Chain-Length-Dependent Photophysical Properties of α,ω-Di(4-pyridyl)polyenes: Effects of Solvent Polarity, Hydrogen Bond Formation, Protonation, and N-Alkylation

• Published in: Journal of fluorescence 2022, Vol.32 (1), p.95-108
• Main Author: Sonoda, Yoriko
• Format: Article
• Language: English
• Subjects: Absorption; Acetonitrile; Alkylation; Analytical Chemistry; Biochemistry; Biological and Medical Physics; Biomedical and Life Sciences; Biomedicine; Biophysics; Biotechnology; Blue shift; Bonding strength; Chains; Charge transfer; Conjugation; Fluorescence; Hexanes; Hydrogen bonds; Methanol; Original Article; Protonation; Solvent effect; Solvents
• ISSN: 1053-0509; 1573-4994
• DOI: 10.1007/s10895-021-02824-y

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( n 2 ) = (ε − 1)/(2ε + 1)−( n 2  − 1)/(2 n 2  + 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