Kepeng Chen

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 τ_T=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.

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).

The recent developments in charge separation, long-lived charge transfer state formation and charge recombination-induced intersystem crossing are summarized.

Efficient triplet photosensitizers are important for fundamental photochemical studies and applications such as triplet–triplet annihilation upconversion (TTA UC), photoredox catalytic organic reactions and photovoltaics. We now report a series of phosphorus corrole compounds as efficient visible light-harvesting metal-free triplet photosensitizers. While the heavy-atom-free phosphorus corroles show absorption in the visible spectral region (centered at 573 nm) and have a decent triplet state quantum yield (Φ_Δ = 49%), iodo-substitution on the corrole core induces red-shifted absorption (589 nm) and improves intersystem crossing significantly (Φ_Δ = 67%). Nanosecond transient absorption spectra confirm triplet state formation upon photoexcitation (τ_T = 312 μs) and the iodinated derivatives also display near IR phosphorescence in fluid solution at room temperature (λ_em = 796 nm, τ_p = 412 μs). Both singlet oxygen (¹O₂) and superoxide radical anions (O₂ ⁻˙) may be produced with the phosphorus corroles, which are competent photocatalysts for the oxidative coupling of benzylamine (the Aza Henry reaction). Very efficient TTA UC was observed with the phosphorus corroles as triplet photosensitizers and perylene as the triplet acceptor, with upconversion quantum yields of up to Φ_UC = 38.9% (a factor of 2 was used in the equation) and a very large anti-Stokes effect of 0.5 eV.

In this review, recent progress in heavy atom-free triplet photosensitizers was summarized. The general approaches include attaining S₁/T_n states sharing similar energy levels or proper molecular geometry to satisfy the angular momentum reservation in intersystem crossing (ISC). ISC via the higher singlet excited state (S_n, n > 1) → T_m (m > 1), which is a rarely reported phenomenon, was also discussed. The ISC of some Bodipy dimers was proposed to be via the ‘doubly excited state’, but recent studies show that the ISC mechanism of these Bodipy dimers is charge separation/recombination. These new findings in the study of triplet photosensitizers are useful for photovoltaics, photodynamic therapy and photocatalysis, as well as in fundamental photochemistry studies.