For organic emitters, extending the π-conjugation length is an effective way to redshift the emission. The non-bonding characters in MR-TADF emitters, however, would limit the increasement of conjugation even with enlarged planar structures, making it challenging for DR/NIR emission.17,20 Enhancing the CT character in MR-TADF emitters is an alternative way to achieve redshifted emissions, which however, would weaken the MR effect and thus broadening the emission spectra22. To fulfill the requirement of a small energy gap and a narrow emission bandwidth simultaneously, a multiple boron (B) and nitrogen (N) atoms embedded polycyclic heteroaromatic motif was proposed here, enabling multiple B and N centers and the modulation of their arrangement around a central phenyl ring. As illustrated in Fig.2a, besides the mutually ortho-positioned B and N atoms, which is the basic requirement for MR effect, linear B-phenyl-B and N-phenyl-N structures were adopted. Previous works have demonstrated the formation of intramolecular dimeric radical between donors (or acceptors) in para positions due to the enhanced electronic coupling between a cation (or an anion) and a neutral moiety in linearly position27,28. The resulted delocalized excited states will significantly narrow energy gap and thus facilitating red-shifted emission. The distributions of frontier energy levels were calculated and it was interesting to note that on the central phenyl ring, both HOMO and LUMO showed π-bond characters (Fig.2b), which arises from the coupling of electrons on para-positioned N atoms or B atoms as described above. The mutually ortho-positioned B and N atoms, meanwhile, induce MR effect on the peripheral skeleton, leading to the localization and separation of the HOMO and LUMO on different atoms. These non-bonding characters facilitate to lower vibration frequency for shallow PES in molecules as aforementioned and thus eliminate nonradiative transitions. The targeted emitters with hybridized π-bonding/ nonbonding molecular orbitals thereof possess the potential to fundamentally overcome the luminescent boundary set by the energy gap law. The energy gaps of the two targeted emitters were predicted by the time-dependent density functional theory (TD-DFT) calculations, obtaining small S1 values of 2.01 eV and 2.07 eV for R-BN and R-TBN, respectively (Supplementary Tables 3).
Fig. 2 | Molecular design strategy of DR/NIR MR-TADF emitters. a, Scheme of the formation of delocalized excited states with B-phenyl-B and N-phenyl-N structures. b, The comparison between conventional PAH, MR-TADF emitters with and without B-phenyl-B and N-phenyl-N structures. For conventional PAH, taking 3,8,13,18-tetraphenylbenzo[5,6]indeno[1,2,3-cd]benzo[5,6]indeno[1,2,3-lm]perylene (DBP) as an example, π-bonds molecular orbitals can be observed. For MR-TADF emitters without B-phenyl-B or N-phenyl-N structures, non-bonding orbitals are obtained.
Additionally, the geometric changes of molecular conformation between S0 and S1 states and structural relaxation at the S1 state are extremely small with reorganization (λS) /structural relaxation (λS*) energies were calculated to be merely 0.10 and 0.09 eV, 0.08 and 0.08 eV for R-BN and R-TBN, respectively (Supplementary Figs. 2-4), significantly smaller than typical DR/NIR organic molecules (0.26-0.52 eV)12,29. Those extremely small λS and λS* facilitate narrow bandwidth emission with high radiative transitions and thigh oscillator strength (f) values of 0.25 and 0.33 for R-BN and R-TBN were obtained, respectively. Moreover, similar reference material with multiple B, N atoms but not linear N-phenyl-N and B-phenyl-B structure was also calculated and only nonbonding characters as well as a large S1 energy of 2.9 eV were obtained, suggesting the N-phenyl-N and B-phenyl-B structure motif is essential for the formation of delocalized excited states. (Fig.2b)26
Fig. 3 | Structure and photophysical properties of R-BN and R-TBN. a, ORTEP drawing of R-BN (up) and R-TBN (down) obtained by X-ray crystallographic analysis. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity. b, UV/Vis absorption and fluorescence (298 K) spectra of R-BN and R-TBN in toluene. c, Transient PL decay spectra of R-BN/R-TBN doped into CBP films (3 wt%) at room temperature. d, Transient absorption (TPA) spectra of R-BN (d) and R-TBN (e) doped in PMMA films (3 wt%) at room temperature.
