Synthesis of High Molecular Weight 1,4-Polynaphthalene for Solution

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Synthesis of High Molecular Weight 1,4-Polynaphthalene for Solution-Processed True Color Blue Light Emitting Diode Suman Kalyan Samanta,∥,† Gundam Sandeep Kumar,† Uttam Kumar Ghorai,‡ Ullrich Scherf,§ Somobrata Acharya,*,† and Santanu Bhattacharya*,† †

School of Applied and Interdisciplinary Sciences, Indian Association for the Cultivation of Science, Kolkata 700032, India Department of Industrial Chemistry and Swami Vivekananda Research Centre, Ramakrishna Mission Vidyamandira, Belur Math, Howrah 711202, India § Macromolecular Chemistry and Institute for Polymer Technology, Wuppertal University, Gauss-Strasse 20, 42119 Wuppertal, Germany

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‡

S Supporting Information *

ABSTRACT: Efficient light emitting diodes (LEDs) derived from organic π-conjugated polymers are in the center of light generating research. Facile fabrication of true color blue LEDs is challenging. Herein, for the first time, we report the synthesis of highly soluble new π-conjugated polymers 1,4-polynaphthalenes with high molecular weight following a Ni(0)-mediated Yamamoto coupling reaction. The designed polymer shows a color pure blue luminescence with a high quantum yield of ∼80% in solution and ∼34% in thin film. We demonstrate solution-processed fabrication and ambient condition operation of LEDs using 1,4polynaphthalene as the active layer. True color blue electroluminescence from the LEDs with CIE coordinates (0.16, 0.08) meets the criteria of the National Television Standards Committee (NTSC) for efficient displays.

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polymers with carbazole,12 pyrene,14 poly(p-phenylene),16,17 pphenylenevinylene,15 and triphenylphosphine/triphenylphosphine oxide13 units have also been reported. Naphthalenebased molecules attribute large band gaps; hence, these are suitable for blue luminescence. Accordingly, naphthalenecontaining polymers were developed for blue emitting OLEDs.28−36 Compared to small molecules, polymers with high molecular weight are preferred for device fabrication since small molecules tend to crystallize and phase segregate within devices.37 Although naphthalene-based small molecules are reported for pure blue electroluminescent properties,37 to the best of our knowledge naphthalene-based polymers with high molecular weight for the same are hitherto unknown. Herein, we report on the synthesis of a π-conjugated 1,4polynaphthalene (1,4-PN) of high molecular weight (Mn up to 54900 g mol−1) obtained by Yamamoto coupling of monomers where individual naphthalene units are connected at the 1,4positions (Chart 1). Intense color-pure blue photoluminescence (PL) with quantum yields (QY) as high as 80% in solution phase was achieved from 1,4-PN. Pure blue EL (CIE 0.16, 0.08) from the OLED devices has been obtained using 1,4-PN as the active layer. Although 1,5-PNs38,39 and 2,6PNs40,41 as well as 1,5- and 2,6-linked naphthylene-based ladder polymers42 were synthesized earlier, to the best of our

INTRODUCTION Organic π-conjugated polymers are in the center of research owing to their robust electroluminescence (EL) characteristics offering potential in organic light emitting diodes (OLEDs) applications.1,2 Organic π-conjugated materials are widely suitable for OLED applications as active material because of their band gap tunability that determines the color of the emitted light from the device.3,4 While low band gap polymers are largely synthesized for the generation of luminescence at higher wavelengths in the visible spectrum, suitable polymers with large band gap for the generation of blue luminescence were rarely reported until date.5−17 Color pure blue emitting polymers are extremely important in optoelectronic applications such as color tunable displays or for white light generation by mixing red−green−blue colors.18−22 Therefore, synthesis of color pure blue emitting polymer that meets the ́ Commission Internationale de lEclairage (CIE) coordinates (0.14, 0.08) defined by National Television Standards Committee (NTSC) is important for high precision display applications.3 Existing light emitting polymers span over the visible spectrum; however, meeting the true blue color is nontrivial. Pure blue electroluminescent polymers are relatively rare and mostly composed of fluorene-based polymers.23−27 Therefore, current research in the field of OLEDs relies on developing new blue emitting materials, particularly organic π-conjugated semiconducting polymers. While blue emitting OLEDs were mostly developed from polymers containing fluorene units,5−11 © XXXX American Chemical Society

