Designing conjugation-extended viologens for high molar absorptivity with longer wavelength absorption

Abstract A series of donor-acceptor (D–A) type conjugated molecules was synthesized, with structures D–π–A or A–π–D–π–A where A = N-hexylpyridinium-4-yl and D = electron donor type units, with A and D linked by π-conjugating units: the D–π–A systems are design-testing models for optimizing the A–π–D-π-A systems, where the latter are conjugatively extended variants of viologen. UV–vis absorption spectra and molar absorptivities were determined in varying solvents, and spectra for neat films. The donor units D comprised phenylene, 2,5-thienylene, 3,4-ethylenedioxythiophen-2,5-diyl (EDOT), 1-N-methyl-2,5-pyrrolediyl, and CH CH units. The system with strongest absorptivity at a relatively long absorption maximum and onset was the extended viologen 4-(1-hexyl)pyridinium-CH CH-(2,5-EDOT-CH CH-)-(2,5-thienylene-CH CH )-(2,5-EDOT-CH = CH-)-4-(1-hexyl)pyridinium dibromide, with λmax(methanol) = 576 nm (278,000 M−1 cm−1), low energy onset at 683 nm (1.82 eV). Photoluminescence quantum yields in methanol mostly were quite weak, with solvatochromic behavior indicating strong intramolecular charge transfer character.