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First N‑Borylated Emitters Displaying Highly Efficient Thermally Activated Delayed Fluorescence and High-Performance OLEDs Yi-Jyun Lien,†,‡ Tzu-Chieh Lin,†,§ Chun-Chieh Yang,⊥ Yu-Cheng Chiang,⊥ Chih-Hao Chang,*,⊥ Shih-Hung Liu,§ Yi-Ting Chen,§ Gene-Hsiang Lee,§ Pi-Tai Chou,*,§ Chin-Wei Lu,‡ and Yun Chi*,‡ ‡

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan ⊥ Department of Photonics Engineering, Yuan Ze University, Chungli 32003, Taiwan §

S Supporting Information *

ABSTRACT: Despite the fast boom of thermally activated delayed fluorescence (TADF) emitters bearing borane-based acceptor, so far, no TADF emitter with a direct B−N linkage between N-donor and boryl acceptor has been reported. The latter should simplify the molecular architecture and hence facilitate the synthetic design and versatility. We report here the preparation and characterization of a new series of Nborylated compounds with functional acridine donor unit; namely: ACBM, PACBM, and SACBM. Spectroscopic studies were performed to explore their photophysical properties that exhibited prominent solvatochromism and thermally activated delayed fluorescence. The time-dependent DFT calculation indicated the involvement of substantial intramolecular charge transfer character for which HOMO and LUMO are spatially separated. For compound SACBM, fabrication of green emitting OLED gave CIE chromaticity of (0.22, 0.59) and maximum external quantum efficiency, luminance efficiency and power efficiency of 19.1%, 60.9 cd/A, and 43.6 lm/W, respectively, demonstrating for the first time the highly efficient OLEDs using Nborylated TADF emitters. KEYWORDS: thermally activated delay fluorescence, acridine, borane, solvatochromism, organic light emitting diodes expected to be competitive, giving the delayed fluorescence, which enables efficient utilization of both singlet and triplet excitons despite the absence of third-row transition metal element. Recently, much attention has been paid to TADF emitters bearing borane-based acceptor, which can be classified into either three- or four-coordinated architecture. In the threecoordinated scenario, the lower-lying vacant p-orbital of boron atom served as the efficient electron acceptor for creating the facile intramolecular charge transfer upon excitation.29 This class of TADF molecules may contain either terminal diarylboryl fragment30,31 or borane-based fragments in the middle of the molecules.32 In both cases, they demand bulky substituents attached around the boron atom for improving both chemical and physical stability. Most recently, phenoxaborin,33 phenazaborin,34 and polycyclic 1,4-oxaborine35 or azaborine36 subunits have emerged to be the better choice of the electron acceptor in assembling efficient TADF emitters. For the last two examples, the resulting smaller Stokes shifts

1. INTRODUCTION Organic light-emitting diodes (OLEDs) have received considerable attention in recent years because of their latent applications in the flat-panel displays and solid-state luminaries. Third-row, late-transition-metal-based phosphors were first used to fabricate OLEDs with unitary internal quantum efficiency by their capability in harvesting both singlet and triplet excitons. Later on, thermally activated delayed fluorescence (TADF) emitters become viable alternatives because of the fast reverse intersystem crossing (RISC) induced by the small energy gap between the lowest energy singlet (S1) and triplet excited state (T1), i.e., ΔET‑S. In fact, majority of TADF emitters were realized by incorporating electron donor such as functional arylamine,1,2 carbazole,3 phenoxazine,4−6 acridine,7,8 etc., and electron accepting unit such as carbonyl,9 phosphine oxide,10,11 sulfoxide or sulfonecontaining units,12−15 triazine,16−19 pyrimidine20,21 etc.22−25 into a single molecular identity. When there is a small spatial overlap between the highest occupied molecular orbital (HOMO) of donor and the lowest unoccupied molecular orbital (LUMO) of acceptor, the electron exchange energy is greatly reduced, resulting in the decrease of ΔET‑S.26−28 Provided that ΔET‑S is thermally accessible, RISC is then © 2017 American Chemical Society

Received: June 9, 2017 Accepted: July 21, 2017 Published: July 21, 2017 27090

DOI: 10.1021/acsami.7b08258 ACS Appl. Mater. Interfaces 2017, 9, 27090−27101

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ACS Applied Materials & Interfaces

2. RESULTS AND DISCUSSION 2.1. Syntheses and Characterization. The 9,9-dimethyl, diphenyl, and spiro-fluorene substituted acridines were first synthesized following the literature procedures with slight modification, which involved treatment of either 2-anilinobenzoic acid methyl ester with corresponding Grignard reagents MeMgBr and PhMgBr, followed by acid-catalyzed cyclization reaction,44,45 or lithiation of 2-bromo-N-phenylaniline, followed by coupling with 9-fluorenone and sequential cyclization.33 Next, the N-borylated compounds ACBM, PACBM and SACBM were synthesized via in situ deprotonation of acridine with BuLi in THF at −78 °C, followed by treatment with equal ratio of B(Mes)2F at RT for an extended period of time. All isolated boron compounds are fully characterized by 1H and 13 C NMR spectroscopies, FAB mass spectrometry and elemental analyses. They appeared to be stable upon extended exposure of solution in air, and could be purified by routine silica gel column chromatography. Single crystal of all compounds suitable for X-ray structural determination could be obtained from a layered solution of mixed CH2Cl2 and MeOH at RT. Both ACBM and PACBM afforded one type of single crystals, whereas the spirosubstituted SACBM gave two distinctive crystals, e.g. SACBM_C (colorless) and SACBM_Y (light yellow), which can be hand-separated and distinguished by their morphologies and emission color. Their structural features are depicted in Figures 1−4.

and narrowed full width at half-maximum (fwhm) of emissions were attributed to the highly rigid, planar framework.35,36 Alternatively, four-coordinated boron fragments were obtained by incorporation of an aromatic N-donor unit at the fourth, vacant site of the trivalent boron fragment, giving formation of a boracyclic coordination unit.37 Hence, the lower-lying π*orbital of this N-donor (or boracyclic chelate) can substitute the vacant p-orbital of the three-coordinated boron atom to serve as the electron accepting unit. The 2-pyridylpyrrolide38 and phenylpyridinato39,40 boron compounds with triarylamino donors belong to this class of TADF emitters and have been successively employed in fabrication of efficient OLEDs.

