Acceptor Interactions in D-A-D

Publication Date (Web): March 7, 2019 ... and synthesized organic molecule, 5,5′-([1,2,5]thiadiazolo[3,4-c]pyridine-4,7-diyl)bis(N,N-diphenylthiophe...
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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Enhanced Pi Conjugation and Donor/Acceptor Interactions in D‑A‑D Type Emitter for Highly Efficient Near-Infrared Organic LightEmitting Diodes with an Emission Peak at 840 nm Jianxia Jiang,† Zeng Xu,† Jiadong Zhou,† Muddasir Hanif,† Qinglin Jiang,† Dehua Hu,*,† Ruiyang Zhao,‡ Cong Wang,† Linlin Liu,† Dongge Ma,*,† Yuguang Ma,*,† and Yong Cao† †

Chem. Mater. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/07/19. For personal use only.

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Guangdong 510640, China ‡ College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, 266042, P. R. China S Supporting Information *

ABSTRACT: Restricted by the energy gap rule, near-infrared (NIR) luminescent materials face great challenges. Here, we report a newly designed and synthesized organic molecule, 5,5′([1,2,5]thiadiazolo[3,4-c]pyridine-4,7-diyl)bis(N,N-diphenylthiophen-2-amine) (DTPS-PT), which has strong donor and acceptor interactions for NIR emission applications. The results demonstrate that the higher planarity of the DTPS-PT molecular structure enhances the pi-conjugation and hybridization between the charge transfer state (CT) and localized pi states (LE). As a result, DTPS-PT exhibits NIR emissions from an LE involved in hybridized local and charge transfer (HLCT) states, showing a 79% high fluorescence quantum yield in the low polar solvent tetrachloromethane. For both doped and nondoped devices, the NIR OLEDs based on DTPS-PT achieved “real” NIR emission with the λonset above 700 nm. The best performing OLED device within the doped devices gave a maximum emission peak around 840 nm with a maximum radiance of 2202 mW Sr−1 m−2.

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INTRODUCTION

features, as well as bipolar charge transport and the ability to fabricate devices by vacuum evaporation. However, D−A materials are facing the problem of further development. First, the overlap between the highest unoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) in the D−A chromophore is generally very limited, inevitably resulting in a lower rate of radiation transition, and ultimately leading to a lower luminescence efficiency, or even no emission at all.27 Second, NIR luminescent materials are also faced with the inherent limitations of the energy gap law.28−31 The law predicts that the vibrational overlap between the ground and the excited states causes the quantum efficiency of the chromophore to decrease as the energy gap decreases. In addition, when the D−A molecules are applied to produce NIR electroluminescent devices, the nonradiative transition rate further increases due to the stacking and dipole interactions in the solid state, resulting in lower NIR luminous OLEDs.32 Therefore, lowering the nonradiative transition or increasing the radiative

Recently, near-infrared (NIR) organic light-emitting diodes (OLEDs) have been undergoing intense research due to their special applications in night-vision displays, chemo-sensing, and medical diagnostics.1−7 Organic NIR-OLED materials are mainly based on transition-metal complexes,8−11 organic dyes,12−14 low-band gap polymers,15,16 and organic donor− acceptor (D-A) small molecules.17−22 Although the NIR phosphorescent OLEDs have been reported to achieve the highest external quantum efficiency (EQE), around 24%, they also suffer from instability and serious efficiency roll-offs at high current density due to their long-lifetime triplet exciton quenching effect.23,24 Therefore, it is necessary to develop metal-free low-band gap organic materials considering their chemical modification, easier synthesis, and lower cost nature. There are two molecular design strategies commonly employed to narrow the optical band gap to achieve NIR emission. One is to obtain a more red-shifted chromophore by extending the π-conjugate, but the red shift is limited and causes problems in multistep synthesis, purification, and device fabrication. The second is to construct an NIR chromophore with a D−A structure.25,26 This is a promising class of materials for NIR OLED applications because their band gap levels can be readily tuned from the emission state of CT © XXXX American Chemical Society

