Twisted biaryl-amines as novel host materials for green-emissive phosphorescent organic light-emitting diodes (PhOLEDs)

Abstract

Insertion of rigid biaryl moieties, namely, 2,2′,6,6′-tetramethylbiphenyl and 2,2′,6,6′-tetramethoxylbiphenyl, between two phenyl rings of CBP leads to novel host materials with improved thermal stabilities (T_gs ∼ 111–148 °C). Their high triplet energies (2.63–2.83 eV) permit application as host materials for green phosphors, i.e., Ir(ppy)₃. External quantum efficiency and brightness of the order of ca. 15% and ca. 6000 cd m⁻² have been realized with TetMeTPA and TetOMeCZL, respectively.

---

Introduction

Organic light-emitting diodes (OLEDs) have emerged as a huge commercial enterprise for lighting and displays; the advantages of OLED-based technologies are indeed unrivaled by any other existing technology.^1–10 Recent years have witnessed a progressive shift of focus from conventional fluorescence-based devices to phosphorescence-based ones in the quest for high efficiencies of light emission.^11–15 It is well-established that phosphorescence-based emitters – largely organometallic complexes of Ir, Os, Pt, etc. – can harvest both singlet and triplet states for radiative emission by facile spin–orbit coupling, whereby ∼100% internal quantum efficiency may be realized.^14,15 A drawback associated with triplet emitters is their long lifetime, which leads to deactivation of the excited states by the so-called triplet–triplet annihilation and concentration quenching processes.^14–16 To overcome the latter, the triplet emitters – popularly known as dopants – are dispersed in suitable host matrices.¹⁴ For an organic material to be applied as a host in phosphorescent OLED (PhOLED) devices, there are certain criteria that must be met in addition to the amorphous property with good stabilities (glass transition temperature – T_g and thermal decomposition temperature – T_d) and morphological stabilities. They are:¹⁴ (i) the triplet energy of the host must be higher than that of the dopant to preclude reverse energy transfer, and (ii) the energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the host should be commensurate with those of the adjacent hole- and electron-transporting materials, respectively, for facile charge injection into the host. Despite a tremendous progress in the development of host materials for PhOLEDs, 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) and m-dicarbazolylbenzene (mCP) continue to be the two most common unipolar host materials employed for a green dopant such as fac-tris(2-phenylpyridinato-N,C^2′)iridium(III) Ir(ppy)₃. Both of these are associated with significantly low T_gs, i.e., 62 (ref. 17) and 60 °C (ref. 18) for CBP and mCP, respectively. There have been efforts made to modify CBP to improve its triplet energy as well as T_g. Indeed, the modified CBP derivatives have been exploited as thermally-stable host materials in blue PhOLED devices.^19–21 However, their utility as hosts for green dopants has seldom been explored. This indeed was the motivation for our present investigation.

In continuation of our studies on biaryl-based systems as applied to OLEDs^22–25 and hybrid metal–organic materials,^26–28 we surmised that rigidification of the flexible biphenyl core in CBP should lead to novel host materials with improved T_gs. We, therefore, designed two modified CBP analogs, i.e., TetMeCZL and TetOMeCZL (Scheme 1), by insertion of 2,2′,6,6′-tetramethylbiphenyl and 2,2′,6,6′-tetramethoxybiphenyl scaffolds, respectively, between the two phenyl rings of the biphenyl core in CBP, cf. Scheme 1.²⁹ Two more analogous host materials in which phenylcarbazole groups are replaced with triphenylamine groups, i.e., TetMeTPA and TetOMeTPA, were also synthesized, for it is well-known that the triplet energy of triphenylamine is high and that it imparts good hole-transporting property.³⁰ In view of the fact that the angles between the aromatic planes in all of the target amines in Scheme 1 vary depending on the nature of the substituents, i.e., Me and OMe, the charge carrier properties as well as triplet energies were also expected a priori to manifest accordingly in the device performance results. Herein, we report synthesis, photophysical, electrochemical and thermal properties of CBP analogs in Scheme 1, and demonstrate that these amines, with the exception of TetMeCZL, function very well as host materials for the green-emissive dopant, namely, (Ir(ppy)₃).

