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

Open AccessCCS ChemistryCOMMUNICATION1 Jun 2021?-Stacked Thermally Activated Delayed Fluorescence Emitters with Alkyl Chain Modulation Tong-Tong Wang, Guohua Xie, Hong-Cheng Li, Sheng-Yi Yang, Hao Yan-Ling Xiao, Cheng Zhong, Kumar Sarvendra, Aziz Khan, Zuo-Quan Jiang and Liang-Sheng Liao Wang Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Devices, Soochow University, Suzhou 215123 Google Scholar More articles by this author , Xie *Corresponding authors: E-mail Address: [email protected] Sauvage Center Molecular Sciences, Hubei Lab on Organic Polymeric Optoelectronic Materials, Department Chemistry, Wuhan 430072 Li Yang Zhejiang Hangzhou 310027 Xiao Zhong Sarvendra Khan https://doi.org/10.31635/ccschem.020.202000355 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Molecules bearing separate ?-electron donor (D) acceptor (A) groups that undergo face-to-face D/A interactions have been utilized develop thermally activated delayed fluorescence (TADF) materials. These ?-stacked architectures are constructed various scaffolds, which either a long distance or permitted conrotatory motion. Here, we novel spiro-based scaffold short restricted circumvolution motions because both the rigid spiro-scaffold large rotation hindrance between nearly coplanar D A. We append different alkyl chains, can modulate charge transfer luminescence properties, at nitrogen moiety four TADF molecules, charge-transfer properties. Because introduction solubilized chain, these molecules were used fabricate solution-processed devices, among maximum external quantum efficiency 18.9% was realized. By modulating building blocks, constructs exemplify side chains be considered as solubilizing units, vital impact optoelectronic properties thus offer new route design solution-processable emitters. Download figure PowerPoint Introduction It is commonly observed phenomenon in nature aromatic systems apt ?-stacking, consequence electrons able delocalize under excitation.1 In field materials, variety linkers scaffold, desired covalently grafted fixed within manner favor ?-stacking excited states.2–4 Such include xanthene,5 anthracene,6 naphthalene,7 [2.2]paracyclophane,8 o-carborane,9 so on. (Figure 1a). Recently, through-space ?–? employed construct emitters.10–15 each luminescent an electron nonconjugated molecular framework. The frontier orbitals, namely highest occupied orbital (HOMO) lowest unoccupied (LUMO), separately located units. As consequence, singlet–triplet energy gap (?EST) significantly minimized, favoring fast reverse intersystem crossing (RISC).11,12 However, case when blocks linked through single bond, motion could occur, leads nonradiative decay 1b) states, lowering photoluminescence yields (PLQYs) materials.16,17 Hitherto, reported emitters seldom simple, low-cost organic light-emitting diodes (OLEDs), except some dendrimers polymers (TSCT) units.18,19 Figure 1 | Schematic representation classic molecules. (a) Reported our scaffold. (b) Previously work about (c) New strategy revealed investigation. (d) Synthetic investigated. contribution, designed synthesized small N2-6, N2-8, N3-6, N3-8, 1c). Each contains based fluorene. A donating unit, alkylated diphenylamine, cyclized onto C9 position fluorene means spiro annulation. On C1 position,20 accepting 2,4,6-triphenylpyrimidine (N2) 2,4,6-triphenyl-1,3,5-triazine (N3), introduced creatively. one hand, D–A rather short, (i.e., <3.60 Å), leading steric restricts other units orientate manner, transfer.21 Ds, chain solubilize target spin coating. Interestingly, N-alkyl not generally materials arylamine Ds usually As. cases, appended site fluorene, making available further derivation. Furthermore, found groups, n-hexyl 2-ethylhexyl, tuning orientation. result, all showed good PLQYs superior integrated into OLEDs, device N3-8 featuring PLQY 91% achieved high (EQE) 18.9%. Results Discussion including three steps, shown 1d. First, intermediates 4 5 prepared Suzuki–Miyaura reaction. then underwent successive nucleophilic addition Friedel–Crafts cyclization form afford solubility common solvents, enabling spin-coating procedure uniform films. Diffraction grade crystals obtained, providing unambiguous description their structures. Both N2-6 N3-6 contain n-hexyl, while N2-8 2-ethylhexyl contained. resides surface driven CH/? protons As, N2 N3 respectively. Given bulkier unit than n-hexyl,22–24 it predicted latter should introduce less Notably, distances smaller those where d1, d3, d5, d7 3.446, 3.413, 3.548, 3.485 Å, respectively, indicated 2a. This unpredicted result explained occurrence inferred from corresponding close contacts D, N2, N3. relatively weaker bulky only ethyl undergoes interactions. hexyl group instead orientates perpendicularly