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We present a femtosecond pump-probe UV-Vis broadband transient absorption spectroscopy study of two styryl-substituted BODIPY chromophores with different position of the substituent. The  α -substituted isomer shows typical BODIPY-type spectral features, such as sharp absorption and emission bands, a small Stokes shift and an excited-state lifetime in the 4 ns range, which only weakly depends on the solvent. In contrast,  β -styryl-BODIPY features much broader steady-state absorption and emission spectra and a larger Stokes shift, particularly in polar solvents. Transient absorption spectroscopy including a complete global kinetic analysis reports a substantial decrease in S 1  lifetime to 300 ps for polar solvents upon change from  α - to  β -substitution. In the case of the  α -isomer, TD-DFT calculations identify a typical “cyanine-type” electron rearrangement upon S 0  → S 1  excitation accompanied by a slight reduction in dipole moment. In contrast, the same transition in the  β -isomer shows strong intramolecular charge transfer (ICT) character involving a substantial increase in dipole moment. Assuming a simple energy-gap-law argument, the accelerated nonradiative decay of the  β -isomer in polar solvents may be linked to the decrease of the S 1 (ICT)-S 0  energy difference. BODIPY dyes with a conjugated substituent in  β -position therefore suffer a substantial loss in fluorescence brightness in polar environments compared with their  α -substituted counterparts. This might limit their applicability in fluorescence imaging.

Exploring Differences in Excited-State Properties of Styryl-BODIPY Chromophores upon Change from α - to β -Substitution