Scheme 1 Synthesis of the target diarylaminophenyl-functionalized biaryl hosts.

Results and discussion

Synthesis of diarylaminophenyl-functionalized biaryl hosts

Two-fold iodination of the core scaffolds, namely, 2,2′,6,6′-tetramethoxybiphenyl and 2,2′,6,6′-tetramethylbiphenyl, with N-iodosuccinimide afforded the corresponding diiodo-biaryls in excellent isolated yields; 2,2′,6,6′-tetramethoxybiphenyl²⁷ and 2,2′,6,6′-tetramethylbiphenyl²⁸ were prepared by literature-reported procedures. Diiodo-biaryls were subjected to Suzuki coupling with appropriate boronic acids to afford diarylaminophenyl-functionalized biaryl hosts in 82–98% isolated yields, Scheme 1.

Photophysical properties

In Fig. 1 are shown UV-Vis absorption and fluorescence spectra of the biaryl-amines recorded in dilute DCM (ca. 10⁻⁵ M) solutions. A careful analysis of the absorption and fluorescence spectra reveals that the optical features of the carbazole derivatives, i.e., TetMeCZL and TetOMeCZL, are considerably different from those of triphenylamines, i.e., TetMeTPA and TetOMeTPA. Insofar as their absorption spectra are concerned, both carbazole derivatives exhibit similar structured absorptions; the absorption maxima for both compounds lie at ca. 293 nm, with two more overlapping absorption bands of lower optical density at 328 and 342 nm. Similarly, the absorption profiles of both triphenylamino-derivatives are similar with their absorption maxima within a small range of 308–313 nm. They are, however, broad and featureless in contrast to those of carbazole derivatives. A similar trend is also observed in the fluorescence spectra of all of these compounds. Both TetMeCZL and TetOMeCZL exhibit similar fluorescence spectra that are characterized by two closely spaced bands at ca. 352 and 367 nm. In a similar manner, the triphenylamino-derivatives, i.e., TetMeTPA and TetOMeTPA, display comparable emission spectra that are broad in nature and red-shifted relative to those of the carbazole compounds. The structured absorption as well as emission for carbazole derivatives when compared to those of triphenylamine derivatives should be ascribed to the rigid structural attributes. The fluorescence quantum yields of the compounds in DCM, calculated relative to anthracene as the standard, were found to be in the range of 8.4–15.2, cf. Table 1.

Fig. 1 Normalized absorption (a) and emission (b) spectra of TetMeTPA, TetOMeTPA, TetMeCZL and TetOMeCZL recorded in DCM (ca. 1 × 10⁻⁵ M) at rt; the excitation wavelength was 290 nm.

Table 1 Photophysical, electrochemical and thermal properties of the biaryl-amines

Compound	λmax (UV)^a (nm)	Band gap^b (eV)	λmax (PL) soln^a (nm)	Φfl soln^c (%)	ET (eV)	HOMO^d/LUMO^e (eV)	Td^f (°C)	Tg^g (°C)
a Absorption and fluorescence spectra were recorded in dilute DCM solutions (ca. 10^−5 M).b Band gap energies were calculated from red edge absorption onset values using the formula E = hc/λ.c Quantum yields were determined for excitation at 290 nm relative to anthracene as the standard.d HOMO energies were calculated from oxidation potentials in the CV spectra.e LUMO energies were calculated by subtracting the band gap energies from HOMO energies.f From TGA.g From DSC.
TetMeTPA	308	3.44	385	8.4	2.70	5.29/1.85	384	111
TetOMeTPA	313	3.40	389	15.2	2.63	5.24/1.84	406	123
TetMeCZL	293	3.47	353, 368	10.0	2.83	5.17/1.70	472	140
TetOMeCZL	293	3.46	351, 366	12.1	2.73	4.98/1.52	402	148

---

In Fig. 2 are shown phosphorescence spectra of the compounds recorded in dilute 2-methyltetrahydrofuran solutions (ca. 10⁻⁵ M) at 77 K; it should be noted that a small amount of iodomethane was added to the solutions of the hosts in 2-MeTHF to induce heavy atom effect while recording the phosphorescence spectra. In the absence of iodomethane, structured emission profiles were not obtained. The triplet energies (E_Ts) of the hosts, calculated from the 0–0 transitions, are in the range of 2.63–2.83 eV, which are higher than that of the conventional green dopant, i.e., Ir(ppy)₃ (E_T ∼ 2.42 eV).³¹

Fig. 2 Phosphorescence spectra of the hosts in 2-methyltetrahydrofuran (ca. 1 × 10⁻⁵ M) at 77 K. MeI (10 μL to 5 mL solution of the hosts in 2-methyltetrahydrofuran) was added to induce heavy atom effect.