In an aim to broaden the horizon of design strategy we set out to develop borane-based TADF materials with a much simplified molecular architecture, i.e. those with a direct B−N linkage between N-donor and boryl acceptor.41 Of particular interest are the N-borylated pyrroles and carbazoles, i.e. PrB(Mes) 2 and CzB(Mes) 2 , both of which exhibited fluorescence with large solvent dependent Stokes shift, deriving from the twisted intermolecular charge transfer excited states (TICT).42 Moreover, attachment of electron withdrawing B(FMes)2 group, FMes = 2,4,6-tris(trifluoromethyl)phenyl, resulted in a significant red-shifted emission, indicative of an even greater charge transfer character. Unfortunately, despite the prerequisite TICT being designed, no TADF properties have yet been detected. This is possible due to the smaller size of carbazole, which is known to exhibit better HOMO−LUMO overlap and hence affords relatively larger ΔET‑S; particularly when there existed less steric interaction between carbazole and nearby acceptor units.43 Alternatively, from chemistry point of view, the N-borylated acridines, e.g. ACBM, PACBM and SACBM shown below, are expected to be a more rational design in exhibiting TADF property because of the increased steric congestion between peri-H atoms of acridine and B(Mes)2 acceptor. Additionally, various 9,9-substituents were also incorporated on acridine in an attempt to probe their structure−luminescence relationship.

Figure 1. Structural drawing of ACBM with ellipsoids shown at the 40% probability, selected bond distances: B(1)−N(1) = 1.443(2), B(1)−C(16) = 1.611(2), B(1)−C(25) = 1.590(2), N(1)−C(1) = 1.441(2) and N(1)−C(13) = 1.456(2) Å; dihedral angle between the N(1)−C(1)−C(13) and B(1)−C(16)−C(25) triangular planes (θ): 17.58°; C(7)···N(1)−B(1) nonbonding angle (ϕ): 151.78°.

As can be seen, the boron center in all four X-ray structures displays trigonal planar arrangement, i.e., sum of bond angles around B(1) atom being close to 360°. Despite the fact that the B−C bond distances of B(Mes)2 unit are similar in lengths to the Ph2B unit in the four-coordinate boron compounds reported in literature,46−49 the B−N distances falls in the range of 1.437(2)−1.466(3) Å, which are notably shorter than the B−N bond (∼1.634 Å) of the corresponding fourcoordinate compounds. This could be due to the formation of an additional B−N dative bonding interaction exerted by the lone-pair donation from the nitrogen atom to the vacant porbital of boron atom. Moreover, these B−N distances are slightly elongated than the B−N distance of aliphatic aminoborane such as Me2N-BMe2 (1.403(1) Å),50 but are comparable to that of aromatic counterparts Ph 2NBPh2 (1.441(2) Å) and CzBPh2 (1.443(2) Å).51 As depicted in 27091


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recorded to be 17.58, 14.32, 31.93, and 49.86° and to be 151.78, 146.96, 162.40, and 179.40°, for ACBM, PACBM, SACBM_C, and SACBM_Y, respectively. These variations of bond angles, i.e. the dihedral angle θ and C···N−B nonbonding angle ϕ seem to be reversely proportional to the observed B−N bond distances, indicative of the competition between πinteractions of the C−N bonds of acridine and the partial N−B π-bonding to the B(Mes)2 acceptor. In another word, the smaller θ and ϕ afforded the so-called quasi-axial conformation,52−54 in which they existed an increased N → B dative bonding interaction and reduced π-conjugation within the acridine donor fragment. In contrast, those with the planar acridine fragment are considered to adopt the quasi-equatorial arrangement and with reduced N → B dative interaction at the ground state, because of the much twisted arrangement between acridine donor and B(Mes)2 acceptor. Thermal properties of these N-borylated compounds were investigated by using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a heating rate of 15 °C min−1. The TGA and DSC graphs were displayed in Figure S1 and the calculated data are depicted in Table S1. The lowest decomposition temperature (Td, which corresponds to 5% weight loss) of 246 °C was observed for PACBM, evidencing an adequate thermal stability under investigated condition, particularly to the parent sample, i.e., ACBM, which showed the highest Td of 325 °C. On the other hand, both PACBM and SACBM showed higher Tm and Tg than those of ACBM, which is apparently due to the higher rigidity of phenyl substituents. 2.2. Photophysics. Photophysical properties of these Nborylated compounds were analyzed by UV−vis absorption and photoluminescence studies as the crystalline sample, spincasted thin film and in various solvent (e.g., cyclohexane, toluene and CH2Cl2) at RT. The corresponding spectra are displayed in Figure 5. As can be seen, the absorption onset of ACBM and PACBM emerged at approximately 400 nm in all solvents, and their absorption extinction coefficient gradually increased upon shifting to the shorter wavelength. In sharp contrast, the onset of SACBM absorption occurred at the much lower energy region near 450 nm and the extinction coefficient increased rapidly, forming a broadened profile in the region between 330−380 nm. This band is assigned to a charge transfer (CT) transition, of which the absorption peak wavelength and profile are nearly independent to the solvent polarity, while the emission is expected to be subjected to changes of solvent polarity due to the CT character in the excited state. Further insight into the transition properties was gained by computational approach. Based on the density functional theory (DFT) calculation, the lowest singlet optical transition S0 → S1 is assigned to HOMO → LUMO for ACBM, PACBM and SACBM (Figure 6 and Tables S2−S4 and Figures S2−S4). For ACBM, PACBM, and SACBM, the electron density distributions in HOMO and LUMO are mainly localized at acridine (donor) and B(Mes)2 (acceptor) fragments, respectively, manifesting the charge transfer property in the lowest lying excited state. Despite the sameness of the transition, however, the difference lies in the percentage of orbital overlap between HOMO and LUMO. Figure S5 shows the HOMO/ LUMO overlap area, in which SACBM reveals the smallest overlap among three titled boron complexes, indicating its largest charge separation and hence the smallest transition gap, which is consistent with the absorption as well as the calculated data shown in Tables S2−S4. In sharp contrast to the

