Exploring Differences In Excited-State Properties Of Styryl-Bodipy Chromophores Upon Change From Alpha- To Beta-Substitution

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 alpha-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, beta-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 alpha- to beta-substitution. In the case of the alpha-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 beta-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 beta-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 beta-position therefore suffer a substantial loss in fluorescence brightness in polar environments compared with their alpha-substituted counterparts. This might limit their applicability in fluorescence imaging.