Received: May 18, 2018 Revised: August 23, 2018

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DOI: 10.1021/acs.macromol.8b01057 Macromolecules XXXX, XXX, XXX−XXX

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proceed through a common intermediate 4, the dibromo derivative of alkylated 2,3-dihydroxynaphthalene (Scheme 1). Compound 4 was synthesized first by alkylation of 2,3dihydroxynaphthalene with n-octyl bromide to produce compound 3 followed by bromination with bromine in dichloromethane. It was then coupled with 1-bromo-4iodonaphthalene (5) by Kumada coupling to obtain the monomer 6. The monomer 6 was used for Yamamoto coupling using Ni(COD)2 following two different reaction conditions: (i) conventional heating and (ii) microwave-assisted heating to produce the polymer 1 (denoted as P1a and P1b, respectively; details in the Experimental Section, Supporting Information). The microwave heating resulted in higher molecular weights (Mn = 54900 g mol−1) compared to the conventional heating (Mn = 28700 g mol−1).49 Polymer 2 was synthesized in a single step by coupling the intermediate 4 with 1,4-dibromonaphthalene in Kumada coupling condition (denoted as P2a), which resulted in only low yields (Scheme 2). However, in the case of Suzuki

Chart 1. Chemical Structures of the 1,4-Polynaphthalenes

knowledge, this is the first conventional synthesis of 1,4-PN with high molecular weight, high solubility, and well-defined structure. First synthesis of 1,4-PN was accomplished by Sato et al. using Kumada coupling,43 which produced mostly insoluble materials because of the absence of any solubilizing functionality such as aliphatic chains. Bergmann cyclization for the preparation of 1,4-PNs often produces undesired side products.44,45 Oxidative coupling to produce chiral 1,4-PNs was described; however, the average molecular weight Mn reached only up to 5200 g mol−1.46 Oxidative coupling of naphthalene using FeCl3 led to undesired products,47 while electropolymerization of naphthalene derivatives produces insoluble polymers having undefined structures.48 Previous attempts to produce aliphatic chain-functionalized 1,4-PNs via Yamamoto coupling reaction resulted in low molecular weight polymers with degrees of polymerization (Dp) ∼ 6.40 We synthesized 1,4-PNs by three different reaction procedures: Kumada coupling using Grignard-type monomers, Suzuki coupling using a Pd(PPh3)4 catalyst, and Yamamoto coupling using Ni(COD)2 as coupling reagent. However, high molecular weight polymers with Mn of up to 54900 g mol−1 were achieved only by Yamamoto-type coupling.

Scheme 2. Schematic Diagram for the Synthesis of 1,4Polynaphthalene (P2a and P2b)

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RESULTS AND DISCUSSION Synthesis. The 1,4-PNs were synthesized by three different reaction procedures: Kumada coupling, Suzuki coupling, and Yamamoto coupling. These three different reaction schemes

coupling, the intermediate 4 was first converted into a diboronic ester (monomer 7) which was then reacted with 1,4-dibromonaphthalene as comonomer to produce polymer 2

Scheme 1. Reaction Scheme for the Synthesis of 1,4-Polynaphthalenes (P1a and P1b)

B

DOI: 10.1021/acs.macromol.8b01057 Macromolecules XXXX, XXX, XXX−XXX

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lifetimes of ∼2.37 and ∼2.41 ns for solution and solid state, respectively, were obtained by time correlated single photon counting (TCSPC) method (Figure S17). The comparable lifetimes indicate similar radiative pathways for solution and solid state and also rules out any significant aggregationinduced emission phenomena. Cyclic Voltammetry. We have estimated the lowest unoccupied molecular orbital (LUMO) energy level of P1a from the onset of the cyclic voltammetry (CV) reduction wave (Figure 2). LUMO (ECV LUMO) was obtained at −2.48 eV with

with improved yield (denoted as P2b). The polymers obtained in different reaction pathways are characterized accordingly (Figures S1−S11), and their molecular weights are listed in Table 1. The average molecular weight obtained by Kumada or Table 1. Molecular Weights and TGA Analyses of the Polymers Obtained in Different Reaction Pathways polymer