Figure 2. Structural drawing of PACBM with ellipsoids shown at the 40% probability, selected bond distances: B(1)−N(1) = 1.437(2), N(1)−C(1) = 1.449(2), N(1)−C(13) = 1.443(2), B(1)−C(26) = 1.603(2) and B(1)−C(35) = 1.604(3) Å; dihedral angle between the N(1)−C(1)−C(13) and B(1)−C(26)−C(35) planes (θ): 14.32°; C(7)···N(1)−B(1) nonbonding angle (ϕ): 146.96°.

Figure 3. Structural drawing of SACBM_C with ellipsoids shown at the 40% probability, selected bond distances: B(1)−N(1) = 1.454(2), B(1)−C(26) = 1.586(3), B(1)−C(35) = 1.585(3), N(1)−C(7) = 1.444(2) and N(1)−C(13) = 1.437(2) Å; dihedral angle between the N(1)−C(7)−C(13) and B(1)−C(26)−C(35) triangular planes (θ): 31.93°; C(1)···N(1)−B(1) nonbonding angle (ϕ): 162.40°.

Figure 4. Structural drawing of SACBM_Y with ellipsoids shown at the 40% probability, selected bond distances: B(1)−N(1) = 1.466(3), B(1)−C(26) = 1.575(3), B(1)−C(35) = 1.580(3), N(1)−C(7) = 1.429(3), and N(1)−C(13) = 1.423(3) Å; dihedral angle between the N(1)−C(7)−C(13) and B(1)−C(26)−C(35) planes (θ): 49.86°; C(1)···N(1)−B(1) nonbonding angle (ϕ): 179.40°.

Figures 1−4, the dihedral angle (θ) between acridine and B(Mes)2 unit and the C···N−B nonbonding angle (ϕ) are 27092


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Figure 5. Absorption and emission spectra of (a) ACBM, (b) PACBM, and (c) SACBM; absorption in cyclohexane (magenta), toluene (black) and CH2Cl2 (blue), emission spectra of crystalline solid (red) and spin-casted thin film (green) from CH2Cl2 solution at 298 K. There are two kinds of crystalline samples of SACBM, for which the emission are depicted as SACBM_C (red solid-cycle) and SACBM_Y (red hollow-cycle), respectively.

Figure 6. Frontier molecular orbitals (HOMO and LUMO) for ACBM, PACBM, and SACBM in cyclohexane.

previously reported four-coordinate borane complexes,38−40 for which the boron atom has rather small contribution on LUMO (38%). In other words, much different from the previous cases of four-coordinate boron complexes in which the boron atom acted virtually as a bridge in supporting donor and acceptor, the boron atom in these N-borylated complexes does serve as the genuine acceptor fragment, for which the situation is common to many TADF molecules bearing three-coordinate boron,30−32,55 simplifying the design strategy. The above proposed photoinduced charge-transfer property is manifested by the corresponding emission, of which the peak wavelength is greatly dependent on their physical states and environment, cf. Table 1. The spectra underwent a notable redshift, i.e., from crystalline sample to spin-casted thin film or upon dissolution in solvents ranging from nonpolar to polar. For example, the emission maximum of ACBM occurred at 439 nm (crystalline solid), 529 nm (thin film), 533 nm (in cyclohexane), 558 nm (in toluene), and 580 nm (in CH2Cl2),

for which the solution spectra can be ascribed to the larger change of dipole moment in the electronically excited state versus that of the ground state. As a result, upon photoinduced charge transfer the solvent relaxation takes place and solvent molecules having higher polarity result in more stabilization and hence the larger Stokes shifted emission. Similar behavior was observed for the diphenyl substituted analogue PACBM. Importantly, the spiro-SACBM exhibited two crystalline samples (see Figures 3 and 4) with distinctive emission characteristics, for which SACBM_C gave a peak wavelength at 482 nm, whereas SACBM_Y showed a much red-shifted emission peak maximum at 540 nm; the latter is close to that observed in the spin-casted thin film (532 nm), and in all solutions, cf. cyclohexane (527 nm), toluene (538 nm) and CH2Cl2 (560 nm). This red-shifting in emission is attributed to the effective CT process occurred, a result of the much perpendicularly arranged acridine of SACBM vs the B(Mes)2 acceptor unit. This phenomenon is reminiscent of a number of TADF compounds bearing structurally similar phenothiazine donor,52−54 for which the distortion, as 27093


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ACS Applied Materials & Interfaces Table 1. Selected Photophysical Properties of ACBM, PACBM, and SACBM Recorded at RT ACBM

PACBM

SACBM

state

λem [nm]

PLQY [%]

τ [ns] and pre-exp. factor

crystalline spin-casted thin filmb vac. deposited thin filmc codeposited thin filmd cyclohexanee toluenee CH2Cl2f crystalline spin-casted thin filmb vac. deposited thin film codeposited thin film cyclohexane toluene CH2Cl2 crystal_Cf crystal_Yf spin-casted thin film vac. deposited thin film codeposited thin film cyclohexane toluene CH2Cl2

