Synthesis, photophysical, theoretical and electroluminescence study of triphenylamine-imidazole based blue fluorophores for solution-processed organic light emitting diodes

Accepted Manuscript Synthesis, photophysical, theoretical and electroluminescence study of triphenylamine-imidazole based blue fluorophores for solution-processed organic light emitting diodes Jairam Tagare, Deepak Kumar Dubey, Jwo-Huei Jou, Sivakumar Vaidyanathan PII:

S0143-7208(18)31602-4

DOI:

10.1016/j.dyepig.2018.09.007

Reference:

DYPI 6989

To appear in:

Dyes and Pigments

Received Date: 20 July 2018 Revised Date:

31 August 2018

Accepted Date: 4 September 2018

Please cite this article as: Tagare J, Dubey DK, Jou J-H, Vaidyanathan S, Synthesis, photophysical, theoretical and electroluminescence study of triphenylamine-imidazole based blue fluorophores for solution-processed organic light emitting diodes, Dyes and Pigments (2018), doi: 10.1016/ j.dyepig.2018.09.007. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, photophysical, theoretical and electroluminescence study of triphenylamine-imidazole based blue fluorophores for solution-processed

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organic light emitting diodes Jairam Tagarea, Deepak Kumar Dubeyb, Jwo-Huei Joub and Sivakumar Vaidyanathana* a

Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan-30013.

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b

Optoelectronic laboratory, Department of Chemistry, National Institute of Technology Rourkela, India.

*

To whom correspondence should be addressed. Email: [email protected] (V. Sivakumar) Tel:

+91-661-2462654 1

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Abstract The deep blue organic light emitting diodes (OLEDs) are necessary for solid-state lighting (SSL)

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and flat panel displays. In the practical application of OLEDs, high efficiencies and luminance with simple device structure, are highly desirable. Hence, we have designed and synthesized three blue (Donor-Acceptor) fluorophores with linear, bent and star-shaped molecular geometry

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by incorporating triphenylamine (TPA) as a hole transporting unit and imidazole as an electron transporting unite with electron withdrawing Ph-CF3 moiety at the N1 position of imidazole unit.

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The synthesized fluorophores were structurally characterized by nuclear magnetic resonance spectroscopy (NMR) and mass spectroscopy. The linear-shaped fluorophore structurally characterized by single crystal X-ray diffraction. Photophysical, electrochemical and electronic (theoretical) properties of the synthesized fluorophores were systematically investigated. All the fluorophores are showing deep blue emission in DCM solution. The synthesized fluorophores are

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shown good quantum yield (Φ) in solution and solid phase. The structurally simple OLEDs devices were fabricated by using these fluorophores as emitters. All the fluorophores showed deep blue electroluminescence (EL) emission with good device efficiency. Among all the

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devices, based on TBIMTPA with a 5 wt % dopant concentration exhibited better performance in

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the series with a maximum luminance of 994 cd/m2, a current efficiency of 2.6 cd/A, a power efficiency of 1.0 lm/W and an EQE of 3.2% at brightness of 100 cd/m2. These results indicate that the introduction of imidazole-based accepter on the TPA core is a favorable design strategy towards deep blue emitters. Keywords:

TPA,

imidazole,

fluorophore,

electroluminescence.

2

DFT

computations,

photoluminescence

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Introduction: The solution processable organic light emitting materials, which possess simple molecular

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structure and good solubility, have attracted much attention from both industrial and scientific communities as efficient candidates for low-cost large scale flat panel displays and solid state lightings (SSL) [1-11]. Efficient red, green and blue materials are highly essential for white

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OLEDs and a full-color display. At present, efficient green and red materials well developed, however, development of efficient blue materials are still challenged, due to the intrinsic wide

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bandgap and contradictory optical and electrical properties [12-16]. The development of intense deep blue materials is one of the important tasks because the high efficiency of the blue OLED devices is favorable to reduce energy consumption and extend the color gamut of full-color OLEDs displays [17-19]. For the blue OLEDs television display, Commission International de L’Eclairage (CIE) color coordinates of (0.14, 0.08) are required by the National Television

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System Committee (NTSC) and CIE of (0.15, 0.06) are required by European Broadcasting Union (EBU) standards [20-25]. Therefore, blue emissive fluorophores showing CIE coordinates with a y-component smaller than 0.1 are highly desired for brilliant full-color display.

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In the past two decades, high-performance OLEDs devices were fabricated by using a vacuum

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deposition technique [26-37]. However, this method is comparatively expensive, difficult to apply for flexible materials, difficult to apply for large area display and complicated to optimize the conditions for future displays. To overcome these problems Gustafsson et al. first reported flexible OLED fabricated by solution processing technique in 1992 [38]. This device fabrication method is cheap, easy to produce large area display and gives better performance because uniform distribution of device materials, which is beneficial for luminescence at high current

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density [4]. Recently, a number of polymeric and small molecules based solution processed OLEDs devices have been reported [39-53]. Recently, high-performance blue OLEDs reported based on phenanthroimidazole [54-57] and

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diphenyl-imidazole [58-60]. Among these materials, diphenylimidazole-based materials are giving very deep blue EL emission with a less full width of half maximum(FWHM), due to more twisting nature of the diphenyl-imidazole unite [58]. Hence, here in this report, we have designed

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and synthesized three highly twisted diphenyl-imidazole fluorophores (linear (BIMTPA), bent (DBIMTPA) and star-shaped (TBIMTPA)), by taking triphenylamine (TPA) as a hole

