Synthesis and Liquid Crystal and Photophysical Properties - American

Article pubs.acs.org/JPCC

Highly Dichroic Metallomesogen of Dinuclear Platinum Complex: Synthesis and Liquid Crystal and Photophysical Properties Yafei Wang,† Qing Chen,† Yanhu Li,‡ Yu Liu,*,† Hua Tan,† Junting Yu,† Meixiang Zhu,† Hongbin Wu,‡ Weiguo Zhu,*,† and Yong Cao‡ †

College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China ‡ Institute of Polymer Optoelectronic Material and Devices, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: A novel dinuclear platinum complex of (biC 12 ppy-fl)Pt2C12DBM)2 with liquid crystal luminescent property has been prepared, as well as its liquid crystal, optophysical properties and hole mobility have been studied, where biC12ppy-fl is 6,6′-[4,4′-(9,9-didodecyl-9H-fluorene-2,7-diyl)bis(1,4-phenylene)]bis[3-(dodecyloxymethyl)pyridine] and C12DBM is 1,3-bis[4(dodecyloxy)phenyl]propane-1,3-dione. This dinuclear platinum complex displayed a smectic mesophase and excellent opto-physical properties. Compared to the reported mononuclear platinum complex of (CnROppy)Pt(acac) (R = H, F), this dinuclear platinum complex exhibited higher emission quantum yield and higher stability, as well as higher hole mobility of 8.4 × 10−4 cm2/(V s). Moreover, a highly polarized ratio of 10.3 was obtained for the aligned film of (biC12ppy-fl)Pt2(C12DBM)2 on the aligned polyimide at room temperature.

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INTRODUCTION Optoelectronic devices such as light-emitting diodes, photovoltaic cells, sensors, and nonlinear optical systems have attracted much attention owing to their application in display, in lighting, as a sustainable energy source, and so on. In these optoelectronic devices, charge transporting and/or luminescence materials play a key role.1−4 Following this line, many efforts have been focused on developing novel materials, e.g., bipolar-transporting materials, to improve the charge transportation and emission efficiency.5 Recently, liquid crystal (LC) luminescent materials have been developed because of their advantages, such as high charge mobility, easy process, quick response to stimuli, and self-organized capability through intermolecular interactions. Among these developed LC luminescent materials, the metalcontaining LCs (metallomesogens), as a class of important ones, exhibited some significant characteristics with optical, magnetic, and electronic properties of transition metal complexes and anisotropic fluids of LC materials.6−13 As cyclometalated platinum complexes have a planar structure, room-temperature phosphorescence, and diverse excited-state and facilitate metal−metal interactions, the metallomesogens with platinum atom have attracted much attention and become a promising class of metal-containing LCs materials. Their different mesophase behavior and luminescent properties were reported by some groups.9−11,13 Recently, we also developed two series of mononuclear platinum(II)-containing metallomesogens with smectic (Sm) structure and intense polarized luminescence.14a Up to now, © 2012 American Chemical Society

most of these reported metallomesogens with platinum atom were mononuclear platinum complexes. Furthermore, few multinuclear platinum complexes have been presented in the luminescent metallomesogens. In order to obtain a novel luminescent platinum(II)containing metallomesogen with highly polarized emission, a dinuclear platinum complex of (biC12ppy-fl)Pt2(C12 DBM)2 {biC12ppy-fl is 6,6′-[4,4′-(9,9-didodecyl-9H-fluorene-2,7-diyl)bis(1,4-phenylene)]bis[3-(dodecyloxymethyl)pyridine] and C12DBM is 1,3-bis[4-(dodecyloxy)phenyl]propane-1,3-dione; Scheme 1} was designed and synthesized. Its liquid crystalline and photophysical properties, hole mobility, and polarized emission were primarily studied. In this dinuclear platinum complex, the 9,9-dioctylfluorene unit is used as a linkage and bridged two mononuclear platinum complexes. We expected that the fluorene group can improve the carrier mobility and emission quantum yield of its dinuclear platinum complex, because fluorene derivatives are often a type of good luminescent materials with excellent carrier-transporting property and high fluorescent quantum yield. As expected, this dinuclear platinum complex exhibited improved optophysical properties with high quantum efficiency, carrier mobility, and polarized emission. To the best of our knowledge, this is a first report on the metallomesogen of a dinuclear Received: December 12, 2011 Revised: February 7, 2012 Published: February 8, 2012 5908

