Sterically Hindered 2-(2'-Hydroxyphenyl)benzoxazole (HBO) Emitters: Synthesis, Spectroscopic Studies, and Theoretical Calculations

We describe various synthetic pathways to introduce sterically hindered substituents (mesityl, 2,4,6-triisopropylphenyl, anthracene) to the proton donor side of excited-state intramolecular proton transfer (ESIPT)-capable 2-(2'-hydroxyphenyl) benzoxazole (HBO) fluorophores. Two original synthetic approaches were investigated in order to synthesize seven HBO derivatives. Optimization studies concluded that electron rich and bulky phosphine ligands are required to ensure completion of the Suzuki-Miyaura cross-coupling reaction involving a bulky aromatic boronic acid. The photophysical properties of all dyes revealed a strong influence of the nature of the solvent on the optical properties, as protic solvents tend to stabilize enol tautomers and induce dual emission. Our studies confirmed that, unlike the majority of organic dyes, ESIPT fluorophores do not undergo pi-stacking in the solid-state. The nature of the excited-states was explored by ab initio calculations.

Automated summary

This article describes various synthetic pathways to introduce sterically hindered substituents to the proton donor side of ESIPT-capable fluorophores. It also investigates the influence of solvent properties on the optical properties of the dyes. The study confirms that ESIPT fluorophores do not undergo pi-stacking in the solid-state.