Special Issue: Jean-Luc Bredas Festschrift Received: November 23, 2018 Revised: February 24, 2019

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DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

Article

Chemistry of Materials Scheme 1. Chemical Structures and Synthetic Route of DTPS-PTa

(i) Pd (PPh3)4; THF; 80 °C; 24 h.

a

transition rate has become a major research topic for improving NIR luminescent materials. The excited-state design strategy of hybridized local and charge transfer (HLCT) states achieves efficient hybridization by carefully tailoring the D−A molecular structure. Recently, our research team has successfully applied the HLCT strategy to achieve various visible and NIR emission colors.16,33−38 To expand the emission wavelength toward the deep infrared region, significant reduction in the energy gap will inevitably bring about an exponential increase of the nonradiative rate. In this case, the nonradiative transition rate becomes a key factor that restricts the luminous efficiency of the molecule. Excitingly, it seems that this HLCT strategy is inherently compatible with the design principles of high-efficiency NIR luminescent molecules. When LE components are introduced in the CT state to form a special HLCT state, LE components can maintain a comparable radiative rate relative to the accelerated nonradiative rate along with the narrowing of band gap, allowing the higher ηPL of the emissive state. Therefore, the HLCT strategy shows great potential for the design of deep NIR materials by optimizing the fluorescence quantum yield of emitters. From recent literature reports, NIR OLEDs with emissions above 700 nm are mostly attributed to NIR emission, because human eyes have weaker perception of light sources above 700 nm. Also, there are many reports about high-efficiency NIR OLEDs with an emission peak around 670 to 780 nm.18−21,33,37−39 However, full-spectrum NIR OLED emissions are rarely reported because of the particularly broad emission spectrum and peak width of D−A molecules. As a result, most of the reported NIR emission spectra overlap within the visible region. In this work, we report a newly designed and synthesized D-A-D compound of 5,5′-([1,2,5]thiadiazolo[3,4-c]pyridine-4,7-diyl)bis(N,N-diphenylthiophen2-amine) (DTPS-PT), with a central pyridine thiadiazole (PT) as the acceptor and diphenylamine thiophene (TPS) as the donor. In our previous work, we found that a pyridine thiadiazole (PT) group with strong electron-withdrawing properties is very promising for constructing NIR D-A-D luminous materials.37,38 Herein, we make a strong intramolecular charge transfer (ICT) molecule by introducing diphenylamine thiophene as the strong electron donor combined with PT, thereby reducing the optical band gap of the molecule. Consistent with the lower optical band gap, the nondoped electroluminescent (EL) device based on DTPS-PT exhibits an emission around 818 nm. In addition, the planar configuration of the molecule, caused by the influence of a small steric-hindrance group (thiophene), enhances the piconjugation and the hybridization between the local pi-state and the charge transfer states. We achieved a maximum radiance of 2202 mW Sr−1 m−2 at 840 nm with the λonset

around 700 nm with our 40 wt % DTPS-PT-doped OLED device, which is, to the best of our knowledge, one of the highest values ever reported for the NIR-OLEDs emitting over 800 nm. This structural design, based on HLCT strategy, may alter and expand the concept of organic NIR-OLED materials.

