Nondoped Blue Organic Light Emitting Devices with DonorâÏ

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Non-Doped Blue Organic Light Emitting Devices with Donor-#-Acceptor Derivatives as the Emissive Material Venugopal Thanikachalam, Palanivel Jeeva, Jayaraman Jayabharathi, Periyasamy Ramanathan, Annadurai Prabhakaran, and Elayaperumal Saroj Purani Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02549 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 25, 2016

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Industrial & Engineering Chemistry Research

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Non-Doped Blue Organic Light Emitting Devices with Donor-π-Acceptor Derivatives as the Emissive Material Venugopal Thanikachalam*, Palanivel Jeeva, Jayaraman Jayabharathi, Periyasamy Ramanathan, Annadurai Prabhakaran, Elayaperumal Saroj Purani Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India

* Address for correspondence

Dr. V. Thanikachalam Professor of Chemistry Department of Chemistry Annamalai University Annamalainagar 608 002 Tamilnadu, India. Tel: +91 9488476098 E-mail: [email protected]

ACS Paragon Plus Environment


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Non-Doped Blue Organic Light Emitting Devices with Donor-π-Acceptor Derivatives as the Emissive Material Venugopal Thanikachalam*, Palanivel Jeeva, Jayaraman Jayabharathi, Periyasamy Ramanathan, Annadurai Prabhakaran, Elayaperumal Saroj Purani Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamilnadu, India

Abstract Blue

emitting

devices

based

on

donor-linker-acceptor

methoxynaphthylphenanthrimidazole-phenyl-phenylbenzenamine methoxynaphthylphenanthrimidazole-styryl-phenylbenzenamine

geometry

such

(TPA-MPI) (TPA-MPS)

as and

ppwere

synthesized and characterised. Due to the rigid molecular back bone composed of phenanthro[9,10-d] imidazole (acceptor), phenyl (TPA-MPI) and styryl (TPA-MPS) spacers and triphenylamine (donor), these compounds exhibit good thermal stability. The non-doped device based on TAP-MPS exhibit higher efficiencies, 1.73 cd A-1 (η c ), 1.46 lm W-1 (η p ) and 2.11 % (η ex ). The higher efficiency is attributed to the coemission from the intercrossed excited state of the emissive layer.

Keywords: Donor-linker-acceptor; Non-doped device; Thermal stability; Blue emitters; High efficiency.


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1. Introduction Devices based on blue emissive materials with higher efficiencies are currently drawn attention in organic light emitting diodes (OLEDs) [1]. Although conversion of blue to red and green light is possible with the combination of proper dyes generation of blue emitting devices is the major problem in the existing OLED community due to wide band gap and unbalanced electron injection and transport ability [2, 3]. It is known that the doped emitters in OLEDs improved the performance of devices but dopant concentration affects the device stability [4-9]. In addition to that fabrication of doped device is more expensive and phase separation on heating induced ineffective energy transfer from adjacent layers to host and to guest results low efficiency. Thermally and electrochemically stable blue emitters such as carbazole and distyrylarylene derivatives are reported with small quantum efficiencies due to their unbalanced charge injection and transport ability [10-14]. OLEDs based on aromatic heterocyclic organic semiconductors with pyridine, quinoline and oxidiazole moieties as emissive layer with excellent carrier balancing ability but they exhibit low luminance [1517]. Deep blue emitters based on fluorinated anthracene derivatives exhibit high luminance due to large FWHM [29]. Blue emitters with electron withdrawing moiety having shallow HOMO energy, increase the hole injection barrier (HIB) and thus results low efficiency [1627]. Attention were drawn to develop blue emitters together with balanced electron injection and transport properties and to exhibit high brightness with reduced driving voltage. Designing non-doped blue emitter with donor-π-acceptor geometry may be an attractive approach for efficient OLEDs. Herein we report donor-linker-acceptor methodology to design new born non-doped blue emitters namely p-methoxynaphthylphenanthroimidazolephenyl-phenylbenzenamine (TPA-MPI) and p-methoxynaphthylphenanthroimidazole- styryl-



