Synthesis, Spectroscopy, Crystal Structure Determination, and Quantum Chemical Calculations of BODIPY Dyes with Increasing Conformational Restriction and Concomitant Red-Shifted Visible Absorption and Fluorescence Spectra

Starting from the conformationally unconstrained compound 3,5-di-(2-bromophenoxy)-4,4-difluoro-8-(4-methylphenyl)- 4-bora-3a,4a-diaza-s-indacene (1), two BODIPY dyes (2 and 3) with increasingly rigid conformations were synthesized in outstanding total yields through palladium catalyzed intramolecular benzofuran formation. Restricted bond rotation of the phenoxy fragments leads to dyes 2 and 3, which absorb and fluoresce more intensely at longer wavelengths relative to the unconstrained dye 1. Reduction of the conformational flexibility in 2 and 3 leads to significantly higher fluorescence quantum yields compared to those of 1. X-ray diffraction analysis shows the progressively more extended planarity of the chromophore in line with the increasing conformational restriction in the series 1 -> 2 -> 3, which explains the larger red shifts of the absorption and emission spectra. These conclusions are confirmed by quantum chemical calculations of the lowest electronic excitations in 1-3 and dyes of related chemical structures. The effect of the molecular structure on the visible absorption and fluorescence emission properties of 1-3 has been examined as a function of solvent by means of the new, generalized treatment of the solvent effect (J. Phys. Chem. B 2009, 113, 5951-5960). Solvent polarizability is the primary factor responsible for the small solvent-dependent shifts of the visible absorption and fluorescence emission bands of these dyes.