Dekhtyar M, Rettig W, Rothe A, Kurdyukov V, Tolmachev A
Institute of Organic Chemistry , National Academy of Sciences of Ukraine , Murmanskaya str. 5 , Kiev , 02094 , Ukraine.
Institute of Chemistry , Humboldt University of Berlin , Brook-Taylor-Strasse 2 , 12489 Berlin , Germany.
J Phys Chem A. 2019 Apr 4;123(13):2694-2708. doi: 10.1021/acs.jpca.8b10660. Epub 2019 Mar 18.
Derivatives of Brooker's merocyanine (BM) have been investigated, which possess different donors and acceptors and therefore vary their donor-acceptor strength S. The 00 energies have been extracted from the spectra and compared. Under basic conditions, where the neutral (merocyanine) form is present, the absorption energies for all compounds are similar, whereas there is a large difference for acidic conditions where the cationic (cyanine) form is present. This behavior could be explained by a simple theoretical model involving the dependence of the excitation energy Δ E on S. This model can be generalized to describe in a consistent way two different well-known classes of neutral chromophores with a certain degree of charge separation, namely merocyanine I (TICT) and merocyanine II (often betainic) compounds. Merocyanines I are characterized by a medium polar aromatic ground state and a zwitterionic quinoid excited state and hence positive solvatochromism, whereas merocyanines II are formally characterized by a zwitterionic aromatic ground state and a less polar quinoid excited state and, accordingly, by negative solvatochromism. On increasing the donor-acceptor strength S sufficiently, merocyanines II can, however, move to the so-called overcritical region with the excited state dominated by the zwitterionic valence bond structure. For many of the merocyanine II molecules investigated here, a weakly positive solvatochromic behavior is observed indicating that the ground state contains less of the zwitterionic valence bond wave function than the excited state and that these compounds belong to the overcritical region. The fluorescence spectra have been analyzed in terms of the Franck-Condon model and confirm these conclusions.
对布鲁克菁染料（BM）的衍生物进行了研究，这些衍生物具有不同的供体和受体，因此其供体 - 受体强度S有所不同。已从光谱中提取并比较了00能量。在中性（菁染料）形式存在的碱性条件下，所有化合物的吸收能量相似，而在阳离子（花青）形式存在的酸性条件下则存在很大差异。这种行为可以用一个简单的理论模型来解释，该模型涉及激发能ΔE对S的依赖性。这个模型可以推广，以一致的方式描述两类具有一定电荷分离程度的不同的著名中性发色团，即菁染料I（TICT）和菁染料II（通常为甜菜碱型）化合物。菁染料I的特征是具有中等极性的芳香基态和两性离子醌型激发态，因此具有正溶剂化显色性，而菁染料II的形式特征是两性离子芳香基态和极性较小的醌型激发态，相应地具有负溶剂化显色性。然而，当供体 - 受体强度S充分增加时，菁染料II可以移动到所谓的超临界区域，其激发态由两性离子价键结构主导。对于这里研究的许多菁染料II分子，观察到弱正溶剂化显色行为，这表明基态中两性离子价键波函数比激发态少，并且这些化合物属于超临界区域。已根据弗兰克 - 康登模型对荧光光谱进行了分析，并证实了这些结论。
