Impact of Electronic Substitution and Planarization on the Photophysical Properties of 2‐(2′‐Hydroxyphenyl)benzoxazole or 2‐(2′‐Hydroxyphenyl)‐Dimethylindole

We present the synthesis along with structural and photophysical properties of a series of 2‐(2′‐hydroxyphenyl)benzoxazole (HBO) and dimethylindole (HDMI) derivatives, in an effort to modulate their fluorescent properties by changing either electronic substitution or introducing planarization via ethynyl bridges. HBO and HDMI dyes are reported to undergo excited‐state intramolecular proton transfer, leading to red‐shifted emission with typically low quantum yield in solution, owing to pronounced non‐radiative deactivation channels. Introducing electron acceptors, such as benzonitrile, emerges as an effective strategy to boost the radiative transitions in these systems. In this context, this study aims at building structure–property relationships by a stepwise investigation of the influence of several parameters, that is, the nature of proton acceptor heterocycle (HBO vs. HDMI), the presence of an ethynyl moiety, and the addition of substituents (CN vs. Me). In each case, the nature of the transition(s) involved in the optical profile was also investigated by theoretical calculations.