Strategies for controlling the triplet states using chemical/physical inputs to achieve external stimuli-switchable triplet–triplet-annihilation upconversion (TTA UC).

A perylene (Pery)–phenoxazine (PXZ) compact orthogonal electron donor/acceptor dyad was prepared to study the relationship between the molecular structures and the spin–orbit charge transfer intersystem crossing (SOCT-ISC), as well as the electron spin selectivity of the ISC process. The geometry of Pery–PXZ (80.0°) is different from the previously reported perylene–phenothiazine dyad (Pery–PTZ, 91.5°), although there is only one atom variation for the two dyads. Pery–PXZ shows a high singlet oxygen quantum yield (84%). Femtosecond transient absorption spectra indicate that the charge separation (CS, faster than 120 fs) is faster than the Pery–PTZ analog (CS, 250 fs) and charge recombination (CR, i.e., SOCT-ISC, 5.98 ns) of Pery–PXZ is slower than the Pery–PTZ analog (CR, 0.9 ns). The intrinsic triplet state lifetime of Pery–PXZ is 242 µs vs the lifetime of 181 µs for the Pery–PTZ analog. Moreover, the triplet state lifetime of Pery–PXZ in the solid polymer matrix is extended to 4.45 ms, which indicates that the triplet state of Pery–PXZ in fluid solution is deactivated not only by the triplet–triplet annihilation effect but also by other factors such as vibration coupled relaxation. Interestingly, with pulsed laser excited time-resolved electron paramagnetic resonance spectroscopy, the electron spin polarization (ESP) pattern of the triplet state of the current dyad is opposite to that of Pery–PTZ. These results demonstrated the rich electron spin chemistry of the ISC of compact electron donor/acceptor dyads, e.g., the ESP is dependent on not only the molecular geometry but also the structure of the electron donor (or acceptor).

Triplet exciton harvesting is crucial in organic light emitting diodes (OLEDs), because the triplet states produced by electron and hole recombination is up to 75% of the total excitons, whereas...