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phenylbenzenamine (TPA-MPS) having electron transporting acceptor phenanthrimidazole moiety, hole transporting donor triphenylamine fragment with different linkers phenyl (TPAMPI) and styryl (TPA-MPS). Influence of the linker in the synthesised materials on optical, optoelectronic and thermal properties and device performances were investigated. Geometry optimization has shown that less tilting of styryl spacer in TPA-MPS increase the intramolecular charge transfer (CT) due to reduced conjugation and thus shows higher efficiency with low turn on voltage. 2. Experiment and Characterization Sigma-Aldrich supplied all the reagents and solvents used for the synthesis of blue emissive materials. NMR and mass spectra were obtained on Bruker 400 MHz NMR spectrometer and Agilent LCMS VL SD in electron ionization mode, respectively. Cyclic voltammetry analyses were carried out to calculate HOMO energy by using CHI 630A potentiostat electrochemical analyzer with platinum electrode and platinum wire as the working electrode and counter electrode, respectively. The Ag/Ag+ electrode is used as the reference electrode at a scan rate of 100 mV s-1 with 0.1M tetrabutylammoniumperchlorate in CH 2 Cl 2 as the supporting electrolyte. The UV-visible spectra was recorded using Perkin Elmer Lambda 35 UV-vis spectrophotometer and corrected for background absorption due to solvent. Perkin Elmer Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere attachment was employed to record UV-vis diffuse reflectance spectra. Photoluminescence spectra were recorded on a Perkin Elmer LS55 fluorescence spectrometer. The PL quantum yield was calculated in dichloromethane with 0.5 M H 2 SO 4 solution of quinine (0.54) as 2

      reference using the following equation: φunk = φstd  I unk   Astd   ηunk  where φunk and φstd is  I std   Aunk   ηstd  ,

the radiative quantum yield of the sample and standard, I unk and I std are the integrated emission intensities of the sample and standard, respectively.


A unk and A std are the

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absorbances of the sample and standard, respectively and η unk and η std are the refractive index of the sample and standard solutions. The solid state quantum yield has been measured on the quartz plate using an integrating sphere. Thermogravimetric analyses (TGA) was carried out on a Perkin Elmer thermal analysis system at a heating rate of 10° C min-1 with nitrogen flow rate of 100 mL min-1. Differential scanning calorimetric (DSC) analysis was recorded with NETZSCH (DSC-204) at 10° C min-1 under nitrogen atmosphere (100 mL min1

). The fabricated film thickness as well as the roughness were measured using atomic force

microscope (Nano surf Easy scan2) AGILENT-N9410A-5500. Density functional theory (DFT) calculations were performed with Gaussian-03 package [28]. 2.1 Synthesis of phenanthroimidazole-π-triphenylamine derivatives: Blue emitting materials 2.1.1

2-(4-bromophenyl)-1-(p-methoxynaphthalen-1-yl)-1H-phenanthro-[9,10-d]imidazole

(BMNP) The product (BMNP) was prepared by refluxing 9,10-phenanthrenequinone (5 mmol), 4-bromobenzaldehyde (5 mmol), 4-methoxynaphthalen-1-amine (6 mmol) and ammonium acetate (61 mmol) in ethanol (20 mL) for 12 h under nitrogen atmosphere (Scheme - 1). After cooling, the reaction mixture was poured into ice. The separated solid was washed with alcohol and dried to give pale yellow solid. The solid was purified by column chromatography (hexane:ethylacetate 9:1). M.P. 230 ºC. Anal. calcd. for C 32 H 21 BrN 2 O: C, 72.60; H, 4.00; N, 5.29.Found: C, 72.57; H, 3.96; N 5.25. 1H NMR (400 MHz, CDCl 3 ): δ 3.96 (s, 3H), 7.08 (d, J=8.0 Hz, 2H), 7.21-7.33 (m, 1H), 7.38 (d, J=8.1 Hz, 2H), 7.43-7.55 (m,7H), 7.60-7.67 (m, 2H), 7.74 (d, J=7.5Hz, 1H), 8.71 (d, J=8.2 Hz, 1H), 8.75 (d, J=8.0 Hz, 1H), 8.84 (d, J=8.5 Hz, 1H).

C NMR (400 MHz, CDCl 3 ): δ 55.9, 115.2, 120.7, 122.5,

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123.0, 123.2, 123.4, 124.3, 125.1, 125.4, 125.7, 126.4, 127.0, 127.3, 127.5, 128.1, 128.3, 128.5, 129.3, 129.6, 130.1,130.7, 130.9, 131.3, 137.5, 149.7, 160.4. MS:m/z. 529 [M+]. calcd.528.31.