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EXPERIMENTAL SECTION

General Methods. All of the solvents and reagents used for synthesis were purchased from Aldrich or Acros and used as received with necessary purification. We collected 1H and 13C NMR data with a Bruker AVANCE Digital 600 MHz NMR workstation and used tetramethylsilane (TMS) as the internal standard for all of the data. With a Bruker Autoflex III Smartbeam, we gathered data from a matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometer. For differential scanning calorimetry (DSC), we used a Netzsch DSC 209 apparatus with a heating and cooling rate of 10 °C min−1 under N2 flow. For thermogravimetric analyses (TGA), we collected data from a Netzsch TG 209 apparatus under a N2 flow at a heating rate of 10 °C min−1. We obtained cyclic voltammetry (CV) measurements on a CHI 760D electrochemical workstation using a glassy carbon working electrode and Pt wire counter electrode. Our scanning rate was 50 mV s−1 against an Ag/Ag+ (0.01 M of AgNO3 in acetonitrile) reference electrode in a nitrogen-saturated anhydrous DMF and dichloromethane (DCM) mixed solution with 0.1 mol L−1 Bu4NPF6 as the electrolyte. The measurement was calibrated against the ferrocene/ferrocenium redox system. We recorded absorption spectra on a UV-3600 spectrophotometer from Shimadzu and photoluminescence (PL) spectra on a Jobin-Yvon spectrofluorometer. Device Fabrication and Characterization. We further purified synthesized NIR materials by sublimation before device fabrication. Devices with active areas of 16 mm2 were grown on patterned ITO glass substrates with a sheet resistance of 10 Ω sq−1. Before fabricating the NIR-OLEDs, we cleaned ITO glass substrates with detergent and deionized water, dried them in an oven at 120 °C for 1 h, treated them with UV−ozone for 15 min, and finally loaded them into a deposition chamber with a basic pressure of 1 × 10−4 Pa. We fabricated organic layers by evaporating them at a rate of 1−2 Å s−1 and depositing a layer of Liq with a 2 nm thickness at a rate of 0.1 Å s−1. Finally, we deposited Al at a rate of approximately 5.0 Å s−1 for the cathode. The current−voltage−brightness characteristics and electroluminescence (EL) spectra were measured with a Keithley 2400 source meter and a PR745 SpectraScan colorimeter. We carried out all of the measurements under ambient conditions and at room temperature. Synthesis of DTPS-PT. A two-neck flask was charged with 4,7dibromo[1,2,5]thiadiazolo[3,4-c]pyridine (1.0 g, 3.42 mmol), N,Ndiphenyl-5-(tributylstannyl)thiophen-2-amine (4.63 g, 8.55 mmol), Pd(PPh3)4 (0.40 g, 5 mol %), and 30 mL of THF. The reaction mixture was heated at 90 °C and refluxed under a nitrogen atmosphere for 24 h. After cooling to room temperature, the reaction mixture was poured into water and extracted with dichloromethane consecutively. After evaporation of the solvent, the residue was purified by column chromatography on silica gel using a petroleum ether/dichloromethane eluent (4:1), resulting in a 35% yield (0.76 g) of pure product compound DTPS-PT. 1H NMR (CDCl3, 600 MHz, B

DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Single-crystal structure of the compound DTPS-PT. [CCDC 1864171 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]

Figure 2. (a) UV−vis spectra and PL spectra of DTPS-PT in n-butyl ether solution and evaporated film. (The absorption intensity is normalized relative to the absorption band of the shortest wavelength.) (b) Transient PL spectra of DTPS-PT, (c) PL spectra of DTPS-PT in different dope concentration evaporated films (host: Alq3; dopant: DTPS-PT); and (d) relevant density functional theory (DFT/M06-2X/def2TZVP) calculations combined with the natural transition orbital (NTO) analysis of DTPS-PT single-crystal structure, where f is the oscillator strengths and the weights of the hole-particle are given for the S0 → S1 excitations. δ): 8.44 (s, 1H, Py-H), 8.44−8.38 (d, H, J = 8.41 Hz, Th−H), 7.79− 7.78 (d, 2H, J = 7.78 Hz, Th−H), 7.28−7.25 (m, 4H, J = 7.27 Hz, Ar−H), 7.24−7.20 (m, 8H, J = 7.22 Hz, Ar−H), 7.16−7.14 (m, 4H, J = 7.15 Hz, Ar−H), 7.09−7.07 (t, 2H, J = 7.07 Hz, Ar−H), 7.02−6.99 (t, 2H, J = 6.99 Hz, Ar−H), 6.64−6.63 (d, 1H, J = 6.63 Hz, Th−H), 6.55−6.54 (d, 1H, J = 6.54 Hz, Th−H). 13C NMR (CDCl3, 600 MHz, δ): 153.78, 152.40, 146.98, 146.47, 145.76, 128.57, 128.28, 125.64, 123.72, 122.61, 122.25, 119.08, 117.79, 115.68. MALDITOF-MS (m/z): C37H25N5S3, 635.1592 (calcd: 635.1272). Elemental analysis calculated [%] for C37H25N5S3: C, 69.89; H, 3.96; N, 11.01; S, 15.13, found: C, 69.88; H, 3.97; N, 11.03; S, 15.10