439 529 536 527 533 558 580 440 513 511 496 522 537 558 482 540 532 531 518 527 538 560

77.5 44.3

7 (1) 26 (0.9948), 3916 (0.0052) 28 (0.9931), 5251 (0.0069) 19 (0.9963), 3026 (0.0037) 49 (0.9828), 4845 (0.0172) 49 (0.9870), 4338 (0.0130) 50 (0.9872), 5123 (0.0128) 6 (1) 21 (0.9935), 1121 (0.0065) 22 (0.9938), 4536 (0.0062) 13 (0.9963), 1616 (0.0037) 47 (0.9831), 1395 (0.0169) 49 (0.9870), 2850 (0.0130) 54 (0.9865), 4014 (0.0135) 8 (0.9999), 1351 (0.0001) 35 (0.9870), 3880 (0.0123) 28 (0.9933), 3003 (0.0067) 31 (0.9884), 3692 (0.0116) 19 (0.9958), 2643 (0.0042) 49 (0.9740), 4798 (0.0260) 45 (0.9835), 4278 (0.0165) 51 (0.9825), 3991 (0.0175)

76.2 93.1 80.2 90.0 74.7 73.5 37.6 73.2 79.4 89.1 48.1 99.7 65.0 99.0 97.8 94.2 53.9

ΔET‑Sa (kcal/mol) −2.46; −2.29; −2.67; −1.74; −1.91; −1.92;

− −2.57 − − − −

−2.33; −2.36; −2.66; −1.75; −1.91; −1.91; −3.78; −1.91; −2.31; −1.98; −2.58; −1.50; −1.77; −1.74;

− −2.61 − − − − − − − −1.22 − − − −

a ΔET‑S values were obtained from either the hypothesis of pre-equilibrium between S1 and T1 states or from the onsets of fluorescence and phosphorescence in thin film. bThese films were obtained by spin-casting from CH2Cl2 solution. cThese were prepared using vacuum thermodeposition. dThese were prepared via codeposition alone with the corresponding host material. eSolution data were measured after three freeze− pump−thaw degassing cycles. fThe sample is hand-picked for the measurement.

Figure 7. Transient decay characteristics of ACBM: (a) spin-casted thin film, degassed (b) cyclohexane, (c) toluene, and (d) CH2Cl2 at 298 K. IRF: instrument response function (red line). Fitting curve (blue line).

geomerty optimization from the SACBM_C structure (with a relatively more distorted quasi-equatorial conformation), which ended up affording an SACBM_Y-like structure shown in Figure 6 and Figure S6. This implicates that the SACBM_Y is a thermodynamically stable isomer (i.e., quasi-equatorial), where-

determined by X-ray structural and theoretical analysis, produced two ground-state conformers (cf. the quasi-axial and quasi-equatorial arrangements) with distinctive energy gaps between the lowest singlet excited state and lowest triplet excited state. In our computational approach, we executed the 27094


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Figure 8. (a) Chemical structures of the component materials; (b) schematic OLED architectures employed for different TADF emitters.

exponential value (∼1.23%, see Table 1), and manifests a more effective TADF process in crystalline SACBM_Y, giving rise to an extremely high Q. Y. of 99.7%. In degassed solution, as shown in Figure 7 and Figures S7 and S8, all three boron complexes exhibit a prompt and a slow delayed fluorescence. Apparently, the steady-state fluorescence is significantly decreased in the aerated solution (see Table 1 and Figure S9), indicating the quenching of the T1 state by molecular oxygen via energy transfer process. The combination of these two observations concludes the TADF character in solution. Comprehensive temperature-dependent emission dynamics of ACBM, PACBM, and SACBM in degassed toluene (Figure S10 and Table S6) have been performed. As a result, the ratio of delayed component (cf. prompt fluorescence) decreased upon lowering the temperature from 40 to 0 °C with an increase of the corresponding lifetime, further affirming the TADF behavior of the titled boron complexes. Qualitatively, under the assumption of equilibrium between S1 and T1 states, the difference in energy ΔET‑S can be estimated according to the relationship between ΔET‑S and S1T1 equilibrium constant Keq expressed as ΔET‑S = −RTln(Keq/ 3) where the equilibrium constant Keq can be deduced from the pre-exponential factors for prompt fluorescence versus delayed fluorescence components. The results (see Table 1) deduce a ΔET‑S value in the range from −1.5 to −2.0 kcal/mol, showing effective thermally activated energy for T1 → S1 reverse intersystem crossing. With this result, it is highly possible that the acridine of all three studied boron compounds has already converted to the quasi-equatorial conformation in both solution and spin-casted thin film for better reduction of the spatial overlap between HOMO and LUMO. 2.3. Electrochemical Studies. Cyclic voltammetry measurements were conducted and the corresponding voltammograms and numerical data are depicted in Figure S11 for scrutiny. As can be seen, the anodic sweep resulted in the formation of two reversible oxidation peaks for ACBM and