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transporting group and imidazole as an electron transporting group with Ph-CF3 substitution at the N1 position of imidazole group. TPA is shown good hole transporting nature and high thermal, photochemical and morphological stabilities [61-64] and imidazole group also one of the most used electrons deficient heterocyclic group for efficient OLEDs [65-67]. Herein this

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molecular design strategy we have chosen electron withdrawing Ph-CF3 group at the N1 position of imidazole unit, it will increase the electron accepting capacity of imidazole unite, thus improve the electron transporting properties of the fluorophores as well as the efficiency of the

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device. The synthesized fluorophores were structurally characterized by NMR and mass spectroscopy. The BIMTPA fluorophore structurally characterized by single crystal X-ray

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diffraction. Photophysical and electrochemical properties of the synthesized fluorophores were verified by density functional theory (DFT) and time dependent-DFT (TD-DFT) computations. All the fluorophores were shown deep blue emission in DCM solution and good quantum yield. Moreover, these fluorophores were tested as deep-blue emitting dopants in solution processable multilayer OLEDs and found to exhibit good device performances with good device efficiency and deep blue EL emission.

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Experimental section

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General Information for synthesis All the reactions were performed under nitrogen atmosphere. Solvents were carefully dried and distilled from appropriate drying agents prior to use. Commercially available reagents (Sigma

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Aldrich and Alfa Aesar) were used without further purification unless otherwise stated. All the reactions were monitored by thin-layer chromatography (TLC) with silica gel 60 F254 Aluminium

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plates (Merck). Column chromatography was carried out using silica gel (Sigma-Aldrich). Measurements: 1

H,

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C, and

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F NMR spectra were recorded using an AV 400 Avance-III 400MHz FT-NMR

Spectrometer (Bruker Biospin International, Switzerland) with tetramethylsilane (TMS) as a standard reference. The FTIR spectra were recorded on a Perkin–Elmer RX-I FTIR

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spectrophotometer. Elemental analysis was obtained using Elementary Analysis Systeme, Germany/Vario EL spectrometer. The mass spectra were recorded by LC-MS (Perkin–Elmer,

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USA/Flexer SQ 300 M). The absorption spectrum of the target fluorophores in solution phase and solid were measured by using UV-visible spectrometer (Shimadzu Corporation, Japan/UV-

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2450 Perkin Elmer, USA/Lamda 25). The photoluminescence excitation and emission spectra were recorded by Horiba Jobin Yvon, USA/Fluoromax 4P spectrophotometer. The absolute quantum yields were determined by using Edinburgh Instruments, spectrofluorometer, FS5, Integrating Sphere SC-30. The CIE color coordinates were calculated by using PL emission data (MATLAB software). The electrochemical properties of the fluorophores were measured by using cyclic voltammetry (CV), AUTOLAB 302N Modular potentiostat, at RT in dimethylformamide solution (DMF). The working (glass-carbon rod), auxiliary (counter, Pt 5

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wire), and reference (Ag/AgCl) electrodes were used for CV analysis. The DMF which contains 0.1 M Bu4NClO4 was used as the supporting electrolyte, and the scan rate was maintained at 100 mV s−1. The optimize structures and HOMO-LUMO energy levels of the fluorophores were

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calculated by using DFT calculation with B3LYP/6-31G (d, p) basis set. After confirming the ground state geometry of the fluorophores, we were vertically excited the fluorophores to get the first excited state (S1 and T1) using time-dependent density functional theory (TD-DFT).

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Results and Discussion Synthesis and Characterization:

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The TPA-CHO, TPA-2CHO and TPA-3CHO intermediates were synthesized according to the literature (synthesis procedure for all the intermediates given in the supplementary information) [68-70]. The target BIMTPA, DBIMTPA, and TBIMTPA fluorophores were synthesized by condensation between benzil, 3-(trifluoromethyl)benzenamine and aldehyde intermediates in the

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presence of ammonium acetate and acetic acid. The synthetic strategy to obtain the target fluorophores is illustrated in Scheme 1. The synthesized fluorophores are characterized by nuclear magnetic resonance spectroscopy and mass spectrometry

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Synthesis of BIMTPA: Amine (0.55g, 1.3eq) was added to a stirred solution of TPA-CHO (0.6g, 1eq) in glacial acetic acid (30 ml) at room temperature (RT). To this reaction mixture,

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subsequently, ammonium acetate (1.68g, 10 eq) and benzil (0.5g, 1.1 eq) were added. The resulting mixture was stirred for 12 hr at 110 °C. The reaction was monitored by thin layer chromatography for the completion of the reaction. After cooling, the reaction mixture was poured into the minimum amount of cold distilled water and then ammonium hydroxide solution was added to neutralize the crude compound. The formed solid was filtered and dissolved in dichloromethane (DCM). This was followed by drying with anhydrous sodium sulfate and the

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solvent was evaporated to get the crude compound. The resultant compound was purified by column chromatography by using silica gel (100-200 mesh) and eluent methanol in dichloromethane (DCM) (1:9).

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Yield: 63% with pale brown coloured fine solid, 1H-NMR (400 MHz, CDCl3, TMS, δ ppm): 7.61 (d, J=7.2 Hz, 2H), 7.54 (d, J=8.0 Hz, 1H), 7.41 (t, J=8.0 Hz, 1H), 7.29-7.21 (m, 14H), 7.14-7.03 (m, 8H), 6.95 (d, J=8.8 Hz, 2H),

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C-NMR (100 MHz, CDCl3, TMS, δ ppm) 148.26, 147.26,

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147.01, 138.45, 137.73, 134.12, 131.65, 131.55, 131.32, 131.14, 130.27, 130.08, 129.92, 129.59, 129.32, 128.61, 128.29, 128.21, 127.39, 126.79, 125.44, 125.41, 124.82, 124.76, 124.72, 123.37, 19

F-NMR (400 MHz, CDCl3, TMS, δ ppm) -62.68 (s, 3F). IR (KBr, ν /

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123.29, 122.46, 28.29,

cm-1): 3049, 1599, 1488, 1452, 1337, 1272, 1171, 1130, 1064, 890, 833, 754, 696. CHNS Analysis: Anal. Calc. for C40H28F3N3: C, 79.06; H, 4.64; N, 6.92; Found: C, 79.33; H, 4.81; N, 7.11 %. EI-MS: m/z exp. (calc.). 607.67 found 627.29 [M+H2O]+.