Electrochemical properties

Electrochemical properties of the amines were examined by cyclic voltammetry in dilute DCM (ca. 1 × 10⁻³ M) solutions. All of them were found to show oxidation peaks upon anodic sweep, but no conspicuous reduction peaks upon cathodic sweep within the limits of the applied potential window. The triphenylamino-derivatives, i.e., TetMeTPA and TetOMeTPA, exhibited reversible oxidation, while the carbazole derivatives, i.e., TetMeCZL and TetOMeCZL, displayed irreversible oxidation, cf. Fig. S2.† The HOMO energies of the amines – calculated from following the relation: E_HOMO = −[(E_ox − E_1/2,Fc/Fc⁺) + 4.8] eV – were found to be in the range of 4.98–5.29 eV, cf. Table 1. The LUMO energies were calculated by subtraction of the band gap energies from the HOMO energies; the band gap energies were in turn obtained from the red edge of the absorption in each case.

Thermal properties

Thermal properties of the amines were investigated by thermogravimetric and differential scanning calorimetric analyses at a heating rate of 10 °C per min under inert atmosphere. All the amines were found to exhibit very high thermal decomposition temperatures (T_ds) in the range of 384–472 °C (Fig. S3†); the glass transition temperatures (T_gs) of the compounds were found to be moderate, which ranged between 111–148 °C, cf. Fig. S4.† It is noteworthy that these values are much higher than those of CBP and mCP. Increased molecular mass as well as structural rigidity are seemingly responsible for the high thermal stabilities.

Performance as host materials for electroluminescence

The higher triplet energies of TetMeTPA, TetMeCZL and TetOMeCZL than that of bis[(4′,6′-difluorophenyl)-pyridinato-N,C^2′]iridium(III)picolinate (FIrpic, E_T = 2.65 eV)³² should in principle permit their use as host systems for facile energy transfer to the latter. The main drawback, however, is that they possess high HOMO energies. We have found in our recent investigations that the host systems with higher HOMO levels than that of the dopant are highly deleterious for device functioning.³³ Indeed, the devices fabricated with biaryl-based hosts and FIrpic as a dopant yielded very poor results in agreement with our previous observations. The ability of biaryl-amines to function as hosts for the green dopant, i.e., Ir(ppy)₃, was examined by fabrication of devices in which the host dispersed with Ir(ppy)₃ constitutes emissive layer. To begin with, three devices of the following configurations were fabricated with TetMeTPA as the host: (A) ITO/NPB (45 nm)/host:Ir(ppy)₃ (7–8%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), (B) ITO/HATCN (10 nm)/TAPC (45 nm)/host:Ir(ppy)₃ (7–8%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), and (C) ITO/TAPC (45 nm)/host:Ir(ppy)₃ (7–8%, 30 nm)/TmPyPB (45 nm)/LiF (2 nm)/Al (150 nm), where N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) and di-[4-(N,N-di-p-tolyl-amino)-phenyl]cyclohexane (TAPC) serve as hole-transporting materials, dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN) as a hole-injection material, 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) functions as an electron-transporting material and LiF/Al as the composite cathode. In Table 2 are collected the results of fabricated devices. As may be seen, the devices fabricated with TetMeTPA as the host lead to comparable results in configurations A and C. Therefore, devices of configurations A and B were fabricated for the remaining compounds to compare their efficacy as host materials for Ir(ppy)₃ as the phosphor. EL spectra and I–V–L characteristics of the devices of configurations A and B are shown in Fig. 3 and 4, respectively.

