Synthesis, structure and spectroscopic properties of BODIPY dyes incorporating the pentafluorosulfanylphenyl group

Abstract

Six compounds (BD1–BD6) are discussed based on the borondipyrromethene (BODIPY) core and functionalised in the meso position or the 3,5-positions with pentafluorosulfanylaryl subunits. The SF₅ unit in the aryl ring acts an electron-withdrawing group and its effect on the properties of the BODIPY derivatives is described. Two crystal structures were obtained for the simplest compounds in which the meso position of the BODIPY is either pentafluorosulfanylphenyl (BD1) or pentafluorosulfanylphenylethynylphenyl (BD2). The crystal packing features of the two structures were used to rationalise the strong solid-state emission observed for the two compounds in terms of exciton-coupled dimers. Solution phase photophysical properties of the compounds are also described, and trends within them are discussed in terms of the negative hyperconjugation and the “non-bonded” resonance effect of the SF₅ group. The long-wavelength absorbing and fluorescing dyes (BD4–BD6) are π-conjugated at the 3,5 positions, but they do not display typical solvatochromic behaviour associated with push–pull chromophores. The alterations in their absorbance and fluorescence maxima with change in solvent are adequately fitted to the solvent polarizability function (f(n²) = (n² − 1)/(2n² + 1)), where n is the refractive index. The good linear fit supports no major change in the electronic transition dipole moment between the ground and excited states. Density functional theory (DFT) calculations were consistent with localised low-energy π–π* transitions rather than any charge-transfer transitions despite the existence of the electron-withdrawing SF₅ group(s).