TADF molecules based on ortho-disposed donors and acceptors were shown by Bredas and coworkers to contain emissive excited states that comprise both TBCT and TSCT character. Yang et al. reported a series of highly twisted emitters, SF34oTz, SF23oTz and SF12oTz, containing indolin-fused spirobifluorene donors (Figure ). According to the DFT computations, the ratios of TBCT/TSCT were initially calculated by integrating the transition density localized on/not on the phenylene bridge. Among them, SF34oTz (FPL: 65% in 10 wt% DPEPO) showed a dominant TSCT (96.8% as suggested by TD-DFT calculations) and a relative larger ΔEST of 0.29 eV, whereas SF23oTz (FPL: 86% in 10 wt% in DPEPO; ΔEST: 0.08 eV) and SF12oTz (FPL: 92% in 10 wt% in DPEPO; ΔEST: 0.05 eV) exhibit higher TBCT contributions, with the ratio increasing to 21% and 32%, respectively. The higher TBCT contribution in SF12oTz leads to much higher ratio of delayed fluorescence (79.1% vs. 39.2%) compared with SF23oTz, which the authors asserted was due to the more effective channel of TBCT over TSCT to realize charge transfer from donors to the acceptor. However, the largest ΔEST of 0.29 eV for SF34oTz in the series does not align with the expectation that a compound with a greater TSCT contribution should have the smallest ΔEST in theory. Among the three The trend in device efficiencies align with that of the FPL. Many OLEDs that contain TADF compounds that emit from an excited state containing some TSCT character still suffer from low device performance. Recently, Kaji et al. and Liao et al. elucidated the importance of the relative orientation of donor and acceptor groups on the electronic coupling between the two in TSCT compounds that influences also the magnitude of the non-radiative decay. Similar to the other TSCT molecules discussed in this section, DM-B (FPL: 96% in 20 wt% in DPEPO; ΔEST: 0.17 eV) and DM-Bm (FPL: 92% in 20 wt% in and DM-Z (30 wt% in DPEPO) exhibit considerably lower maximum efficiencies of 4.3% and 3.2%, respectively. Kaji et al. reported a TSCT compound TpAT-tFFO (Figure ) based on the design strategy of controlling the distance and relative orientation between the adjacent tilted donor and acceptor moieties attached via the triptycene (Tp) scaffold. According to the DFT calculations, there is an increase in the energy levels of 1 CT and 3 CT states with increasing distance between the donor and acceptor (Figure top). By contrast, the 3 LE state only shows a weak dependence on the donor-acceptor distance. RISC is mediated by the intervening 3 LE state between the 1 CT and 3 CT states and the high kRISC of 1.2 × 10 7 s -1 is ensured by the energy level matching of the three states. Importantly, the authors showed by DFT calculations that tilted face-to-face orientation between the donor and acceptor greatly enhances the SOC between the 1 CT and 3 LE states, which is not the case for the co-planar conformation. TpAT-tFFO exhibited strong sky-blue emission with a lPL of 485 nm. The ΦPL in toluene markedly increased from 1.8% to 84% after 30 min of Ar bubbling, while the ΦPL of the film 25 wt% doped into the mCBP host and for the neat film were determined to be 76% and 71%, respectively. A device using an optimized doping concentration of 25 wt% showed an EQEmax of 19.2%. A gentle efficiency roll off was obtained with EQEs of 19.1% and 18.1% at 100 cd m -2 and 1,000 cd m -2 , respectively; moreover, EQE of 14.4% and 11.6% were retained even at high luminance of 10,000 cd m -2 and 20,000 cd m -2 . where 3 LED and 3 LEA denote the 3 LE states on the donor and acceptor segments, respectively.