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Synthesis of DBIMTPA: Amine (0.58g, 2.2eq) was added to a stirred solution of TPA-2CHO (0.5g, 1eq) in glacial acetic acid (30 ml) at room temperature (RT). To this reaction mixture, subsequently, ammonium acetate (2.54g, 20 eq) and benzil (0.76g, 2.2 eq) were added. The

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resulting mixture was stirred for 12 hr at 110 °C. The reaction was monitored by thin layer chromatography for the completion of the reaction. After cooling, the reaction mixture was

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poured into the minimum amount of cold distilled water and then ammonium hydroxide solution was added to neutralize the crude compound. The formed solid was filtered and dissolved in DCM. This was followed by drying with anhydrous sodium sulfate and the solvent was evaporated to get the crude compound. The resultant compound was purified by column chromatography by using silica gel (100-200 mesh) and eluent methanol in dichloromethane (DCM) (1:9).

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Yield: 54% with pale yellow coloured fine solid,

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H-NMR (400 MHz, CDCl3, TMS, δ ppm):

7.62 (d, J=6.8 Hz, 2H), 7.55 (d, J=8.0 Hz, 1H), 7.41 (t, J=8.0 Hz, 1H), 7.33-7.21 (m, 11H), 7.14 (d, J=6.0 Hz, 2H), 7.09-7.05 (m, 1H), 6.95 (d, J=8.4 Hz, 2H),

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C-NMR (100 MHz, CDCl3,

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TMS, δ ppm) 147.63, 146.86, 146.73, 138.52, 137.66, 134.09, 131.67, 131.53, 131.34, 131.14, 130.23, 130.17, 130.00, 129.64, 129.43, 128.63, 128.33, 12823, 127.39, 126.82, 125.43, 125.39, 125.21, 124.82, 124.78, 124.51, 124.09, 123.92, 123.27, 121.88, 119.17, 29.43,

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F-NMR (400

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MHz, CDCl3, TMS, δ ppm) -62.38 (s, 3F). IR (KBr, ν / cm-1): 3054, 1603, 1481, 1445, 1330,

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1272, 1171, 1130, 1056, 841, 704. CHNS Analysis: Anal. Calc. for C62H41F6N5: C, 76.77; H, 4.26; N, 7.22; Found: C, 77.05; H, 4.47; N, 7.41 %. EI-MS: m/z exp. (calc.). 970.01 found 990.19 [M+H2O]+.

Synthesis of TBIMTPA: Amine (0.46g, 3.2 eq) was added to a stirred solution of TPA-3CHO (0.3g, 1eq) in glacial acetic acid (30 ml) at room temperature (RT). To this reaction mixture,

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subsequently, ammonium acetate (2.8g, 40 eq) and benzil (0.61g, 3.2 eq) were added. The resulting mixture was stirred for 12 hr at 110 °C. The reaction was monitored by thin layer chromatography for the completion of the reaction. After cooling, the reaction mixture was

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poured into the minimum amount of cold distilled water and then ammonium hydroxide solution was added to neutralize the crude compound. The formed solid was filtered and dissolved in

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DCM. This was followed by drying with anhydrous sodium sulfate and the solvent was evaporated to get the crude compound. The resultant compound was purified by column chromatography by using silica gel (100-200 mesh) and eluent methanol in dichloromethane (DCM) (1:9). Yield: 50 % with brown coloured fine solid, 1H-NMR (400 MHz, CDCl3, TMS, δ ppm): 7.60 (d, J=6.8 Hz, 2H), 7.55 (d, J=7.6 Hz, 1H), 7.42 (t, J=8.0 Hz, 1H), 7.31-7.20 (m, 10H), 7.14 (d, J=6.4

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Hz, 2H), 6.93 (d, J=8.8 Hz, 2H),

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C-NMR (100 MHz, CDCl3, TMS, δ ppm) 147.05, 146.72,

138.54, 137.57, 134.01, 131.67, 131.50, 131.34, 131.11, 130.24, 130.14, 130.07, 129.66, 128.63, 128.35, 128.23, 127.40, 126.85, 125.37, 124.88, 124.69, 124.55, 123.80, 29.66,

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F-NMR (400

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MHz, CDCl3, TMS, δ ppm) -62.69 (s, 3F. IR (KBr, ν / cm-1): 3068, 1603, 1479, 1449, 1377, 1284, 1130, 1068, 842, 739, 698. CHNS Analysis: Anal. Calc. for C83H53F9N7: C, 75.56; H, 4.05; N, 7.43; Found: C, 75.87; H, 4.26; N, 7.59 %. EI-MS: m/z exp. (calc.). 1332.36 found

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1352.41 [M+H2O]+.

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Scheme 1. Synthetic routes for the triphenylamine derivatives

Structural Characterization: The crystal data and structure-refinement parameters of BIMTPA fluorophore are presented in Table 1. The ORTEP diagram of the BIMTPA fluorophore is shown in Fig. 1 and corresponding bond lengths and bond angles from the single-crystal XRD data are listed in Table S1 and S2 (In supplementary information). Rest of the fluorophores crystal studies 10

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could not be carried out. Since the obtained crystals are very poor and not suitable for X-ray

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diffraction.

