Electron transfer and intersystem crossing in Bodipy dimers: a study of their photophysical properties using steady state and transient optical and electron paramagnetic resonance spectroscopic methods

Abstract

The photophysical properties of two new Bodipy dimers are investigated using a variety of techniques, including steady-state UV-vis absorption and fluorescence spectroscopy, femtosecond and nanosecond transient absorption spectroscopy, and pulse laser-excited time-resolved electron paramagnetic resonance (TREPR) spectroscopic methods. The dimers are formed by the Bodipy units rigidly linked by the orthogonal phenylene bridge. One of the dimers is composed of iodinated units, and the other is not. The dimerization of the non-iodinated Bodipy leads to a strong quenching of the fluorescence compared to the monomer, indicating the presence of a new electronic state relaxation pathway, which we assign to charge transfer processes. This conclusion is supported by the femtosecond transient absorption (fs-TA) spectroscopy and TREPR studies. However, the triplet yield of this dimer formed by charge recombination is not high (4.4%). The iodination of the Bodipy units leads to a significant difference in the photophysical properties of the studied chromophores as a result of the enhanced intersystem crossing (ISC, the singlet oxygen quantum yield: 69.8%) induced by the spin–orbit coupling of the iodine atom due to the heavy atom effect. As a result, the metastable T₁ triplet state is formed after photoexcitation (the rate constant for S₁ → T₁ is 0.2 ps⁻¹), and the processes of charge separation and recombination have a low quantum yield (3.2%). TREPR spectra reveal that IDPB exhibits a weak charge transfer (CT) signal in polar solvents. The absence of heavy atoms in DPB decreases the triplet state formation efficiency and enhances the formation of the CT state. This observation is consistent with fs-TA spectral data. Overall, these results suggest that the triplet state may form indirectly through the CT state. Low-temperature TREPR spectra observed for the triplet and CT states in a polar solvent exhibit similar decay rates. This indicates that these two states may have similar energies and could achieve dynamic equilibrium under certain conditions. The spin density of the metastable T₁ state of the studied dimers is localized on one Bodipy unit, as confirmed by the analysis of the zero-field splitting (ZFS) parameters derived from the TREPR spectra. This information is useful for an in-depth understanding of the triplet state and intersystem crossing in chromophore dimers or oligomers.