Investigation Of Delayed Emission From Singlet And Triplet States In Pure Organic Molecules: Design, Synthesis, Photophysical and Electroluminescent Applications

Abstract

The current thesis addresses the molecular design, synthesis, and photophysical analyses of a novel class of pure organic thermally activated delayed fluorescence emitters for use in solution-processed organic light-emitting diodes (OLEDs). In addition, the research focuses on the design, synthesis, and photophysical analysis of a novel class of pure organic room-temperature phosphorescent materials. 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 in TADF OLEDs and pure organic RTP materials, a literature review of recent trends, and a brief summary of the objectives of this thesis are presented. In this chapter, the basic purpose and significance of the planned work of the thesis are summarized. In chapter 2, two TADF emitters are synthesized utilizing new design strategy of twisted interlocked acceptor core integrated with carbazole (KCCz) and tert. butyl-carbazole (KCTBC) as donors. Twisting of acceptor core by two methyl groups resulted in complete separation of HOMO and LUMO along with cyanide group facilitate in generating low-lying triplet exited states as suggested by theoretical simulation. These emitters showed deep blue emission with good quantum yield in film as well as good thermal stability. According to the electrochemical investigation, both of the emitters exhibit distinct oxidation and reduction behaviors which are in good agreement with theoretical values (by DFT). To assess these emitters in electroluminescence application, non-doped/doped OLED devices were fabricated. A doped device based on KCCz showed EQEmax of 3.9% and CIE coordinates of (0.16, 0.13). However, doped device based on KCTBC showed EQEmax of 9.0% along with low Efficiency roll-off with long operational device half lifetime of 72 minutes at initial brightness of 1,000 cd m-2, and CIE coordinates of (0.17, 0.13). In addition, with 12.5 wt% of 4CzFCN as assistant dopant/co-host to enhanced the performance of the KCTBC based device with an EQEmax of 13.9% and CIE coordinates of (0.18, 0.13). Further, a high-efficiency warm white OLED adopting the TADF hybrid approach is realized with EQEmax of 9.0 %. In chapter 3, to tune the color in towards red end of spectrum, one benzene ring was inserted between acceptor (KC) and donors, carbazole/diphenyl amine. This design further tuned the spectra from deep-blue to red shifted, blue region because of enhancement of -conjugation. The cyclic voltammetry demonstrates the redox behavior of the emitters. The 7.5 wt% KCPhCz doped device (in CBP) exhibited a PEmax of 22.7 lm/W, CEmax of 32.5 cd/A, EQEmax of 10.8%, and Lmax of 4,850 cd/m2 with CIE color coordinates of (018, 0.19). The 7.5 wt% KCPhDPA doped device (in CBP) exhibited a PEmax of 39.2 lm/W, CEmax of 49.8 cd/A, EQEmax of 15.3%, and Lmax of 15,530 cd/m2. In chapter 4, two new TADF emitters are designed utilizing the strategy of very strong donors, Phenoxazine and Phenothiazine integrated with the interlocked acceptor core KC. The two new TADF emitters based on the donors, Phenoxazine (KCPOZ) and Phenothiazine (KCPTZ) emitted in yellow region. Unsymmetrical and twisted molecular structure aided twisted intramolecular charge transfer in
film. Narrow ΔEST in both the emitters enabled efficient triplet exciton population and RISC to manufacture high-efficiency devices. The 7.5 wt% KCPTZ doped device exhibited a PEmax of 51.54 lm/W, CEmax of 65.6 cd/A, EQEmax of 21.9%, and Lmax of 11,640 cd/m2. A doped (5% in CBP) OLED device based on KCPOZ showed the best performance among both. The 5.0 wt% KCPOZ doped device exhibited a PEmax of 85.6 lm/W, CEmax of 95.2 cd/A, EQEmax of 31.5%, and Lmax of 18,240 cd/m2. Both emitters were also employed as sensitizers for TBRb, an orange TADF emitter, to improve orange device performance. EQEmax increased from 5% to 20% and 18.0% when KCPOZ and KCPTZ concentrations climbed from 0% to 10%. At 100 cd/m2, the KCPOZ device had an estimated half lifetime of 19,844 hours while the KCPTZ device had a lifetime of 10,550 hours. This work demonstrates using unconventional ways to design molecular core structures integrated with appropriate donors to enable high efficiency in the OLED device with a longer lifetime. In chapter 5, two new TADF emitters with the strategy of methoxy substituted double-twisted pyridine-cyano core as weak acceptor unit integrated strong donor by integrating double-twisted pyridine-cyano core as acceptor unit with phenoxazine (PyCN-POZ) and dimethyl acridine (PyCN-DMARC) were designed. PL spectra in solution as well as in neat film suggested emission of PyCN-DMARC in yellow region while that of PyCN-POZ in red region. Cyclic voltammetry analysis has been carried out to know the low and high lying HOMO-LUMO energy levels for the optimization of devices. Both the emitters showed very narrow EST by DFT calculations which suggests these two emitters can be very potential for 3rd generation OLEDs. In chapter 6, two positional isomers (benzaldehyde-alkyl spacer-carbazole) namely, pC4CZ and mC4CZ was designed and synthesized. It is found that these positional isomers with alkyl spacer shows different phosphorescence properties at room temperature in terms of lifetime and Quantum efficiency. Both shows RTP behavior in crystal form with lifetime of 384 and 79 ms and phosphorescence quantum yield of 3.8% and 0.3% respectively. Combined with crystals investigations, rate constant calculations and theoretical studies, it was found that greater phosphorescence lifetime of pC4CZ was due to their strong intermolecular electronic interactions which leads to its reduced krP and knrP by means of stabilized emissive states in dimers. Whereas smaller phosphorescence lifetime of mC4CZ crystals was owing to its weak intermolecular electronic interactions which caused higher krP and knrP. Further, it has been shown that pC4CZ can be very good candidate for anticounterfeiting applications. In chapter 7, the replacement of the carbonyl group with cyanide was executed to get RTP. These positional isomers with alkyl spacer show different phosphorescence properties at room temperature. Rigorous molecular packing investigation of their single crystal tells their diverse RTP behavior by means of the different types of interactions they make in the crystalline state. Also, theoretical investigations of their monomeric as well as different dimeric forms confirms stabilized emissive states in OCNCZ leads to its longer lifetime by efficient kisc. Further, it has been shown that, owing to its longer afterglow, OCNCZ can be potential candidate for Data security/Anticounterfeiting applications. Chapter 8 contains the work's summary and conclusion. The present dissertation focuses on the design, synthesis, and photophysical studies of a novel/new class of D-A pure organic TADF and RTP emitters, with the goal of studying TADF emitters in solution-processed OLED. This chapter provides a summary of the present investigation's observations and conclusions.