dx.doi.org/10.1021/jp211976x | J. Phys. Chem. C 2012, 116, 5908−5914

The Journal of Physical Chemistry C

Article

Scheme 1. Synthetic Route of (BiC12ppy-fl)Pt2(C12DBM)2

and a negative electrode, respectively. PEDOT [poly(3,4ethylenedioxythiophene)/poly(styrenesulfonate)] was used as a buffer layer. During device fabrication, the ITO glass was precleaned according to the literature method.14c PEDOT was first spin-coated onto the precleaned ITO surface and backed at 80 °C for 12 h under vacuum to increase substrate smoothness. The (biC12ppy-fl)Pt2(C12-DBM)2 layer was then spin-coated on the ITO/PEDOT electrode as an emission layer. The MoO3/Al cathode was finally deposited on the emission layer by vacuum evaporation under ∼10−4 Pa. The spin-coated thickness was measured with a surface profiler (Tencor AlphaStep 500). All devices were fabricated in a controlled N2-filled glovebox (Vacuum Atmosphere Co.) with oxygen and water contents less than 1 ppm. Current density (J)−voltage (V) data were collected by a Keithley 236 source meter. 2-(4-Bromophenyl)-5-(dodecyloxymethyl)pyridine (3). To a mixture of 4-bromophenylboronic acid (3.0 g, 15.0 mmol), compound 2 (5.3 g, 15.0 mmol), and tetrakis(triphenylphosphine) palladium (0.35 g) was added a degassed mixture of toluene (30 mL), ethanol (15 mL), and 2 M potassium carbonate aqueous solution (15 mL). The mixture was refluxed for 24 h under the protection of nitrogen. After cooling to room temperature (rt), the mixture was poured into water (100 mL) and extracted with DCM (3 × 30 mL). The combined organic layer was dried over anhydrous MgSO4 and filtered. The filtrate was evaporated to remove the solvent and the residue was passed through a flash silica gel column using PE/EtOAc (20:1) as the eluent to give a white solid (5.4 g, 83.4%). 1H NMR (400 MHz, CDCl3, TMS) δ (ppm): 8.65 (s, 1H), 7.89 (d, 2H, J = 8.0 Hz), 7.81 (d, 1H, J = 7.6 Hz), 7.72 (d, 1H, J = 8.0 Hz), 7.61 (d, 2H, J = 8.0 Hz), 4.55 (s, 2H), 3.52 (t, 2H), 1.64 (t, 2H), 1.36−1.25 (m, 18H), 0.89 (t, 3H).

cyclometalated platinum complex with polarized emission to date.

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EXPERIMENTAL SECTION Materials and Instruments. All reagents were purchased from Aldrich, Acros, or TCI. All reactions and manipulations were carried out under N2 and Schlenk techniques. Solvents were dried and purified by using standard methods when necessary. 1H NMR spectra were measured in CDCl3 solution on a Bruker DPX (400 MHz) NMR spectrometer with tetramethylsilane (TMS) as the internal standard. The DSC (differential scanning calorimetry) curve was recorded at 10 °C min−1 during the first cooling and subsequent heating process under N2 atmosphere. Thermogravimetric analyses (TGA) were measured under nitrogen atmosphere at a scan rate of 20 °C min−1. The UV absorption and photoluminescence spectra were measured with a Varian Cray50 and Perkin-Elmer LS50B luminescence spectrometer, respectively. Procedures of the (biC12ppy-fl)Pt2(C12DBM)2 Aligned Films. According to the literature,15 the (biC12ppy-fl)Pt2(C12 DBM)2 aligned films on glass (or aligned polyimide) substrate were made by two processes. First, the platinum(II) complex was formed an aligned liquid crystalline thin film on the substrate at elevated temperature within a thermotropic mesophase. Second, this aligned liquid crystalline thin film was quickly cooled down to room temperature and was kept a liquid crystalline state to form the (biC12ppy-fl)Pt2(C12DBM)2 aligned film. Device Fabrication and Characterization of Space Charge Limit Current (SCLC). The device of ITO/PEDOT (50 nm)/(biC12ppy-fl)Pt2(C12DBM)2 (50 nm)/MoO3 (10 nm)/Al (90 nm) was fabricated. In this device, ITO (indium tin oxide) glass and the MoO3/Al layer are used as a positive 5909



