Impact of the Keto–Enol Tautomeric Equilibrium on the BODIPY Chromophore

An intramolecular tautomeric fluorescent BODIPY sensor has been designed and synthesized. The obtained BODIPY dye is a combination of the 4-bora-<i>3a</i>,<i>4a</i>-diaza-<i>s</i>-indacene core and a diketone fragment. The study of conformational equilibria in the ground and excited states has been completed for a broad range of solvent polarity by steady state and NMR methods as well as by DFT and TD-DFT calculations. The interpretation of the unique emission observed in hydrogen bond accepting solvents upon the excitation of the fluorescent dye in the S₀–S₂ transition has been accomplished. The Jablonski diagram has been analyzed for the observed processes in the BODIPY dye studied on the basis of DFT and TD-DFT calculations

8‑HaloBODIPYs and Their 8‑(C, N, O, S) Substituted Analogues: Solvent Dependent UV–Vis Spectroscopy, Variable Temperature NMR, Crystal Structure Determination, and Quantum Chemical Calculations

The UV–vis electronic absorption and fluorescence emission properties of 8-halogenated (Cl, Br, I) difluoroboron dipyrrin (or 8-haloBODIPY) dyes and their 8-(C, N, O, S) substituted analogues are reported. The nature of the <i>meso</i>-substituent has a significant influence on the spectral band positions, the fluorescence quantum yields, and lifetimes. As a function of the solvent, the spectral maxima of all the investigated dyes are located within a limited wavelength range. The spectra of 8-haloBODIPYs display the narrow absorption and fluorescence emission bands and the generally quite small Stokes shifts characteristic of classic difluoroboron dipyrrins. Conversely, fluorophores with 8-phenylamino (<b>7</b>), 8-benzylamino (<b>8</b>), 8-methoxy (<b>9</b>), and 8-phenoxy (<b>10</b>) groups emit in the blue range of the visible spectrum and generally have larger Stokes shifts than common BODIPYs, whereas 8-(2-phenylethynyl)­BODIPY (<b>6</b>) has red-shifted spectra compared to ordinary BODIPY dyes. Fluorescence lifetimes for <b>6</b>, <b>8</b>, and <b>10</b> have been measured for a large set of solvents and the solvent effect on their absorption and emission maxima has been analyzed using the generalized Catalán solvent scales. Restricted rotation about the C8–N bond in <b>7</b> and <b>8</b> has been observed via temperature dependent ¹H NMR spectroscopy, whereas for <b>10</b> the rotation about the C8–O bond is not hindered. The crystal structure of <b>8</b> demonstrates that the short C8–N bond has a significant double character and that this N atom exhibits a trigonal planar geometry. The crystal structure of <b>10</b> shows a short C8–O bond and an intramolecular C–H···π interaction. Quantum-chemical calculations have been performed to assess the effect of the <i>meso</i>-substituent on the spectroscopic properties