Boron Dipyrromethene Analogs with Phenyl, Styryl, and Ethynylphenyl Substituents: Synthesis, Photophysics, Electrochemistry, and Quantum-Chemical Calculations

Seven fluorescent boradiazaindacene-based compounds with one or two phenyl, ethenylphenyl, and ethynylphenyl substituents at the 3- (or 3,5-) position(s) were synthesized via palladium-catalyzed coupling reactions with the appropriate 3,5-dichloroBODIPY derivative. The effect of the various substituents at the 3- (or 3,5-) position(s) on the spectroscopic and photophysical properties were studied as a function of solvent by means of UV/vis absorption, steady-state, and time-resolved fluorometry, and theoretical modeling. The emission maxima of the symmetrically 3,5-disubstituted dyes are shifted to longer wavelengths (by 30 to 60 nm) relative to the related asymmetrically 3,5-disubstituted ones. Introduction of styryl substituents causes the largest red shift in both the absorption and emission spectra. BODIPY derivatives with ethynylaryl groups also shift the spectral maxima to longer wavelengths compared to aryl-substituted ones but to a lesser degree than the styryl compounds. The quantum-chemical calculations confirm these trends and provide a rationale for the spectral shifts induced by substitution. The fluorescence quantum yields of the ethenylaryl and ethynylaryl analogs are significantly higher that those of the aryl-substituted dyes. The 3,5-diethynylaryl dye has the highest fluorescence quantum yield (1.0) and longest lifetime (around 6.5 ns) among the BODIPY dyes studied. The differences in the photophysical properties of the dyes are also reflected in their electrochemical properties where the symmetrically 3,5-disubstituted dyes display much lower oxidation potentials when compared to their asymmetric counterparts.