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Open AccessCCS ChemistryCOMMUNICATION1 Jun 2021

π-Stacked Thermally Activated Delayed Fluorescence Emitters with Alkyl Chain Modulation

    CCS Chem. 2021, 3, 1757–1763
    https://doi.org/10.31635/ccschem.020.202000355

    Molecules bearing separate π-electron donor (D) and acceptor (A) groups that undergo face-to-face D/A interactions have been utilized to develop thermally activated delayed fluorescence (TADF) materials. These π-stacked D/A architectures are constructed on various scaffolds, which have either a long D/A distance or permitted conrotatory motion. Here, we develop a novel spiro-based scaffold with a short D/A distance and restricted circumvolution motions because of both the rigid spiro-scaffold and large rotation hindrance between the nearly coplanar D and A. We append different alkyl chains, which can modulate charge transfer and luminescence properties, at the nitrogen of the D moiety to develop four TADF molecules, which can modulate charge-transfer and luminescence properties. Because of the introduction of the solubilized alkyl chain, these molecules were used to fabricate solution-processed devices, among which a maximum external quantum efficiency of 18.9% was realized. By modulating interactions between the D/A building blocks, these TADF constructs exemplify that the alkyl side chains of TADF molecules, which used to be considered as solubilizing units, have vital impact on the optoelectronic properties and thus offer a new route to the design of solution-processable TADF emitters.

    Introduction

    It is a commonly observed phenomenon in nature that aromatic systems are apt to undergo π-stacking, as a consequence of which electrons are able to delocalize under excitation.1 In the field of optoelectronic materials, a variety of linkers have been used as the scaffold, in which the desired aromatic systems are covalently grafted and fixed within a short distance in a face-to-face manner to favor the π-stacking interactions and charge transfer in excited states.2–4 Such linkers include xanthene,5 anthracene,6 naphthalene,7 [2.2]paracyclophane,8 or o-carborane,9 and so on. (Figure 1a). Recently, these through-space π–π interactions have been employed to construct thermally activated delayed fluorescence (TADF) emitters.10–15 In each of these luminescent molecules, an electron donor (D) and an acceptor (A) are grafted in a nonconjugated manner on to a rigid molecular framework. The frontier molecular orbitals, namely the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), are thus separately located on the D/A units. As a consequence, the singlet–triplet energy gap (ΔEST) is significantly minimized, favoring fast reverse intersystem crossing (RISC).11,12 However, in the case when both D/A blocks are linked through a single bond, conrotatory motion could occur, which leads to nonradiative decay (Figure 1b) of the excited states, lowering photoluminescence quantum yields (PLQYs) of the TADF materials.16,17 Hitherto, reported π-stacked TADF emitters have seldom been used in simple, low-cost solution-processed organic light-emitting diodes (OLEDs), except some π-stacked dendrimers and polymers with through-space charge transfer (TSCT) between D/A units.18,19

    Figure 1

    Figure 1 | Schematic representation of classic molecules. (a) Reported scaffold and our scaffold. (b) Previously reported work about π-stacked TADF emitters. (c) New molecular design strategy revealed in this investigation. (d) Synthetic route of the four molecules investigated.

    In this contribution, we designed and synthesized four small molecules, namely N2-6, N2-8, N3-6, and N3-8, (Figure 1c). Each of these TADF molecules contains a spiro-scaffold based on fluorene. A π-electron donating unit, namely alkylated diphenylamine, is cyclized onto the C9 position of the fluorene by means of spiro annulation. On the C1 position,20 a π-electron accepting unit, namely either 2,4,6-triphenylpyrimidine (N2) or 2,4,6-triphenyl-1,3,5-triazine (N3), is introduced creatively. On the one hand, the D–A distance is rather short, (i.e., <3.60 Å), leading to large steric hindrance that restricts circumvolution motions of both these units. On the other hand, the D/A units orientate in a π-stacking or face-to-face manner, favoring D/A charge transfer.21 In each of the Ds, an alkyl chain is introduced to solubilize the target TADF molecules for spin coating. Interestingly, this N-alkyl chain is not generally used in solution-processable TADF materials because the nitrogen on arylamine Ds is usually occupied by electron As. In our cases, the A is appended at the C1 site of fluorene, making the nitrogen on the D available for further derivation. Furthermore, we found that different alkyl groups, n-hexyl and 2-ethylhexyl, are able to modulate the D/A interactions by tuning the orientation. As a result, all of these four emitters showed good PLQYs and superior TADF properties. These molecules are integrated into the solution-processed OLEDs, among which the device based on N3-8 featuring a highest PLQY of 91% achieved a high external quantum efficiency (EQE) of 18.9%.

