The natural transition orbital (NTO) analysis performed on the excited states uncover that, for S1 state, the highest occupied natural transition orbital (HONTO) is concentrated on SXAC, while the lowest unoccupied natural transition orbital (LUNTO) is mainly distributed on carbonyl and two adjacent benzenes (Fig. and Supplementary Fig. ). Due to weakened D-A interaction and thus lowered CT effect of S1 state, the LE feature of CCO-1 in S1 state is enhanced accordingly, accompanied by a larger overlap integral of norm of HONTO and LUNTO (OIH/L) of 0.132 than that of CBP-1 (0.117), which is conducive to radiative transition. The calculated T1 energy level of CBP-1 is much lower than that of CCO-1, and thus the theoretical ∆Est of CBP-1 (0.17 eV) becomes larger than that of CCO-1 (0.08 eV). The lower T1 energy level of CBP-1 discloses a larger exchange integral (K), which can endow the T1 state of CBP-1 with increased LE component . The OIH/L for the T1 state of CBP-1 is calculated to be 0.615, larger than that of CCO-1 (0.486), which also validates a higher LE component of the T1 state. Due to more different transition characteristics between S1 and T1 states, the SOC constant of CBP-1 (0.77 cm -1 ) is larger than that of CCO-1 (0.36 cm -1 ), suggesting the bridging oxygen atom does not contribute to SOC. But the ring-closed structure by oxygen atom can influence excited-state energy levels, leading to a smaller ∆Est for CCO-1. So, the kRISC from T1 to S1 state of CCO-1 is calculated to be 2.46 × 10 6 s -1 , larger than that of CBP-1 (9.14 × 10 5 s -1 ), despite its smaller SOC constant. Moreover, the T2 energy level of CCO-1 lies close and slightly higher to its S1 energy level, and the gap between T1 and T2 energy levels is only 0.12 eV, small enough for vibronic coupling to induce SOC interaction between T2 and S1 states (Fig. and Supplementary Table ). Therefore, T1 and T2 wavefunctions can be mixed by vibronic coupling, opening supplementary RISC channel from T2 to S1 states in CCO-1 . However, the T2 energy level of CBP-1 (2.53 eV) is almost equal to T1 energy level (2.51 eV), which makes it possible to form a conical intersection between T2 and T1 states. This virtually will provoke consecutive and fast internal conversion (IC) process to compete with RISC process . Therefore, in comparison with CBP-1, CCO-1 is more likely to have a faster RISC process with the aid of its smaller ∆Est and double RISC channels. CCO-2 and CCO-3 have similar theoretical results to CCO-1 (Supplementary Table ). The transition dipole moments (TDMs) of CCO-1 and CBP-1 are calculated in solid state based on crystal structures. The two-layer ONIOM approach combining quantum mechanics and molecular mechanics (QM/MM) is employed in calculation, where the central molecule is defined as the active QM part, and the surrounding molecules act as MM part. According to Born-Oppenheimer approximation, the TDM is influenced by nuclear vibration configuration, electron spin configuration and electron orbital wavefunction, and the large overlap of transition orbitals is favorable for gaining high value of TDM (µ). Besides, oscillator strength (f) is related to µ according to equation (1) :