R-BN and R-TBN were synthesized in two steps from commercially available starting materials and no palladium or other transition metal catalysts were needed, making them cost effective. The single crystal X-ray diffraction analyses demonstrated that R-BN combines the characteristics of a fusion structure and a sterically hindered structure, where small torsional angles (~20°) of the carbazolyl groups could be found (Fig. 3a and Supplementary Fig. 5). This limited torsional flexibility can be another major factor in reducing the non-radioactive decay of R-BN/R-TBN. Photophysical properties of R-BN and R-TBN in toluene with a concentration of 10-5 M were measured and listed in Table 1. Strong absorption bands with a maximum at 629 nm (log ε= 5.13, where ε is the molar excitation coefficient) and 651 nm (log ε= 4.87) were recorded for R-BN and R-TBN, respectively (Fig. 3b and Supplementary Fig. 6). Arising from the small optical energy gaps of 1.87 eV and 1.79 eV for R-BN and R-TBN, deep red fluorescence emission peaked at 662 nm and 692 nm were observed, evidencing the molecular design strategy above. More importantly, nearly 100% PLQYs were measured for both emitters, suggesting the greatly suppressed nonradiative decay rates. Their small full-width at half-maximums (FWHMs) of both 38 nm reflect the narrow energy bandwidths of both S0 and S1 states as illustrated in Fig. 1b and thus the minimized the vibronic coupling between the S0 and S1 states. Also, small stokes shifts of 33 nm and 41 nm were also exhibited for R-BN and R-TBN, respectively, suggesting the limited vibrational relaxation at the S1 state. All those parameters combined to overcome the limitation of energy gap law as aforementioned. The triplet energies were obtained from phosphorescence spectra recorded in a frozen toluene (77 K) matrix with a delay time of 10 ms, being 1.69 eV and 1.63 eV for R-BN and for R-TBN, respectively (Supplementary Fig. 10). The corresponding ΔEST values were 0.14 eV for R-BN and 0.17 eV for R-TBN. Such small ΔEST values are favorable for exciton up-conversion from T1 to S1 at ambient condition. These characteristics are promising for efficient DR/NIR emitters.
To evaluate the properties of TADF, PL properties of 3 wt% R-BN/R-TBN doped films with 4,4’-di(9H-carbazol-9-yl)-1,1’-biphenyl (CBP) as the wide-energy gap host were measured. High PLQYs of unity were maintained for both films though their significantly redshifted emission peaking at 672 nm and 698 nm for R-BN and R-TBN, respectively, suggesting the strong ability of those molecules in beating the limitation of energy gap law (Supplementary Fig. 11). Also, a relatively larger FWHMs of 48 nm and 49 nm were recorded for R-BN and R-TBN, respectively, partly attributed to the interaction between host and dopant. The TADF characteristics of the R-BN and R-TBN were recorded and depicted in Supplementary Fig. 12. Both prompt and delayed fluorescence components were clearly identified, which can be unambiguously assigned to the TADF emission. The quantum yields (ΦF = 0.82 and ΦDF = 0.18 for R-BN, ΦF = 0.73 and ΦDF = 0.27 for R-TBN) and lifetimes (τF = 4.7 ns and τDF = 0.31 ms for R-BN, τF = 10.3 ns and τDF = 0.71 ms for R-TBN) of the fluorescence and TADF components were determined based on the total PLQY and ratio of the integrated area of each component in the transient spectra to the total integrated area30. Based on these measurements, the rate constants of fluorescence (kr), intersystem crossing (kISC), reverse intersystem crossing (kRISC) and nonradiative transition (knr) were calculated to be 1.7 × 108, 3.9 × 107, 3.9 × 103 s−1, and <3.3× 102 s−1 for R-BN, 7.1 × 107, 2.6 × 107, 1.9 × 103 s−1 and <1.0× 102 s−1 for R-TBN, respectively, using the methodology provided in the literature31. The large kr values are consistent with the large molar excitation coefficient (log ε = 5.13 for R-BN and 4.87 for R-TBN, respectively) and large oscillator strengths.
