Purely Organic Triplet Emitters: From Fundamental Molecular Design to Performance Amplification in Modern Applications

Abstract

Emissive materials are the essential functional components in many modern and emerging technologies such as organic light-emitting diodes (OLEDs), solid-state lighting, sensors, and data encryption. Metal-free purely organic phosphors (POPs), as a promising novel candidate, have brought milestone evolution to emissive materials in the past decade. Compared to prevailing organo-metallic counterparts, these emitters have various advantages such as large design windows, easy processability, and more environmental friendliness. However, realizing the full capacities of POPs remains challenging due to the limited fundamental understanding of their emission mechanism and the dearth of systematic molecular design blueprint, significantly restricting their overall performance in state-of-the-art emitting systems. This dissertation research focuses on developing advanced design rules of POPs combining a renovated interpretation of the photophysical laws regularizing their performance and a computation-driven design pipeline. This dissertation begins with reviewing the past developments on POPs and the present challenges towards the design of “fast and efficient” POPs. The molecular design factors affecting the internal and external efficiencies of POPs were summarized by connecting the theoretical descriptors influencing SOC with the molecular orbitals and functional moieties. Our recent research developments on POPs were discussed from a practical engineering perspective in various modern applications including sensor, imaging, data encryption, display devices, as well as from a fundamental scientific perspective by addressing how molecular level manipulation affects the application merits of POPs. To perform structural expansions on POPs and explore their potential in sensing and patterning systems, purely organic triplet emission is combined with the excited-state intramolecular proton transfer (ESIPT) phenomenon to create dual-emissive “ESIPT triplet emitters” exhibiting up to 50% delayed emission quantum yield (Φ_d). These unique emitters presented tunable triplet emission which could be switched on and off by controlling the matrix acidity. Switchable triplet emission systems controlled by acid vapor annealing as well as photopatterning systems capable of generating facile and high-contrast emissive patterns have been devised and demonstrated. To explore the potential of POPs in encryption systems, the oxygen quenching characteristic of POPs was adapted to sensitize singlet oxygen species and thus created photo-responsive encoding systems consist of bright green POPs with 19% Φ_d and oxygen-permeable polymeric matrix: excitation of POPs leads to area-selective consumption of oxygen in the matrix, leading to localized phosphorescence enhancement and subsequent reversible recording of emissive patterns. Last, to realize POPs’ advantages in devices, POP-based OLEDs were constructed with a tailor-designed novel fluorene-based POP with efficient spin-orbit coupling and 24.0% Φ_d. Effects of OLED host materials on the phosphor were investigated in terms of color purity, suppression of exciplex emission, and restraint of molecular motion. Bright green phosphorescence emission (1430 cd/m2 at 100 mA/cm2) was realized with 2.5% maximum external quantum efficiency. Finally, the fundamental molecular design issues of “fast and efficient” POPs were discussed. A novel design concept of “Heavy atom oriented orbital angular momentum manipulation (HAAM) ” was presented to address the significance of synergetic interplay between heavy atoms and ΔL-satisfying moieties in the promotion of SOC, rather than acting autonomously. Its implementation demonstrated rational creation of milestone molecules with intrinsic SOC efficiencies over 200 cm-1 and experimental τ_ph approaching 200 µs, while maintaining near-unity room-temperature Φ_ph. This implies that if a proper SOC manipulation is implemented through the HAAM concept, POPs could potentially have similar ISC and phosphorescence efficiencies to their organometallic counterparts.