Multiple Resonance Deep-red Emitters with Hybridized π-bonding/ non-bonding Orbitals to Surpass the Energy Gap Law

Ecient organic emitters in the deep-red to near infrared region are rare due to the ‘energy gap law’. Here, multiple boron (B)- and nitrogen (N)-atoms embedded polycyclic heteroaromatics featuring hybridized π-bonding/ non-bonding molecular orbitals are constructed, providing a way to overcome the above luminescent boundary. The introduction of B-phenyl-B and N-phenyl-N structures enhances the electronic coupling of those para-positioned atoms, forming restricted π-bonds on the phenyl-core for delocalized excited states and thus a narrow energy gap. The mutually ortho-positioned B- and N-atoms also induce a multiple resonance effect on the peripheral skeleton for the non-bonding orbitals, creating shallow potential energy surfaces to eliminate the high-frequency vibrational quenching. The corresponding deep-red emitters with peaks at 662 nm and 692 nm exhibit narrow full-width at half-maximums of 38 nm, high radiative decay rates of ~10 8 s -1 , ~100% photo-luminance quantum yields and record-high maximum external quantum eciencies of >28% in a normal planar organic light-emitting diode structure, simultaneously. and R-TBN in toluene. c, Transient PL decay spectra of R-BN/R-TBN doped into CBP lms (3 wt%) at room temperature. d, Transient absorption (TPA) spectra of R-BN (d) and R-TBN (e) doped in PMMA lms (3 wt%) at room temperature. by the gap law’. B-phenyl-B and N-phenyl-N structures ortho-positioned B and N atoms, hybridized π-bonding/ nonbonding molecular orbitals are recorded, narrowing energy gap for DR/NIR emission by the delocalized excited states but also nonradiative transitions by suppressing vibration coupling due to the shallow potential energy curve induced by MR effect. Deep red emitters showed high PLQYs of 100±2% and maximum EQE of >28% with narrow bandwidth emission spectra. This here provides a strategic implementation multiple in polycyclic heteroaromatics, showing viable potential to generate DR/NIR transitions.


Introduction
The energy gap law 1,2 , recognized as the fact that nonradiative transitions will signi cantly increase with the decreased energy gap, has created a formidable barrier to produce deep-red (DR)/ near-infrared (NIR) organic emitters with high photo-luminance quantum yields (PLQYs), despite their great demand for applications in night vision displays, biomedical imaging, optical communications and computing 3-7 . This effect is strong even for rigid systems, as was observed in the nonradiative deactivation of polycyclic aromatic hydrocarbons (PAHs) 8 . In those large conjugation PAH structures, the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO, respectively) are primarily localized between atoms, forming π-bonds with bonding/antibonding character. The resulted interactions between the electronic and nuclear vibrational motion will induce signi cant C=C torsion or C-H vibration with high frequency, forming deep potential energy surfaces (PES) as illustrated in Fig. 1a. In the absence of a zero-order surface crossing, such deep PES will facilitate the wavefunction overlap of the zerovibration level (v 0 ) of the excited state (singlet or triplet, that is S 1 or T 1 ) and the high isoenergetic vibration level (v n ) of the ground state (S 0 ), consequently relaxing the excited states in a nonradiative decay pathway. For DR/NIR emitters with emission peaks >650 nm, the small energy gap between v 0 s (ΔEv 0-0 ) of S 1 (or T 1 ) and S 0 will exponentially accelerate this nonradiative transitions since only a few vibrational ladders in S 0 are required to satisfy the energy requirement for vibrionic coupling with v 0 of excited states, drastically reducing the emission intensity 9,10 .. processes of PAHs with π-bonding and deep PES. In this type emitters, only a few vibrational ladders in S 0 are required to satisfy the energy requirement for vibrionic coupling with v 0 of S 1 , drastically increasing nonradiative transitions and reducing the emission intensity. b, Scheme of excited states decay processes of MR TADF emitters with non-bonding and shallow PES. In this type emitters, owing to the reduced vibrational frequency, it is hard for vibrational ladders in S 0 to reach the energy requirement for vibrionic coupling with v 0 of S 1 , thus bene ting to eliminate nonradiative transitions.
Breakthroughs to surpass energy gap law have been made in planar platinum (Pt) complexes with dimers or oligomers to increase the exciton delocalization length, which can decouple the exciton band from high vibrational ladders in S 0 state and thus suppress nonradiative transitions 11,12 . This conceptually advancements in molecular design leads to NIR-emitters with impressive PLQYs of over 80% under emission peaks of 740 nm and high performance NIR organic light-emitting diodes (OLEDs). Recently, DR/NIR thermally activated delayed uorescence (TADF) emitters with PLQYs >80% and peaks at ~650 nm have also been reported [13][14][15][16] . Determined by the onset of their wide emission spectra, the relatively larger ΔEv 0-0 s of TADF emitters than PAHs with similar emission peaks are bene cial for suppressing the nonradiative transitions, though at the cost of the color purity. On the other hand, the charge transfer features of TADF emitters would always lead to large reorganization energies (λs) due to the large structure relaxation and thus much smaller radiative decay rates (k r s) than that of PAHs. Therefore, it remains challenging to develop high color purity DR/NIR emitters with PLQYs up to 100%.
Quite recently, a new class of boron (B)-and nitrogen (N)-atoms embedded PAHs have been reported by Hatakeyama et al, in which the ortho-positioned electron donating N-atom and electron de cient B-atom induce complementary multiple resonance (MR) effects for the offset electron density distributions of HOMO and LUMO orbitals by one atom in an alternating pattern [17][18][19][20][21][22][23][24][25][26] . Such MR emitters not only inherit the merits of small λs of PAHs for high k r s, but also minimizes the bonding/antibonding character between adjacent atoms. The so-called non-bonding character could signi cantly lower the vibration frequency in the molecules, leading to shallow PES of S 0 and S 1 states. In this regard, emitters with sharp photo-luminescence (PL) and electro-luminescence (EL) emissions can be expected. From another perspective, as illustrated in Fig.1b, for such shallow PES, more vibrational ladders in S 0 are required to reach the vibrionic coupling with v 0 of S 1 state, which is almost prohibited owing to their non-bonding characters, even for DR/NIR emitters. As a result, the high-frequency vibrational quenching can be theoretically eliminated in well-designed MR emitters, providing a way to surpass the energy gap law (Supplementary Fig. 1 and Supplementary Tables 1-2). Another bonus is that the alternated distributions of HOMO and LUMO render small energy gaps (ΔE ST s) between S 1 and T 1 levels in these compounds and thus e cient TADF emissions.