To gain further insight into the nature of electronic excited states, femtosecond pump-probe transient absorption (TA) measurements for R-BN and R-TBN in doped films (3wt% in poly(methyl methacrylate)) were carried out. As shown in Fig. 3d, e, both TA spectra of R-BN and R-TBN showed broad excited-state absorption (EAS) signals, overlapping with two ground state bleach (GSB) bands as well as a stimulated emission (SE) band. Specifically, for R-TBN in doped film, the SE signal at 735 nm rises rapidly and then decay after 3 ps, along with the 675 nm ESA signal decay quickly, corresponding to the vibrational relaxation process from the high vibration levels of S1 state to the zero vibration level of the S1 state, and then undergo a ultrafast radiative process within 1.8 ns, in line with its high kr and PLQY mentioned above. On the other hand, during the following 0.2-10 ps, the S0→S1 GSB signal around 640 nm exhibited a neglectable peak shift from 641 nm to 645 nm, corresponding to the depopulation of excitons on the vibrational states. This also evidences that both R-BN and R-TBN possess shallow potential energy surface, resulting in the narrow bandwidth emission.
Fig. 4 | OLED performance. a, The energy-level diagrams and the emitter structures of the devices. b, The EL spectra of the optimized DR/NIR devices. c, EQE versus current density characteristics. d, Radiance versus current density characteristics. e, CIE x (left)/EQE (right) summary of DR/NIR TADF-OLEDs with emission peak >650 nm (for references, see Supplementary Table 5).
OLEDs were further constructed to evaluate the performances of those two emitters with the following structures of Indium tin oxide (ITO)/ 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HATCN, 10 nm)/ 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 60 nm)/ tris(4-carbazolyl-9-ylphenyl)amine (TCTA, 10 nm)/ CBP: 30wt% Ir(mphmq)2tmd: 3wt% R-BN/R-TBN (30 nm)/ 4,6-bis(3-(9H-carbazol-9-yl)phenyl)pyrimidine (CzPhPy, 10 nm)/ 4,6-Bis(3,5-di(pyridin-4-yl)phenyl)-2-methylpyrimidine (B4PyMPM, 50 nm)/ LiF (0.5 nm)/ Al (150 nm). And the device energy levels were provided in Fig. 4a. Electro-luminance spectra with peaks at 664 nm and 686 nm were recorded for R-BN and R-TBN based devices with small FWHMs of 48 nm and 49 nm as illustrated in Fig. 4b, leading to CIE coordinates of (0.719, 0.280) and (0.721, 0.278), respectively. The CIEx here outperforms all reported values of DR TADF emitters with even red-shifted emission peaks benefiting from their narrow emission bandwidth4,13-16,32-38. For deep red emitters, the radiance is also an important parameter to evaluate their brightness. Maximum radiance of 6.5 × 105 mW sr−1 m−2 for R-BN and 7.3 × 105 mW sr−1 m−2 for R-TBN devices were recorded as depicted in Fig.4c. Unprecedently high maximum EQEs of 28.4% and 28.1% were observed for R-BN and R-TBN based devices (Fig.4d), respectively. As revealed in Fig. 4e, to the best of our knowledge, those values are the record-high values among all reported results of devices utilizing TADF emitters with peaks >650 nm4,13-16,32-38. Those state-of-the-art performances obtained here testify the great potential of the molecular design strategy.