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5-(Dodecyloxymethyl)-2-[4-(4,4,5,5-tetramethyl-1,3,2dioxaborolan-2-yl)phenyl]pyridine (4). A mixture of compound 5 (3.0 g, 6.9 mmol), bis(pinacolato)diboron (1.9 g, 7.6 mmol), potassium acetate (2.0 g, 20.8 mmol), [1,1′bis(diphenylphosphino)ferrocene] dichloropalladium complex with dichloromethane (1:1) (0.17 g, 0.21 mmol), and dimethyl sulfoxide (DMSO, 60 mL) was stirred at 80 °C for 19 h under nitrogen atmosphere. The resulting mixture was cooled to rt, poured into ice−water (200 mL), and then extracted with DCM (3 × 30 mL). The combined organic layer was dried over anhydrous MgSO4 and filtrated. The filtrate was evaporated to remove the solvent and the residue was passed through a flash silica gel column using PE/EtOAc (10:1) as the eluent to give the resulting product as a white solid (1.3 g, 38.6%). 1H NMR (400 MHz, CDCl3, TMS) δ (ppm): 8.65 (s, 1H), 8.01 (d, 2H, J = 7.6 Hz), 7.92 (d, 2H, J = 7.6 Hz), 7.76 (s, 2H), 4.56 (s, 2H), 3.52 (t, 2H), 1.64−1.61 (m, 2H), 1.37 (s, 12H), 1.25−1.16 (m, 18H), 0.89 (t, 3H). 6,6′-[4,4′-(9,9-Didodecyl-9H-fluorene-2,7-diyl)bis(4,1phenylene)]bis[3-(dodecyloxymethyl)pyridine] (6). To a mixture of compound 6 (1.3 g, 2.7 mmol), compound 7 (0.8 g, 1.2 mmol), and tetrakis(triphenylphosphine)palladium (10 mg) was added a degassed mixture of toluene (20 mL), ethanol (10 mL), and 2 M potassium carbonate aqueous solution (10 mL). The mixture was refluxed for 24 h under the protection of nitrogen. After cooling to rt, the mixture was poured into water (100 mL) and extracted with DCM (3 × 30 mL). The combined organic layer was dried over anhydrous MgSO4 and filtered. The filtrate was evaporated to remove the solvent and the residue was passed through a flash silica gel column using PE/EtOAc (10:1) as the eluent to give a white solid (0.8 g, 55.2%).1H NMR (400 MHz, CDCl3, TMS) δ (ppm): 8.70 (s, 2H), 8.15 (d, J = 8.2 Hz, 4H), 7.83−7.81 (m, 10H), 7.69 (t, 4H), 4.60 (s, 4H), 3.57 (t, 4H), 2.11−2.07 (m, 4H), 1.68−1.65 (m, 4H), 1.39−1.10 (m, 68H), 0.92−0.77 (m, 20H). 1,3-Bis[4-(dodecyloxy)phenyl]-3-hydroxyprop-2-en-1one (9). A mixture of compound 9 (2.9 g, 9.4 mmol), compound 10 (3.0 g, 9.4 mmol), NaH (60%, 1.8 g, 47.0 mmol), and dimethoxymethane (DME, 70 mL) was refluxed for 10 h under nitrogen atmosphere. After cooling to rt, water (5 mL) was added dropwise with stirring, followed by HCl (10% aqueous) to adjust the solution to pH 2. Then the mixture was extracted with DCM (3 × 30 mL) and the combined organic layer was dried over anhydrous MgSO4 and filtered. The filtrate was evaporated to remove the solvent and the residue was purified by recrystallization from ethanol and DCM (1:1 v:v) to give a white solid (4.7 g, 85.6%). 1H NMR (400 MHz, CDCl3, TMS) δ (ppm): 17.2 (s, 1H), 7.96 (d, J = 8.7 Hz, 4H), 6.97 (d, J = 8.8 Hz, 4H), 6.73 (s, 1H), 4.04 (t, 4H), 1.85−1.77 (m, 4H), 1.47−1.27 (m, 36H), 0.90−0.87 (m, 6H). (BiC12ppy-fl)Pt2(C12DBM)2. To a mixture of K2PtCl4 (0.2 g, 0.5 mmol) and water (4 mL) was added a solution of compound 6 (0.6 g, 0.5 mmol) and 2-ethoxyethanol (12 mL). The mixture was stirred under inert gas atmosphere at 80 °C for 24 h. After cooling to rt, the colored precipitate was filtered off and washed with water and hexane to gain a [(biC12ppyfl)Pt2Cl2]2 dimer as a yellow solid (0.45 g), which was directly used in the following process. A mixture of [(biC12ppyfl)Pt2Cl2]2 (0.45 g, 0.14 mmol), compound 9 (0.5 g, 0.8 mmol), and sodium carbonate (0.09 g, 0.8 mmol) was stirred in 2-ethoxyethanol (15 mL) under inert gas atmosphere at 100 °C for 24 h. After cooling to rt, the mixture was extracted with

