Tuning the Photophysical Properties of 2‐Quinolinone‐Based Donor–Acceptor Molecules through N ‐ versus O ‐Alkylation: Insights from Experimental and Theoretical Investigations

A series of 2‐quinolinone‐based molecular systems with three different acceptor groups and N/ O ‐alkylated quinolinone compounds have been synthesised in an attempt to understand their optical properties. All the compounds were characterised by 1 H NMR, 13 C NMR and mass analysis. Absorption measurements revealed that charge‐transfer transition was observed by introducing electron‐withdrawing acceptor groups. Alkylation of 2‐quinolinone at the O‐position thwarts the charge‐transfer transitions, whereas N ‐alkylation retains the charge‐transfer property. The presence of resonance zwitterionic forms in the unalkylated and N ‐alkylated quinolinone compounds play an important role in charge‐transfer transition. The effects of solvents on the absorption and emission properties of these compounds were probed through Lippert Mataga and E T (30) correlation. The Stokes shifts of O ‐alkylated compounds were larger than those of the unalkylated and N ‐alkylated quinolinone compounds. The observed higher quantum yield and Stokes shift for these compounds will make them ideal fluorescent probes. Incorporation of acceptor groups and alkylation in the quinolinone moiety alters their energy levels. Good thermal stability was observed for both unalkylated and alkylated quinolinone compounds. The trends observed in the photophysical and electrochemical properties were supported by theoretical studies. The observed tunable optical properties, which were achieved through simple N ‐ vs. O ‐alkylation, results in large Stokes shifts and thermal stability, which means that the 2‐quinolinone‐based molecular systems are expected to emerge as potential candidates for photovoltaic and biological applications.