Results
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 spectra 22 . To ful ll 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 position 27,28 . The resulted delocalized excited states will signi cantly 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 orthopositioned 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-  To evaluate the properties of TADF, PL properties of 3 wt% R-BN/R-TBN doped lms with 4,4'-di(9Hcarbazol-9-yl)-1,1'-biphenyl (CBP) as the wide-energy gap host were measured. High PLQYs of unity were maintained for both lms though their signi cantly 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 uorescence components were clearly identi ed, which can be unambiguously assigned to the TADF emission. The quantum yields (Φ   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) 2 tmd: 3wt% R-BN/R-TBN (30 nm)/ 4,6-bis(3-(9H-carbazol-9yl)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. Electroluminance 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 CIE x here outperforms all reported values of DR TADF emitters with even red-shifted emission peaks bene ting from their narrow emission bandwidth 4,[13][14][15][16][32][33][34][35][36][37][38] . For deep red emitters, the radiance is also an important parameter to evaluate their brightness.
Maximum radiance of 6.5 × 10 5 mW sr −1 m −2 for R-BN and 7.3 × 10 5 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 nm 4,[13][14][15][16][32][33][34][35][36][37][38] . Those state-of-the-art performances obtained here testify the great potential of the molecular design strategy. The probe was generated by home-built broadband visible (500-770 nm), pumped by the frequencydoubled output (400 nm) of the Ti:sapphire laser. The delay between the pump and probe pulses was varied using a Stanford DG645 delay generator for the nanosecond measurements, whereas a mechanical delay stage (Thorlabs DDS300-E/M) was used to delay the probe with respect to the pump for the picosecond measurements. The transmitted probe pulses were collected with a silicon dual-line array detector (Hamamatsu S8381-1024Q), which was driven and read out by a custom-built board (Stresing Entwicklungsbüro).

Discussion
Electrochemical measurements. Cyclic voltammetry were performed on a CHI 660 instrument, using a platinum (Pt) electrode as the working electrode, a Pt wire as the auxiliary electrode and an Ag/Ag + electrode as the reference electrode. The oxidation/reduction potentials were measured in dry dichloromethane/DMF solutions with 0.1 M of TBAPF 6 (tetrabutylammonium hexa uorophosphate) as a supporting electrolyte at a scan rate of 100 mV s -1 .  Figure 1 Relationship between PES and excited states decay processes. a, Scheme of excited states decay processes of PAHs with π-bonding and deep PES. In this type emitters, only a few vibrational ladders in S0 are required to satisfy the energy requirement for vibrionic coupling with v0 of S1, drastically increasing nonradiative transitions and reducing the emission intensity. b, Scheme of excited states decay processes of MR TADF emitters with non-bonding and shallow PES. In this type emitters, owing to the reduced vibrational frequency, it is hard for vibrational ladders in S0 to reach the energy requirement for vibrionic coupling with v0 of S1, thus bene ting to eliminate nonradiative transitions.