Mn [g mol−1]

Đ

P1a P1b P2a P2b

28700 54900 3500 4500

2.4 2.9 1.4 1.4

Dp (no. of naphthalene units) 45 86 7 9

(135) (258) (14) (18)

Td (°C) 350 350 300

Suzuki coupling was significantly lower compared to the Yamamoto coupling, as indicated by the monomodal size distribution of the GPC traces (Figure S12), possibly due to the steric bulk of the aliphatic chains present close to the reaction center in the intermediate 4. Because of the presence of two n-octyl chains per repeat unit of the polymer, 1,4-PNs generated from different pathways are readily soluble in chlorinated solvents such as dichloromethane or chloroform and aromatic solvents like benzene, toluene, or o-dichlorobenzene (ODCB). Thermogravimetric analysis (TGA) showed decomposition onset temperatures of ca. 350 °C for both polymer P1a and P1b and 300 °C for polymer P2b, indicating the high thermal stability (Figures S13 and S14). Polymer P1b exhibited a weak glass transition at ca. 166 °C while polymer P2b at 85 °C in differential scanning calorimetric analysis (Figures S15 and S16). UV−Vis and PL Studies. We have studied the optical properties of the 1,4-PNs by UV−vis and PL spectroscopy considering polymer P1a having optimum molecular weight. The absorption spectrum of P1a in ODCB solution showed a single peak with a maximum at ∼320 nm (Figure 1a). The

Figure 2. Cyclic voltammetry reduction wave of polymer P1a for the estimation of LUMO. Red dotted lines indicate onset reduction potential.

respect to the redox potential of ferrocene measured from vacuum level (−4.8 eV).12,15 Because the corresponding oxidation potential was not obtained from CV, the highest occupied molecular orbital (HOMO) energy level was calculated considering the optical band gap (EOPT = 3.51 g eV) obtained from the onset of the absorption spectrum. HOMO was calculated to be −5.99 eV using the equation CV OPT EOPT HOMO = ELUMO − Eg . In the 1,4-PN backbone, the naphthalene units are assumed to have high torsion angle due to the steric interaction caused by the peri-hydrogens, thus reducing the π-conjugation among the main chain, in correlation to the large band gap of the polymers. The large band gap and the intense blue PL are unique features for the use of such materials in lighting devices. OLED Device Fabrication. To demonstrate the application potential, we have fabricated OLEDs using polymer P1a as active layer. The device structure consists of ITO/ PEDOT:PSS/PVK/P1a/BPhen/LiF/Al, where ITO is used as anode and Al as cathode (Figure 3a). All the layers employed in the device were deposited from the solution phase by spin-coating except the BPhen layer and the LiF/Al electrodes (details in the Experimental Section, Supporting Information). The PEDOT:PSS acts as a hole injection layer, and the PVK layer favors the hole transport (HTL) while effectively blocking the electrons. The Bphen acts as an electron injection/transport layer (ETL) within the device. A schematic representation of energy levels of the components within the device is shown in Figure 3b. The favorable energy levels allow the electrons and holes to recombine radiatively within the active polymer layer. The device characterizations were performed at room temperature under ambient conditions. PL and EL spectra at various voltages are very similar, suggesting that the EL is occurring from the active layer of polymer P1a (Figure 3c). This rules out the possibility for participation of the PVK layer in the emitting process which

Figure 1. (a) UV−vis absorption spectrum of P1a in ODCB solution. (b) PL spectra of the polymer in solution and in thin film. Insets show photographs of thin film and solution (a) in daylight and (b) under 365 nm UV light illumination.