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2.1.2

2-(4-bromostyryl)-1-(p-methoxynapththalen-1-yl)-1H-phenanthro[9,10-d]imidazole

(BSMNP) BSMNP was prepared using the methodology similar to that of BMNP by replacing 4-bromobenzaldehyde with 4-bromocinnamaldehyde. M.P. 248 ºC. Anal. calcd. for C 34 H 23 BrN 2 O: C, 88.76; H, 4.97; N, 6.27. Found: C, 88.73; H, 4.95; N 6.25. 1H NMR (400 MHz, CDCl 3 ) : δ 3.98 (s, 3H), 6.70 (d, J=8.4 Hz, 1H), 7.17 (d, J=7.6 Hz, 1H), 7.25 (d, J=9.4 Hz, 2H), 7.42-7.50 (m, 5H), 7.52-7.61 (m, 4H), 7.80 (d, J=8.2 Hz, 1H), 7.98 (d, J=9.8 Hz, 1H) 8.14 (d, J=10.2 Hz, 2H), 8.62 (d, J=8.2 Hz, 2H), 9.5 (t, J=7.8 Hz, 1H).13C NMR (100 MHz, CDCl 3 ): δ 116.78, 121.53, 121.96, 122.41, 122.88, 123.22, 123.54, 124.95, 125.22, 125.95, 126.12, 126.80, 126.95, 127.27, 127.36, 127.49, 127.77, 128.09, 128.34, 128.43, 128.64, 129.82, 131.83, 133.35, 148.77. MS: m/z. 446 [M+]. calcd. 446.18. 2.1.3 p-methoxynaphthylphenanthrimidazole- phenyl-phenylbenzenamine (TPA-MPI) A mixture of 2-(4-bromophenyl)-1-(p-methoxynaphthalen-1-yl)-1H-phenanthro-[9,10-d] imidazole (4.5 mmol), 4-(diphenylamino)phenylboronic acid (7.5 mmol), Pd(PPh 3 ) 4 (0.25 mmol) and aqueous Na 2 CO 3 (15 mL) in toluene:ethanol (20:15 mL) was refluxed in nitrogen atmosphere for 18 h. The reaction mixture was cooled and extracted with dichloromethane. The extract was concentrated and the residue was purified by column chromatography (petroleum ether: CH 2 Cl 2 , 1:1). M.P. 281 ºC. Anal. calcd. for C 50 H 35 N 3 O: C, 86.55; H, 5.08; N, 6.06. Found: C, 72.57; H, 3.96; N, 5.25. 1H NMR (400 MHz, CDCl 3 ): δ 4.08 (s, 3H), 6.52-6.57 (m, 4H), 6.62 (d, J=8.6 Hz, 2H) 6.72 (d, J=8.3 Hz, 2H), 6.84-6.92 (m, 4H), 6.98 (d, J=7.7 Hz, 2H) 7.16 (d, J= 8.6 Hz, 2H), 7.31-7.42 (m, 1H), 7.47 (d, J=8.0 Hz, 2H), 7.52-7.64 (m, 7H), 7.69-7.76 (m, 2H), 7.82 (d, J=7.9 Hz, 1H), 8.79 (d, J=8.2 Hz, 1H), 8.83 (d, J=8.6 Hz, 1H), 8.92 (d, J=8.4 Hz, 1H). 13C NMR (400 MHz, CDCl 3 ): δ 59.1, 118.4, 123.9, 125.7, 126.2, 126.4, 126.8, 127.7, 128.5, 128.8, 129.1, 129.7, 130.3, 130.6, 130.9, 131.5,