Table 2 Device performance results for PhOLED devices constructed with the host materials

Compound	Device^a	Von^b	η^maxex^c/ηex^100^d	ηp^e	ηl^f	Lmax^g	λmax^h	CIE^i (x, y)
a A and B refer to the device configurations, see text.b Turn-on voltage (V).c Maximum external quantum efficiency (%).d External quantum efficiency (%) at a luminance of 100 cd m^−2.e Maximum power efficiency (lm W^−1).f Maximum luminous efficiency (cd A^−1).g Maximum luminance achieved (cd m^−2).h λ^ELmax (nm).i 1931 chromaticity coordinates measured at 8 V.
TetMeTPA	A	3.5	11.8/10.1	31.0	34.5	3760	504	0.27, 0.60
	B	3.5	15.3/11.9	41.2	45.9	1790	504	0.27, 0.60
	C	3.5	10.6/10.4	26.2	34.4	4310	504	0.28, 0.60
TetOMeTPA	A	5.0	4.9/4.8	7.4	15.8	4120	504	0.27, 0.60
	B	5.0	12.8/7.3	20.9	36.6	3720	504	0.27, 0.60
TetMeCZL	A	6.0	2.4/2.4	2.1	7.9	687	508	0.28, 0.55
	B	6.0	2.7/2.6	2.5	8.9	1330	508	0.28, 0.57
TetOMeCZL	A	4.0	14.9/3.2	25.8	32.8	6090	508	0.30, 0.61
	B	5.0	13.3/8.3	20.9	33.3	5530	512	0.29, 0.61
CBP	B	4.0	13.0/12.5	33.3	42.5	22 200	508	0.30, 0.61

---

Fig. 3 EL spectra recorded for the devices fabricated with biaryl-based amines as hosts in configurations A (a) and B (b).

Fig. 4 Current density vs. voltage (a) and luminance vs. voltage (b) profiles for the PhOLED devices of configurations A and B.

A perusal of the data in Table 2 sufficiently brings out the fact that all amines with the exception of TetMeCZL function nicely as host materials for the green dopant, i.e., Ir(ppy)₃. Efficiencies of light emission from the devices are better in configuration B than in A for all the compounds with the exception of TetMeCZL. Device B fabricated with the triphenylamino-derivatives, i.e., TetMeTPA and TetOMeTPA, exhibit maximum luminous efficiencies of 45.9 and 36.6 cd A⁻¹, respectively, which are much better than those obtained for the devices fabricated with similar amine hosts based on Tröger's base reported recently by us.³³ External quantum efficiencies at the peak value and also measured at a practical luminance of 100 cd m⁻² are collected in Table 2. Although TetMeTPA, TetOMeTPA and TetMeCZL exhibit stable external quantum efficiencies with low roll-off in the luminance range of 1–100 cd m⁻², that of TetOMeCZL is characterized by large fluctuation, suggesting the instability of the device in this luminance range cf. Fig. S6.† The performance of TetOMeCZL as a host is, however, more impressive than others at higher values of luminance and current density, cf. Fig. S6 and S8,† which suggests that the devices fabricated with TetOMeCZL as the host material are more stable at higher brightness. Despite the drawback with mismatching of the HOMO–LUMO levels (Fig. 5), the impressive performance exhibited by TetOMeCZL points to highly efficient energy transfer to the dopant, i.e., Ir(ppy)₃. The poor performance with the analogous TetMeCZL is indeed intriguing. We wondered if severe structural changes during sublimation under high vacuum are a cause. The vacuum sublimed films of TetMeCZL were indeed found to exhibit broad emission, which extends from 350 to 625 nm. Presumably, TetMeCZL undergoes, with sublimation at high temperatures, structural reorganization or fractional decomposition in the solid state to account for the observed poor device performance results, cf. ESI.†

Fig. 5 Energy level diagrams of the devices for configurations A and B (hosts from left to right are TetMeTPA, TetOMeTPA, TetMeCZL, TetOMeCZL and CBP).