Table 1. Photophysical properties of the material: photoluminescence emission, quantum yields (ϕF), lifetime (τF), radiative (kr) and non-radiative (knr) decay rates

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DTPS-PT

tetrachloromethanea

n-butyl ethera

λem (nm) τ (ns) ϕflb (%) kr (107) (s−1)c knr (107) (s−1)c

649 5.45 78.5 14.40 3.94

729 2.04 11.3 5.54 43.5

Concentration of 1.0 × 10−5 M. bAbsolute quantum yields determined with a calibrated integrating sphere system. ckr and knr were obtained by kr = ϕF/τF; τF−1 = kr + knr. a

RESULTS AND DISCUSSION Synthesis and Single Crystal Structures Analysis. The synthetic routes of DTPS-PT are shown in Scheme 1 through a one-step Stille-coupling reaction of N,N-diphenyl-5(tributylstannyl)thiophen-2-amine with the 4,7-dibromopyridal[2,1,3]thiadiazole. The final product was purified by silica gel column, chromatography, preparative HPLC, and vacuum sublimation in sequence. The structure and purity of the final product was fully confirmed by 1H NMR, 13C NMR,

mass spectrometry, and elemental analysis (Figure S1 and Figure S2). The molecular structure of DTPS-PT is further confirmed by the single crystal analysis. As shown in Figure 1, the unit cell of DTPS-PT is triclinic, space group P1̅, and contains two molecules. Due to the small bilateral steric hindrance between the thiophene group and PT ring, DTPSPT exhibits relatively small twist angles (12.98° and 10.95°) C

DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

between the donor TPS and the acceptor PT, resulting in a relatively planar molecular structure. In a cell, two DTPS-PT molecules are inversely packed, and the vertical distance between inversely neighboring PT rings is 3.6 Å. The whole crystal of DTPS-PT shows the tightest stacking along the a-axis and strong pi−pi interactions between PT rings, while hydrogen bonding (Figure 1c) between molecules is the driving force of three-dimensional order. The specific crystal data are summarized in Table S1. Photophysical Properties. The comparison of UV−vis and PL spectra of DTPS-PT in moderate polar solvents (nbutyl ether) and evaporated film are shown in Figure 2a. The UV−vis spectra of DTPS-PT involved high-energy and lowenergy absorption bands. The high-energy absorption bands (279−377 nm) are associated with the π−π* electronic transitions, largely localized on the donor chromophore. The low-energy absorption bands (594 and 614 nm for the solution and film) can be assigned to the S1 ← S0 transition, with some intermolecular CT-like character. The PL spectra showed NIR emission at 729 and 803 nm for the solution and film, respectively. The evaporated film exhibits obvious red shift both in UV−vis and in PL spectra when compared to that in solution, which is ascribed to the domination of a more planar D−A conformer in the aggregated state. Intermolecular interactions in solid films also caused broadening of the absorption spectrum. Furthermore, the high molar extinction coefficient (ε = 2.83 × 104 in n-butyl ether) of this CT transition indicates a strong and effective intramolecular charge transfer interaction. We also characterized the PL of DTPS-PT in various solvents (Figure S3 and Table S2). With an increase of solvent polarity from low polarity hexane to high-polarity dimethylformamide, we observed an obvious bathochromic shift and NIR emission, indicating typical CT characters of the emissive states of the DTPS-PT. In addition, we examined the solvatochromic effect using Lippert−Mataga model.40,41 From the fitted lines (Figure S4) of Stokes shift versus the orientation polarizability (f), the dipole moment of the S1 exciton can be estimated by the slope of the fitted line of

Figure 3. (a) Cyclic voltammogram of DTPS-PT in CH2Cl2 and DMF mixed solutions. The scan rate was 50 mV s−1. (b) Frontier molecular orbitals of DTPS-PT.

