Aggregation‐induced narrowband isomeric fluorophores for ultraviolet nondoped OLEDs by engineering multiple nonbonding interactions

Traditional donor‐acceptor type organic luminescent materials usually suffer from unfavorable spectral broadening and fluorescence quenching problems arising from strong inter/intra‐chromophore interactions in aggregation state. Here, two ultraviolet carbazole‐pyrimidine isomers (named o‐DCz‐Pm and m‐DCz‐Pm) with novel aggregation‐induced narrowband phenomenon are constructed and systematic investigated by experiments and theoretical simulations. Benefitting from strengthened steric hindrance and multiple noncovalent interactions, the nonradiative decay, vibrational motion, and structural relaxation of singlet state can be effectively suppressed in aggregation state. Consequently, the electroluminescence peak of 397 nm, full width at half maximum of 21 nm and external quantum efficiency of 3.4% are achieved simultaneously in nondoped o‐DCz‐Pm‐based device. This work paves an avenue toward the development of high‐performance narrowband nondoped ultraviolet materials and organic light‐emitting diodes.

8][9][10][11][12][13] This is undesirable as it raises the efficiency loss of OLED displays seriously.Thus, the development of efficient narrow emissive materials is an important and challenging task for wide-color gamut displays.
[16][17][18] Recently, represented by polycyclic boron-nitrogen (B-N) frameworks with covalent binding method, rigid heteroatom-doped aromatic emitters featuring the multiple resonance (MR) effect have been well developed due to their extraordinary narrowband emission character with the small FWHMs (≤30 nm).The distinct MR effect makes the strategic separation of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) be distributed in the individual atoms, minimizing the structural relaxation and vibronic coupling between the ground (S 0 ) state and S 1 state, which enables a narrowband emission.The serious efficiency roll-off in the device originating from triplet-related exciton quenching processes and complicated device fabrication involving two-or triple-source co-doping method limit their applications.Of note, it is an equally very important strategy for realizing organic emitters with narrow emission bandwidths to suppress the intrinsic vibronic coupling and structural relaxation of the S 1 state through the flexible regulation of nonbonding interactions.For example, Yang et al. reported a deep-blue donor-acceptor (D-A) type triphenyl-substituted imidazole derivative N,N-diphenyl-4′- (1,4,5-triphenyl-1H-imidazol-2-yl)-[1,1′-biphenyl]-4-amine (TPA-PIM) by suppressing vibration splitting using twisted molecular structure with bulky substituents, exhibiting a narrow EL emission at 420 nm with a FWHM of 35 nm. [21]Fleetham and coworkers reported a series of efficient blue OLEDs based on the tetradentate platinum complexes, showing the emission peaks of 448-454 nm and the narrow FMHMs of 24-49 nm owing to the confinement of vibrational coupling by the steric effect of rigid ligands. [22]Wang et al. designed a sandwich-type molecule 2,12-bis(9-(5,9-dioxa-13bboranaphtho[3,2,1-de]anthracen-7-yl)-3,6-di-tert-butyl-9Hcarbazol-1-yl)benzo [5,6] [1,4]oxazino [2,3,4-kl]phenoxazine (BNB-m) with a FWHM of down to 0.28 eV by reducing intramolecular motions and enhancing through-space electronic coupling, resulting from the presence of strong noncovalent π-stacking interactions. [23]Rault-Berthelot et al. reported an emitter dispiroxanthene-indenofluorene (DSX-IF) with narrow EL spectrum with the FWHM of 40 nm by using the highly rigid spiro structure to suppress excimer formation. [24]Nevertheless, most of the molecules in early reports shows preferred narrowband emission in the host-guest doping OLEDs instead of nondoped OLEDs, likely due to the strong intermolecular aggregation arising from the rigid and planar core structures.Meanwhile, high efficiency and narrowband blue, green, and red OLEDs have been widely studied, narrowband ultraviolet-emitting molecules with the maximum emission peaks small than 400 nm are still far behind. [27]Indeed, it is urgent to propose corresponding strategy to realize nondoped ultraviolet emitters and devices with narrowband emission.
Different from traditional donor-acceptor type fluorophores with broad FWHM and redshifted emission peak in aggregation state, we herein reported a novel aggregation-induced narrowband phenomenon based on carbazole-pyrimidine (Cz-Py) isomeric emitters (o-DCz-Pm and m-DCz-Pm) with ortho and meta substitution patterns of carbazole about the 5-phenylpyrimidine acceptor.A systematic structure-property relationship about isomeric and aggregation effects has been investigated by experimental and theoretical methods.The conflicting issues among wide bandgap, color purity and intramolecular CT effect have been well addressed by strengthened steric hindrance effect and multiple noncovalent interactions in aggregation state.As a result, using ortho-substituted o-DCz-Pm as pure emissive layer, the nondoped device not only emits the stable ultraviolet light with an EL peak of 397 nm and FWHM of 21 nm, but also achieves a higher external quantum efficiency (EQE) of 3.4%.

