Molecular Design, Synthesis and Photophysical Investigations of Blue Emitting Fluorophores for Organic Light Emitting Diodes and Sensing

Abstract

The current thesis work addresses the potential for utilising new class of unipolar and bipolar blue emissive organic materials in applications for chemosensors and organic light emitting diodes (OLEDs) through molecular engineering and production. In chapter 1, a general overview of the development of the new generation and several molecular design strategies based on blue fluorophores: introduction, literature survey of recent trends and brief objectives of the present thesis work was discussed. The design and production of blue emissive materials based on imidazoles (phenanthroimidazole/diphenylimidazole), as well as their use in blue OLEDs, phosphorescent organic light emitting diodes (PhOLEDs) and chemosensors, were the key topics covered in the introduction. This chapter provided a summary of the thesis‟s principal objective and significance. In chapter 2, a series of luminophores (construction of diphenylimidazole (m-CF3PTPI) groups functionalizing with N1-positions of imidazole moiety) were designed and synthesized for optoelectronics and selective nitroaromatic detection. All the luminophores showed deep blue emission in solution, thin-film and solid phase with reasonable quantum yield as well as good thermal stability (5% weight loss at 258 296○C). From electrochemical analysis as well as theoretical calculations, the HOMO-LUMO energy gaps are found to be good agreement and all of them showed good triplet energy. The luminophores can be explored as host for phosphorescent organic light emitting diodes (PhOLEDs). Furthermore, the m-CF3PTPI derivatives were used as emitters for fluorescent OLEDs and host (m-CF3PTPI-1 and m-CF3PTPI-2) for triplet dopant in PhOLEDs. All the doped devices exhibited near-UV emissions with EL peaks raising ~380395 nm with Commission International deL'Eclairage (CIE) coordinate of (CIEy ~0.09). Of all the devices, m-CF3PTPI-5 (3 wt%) based device displayed maximum EQE (EQEmax) of 2.8%, power efficiency (PEmax) of 0.9 lm/W, current efficiency (CEmax) of 1.3 cd/A, and the brightness of 953 cd/m2. Moreover, the device was further optimized using different host approach. Amongst, SimCP2 displayed the best performance by achieving a high EQEmax of 4.0% that is close to the theoretical limit of fluorescent materials. Furthermore, m-CF3PTPI-1 and m-CF3PTPI-2 possessing triplet energy of 2.67 and 2.63 eV, respectively, were used as host for efficient green (Ir(ppy)3-2.4 eV) PhOLEDs. The m-CF3PTPI-2 based device achieved EQEmax (4.8%), CEmax (17.5 cd/A), PEmax (13.4 lm/W), and maximum brightness (Lmax) of 4695 cd/m2, much higher than that of counterpart m-CF3PTPI-1 based OLED. Taking advantage of the structural functionality, the luminophores were used for the detection of nitro aromatic compounds (including picric acid (PA)). All the luminophores showed good selectivity and high sensitivity towards the PA and the sensing mechanism was carefully investigated by using nuclear magnetic resonance (NMR) spectroscopy and DFT analysis. In chapter-3, as mentioned earlier, the efficient near ultraviolet light-emitting materials are pivotal for organic light emitting devices (OLEDs) because of its long life time, energy saving and for high-quality flat panel displays. In the present investigation, a series of luminophores having electron donating and electron withdrawing group with N1 functionalization were designed which can act as host as well as chemosensors. All TPTI luminophores showing deep blue emission in respective phase (solution, thin-film and solid) with reasonable quantum yield. Electrochemical analysis and theoretical calculations show similar trend for HOMO-LUMO energy gaps calculation and all of them having high triplet energy (2.76-2.91 eV). High triplet energy of the TPTI luminophores are extensively apply for green (Ir(ppy)3 2.4eV) emitting materials. It has been observed that the host TPTI-1 possess the highest efficiency and luminance among all the four hosts in all respects at 15.0 wt% with a maximum power efficacy (PEmax) 24.0 lmW-1, maximum current efficacy (CEmax) 38.2 cdA-1, and maximum external quantum efficiency (EQEmax) 6.8 % with a maximum luminance of 7549 cdm-2 at a turn-on voltage 4.4 V. To have the use of structural functionality, the TPTI luminophores were used for the recognition of nitro aromatic compounds (including picric acid (PA)). All the luminophores showed good selectivity and high catching towards the PA and the sensing mechanism was thoroughly investigated theoretically and experimentally. In chapter-4, a series of N1-functionalized imidazole have been synthesized in very good yield and due to the amphoteric nature it is selectively recognizing the varieties of nitroaromatic compounds. The fluorescence spectra of all luminophores are highly sensitive towards picric acid compared to other nitroaromatics compounds. The fluorescence method is fast response and low cost as well as high selectivity towards other nitroaromatic compounds. Electrochemical analysis were performed and verified by DFT calculations. The electrochemical properties were studied through cyclic voltammetry to calculate the HOMO-LUMO gap and the same confirmed by theoretical study. The stern-volmer quenching
