The neat films of CCO-1, CCO-2 and CCO-3 exhibit blue PL emissions with peaks at 471, 469 and 464 nm, and ΦPLs of 68%, 72% and 66%, respectively. The PL spectrum is blue-shifted progressively by adding tert-butyl groups, which is attributed to the reduced intermolecular π-π interactions. The 20 wt% doped films of these materials in diphenyl-4-triphenylsilylphenyl-phosphine oxide (DPEPO) host show stronger blue PL emissions peaking at about 470 nm and excellent ΦPLs reaching 99%. By contrast, CBP-1 shows slightly red-shifted PL spectra with wider FWHM by about 6 nm and lower ΦPLs than CCO-1 no matter in neat or doped films. The neat films of CCO-1, CCO-2 and CCO-3 have small ∆Ests of 63-66 meV, while their doped films hold even smaller ∆Ests of 33 meV. A low doping concentration is also found to be conducive to reducing ∆Ests of these materials. Taking CCO-1 for instance, its ∆Est is decreased from 45 to 33 meV by lowering doping concentration from 40 to 20 wt% in DPEPO host (Supplementary Fig. -18 and Table ). This phenomenon is actually associated with the different transition characteristics of S1 and T1 states, which induce different variations of S1 and T1 energy levels in response to polarity change of microenvironments at different doping concentrations (Supplementary Table ). In neat films and 20 wt% doped films, CCO-1, CCO-2 and CCO-3 exhibit prominent delayed fluorescence with short lifetimes (τDFs) of 1.61-1.69 μs and 1.80-1.91 μs, respectively, close to those in toluene solutions (1.54-1.64 µs) but much shorter than those of CBP-1 (5.41 and 7.55 μs) (Table , Fig. and Supplementary Fig. , Table ). The kRISCs of CCO-1, CCO-2 and CCO-3 in 20 wt% doped films are in the range of 13.74-14.16 × 10 5 s -1 , being more than 5-fold larger than that of CBP-1 (3.43 × 10 5 s -1 ). Concerning the same ∆Ests (33 meV) of CCO-1 and CBP-1, the double channel RISC process opened by vibronic coupling between T1 and T2 states should account for the larger kRISC of CCO-1. They show decreased kRISCs of 11.6-11.9 × 10 5 s -1 in 40 wt% doped films and even smaller kRISCs of 7.68-8.12 × 10 5 s -1 in neat films (Supplementary Table ). This tendency can be ascribed to the increased ∆Ests in highly doped films and neat films. The krs of CCO-1, CCO-2 and CCO-3 gradually increase from 1.54-1.66 × 10 7 s -1 in 20 wt% doped films, to 1.86-1.87 × 10 7 s -1 in 40 wt% doped films, and to 2.34-2.73 × 10 7 s -1 in neat films. Besides, benefitting from the larger f values, CCO-1, CCO-2 and CCO-3 also have larger krs than CBP-1, which help to suppress SSA process.