as SACBM_C is a dynamically stable structure, existing because of the slightly increased π-donation from acridine N atom to the B(Mes)2 unit. This puckered acridine moiety in SACBM_C gives a higher emission gap than that of SACBM_Y (see Figure 5c). Figure 7 and Figures S7 and S8 display the transient photoluminescence decay dynamics for ACBM, PACBM and SACBM in different physical states such as crystalline, spincasted thin film and in degassed solution. To analyze the TADF behavior, their corresponding lifetime data are listed in Table 1. In crystalline state, the emission of ACBM and PACBM showed a single exponential decay kinetics of ∼6−7 ns, which was attributed to the prompt fluorescence. Owing to the imposed lattice energy, the lack of TADF character in crystalline ACBM and PACBM may be attributed to the relatively small θ angle between quasi-axial arranged acridine and B(Mes)2 unit (17.58 and 14.32°) and nonbonding ϕ angle of 151.78 and 146.96° that enhances the N → B dative interaction at the ground state but reduces the charge transfer character upon excitation. As a result, the energy gap between singlet (S1) and triplet (T1) manifolds may be more separated for ACBM and PACBM, shutting off the reverse intersystem crossing and hence the TADF process. Conversely, both crystalline SACBM_C and SACBM_Y with quasi-equatorial acridine unit show increased θ of 31.93° and 49.86° and ϕ of 162.40° and 179.40°, and enlarged HOMO−LUMO spatial separation vs both ACBM and PACBM, respectively. The crystalline SACBM_C shows a sky-blue emission (λem = 482 nm) and dual decay components of τ1 = 8.7 ns and τ2 = 1.3 μs. However, the fitted data show rather small pre-exponential value (0.012%) for the 1.3 μs decay component (see Table 1 and Figure S8), indicating its slightly ineffective TADF process. On the contrary, crystalline SACBM_Y shows two obvious decay time constants (τ1 = 35.1 ns, τ2 = 3.9 μs), in which the 3.9 μs decay component has a substantial contribution to the pre27095


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Figure 9. (a) EL spectra, (b) current density−voltage−luminance (J−V−L) characteristics, (c) external quantum efficiency vs luminance, (d) luminance/power efficiencies vs luminance for devices A, B, and C.

Table 2. EL Characteristics of OLEDs with the Studied N-Borylated TADF Emitters external quantum efficiency (%)

a

luminance efficiency (cd/A)

power efficiency (lm/W)

CIE1931 (x, y)

emitter

a

b

a

b

a

b

Von (V)

ACBM PACBM SACBM

11.2 4.1 19.1

10.2 4.1 18.8

36.3 10.1 60.9

33.1 9.9 59.7

28.5 6.4 43.6

20.3 6.3 37.8

4.1 3.8 4.2

c

max. luminance (cd/m ) [@V]

b

d

2525 [12.0] 9235 [12.2] 10338 [12.8]

0.33, 0.56 0.22, 0.41 0.22, 0.59

0.34, 0.54 0.23, 0.41 0.22, 0.58

2

Maximum efficiencies. bMeasured at a brightness of 100 cd/m2. cTurn-on voltage measured at 1 cd/m2. dRecorded at 1000 cd/m2.

(ET) of 2.56, 2.71, 2.93, and 2.9 eV, respectively. Furthermore, 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC)63 and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB)64 with high carrier transport capability and wide bandgaps were also selected as the hole transport layer (HTL) and electron transport layer (ETL) to facilitate exciton confinement. In addition, a commonly used MoO3 was doped into TAPC to reduce the interfacial energy barrier and thus facilitate hole injection into the organic layer.65,66 Furthermore, because the thin films constructed in the stacked OLED architecture possess different refractive indices, their thickness would seriously affect the light out-coupling of the OLEDs.67,68 Thus, all HIL, HTL, and ITO layers were carefully adjusted to improve the output of radiation. The doping concentrations of TADF emitters in EML were also properly fine-tuned for optimization. Overall, mCP was the appropriate host for PACBM, while 26DCzppy suited both ACBM and SACBM. Basically, the suitable host−guest system should fulfill three general criteria, i.e., fast host−guest energy transfer, effective exciton confinement in the emitting layer and balanced carrier transports during operation. Among these factors, the carrier transport capabilities of the host−guest system are of particular importance. Therefore, although both mCP and PYD-2Cz possess higher ET than that of bluish-green PACBM, our results indicated that mCP could achieve better carrier balance for

three peaks for both PACBM and SACBM, for which the one with the highest peak potential at 0.66 V, 0.77 and 0.71 V can be ascribed to the formation of the radical cation of oxidized acridine, while other peaks with lowered oxidation potentials may be attributed to the dimerization of acridine radical cations following the initial oxidation. Such an observation is consistent with that observed for the functional triphenylamine and acridine-containing molecules.44 On the other hand, the reversible reduction of all boron compounds occurred in the region of −2.65 to −2.76 V, implying that their LUMO is similar in property and presumably is residing over the B(Mes)2 fragment. Moreover, reductions of SACBM is observed to be more facile at −2.65 V vs that of ACBM and PACBM at −2.75 V and −2.76 V. Notably, their relative reductive peak potentials mirror the LUMO energies obtained according to DFT calculation. 2.4. Fabrication of OLEDs. To investigate the electroluminescence (EL) performance of these N-borylated acridine compounds, we optimized OLEDs using different device architectures (cf. Figure 8). In general, to achieve adequate host−guest exothermic energy transfer,56 several host materials with wide triplet energy gap (ET) values were first evaluated, including 4,4′-N,N′-dicarbazolebiphenyl (CBP),57 2,6-bis(3(9H-carbazol-9-yl)phenyl)pyridine (26DCzppy),58,59 2,6-di(9H-carbazol-9-yl) pyridine (PYD-2Cz),60 and 3-bis(9carbazolyl)benzene (mCP),61,62 each with triplet energy gaps 27096


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practical luminance levels of 100 and 1000 cd/m2, respectively. Its small efficiency roll-off demonstrates the high potential of SACBM in EL applications.

