Here, we present a high-performance pure blue MR-TADF emitter design strategy by fusing boron and carbonyl MR-TADF cores where the boron and nitrogen atoms are disposed para to each other in a B-p-N manner. By fusing the two MR-TADF emitters of DOBNA and DiKTa into a single molecule, DOBDiKTa, not only are the MR-TADF properties conserved, but a desired color tuning to the blue and improved RISC rates were also achieved. As shown in Figure , the boron atom is embedded para to the nitrogen atom of DiKTa, thus the emission is blue-shifted to 445 nm between DiKTa (lPL = 451 nm) and tBuDOBNA (lPL = 397 nm) in toluene. The emission of 1.5 wt% doped film DOBDiKTa in a 1:1 mixture of 1,3-bis(Ncarbazolyl)benzene:2,8-Bis(diphenyl-phosphoryl)-dibenzo[b,d]thiophene (mCP:PPT) as a cohost system is 461 nm (FWHM of 38 nm) , which is nearly identical to the emission observed in mCP at the same doping concentration (lPL = 460 nm with FWHM of 37 nm). Compared with DiTKa, OLEDs using the mCP:PPT co-host with DOBDiKTa as the emitter show an increased EQEmax of 17.4% and bluer emission at lEL of 458 nm (FWHM of 38 nm) associated with improved Commission Internationale de l' Éclairage (CIE) coordinates of (0.14, 0.12). The molecular design for DOBDiKTa was rationalized through a ground-and excited-state properties computational study using a combination of gas-phase DFT (PBE0/6-31G(d,p)) and wavefunction-based (SCS-ADC(2)/cc-pVDZ) methods, respectively (Figure ); indeed, we have previously demonstrated DFT methods are not suitable for accurately modelling the excited states of MR-TADF compounds and so a higher level of theory is required. From Figure , the highest occupied molecular orbitals (HOMO) is distributed over the entire pconjugated system and the lowest unoccupied molecular orbital (LUMO) is mostly localized on the DiKTa fragment, similar to that calculated for DiKTa (Figure ). The calculated HOMO and LUMO levels are -5.95 and -2.03 eV, respectively, which are destabilized compared to those of DiKTa (HOMO: -6.20 eV and LUMO: -2.23 eV) and stabilized compared to those of DOBNA (HOMO: -5.68 eV and LUMO: -1.71 eV). In the S1 and T1 excited states, the electron density is localized on the DiKTa moiety (Figure ) and the corresponding difference density plots also reveal the expected alternating increasing and decreasing electron density pattern that is characteristic of excited states of short-range charge transfer (SRCT) character. The calculated S1 and T1 energies of DOBDiKTa are 3.46 and 3.27 eV, respectively, which are almost the same as those of DiKTa (S1 = 3.46 eV and T1 = 3.20 eV, Figure ) and a modestly stabilized to those of DOBANA of (S1 = 3.65 eV and T1 = 3.44 eV, Figure ), indicating that a similar emission energy to DiKTa is expected. The DEST of DOBDiKTa is 0.18 eV, which is smaller than both of DiKTa (0.26 eV) and DOBNA (0.20 eV). The oscillator strength for the S0-S1 transition in DOBDiKTa is larger at 0.30 than in both DiKTa (0.20) and DOBNA (0.17), implying a higher FPL in the former. DOBDiKTa shows a small structural relaxation at the optimized singlet excited state geometry compared to the ground state geometry (Figure ), which suggests that this compound should exhibit very narrowband emission. The spin-density distribution (SDD) of the T1 state of DOBDiKTa is localized on the DiKTa fragment. The benefit of the carbonyl groups in DOBDiKTa is evidenced by the much large SOC matrix elements (SOCME) of 1.64 cm -1 compared with DOBNA of 0.06 cm -1 (SOCME of DiKTa is 1.83 cm -1 ) between S1 and T1 (Figure )。 There are additionally five intermediate triplet states that lie between S1 and T1. The SOCME values range from 1.88 to 4.97 cm -1 , which are of intermediate values to those of DiKTa (<S1|ĤSOC|T2> = 6.32 cm -1 , <S1|ĤSOC|T3> = 7.77 cm -1 , <S1|ĤSOC|T4> = 7.59 cm -1 ) and DOBNA (<S1|ĤSOC|T2> = 0.45 cm -1 and <S1|ĤSOC|T3> = 0.08 cm -1 ), and point to a RISC mechanism via spin-vibronic coupling involving one or more of these states. The energies of the frontier molecular orbitals were inferred from the electrochemistry, measured by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in degassed DMF. The electrochemical data reported versus SCE are summarised in Table . As shown in Figure , there is an irreversible oxidation and a quasi-reversible reduction wave, with associated peak potentials from the DPV of 1.47 and -1.50 eV, respectively. The corresponding HOMO and LUMO levels are -5.78 and -2.80 eV, respectively, leading to an intermediate HOMO-LUMO gap of 2.98 eV compared to those tBuBONA (3.00 eV) and tBuDiKTa (2.85 eV). The room temperature UV-visible absorption, steady-state photoluminescence (PL) and 77 K phosphorescence spectra of DOBDiKTa in dilute toluene solution are depicted in Figure . The absorption spectrum possesses three distinguished peaks at 335, 374 and 429 nm (Figure ). The absorption band at 300-350 nm is assigned to p-p * transitions over the whole skeleton. The absorption band at 374 nm is assigned to the SRCT transition localized on the tBuDOBNA unit of DOBDiKTa given its similar energy to that found the absorption spectrum of tBuDOBNA (383 nm) as shown in Figure . The lowest energy absorption centered at 429 nm is the SRCT transition on the DiKTa fragment (Figure ), according to the similar energy of this band to that of DiKTa (433 nm). Thus, though the two MR-TADF fragments of tBuBONA and DiKTa are annulated in DOBDiKTa, in the ground state absorption spectra, they act essentially as independent chromophores. The steady-state PL of DOBDiKTa in toluene evidences a narrowband blue emission, with lPL of 449 nm and a full width at half maximum (FWHM) of 26 nm. This emission is in between that of tBuDOBNA (397 nm) and DiKTa (451 nm). The mirrored absorption and emission spectra, the small Stokes shift of 20 nm (0.12 eV, shown in Figure ) and small FWHM all reflect the small degree of geometric reorganization in the excited state, which is confirmed by the small configuration difference between S1 and S0 shown in Figure . The change in PL as a function of solvent polarity (Figure ) reflects a modest positive solvatochromism that is consistent with an emissive excited state of SRCT character that is a hallmark of MR-TADF emitters. The energies of the S1 and T1 states were determined from the onsets of the prompt fluorescence and delayed emission in toluene glass at 77 K (Figure ). The calculated S1 and T1 energies are 2.87 and 2.66 eV, respectively, revealing a modest DEST of 0.21 eV, which is in between those of DiKTa (0.22 eV) and tBuDOBNA (0.20 eV) (Figure ). Steady-state and time-resolved PL in both aerated and degassed in toluene are shown in Figures and. The PL spectrum is somewhat quenched in air with the photoluminescence quantum yield, FPL, decreasing from 48% to 29%.