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131.7, 131.9, 132.6, 132.9, 133.4, 134.1, 134.3, 134.7, 140.9, 153.1, 163.08. MS: m/z. 529 [M+]. calcd. 528.31. 2.1.4 p-methoxynaphthylphenanthrimidazole- styryl-phenylbenzenamine (TPA-MPS) TPA-MPS was synthesised using the methodology similar to that of TPA-MPI. M.P. 298 ºC., Anal. calcd. for C 52 H 37 N 3 O: C, 86.76; H, 5.18; N, 5.84. Found: C, 88.73; H, 4.95; N 6.25. 1H NMR (400 MHz, CDCl 3 ): δ 4.12 (s, 3H) 7.33-7.38 (m, 4H), 7.44 (d, J=8.2 Hz, 2H), 7.54 (d, J=7.6 Hz, 2H), 7.59 (d, J=8.6 Hz, 2H), 7.61-7.66 (m, 4H) 7.68 (d, J=8.6 Hz, 2H), 7.72- 7.80 (m, 4H), 7.90 (d, J=8.2 Hz, 1H) 7.98 (m, 5H), 8.04-8.12 (m, 5H), 8.50 (d, J=8.2 Hz, 1H), 8.66 (d, J=7.4 Hz, 1H), 9.54 (t, J=7.8 Hz, 1H).

C NMR (100 MHz, CDCl 3 ): δ 59.8 119.98,

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124.73, 125.16, 125.61, 126.08, 126.30, 126.64, 127.01, 127.28, 128.02, 128.19, 128.47, 128.62, 128.94, 129.03, 129.16, 129.44, 129.75, 130.00, 130.09, 130.31, 131.49, 133.50, 135.02, 141.22, 153.44, 163.51. MS: m/z. 446 [M+]. calcd. 446.18. 2.2 Fabrication of non-doped devices The fabricated device configuration is as follows: (a) ITO/ NPB (70 nm)/TPA-MPI (device I)/ TPA-MPS (device II) (100 nm) /TPBI (20 nm)/LiF (0.5 nm)/Al; (b) ITO/ TPA-MPS (70 nm)/ Alq 3 (60 nm)/ LiF (0.5 nm)/ Al (80 nm) (device III); (c) ITO/ NPB (70 nm)/ Alq 3 (60 nm)/ LiF (0.5 nm)/ Al (80 nm) (device IV). The layers deposition was made using thermal evaporation method and thickness was measured with quartz crystal digital thickness monitor. Keithley 2400 was employed for current density versus voltage measurements in ambient atmosphere. 3. Results and Discussion Geometry optimisation of TPA-MPI and TPA-MPS has been carried out by Gaussian-03 and their optimised geometry is displayed in Figure 1. The p-methoxynaphthyl moiety at nitrogen (N-22) is tilted by an angle of 80° in TPA-MPI and 84° in TPA-MPS with respect to phenanthrimidazole unit. At this large angle fluorescence quenching was suppressed due to



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aggregation in the film. The optimized geometries have shown that the dihedral angle between styryl and phenanthrimidazole unit in TPA-MPS and TPA-MPI is 3° and 19°, respectively. It is expected that the lesser dihedral angle (3°-TPA-MPS) reduced the conjugation there by increasing the intramolecular change transfer from triphenylamine to phenanthrimidazole via styryl spacer. Thermal morphological stability of TPA-MPI and TPA-MPS thin films were examined by atomic force microscopy (AFM) measurement at room temperature and at 100 ° C for 18 h (Figure 2). The root-mean-square roughness (RMS) of TPA-MPI (0.31 nm) and TPA-MPS (0.28 nm) thin film smooth surfaces showed no substantial changes before and after annealing (100°C) (Figure 2). This high quality amorphous and morphology are unique for efficient device performances by reducing the defects formation in emissive layer and thus, it is expected that D– π –A materials will lower the turn on voltage in the device performances [30-31]. TGA and DSC measurements of these compounds reveal that they are thermally stable materials due to the bulky phenanthrimidazole block along with triphenylamine and phenyl and styryl linkers (Figure 2). Thermal decomposition temperatures (T d5 ) of TPA-MPI and TPA-MPS are of 442 and 451°C, respectively. From the DSC curve the melting temperatures (T m )/ glass transition temperatures (T g ) have been observed at 281/130 °C and 298/139 °C for TPA-MPI and TPAMPS, respectively (Table 1). Thermal measurements (high T g , T d5 and T m ) also show that these materials are very stable and have better film form ability for device fabrication as suggested by AFM measurements. Figure 3 displayed the UV and photoluminescence spectra of TPA-MPI and TPA-MPS. It is reported that for D-π-A conjugated molecules (π-π* transition), the extinction coefficients are higher when there is increase of conjugation length [32]. In the present study compared with parent compounds (BMNP and BSMNP) the new born blue emitting materials. TPA-MPI and TPA-MPS show very strong absorption peak at 396 (ε max =61280 cm-1 M-1) and 391nm (ε max =70240 cm-1 M-1).


