D–π–A–π–D Dyes with a 1,3,2‐Dioxaborine Cycle in the Polymethine Chain: Efficient Long‐Wavelength Fluorophores

Abstract

The nitrile group in newly synthesized 5‐cyano‐2,2‐difluoro‐4,6‐dimethyl‐1,3,2‐dioxaborine is shown to significantly increase the acidity of the methyl groups. This allows the cyanine condensation with both methyl groups to form D–π–A–π–D dyes in which the dioxaborine cycle is a part of the polymethine chain. There, the reactivity of the remaining methyl group in the semi‐product is reduced enough to perform the reaction stepwise, giving access to nonsymmetric derivatives. The synthesis of dyes with various electron donor termini [indole, benzothiazole, pyran, 4‐(dialkylamino)phenyl] and with different polymethine chain lengths is reported. The obtained compounds show intense absorption and fluorescence in the red and NIR region, their fluorescence brightness (the product of the molar extinction and the fluorescence quantum yield) attaining a value of 200,000 m–1 cm–1. They are characterized by small‐scale positive solvatochromism, yet their fluorescence quantum yield is much more sensitive to ambient polarity. Time‐dependent (TD)DFT calculations have been performed to study the electronic structure of the new dioxaborines and reveal the nature of the long‐wavelength electronic transitions in those molecules.