A comparison of the device results in Table 2 points to an overall superior performance of TetMeTPA in terms of efficiencies. A closer look at the energy level diagram in Fig. 5 reveals that the HOMO and LUMO energies of Ir(ppy)₃ are buried within those of TetMeTPA, leading to favorable conditions for energy transfer. Further, the LUMO of TetMeTPA has the lowest energy gap when compared with other amines, which may permit better injection of electrons from TmPyPB layer to the host. It is noteworthy that the performance of CBP as a control host is much better than any of the materials investigated. Of course, its lower HOMO level relative to that of Ir(ppy)₃ and a relatively small gap between the LUMO levels of CBP and TmPyPB contribute to its better performance as the host. It appears that the choice of TmPyPB – one of the most frequently employed ETMs – as an ETM is not the right one for these materials to function as hosts. ETMs with high LUMO energies such as 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD, LUMO = 2.4 eV),³⁴ diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, LUMO = 2.5 eV),²¹ etc. should lead to improvement of device performance results; of course, introduction of bifunctionality into the molecular structure is another approach to improve the efficiencies by balanced carrier transport.^35–38 We are currently exploring these exciting opportunities. Nonetheless, the utility of modified-biaryl-based amines as high T_g host materials is compellingly borne out from limited device fabrication results.

Conclusions

Four novel host materials were designed and synthesized based on two rigid scaffolds, namely, 2,2′,6,6′-tetramethylbiphenyl and 2,2′,6,6′-tetramethoxylbiphenyl, with an objective to develop amorphous host materials characterized by high T_gs. Indeed, T_gs of the biaryl-based amines have been found to be high in the range of 111–148 °C; these values are significantly higher than those of the prototype, yet popular host materials, i.e., CBP and mCP. Clearly, the rigidification as well as increased molecular mass manifest in their high thermal stabilities. The high triplet energies (2.62–2.83 eV) permit their utility as host materials for the green phosphor, i.e., Ir(ppy)₃. With a limited experimentation of the fabrication of devices, external quantum efficiencies of the order of ca. 15% have been achieved with TetMeTPA. Although CBP performs relatively better than the host materials designed and examined, there is no reason to doubt that more experimentation with high LUMO materials will lead to better device performance results. Otherwise, the fact that insertion of a rigid twisted biaryl moiety between two phenyl rings of CBP leads to novel hosts with improved thermal stabilities is compellingly borne out.

Experimental section

Synthesis of 3,3′-diiodo-2,2′,6,6′-tetramethoxybiphenyl

2,2′,6,6′-Tetramethoxybiphenyl was synthesized by following the literature-reported procedure.²⁷ Accordingly, 2,2′,6,6′-tetramethoxybiphenyl (0.50 g, 1.82 mmol) was added to a 25 mL round bottom flask charged with 7 mL of acetonitrile. To this solution, N-iodosuccinimide (0.83 g, 3.74 mmol) was added in small portions. After the addition of NIS was complete, 0.1 mL of trifluoroacetic acid (TFA) was added to the reaction mixture, and was allowed to stir for 3 h. At the end of this period, crushed ice was added to the reaction mixture. The organic matter was extracted two times with chloroform, and the combined organic extract was dried over anhydrous Na₂SO₄. The solvent was removed in vacuo to obtain the crude product, which was subjected to a short pad silica gel filtration to obtain 3,3′-diiodo-2,2′,6,6′-tetramethoxybiphenyl as a white solid, yield 0.95 g (98%); IR (KBr) cm⁻¹ 2933, 2832, 1562, 1481, 1461, 1400, 1389; ¹H NMR (CDCl₃, 400 MHz) δ 3.49 (s, 6H), 3.71 (s, 6H), 6.57 (d, J = 8.72 Hz, 2H), 7.75 (d, J = 8.72 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 56.0, 60.5, 80.9, 108.9, 118.6, 138.7, 158.1, 158.8; EI-MS⁺ m/z [M]⁺ calcd for C₁₆H₁₆I₂O₄ 525.9138, found 525.9145.

Synthesis of 3,3′-diiodo-2,2′,6,6′-tetramethylbiphenyl

2,2′,6,6′-Tetramethylbiphenyl was synthesized by following the literature-reported procedure.²⁸ Following the procedure described above, the reaction of 2,2′,6,6′-tetramethbiphenyl (2.0 g, 9.52 mmol) with N-iodosuccinimide (5.3 g, 23.81 mmol) in the presence of 1 mL of TFA in 15 mL of acetonitrile led to formation of 3,3′-diiodo-2,2′,6,6′-tetramethylbiphenyl as a colorless solid, yield 3.66 g (95%); IR (KBr) cm⁻¹ 2970, 2914, 2852, 1436, 1377; ¹H NMR (CDCl₃, 400 MHz) δ 1.81 (s, 6H), 2.03 (s, 6H), 6.84 (d, J = 8.00 Hz, 2H), 7.73 (d, J = 8.00 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 19.7, 25.4, 98.9, 129.4, 135.8, 138.00, 138.10, 141.1; EI-MS⁺ m/z [M]⁺ calcd for C₁₆H₁₆I₂ 461.9341, found 461.9340.