Due to the longer

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conjugation length in TPA-MPS the π-π* transitions are higher in intensity and the higher extinction coefficients are expected to reduce film thickness. Intramolecular charge transfer from donor (triphenylamine) to acceptor (phenanthrimidazole) is likely to be the reason for strong absorption at 396 and 391nm and absorption around 250 nm is assigned to π-π transition [32]. The solid film of TPA-MPI and TPA-MPS shows absorption at 398 and 392 nm, respectively. Compared with the solution the negligible absorption shifts of TPA-MPI and TPA-MPS in their corresponding film reveal the suppressed π-π stacking in the solid state [33]. Room temperature absorption and emission of TPA-MPI and TPA-MPS in different solvents are also shown in Figures S1 and S2, respectively. The three absorption bands observed in hexane around 24937, 25252 and 39525 cm-1 (TPA-MPI) and 25062, 25575 and 40000 cm-1(TPA-MPS) are assigned to 1(π- π*) states correspond in Platt’s notation to the 1L b , 1La and 1B a excited states. The low and high energy transitions, 1Lb ← S 0 , 1La ←S 0 and 1Ba ← S 0 respectively, with a relatively high probability can be clearly observed in the absorption spectra of TPA-MPI and TPA-MPS [33]. The low-energy absorption region strongly supports the presence of charge transfer (CT) state and a long wave shoulder attributed to the 1CT← S 0 transition is also observed. The parent compounds shows blue emission at 400 nm (BMNP) and 375 nm (BSMNP) with vibronic structures whereas deep blue emission without vibronic features were observed for TPA-MPI and TPA-MPS at 466 and 445 nm, respectively (large red shift of 66 and 70 nm compared with BMNP and BSMNP). The observed larger red shift further supports the intramolecular charge transfer via the less tilted geometry of TPA-MPI and TPA-MPS. The obtained smaller band gap [2.70 eV (TPA-MPI); 2.60 eV (TPA-MPS)] of TPA-MPS is due to the less twisted conformation of TPA-MPS as confirmed by the theoretical studies. When the solvent polarity increased the blue emissive materials (TPA-MPI and TPA-MPS) exhibit a



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larger red shift (Figure 3) which support the existence of charge transfer in these molecules [34]. From the plot of Stokes shift against solvent polarity function the ground state dipolemoment can be calculated [34]: 𝜈𝜈𝜈 hc(ῦ abs - ῦ flu ) = hc(ℎ𝑐ῦ𝜈𝜈𝜈 abs -ℎ𝑐ῦ𝑓𝑓𝑓 ) R

+

2 (µ e - µ g )2 / 𝑎o3 [(ε – 1/ 2 ε + 1) – ½ (n2 -1/ 2n2 +1)]

where µ g and µ e are ground state and excited state dipolemoment, ῦ abs and ῦ𝜈𝜈𝜈 abs are solvent𝜈𝜈𝜈 equilibrated absorption maxima and extrapolated to gas phase, ῦ flu and ῦ𝑓𝑓𝑓 are solvent

equilibrated fluorescence maxima and extrapolated to gas-phase, respectively, a o is Onsager cavity and ε and n are the solvent dielectric constant and refractive index, respectively. Figure 3 shows the non-linear correlation obtained from the plot of Stokes shift against solvent polarity function. The non-linear correlation reveals that there is transformation of the fitted line between ethyl ether and methylene chloride. The ground state dipole (μ g ) of the blue emitting materials, TPA-MPI and TPA-MPS could be estimated from density functional theory calculation as 3.15 and 5.05 D, respectively. The calculated μ e is 20.6 and 24.8 D in high polar solvents. Lippert–Mataga plot of these compounds in low polar solvents shows that the observation of linear correlation with zero slope revealing no change of μ e and μ g . The small μ e estimated from the small slope is close to 3.15 and 5.05 D for TPA-MPI and TPA-MPS and this small μ e is due to the presence of local exciton (LE) transition [34, 36]. The calculated large μ e in high polar solvents (20.6 and 24.8 D) is very close to μ e of charge transfer molecule, 4-(N,N-dimethylamino)-benzonitrile (23.0 D) [34]. In high polar solvents these compounds shows non-linear correlation which reveal that these compounds possess both local exited state (LE) and charge transfer exited state (CT). All these results show that CT dominates in more polar medium whereas LE dominates in low polar medium. There is a mixed contribution of LE and CT occurs in medium polar solvents (ethyl ether and methylene choloride). Free energy change of salvation (∆G solv ) and reorganization energy (λ) have been determined (Tables S1&S2) in various solvents. Marcus reported that E (A) = ∆G solv + λ 1