Fig. 1. ORTEP Diagrams of BIMTPA (50 % probability ellipsoids; H atoms and cocrystallized solvent molecules are omitted). N atoms blue in color. (CCDC 1842426)

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Table 1. Crystal data and structure-refinement parameters of BIMTPA fluorophore. Identification code

BIMTPA C80H56F6N6

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Empirical formula Formula weight

1215.31

Temperature/K

293(2)

Crystal system

triclinic P-1

Space group a/Å

9.8918(8)

b/Å

11.4721(10)

c/Å

14.8396(11)

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75.484(7)

β/°

76.048(7)

γ/°

84.058(7)

Volume/Å3

1580.5(2) 1

Z

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α/°

ρcalcg/cm3

1.277

µ/mm-1

0.087

F(000)

632.0

0.366 × 0.271 × 0.214

Radiation

MoKα (λ = 0.71073) 3.68 to 56.52

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2Θ range for data collection/°

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Crystal size/mm3

-12 ≤ h ≤ 12, -11 ≤ k ≤ 14, -15 ≤ l ≤ 18

Index ranges Reflections collected Independent reflections Data/restraints/parameters

6822 [Rint = 0.0197, Rsigma = 0.0427] 6822/0/415 1.038

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Goodness-of-fit on F2

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Final R indexes [I>=2σ (I)]

R1 = 0.0721, wR2 = 0.1785

Final R indexes [all data]

R1 = 0.0979, wR2 = 0.2006

Largest diff. peak/hole / e Å-3

0.58/-0.51

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Fourier transform infrared (FTIR) Spectroscopy: The FTIR spectrum of all the fluorophores was measured from 400– 4000 cm–1, the

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corresponding FTIR spectra are shown in Fig. 2 and the vibrational frequencies are provided in Table 2. The frequency approximately 1600 cm-1 corresponding to C=N functional group of the imidazole ring. The 1130 frequency of all the compounds correspond to the C-CF3 group [71]. The peak around 3050 cm-1 corresponds to the aromatic C-H stretching frequency of the fluorophores. The peak at around 700 cm -1 is likely due to the aromatic C– H bending frequencies of the fluorophores.

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Fig. 2. FTIR spectra of the BIMTPA, DBIMTPA and TBIMTPA fluorophores. Table 2. The major FTIR bands [cm–1] for BIMTPA, DBIMTPA and TBIMTPA fluorophores. BIMTPA

DBIMTPA

TBIMTPA

1599

1603

1603

1130

1130

1130

C-F stretch

1337

1330

1377

C-N stretch

1272

1272

1284

3049,1488, 1452,

3054,1481, 1445,

3068, 1779, 1449,

1171

1171

1064, 890, 833, 754,

1056, 841, 704.

C=N stretch

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C-CF3 stretch

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Bonding

Aromatic C-H, C-C, C=C, stretching frequency C–H bending frequency

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1068, 842, 739, 698

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Thermal properties: The thermal properties of synthesized fluorophores were examined by thermogravimetric analysis (TGA). TGA measurements were carried out from 0 to 800 °C at a scanning rate of 10

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°C/min under a nitrogen atmosphere and the abtained thermogravimetric curves are shown in Fig. 3 and corresponding data is listed in Table 3. All the luminophores showed good thermal stability with high thermal decomposition temperatures (Td corresponding to 5%) which fell in

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the range of 410-445 °C. The temperature for 5% weight loss was found to be 410, 430 and 445 °C for BIMTPA, DBIMTPA, and TBIMTPA, respectively. Among all the fluorophores

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TBIMTPA fluorophore showed high thermal stability (Td corresponding to 5% = 445 °C) in this series, the significant enhancement of Td can be attributed to the addition of two diphenylimidazole moieties, which are likely to improve their morphological stability greatly.

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Thermal stability of the material is important for applications in organic electronic devices.

Fig. 3. Thermogravimetric curves of the fluorophors.

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Photophysical properties: The absorption and luminescence properties of the synthesized fluorophors were investigated by UV-vis and fluorescence spectroscopy in dichloromethane (DCM) solution as well as in solid

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phase. All the fluorophores are shown three absorption bands (Fig.4a), the higher energy absorption bands (~240 and ~300 nm) occur, due to the π-π* transition of phenyl ring [72] and lower energy absorption band arise, due to the π-π* transition of TPA and imidazole moieties.

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The absorption band at a lower energy bent (DBIMTPA) and star-shaped (TBIMTPA) fluorophores are red shifted around 20 nm, due to the increase of the conjugation of the

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fluorophores. The absorption spectra of the synthesized fluorophores in solid phase were found to be broad and red-shifted (320 nm and 396 nm) as compared with the solution (Fig. 4b). the obtained redshift absorption can be explained by their relative strong π-π stacking interactions [73]. Calculated UV/Vis absorption spectra of the fluorophores in the gas phase shown in Fig.

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4d, the absorption behavior of synthesized fluorophores is well lined with the experimental analysis. The optical band gap of the synthesized fluorophores calculated from diffuse reflectance spectra (DRS) with the help of the Kubelka-Munk function (Fig. S16 in

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supplementary information) [74]. The obtained band gaps of the fluorophores are tableted in Table 6, these band gaps are well aligned with the experimental (CV) band gap.