DCM and the mixed organic layer was dried over anhydrous MgSO4 and filtered. The filtrate was evaporated to remove the solvent and the residue was passed through a flash silica gel column using PE/DCM (1:2) as eluent to gain (biC12ppyfl)Pt2(C12DBM)2 as a yellow solid (80.1 mg, 20.6%). 1H NMR (400 MHz, CDCl3, TMS) δ (ppm): 9.21 (s, 2H), 8.09 (t, 8H), 7.95 (d, J = 8.68 Hz, 2H), 7.82 (d, J = 7.88 Hz, 4H), 7.65 (d, J = 8.28 Hz, 2H), 7.50 (d, J = 7.52 Hz, 2H), 7.16 (t, 2H), 7.00− 6.93 (m, 12H), 6.72 (s, 2H), 4.62 (s, 4H), 4.07 (t, 8H), 3.59 (t, 4H), 1.86−1.83 (m, 12H), 1.78 (t, 8H), 1.50−1.26 (m, 144H), 0.91 (t, 24H). 13C NMR (100 MHz, CDCl3, TMS) δ (ppm): 178.69, 177.85, 167.56, 161.48, 161.35, 146.09, 144.73, 139.41, 137.04, 132.49, 131.62, 130.89, 130.54, 129.13, 128.87, 128.78, 123.48, 122.94, 117.97, 114.36, 114.34, 114.16, 95.67, 71.18, 69.71, 68.21, 31.89, 29.77, 29.67, 29.64, 29.61, 29.58, 29.56, 29.5, 29.40, 29.38, 29.32, 29.23, 29.21, 26.17, 26.05, 26.03, 22.65, 14.05. Anal. Calcd for C163H240N2O10Pt2: C, 70.48; H, 8.71; N, 1.01. Found: C, 69.5; H, 8.76; N, 1.11