    Results and Discussion

    The four molecules, including N2-6, N2-8, N3-6, and N3-8, were synthesized by three steps, which are shown in Figure 1d. First, the intermediates 4 and 5 were prepared through Suzuki–Miyaura reaction. The intermediates 4 and 5 then underwent successive nucleophilic addition and Friedel–Crafts cyclization to form the target spiro molecules. The alkyl chains afford all these four molecules good solubility in common organic solvents, enabling the spin-coating procedure to form the uniform films.

    Diffraction grade single crystals of N2-6, N2-8, N3-6, and N3-8 were obtained, providing unambiguous description for their structures. Both N2-6 and N3-6 contain an n-hexyl, while in N2-8 and N3-8 a 2-ethylhexyl is contained. Each of the n-hexyl chains in N2-6 and N3-6 resides on the π-electron surface of the A units, driven by CH/π interactions between the protons in n-hexyl and the As, N2 and N3 in N2-6 and N3-6, respectively. Given that 2-ethylhexyl is considered as a bulkier unit than n-hexyl,22–24 it was predicted that the latter should introduce less steric hindrance between the D and A. Notably, the D–A distances in the case of N2-8 and N3-8 are observed to be smaller than those in N2-6 and N3-6, where d1, d3, d5, and d7 are observed to be 3.446, 3.413, 3.548, and 3.485 Å, respectively, indicated in Figure 2a. This unpredicted result could be explained by CH/π interactions between the alkyl chains and the A. In the case of N2-6 or N3-6, the occurrence of CH/π interactions is inferred from the corresponding close contacts between the protons in the n-hexyl chain and the D, N2, or N3. Such interactions are relatively weaker in N2-8 and N3-8, because the 2-ethylhexyl is so bulky that only the ethyl side chain undergoes CH/π interactions. The hexyl group in 2-ethylhexyl instead orientates perpendicularly to the A in N2-8 and N3-8. Moreover, the intermolecular distance between the As is less than 3.60 Å, driven by π–π stacking in solid state. The occurrence of intramolecular noncovalent interactions in these four molecules is further confirmed by theoretical calculations. As shown in Figures 2b and Supporting Information Figure S2, all molecules present obvious intramolecular attractive interactions (green region) and larger steric hindrance (brown region) between D and A segments,25 restricting the intramolecular vibrations that prevent nonradioative decay of the excited states. The distributions of HOMO/LUMO are verified by density functional theory calculation results. As shown in Supporting Information Figure S3, in each of these four molecules, HOMO and LUMO are located on D and A, respectively. Because the D and A are well separated in space, the ΔEST is decreased. Besides, natural transition orbital (NTO) simulation of the lowest singlet states (S1) and the lowest triplet states (T1) is also calculated based on the crystal data. For the S1 state of all materials, the hole (red region) and particle (green region) are distributed on D and A part, respectively, proving the charge-transfer nature of fluorescence emission. Compared with N2-6 and N3-6, the A part in N2-8 and N3-8 exhibits obvious torsion because of the introduction of the ethyl group. The proportions of TSCT/through-bond CT (TBCT) could also be characterized by integrating the transition density, and the proportions of TSCT/TBCT in the S1 state of N2-6, N2-8, N3-6, and N3-8, are 95%/5%, 97%/3%, 97%/3%, and 98%/2%, respectively, indicating that the D/A spatial interaction is absolutely dominant in this system.

    Figure 2

    Figure 2 | (a) Crystal structure and (b) the functions of reduced density gradient (RDG) and sign(λ2)ρ for four molecules. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centers supplementary publication no. CCDC-1972748 (N2-6), CCDC-1972704 (N2-8), CCDC-1972788 (N3-6), CCDC-1972710 (N3-8). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre: https://www.ccdc.cam.ac.uk/structures-beta/.

