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%).

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.

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An entirely earth‐abundant chromophore‐relay water oxidation catalyst triad system, which is robust and efficient at neutral pH, is presented. The synthesis involves the coordination of a porphyrin derivative to a bridging Fe(CN)5 group, which is then reacted with Co ions to prepare a covalently linked chromophore–Prussian blue analogue assembly. Light‐driven water oxidation studies in the presence of an electron scavenger indicate that the triad is active and it maintains a steady activity for at least three hours. Transient absorption experiments and computational studies reveal that the Fe(CN)5 group is more than a linker as it takes part in electron‐transfer and co‐operates with porphyrin in the charge separation process.

Intersystem crossing (ISC) was observed for naphthalimide (NI)‐derived Tröger's base, and the ISC was confirmed to occur by a spin‐orbital charge‐transfer (SOCT) mechanism. Conventional electron donor/acceptor dyads showing SOCT‐ISC have semirigid linkers. In contrast, the linker between the two chromophores in Tröger's base is rigid and torsion is completely inhibited, which is beneficial for efficient SOCT‐ISC. Femtosecond transient absorption (TA) spectra demonstrated charge‐separation and charge‐recombination‐induced ISC processes. Nanosecond TA spectroscopy confirmed the ISC, and the triplet state is long‐lived (46 μs, room temperature). The ISC quantum yield is dependent on solvent polarity (8–41 %). The triplet state was studied by pulsed‐laser‐excited time‐resolved EPR spectroscopy, and both the NI‐localized triplet state and triplet charge‐transfer state were observed, which is in good agreement with the spin‐density analysis. The Tröger's base was confirmed to be a potent photodynamic therapy reagent with HeLa cells (EC50=5.0 nm).