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RESULTS AND DISCUSSION Synthesis. The intermediates of compounds 2 and 5 were prepared in our previous work.14a,b Cyclometalated ligand 6 was obtained by Suzuki coupling reaction in a high yield.16 Ancillary ligand 9 was prepared by condensation reaction used NaH as alkali in a mode-rate yield.10 The dinuclear platinum complex of (biC12ppy-fl)Pt2(C12DBM)2 was prepared by a chloride cleavage of the Pt4((biC12ppy-fl)2(μ-Cl)4 dimer with the HC12DBM ancillary ligand in a molar ratio of 1:6, in which the dimer was obtained by a cyclometalated reaction of the biC12ppy-fl ligand and K2PtCl4 in a molar ratio of 1:1. The NMR spectra (Supporting Information, Figure S1) and elemental analysis confirmed that this platinum complex was successfully achieved. Thermal Property. The thermal stability of (biC12ppyfl)Pt2(C12DBM)2 was examined via thermogravimetric (TG) analyses under nitrogen atmosphere with a scanning rate of 20 °C min−1. The recorded TG curve is shown in Figure S2 of the Supporting Information. A decomposition temperature (Td) of 301 °C is observed, which correspond to a 5% weight loss. We note that the Td level of the mononuclear platinum complex of (C12Oppy)Pt(acac) is 292 °C.14a It means that the dinuclear platinum(II) complex possesses better thermal stability than its corresponding mononuclear platinum complexes. Therefore, incorporating a fluorene unit into the middle of the mesogenic dinuclear platinum(II) complex has a positive influence on the thermal stability. Liquid Crystalline Properties. The thermal and liquid crystalline behavior of two platinum(II) complexes were characterized by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The nature of the mesophase was further confirmed by wide-angle X-ray diffraction (WXRD). The DSC thermograms of (biC12ppy-fl)Pt2(C12DBM)2 were measured at a scan rate of 10 °C min−1 during the first cooling and subsequent heating process. The recorded DSC thermograms are shown in Figure 1a and the corresponding data are summarized in Table 1. Three endothermic peaks are observed at 24, 69, and 117 °C, respectively. The peak at about 69 °C with a small enthalpy (ΔH) is due to the transition from crystal phase to smectic phase. Another peak at about 117 °C with a larger ΔH is attributed to the transition from smectic phase to isotropic phase transition. It implies that this dinuclear cyclometalated platinum complex formed a higher-order 5910



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Figure 2. Polarized optical textures after the cooling process for (biC12ppy-fl)Pt2(C12DBM)2 at 113 °C and powder X-ray diffraction patterns of (biC12ppy-fl)Pt2(C12DBM)2 at 110 °C.

temperatures were carried out. The powder WXRD patterns of (biC12ppy-fl)Pt2(C12DBM)2 at 318, 333, 383, and 408 K are shown in Figure 2 and Figure S3 (Supporting Information). A distinct diffuse scattering halo (h0) is observed in the wideangle region at 383 K. The halo at 21.7o (d = 4.09 Å) corresponds to the liquid-like order of the loose lateral interactions between rigid parts of the complex.18 On the other hand, a slightly weak reflection peak at about 6.35o (d = 13.9 Å) and a sharply intense reflection peak at about 3.10o (d = 28.5 Å) are observed in the small-angle region. The reciprocal spacings of the two small-angle reflection peaks are about in a ratio of 1:2. It indicates that this platinum complex formed lamellar structure under this temperature. Comparing the estimated molecular length (63.4 Å) and the XRD data for (biC12ppy-fl)Pt2(C12DBM)2, the resulting liquid crystal seems to have the interdigitated molecular arrangement via strong π−π stacking of rigid cores and hydrophobic interaction of side chains. Photophysical Properties. Figure 3 shows the UV−vis absorption and photoluminescence (PL) spectra of (biC12ppyfl)Pt2(C12DBM)2. Three structured UV absorption bands are observed in DCM solution. The first one around 282 nm originates from the intraligand π−π* transitions of the ppy unit, the second one around 350 nm is assigned to the π−π* transitions of the DBM unit, and the final weak one around 403 nm is attributed to the metal-to-ligand charge transfer (MLCT) transitions.15,19 Compared to our reported mononuclear platinum complexes,14a this dinuclear platinum complex presents a distinctly red-shifted absorption spectral profile. According to the MLCT absorption of (biC12ppy-fl)Pt2(C12DBM)2, we chose an excitation light with a wavelength of 400 nm to excite the dinuclear platinum complex. Obviously, the PL profile of (biC12ppy-fl)Pt2(C12DBM)2 in DCM exhibits dual emission bands at 493 and 525 nm under this excitation

Figure 1. DSC curve of (biC12ppy-fl)Pt2(C12DBM)2 (a) at a scan rate of 10 °C/min and (b) at different scan rates on heating and cooling processes.

