Molecular Engineering of Organic Fluorophores for Blue Organic Light Emitting Diodes: Synthesis, Photophysical and Electroluminescence Investigation

Abstract

The current thesis investigates the molecular engineering and synthesis of a novel class of blue organic fluorophores with the goal of employing them in blue and white organic light emitting diodes (OLEDs). In chapter 1, a general overview of the development of the new generation and several molecular design strategies, fundamental principles used to achieve high efficiency, a literature survey of recent trends, and brief objectives of the present thesis work were discussed. The design and synthesis of imidazole-based blue fluorescent materials, as well as their applications in blue OLEDs, were highlighted in the introduction. The main aim and importance of the proposed work of the thesis were summarized in this chapter. In chapter 2, two sets of new deep-blue phenanthroimidazole (PI)/benzilimidazole(BI) (A) and triphenylamine (TPA) (D) based fluorophores were designed and synthesized. In the first set of fluorophores, two phenanthroimidazole linearly connected with the hole transporting TPA with the aromatic ring as a spacer and different functional groups at N1 position and of the PI (p-tolyl benzene and p-triflurobenzene) moiety (4-PIPCFTPA and 4-PIPTPA). In the second set of fluorophores, the hole transporting TPA unit linearly connected with electron-transporting PI and BI through the aromatic ring as spacer linearly between D-A with m-triflurobenzene moiety at the N1 position of both imidazole (4- PIMCFTPA and 4-BICFTPA). The synthesized fluorophores were structurally characterized by spectroscopic methods. These fluorophores are thermally stable with the thermal decomposition temperature (Td) of ~446 C. The photophysical, electronic (theoretical), electrochemical (CV), and electroluminescence properties of all the fluorophores were thoroughly investigated, and the N1 substituted moieties play a vital role as it affects the photophysical as well as electronic properties of the synthesized fluorophores. These fluorophores showed deep-blue photoluminescence (PL) emission and high photoluminescence quantum yield (PLQY) in the solution as well as in the solid phase. The HOMO-LUMO energy level of the fluorophores was calculated using electrochemical studies and the same was compared with the theoretical calculation density-functional-theory (DFT). OLED devices were fabricated using solution processes to demonstrate these materials as emissive materials. Fabricated doped devices in the first set, with 4-PIPCFTPA demonstrated superior performance as compared to that of 4-PIPTPA. The best electroluminescence (EL) performance is displayed by the device fabricated with 4-PIPCFTPA (1 wt% in the CBP host) and (3 wt% in the CBP host) with a maximum external quantum efficiency (EQEmax) of 2.7 and 2.9%, respectively and CIE coordinates of (0.17, 0.07) and (0.18, 0.12) were observed, respectively. In the second set of fluorophores, doped devices fabricated with 4-PIMCFTPA demonstrated good EL performance as compared with BI–based emitter. The EL performance based on 4-PIMCFTPA (1 wt% in the CBP host) with an EQEmax of 1.7% CIE coordinates of (0.17, 0.06). The observed EL properties of these emitters reveal their potential as efficient deep-blue emitters for display applications. In chapter-3, two sets of novel NUV and deep-blue light emitting fluorophores based on phenanthroimidazole (PI)/benzilimidazole(BI) (A) integrating with triphenylamine (TPA) (D) emitters were designed and synthesized with meta linking between PI/BI and TPA D--A design strategy which efficiently shortens the -conjugation length, which results in emission in the higher energy end of the emitters. In the first set of compounds three deep blue 4-BIPTPA (para-linked), 3-BIPTPA, and 3-PIPTPA (meta-linked) by integrating a TPA moiety with BI/PI with the different linking arrangement between donor-acceptor and functionalization at the N1 position of the imidazole with electron-donating (p-tolyl benzene) moiety has been successfully designed and synthesized and in the second set of compounds two efficient NUV/deep-blue fluorophores with electron-transporting PI/BI and the hole transporting TPA unit through meta linking D--A design strategy with m-triflurobenzene moiety at the N1 position of both imidazole moiety (3-PIMCFTPA and 3-BICFTPA). Experimental