Synthesis and characterization
As shown in Scheme

Photophysical properties
To investigate the influence of different substitution positions on photophysical properties, the ultraviolet-visible absorption (Abs.) and fluorescence (Fluo.)spectra of o-DCz-Pm and m-DCz-Pm were first measured in dichloromethane solution with the concentration of 10 -5 M at room temperature.As shown in Figure 1A and B, both compounds with the E g s of 3.59 eV show similar absorption bands in the range of 220-350 nm where the absorption peak at around 290 nm can be assigned to the n-π* transition of carbazole unit and the absorption band from 320 to 350 nm can be associated with the π-π* transition between the carbazole and adjacent phenyl groups. [28]Interestingly, it was noticed that two compounds exhibit broad and structureless emissions, suggesting their CT state characteristics.The emission peak of o-DCz-Pm is 406 nm with the FWHM of 59.4 nm, showing a distinct blue-shift and spectral narrowing effect compared with that of m-DCz-Pm (418 nm, FWHM: 66.8 nm).In terms of photoluminescence quantum yield (PLQY), o-DCz-Pm   high emission intensity.[31] When increasing the f w from 90% to 99%, the emission intensity of o-DCz-Pm is enhanced, showing an atypical aggregation enhanced emission (AEE) characteristic, [32][33][34][35] which could be due to the formation of stable nanoparticles that prevent the interactions between molecules and water (Figure S4b).As for m-DCz-Pm, even if there is a slight rise at f w of ≈60% comparable to that in THF solution, AEE phenomenon is inconspicuous.Most notably, when adding a large amount of deionized water (f w ≥ 60%), glamorous aggregation-induced narrowband increasing the f w , which is different from m-DCz-Pm and the reported BODIPY derivatives with aggregation-induced emission effect. [36]