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and E (F) = ∆G solv - λ 0 , where E(A) and E(F) are absorption maxima and fluorescence maxima in cm−1, respectively, ∆G solv denotes difference in free energy of ground and excited states in a given solvent and λ is reorganization energy. E (A) + E (F) = 2∆G solv ; E (A) - E (F) = 2 λ ( under, λ 0 ≈ λ 1 ≈ λ,): difference between ∆G hex and ∆G solv should give R

the free energy change required for hydrogen bond formation. The plot of ∆ (∆G solv ) versus

f(Ɛ,n)has been depicted in Figure S3. The definite reorganization energies reveal the interaction between reorientation of solvent cell with low and medium frequency motion of solute (Figure S3). The mixed contribution of LE and CT occurs in medium polar solvents is further supported by potential energy curve (PEC) of TPA-MPS derivative (Figure 4). Analysis of potential energy curve and electron density distribution of HOMO-LUMO orbitals shows that the low energy geometry of TPA-MPI corresponds to 55° in which the electron cloud of HOMO and LUMO is populated in entire molecule which shows the crossed transition between the intercrossed local exciton (LE) and charge transfer exciton (CT). The higher energy conformation of TPA-MPI at 90° at which the electron density of HOMO is populated on triphenylamine whereas the electron density of LUMO is populated on phenanthrimidazole. The electron density contribution evidenced that the charge transfer is from TPA to acceptor phenanthrimidazole as it is confirmed by optical studies [37]. The twisting conformation at 90° is unstable than that of TPA-MPI at 55°. In electrostatic potential map, the dark red surface represent the negative potential region and blue surface represents small negative potential as compared to oxygen atom. The green region is midway between red and dark blue colour. The positive and negative part of these materials determined by surface map also supports the charge migration (Figure 4). Using the onset potential of 0.24 V (TPA-MPI) and 0.45V (TPA-MPS), the calculated HOMO energies are 5.04 and -5.25 eV (E HOMO = E ox + 4.8 eV) [35] and the corresponding LUMO energies are 2.31 and -2.60 eV (E LOMO = HOMO + 1239/λ onset ) [35]. High fluorescence quantum yield



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obtained for TPA-MPI (0.70) and TPA-MPS (0.75) in both solution and film further supports the coemission from the LE and CT of the emissive materials. From Figure S4 it is shown that the electron cloud densities of HOMOs are mainly localized on entire molecule of the blue emitting materials TPA-MPI and TPA-MPS. The electron cloud densities of HOMO+1 are localized on entire molecule of TPA-MPI whereas on p-methoxynaphthylamine at N-37 and partly with some phenanthrimidazole carbons of TPA-MPS. The electron density distributions of HOMO+2 are distributed on electrondonating TPA and partly on phenanthrimidazole of both TPA-MPI and TPA-MPS. The electron distribution on LUMO, LUMO+1 and LUMO+2 are localized on TPA and spacer groups. This distribution implied that LE transition of the centre fragment and CT transition from phenanthrimidazole to centre fragment which strongly support the co-emissive from LE and CT. The calculated HOMO of TPA-MPI and TPA-MPS are -4.94 and -4.86 eV and their LUMOs are -2.21 and -2.30 eV, respectively. The calculated HOMO of TPA-MPS is 0.08 eV lower than TPA-MPI and LUMO energy (0.09 eV) higher than TPA-MPI. Thus, there is decrease of energy gap of TPA-MPS expected to increase the efficiency. The electroluminescent performance of the fabricated devices using TPA-MPI (I) and TPA-MPS (II) as emissive layer is displayed in Figure 5. Device with TPA-MPS shows maximum efficiencies (η c = 1.73 cd/A; η p = 1.46 lm/W; η ex = 2.11%) at 4.6 V, deep HOMO level of TPA-MPS (-5.25 eV) with that of NPB (5.40 eV) is the reason for higher efficiencies. Because of shallow HOMO of TPA-MPS the leakage of holes to electron transport layer is reduced so that the effective recombination of electron and hole in the emissive layer of the fabricated devices results in higher efficiencies. These results show that TPA-MPS is a potential hole-transporting material (HTL) which is studied by the device fabrication [TPA-MPS/Alq 3 , device III] and analysed with the control device [NPB/Alq 3, device IV]. Device III shows maximum efficiencies (3.7 cd/A, 2.2 lm/W, 1.18 %) than the


