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

Abstract

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 S1 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 S0 → S1 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 S1(ICT)-S0 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.

Synthesis and characterization of aryl-substituted BODIPY dyes displaying distinct solvatochromic singlet oxygen photosensitization efficiencies

“…Therefore, the excited state dynamics of the investigated BODIPY can be described with a series of sequential steps, see Scheme 3. In agreement with previous works [33,[38][39][40], we propose that after light absorption to the excited Franck-Condon state (FC); (i) fast FC relaxation and solvent-related reorganization occurs to form the locally excited state (transient lifetime values ranging from 0.44 ps to 4 ps, τ 1 ); (ii) from here charge separation concomitantly with conformational evolution (associated with BODIPY-core structural changes in the excited state) give rise to the charge transfer state (CT) discussed above (τ 2 in the 50-87 ps range). Indeed, although the methyl groups at the 1,3,5-positions hinder the rotation of the substituent groups at the meso-and β-positions, it was shown that bending of the BODIPY-core occurs in the excited state [38].…”

β-Boronic Acid-Substituted Bodipy Dyes for Fluorescence Anisotropy Analysis of Carbohydrate Binding

“…58 This opens new opportunities in fluorescence spectroscopy for our other Bodipys, which possess reactive side groups at the βposition. 57,59,60 ■ EXPERIMENTAL SECTION…”

“…Moreover, questions on molecular stiffness, which are discussed in cell recognition by means of the glycocalyx, can be assessed by time-resolved fluorescence anisotropy . This opens new opportunities in fluorescence spectroscopy for our other Bodipys, which possess reactive side groups at the β-position. ,, …”