ESIPT fluorescent dyes with adjustable optical properties: Substituent and conjugation effects

Based on the design and synthesis of 10-hydroxybenzo[h]quinoline (3) derivatives, we demonstrate a prototypical system to investigate both substituent and conjugation effects on excited-state intramolecular proton transfer emission through a combination of experimental and theoretical studies. On the one hand, adding an electron-withdrawing substituent at the phenol ring in 3 results in a decrease of HOMO, and hence, an increase in the energy gap of the keto-tautomer emission (o-4 and p-4). An opposite effect is observed when an electron-donating substituent is added (1). On the other hand, the elongation of the p-conjugation length by the installed 2-methylenemalononitrile group (o-2 and p-2) induces further π-electron delocalization, and thus a smaller emission energy gap. Time-dependent density functional theory calculations on these proton transfer dyes are reported in order to rationalize their electronic structure and optical properties.