Synthesis of TetMeTPA

A mixture of 15 mL toluene, 10 mL ethanol and 5 mL distilled water contained in a two-necked round bottom flask was degassed thoroughly by bubbling N₂ gas for 15 min. To the degassed solution were added 3,3′-diiodo-2,2′,6,6′-tetramethylbiphenyl (0.50 g, 1.08 mmol), (4-(diphenylamino)phenyl)boronic acid (1.25 g, 4.32 mmol), NaOH (0.26 g, 6.49 mmol) and Pd(PPh₃)₄ (0.13 g, 0.11 mmol). The resulting mixture was heated at 100 °C for 36 h. At the end of this period, the reaction mixture was cooled to rt and the organic solvent was removed in vacuo. The residue was extracted with chloroform three times and the combined organic extract was dried over anhydrous Na₂SO₄. Evaporation of the solvent in vacuo afforded the crude product, which was subjected to silica gel column chromatography to obtain TetMeTPA as a white solid, yield 0.62 g (82%); IR (KBr) cm⁻¹ 3031, 2917, 1588, 1510, 1493, 1314; ¹H NMR (CDCl₃, 500 MHz) δ 1.90 (s, 6H), 1.96 (s, 6H), 7.02 (t, J = 7.0 Hz, 4H), 7.11–7.29 (m, 28H); ¹³C NMR (CDCl₃, 100 MHz) δ 17.7, 20.0, 122.7, 123.3, 124.3, 127.3, 128.3, 128.5, 129.2, 130.3, 132.7, 134.2, 136.8, 139.8, 141.1, 146.2, 147.8; ESI-MS⁺ m/z [M + H]⁺ calcd for C₅₂H₄₅N₂ 697.3582, found 697.3586.

Synthesis of TetMeCZL

The reaction was carried out with 3,3′-diiodo-2,2′,6,6′-tetramethylbiphenyl (1.2 g, 2.60 mmol), (4-(9H-carbazol-9-yl)phenyl)boronic acid (2.98 g, 10.39 mmol), NaOH (0.62 g, 15.58 mmol) and Pd(PPh₃)₄ (0.45 g, 0.39 mmol) in a degassed mixture of 24 mL dioxane, 16 mL ethanol and 8 mL distilled water at 100 °C for 36 h. Typical work-up and isolation procedure as described for the synthesis of TetMeTPA was followed to obtain TetMeCZL as a colorless solid, yield 1.54 g (86%); IR (KBr) cm⁻¹ 3047, 2913, 1596, 1515, 1479, 1452, 1362; ¹H NMR (CDCl₃, 500 MHz) δ 2.03 (s, 6H), 2.07 (s, 6H), 7.30–7.33 (m, 8H), 7.43–7.47 (m, 4H), 7.53 (d, J = 7.95 Hz, 4H), 7.60–7.65 (m, 8H), 8.18 (d, J = 7.3 Hz, 4H); ¹³C NMR (CDCl₃, 125 MHz) δ 17.7, 20.1, 109.8, 119.9, 120.3, 123.4, 125.9, 126.6, 127.6, 128.6, 130.9, 132.7, 134.9, 136.1, 139.5, 140.9, 141.1, 141.7.

