Near‐IR‐Absorbing BODIPY‐5,10‐Dihydrophenazine Compact Electron Donor/Acceptor Dyads and Triads: Spin‐Orbit Charge Transfer Intersystem Crossing and Charge‐Transfer State

Abstract

Abstract5,10‐dihydro‐5,10‐dimethylphenazine (PZ) and boron dipyrromethene (Bodipy) were linked to prepare compact electron donor/acceptor dyads and triads. The effect of the molecular geometry on the UV‐Vis absorption, fluorescence and the spin‐orbit charge transfer intersystem crossing (SOCT‐ISC) efficiency were studied. Interestingly, a broad near‐IR charge transfer (CT) absorption band (centred at 670 nm) was observed for the dyad/triad showing more coplanar geometry. Triplet state formation was observed in low polar solvents for the triad with orthogonal geometry (triplet state quantum yield is 30 % and lifetime is τT=210 μs). Femtosecond transient absorption spectra indicated fast charge separation (CS, 2 ps) and slow charge recombination (CR, >1.5 ns). The low‐lying 3CT state of the dyads/triads in polar solvent (1.23 eV vs. the 1.65∼1.69 eV of the 3LE state) was confirmed by intermolecular triplet photosensitizing experiments and the spin density surface analysis. These results are useful for design of new broadband, near‐IR absorbing heavy atom‐free triplet photosensitizers.