Studies on Triptycene-Fused and Polycyclic Aromatic Hydrocarbon-Appended Multi-Resonance Emitters for Highly Efficient Blue OLEDs

Abstract

This thesis represents research on multi-resonance (MR) organoboron emitters fused with triptycene (Tp) and appended with polycyclic aromatic hydrocarbon (PAH) groups. It explores their photophysical, electrochemical and electroluminescence characteristics. Chapter one provides a brief introduction to organic light-emitting diodes (OLEDs), MR emitters, and triptycene in general. Chapter two describes the synthesis of a novel emitter in which Tp groups have been fused into the core of mono-boron MR emitter. The emitter, namely Tp-DABNA, exhibited intense deep blue emission, a small singlet-triplet energy gap (ΔEST) and a narrower full width at half maxima (FWHM) compared to the famous bulky emitter t-DABNA. Benefitting from a rigid and large Tp scaffold, one can expect that Tp-DABNA could suppress unwanted Dexter energy transfer (DET) while maintaining efficient Forster resonance energy transfer (FRET). Highly efficient deep blue OLEDs were realized in TADF-OLED and sensitized hyperfluorescence (HF)-OLED, displaying higher performances and reduced efficiency roll-offs than that of t-DABNA. Chapter three discusses the impact of isomeric structures on Tp-fused blue MR emitters. Following the result of Tp-DABNA, we explored Tp groups in different positions, specifically, at the 2,3- and 3,4- position relative to the boron center. It was demonstrated that the distinct position of Tp-fused MR emitters resulted in dissimilar photophysical properties. One isomer of 4b exhibited higher color purity while the others experienced faster reverse intersystem crossing (RISC) (7b). An initial study of blue TADF-OLED devices was conducted, revealing that the 3,4-position isomer (7b) had reduced efficiency roll-offs compared to its counterpart (4b). Chapter four presents the synthesis of asymmetric Tp-fused MR emitters (1-4) with the aim of manipulating their photophysical behaviors. While the Tp moieties were fixed on half of the mono- boron MR emitters, various N-containing functional groups were facilely introduced to control the energy gap (Eg). It was shown that a shift in emission wavelength from deep- to sky-blue could be achieved depending on the substituents and extension of π-conjugated system. Chapter five discusses mono-boron MR emitters in which a PAH group such as pyrenyl and anthryl, was appended in the para position to the boron center of the t-DABNA compound. The emitters, namely t-DABNA-PAH, displayed sky-blue fluorescence with a high photoluminescence quantum yield (PLQY) and a narrow FWHM in solution. Notably, t-DABNA-PAH possessed low-lying triplet (T1) energy, which is beneficial in suppressing the accumulation of long-lived T1 exciton, thus eliminating their TADF characters. Theoretical studies suggest that the low T1 energy is localized at the PAH moiety.