Long-Lived Local Triplet Excited State and Charge Transfer State of 4,4′-Dimethoxy Triphenylamine-BODIPY Compact Electron Donor/Acceptor Dyads

The spin–orbit charge transfer intersystem crossing (SOCT-ISC) and the formation of a long-lived charge transfer (CT) state were studied with a series of 4,4′-dimethoxy triphenylamine-BODIPY compact electron donor/acceptor dyads. Different torsion freedoms were applied in the dyads to tune the electronic coupling between the donor and acceptor, and a red-shifted CT absorption band was observed for one dyad. The dyads show solvent polarity-dependent singlet oxygen photosensitizing ability (quantum yields 3%–79%). Nanosecond transient absorption spectra of the dyad in nonpolar solvent confirm the formation of triplet states. The intrinsic triplet state lifetime is up to 383 μs (in fluid solution), which is much longer than that accessed with the heavy atom effect (276 μs). Intermolecular triplet photosensitizing of the dyads in a polar solvent produces a long-lived 3CT state (lifetime, τCT = 8.0 μs supported by the electron spin density surface analysis). The triplet state lifetime of the dyads doped in a Clear Flex 50 polymer film is exceptionally long (7.6–11.4 ms), and formation of a long-lived CT state (37 μs) was observed. Triplet–triplet annihilation upconversion was performed with the electron donor/acceptor dyads used as the triplet photosensitizer and perylene used as the triplet acceptor; the upconversion quantum yield is up to 15.8%.