Synthesis, Photophysical Properties and Solvatochromism of Meso-Substituted Tetramethyl BODIPY Dyes

The 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorescent dyes (BODIPYs) were first synthesized almost 50 years ago; however, the exploration of their technological application has only begun in the last 20 years. These dyes possess interesting photophysical properties, increasing interest in their application as fluorescent markers and/or dyes. Herein, we report the synthesis of tetramethyl BODIPY and four meso-substituted dyes (2-thienyl, 4-pyridinyl, 4-fluorophenyl and 4-nitrophenyl derivatives). Their photophysical characterization (absorption spectra, emission spectra, fluorescence quantum yields and time-resolved fluorescence) and solvatochromic behavior were studied. Absorption and emission were barely affected by substituents, with a slightly higher stokes shift observed in the substituted dyes. Substitutions could be associated with a shorter fluorescence lifetime and lower quantum yields. Good correlations were observed between the Catalán solvent descriptors and the photophysical parameters. Also, better correlation was observed between the solvent polarizability descriptor (SP) and photophysical parameters. Overall, only slight solvatochromism was observed. The 4-pyridinyl derivative was the subject of a relatively significant solvatochromism regarding the wavelengths of the emission spectra, with the observation of a bathochromically shifted emission in methanol. The fluorescence quantum yield of the 4-nitrophenyl substituted BODIPY was approximately 30 times higher in hexane, which may be of interest for practical applications.