Table 2. Electroluminescence Data of DTPS-PT (device 1, device 2) device−dopant concentration (%)

Vona [V]

λemb [nm]

fwhmc [nm]

Rmaxd [mW·Sr−1·m−2]

1−10 1−20 1−40 1−60 1−100 2−10 2−20 2−40 2−60 2−100

3.8 3.2 3.0 3.1 3.3 3.7 3.5 3.4 3.7 3.7

798 820 842 840 816 804 820 840 832 818

193 198 200 198 194 193 198 200 201 196

641 656 757 686 644 1631 2266 2202 1297 590

Von: turn-on voltage, measured at 1 mW Sr−1 m−2. bλem: Obtained at 5 V. cfwhm: full width at half-maximum. dR: radiance. Devices 1− 1∼5: ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/(host) Alq3:xDTPS-PT (dopant) (x = 10%, 20%, 40%, 60%, 100%, 20 nm)/ Bphen (55 nm)/Liq (2 nm)/Al. Devices 2−1∼5: ITO/HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/(host) Alq3:xDTPS-PT (dopant) (x = 10%, 20%, 40%, 60%, 100%, 20 nm)/BmPyPB (55 nm)/Liq (2 nm)/Al. a

Figure 4. (a) Structures of the devices as well as the energy levels of used materials. (b) Chemical structures of the materials used in the devices. (c) Electroluminescence spectrum of DTPS-PT. (d) Radiance−voltage and radiance/current density−voltage characteristics of the NIR OLED device. D

DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

solutions) and ferrocene (Fc) as the internal standard. The highest unoccupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels estimated from electrochemical results are −3.58 eV and −4.9 eV for DTPS-PT. The theoretically calculated (DFT) HOMO energy levels are mainly distributed on the donor-based whole molecular skeleton, while the LUMO energy levels are mainly distributed on the acceptor PT ring and its adjacent thiophene group. This resulted in a lower HOMO−LUMO gap of 1.32 eV, which is small enough to provide NIR emission. Electroluminescence Properties. Based on the encouraging PL properties, we evaluated the OLED’s performance of DTPS-PT with the following device configuration: ITO/ HATCN (5 nm)/TAPC (50 nm)/TCTA (5 nm)/Alq3:emitter molecules (20 nm)/Bphen (BmPyPB) (55 nm)/Liq (2 nm)/ Al, where HATCN is dipyrazino(2,3-f:2′,3′-h)-quinoxaline2,3,6,7,10,11-hexacarbonitrile as the anode buffer layer; TAPC (di-[4-(N,N-ditolyl-amino)-phenyl] cyclohexane) as the holetransport layer, and TCTA (4,4′,4″-tri(N-carbazolyl)triphenylamine) as the electron/exciton-blocking layer; and BmPyPB (1,3-bis[3,5-di(pyridin-3-yl)phenyl]benzene) and Bphen (4,7-diphenyl-1,10-phenanthroline) as the electrontransporting layer (ETL)/hole-blocking layer. These devices have excellent pure NIR emission and very low turn-on voltage (3.8 V), which means that the device structure is reasonable and there is a small injection barrier between the emitter and the transfer layer. As mentioned above, the doped film exhibits significant emission enhancement (Figure 2c). Therefore, devices were first optimized by changing doping concentration. Since Alq3 is a commonly used host material with excellent properties, we used it as a host material to form a host−guest system with DTPS-PT. The device of pristine DTPS-PT exhibited NIR emission at 816 nm, with the maximum radiance of 644 mW Sr−1 m−2. As expected, doping significantly improved the device performance, especially at the dopant concentration of 40%, where the DTPS-PT-based device not only had the lowest voltage and the highest radiance, but it also possessed a deeper red emission. From Table 2, we can see the 100% electroluminescence of the doped device shows a blue shift when compared to those of 40% and 60% doped film. This trend was also observed for the doped films (Figure 2c). We proposed the following assumptions about this phenomenon. When the doping concentration changed from 100% to 40%, the concentration of the emitter molecules is large and the planar molecules are more likely to aggregate, so the aggregation between the emitters is disturbed little with the change of doping concentration. In the 40% and 60% doped films, the relatively polar surrounding environment (Alq3) induced a decrease of the first excited state energy and then a red shift in luminescence are observed as compared with that of pristine film (100% doped films). While in the 20% and 10% doped films, the Alq3 gradually becomes a main component and the aggregations between the emitter molecules that were destroyed. Thus the emissions of 20% and 10% doped films turned into a blue shift. This phenomenon can also be defined as a solid solvation effect. Next, the device was further optimized by using electron transporting materials of BmPyPB to replace Bphen. We investigated the final EL spectra, radiance vs voltage, current density−voltage characteristics of the OLED devices (Figure S8) and summarize the data in Table 2. Device 2 with BmPyPB as an electron-transporting layer (ETL) performed

