The design of rigid electron acceptor with weak electron-withdrawing ability is critical in this work. According to previous reports, electron-withdrawing benzoyl is favourable for the construction of delayed fluorescence materials due to the intrinsic n-π* transition of its carbonyl, which exert positive impacts over strengthening SOC effect and facilitating RISC process . In addition, direct attaching electron-rich carbazole to carbonyl in benzoyl can weaken its electron-withdrawing ability, which is conducive to building efficient delayed fluorescence materials with blue-shifted emission by introducing another electron donor on benzoyl via its phenyl bridge . But the carbazole-benzoyl framework is relatively flexible, which is not perfect in view of reducing nonradiative decay. Therefore, a rigid polycyclic structure with lowered electronwithdrawing ability is envisioned by ring-closure with an oxygen linkage in carbazole-benzoyl framework. The selection of oxygen linkage is because its lone pair of electrons can delocalize to carbonyl to further weaken the electron-withdrawing ability. The positions for ring-closure are analyzed by relaxed scan on the control material (CBP-1), performed by adjusting the dihedral angle between carbazole and carbonyl step by step (Supplementary Fig. ). The results suggest that a stable molecular configuration with the lowest energy can be achieved by ring-closure at the C6 and C27 positions as illustrated in Fig. . The carbonylcontaining polycyclic electron acceptor CCO-α, target materials (CCO-1, CCO-2 and CCO-3) and control material (CBP-1) are prepared according to the synthetic routes outlined in Supplementary Fig. . The detailed procedures and characterization data are described in Supplementary Materials. All the final materials are thermally stable with high decomposition temperatures over 450 o C at 5 wt% initial weight loss and no clear glass-transition temperature (Supplementary Fig. ). The single crystal structures unveil that SXAC donor is connected to CCO-α acceptor in a twisted conformation with large dihedral angles of 77.5-83.9 o . CCO-α has a planar structure, and adopts a face-to-face stacking manner in crystals, which facilitates carrier transport . The inter-plane distance between two adjacent CCO-α moieties is increased from 3.47 Å of CCO-1 to 3.62 Å of CCO-2 and to 4.30 Å of CCO-3 (Fig. and Supplementary Fig. ), indicating tertbutyl groups have effectively modulated the intermolecular π-π interaction of CCO-α. To investigate the influence of ring-closed structure, the reorganization energy (λ) and Huang-Rhys factor (S) between S0 and S1 states are calculated for these materials, which can quantify the displacements of the nuclear coordinates (D) . The sum of λ and S for the normal mode of S0/S1 states are reduced from 0.253/0.298 eV and 23.80/34.36 of CBP-1 to 0.187/0.183 eV and 4.14/4.28 of CCO-1. And the root mean squared displacement (RMSD) between S0 and S1 states is decreased from 0.557 Å of CBP-1 to 0.230 Å of CCO-1. By decomposing the total λ and S into the contribution of each vibration mode, it is found that the vibrations of carbazole-benzoyl framework are mainly responsible for conformation change in CBP-1 (Fig. and Supplementary Fig. ). But in CCO-1, the twisting vibrations at low frequencies (< 200 cm -1 ) and the stretching and scissoring vibrations at high frequencies (>1000 cm -1 ) of benzoyl and carbazole are greatly restricted, indicative of significantly improved rigidity by ring-closure (Supplementary Fig. ). The atomic dipole moment corrected Hirshfeld charge population analysis discloses that the negative charge on the oxygen atom of carbonyl is increased from -0.305 in CBP-1 to -0.319 in CCO-1, while the positive charge on the carbon atom of carbonyl is decreased from +0.229 in CBP-1 to +0.174 in CCO-1, disclosing the electron-withdrawing ability of CCO-α is lowered intrinsically. However, with the inductive effect of the oxygen bridge in CCO-α, the LUMO energy level (ELUMO) of CCO-α is lowered than that of CBP-α (Supplementary Fig. ). While the energy level of HOMO (LUMO) of a D-A-type molecule is driven in a first approximation by both the relative energy and the electronic interaction between the HOMOs (LUMOs) of the electron donor (D) and acceptor (A) fragments, and the electronic interaction of CCO-1 and CBP-1 is similar due to the close dihedral angles between D and A fragments (Supplementary Fig. ). Hence, the lower relative energy of CCO-α with respect to CBP-α results in a lower ELUMO of CCO-1. An identical reasoning can be adopted for the evolution of HOMO energy level (EHOMO) of CBP-1 and CCO-1. However, due to the small orbital coefficients in HOMO at the C6 and C26 positions of CBP-1 (Supplementary Fig. ), the influence of the inductive effect on EHOMO is weaker in comparison with that on EHOMO. Besides, the EHOMO of CCO-α is further destabilized by the mesomeric effect of the oxygen bridge, due to the low-lying energy level of the oxygen's lone pair in relation to this orbital . Thus, the decrease in EHOMO of CCO-α is smaller than that of ELUMO. Overall, the HOMO-LUMO gap of CCO-1 (3.68 eV) is decreased compared with that of CBP-1 (3.69 eV), but the difference between them is actually neglectable, which will avoid the red-shifted emission because of the expansion of conjugation. Concerning the smaller S and λ resulting from the increased molecular rigidity and weakened D-A interaction, CCO-1 is anticipated to achieve stronger and bluer emission with a narrower FWHM than CBP-1.