A long-lived triplet excited state of the well-known fluorophore boron dipyrromethene (Bodipy) was observed for the first time via efficient radical-enhanced intersystem crossing (EISC). The triplet state has been obtained in two dyads in which the Bodipy unit is linked to a nitroxide radical, 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), with two different length spacers. The photophysical properties were studied with steady-state and time-resolved transient optical spectroscopies and electron spin resonance (cw-ESR and TR-ESR). The fluorescence of Bodipy units is significantly quenched in the dyads, and the spin-polarized TEMPO signals were observed with TR-ESR, generated by a radical triplet pair mechanism. Efficient EISC (Φ = 80%) was observed for the dyad with a shorter linker, and the triplet state lifetime of the Bodipy chromophore is exceptionally long (62 μs). The EISC takes 250 ps. Poor ISC was observed for the dyad with a longer linker. The efficient ISC and long-lived triplet excited state in this flexible system are in stark contrast to the previously studied rigid EISC systems. The EISC effect was employed for the first time to perform triplet-triplet annihilation (TTA) upconversion (quantum yield Φ = 6.7%).

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We prepared perylene (Pery)-phenothiazine (PTZ) compact donor/acceptor dyads with connection at either N-or 2-C positions of the PTZ moiety to attain molecular conformation restriction and to study the relationship between mutual chromophore orientation and spin−orbit charge-transfer intersystem crossing (SOCT-ISC) efficiency. In Pery-N-PTZ, the linkage is at the N-position of the PTZ moiety, and the molecule adopts an orthogonal geometry (φ = 91.5°), whereas in Pery-C-PTZ, the connection is at the 2-C position, resulting in a more planar geometry (φ = 60.6°). A diphenylamino derivative (Pery-DPA) was also prepared in which a N atom is fully π-conjugated with the perylene moiety. Highly solvent polarity-dependent singlet oxygen production was observed for the dyads (Φ Δ = 3−60%), which is an indication of the SOCT-ISC mechanism. The potential energy curve of the torsion about the C−N/C−C linker indicated different energy landscapes for the dyads; interestingly, we found that nonorthogonal geometry also induces efficient SOCT-ISC, which is different from previous studies. The ultrafast charge separation process (<100 fs) and the ISC rate (0.27 ps) were observed by femtosecond transient absorption spectroscopy. Time-resolved electron paramagnetic resonance spectroscopy further confirmed the SOCT-ISC mechanism. With perylenebisimide as the triplet acceptor and the dyads as the triplet photosensitizer, the triplet−triplet annihilation-induced delayed fluorescence was observed, with the luminescence lifetime up to 71 μs.

A compact naphthalenediimide (NDI)–2,2,6,6‐tetramethylpiperidinyloxy (TEMPO) dyad has been prepared with the aim of studying radical‐enhanced intersystem crossing (EISC) and the formation of high spin states as well as electron spin polarization (ESP) dynamics. Compared with the previously reported radical–chromophore dyads, the present system shows a very high triplet state quantum yield (ΦT=74 %), a long‐lived triplet state (τT=8.7 μs), fast EISC (1/kEISC=338 ps), and absorption in the red spectral region. Time‐resolved electron paramagnetic resonance (TREPR) spectroscopy showed that, upon photoexcitation in fluid solution at room temperature, the D0 state of the TEMPO moiety produces strong emissive (E) polarization owing to the quenching of the excited singlet state of NDI by the radical moiety (electron exchange J>0). The emissive polarization then inverts into absorptive (A) polarization within about 3 μs, and then relaxes to a thermal equilibrium while quenching the triplet state of NDI. The formation and decay of the quartet state were also observed. The dyad was used as a three‐spin triplet photosensitizer for triplet–triplet annihilation upconversion (quantum yield ΦUC=2.6 %). Remarkably, when encapsulated into liposomes, the red‐light‐absorbing dyad–liposomes show good biocompatibility and excellent photodynamic therapy efficiency (phototoxicity EC50=3.22 μm), and therefore is a promising candidate for future less toxic and multifunctional photodynamic therapeutic reagents.

2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad were prepared, with the iodoBodipy moiety as the singlet/triplet energy donor and the PBI moiety as the singlet/triplet energy acceptor. IodoBodipy undergoes intersystem crossing (ISC), but PBI is devoid of ISC, and a competition of intramolecular resonance energy transfer (RET) with ISC of the diiodoBodipy moiety is established. The photophysical properties of the compounds were studied with steady-state and femtosecond/nanosecond transient absorption and emission spectroscopy. RET and photoinduced electron transfer (PET) were confirmed. The production of the triplet state is high for the iodinated dyad and the triad (singlet oxygen quantum yield ΦΔ = 80%). The Gibbs free energy changes of the electron transfer (ΔGCS) and the energy level of the charge transfer state (CTS) were analyzed. With nanosecond transient absorption spectroscopy, we confirmed that the triplet state is localized on the PBI moiety in the iodinated dyad and the triad. An exceptionally long lived triplet excited state was observed (τT = 150 μs) for PBI. With the uniodinated reference dyad and triad, we demonstrated that the triplet state localized on the PBI moiety in the iodinated dyad and triad is not produced by charge recombination. These information are useful for the design and study of the fundamental photochemistry of multichromophore organic triplet photosensitizers.