constant and detection limit were found to be 6.68 x 106 M-1 and 446 ppb. The capability of luminophores to recognised PA was estimated by fluorescence, DFT analysis as well as 1H NMR spectroscopies. The potential luminophores were synthesized for the convenient and promising multifunctional sensors for toxic and explosive nitroaromatics analytes. In chapter-5, Phenanthroimidazole (PI) has great proficiency to design efficient near UV deep blue emitting materials due to its bipolarity behavior. The tuning of N1 and C1 Centre of the PI further reveals the improvement of the optical and electronical properties. Thorough investigation is also executed to explore the influence of alkyl chain on the optical and electroluminescence (EL) properties of these emissive materials. The incorporation of alkyl chain in the materials can tune the optical properties, leading to better solid state emission and enhance the photoluminescence quantum yield (PLQY) of thin film ( 74%) as compared to solution ( 65%). The EL spectra also exhibited between 395-420 nm (in the near UV spectral region). Furthermore, boost the device efficiency and color purity, the OLED device is fabricated by doping CBP as host matrix. Selectively, CBP is used as a host because of the similar energy level (HOMO and LUMO) of the synthesized emitters which assist efficient charge trapping. Predominantly, the emitters in OLED device blended with 0.5 wt% concentration, achieved brilliant device efficiency and luminance properties. Among all the synthesized fluorophores, PIPP (0.5 wt%) doped OLED device showed 4.4% of maximum external quantum efficiency (EQEmax), 1202 cd/m2 of highest luminance, 2.2 lm/W of power efficiency (PE), 2.7 cd/A of current efficiency (CE) with Commission International de L‟Eclairage (CIE) coordinates x= 0.17, y= 0.10 (nearer to NTSC standard value). In chapter-6, For full-colour organic light emitting diodes (OLEDs) displays, it is requisite to synthesize new deep blue emitters with high luminous performance. The hybridized local and charge transfer (HLCT) is a remarkable excited state that intermix the local and charge transfer state to assure eminent fluorescence quantum yield. The ambition of this study is to implement a subtle interpretation of photophysical and electroluminescence properties. The density functional theories (DFT) were studied to apprehend more about their geometry and orientations. Herein, two sets of deep blue emitting fluorophores with twisted geometry (Donor-π-Acceptor). In the first set of fluorophores, three blue emissive materials are connected with the triphenylamine (TPA) which act as donor, 9,9 diethylfluorene act as spacer and phenanthroimidazole act as acceptor. The electroluminescence (EL) spectra of the device are very similar to spectra of photoluminescence (PL) in the solution phase. Among all, the best performing 15 wt% MCFBI-fl-TPA based OLED device illustrates a maximum luminance of 3290 cd/m2. The device illustrate 8.2 lmW-1 of power efficiency (PE), 7.9 cdA-1 of current efficiency (CE) and 3.5% of high external quantum efficiency (EQE) with chromaticity coordinates (x, y) of the device are found to be (0.20, 0.23) emits deep blue colour. In the second set of fluorophores, three blue emissive materials are connected with the carbazole (CBZ) which act as donor, 9,9 diethylfluorene act as spacer and phenanthroimidazole act as acceptor. Some degree of overlap of electron cloud between HOMO and LUMO of the fluorophores which results radiative transition rate for this fluorophores to obtain high photoluminescence quantum yield (PLQY) and also favour the locally excited (LE) character. The optical properties show that the peak shows vibrational emission which is located due to the locally excited (LE) character. The PL emission peaking around 418 nm in the DCM solution and 430 nm in solid phase due to the π-π interaction of the fluorophores. The higher quantum yield reveals the coexistence of both LE and CT character in the fluorophores. In chapter 7, deals with the summary and conclusion of the present thesis work. The present thesis works deals with molecular engineering and synthesis of new class of unipolar and bipolar D-π-A) blue organic fluorophores combine with sensing mechanism, with the aim of exploring their potential application in blue OLED. The observations and the conclusions derived from the present investigations are summarized in this chapter.