Synthesis of TetOMeTPA

The reaction was carried out with 3,3′-diiodo-2,2′,6,6′-tetramethoxybiphenyl (0.4 g, 0.76 mmol), 4-(diphenylamino)phenyl boronic acid (0.88 g, 3.04 mmol), NaOH (0.18 g, 4.56 mmol) and Pd(PPh₃)₄ (0.13 g, 0.11 mmol) at 100 °C for 36 h in a degassed solvent mixture comprising of 15 mL toluene, 10 mL ethanol and 5 mL distilled water. Typical work-up and isolation procedure as described for the synthesis of TetMeTPA was followed to obtain TetOMeTPA as a colorless solid, yield 0.57 g (98%); IR (KBr) cm⁻¹ 3034, 2933, 2833, 1590, 1513, 1490, 1325, 1274; ¹H NMR (CDCl₃, 500 MHz) δ 3.31 (s, 6H), 3.79 (s, 6H), 6.84 (d, J = 8.55 Hz, 2H), 7.00 (t, J = 7.30 Hz, 4H), 7.09–7.14 (m, 12H), 7.23–7.27 (m, 8H), 7.36 (d, J = 8.55 Hz, 2H), 7.48 (d, J = 8.55 Hz, 4H); ¹³C NMR (CDCl₃, 125 MHz) δ 55.9, 60.2, 106.6, 118.1, 122.6, 123.5, 124.2, 127.0, 129.2, 129.8, 130.3, 133.2, 146.2, 147.8, 156.6, 157.5; ESI-MS⁺ m/z [M]⁺ calcd for C₅₂H₄₄N₂O₄ 760.3301, found 760.3306.

Synthesis of TetOMeCZL

To a degassed mixture of toluene (24 mL), ethanol (16 mL) and distilled water (8 mL) were added 3,3′-diiodo-2,2′,6,6′-tetramethoxybiphenyl (1.1 g, 2.09 mmol), (4-(9H-carbazol-9-yl)phenyl)boronic acid (2.4 g, 8.36 mmol), NaOH (0.50 g, 12.54 mmol) and Pd(PPh₃)₄ (0.36 g, 0.31 mmol). The resulting mixture was heated at 100 °C for 36 h, subsequent to which typical procedure as described for isolation of other compounds was followed to obtain TetOMeCZL as a colorless solid, yield 1.64 g (96%); IR (KBr) cm⁻¹ 3046, 2935, 2835, 1594, 1518, 1491, 1478, 1452; ¹H NMR (CDCl₃, 400 MHz) δ 3.42 (s, 6H), 3.87 (s, 6H), 6.95 (d, J = 8.24 Hz, 2H), 7.30 (td, J₁ = 7.68 Hz, J₂ = 0.92 Hz, 4H), 7.34 (td, J₁ = 7.68 Hz, J₂ = 0.92 Hz, 4H), 7.49–7.52 (m, 6H), 7.61 (d, J = 8.24 Hz, 4H), 7.87 (d, J = 8.72 Hz, 4H), 8.16 (d, J = 7.80 Hz, 4H); ¹³C NMR (CDCl₃, 125 MHz) δ 56.0, 60.5, 106.8, 109.9, 118.2, 119.8, 120.3, 123.3, 125.9, 126.64, 126.69, 130.4, 130.7, 135.9, 138.1, 140.9, 156.7, 158.0; ESI-MS⁺ m/z [M + NH₄]⁺ calcd for C₅₂H₄₄N₃O₄ 774.3331, found 774.3337.

Acknowledgements

J. N. M. is thankful to SERB, New Delhi, for generous financial support. S. J. thanks CSIR, New Delhi, for a senior research fellowship. We acknowledge the optoelectronic device fabrication and testing by the scientific instrument facility at the Institute of Chemistry, Academia Sinica, Taipei.