PACBM. Similarly, 26DCzppy could realize optimal carrier balance in both devices A and C. As a result, OLED devices A and C (e.g., with ACBM and SACBM emitter) display green emission and their device architecture consist of ITO (70 nm)/ TAPC doped with 20 wt % of MoO3 (20 nm)/TAPC (70 nm)/26DCzPPy and 8 wt % ACBM or 4 wt % SACBM (20 nm)/ TmPyPB (50 nm)/ LiF (0.8 nm)/Al (150 nm), for which LiF and aluminum were employed as the electron injection layer and reflective cathode. On the contrary, device B showed a slightly higher energy emission with bluish-green color and, hence, the optimized structure was set to ITO (120 nm)/ TAPC doped with 20 wt % of MoO3 (40 nm)/TAPC (30 nm)/mCP doped with 6 wt % of PACBM (30 nm)/ TmPyPB (50 nm)/ LiF (0.8 nm)/Al (150 nm). The chemical structures of the materials used in the devices, along with the schematic OLED architectures are presented in Figure 8, whereas the EL characteristics and the numeric data are given in Figure 9 and Table 2, respectively. Figure 9a shows the EL spectrum of devices measured at 1000 cd/m2, for which the profile is analogous to the corresponding PL profiles, indicating that the exciton was well confined within the EML.69 Furthermore, the fwhm of the EL spectra of ACBM, PACBM, and SACBM emitters were calculated to be 3462 cm−1, 3801 cm−1, and 2887 cm−1, respectively. Clearly, the spiro-substituted SACBM gave the smallest fwhm and its CIE coordinate presented a pure green emission compared to the ACBM counterpart; thus SACBM meets the display application requirement for better color purity with the green-emitting gamut. Figure 9b depicts the current density−voltage−luminance (J−V−L) characteristics. Green-emitting ACBM and SACBM devices (i.e., both A and C) exhibited rather high turn-on voltages of 4.1 and 4.2 V, due to the poor carrier transport capability of 26DCzppy and thicker HTL.58,59 In contrast, the turn-on voltage of B was reduced to 3.8 V even using a higher energy emitter PACBM, due to the thinner device structure as well as better carrier transport capability of mCP.61,62 Figure 9c, d compares the efficiency and luminescence of all fabricated OLEDs. Bluish-green emitting B gave poor maximum efficiencies of 4.1% (10.1 cd/A, 6.4 lm/W), which is partially attributed to the possession of the most distorted quasi-axial acridine in PACBM. Furthermore, we measured the PLQY of all TADF emitters doped in the respective host materials, and with a thickness of 80 nm. The respective PLQY of ACBM, PACBM, and SACBM were estimated to be 0.76, 0.38, and 0.99. Thus, the relatively lower external quantum efficiency (EQE) obtained in PACBM-based device might be also due to the incompleted host-to-guest energy transfer since the triplet energy of mCP was only slightly higher than that of PACBM, leading a small residue emission of mCP around 350 nm in the EL spectrum. Although this mCP device led the incompleted energy transfer, it still afforded better carrier balance compared to the device that employed alternative host material PYD-2Cz. In contrast, green-emitting A and C showed much improved OLED performances, among which device A exhibited a peak efficiency of 11.2%, (36.3 cd/A, 28.5 lm/W), whereas much superior OLED efficiencies of 19.1% (60.9 cd/A, and 43.6 lm/ W) were achieved in C. The PLQY of SACBM was estimated to be 0.99 in doped thin film, showing good agreement with the superior efficiencies of C. Furthermore, it is worth noting that device C maintains forward efficiencies of 18.8% (59.7 cd/A, and 37.8 lm/W) and 14.3% (45.5 cd/A, and 23.5 lm/W) at

3. CONCLUSION In summary, we have strategically designed and synthesized a new series of N-borylated compounds, ACBM, PACBM, and SACBM, with distinctive functional acridine donor unit. SACBM reveals two isomers, SACBM_C (colorless) and SACBM_Y (light yellow) in the crystal form, in which TADF is prominent in SACBM_Y where donor (D) and acceptor (A) are much perpendicularly aligned, whereas the D/A tilted SACBM_C exhibits much less TADF efficiency, manifesting the importance of spatial separation between donor and acceptor chromophores. This viewpoint is further supported to the lack of TADF for ACBM and PACBM in crystalline form, in which the smaller dihedral angle θ between acridine and B(Mes)2 acceptor and C···N−B nonbonding angle ϕ have restricted the acridine fragment to adopt the quasi-axial conformation, probably due to the lattice constraint. In sharp contrast, the lattice constraint was eliminated or reduced in solution, spin-casted and thermo-deposited thin film and certain codeposited thin film (e.g., ACBM and SACBM), and allowed the occurrence of prominent TADF due to the formation of quasi-equatorial arranged acridine. According to the anomalous red shift of the title complexes in nonpolar solvent, the twisted acridine moiety (confirmed by X-ray analyses) may undergo certain planarization motion in the excited state. This semilarge amplitude motion is prohibited in the strictly rigid crystal (or powder) form, but can be partially allowed in the film, giving the red-shifted emission in the film (cf. solid powder). In fact, this contribution presents an unique example on the conformational isomerization for acridine donor, as well as its potential influence to the TADF emission. Finally, OLED fabricated with SACBM gave bright green emission with CIE chromaticity of (0.22, 0.59) and maximum external quantum efficiency, luminance efficiency and power efficiency of 19.1%, 60.9 cd/A, and 43.6 lm/W, respectively. These results demonstrate for the first time that N-borylated emitters are also capable to form highly efficient TADF and suitable for making efficient OLEDs. 4. EXPERIMENTAL SECTION 4.1. Syntheses. General Procedures. All reactions were conducted under nitrogen atmosphere unless otherwise noted. Solvents were dried over appropriate drying agents and distilled prior to use. Commercially available reagents were used without further purification. 9,9-Dimethyl-9,10-dihydroacridine was obtained from esterification of N-phenylanthranilic acid in methanol to afford 2-anilinobenzoic acid methyl ester, followed by treatment with methylmagnesium bromide and cyclization in the presence of sulfuric acid,44 whereas 9,9diphenyl-9,10-dihydroacridine was obtained from treatment of 2anilinobenzoic acid methyl ester with phenylmagnesium bromide and cyclization in the presence of sulfuric acid.45 10H-Spiro[acridine-9,9fluorene] was obtained from a multistep protocol starting from 2bromo-N-phenylaniline, which was synthesized from coupling of 1bromo-2-iodobenzene and aniline in the presence of [Pd(dppf)2Cl2]. After being lithiated with n-BuLi in hexane (2.5 M), it was coupled with 9-fluorenone and then cyclized in the presence of methanesulfonic acid.33 All reactions were monitored by precoated TLC plates (0.20 mm with fluorescent indicator F254). Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment (FAB) mode. 1H and 13C NMR spectra were recorded on a Varian Mercury-400 or an INOVA-500 instrument. 1H NMR spectra were obtained in CDCl3 and calibrated using residual 27097