and theoretical investigation reveals materials with efficient NUV/deep-blue emission and good bipolar carrier transporting properties. In addition, doped and non-doped OLEDs were fabricated using the solution processes technique to demonstrate these materials as emissive materials. In the first set of fluorophores, the doped OLED devices based on 4-BIPTPA emitter as dopant showed deep blue emissions with excellent device performance of EQEmax of 6.2%, and CIE coordinates of (0.17, 0.14) at 5 wt% doping concentration and doped device based on 3-PIPTPA shows deep blue emission with EQEmax of 6.0%, and CIE coordinates of (0.17, 0.09) at 1 wt%. In the second set of fluorophores, the 3-PIMCFTPA showed the best EL performance with EQEmax of 5.7% and 3.4% with Commission Internationale d’Énclairage (CIE) coordinates of (0.17, 0.10) and (0.17, 0.02) respectively. Moreover, the 3-PIMCFTPA possesses pure violet emission (385 nm) with high performance allowing the emitter to be used to realize high-performance hybrid white OLEDs. The 1wt% yellow emitter-based device displayed pure white light with an EQEmax of 12.0% with a CIE coordinate (0.33, 0.37) at 1000 cd m–2 . These results of emitters make them potential for smart displays and lighting applications. In chapter-4, A series of deep blue fluorophores (PTPIBI and m-CF3PIBI) based on the hybridized local and charge transfer (HLCT) characteristics were designed and synthesized by incorporating weak donor phenanthroimidazole (PI) and weak acceptor benzimidazole (BI) to study their potential application in deep-blue organic electronic devices. The systematic investigation of photophysical and theoretical properties of both the fluorophores indicates that the materials possess locally excited (LE) and charge transfer (CT) excited states character. The synthesized emitters showed high thermal stability and intense blue emission with high quantum yields. fabricated solution-processed undoped and doped OLED devices with newly synthesized emitters and observed that the PTPIBI demonstrated superior performance as compared to that of m-CF3PIBI. The best electroluminescent (EL) performance is displayed by the device fabricated with PTPIBI (5 wt% in the TCTA host), showing an EQEmax of 2.4%, CIE coordinates of (0.15, 0.08), and maximum luminance of 2,507 cd m–2 . In addition, deep blue pure organic light-emitting material based on donor--acceptor weak donor benzilimidazole and weak acceptor BI(m-CNBIBI) has been designed and synthesized. The synthesized m-CNBIBI was well characterized by spectroscopic techniques and theoretical, optical, thermal stability, redox, and EL characteristics were systematically explored. The fully twisted m-CNBIBI emitter can emit pure blue emission in the solution and solid phase having good PLQY. The new deep blue emissive material displays a positive solvatochromism effect indicating material with charge transfer properties in the excited state. Moreover, a deep blue organic light-emitting device (OLED) was fabricated by using a low-cost solution-processed technique. The doped device shows good EL performance with EL emission in the deep blue region with color purity Commission International de l’Eclairage (CIE) of (0.16, 0.08) which is close to the National Television Standard Committee (NTSC) standard (0.14, 0.08). In chapter-5, a series of pure deep-blue organic fluorescence light-emitting materials which are thermally stable and have improved photophysical properties and with hybrid local and charge-transfer (HLCT) states were designed and synthesized by using weak donor and weak acceptor designed strategy (PTBIBI and MCFBIBI). Additionally, a thermally stable deep blue emissive material (MCNPIBI) by integrating moderate donor and acceptor namely phenanthroimidazole (PI) and benzimidazole with cyanophenyl (-CN) group at the N1 position of the PI to tune the CT component in the excited states. Systematic theoretical and photophysical study reveals the MCNPIBI with HLCT excited states. Time dependent density functional theory (TD–DFT) calculation suggests that the reverse intersystem crossing (RISC) process in MCNPIBI occurs from high-lying triplet states to singlet states. Furthermore, the synthesized deep blue emissive materials were employed as dopants in multilayer organic light-emitting diode (OLED) devices resulting in deep-blue EL with emission wavelength of 447 nm and CIE coordinates of (0.15, 0.08) which are close to standard values for blue emitters as suggested by NTSC (0.14, 0.08). The doped devices based on PTBIBI and MCFBIBI display a reasonably good device performance having the CIE coordinate of (0.15, 0.06) in the deep blue region and maximum luminance of 6599 cd m−2 and 3305 cd m−2 , maximum current efficiency (CEmax) of 1.95 and 1.54 cd A–1 , maximum power efficiency (PEmax) of 1.59 and 1.39 lm W–1 , and EQEmax of 3.59 and 2.67 %, respectively. The OLED device based on MCNPIBI displays CEmax of 2.78 cd A–1 , PEmax of 1.94 lm W–1 , and EQEmax of 3.69% respectively. In addition, the OLED device has low turn in voltage of 3.8 V. In chapter-6, two sets of D--A novel deep-blue light-emitting fluorophores based on benzothiazole as acceptor core and phenanthroimidazole (PI)/benzilimidazole(BI) as donor cores were designed and synthesized. In the first set of fluorophores three blue-light emitting materials by integrating BI as a weak donor and benzothiazole as a weak acceptor moiety (PHBISN, PTBISN, and m-CFBISN) with different functionalization at the N1 position of imidazole (benzene, p-tertbutyl benzene, and m triflurobenzene) to tune the electrical and photophysical properties and in the second set of fluorophores, three blue emissive materials with electron-donating and stable PI as weak donor integrated with electron accepting benzothiazole as a weak acceptor (PTPISN, m-CFBISN, and m-CNBISN) with varying substituents at the N1 position of imidazole (p-tertbutyl benzene, m-triflurobenzene, and m-cyanobenzene) to tune the properties of the materials have been designed and synthesized. All the synthesized fluorophores were structurally confirmed by spectroscopic techniques. All the fluorophores exhibited good thermal properties with high thermal degradation temperatures. An optical study reveals that the fluorophores are capable of emitting in the blue spectral region in solution and solid. The quantum chemical investigations were also performed with the aim to understand the electronic structures of the fluorophores. In addition, doped and non-doped OLEDs were fabricated using the solution processes technique to demonstrate these materials as emissive materials. The solution-processed OLED device was fabricated by using these fluorophores as emissive dopants shown good device performance with bright deep-blue electroluminescence with CIEy of (~0.08), which is near to the blue standard (0.14, 0.08) of the National Television System Committee (NTSC). The obtained results suggest that the introduction of benzothiazole moiety is a promising design strategy for obtaining new deep-blue emitters. In chapter-7, a series of three sky-blue fluorophores based on Imidazo[1,5-a]pyridine (ImPy) decorated with aromatic π-system (C3 position of ImPy is decorated with naphthalene, methoxy naphthalene, and pyrene) was designed and synthesized by the one-pot synthesis method. The fluorophores were structurally characterized by spectroscopic techniques. The density functional theory (DFT) calculations were carried out to understand the HOMO-LUMO energy level as well as excited energy level (singlet and triplet) of the molecules, detailed computational study reveals that the compounds showed a wide energy gap (>3 eV). Their photophysical and electrochemical properties, including UV-Vis, photoluminescence, and cyclic voltammetry were systematically studied. The presently studied fluorophores exhibited good thermal and electrochemical stability. The systematic photophysical study reveals that the compounds showed sky-blue emission with CIE color coordinates (x = 0.16/0.20, y = 0.22/0.32). ImPy derivatives were used as an emitter to fabricate the sky-blue emission OLED. Novel synthesized Impy derivatives demonstrate a good performance for sky blue emission OLED. Among the three emitters, the highest performance was observed with ImPy-3 demonstrating the maximum EQEmax of 4.3%, PEmax of 4.7 lm W–1 , and CEmax of 8.4 cd A–1 at 100 cd m–2 . Chapter 8 deals with the summary and conclusion as well as the future prospects of the work. The present thesis works deals with molecular engineering and synthesis of novel/new class of bipolar strong D-A and weak D-A blue organic fluorophores, with the aim of exploring their potential application in blue and white OLED. The observations and the conclusions derived from the present investigations are summarized in this chapter.