Theoretical calculations
As for the optimized S 0 geometries in a vacuum (Figure S5), a notable difference of the dihedral angles is found, where the dihedral angles of 54.67 and ∼62 • between the pyrimidine/carbazole planes and adjacent central benzene ring in o-DCz-Pm, larger than that of m-DCz-Pm (36.03 and 49.97 • ).As depicted in Figure 3, the two isomers show a similar FMOs distributions that the HOMOs are mainly located on the two carbazole units and slightly extended to the central benzene, and the LUMOs are resided on the pyrimidine and central benzene, resulting in the limited FMOs overlap.o-DCz-Pm with the small average changes in bond length (0.01104 Å) and torsional angle (6.75 • ) shows a much smaller root-mean squared displacement (RMSD) of 0.2384 Å than that of m-DCz-Pm (RMSD = 0.6645 Å), illustrating that the geometric change between S 0 and S 1 states in o-DCz-Pm is smaller than that in m-DCz-Pm.It can be seen from the potential energy surface that there exists a much larger energy barrier in o-DCz-Pm than in m-DCz-Pm for torsional motions between the donor/acceptor and central benzene (Figure S6).Furthermore, the reduced density gradient (RDG) analysis of the optimized S 0 geometries reveals that the presence of more obvious intramolecular noncovalent interactions and large steric hindrance effect (corresponding to the larger green and brown regions and larger spikes (red dashed circles)) in the ortho-substituted o-DCz-Pm (Figure S7a and b).These results signify that the structural variations of o-DCz-Pm molecule from S 0 to S 1 can be more effectively suppressed compared to m-DCz-Pm with the meta-substitution pattern, which could benefit to improving PLQY and narrowing emission spectra, being quite consistent with the photophysical data.
The NTOs analysis of singlet and triplet excited states in a vacuum are displayed in Figure S8 and S9 and Table S1 and S2.The transitions from S 0 to S 1 of the two isomers are dominated by the CT state with the accounts of 84.47% and 77.24%, while the S 0 →T 1 transitions show LE-dominated feature (98.09%) for o-DCz-Pm, and hybrid CT and LE feature for m-DCz-Pm, respectively.Interestingly, due to the overlap of NTOs on central benzene and intramolecular noncovalent interaction, the CT component in S 1 state are combined with through-bond charge transfer (TBCT) and through-space charge transfer (TSCT) effects where the proportions of TBCT/TSCT are 29.59%/70.40%for o-DCz-Pm, and 54.30%/45.69%for m-DCz-Pm, respectively, which is similar to reported multi-(donor/acceptor) emitters. [39]n order to make further elucidation of mechanism of AINB effect, theoretical studies on aggregation state were performed.Molecular dynamics simulations for aggregates were carried out by the GROMACS 2018.8 program. [40,41][44] Detailed steps are given in the Supporting Information.The representative structures of aggregation state, the QM/MM model where the center molecule and 7 Å outside the center molecules were selected to construct a system, and cluster analysis for the last 10 ns of the simulation process are shown in Figure S10.By comparison with the geometric changes in a vacuum, the average bondlength changes from S 0 to S 1 state are decreased to 0. the amorphous film (Figure 3 and S5).The more limited structural variation between S 0 and S 1 in rigid environments (aggregation state) is beneficial for hindering the nonradiative energy consumption path and narrowing the emission spectrum, which agrees well with the AEE and AINB effects of o-DCz-Pm.Intriguingly, as shown in Figure S11 and S12 and Table S3 and S4, the CT component in S 1 state is reduced to 75.84% for o-DCz-Pm in aggregation state, where the TBCT and TSCT proportions are varied to 69.09% and 30.90% dramatically.However, the CT component in S 1 state for m-DCz-Pm is increased to 88.82%, while its TBCT/TSCT proportions have a slight change.This should be one of the main reasons for the low k nr rate of o-DCz-Pm neat film.
Meanwhile, for illustrating the interrelationship between geometrical structures and spectral narrowing effect, the reorganization energies (E reo , E I + E II ) and Huang-Rhys (HR) factors in the vacuum and amorphous solid-state phase were further investigated (Figure 4 and S13-S17).Due to the smaller structural variations between S 0 and S 1 in amorphous state, the values of E reo for o-DCz-Pm and m-DCz-Pm are reduced to 0.509 and 0.549 eV, respectively.After the vertical transition from S 0 to S 1 , the structural relaxations of S 1 state are restrained obviously in amorphous film compared with that in a vacuum, and except for a few vibration modes with small HR factors, all the twisting vibration (30, 23 cm −1 ) and rotation vibration (106 and 6 cm −1 ) in low-frequency mode for o-DCz-Pm and m-DCz-Pm, and high-frequency stretching vibrations (1650 and 1658 cm −1 for o-DCz-Pm) are effectively suppressed (Figure 4 and S16, S17), which endows the values of E II down to 0.042 and 0.250 eV in amorphous state.The existence of stretching vibration (1664 cm −1 ) in high-frequency mode for m-DCz-Pm contributes an additional value to E II , leading to broaden emission compared with o-DCz-Pm.These results illustrate that the formation of AINB effect can be attributed to the limited structural relaxation of S 1 state and the small vibronic coupling.
To reveal the aggregation structure, single crystal of o-DCz-Pm and m-DCz-Pm was obtained through slow solvent evaporation in the dichloromethane/acetone mixture and confirmed by single-crystal X-ray analysis.The related crystal parameters have been submitted to the Cambridge Crystallographic Data Centre (numbers: 2174440 and 2261528) and listed in Table S5.As given in Figure 5 and S18, o-DCz-Pm displays a highly twisted molecular conformation with the dihedral angles larger than 70 • , resulting from the steric hindrance effect caused by the nearby substituted-positions of donor and acceptor.There exists the intramolecular throughspace π-π interactions between the pyrimidine and carbazole units with close distance of 3.095-3.318Å, and multiple intramolecular C-H⋅⋅⋅π interactions with the distances of 2.821-2.895[47] What's more, there shows the obvious intermolecular interactions and large steric hindrance (corresponding to the green and brown region of RDG analysis in Figure 5B) caused by multiple C-H⋅⋅⋅π and C-H⋅⋅⋅N interactions with the distances of 2.792-2.898Å.Meanwhile, there also exists multiple intermolecular interactions (2.551-3.288Å) in m-DCz-Pm single crystal (Figure S19), matching well with the RDG analysis of the optimized S 0 geometry.This can be beneficial for suppressing molecular vibration and structural relaxation, realizing spectral narrowing in aggregation state.
In view of their excellent AINB effects, we further evaluated the EL performance with o-DCz-Pm and m-DCz-Pm as nondoped emissive layers.The optimized device  S6.
As expected, both devices A and B emit near-ultraviolet light with the EL peaks of 397 and 392 nm, respectively.With the increase of applied voltages from 5 to 8 V, EL spectra are steady, illustrating that the o-DCz-Pm and m-DCz-Pm have excellent spectral stability (Figure S21).More significantly, o-DCz-Pm-based device A shows wonderful color purity with a narrowband FWHM of 21 nm, while m-DCz-Pm-based device B displays a relatively wide FWHM of 40 nm, which is coincident with photophysical properties and theoretical calculations.Due to the low k nr and high PLQY in o-DCz-Pm neat film, device A achieves a better EL efficiencies of 3.4%, 0.9 cd A -1 , and 0.7 lm W -1 for EQE, current efficiency, and power efficiency, respectively.The EQE of device A is as 5.67 times as that of device B. This result is one of the best for reported organic ultraviolet-emitting emitters involved in nondoped OLEDs with narrowband emission and EL peaks smaller than 400 nm.The efficiency roll-off at high current density should be attributed to the imbalanced hole/electron mobilities (Figure S23).In addition, the doped OLEDs with DPEPO as host were also fabricated.
As shown in Figure S24 and summarized in Table S6.Compared with the nondoped OLEDs, the EL peaks of o-DCz-Pm-and m-DCz-Pm-based doped devices (C and D) have a slight change, but their FWHMs have a significant increase, showing the values of 38 and 44 nm, respectively.As for o-DCz-Pm emitter, the EQE of nondoped device A is also higher than that of doped device.These results further illustrate the AINB-active emitters have great potential applications in simple and nondoped OLEDs with high color purity.