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The emission spectra of the synthesized fluorophores in DCM solution are shown in Fig. 3a and corresponding data summarised in Table 3. All the fluorophores are shown intense blue emission peaking at 400 nm (BIMTPA), 410 nm (DBIMTPA) and 430 nm (TBIMTPA), respectively. The emission band of the TBIMTPA fluorophore 20 nm red shifted, due to increasing of conjugation of the fluorophore. The solid emission spectra of the fluorophores also shown similar behavior with peaking wavelength of 485 nm (BIMTPA), 520 nm (DBIMTPA) and 538 nm (TBIMTPA),

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respectively. The observed red-shift can be explained by their strong π-π stacking interactions [75, 76] The CIE color chromaticity of the synthesized fluorophores as shown in Fig. 4c and corresponding CIE color coordinates (x and y) are tableted in Table S3 in the supplementary

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information. The absolute quantum yields (Ф) of synthesized fluorophores were also measured using integrating sphere in both solution and solid phase. The measured Ф digital images are shown in Fig. S11 (solution) and Fig. S12 (solid) (in supplementary information) and calculated

equation (1) [77].

η=

λ

λ



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Ф=

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values are tableted in Table 3. The Ф of the fluorophores calculated by using the following

λ

Ф

λ

(1)

λ

λ Ф

Where, L0(λ) is the integrated excitation profile (sample is directly excited by the incident beam)

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and Li(λ) are the integrated excitation profile attained from the empty integrated sphere. E0(λ) is the integrated luminescence of powder caused by direct excitation and Ei(λ) is indirect illumination from the sphere, respectively. Among all the fluorophores the star-shaped

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(TBIMTPA) fluorophore shown high Ф (42%) in solid phase as compared with the other fluorophores.

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Fig. 4 (a) UV absorption and PL spectra of fluorophores in DCM solution, (b) UV absorption and PL spectra of fluorophores in the solid state, (c) The CIE chromaticity of the fluorophores in

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solutions and solid phase and (d) Calculated UV/Vis absorption spectra of the fluorophores in the gas phase

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Table 3. Key photophysical properties of BIMTPA, DBIMTPA, and TBIMTPA Tda

Abs (nm)

PL

Abs (nm)

410

240, 298,

240, 296,

240, 286, 360

a

(%)

320, 396

485

22

40

410

320, 396

520

36

39

430

320, 396

538

33

42

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445

(%)

400

356 TBIMTPA

Solution Solid

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430

PL (nm)

340 DBIMTPA

quantum yield

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(nm) BIMTPA

Absolute

Solid

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Fluorophores

Solution

Thermal decomposition temperature corresponding to 5% weight loss.

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Theoretical calculations:

To understand the structure-property of the fluorophores at the molecular level, the geometric and electronic properties of the fluorophores were studied by density functional theory (DFT)

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and time-dependent DFT (TD-DFT) calculation by using B3LYP/6-31G (d, p) basis set [78]. The optimized geometries of fluorophores are shown in Fig. 5. The calculated highest occupied

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molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energy levels, energy gaps (Eg) and excited energy levels (singlet and triplet) of the fluorophores are summarized in Table 4. The Fig. 6 illustrates the spatial distributions of frontier molecular orbitals (HOMO and LUMO) energy levels of the fluorophores. The HOMO levels of all the fluorophores mainly populated on electron-donating TPA moiety. The LUMO levels of all the fluorophores mainly located on electron-accepting imidazole-N1 substitution moiety. The

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complete separation between HOMO and LUMO energy levels is beneficial to the efficient hole and electron transporting properties. Thus favorable for the EL performance of the OLEDs [79, 80]. Excited Singlet and triplet energy levels were calculated by using the TD-DFT method and

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the corresponding values were mentioned in Table 4. The calculated vertical excitation wavelengths, orbital contribution and their oscillator strength (f) of the fluorophores are listed in Table 5. In addition, atom coordinates of all the fluorophores were given in supplementary

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information (SI7 in supplementary information).

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Fig. 5. Optimized structures of BIMTPA, DBIMTPA, and TBIMTPA fluorophores.

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Table 4. Calculated Frontier Molecular Energy Levels of the fluorophores. HOMO

LUMO

HOMO-1

LUMO+1

Eg

S1 (Gas)

T1 (Gas)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

(eV)

BIMTPA

-4.88

-1.14

-5.50

-1.03

3.74

3.24

2.69

DBIMTPA

-4.85

-1.37

-5.36

-1.25

3.48

3.03

2.55

TBIMTPA

-4.86

-1.27

-5.37

-1.23

3.67

3.14

2.61

Fluorophores

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Fig. 6. Electron Density Contours of frontier molecular orbitals (FMO)

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Table 5. The computed vertical transitions and their oscillator strengths (ƒ) and configuration of the fluorophores.

Gas

λmax nm

ƒ

382.20

0.0971

HOMO→ LUMO (69.79%)

3.3635

368.62

0.1665

HOMO→ LUMO+1 (69.45%)

3.6670

338.11

0.5053

3.8995

317.95

0.1038

459.78

-

Triplet

Gas

2.6966

DBIMTPA Singlet

Gas

3.0311 3.1328

Gas

0.0484

395.76

0.3924

HOMO→ LUMO+1 (68.21%)

379.65

0.1033

HOMO→ LUMO+2 (68.22%)

3.3670

368.23

0.3297

HOMO→ LUMO+3 (68.59%)

3.5324

350.99

0.1890

HOMO→ LUMO+4 (64.38%)

2.5590

484.50

-

3.1484

393.80

0.3212

HOMO-1→ LUMO+4(13.77%) HOMO→ LUMO (14.58%) HOMO→ LUMO+1 (47.36%) HOMO→ LUMO+2 (16.54%) HOMO→ LUMO+3 (29.58%) HOMO→ LUMO (67.80%)