    The photophysical properties of these four molecules were influenced by both the alkyl chains and As, which are summarized in Table 1. The UV–vis absorption and photoluminescence (PL) spectra are shown in Figures 3a–3d. The strong absorption bands observed between 250 and 330 nm are attributed to n–π* and π–π* transitions of the conjugated structure. All the four molecules exhibit broad and featureless fluorescence profiles. The peak wavelengths (λmaxs) are observed to be 461, 470, 485, and 495 nm in the case of N2-6, N2-8, N3-6, and N3-8, respectively. Compared with N3-6 and N3-8, λmax values of N2-6 and N2-8 are observed to be relatively shorter, because the A in the latter two molecules, N2 series, is less electron-withdrawing, compared with N3 series in the former two molecules. The S1 values of N2-6 and N2-8 are therefore higher than those in N3-6 and N3-8. The λmax values of N2-8 and N3-8 are generally larger than those of N2-6 and N3-6, because the former two molecules have the shorter D/A distance and consequently enhanced intramolecular interactions.

    Table 1 | Optical and Thermal Physical Properties of Four Materials
    Emitter λabsa (nm) λflb (nm) λphosc (nm) S1/T1d (eV) ΔESTe (eV) Egf (eV) HOMOg (eV) LUMOh (eV)
    N2-6 269, 310 461 442,469 3.08/2.81 0.27 3.60 −5.13 −1.53
    N2-8 268, 310 470 440,466,501 2.99/2.83 0.16 3.59 −5.14 −1.56
    N3-6 278, 309 485 471 2.97/2.79 0.18 3.59 −5.14 −1.56
    N3-8 277, 312 495 468, 492 2.95/2.81 0.14 3.56 −5.15 −1.59

    aMeasured in dichloromethane solution (10−5 M) at room temperature.

    bMeasured in toluene solution (10−5 M) at room temperature, excitation wavelength 340 nm.

    cMeasured in toluene solution (10−5 M) at 77 K, excitation wavelength 340 nm.

    dCalculated from the onset of fluorescence and lowest vibrational state of phosphorescence spectra.

    eCalculated from S1–T1.

    fCalculated from the onset of absorption spectra.

    gCalculated from CV data.

    hCalculated from HOMO and Eg.

    Figure 3

    Figure 3 | Absorption and fluorescence spectra at room temperature and phosphorescence spectra at 77 K: (a) N2-6, (b) N2-8, (c) N3-6, and (d) N3-8.

    In addition, the fluorescence spectra of these four molecules are highly dependent on the solvent polarities ( Supporting Information Figure S4), a factor having great impact on D–A charge transfer. The fluorescence and phosphorescence spectra at 77 K were also measured ( Supporting Information Figure S5a). The ΔEST values of four molecules are calculated to be 0.27, 0.16, 0.18, and 0.14 eV for N2-6, N2-8, N3-6, and N3-8, respectively. The ΔEST values in the case of N2-6 and N3-6 are larger than those of N2-8 and N3-8, probably because D/A distances in the former two molecules are larger.26 Eventually, these four emitters were doped (10 wt %) in a host 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-phenyl)-9,9-dimethyl-9,10-dihydro-acridine (CzAcSF). The fluorescence spectra of the 10 wt % doped films in CzAcSF at room temperature are shown in Supporting Information Figure S5b. The PLQYs were measured to be 76%, 82%, 83%, and 91% for N2-6, N2-8, N3-6, N3-8 in doped films, respectively. Such high PLQYs indicate that nonradiative decays are effectively suppressed in our spiro compounds. The PLQYs of N2-8 and N3-8 are larger than those of N2-6 and N3-6, respectively. Again, this is because the former two molecules have the shorter D/A distances, which helps to restrict the molecular rotation and thus suppress nonradiative decays.

