Synthesis of T-shaped Oxazolonaphthoimidazo[1,2-a] pyridines using Lactic Acid as Bio-based Green Solvent: An Insight into Photophysical Studies

A simple and chemoselective method of synthesizing T-shaped oxazolonaphtho[1', 2': 4,5] imidazo[1,2-a] pyridines in a one-pot selective fashion was developed in good yields. Lactic acid was used as an alternative solvent to acetic acid, which exhibited advantages such as bio-based origin, ease of isolating the product and superior synthetic efficiency. Derivatives containing electron donating and withdrawing analogues were achieved in good yields and the synthetic strategy was highly compatible with various functionalities. Their electronic, optical and electrochemical properties were studied by a combination of absorption, emission spectroscopic techniques and fluorescence quantum yield measurements. Most of them displayed fluorescence in solution state, which could be modulated by structural changes and the nature of the solvents polarity. Further, the introduction of a strong donor/acceptor groups caused a significant shift in absorption and emission spectra, and the effect was remarkable, especially in the case of dialkylamino/cyano substituents, suggesting an increased intramolecular charge transfer character. Additionally, we observed dramatic colour changes and luminescence switching with the addition of acid, suggesting the potential ability of these molecules to function as colorimetric and luminescence pH sensors.