CONCLUSION
In summary, a novel aggregation-induced narrowband phenomenon was found in the Cz-Py isomeric emitters (o-DCz-Pm and m-DCz-Pm) with different substitution patterns.Theoretical and experimental investigations reveal that the structural relaxation of S 1 state and vibrational coupling strength can be effectively suppressed in aggregation state, realizing the reduction of k nr and spectral narrowing, resulting from steric hindrance effect and multiple intramolecular and intermolecular noncovalent interactions.In comparison to m-DCz-Pm, o-DCz-Pm with an ortho substitution pattern exhibits a narrower emission with the FWHM of 21 nm, and a higher EQE of 3.4% in the nondoped devices, which is related to the large steric hindrance, small recombination energy, low k nr and high PLQY of o-DCz-Pm in aggregation state.This work provides a promising way to address the aggregation caused quenching and spectral broadening issues in the D-A type organic emitters.

A C K N O W L E D G M E N T S
This work was supported by grants from the National Natural Science Foundation of China (grant numbers: 52002804, 52103220, 52103017, and 22022501), Shandong Provincial Natural Science Foundation (grant numbers: ZR2023QE078, ZR2022ZD37, and ZR2019ZD50) and Natural Science Foundation of Qingdao (grant number: 23-2-1-75-zyyd-jch).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interests.