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reflections collected, 6085 independent reflections (Rint = 0.0339), max. and min transmission = 0.7533 and 0.6154; restraints/parameters = 0/412, GOF = 1.011, final R1[I > 2σ(I)] = 0.0509 and wR2(all data) = 0.1255; Largest diff. peak and hole = 0.229 and −0.218 e Å−3. Selected Crystal Data of SACBM_Y. C43H38BN; M = 579.55; monoclinic; space group = P21/n; a = 7.5530(2), b = 15.3830(3), c = 28.0519(6) Å; β = 94.1191(8)°; V = 3250.87(13) Å3; Z = 4; ρcalcd = 1.184 Mg m−3; F(000) = 1232; crystal size = 0.453 × 0.135 × 0.132 mm3; λ(Cu−Kα) = 1.54178 Å; T = 200(2) K; μ = 0.505 mm−1; 15082 reflections collected, 6138 independent reflections (Rint = 0.0254), max. and min transmission = 0.7536 and 0.5994; restraints/parameters = 0/413, GOF = 1.120, final R1[I > 2σ(I)] = 0.0627 and wR2(all data) = 0.1874; Largest diff. peak and hole = 0.370 and −0.350 e Å−3. 4.2. Photophysical Measurements. Steady-state absorption and emission spectra in both solution and solid were recorded with a Hitachi (U-3310) spectrophotometer and an Edinburgh (FS-920) fluorimeter, respectively. Lifetime studies were performed with an Edinburgh (FL-900) photon-counting system with a hydrogen- or nitrogen-filled excitation source with 40 kHz repetition rate. 4.3. OLED Fabrication. Organic materials and the indium tin oxide (ITO)-coated glass with sheet resistance of ∼15 Ω/square were purchased from Lumtec and Shine Materials Technology. The ITO substrate was washed with deionized water and acetone in sequence, followed by treatment with UV-Ozone for 5 min. All organic materials were subjected to temperature gradient sublimation. The organic and metal layers were deposited onto the ITO-coated glass substrate by thermal evaporation and the device fabrication was completed in a single cycle without breaking the vacuum. Shadow mask was used to define the active area (2 × 2 mm2) of the device. Current density− voltage−luminance characterization was measured using a Keithley 238 current source-measure unit and a Keithley 6485 pico-ammeter equipped with a calibrated Si photodiode. The electroluminescent spectra were recorded using an Ocean Optics spectrometer. 4.4. Single-Crystal X-ray Diffraction Studies. Single-crystal Xray diffraction study was measured with a Bruker SMART Apex CCD diffractometer using (Cu−Kα) radiation (λ = 1.54178 Å). The data collection was executed using the SMART program. Cell refinement and data reduction were performed with the SAINT program. An empirical absorption was applied based on the symmetry-equivalent reflections and the SADABS program. The structures were solved using the SHELXS-97 program and refined using the SHELXL-97 program by full-matrix least-squares on F2 values. The structural analysis and molecular graphics were obtained using the SHELXTL program on a PC. 4.5. Computational Method. All the calculations were performed with the Gaussian 09 program package. The geometry optimization of ground states for the three complexes was simulated with density functional theory (DFT) at the B3LYP/6-31g(d) levels. The calculated lowest optical transitions were performed with the timedependent density functional theory (TD−DFT) method. The solvent effect was based on the polarizable continuum model (PCM), which was implemented in the Gaussian 09 program.

