Thiophene-substituted quinoxaline donor-acceptor dyes: Synthesis, NMR spectroscopy, X-ray crystallography, and photophysical properties

Structure-property relationship studies of fluorescent molecules containing donor-acceptor units are important for informing the design of organic materials for custom applications. We report the synthesis and characterization of a series of novel thiophene-substituted quinoxaline donor-acceptor dyes with varied substituent positioning. The dyes were synthesized using microwave-assisted methods in one or two steps from commercially available precursors. In solution, proton NMR spectroscopy studies revealed significant upfield shifts for certain thiophene ring protons in the o-disubstituted quinoxaline derivatives compared to their monosubstituted analogs. The upfield shifts resulted from restricted rotation about the thiophene-quinoxaline C–C bonds due to steric congestion leading to some protons being shielded by neighboring aromatic rings. In the solid state, single-crystal X-ray diffraction studies of the o-disubstituted derivatives showed that they exhibit thiophene ring disorder. These dyes are weakly emissive in solution, and their photophysical characterization using UV–vis and fluorescence spectroscopy in solvents of varying polarity revealed slight to moderate emission solvatochromism, with Lippert-Mataga analysis estimated Δμ values between 3 and 6 D. The dyes with thiophene rings attached to the benzene side of the quinoxaline ring exhibited a higher degree of solvatochromism than those with thiophene rings attached to the pyrazine side, illustrating the impact that substituent positioning can have on a molecule's photophysical properties. DFT and TD-DFT computational results were consistent with the observed photophysical properties and electronic structures of the dyes and aided in rationalization of the trends.