experimental points. Notably, DTPS-PT exhibits two sections with a slope of 3467 (r = 0.82) and 9694 (r = 0.99), corresponding to the excited state dipole moment (μe) of 7.7 and 12 D. In low polarity solvents, the μe of 7.7 D is slightly larger than that of the usual LE excited state, implying the emissive state of DTPS-PT is a LE-like state involving a slight CT component.42 In high polarity solvents, due to the increase of CT component the μe of DTPS-PT increase to 12 D, which is still obviously smaller than that of typical CT state (4-(N,Ndimethylamino)-benzonitrile (DMABN), 23 D).42 These indicated that the S1 state of DTPS-PT is a hybridization of LE and CT states (HLCT state) rather than a pure LE or CT state. Therefore, DTPS-PT forms an LE-dominated HLCT state in low-polarity solvents and CT-dominated HLCT state in high polarity solvents, respectively. This corresponds to DTPS-PT’s relatively high PL quantum yield in low and medium polarity solvents (Table S2). Moreover, we measured the transient PL spectra of DTPS-PT in tetrachloromethane and n-butyl ether (Figure 2b) solutions. Both spectra exhibit single-exponential fluorescence decay processes with no delayed lifetime component. The lifetimes in tetrachloromethane and n-butyl ether solutions are estimated to be 5.45 and 2.04 ns, and the fluorescence quantum yields are 0.785 and 0.113, respectively. In addition, DTPS-PT has a higher radiation transition rate and a lower nonradiative transition rate in the tetrachloromethane (Table 1). As the polarity of the solvent increases, it inevitably leads to a decrease in the molecular radiation transition rate and an increase in the nonradiative transition rate. Fortunately, the change of the molecule in n-butyl ether solution is quite acceptable. It is well-known that the PLQYs of most NIR emitters suffer sharp decreases in neat films compared with those in dilute solution due to aggregation, causing quenching effects. Therefore, doping is especially important for the application of NIR materials. In Figure 2c, we show that the emission intensity of doped films is generally much higher and increases further with the decrease of DTPS-PT concentration (Figure S5 is the absorption spectrum of these films). In addition, we were pleasantly surprised to find that when the dopant concentration is beyond 40%, doping does not cause a blue shift of the emission. Even if the dopant concentration is further decreased to 10%, the emission peak can reach 773 nm, which is still an ideal NIR emitter. To understand the structural and electronic properties of the molecule, we characterized the relevant molecular natural transition orbitals (NTO) distribution of DTPS-PT according to the DFT-optimized geometry (Figure 2d) and single-crystal structure (Figure S6). The DFT-optimized geometry of DTPSPT is more planar than that in single-crystal. This planar molecular structure is beneficial to enhance pi-conjugation and affects the distribution of NTO. For the S1 state of DTPS-PT in both DFT-optimized and single-crystal geometries, hole and particle NTOs showed an excellent balance between spatial separation and orbital overlap. The easily distinguishable separated orbitals led to a CT character with a larger dipole moment. However, sufficient orbital overlaps induced LE character and ensured a reasonable radiative-transition rate. Thus, the S1 ← S0 transition of DTPS-PT shows a large oscillator strength (0.80 and 0.72 for DFT-optimized geometry and single-crystal structure, respectively). Electrochemical Properties. Using cyclic voltammetry, we investigated the electrochemical properties of DTPS-PT (Figure 3) in 0.1 M n-Bu4NPF6 (CH2Cl2 and DMF mixed E

DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials Notes

better than Bphen, with maximum irradiance of the device increasing exponentially. Above all, the DTPS-PT-based doping device showed real NIR emission with a maximum peak at 840 nm, the λonset of the spectrum extending to 700 nm, and the radiance achieving 2202 mW Sr−1 m−2 (Figure 4c,d). The relationship between irradiance/current density and voltage (Figure 4d) shows that, as voltage increases, the output radiation power of the NIR OLED device is stable and maintains an average 0.55 mW Sr−1 A−1. It is worth noting that the doped DTPS-PT device exhibits a “rare” NIR region emission spectrum, which is among the best results in NIR fluorescent OLEDs (FOLEDs) with similar device structure and EL spectra (Table S3 summarized the electroluminescent device performance of organic small-molecule NIR emitters).

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to the Natural Science Foundation of China (21334002, 51521002, 51403063), the Ministry of Science and Technology of China (2013CB834705, 2015CB655003), the Fundamental Research Funds for the Central Universities (2015ZP001, 2015ZM042), Introduced Innovative R&D Team of Guangdong (201101C0105067115), Major Science and Technology Project of Guangdong Province (2015B090913002), Foundation of Guangzhou Science and Technology Project (201504010012), and China Postdoctoral Science Fund (Grant No. 2014M562174) for their support. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

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CONCLUSIONS In summary, the DTPS-PT (D-A-D type molecule) was synthesized and characterized by the 1H NMR, 13C NMR, mass spectrometry, and elemental analysis. The molecular properties were explored by the UV−vis, PL, cyclic voltammetry, X-ray crystallography, and DFT calculations. Photophysical and DFT analysis demonstrate that the S1 state of DTPS-PT shows HLCT character. The near-planar arrangement of the PT ring and donor groups cause considerable orbital overlap between these moieties and further enhance the radiative-transition rate and fluorescence efficiency. Therefore, the fluorescence quantum yields of DTPS-PT are as high as 0.79 in the tetrachloromethane solvent. Doping has a positive effect on the emission properties due to the flexible planar molecular structure. With the decrease of doping concentration, a high-performance device was achieved with increased emission intensity and a red-shift of the emission peak. When the dopant concentration increased to 40%, we attained a maximum radiance of 2202 mW Sr−1 m−2 with the maximum peak around 840 nm. To the best of our knowledge, these doped devices are record-setting among the NIR FOLEDs with similar EL wavelengths. Our results reveal a deep insight into a practical strategy to design high-efficiency NIR fluorescent OLED materials, bypassing the energy gap law on the basis of donor−acceptor molecular architecture using the HLCT state as the emissive state.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b04894.

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REFERENCES

Structure characterization, photophysical properties, theoretical calculations, thermal and morphological properties, and electroluminescence performances (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(D.H.) E-mail: [email protected]. *(D.M.) E-mail: [email protected]. *(Y.M.) E-mail: [email protected]. ORCID

Zeng Xu: 0000-0001-7552-8836 Linlin Liu: 0000-0002-2877-0774 Yuguang Ma: 0000-0003-0373-5873 F

DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.chemmater.8b04894 Chem. Mater. XXXX, XXX, XXX−XXX