polymer solution and thin film appeared completely transparent in visible light (inset, Figure 1a). An intense PL spectrum was observed in solution with a peak at ∼403 nm (Figure 1b). Interestingly, the solid-state PL spectrum replicates the solution spectrum showing a similar peak position which indicates absence of the aggregation phenomena. Both the solution and the film exhibited strong deep blue emission under UV light illumination (365 nm, inset of Figure 1b). In addition, the PL QY of polymer P1a was measured both in solution and in the solid state using an integrating sphere which showed high QY of ∼80% in ODCB solution. Interestingly, the strongly emissive property was also retained in the solid state showing a QY of ∼34%. The average PL C

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detectable starting from a threshold voltage of 4 V showing a maximum of ∼756 cd m−2 at 15 V. The device operated in the ambient showed a maximum current efficiency of ∼5.76 cd A−1 and external quantum efficiency (EQE) of 1.18% at an applied bias of 9 V (Figure 4b). The corresponding CIE chromaticity coordinates diagram indicates a voltage-independent deep blue emission of the LEDs (Figure 3c, inset). Importantly, CIE coordinates of (0.16, 0.08) are close to the standard blue-light CIE coordinates of (0.14, 0.08) defined by the NTSC. Thus, our preliminary device results demonstrate bright deep blue EL at relatively low turn-on voltages within the full range of operating voltages. The maximum luminous efficiency (LEmax ∼ 5.76 cd A−1) achieved is comparable with or better than the polymeric deep blue emitters reported in the literature having CIE coordinates close to (0.14, 0.08).23−26 In summary, we describe for the first time the synthesis of πconjugated, soluble 1,4-PNs of high molecular weight (Mn up to 54900 g mol−1) and well-defined structure following a Ni(0)-mediated Yamamoto coupling reaction. Polymer P1a exhibited deep blue PL with QY ∼ 80% in solution and ∼34% in solid state. This large band gap (3.51 eV) polymer was used for LED device fabrication, showing a maximum luminous efficiency of ∼5.76 cd A−1 and EQE of 1.18% measured under ambient conditions. The devices exhibited deep blue EL with CIE coordinates of (0.16, 0.08), thus meeting the standard defined by the NTSC. The CIE shows the best deep blue emitters for high molecular weight polymer LED applications until now. Therefore, the results on our newly synthesized 1,4PNs indicate high potential for applications in organic optoelectronics.

Figure 3. (a) Multilayer device architecture and (b) flat energy-level band diagram of the polymer P1a OLED. The energy levels are represented with respect to vacuum level. (c) EL spectra of the OLED devices operated under different bias from ∼5−15 V and compared with the PL spectrum. Inset shows CIE chromaticity coordinates of the EL from the OLED device. (d) True color photographic images showing intense pure blue OLEDs under various operating voltages.

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was further confirmed from a reference device without a P1a layer showing negligible luminance (Figure S18). The devices containing P1a exhibited intense EL over a wide range of operating voltages. The photographs of the active device showed the emission of deep blue light, the intensity of which increases with the bias voltage (Figure 3d). Interestingly, the LEDs glow uniformly over the entire active device area without any inhomogeneity, demonstrating the prevention of electrical shunting paths, where injected charge carriers bypass the emitter polymer. OLED Device Characteristics. The current density versus voltage characteristics of the devices showed LED responses with an effective threshold voltage below 4 V (Figure 4a). Above the turn-on voltage, the luminance versus voltage plot is mostly linear in a double-logarithmic scale. Luminance was

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01057. Spectral data of the newly synthesized molecules and polymers (1,4-polynaphthalenes); Figures S1−S18 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.B.) E-mail [email protected]. *(S.A.) E-mail [email protected].

Figure 4. Device performance characteristics of polymer P1a OLEDs showing (a) current density and luminance vs voltage characteristics and (b) current efficiency and external quantum efficiency vs current density characteristic curves. D

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Somobrata Acharya: 0000-0001-5100-5184 Santanu Bhattacharya: 0000-0001-9040-8971 Present Address ∥

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur 721302, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. acknowledges the Alexander von Humboldt foundation for a postdoctoral fellowship and IACS for a Research Associate fellowship. S.A. acknowledges SERB for funding. G.S.K. acknowledges DST-INSPIRE Fellowship. Dr. Michael Forster and Anke Helfer are gratefully acknowledged for FT-IR and GPC measurements.

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