3.1726

390.79

0.3620

HOMO→ LUMO+1 (68.57%)

3.2366

383.07

0.1035

HOMO→ LUMO+2 (67.82%)

3.3009

375.61

0.2904

HOMO→ LUMO+3 (67.52%)

3.3667

368.26

0.1753

2.6139

474.32

-

HOMO-2→ LUMO+4(10.35%) HOMO→ LUMO+4 (67.85%) HOMO→ LUMO (33.03%) HOMO→ LUMO+3 (34.57%)

EP

Triplet

409.05

HOMO→ LUMO (11.69%) HOMO→ LUMO+2 (67.49%) HOMO→ LUMO+3 (11.05%) HOMO-1→ LUMO (26.43%) HOMO→ LUMO+3 (55.54%) HOMO-1→ LUMO+3(69.79%) HOMO→ LUMO (32.82%) HOMO→ LUMO+1 (40.37%) HOMO→ LUMO+2 (31.79%) HOMO→ LUMO+3 (15.14%) HOMO→ LUMO (68.86%)

TE D

3.2657

Gas

AC C

TBIMTPA Singlet

Triplet

Gas

Configuration

RI PT

BIMTPA Singlet

Energy (eV) 3.2440

SC

State

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Fluorophores

21

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Electrochemical Properties: The electrochemical properties of the synthesized fluorophores were investigated in dimethylformamide (DMF) solution by cyclic voltammetry (CV) analysis. The CV analysis was

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carried out by using 0.1 M Tetrabutylammonium perchlorate (Bu4NClO4) as supporting electrolyte at a scan rate of 100 mV/s. The cyclic voltammograms of the fluorophores shown in Fig. 7 and the corresponding data are summarized in Table 6. The onset oxidation and reduction

SC

potential of the BIMTPA, DBIMTPA, and TBIMTPA fluorophores are 1.36 eV, 1.21 eV, 1.10 eV and -0.99 eV, -1.12 eV, -1.06 eV, respectively, which clearly indicate their potential bipolar

M AN U

carrier transporting nature. By using these values (onset oxidation and reduction potential) we have calculated HOMO and LUMO energy level of the synthesized fluorophores. The HOMO and LUMO levels calculated by using the equation (2) and (3) reported by de Leeuw et al., [8183].

=− E

+ 4.4 eV

(2)

E

=− E

+ 4.4 eV

(3)

TE D

E

EP

The HOMO/LOMO energy levels of the BIMTPA, DBIMTPA, and TBIMTPA fluorophores are -5.76 eV, -5.61 eV, -5.50 eV and -3.41 eV, -3.28 eV, -3.34 eV, respectively. The absorbed

AC C

energy bandgap are 2.35 eV (BIMTPA), 2.33 eV (DBIMTPA), and 2.16 eV (TBIMTPA). These band gaps are aligned with the optical (DRS) band gap (Table. 6). The comparison of HOMO and LUMO energy level diagram is shown in Fig. 8.

22

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AC C

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Fig. 7. Cyclic voltammograms of BIMTPA, DBIMTPA and TBIMTPA fluorophores.

Fig. 8. HOMO-LUMO energy gap diagram of BIMTPA, DBIMTPA and TBIMTPA fluorophores. 23

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Table 6. Electrochemical properties of BIMTPA, DBIMTPA, and TBIMTPA luminophores. Eredb

HOMO

LUMO

E gc

Egd

(V)

(V)

(eV)

(eV)

(eV)

(eV)

BIMTPA

1.36

-0.99

-5.76

-3.41

2.35

2.45

DBIMTPA

1.21

-1.12

-5.61

-3.28

2.33

2.38

TBIMTPA

1.10

-1.06

-5.50

-3.34

2.16

2.20

SC

a

RI PT

Eoxa

Fluorophores

The onset oxidation potential, b the onset reduction potential,

c

electrochemical bandgap determined from cyclic voltammetry, d optical energy

Electroluminescence characteristics:

M AN U

bandgap estimated from the solid-state absorption spectra

To evaluate carrier-transport properties of synthesized blue fluorophores BIMTPA, DBIMTPA,

TE D

and TBIMTPA, hole-only and electron-only devices were fabricated. The configuration of the hole-only device and electron-only devices are indium tin oxide (ITO)/PEDOT:PSS (35nm)/ BIMTPA, DBIMTPA, and TBIMTPA (25nm)/LiF (0.9nm)/Al (100 nm) and ITO/4,4՜ -N,N՜ -

EP

bis[N-(1-naphthyl)-N-phenylamino]biphenyl (TPBi) (35 nm)/ BIMTPA, DBIMTPA, and TBIMTPA (25nm)/TPBi (35 nm)/ LiF (0.9nm)/Al (100 nm), respectively. The current density–

AC C

voltage characteristics of these the hole-only devices and electron-only devices are shown in Fig. 9(a) and (b), respectively. Amongst all the synthesized fluorophores, compound TBIMTPA displays the superb and comparable hole and electron transporting property, i.e. in the order of 2x103 mA/cm2. This indicates that the OLED device based TBIMTPA fluorophores can effectively balance formation and recombination of excitons in the emissive layer, which is beneficial for good device performance.

24

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RI PT

ACCEPTED MANUSCRIPT

Fig. 9. The current density versus voltage curves of the hole-only (a) and electron-only (b) devices for compounds BIMTPA, DBIMTPA and TBIMTPA.

