Impact of alkyl chains, electron donor substituents and acidic group on aggregation-induced emission of carboxylic acid functionalized quinoxalines

We report an optical study of quinoxaline-based luminogens derived from 2,3-bis(aryl)-quinoxaline-6-carboxylic acid (1a, 1b, and 2) in solution and solid state. These compounds were fluorescent in solid films and in solutions with emission bands ranging from blue to green (445-517 nm). They showed a positive solvatochromism in the PL spectra, indicative of intramolecular charge transfer (ICT) in the excited state. 1a, 1b, and 2 exhibited Aggregation-Induced Enhanced Emission (AIEE) characteristics with a drastic change in properties by increasing alkyl chains and donor alkoxy groups. Their carboxylate potassium salts had a blueshift emission compared to the acid carboxylic form and a significant quenching in PL intensity in protic polar solvents. The salt of 1a presented an Aggregation-Induced Emission (AIE) profile, its weak fluorescence in water had an expressive enhancement by adding THF. Theoretical calculations were also carried out considering implicit solvent effects. The optimized structures of compounds and their simulated UV-vis absorption spectra showed no significant differences in water and acetonitrile. These results are in good agreement with the solvatochromism experiments. The calculated HOMO-LUMO energy gaps were 3.47 eV in both solvents, and the energy levels for HOMO and LUMO are slightly lower in water.