Efficient Radical-Enhanced Intersystem Crossing in an NDI-TEMPO Dyad: Photophysics, Electron Spin Polarization, and...

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 (Φ_T =74 %), a long-lived triplet state (τ_T =8.7 μs), fast EISC (1/k_EISC =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 Φ_UC =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.