S C H E M E 1 F I G U R E 1
Molecular structures and synthetic routes to o-DCz-Pm and m-DCz-Pm.(i) Pd(PPh 3 ) 4 , 2 M K 2 CO 3 (aq), toluene: CH 3 CH 2 OH = 2:1, 110 • C, 26 h; (ii) Cs 2 CO 3 , DMF, 155 • C, 16 h.Normalized absorption and PL spectra of o-DCz-Pm (A) and m-DCz-Pm (B) in dichloromethane solution.Concentration: 10 μM.Normalized absorption and PL spectra (C) and transient PL decay spectra (D) of o-DCz-Pm and m-DCz-Pm neat films.shows a relatively higher PLQY of 17% than m-DCz-Pm (9%).Meanwhile, the solvatochromic effect was performed in dilute solutions (10 μM) with different polarity solvents (FigureS3).As the solvent polarity increases from nonpolar hexane to polar acetonitrile, two compounds show insignificant change in absorption spectra, indicating that the ground state is insensitive to solvent polarity.Whereas the emission peaks exhibit significant bathochromic shift from 374 to 415 nm for o-DCz-Pm and from 368 to 444 nm for m-DCz-Pm, confirming the CT characters of both compounds in excited state.Furthermore, the S 1 and first triplet (T 1 ) energy levels are determined by the edges of the low temperature fluorescence and phosphorescence spectra at 77 K, which are 3.47 and 3.06 eV for o-DCz-Pm, 3.53 and 2.97 eV for m-DCz-Pm, respectively.The singlet-triplet energy offsets (ΔE ST s) between S 1 and T 1 state can be calculated to be 0.41 and 0.56 eV.Such large values illustrate that the thermal activated delayed fluorescence mechanism can be excluded, which is also proved by the single-exponential fluorescence lifetimes of 6.9 and 5.0 ns for o-DCz-Pm and m-DCz-Pm (FigureS4a).The photophysical properties of o-DCz-Pm and m-DCz-Pm neat films were further studied.As shown in Figure1C,D, absorption spectra of them exhibit similar features to that in dichloromethane solution, while the photoluminescence (PL) spectra exhibit the desired ultraviolet emission with peaks of 399 and 393 nm for o-DCz-Pm and m-DCz-Pm.Curiously, o-DCz-Pm film shows the remarkable vibrational TA B L E 1 Summary of photophysical, thermal and electrochemical properties of o-DCz-Pm and m-DCz-Pm.
Abbreviations: HOMO, highest occupied molecular orbitals; LUMO, lowest unoccupied molecular orbitals.a Measured in dichloromethane solution (10 μM)/ neat film at 300 K; .b Estimated from the edges of fluorescence and phosphorescence spectra in dichloromethane solution (10−5 M) at 77 K; .c HOMO: obtained from CV measurement; Eg: calculated from the absorption onset; LUMO: calculated by ELUMO = EHOMO + Eg; .d Absolute PLQY.e Calculated by the equations of  PL = k r k r +k nr and  = 1 k r +k nr in solution and neat film.
(AINB) phenomenon is generated in o-DCz-Pm and m-DCz-Pm, where the FWHMs are narrowed gradually from 55.2 to 33.0 nm for o-DCz-Pm, and from 63.8 to 48.8 nm for m-DCz-Pm, respectively, along with the increase of f w (Figure 2D).Interestingly, aggregation-induced LE state emission of o-DCz-Pm is obviously activated where the fine vibrational structure peaks gradually appears with F I G U R E 3 The energies and electron distributions of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs), and root mean squared deviations (RMSDs) of the optimized structures in the S 0 and S 1 states of o-DCz-Pm and m-DCz-Pm in vacuum and amorphous film from DFT and TD-DFT calculations at the B3LYP-D3(BJ)/6-31G(d,p) level (isovalue = 0.02).
00953 and 0.01164 Å for o-DCz-Pm and m-DCz-Pm, resulting in a much lower RMSD values (0.1316 and 0.1170 Å) in F I G U R E 4 The optimized S 0 and S 1 structures and reorganization energies (A and D), calculated Huang-Rhys factors (B and E), and reorganization energies (E II ) (C and F) of S 1 vibrational relaxation in a vacuum (A, B, and C) and amorphous state (D, E, and F).Vibration modes with large contribution to reorganization energies are shown as insets.

-DCz-Pm and m-DCz-Pm with
1, the two new emitters o