    The TADF properties are investigated by using transient PL spectroscopy. In Supporting Information Figure S6, the luminescence spectra of these doped films show both prompt and delayed components. Their prompt (τp) and delayed (τd) lifetimes are 21.97 ns and 1.01 μs for N2-6, 22.12 ns and 1.18 μs for N2-8, 23.05 ns and 1.29 μs for N3-6, 23.65 ns and 1.50 μs for N3-8, confirming the TADF nature of these materials. The rate constants of four emitters are calculated and shown in Supporting Information Table S1. The electrochemical properties of these four molecules were measured by cyclic voltammetry ( Supporting Information Figure S7), and the thermodynamic properties were characterized by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) ( Supporting Information Figure S8). All the materials exhibit suitable energy levels and good thermal stabilities, which are crucial for device applications. In addition, the morphologies of the doped films were tested by atomic force microscopy (AFM). The films of the four materials doped in CzAcSF display roughnesses of 1.02 nm for N2-6, 0.24 nm for N2-8, 0.26 nm for N3-6, and 0.22 nm for N3-8, determined from an area of 5 μm × 5 μm ( Supporting Information Figure S9), indicating that four emitters with alkyl chains have good solubility and film morphology.

    To investigate the electroluminescence of the TADF emitters, we fabricated solution-processed devices with the following configurations: indium tin oxide (ITO)/poly(3,4-ethylenedioxy-thiophene) poly(styrene-sulfonate) (PEDOT:PSS, 40 nm)/CzAcSF:emitter (90∶10, 50 nm)/bis[2-(diphenylphosphino) phenyl] ether oxide (DPEPO, 10 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]-benzene (TmPyPB, 50 nm)/8-hydroxyquinolinato lithium (Liq, 1 nm)/Al (100 nm). The molecular structures of the charge carrier transporting/injecting materials are shown in Supporting Information Figure S10. The electroluminescence spectra and the EQE–current density curves of the devices are presented in Figures 4a and 4b. All the device data are summarized in Supporting Information Table S2.

    Figure 4

    Figure 4 | (a) The electroluminescence spectra and (b) EQE–current density curves of the solution-processed devices with the four TADF emitters.

    It is worth noting that the device based on N3-8 achieves the best performance, featuring a maximum EQE, current efficiency, and power efficiency of 18.9%, 43.1 cd/A, and 27.1 lm/W, respectively. Furthermore, the device based on N2-8 also shows a satisfactory EQE of 17.6%, certifying that this molecular scaffold is suitable for constructing efficient TADF emitters. The device based on N2-8 exhibits the largest efficiency roll-off because of the imbalanced injection of holes and electrons in the emitting layer. The reference material, DM-B,27 was also evaluated in a device with the identical architecture; it exhibited a lower maximum EQE of 14.03% partially due to the much poorer film morphology than that of N3-6 or N3-8 featuring a long chain ( Supporting Information Figure S12). The introduction of the ethyl group in N2-8 and N3-8 definitely accounts for the excellent electroluminescent performance since it enhances the intra- and intermolecular charge transfer by deliberately decreasing the distance between D and A. The improved solubility and film morphology induced by the ethyl group are also vital to the improvements of device performance.

    Conclusion

    We utilize the spiro-structure to construct a series of π-stacked small molecules suitable for solution-processed TADF OLEDs. This design leads to a shortened D/A distance that inhibits the A from free rotation. It is noteworthy that the alkyl chains on these four molecules have an impact on their optoelectronic properties. Intramolecular CH/π interactions occur between each of the alkyl chains grafted on the D and the A moieties. Such noncovalent forces actually push the A away from the D. Consequently, the photoluminescent and electroluminescent properties, for example PLQYs, colors, and luminous efficacies, are deliberately modulated, indicating a new approach to regulate the features of the TADF emitters. In the solution-processed devices encouraging maximum EQEs of 14.2% for N2-6, 17.6% for N2-8, 14.7% for N3-6, and 18.9% for N3-8 are achieved, which makes such emitters superior to polymeric π-stacked TADF devices.15,19 This work demonstrates that the D/A spatial controlled to regulate the TADF properties and the performance of the solution-processed devices.

    Supporting Information

    Supporting Information is available.

    Conflict of Interest

    The authors declare no competing financial interests.

    Acknowledgments

    The authors acknowledge the financial support from the National Key R&D Program of China (nos. 2016YFB0400700 and 2016YFB0401002), the National Natural Science Foundation of China (nos. 51773141, 51873139, and 61961160731), and the Natural Science Foundation of Jiangsu Province of China (no. BK20181442). This project was also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology and by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the 111 Project. G. X. acknowledges the fundamental Research Funds for the Central Universities of China (no. 2042019kf0234). The authors gratefully acknowledge the characterization tests helped by Xue-Qi Wang, Xing Chen, and Song Chen.

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