Spectroscopic and photophysical properties of mono- and dithiosubstituted BODIPY dyes

A series of BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes, substituted at the 3 and 5 positions was synthesized and their spectral and photophysical properties were investigated in solvents of different polarity and polarizability. All compounds are characterized by strong absorption bands at 500-580 nm, corresponding to S-0 -> S-1 transition, the typical for the BODIPY chromophore. Emission spectra are mirror images to absorption with maxima at 520-600 nm. Substitution of the BODIPY chromophore by Cl or thioalkyl substituents induces batochromic shifts in both absorption and emission spectra for 50-60 nm and induces higher quantum yields of fluorescence (Phi(f) = 0.1-0.7). The findings were rationalized by TD-DFT computations. The dyes exhibit weak solvatochromic properties which could not be correlated to solvent polarizability, contrary to majority of BODIPY derivatives. Furthermore, estimated singlet excited state lifetimes (tau = 2-6 ns) reveal that radiative and non-radiative rate constants for deactivation from S-1 do not vary with changes in solvent polarity/polarizability. However, the highest values of Phi(f) and tau are generally found in solvents of high polarizability. Presented spectroscopic and photophysical study is important for the use of dyes as chemodosimeters for thiols and cystein, as well as for the rational design of new molecular probes.