Merocyanines, owing to their readily tunable electronic structure, are arguably the most versatile functional dyes, with ample opportunities for tailored design via variations of both the donor/acceptor (D/A) end groups and π-conjugated polymethine chain. A plethora of spectral properties, such as strong solvatochromism, high polarizability and hyperpolarizabilities, and sensitizing capacity, motivates extensive studies for their applications in light-converting materials for optoelectronics, nonlinear optics, optical storage, fluorescent probes, etc. Evidently, an understanding of the intrinsic structure–property relationships is a prerequisite for the successful design of functional dyes. For merocyanines, these regularities have been explored for over 70 years, but only in the past three decades have these studies expanded beyond the theory of their color and solvatochromism toward their electronic structure in the ground and excited states. This Review outlines the fundamental principles, essential for comprehension of the variable nature of merocyanines, with the main emphasis on understanding the impact of internal (chemical structure) and external (intermolecular interactions) factors on the electronic symmetry of the D−π–A chromophore. The research on the structure and properties of merocyanines in different media is reviewed in the context of interplay of the three virtual states: nonpolar polyene, ideal polymethine, and zwitterionic polyene.

中文翻译：

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美罗花青：溶液、固态和气相中的电子结构和光谱


甲菁由于其易于调节的电子结构，可以说是用途最广泛的功能性染料，通过供体/受体 （D/A） 末端基团和π共轭聚甲辛链的变化，为定制设计提供了充足的机会。大量的光谱特性，如强溶剂变色、高极化和超极化以及敏化能力，促使人们广泛研究它们在光电子、非线性光学、光存储、荧光探针等的光转换材料中的应用。显然，了解内在结构-性能关系是成功设计功能性染料的先决条件。对于甲菁，这些规律已经探索了 70 多年，但直到最近三十年，这些研究才扩展到它们的颜色和溶剂变色理论，转向它们在基态和激发态下的电子结构。本综述概述了基本原理，这些原则对于理解美菁的可变性质至关重要，主要强调了解内部（化学结构）和外部（分子间相互作用）因素对 D-π-A 发色团电子对称性的影响。在三种虚拟状态（非极性多烯、理想多甲辛和两性离子多烯）相互作用的背景下回顾了对美菁在不同介质中结构和性质的研究。