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To investigate the electroluminescence (EL) properties of the synthesized deep-blue emitters BIMTPA, DBIMTPA and TBIMTPA, two type OLED devices, non-doped and doped, were fabricated by a solution process with the configurations ITO/PEDOT:PSS/deep-blue emitter/TPBI/ LiF/Al and ITO/PEDOT:PSS/deep-blue emitter doped in CBP/TPBI/ LiF/Al,

TE D

respectively. In the devices, PEDOT:PSS and LiF were used as the hole and electron injecting layers respectively. TPBI was employed as the electron transporting and hole blocking layer in order to restrict the charges recombined in the emitting layer due to its low LUMO energy level

EP

and deep HOMO energy level. ITO and Al acted as an anode and a cathode, respectively. The energy level alignment for the materials used in the devices is depicted in Fig. 10. The

AC C

electroluminescent data of the devices based on synthesized deep-blue emitters are summarized in Table 7.

25

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Fig. 10. Schematic diagram of energy levels of the OLED devices containing the deep-blue

EP

emitters BIMTPA, DBIMTPA, and TBIMTPA (a) as a host and (b) as a dopant within CBP host, respectively.

AC C

The current density-voltage–luminescence (I–V–L) plots of the all the fabricated devices are depicted in Fig. 11 and Fig. 12 and the pertinent data are tabulated in Table 7. The OLED devices fabricated with modified imidazole tri-substituted TPA derivatives (TBIMTPA) showed relatively low turn-on voltages (Von) and high current densities when compared to the mono- and disubstituted fluorophores (BIMTPA and DBIMTPA). This probably points to an effective charge transport in the molecular layers of the TBIMTPA when comparing with other counterparts, it’s because, among all the synthesized dyes, TBIMTPA possesses the smallest 26

ACCEPTED MANUSCRIPT

hole injection barrier(0. 60 eV) from the PEDOT: PSS layer and a small electron injection barrier

AC C

EP

TE D

M AN U

SC

RI PT

(0. 57 eV) from the TPBI layer.

Fig. 11. (a-c) the luminance-current density (L–I) and (d-f) current density-voltage (I–V) plots of the solution processed OLEDs by using synthesized dyes BIMTPA, DBIMTPA, and PBIMTPA at various doping concentrations. 27

AC C

EP

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M AN U

SC

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Fig. 12. (a-c) power efficiency–luminance and (d-f) current efficiency-luminance plots of the solution processed OLEDs by using synthesized dyes BIMTPA, DBIMTPA and TBIMTPA at various doping concentrations. 28

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Furthermore, all the non-doped devices displayed bluish-white emission with high current density and low operating voltage when comparing with doped devices. However, they showed poor luminance and efficiencies when compared to corresponding doped devices. The reason

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why non-doped devices show poor performance may be attributed to the leakage of charge carriers at the interface of respective electrodes without formation of excitons in emitting layer and charge transport balance in the emissive layer. Additionally, it is also noted that as the

SC

doping concentrations of the synthesized dyes increase from 1 wt% to 5 wt%, current densities of the devices dramatically decrease where other electroluminescent properties such as brightness

M AN U

and efficiencies affectedly increase. It indicates a balance carrier injection in the recombination zone and an appropriate host-to-guest energy transfer between the host and guest materials [8486].

The electroluminescence spectra of the fabricated solution-processed OLED devices are shown

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in Fig. 13. The bright deep-blue emission was observed for the OLED devices containing DBIMTPA and TBIMTPA as a guest with peaks centered at ~442 nm and Commission International de l’E´clairage coordinates (CIE) of (0.16, 0.10) and (0.16, 0.08) respectively for a

EP

1 wt% doping. These values match well with those of standard deep blue emission (0.14, 0.08)

AC C

prescribed by the National Television System Committee (NTSC 1987). For the devices doped with BIMTPA, blue emission was observed with wavelength maxima at 452 nm with a CIE coordinate of (0.20, 0.19) for the same doping concentration. The narrow full-width half maxima (FWHM) of the EL spectra are responsible for the high color purity. Furthermore, it is also very interesting to note that, electroluminescence properties of these OLED devices are highly dependent on the chromophore density of the triphenylamine nucleus of the synthesized emitters. The derivative decorated with three imidazole units having electron withdrawing Ph-CF3 moiety 29

ACCEPTED MANUSCRIPT

at the N1 position displayed the most hypsochromic-shifted EL-emission profiles among the series. Remarkably, upon increasing the dopant concentration the EL maxima and color coordinates showed a negligible change, indicating a stable EL even at high doping

RI PT

concentration. Moreover, the EL spectra closely resemble the PL recorded in DCM indicating that the EL originates from the radiative decay of singlet excitons localized on the guest and no presents

among

the

molecules

in

the

film.

AC C

EP

TE D

M AN U

SC

aggregation

Fig. 13. (a-c) EL spectra of the solution processed OLEDs by using synthesized dyes BIMTPA, DBIMTPA, and PBIMTPA at various doping concentrations (d) EL spectra of the 1 wt % containing emitters at brightness 100 cd/m2.

30

ACCEPTED MANUSCRIPT

Overall the device based on TBIMTPA with a 5 wt % dopant concentration exhibited excellent performance in the series with a maximum luminance of 994 cd/m2, a current efficiency of 2.6 cd/A, a power efficiency of 1.0 lm/W and an EQE of 3.2% at the brightness of 100 cd/m2. It

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might be due to lower energy barrier of TBIMTPA with respect to the adjacent hole injection and an electron transport layers as well as host, which favors balanced charge transport and effective transfer of energy from the CBP host to the dopant. Moreover, no CBP emission was noticed in

SC

the EL spectra also indicating a complete energy transfer from the CBP host to dopants [84]. Table 7. Effects of doping concentrations on the operation voltage (OV), power efficiency (PE),

M AN U

current efficiency (CE), and external quantum efficiency (EQE), CIE coordinates, and maximum luminance of the deep blue emitters BIMTPA, DBIMTPA, and TBIMTPA with CBP host are studied.

