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

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 (lambda(ab) = 346 nm) showed red-shifted lambda(ab) compared to the 6-p-diethylaminophenyl substituted derivative 6 (lambda(ab) = 308 nm) because of the efficient donor-Tc-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., lambda(fl) = 406 nm in n-hexane) in nonpolar solvents, twisted intramolecular charge transfer fluorescence (e.g., lambda(fl) = 538 nm in acetonitrile) in polar solvents, and dual fluorescence (e.g., lambda(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 (lambda(fl) = 439 nm, Phi(f) = 0.04) was characterised by solid-state fluorescence due to the inhibition of consecutive intermolecular interactions between neighbouring fluorophores.

Automated summary

Unsymmetrical 1,4-dihydropyrrolo[3,2-b]pyrroles were synthesised and their absorption and fluorescence properties were investigated. Different substituents led to significant differences in the emission wavelength, while the fluorescence properties were also affected by the solvent environment.