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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

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journal contribution
posted on 06.03.2014, 00:00 by Noël Boens, Lina Wang, Volker Leen, Peijia Yuan, Bram Verbelen, Wim Dehaen, Mark Van der Auweraer, Wim D. De Borggraeve, Luc Van Meervelt, Jeroen Jacobs, David Beljonne, Claire Tonnelé, Roberto Lazzaroni, Maria J. Ruedas-Rama, Angel Orte, Luis Crovetto, Eva M. Talavera, Jose M. Alvarez-Pez
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 meso-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 (7), 8-benzylamino (8), 8-methoxy (9), and 8-phenoxy (10) groups emit in the blue range of the visible spectrum and generally have larger Stokes shifts than common BODIPYs, whereas 8-(2-phenylethynyl)­BODIPY (6) has red-shifted spectra compared to ordinary BODIPY dyes. Fluorescence lifetimes for 6, 8, and 10 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 7 and 8 has been observed via temperature dependent 1H NMR spectroscopy, whereas for 10 the rotation about the C8–O bond is not hindered. The crystal structure of 8 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 10 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 meso-substituent on the spectroscopic properties.