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control device IV (2.8 cd/A, 1.7 lm/W, 1.03 %) and the higher efficiencies reporting that TPA-MPS is a potential hole transporting material (HTL). Blue shift of the EL spectra of TPA-MPS compared to that of TPA-MPI may be due to shorter conjugation length of TPA-MPS compared to that of TPA-MPI. Due to less twisted aromatic spacer in TPA-MPS the extent of conjugation is reduced and thus better charge transfer from donor (TPA) to acceptor (phenanthrimidazole) leading to a blue-shifted EL. The similarity of solid EL and PL spectra supports the coemission from the LE and CT excited state of the emissive materials (Figure 6). The maximum external quantum efficiency of 2.11% is due to the coemission from the intercrossed excited state LE and CT of the emissive layer in the device; isoenergies of singlet (1CT) and triplet (3CT)] make the 3CT → 1CT transition as spin-allowed one [17]. In addition to that better nanoscale morphology of the annealed TPA-MPS thin films is attributed to low turn-on voltages. 4. Conclusion We have synthesized newly born blue emitting materials composed of donor-linkeracceptor geometry with phenyl and styryl as spacers and their utilisation in non-doped OLEDs have been investigated. The blue emissive materials exhibit better quantum efficiency and exhibit high thermal stability. The TPA-MPS with styryl as spacer based device exhibits 1.73 cd/A (current efficiency), 1.46 lm/W (power efficiency) and 2.11% (external quantum efficiency) with 4.6 V. The hole-transporting and emissive nature made these new born materials (TPA-MPI and TPA-MPS) as potential blue OLEDs materials. 

Supporting Information: Contents

1. Tables- S1 & S2 2. Figures- S1-S4 This material is available free of charge via the Internet at http://pubs.acs.org.



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

Acknowledgments

The authors thank the Council of Scientific and Industrial Research (01/(2707)/13EMRII), Department of Science and Technology (EMR/2014/000094), Defence Research and Development Organization (Naval Research Board) (213/MAT/10-11) and University Grant Commission (36-21/2008) for financial support.


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Constructing Low-Triplet-Energy Hosts for Highly Efficient Blue PHOLEDs: Controlling Charge and Exciton Capture in Doping Systems, Chem. Mater. 2013, 25, 4966. (e) Sun, M.; Zhu, L.; Kan, W.; Wei, Y.; Ma, D.; Fan, X.; Huang, W.; Xu. H. Ternary donor–acceptor phosphine oxide hosts with peculiar high energy gap for efficient blue electroluminescence., J. Mater. Chem. C, 2015, 3, 9469.



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Table 1. Optical and thermal properties of TPA-MPI and TPA-MPS: their device performances Parameters

TPA-MPI

TPA-MPS

λ ab (nm)

253, 396/

250, 391/

(soln/film)

258, 398

256, 392

466/468

445/446

281/130/442

298/139/451

0.70/0.70

0.75/0.75

-5.04/-4.94

-5.25/-4.86

-2.31/-2.21

-2.60/-2.30

E g (eV)

-2.70/-2.73

-2.60/-2.53

Device

I

II

5.1

4.6

L (cd/m )

1942

2071

η ex (%)

1.30

2.11

η c (cd A-1)

1.53

1.73

η p (lm W-1)

1.26

1.46

EL (nm)

459

442

Photophysical & thermal

λ em (nm) (soln/film) T m /T g / T d (°C) ɸ (soln/film) HOMO (eV) (optical/calculated) LUMO (eV) (optical/calculated)

V 1000 (V) 2

III/IV: V 1000 (V):5.6/5., L(cd/m2) 4815/3158, η ex (%):1.18/1.03, η c (cd A-1):3.7/2.8, η p (lm W-1):2.2/1.70



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