In this study, we investigated the solvent effects on the photophysical properties of α,ω-di(4-pyridyl)polyenes 1–5 having 1–5 double bonds. The solution photoproperties depend strongly on conjugation chain length. The absorption maximum (λ_a) of dipyridylethylene 1 is observed at around 290 nm and only slightly redshifts as the solvent polarity increases, whereas its fluorescence maximum (λ_f) redshifts from 368 nm in hexane to 403 nm in acetonitrile. Although 1 is a centrosymmetric molecule, its fluorescence energy linearly correlates with the Onsager solvent polarity function f(ε) − f(n²) = (ε − 1)/(2ε + 1)−(n² − 1)/(2n² + 1), indicating that the emission originates from an intramolecular charge transfer (CT) excited state. Exceptionally, λ_f in methanol is largely blue-shifted to 341 nm from those in other aprotic solvents. The fluorescence solvatochromism of longer polyenes 2–5 is much less significant than that of 1. Upon protonation and N-alkylation, both the absorption and fluorescence spectra of all five compounds are red-shifted in methanol. The largest shifts in λ_a and λ_f on protonation are observed for pentaene 5 (73 nm) and diene 2 (57 nm), respectively.

Graphic Abstract

中文翻译：

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α,ω-二（4-吡啶基）多烯的链长依赖性光物理性质：溶剂极性、氢键形成、质子化和 N-烷基化的影响

在这项研究中，我们研究了溶剂对具有1-5个双键的 α,ω-二（4-吡啶基）多烯 1-5 的光物理性质的影响。溶液的光性能很大程度上取决于共轭链的长度。联吡啶乙烯1的最大吸收 (λ _a )在 290 nm 附近观察到，并且随着溶剂极性的增加仅略微红移，而其荧光最大值 (λ _f ) 从己烷中的 368 nm 红移到乙腈中的 403 nm。虽然1是中心对称分子，但其荧光能量与 Onsager 溶剂极性函数 f(ε) - f( n ² ) = (ε - 1)/(2ε + 1)-( n ^{2呈线性相关} − 1)/(2 n ²  + 1)，表明发射源于分子内电荷转移 (CT) 激发态。例外的是，甲醇中的 λ _f从其他非质子溶剂中的波长蓝移到 341 nm。较长多烯 2-5 的荧光溶剂化显色性远不如1显着。在质子化和 N-烷基化后，所有五种化合物的吸收和荧光光谱都在甲醇中发生红移。分别在戊烯5 (73 nm) 和二烯2 (57 nm)中观察到质子化时 λ _a和 λ _f的最大变化。

图形摘要