Table 1. Thermal Behaviors of (BiC12ppy-fl)Pt2(C12DBM)2 compounds (biC12ppy-fl) Pt2(C12DBM)2

transitiona

temp (°C)b

Td (°C)

ΔH (J g−1)

Cr−Cr′ Cr′−Sm Sm−Iso

24 69 117

301

15.8 6.16 26.5

a Cr, Cr′ = crystal; Sm = smectic phase; Iso = isotropic liquid. bData refers to the second DSC cycle starting from the crystal.

smectic phase. 17 In order to clarify the type of the corresponding transitions, the DSC thermograms of (biC12ppy-fl)Pt2(C12DBM)2 were recorded at four successive heating and cooling scan rates of 40, 20, 10, and 5 °C min−1 (Figure 1b). The transition temperatures upon cooling and heating process have not been affected by the scan rates. It implies that (biC12ppy-fl)Pt2(C12DBM)2 maybe exhibit a mesophases transition feature. The mesophase was further identified by POM. The polarized optical texture of (biC12ppy-fl)Pt2(C12DBM)2 upon cooling process at 113 °C is shown in Figure 2. Strong birefringence texture and fluidity were observed under this condition. Referred to in reports by Venkatesan et al. and Damm et al. about platinum complexes with β-diketone,9,10 (biC12ppy-fl)Pt2(C12DBM)2 is suggested to form a smectic mesophase at 113 °C on cooling. To further illustrate the phase structures clearly, the wideangle X-ray diffraction (WXRD) experiments with variational 5911



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intrinsic and aggregated emissions at this temperature. When the film is heated to 342 K and then annealed, the emission spectrum is changed to a broad band around 556 nm. The intensity of the intrinsic emission weakened remarkably. The broad emission band is assigned to the aggregation emission, to some extent, together with the contribution of the excimer emission. As the dinuclear complex is able to form an ordered crystal structure and it is very facile for the ordered structure to form the aggregation state, the intense aggregation emission, even excimer emission, is observed in the film under this condition. If the film is sequentially heated to 373 K and then annealed, (biC12ppy-fl)Pt2(C12DBM)2 is available to form a smectic phase. In this self-assembled structure, luminescent cores are, to some extent, surrounded and separated by the alkoxy side chains.23 As a consequence, both intrinsic and aggregated emissions are obviously observed for (biC12ppyfl)Pt2(C12DBM)2 at 373 K. Polarized Emission Properties. Linearly polarized emission is very important in LCD back-lights because it could replace both polarizer and color filter to reduce the power consumption. With this in mind, we carried out here a preliminary study on the polarized emission property of dinuclear platinum complex. Two classes of the aligned films for (biC12ppy-fl)Pt2(C12DBM)2 were made on a glass and an aligned polyimide substrate, respectively. The polarized phosphorescent emissions are observed for both aligned films at room temperature under polarized photoirradiation, as shown in Figure S4 (Supporting Information). Similar PL profiles with three peaks at about 490, 531, and 572 nm but different emission intensities are presented under excitations of the parallel and perpendicular light, respectively. The emission intensity in the parallel direction is 1.3 and 10.3 times that in the perpendicular direction for the aligned films on a glass and an aligned polyimide substrate, respectively. Furthermore, the emission intensity of the parallel direction at the low-lying band over 531 nm is highly increased for the aligned film on an aligned polyimide substrate under photoirradiation, as shown in Figure 5. Our results successfully demonstrate that highly

Figure 3. UV−vis and PL spectra of (biC12ppy-fl)Pt2(C12DBM)2 at 298 K.

light. It is assigned to the intrinsic emission of (biC12ppyfl)Pt2(C12DBM)2. Using Ru(bpy)3(PF6)2 as a standard in DCM (1 × 10−5 M),20,21 (biC12ppy-fl)Pt2(C12DBM)2 gave a quantum yield (Φem) of 4.68%. Interestingly, as expected, this level is distinctly higher than that of the metallomesogens of (C nROppy)Pt(acac) reported by our group. 14a Higher quantum yield for (biC12ppy-fl)Pt2(C12DBM)2 is possibly related to the increased stereohindrance effect, owing to introducing a fluorene group and DBM unit, as well as the bimetallic structure. In general, the molecular stereohindrance effect is available to suppress the luminescent quenching, further to improve luminescent quantum efficiency.22 Unlike the PL spectrum in DCM solution, that in the neat film shows a broad emission band. Besides the high-lying intrinsic emission bands at 494 and 531 nm two low-lying emission bands at 571 and 625 nm are newly observed. Both latter emission bands are attributed to the aggregation and excimer emissions of (biC12ppy-fl)Pt2(C12DBM)2 based on Bolink’s work.21 To further explore the relationship between luminescence and morphology, we studied the luminescence−temperature characteristic of (biC12ppy-fl)Pt2(C12DBM)2. As shown in Figure 4, significantly different PL spectra are observed for the (biC12ppy-fl)Pt2(C12DBM)2-based film under different temperature from 300 to 373K. At 300 K, the emission spectrum displays three typical emission peaks at 494, 530, and 570 nm. This means that (biC12ppy-fl)Pt2(C12DBM)2 gave its

