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 * 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 1,4- polynaphthalene 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. ■ 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 applica- tions 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 l ́ Eclairage (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 polymers with carbazole, 12 pyrene, 14 poly(p-phenylene), 16,17 p- phenylenevinylene, 15 and triphenylphosphine/triphenylphos- phine oxide 13 units have also been reported. Naphthalene- based molecules attribute large band gaps; hence, these are suitable for blue luminescence. Accordingly, naphthalene- containing 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 1,4- polynaphthalene (1,4-PN) of high molecular weight (M n up to 54900 g mol −1 ) obtained by Yamamoto coupling of monomers where individual naphthalene units are connected at the 1,4- positions (Chart 1). Intense color-pure blue photolumines- cence (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-PNs 38,39 and 2,6- PNs 40,41 as well as 1,5- and 2,6-linked naphthylene-based ladder polymers 42 were synthesized earlier, to the best of our Received: May 18, 2018 Revised: August 23, 2018 Article pubs.acs.org/Macromolecules Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b01057 Macromolecules XXXX, XXX, XXX−XXX Downloaded via UNIV OF LOUISIANA AT LAFAYETTE on October 13, 2018 at 05:57:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.