Device Dopant @100/1000 cd m-2 [wt.%] OV[V] PE

TE D

Dopant

CE

-1

BIMTPA Ι-1

CIExy

0.2

(0.21, 0.25)

173

coordinates

-2

(cd m )

9.4

[cd A ] 0.3

9.7

0.3

0.8

1.1

(0.20, 0.19)

573

9.7

0.2

0.9

0.9

(0.19, 0.18)

658

5

9.8

0.2

0.5

0.6

(0.19, 0.18)

541

100

7.7

0.2

0.5

0.2

(0.29, 0.32)

368

100

Ι-3

3

AC C

EP

1

DBIMTA ΙΙ-1

EQE [%]

[lm W ] 0.1

Ι-2 Ι-4

-1

Maximum Luminance

II-2

1

8.7/11.7

0.4/0.3

1.0/0.6

0.8

(0.16, 0.10)

1378

ΙΙ-3

3

9.6/12.1

0.3/0.2

1.1/0.8

1.2

(0.16, 0.12)

1499

ΙΙ-4

5

9.6/11.6

0.6/0.3

1.6/1.1

1.8

(0.18, 0.14)

1530

100

7.6

0.2

0.5

0.3

(0.30, 0.33)

249

ΙΙI-2

1

8.3

0.7

1.8

2.1

(0.16, 0.08)

837

ΙΙI-3

3

8.6/12.1

0.7/0.2

2.0/0.8

2.6

(0.16, 0.10)

1117

III-4

5

8.6

1.0

2.6

3.2

(0.18, 0.12)

924

TBIMTA ΙII-1

31

ACCEPTED MANUSCRIPT

Conclusion: In summary, three new Donor-Acceptor (D-A) fluorophores were designed and synthesized by using triphenylamine as hole transporting moiety and imidazole as electron transporting moiety.

RI PT

By increasing imidazole moiety in TPA core resulted in different physical, chemical and electronic properties. All the fluorophore showed deep blue emission (DCM), high quantum yield, and balance charge transporting characteristics. In addition, the computational

SC

investigation is in good agreement with the experimental outcome. Non-doped and doped devices were fabricated by using these fluorophores as emissive and dopant materials. All the

M AN U

non-doped devices exhibited bluish-white emission with high current density and low operating voltage when comparing with doped devices. However, they showed poor luminance and efficiencies when compared to corresponding doped devices. The bright deep-blue emission was observed for doped OLED devices containing DBIMTPA and TBIMTPA as a guest with peaks

TE D

centered at ~442 nm and Commission International de l’E´clairage coordinates (CIE) of (0.16, 0.10) and (0.16, 0.08) respectively for a 1 wt% doping. These values match well with those of standard deep blue emission (0.14, 0.08) prescribed by the National Television System

EP

Committee (NTSC 1987). Among all the doped devices, the TBIMTPA with a 5 wt % based device exhibited good device performance in the series with a maximum luminance of 994

AC C

cd/m2, a current efficiency of 2.6 cd/A, a power efficiency of 1.0 lm/W and an EQE of 3.2% at brightness of 100 cd/m2. Our results indicate that improvement of the charge transporting properties of the fluorophore by chemical modification is highly necessary for high performance deep blue OLED.

32

ACCEPTED MANUSCRIPT

Device fabrication and measurement For wet-processing, the fabrication process included first spin coating an aqueous solution of PEDOT: PSS at 4000 rpm for 20 s to form a hole-injection layer on a pre-cleaned ITO anode.

RI PT

Before depositing the following emissive layer (EML), the solution was prepared by dissolving the host and guest molecules in tetrahydrofuran at 50 C for 0.25 h with stirring. The resulting solution was then spin-coated at 2500 rpm for 20s under nitrogen, followed by deposition of the

SC

electron-transporting layer TPBi, the electron injection layer LiF, and the cathode Al, by thermal evaporation under less than 10-5 Torr.

M AN U

The luminance, CIE chromatic coordinates, and electroluminescence spectrum of the resulting ultra-deep blue OELDs were measured by using the Photo Research PR-655 spectra scan. A Keithley 2400 electrometer was used to measure the current-voltage (I–V) characteristics. The emission area of the devices was 2.5 mm-2, and only the luminance in the forward direction was

Supporting Information:

TE D

measured.

Supporting Information consists of NMR spectra (1H,

13

C, and

19

F) and mass spectra of the

EP

fluorophores, Quantum yield studies of fluorophores in solution and solid state, and atom

AC C

coordinates of all the fluorophores. Acknowledgment VS

acknowledges

SERB,

Department

of

(EMR/2016/002462) for the financial support.

33

Science

and

Technology

(DST),

India

ACCEPTED MANUSCRIPT

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Table of content (TOC):

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Solution processable multilayer OLEDs were fabricated by employing these fluorophores as emitting dopants in 4,4′-bis(9H-carbazol-9-yl)biphenyl (CBP) host and found to exhibit deep

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blue electroluminescence with high efficiency.

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Highlights:

Donor-Acceptor blue fluorophores were designed and synthesized

•

Fluorophores shown good quantum yield (Φ) in solution and solid phase.

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HOMO and LUMO separation endows them with good carrier transporting property.

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TBIMTPA based OLED showed deep-blue emission with CIE of (0.16, 0.08).

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TBIMTPA (5 wt %) based device exhibited good device performance with EQE of 3.2%.

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•