Figure 5. Polarized emission of the (biC12ppy-fl)Pt2(C12DBM)2 complex in the aligned polyimide.

polarized photoluminescence can be achieved by designing luminescent metallomesogen of dinuclear platinum complex and introducing the proper aligned approach. Hole Mobility. As we know, hole mobility plays an important role in the OLEDs materials. In order to investigate the carrier mobility of this luminescent metallomesogen, here,

Figure 4. Emission spectra of the (biC12ppy-fl)Pt2(C12DBM)2 in its neat film at 300, 342, and 373 K. 5912



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we measured its hole mobility by the SCLC method using a device structure of ITO/PEDOT (50 nm)/(biC12ppy-fl)Pt2(C12DBM)2 (50 nm)/MoO3 (10 nm)/Al (90 nm). For the hole-only devices, SCLC is described by the formula J = (9/ 8)ε0εrμ(V/d3),24 where J is the current density, ε0 is the permittivity under vacuum (ε0 = 8.85 × 10−12 F/m), εr is the dielectric constant (εr = 3), μ is the hole mobility, and d is the sample thickness. As a result, the current−voltage (IV) characteristics taken under pulsed conditions are shown in Figure 6. From the pulsed IV curves and the formula, the hole

*Tel: +86-731-58298280. Fax: +86-731-58292251. E-mail: [email protected] (W.Z.), [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (50973093, 20872124 and 20772101), the Science Foundation of Hunan Province (2009FJ2002), the Specialized Research Fund and New Teachers Fund for the Doctoral Program of Higher Education of China (2009430110004, 200805301013), the Scientific Research Fund of Hunan Provincial Education Department (10A119, 11CY023, and 10B112), and the Postgraduate Science Foundation for Innovation in Hunan Province (2011B263) is acknowledged.

mobility of this dinuclear platinum complex is calculated to be 8.4 × 10−4 cm2/(V s). Notably, the hole mobility for this dinuclear cyclometalated platinum complex is superior to that for the reported platinum complexes, which has a mobility of ca. 10−7−10−4 cm2/(V s).4,25 Therefore, this dinuclear platinum complex could be a promising metallomesogen in application of polarized OLEDs.

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REFERENCES

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Figure 6. Current density vs voltage under pulsed conditions for the device at 298 K. log J is fitted linearly dependent on log V.

CONCLUSIONS

AUTHOR INFORMATION

Corresponding Author

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Article

In summary, the heteroleptic dinuclear cyclometalated platinum complex with luminescent liquid-crystal properties was successfully obtained. An enantiotropic mesophase transition with lamellar smectic-type (Sm) structure was observed for this class of dinuclear cyclometalated platinum complex. More intense UV absorption, higher emission quantum efficiency, and high hole mobility were found for the (biC12ppyfl)Pt2(C12DBM)2. Different emission spectra are observed for the (biC12ppy-fl)Pt2(C12DBM)2 film at different temperatures. Highly polarized emission with a 10.3 polarized ratio was achieved for the aligned film of (biC12ppy- fl)Pt2(C12DBM)2 on an aligned polyimide substrate. Our research suggests that the dinuclear cyclometalated platinum complex is a class of promising luminescent liquid-crystal materials with high polarized ratio.

S Supporting Information *

NMR, TGA, and XRD data, as well as the polarized emission spectrum of (biC12ppy-fl)Pt2(C12DBM)2. This material is available free of charge via the Internet at http://pubs.acs.org. 5913



Article

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Synthesis and Liquid Crystal and Photophysical Properties - American

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