Electron transfer and intersystem crossing in Bodipy dimers: Study of the photophysical properties with 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 (TA) spectroscopy, and pulse laser-excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy 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 electron relaxation pathway, which we relate to charge transfer processes. This conclusion is supported by the fs-TA and TREPR studies. However, the triplet yield of this dimer formed by charge recombination is not high (4.39%). 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: 69.76%) 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 light absorption (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%). 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.