N3-8. Moreover, intermolecular 3.60 stacking solid state. intramolecular noncovalent confirmed theoretical calculations. Figures 2b Supporting Information S2, present obvious attractive (green region) larger (brown segments,25 restricting vibrations prevent nonradioative states. distributions HOMO/LUMO verified density functional theory calculation results. S3, HOMO LUMO A, well separated space, ?EST decreased. Besides, natural transition (NTO) simulation singlet states (S1) triplet (T1) also calculated crystal data. For S1 state hole (red particle distributed part, proving emission. Compared part exhibits torsion group. proportions TSCT/through-bond CT (TBCT) characterized integrating density, TSCT/TBCT 95%/5%, 97%/3%, 98%/2%, indicating spatial interaction absolutely dominant system. 2 Crystal structure functions reduced gradient (RDG) sign(?2)? Crystallographic data deposited Cambridge Data Centers supplementary publication no. CCDC-1972748 (N2-6), CCDC-1972704 (N2-8), CCDC-1972788 (N3-6), CCDC-1972710 (N3-8). obtained free Centre: https://www.ccdc.cam.ac.uk/structures-beta/. photophysical influenced summarized Table 1. UV–vis absorption (PL) spectra 3a–3d. strong bands 250 330 nm attributed n–?* ?–?* transitions conjugated structure. All exhibit broad featureless profiles. peak wavelengths (?maxs) 461, 470, 485, 495 ?max values shorter, two series, electron-withdrawing, compared series former therefore higher shorter consequently enhanced Optical Thermal Physical Properties Four Emitter ?absa (nm) ?flb ?phosc S1/T1d (eV) ?ESTe Egf HOMOg LUMOh 269, 310 461 442,469 3.08/2.81 0.27 ?5.13 ?1.53 268, 470 440,466,501 2.99/2.83 0.16 3.59 ?5.14 ?1.56 278, 309 485 471 2.97/2.79 0.18 277, 312 468, 492 2.95/2.81 0.14 3.56 ?5.15 ?1.59 aMeasured dichloromethane solution (10?5 M) room temperature. bMeasured toluene temperature, excitation wavelength 340 nm. cMeasured 77 K, dCalculated onset vibrational phosphorescence spectra. eCalculated S1–T1. fCalculated gCalculated CV hCalculated Eg. 3 Absorption temperature K: addition, highly dependent solvent polarities ( S4), factor having great transfer. K measured S5a). 0.27, 0.16, 0.18, eV probably larger.26 Eventually, doped (10 wt %) host 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-phenyl)-9,9-dimethyl-9,10-dihydro-acridine (CzAcSF). 10 % films CzAcSF S5b. 76%, 82%, 83%, films, indicate decays effectively suppressed compounds. Again, distances, helps restrict suppress decays. investigated using transient PL spectroscopy. S6, show prompt components. Their (?p) (?d) lifetimes 21.97 ns 1.01 ?s 22.12 1.18 23.05 1.29 23.65 1.50 confirming rate constants S1. electrochemical cyclic voltammetry S7), thermodynamic thermogravimetric analysis (TGA) differential scanning calorimetry (DSC) S8). suitable levels thermal stabilities, crucial applications. morphologies tested atomic force microscopy (AFM). display roughnesses 1.02 0.24 0.26 0.22 determined area ?m × S9), film morphology. To investigate electroluminescence emitters, fabricated devices 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 (DPEPO, nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]-benzene (TmPyPB, nm)/8-hydroxyquinolinato lithium (Liq, nm)/Al (100 nm). structures carrier transporting/injecting S10. EQE–current curves presented 4a 4b. S2. worth noting achieves best performance, EQE, current efficiency, power 18.9%, 43.1 cd/A, 27.1 lm/W, shows satisfactory EQE 17.6%, certifying constructing efficient largest roll-off imbalanced injection holes emitting layer. reference material, DM-B,27 evaluated identical architecture; exhibited lower 14.03% partially due much poorer morphology S12). definitely accounts excellent electroluminescent performance since enhances intra- deliberately decreasing improved induced improvements performance. Conclusion utilize spiro-structure OLEDs. shortened inhibits rotation. noteworthy Intramolecular occur moieties. forces actually push away D. Consequently, photoluminescent example PLQYs, colors, luminous efficacies, modulated, approach regulate features encouraging EQEs 14.2% 17.6% 14.7% achieved, makes such polymeric devices.15,19 demonstrates controlled devices. available. Conflict Interest authors declare no competing financial interests. Acknowledgments acknowledge support National R&D Program China (nos. 2016YFB0400700 2016YFB0401002), Natural Science Foundation 51773141, 51873139, 61961160731), Province (no. BK20181442). project funded Collaborative Innovation Technology Priority Academic Development Higher Education Institutions (PAPD) 111 Project. G. X. acknowledges fundamental Research Funds Central Universities 2042019kf0234). gratefully characterization tests helped Xue-Qi Xing Chen, Song Chen. References Predeep P.Optoelectronics Techniques. 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