可少，并且该过程无助于增强红移，也无法解释其吸收带中观察到的结构。此外，已证实由于单体和低聚物的电子跃迁彼此接近，堆叠低聚物的单体单元之间的平面间距变化会极大地影响它们对光吸收光谱贡献的光谱分辨率。