CHCl3 (δ = 7.24 ppm) as an internal reference. Elemental analysis was carried out with a Heraeus CHN-O-rapid elemental analyzer. 4.1.1. Preparation of ACBM. To a solution of acridine (0.74 g, 3.5 mmol) in THF (10 mL) was slowly added 2.8 mL of 2.5 M solution of n-BuLi in hexaneat −78 °C and stirred for 2 h. After that, dimesitylfluoroborane (1.1 g, 3.9 mmol) in THF (8 mL) was added, and was gradually warmed up to RT and stirred for 12 h. The mixture was evaporated to dryness, and the residue was dissolved into CH2Cl2, washed with H2O, dried over MgSO4, and concentrated. The resulting solid was purified by silica gel column chromatography, eluting with a mixture of CH 2 Cl 2 and hexane (v/v = 1/9), followed by recrystallization from mixed CH2Cl2 and MeOH to afford ACBM (0.85 g, 53%) as a white solid. Characterization of ACBM. 1H NMR (400 MHz, CDCl3, 298 K, δ): 7.46 (d, J = 7.7 Hz, 2H), 7.26 (d, J = 8.1 Hz, 2H), 7.01 (t, J = 7.7 Hz, 2H), 6.75 (t, J = 8.1 Hz, 2H), 6.66 (s, 4H), 2.22 (s, 6H), 2.16 (s, 12H), 1.76 (s, 6H). 13C NMR (100 MHz, CDCl3, 298 K, δ): 140.4, 140.3, 137.6, 135.7, 128.1, 126.2, 125.4, 124.0, 121.8, 110.0, 36.3, 34.4, 22.9, 21.1. MS (FAB): m/z 457.3 [M+]; Anal. Calcd for C33H36BN: C, 86.64; H, 7.93; N, 3.06. Found: C, 86.91; H, 8.04; N, 3.25. Selected Crystal Data of ACBM. C33H36BN; M = 457.44; orthorhombic; space group = P212121; a = 9.35642(2), b = 14.0428(3), c = 19.8797(4) Å; V = 2611.99(9) Å3; Z = 4; ρcalcd = 1.163 Mg m−3; F(000) = 984; crystal size = 0.291 × 0.152 × 0.142 mm3; λ(Cu−Kα) = 1.54178 Å; T = 200(2) K; μ = 0.491 mm−1; 15957 reflections collected, 4962 independent reflections (Rint = 0.0187), max. and min transmission = 0.7533 and 0.6949; restraints/parameters = 0/324, GOF = 1.063, final R1[I > 2σ(I)] = 0.0328 and wR2(all data) = 0.0851; largest diff. peak and hole = 0.175 and −0.191 e Å−3. 4.1.2. Preparation of PACBM. Following the procedure described for ACBM, reaction of 9,9-diphenyl acridine (0.90 g, 2.7 mmol) and dimesitylfluoroborane (0.73 g, 3.0 mmol) afforded PACBM (0.59 g, 38%) as a white solid. Single crystal of PACBM was obtained from a layered solution of mixed CH2Cl2 and MeOH at RT. Characterization of PACBM. 1H NMR (400 MHz, CDCl3, 298 K, δ): 7.44 (d, J = 8.6 Hz, 2H), 7.26−7.23 (m, 6H), 7.04−7.01 (m, 4H), 6.96 (t, J = 14.9 Hz, 2H), 6.89−6.85 (m, 4H), 6.59 (s, 4H), 2.18 (s, 12H), 1.88 (s, 6H). 13C NMR (100 MHz, CDCl3, 298 K, δ): 148.0, 142.6, 140.5, 140.3, 137.3, 135.5, 131.7, 130.6, 127.9, 127.7, 126.4, 126.0, 123.4, 122.1, 56.9, 22.7, 21.0. MS (FAB): m/z 581.3 [M+]; Anal. Calcd for C43H40BN: C, 88.80; H, 6.93; N, 2.41. Found: C, 88.83; H, 6.78; N, 2.79. Selected Crystal Data of PACBM. C43H40BN; M = 581.57; monoclinic; space group = P21/n; a = 8.9156(2), b = 16.0349(3), c = 23.3345(4) Å; β = 99.6606(10)°; V = 3288.61(11) Å3; Z = 4; ρcalcd = 1.175 Mg m−3; F(000) = 1240; crystal size = 0.258 × 0.218 × 0.169 mm3; λ(Cu−Kα) = 1.54178 Å; T = 200(2) K; μ = 0.499 mm−1; 17781 reflections collected, 6196 independent reflections (Rint = 0.0294), max. and min transmission = 0.7533 and 0.6552; restraints/parameters = 0/412, GOF = 1.024, final R1[I > 2σ(I)] = 0.0523 and wR2(all data) = 0.1241; Largest diff. peak and hole = 0.248 and −0.287 e Å−3. 4.1.3. Preparation of SACBM. Following the procedure described for ACBM, reaction of spiro-acridine-fluorene (0.84 g, 2.7 mmol) and dimesitylfluoroborane (0.73 g, 3.0 mmol) afforded SACBM (0.23 g, 15%) as a yellow solid. Single crystal of SACBM was obtained from a layered solution of mixed CH2Cl2 and MeOH at RT. Characterization of SACBM. 1H NMR (400 MHz, CDCl3, 298 K, δ): 7.81 (d, J = 7.4 Hz, 2H), 7.40−7.34 (m, 4H), 7.20 (t, J = 14.9 Hz, 2H), 7.12 (d, J = 7.4 Hz, 2H), 6.73 (s, 4H), 6.66 (t, J = 14.6 Hz, 2H), 6.58 (t, J = 14.6 Hz, 2H) 6.28 (d, J = 7.8 Hz, 2H), 2.27 (s, 12H), 2.24 (s, 6H). 13C NMR (100 MHz, CDCl3, 298 K, δ): 206.9, 158.6, 142.3, 140.5, 139.6, 138.6, 129.6, 128.6, 128.4, 127.3, 127.0, 126.1, 125.9, 123.4, 120.6, 119.7, 29.7, 22.7, 21.2. MS (FAB): m/z 579.6; found: 579.3 [M+]. Anal. Calcd for C43H38BN: C, 89.11; H, 6.61; N, 2.42. Found: C, 89.56; H, 6.53; N, 2.67. Selected Crystal Data of SACBM_C. C43H38BN; M = 579.55; monoclinic; space group = P2/c; a = 14.0414(3), b = 8.3928(2), c = 27.5660(6) Å; β = 95.6628(13)°; V = 3232.71(13) Å3; Z = 4; ρcalcd = 1.191 Mg m−3; F(000) = 1232; crystal size = 0.481 × 0.086 × 0.035 mm3; λ(Cu−Kα) = 1.54178 Å; T = 200(2) K; μ = 0.508 mm−1; 14771

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b08258. Detailed DFT data and the device data of OLEDs with different ETL thicknesses (PDF) Crystallographic information data of N-borylated compounds (CIF)

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. 27098


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Pi-Tai Chou: 0000-0002-8925-7747 Yun Chi: 0000-0002-8441-3974 Author Contributions †

Y.-J.L. and T.-C.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan, under Grant 102-2221-E-155-080-MY3. REFERENCES

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