Tunable far-red fluorescence utilizing π-extension and substitution on the excited state intramolecular proton transfer (ESIPT) of naphthalene-based Schiff bases: A combined experimental and theoretical study

In this work, a series of naphthalene-based Schiff bases exhibiting excited-state intramolecular proton transfer (ESIPT) namely bis(salicylidene)-1,5-diaminonaphthalene (BSD) containing double hydrogen bonds has been designed and synthesized with extending π-conjugation and substituting electron-donating (EDG)/-withdrawing (EWG) groups. Different chemical modifications significantly control photophysical properties particularly fluorescence emission of BSD derivatives. A fluorine substituent as an EWG induces far-reded keto emission in the range of 520–670 nm (peaked at 582 nm) whereas phenyl π-extension and EDGs (diethylamino and julolidine) make keto emission to be blue-shifted compared to BSD. The spectral shifts of keto emission peaks are dependent on the electron-withdrawing ability which gives rise to the narrowing energy bandgaps. Based on density functional theory, calculated results suggest that the observed keto emission peaks of all synthesized compounds are mainly produced by single ESIPT because of thermodynamically and kinetically favorable reactions. Among the synthesized compounds, a BSD containing the fluorine substituent is the best candidate for ESIPT-based fluorescent molecular probes in living cells owing to its far-reded keto emission with the largest Stokes shift.