Long‐Lived Triplet Excited State Accessed with Spin–Orbit Charge Transfer Intersystem Crossing in Red Light‐Absorbing Phenoxazine‐Styryl BODIPY Electron Donor/Acceptor Dyads

Orthogonal phenoxazine‐styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom‐free triplet photosensitizers (PSs) with strong red light absorption (ϵ=1.33×10(5) M(−1) cm(−1) at 630 nm), whereas the previously reported triplet photosensitizers based on the spin‐orbit charge transfer intersystem crossing (SOCT‐ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long‐lived triplet state (τ(T)=333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τ(T)=1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT‐ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light‐harvesting chromophore, which shows red‐light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT‐ISC (charge recombination, CR; 2.3 ns) for BDP‐1. ISC efficiency of BDP‐1 was determined as Φ(T)=25 % (in toluene). The dyad BDP‐3 was used as triplet PS for triplet‐triplet annihilation upconversion (upconversion quantum yield Φ(UC)=1.5 %; anti‐Stokes shift is 5900 cm(−1)).