Solvent-dependent photophysical properties of borondipyrromethene dyes in solution

Four fluorescent dyes derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) with unusual substituents (chloro, methoxy, and malonate) at the 3,5-positions have been synthesized by a novel nucleophilic substitution reaction of 3,5-dichloroBODIPY. Their solvent-dependent photophysical properties have been investigated by means of UV/Vis absorption, steady-state and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity due to an increase in the rate of nonradiative deactivation. The fluorescence rate constants are in the (1.6–2.0) × 10⁸ s⁻¹ range. The borondipyrromethene derivatives show small Stokes shifts (between 440 and 490 cm⁻¹) and narrow absorption (FWHM between 710 and 820 cm⁻¹) and emission bands.