References

1. A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed., 1998, 37, 402.
2. M. T. Bernius, M. Inbasekaran, J. O'Brien and W. Wu, Adv. Mater., 2000, 12, 1737.
3. U. Mitschke and P. Bauerle, J. Mater. Chem., 2000, 10, 1471.
4. Y. Shirota, J. Mater. Chem., 2000, 10, 1.
5. L. S. Hung and C. H. Chen, Mater. Sci. Eng., R, 2002, 39, 143.
6. A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, Chem. Mater., 2004, 16, 4556.
7. G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94.
8. Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953.
9. S.-H. Hwang, C. N. Moorefield and G. R. Newcome, Chem. Soc. Rev., 2008, 37, 2543.
10. M. Zhu and C. Yang, Chem. Soc. Rev., 2013, 42, 4963.
11. M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
12. M. A. Baldo, S. P. Lamansky, E. Burrows, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4.
13. Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson and S. R. Forrest, Nature, 2006, 440, 908.
14. Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943.
15. H. Xu, R. Chen, Q. Sun, L. Wenyong, Q. Su, W. Huang and X. Liu, Chem. Soc. Rev., 2014, 43, 3259.
16. M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B: Condens. Matter Mater. Phys., 2000, 62, 10967.
17. T. Zhang, Y. Liang, J. Cheng and J. Li, J. Mater. Chem. C, 2013, 1, 757.
18. P. I. Shih, C. L. Chiang, A. K. Dixit, C. K. Chen, M. C. Yuan, R. Y. Lee, C.-T. Chen, E. W.-G. Diau and C.-F. Shu, Org. Lett., 2006, 13, 2799.
19. J. He, H. Liu, Y. Dai, X. Ou, J. Wang, S. Tao, X. Zhang, P. Wang and D. Ma, J. Phys. Chem. C, 2009, 113, 6761.
20. N.-J. Lee, D.-H. Lee, D.-W. Kim, J.-H. Lee, S. H. Cho, W. S. Jeon, J. H. Kwon and M. C. Suh, Dyes Pigm., 2012, 95, 221.
21. X. Wang, S. Wang, Z. Ma, J. Ding, L. Wang, X. Jing and F. Wang, Adv. Funct. Mater., 2014, 24, 3413.
22. J. N. Moorthy, P. Venkatakrishnan, D.-F. Huang and T. J. Chow, Chem. Commun., 2008, 2146.
23. J. N. Moorthy, P. Venkatakrishnan, P. Natarajan, D.-F. Huang and T. J. Chow, J. Am. Chem. Soc., 2008, 130, 17320.
24. J. N. Moorthy, P. Venkatakrishnan, P. Natarajan, Z. Lin and T. J. Chow, J. Org. Chem., 2010, 75, 2599.
25. P. Venkatakrishnan, P. Natarajan, J. N. Moorthy, Z. Lin and T. J. Chow, Tetrahedron, 2012, 68, 7502.
26. R. Natarajan, G. Savitha, P. Dominiak, K. Wozniak and J. N. Moorthy, Angew. Chem., Int. Ed., 2005, 44, 2115.
27. S. Seth, P. Venugopalan and J. N. Moorthy, Chem.–Eur. J., 2015, 21, 2241.
28. S. Seth, G. Savitha and J. N. Moorthy, Inorg. Chem., 2015, 54, 6829.
29. For characterization of photophysical and thermal properties of o-methyl-substituted twisted CBP derivatives, see: P. Schrögel, A. Tomkevičiene, P. Strohriegl, S. T. Hoffmann, A. Köhler and C. Lennartz, J. Mater. Chem., 2011, 21, 2266.
30. P.-I. Shih, C.-H. Chien, F.-I. Wu and C.-F. Shu, Adv. Funct. Mater., 2007, 17, 3514.
31. S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau and M. E. Thompson, Inorg. Chem., 2001, 40, 1704.
32. R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett., 2003, 82, 2422.
33. I. Neogi, S. Jhulki, A. Ghosh, T. J. Chow and J. N. Moorthy, ACS Appl. Mater. Interfaces, 2015, 7, 3298.
34. J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt, H. Bässler, M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
35. Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and D. Ma, Angew. Chem., Int. Ed., 2008, 47, 8104.
36. S. Gong, Y. Chen, X. Zhang, P. Cai, C. Zhong, D. Ma, J. Qin and C. Yang, J. Mater. Chem., 2011, 21, 11197.
37. X.-K. Liu, C.-J. Zheng, M.-F. Lo, J. Xiao, C.-S. Lee, M.-K. Fung and X.-H. Zhang, Chem. Commun., 2014, 50, 2027.
38. T. Zhang, Y. Liang, J. Cheng and J. Li, J. Mater. Chem. C, 2013, 1, 757.

---

Footnote

† Electronic supplementary information (ESI) available: CV, TGA and DSC profiles, EL and efficiency plots for the devices constructed, and ¹H and ¹³C NMR spectral reproductions of the compounds reported. See DOI: 10.1039/c5ra21259f.

---