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

5,10-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.