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

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.

Triplet photosensitizers (PSs) have attracted much attention in recent years, due to the significance of these kinds of compounds in fundamental photochemistry studies and applications in photocatalysis, photodynamic therapy (PDT) and triplet–triplet-annihilation upconversion (TTA-UC). Heavy-atom-free triplet PSs are of particular interest, because of the advantages of low cost, low toxicity and long-lived triplet states compared with triplet PSs that rely on the heavy-atom effect to enhance intersystem crossing (ISC). This review article summarizes recent developments in the design and application of heavy-atom-free triplet PSs, including those based on compact orthogonal electron donor–acceptor dyads, or compounds with twisted π-conjugation frameworks, or those that contain thiocarbonyl groups. The ISC mechanisms of these different types of triplet PSs are discussed, and all are based on enhanced spin–orbit coupling. Moreover, the electron spin selectivity of ISC, studied via time-resolved electron paramagnetic resonance (TREPR) spectra, is also introduced. Some new discoveries and the challenges in this area are discussed, for instance, the role of the triplet charge-transfer (³CT) state in the spin–orbit charge-transfer ISC (SOCT-ISC) mechanism, the relationship between the twisted π-conjugation frameworks and the ISC efficiency, and determination of the intrinsic triplet-state lifetimes.

The high non-radiative energy loss is a bottleneck issue that impedes the improvement of organic solar cells. The formation of triplet exciton is thought to be the main source of the large non-radiative energy loss. Decreasing the rate of back charge transfer is considered as an effective approach to alleviate the relaxation of the charge-transfer state and the triplet exciton generation. Herein, we develops an efficient ternary system based on D18:N3-BO:F-BTA3 by regulating the charge-transfer state disorder and the rate of back charge transfer of the blend. With the addition of F-BTA3, a well-defined morphology with a more condensed molecular packing is obtained. Moreover, a reduced charge-transfer state disorder is demonstrated in the ternary blend, which decreases the rate of back charge transfer as well as the triplet exciton formation, and therefore hinders the non-radiative recombination pathways. Consequently, D18:N3-BO:F-BTA3-based device produces a low non-radiative energy loss of 0.183 eV and a record-high efficiency of 20.25%. This work not only points towards the significant role of the charge-transfer state disorder on the suppression of triplet exciton formation and the non-radiative energy loss, but also provides a valuable insight for enhancing the performance of OSCs.

Heavy atom-free dyads showing a red light-absorbing and exceptionally long-lived triplet state based on a spin–orbital charge transfer intersystem crossing mechanism.

The triplet state lifetimes of organic chromophores are crucial for fundamental photochemistry studies as well as applications as photosensitizers in photocatalysis, photovoltaics, photodynamic therapy and photon upconversion. It is noteworthy that the triplet state lifetime of a chromophore can vary significantly for its analogues, while the exact reason was rarely studied. Herein with a few exemplars of typical BODIPY derivatives, which show triplet lifetimes varying up to 110-fold (1.4–160 μs), we found that for these derivatives with short triplet state lifetimes (ca. 1–3 μs), the electron spin polarization (ESP) pattern of the time-resolved electron paramagnetic resonance (TREPR) spectra of the triplet state is inverted at a longer delay time after laser pulse excitation, as a consequence of a strong anisotropy in the decay rates of the zero-field state sublevel of the triplet state. For the derivatives showing longer triplet state lifetimes (>50 μs), no such ESP inversion was observed. The observed fast decay of one sublevel is responsible for the short triplet state lifetime; theoretical computations indicate that it is due to a strong coupling between the T_z sublevel and the ground state mediated by the spin–orbit interaction. Another finding is that the heavy atom effect on the shortening of the triplet state lifetime is more significant for the T₁ states with lower energy. To the best of our knowledge, this is the first systematic study to rationalize the short triplet state lifetime of visible-light-harvesting organic chromophores. Our results are useful for fundamental photochemistry and the design of photosensitizers showing long-lived triplet states.