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

Abstract

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.