3,5-diaryl-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes:: Synthesis, spectroscopic, electrochemical, and structural properties

This research was undertaken to obtain new "BODIPY" dyes that fluoresce at relatively long wavelengths. The title compounds 1a-e were prepared via a divergent route involving Suzuki couplings of arylboronic acids to N-tert-butoxycarbonyl-4-bromopyrrole 2, condensation of the products with an acid chloride, and incorporation of the boron difluoride entity. Two alkyl-substituted systems 7a and 7b were also prepared for comparison; the critical difference between structures 1 and 7 is that the former have an aryl group attached to each pyrrole nucleus whereas the latter only have alkyl substituents on that same ring. UV absorption and fluorescence emission data were compared for compounds 1 and 7. Absorption and fluorescence emission maxima for compounds 1 occur at higher wavelengths than for compounds 7, and the Stokes shifts for the aryl-substituted compounds 1 are larger than for the alkyl-substituted compounds 7. Fluorescence quantum yields measured for compounds 1 are less than for compounds 7, and possible reasons for this are outlined. Other physical data for the compounds were also collected. Oxidation and reduction potentials of the systems were obtained from cyclic voltammetry experiments, and a single-crystal X-ray structure determination was performed for the bisnaphthyl-substituted compound 1b.