Synthesis and fluorescence properties of unsymmetrical 1,4-dihydropyrrolo[3,2-]pyrrole dyes

• Published in: New journal of chemistry 2022-01, Vol.46 (4), p.1533-1542
• Main Authors: Kubota, Yasuhiro, Koide, Kenta, Mizuno, Yuka, Nakazawa, Masato, Inuzuka, Toshiyasu, Funabiki, Kazumasa, Sato, Hiroyasu, Matsui, Masaki
• Format: Article
• Language: English
• Subjects: Acetonitrile; Charge transfer; Chemical compounds; Dichloromethane; Fluorescence; Hexanes; Pyrroles; Solid state; Solvents; Substitutes
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/d1nj04663b

Unsymmetrical 1,4-dihydropyrrolo[3,2- b ]pyrroles were synthesised and their absorption and fluorescence properties were investigated. The 5- p -diethylaminophenyl substituted derivative 5 ( λ ab = 346 nm) showed red-shifted λ ab compared to the 6- p -diethylaminophenyl substituted derivative 6 ( λ ab = 308 nm) because of the efficient donor-π-acceptor structure of the former that facilitates intramolecular charge transfer from the diethylamino group to the ethoxycarbonyl group. The 5-substituted derivative 5 showed solvatochromic fluorescence (from 393 nm in n -hexane to 446 nm in acetonitrile). In contrast, the 6-substituted derivative 6 showed locally excited fluorescence ( e.g. , λ fl = 406 nm in n -hexane) in nonpolar solvents, twisted intramolecular charge transfer fluorescence ( e.g. , λ fl = 538 nm in acetonitrile) in polar solvents, and dual fluorescence ( e.g. , λ fl = 392 and 517 nm in dichloromethane) in chlorinated solvents. As a result, the 6-substituted derivative 6 showed a large Stokes shift in polar solvents (up to 230 nm in acetonitrile). The 6-substituted derivative 6 did not exhibit fluorescence in the solid state, whereas the 5-substituted derivative 5 ( λ fl = 439 nm, Φ f = 0.04) was characterised by solid-state fluorescence due to the inhibition of consecutive intermolecular interactions between neighbouring fluorophores. Despite being regioisomers, unsymmetrical 1,4-dihydropyrrolo[3,2- b ]pyrroles 5 and 6 showed significantly different absorption and fluorescence properties due to the difference of the resonance structure between 5 and 6 .