Enhancing Room Temperature Phosphorescence from Organic Molecules by Internal Heavy Atom Effect and External Agents

Abstract

Room temperature phosphorescence has gained a great deal of attention due to its significant importance in practical application such as phosphorescence organic light emitting diode (PhOLED). Bright room temperature phosphorescence has also been utilized in the field of bio-imaging and oxygen indicators which were not practically achievable by using phosphorescent materials. Heavy-metal complexes such as iridium complex are commonly used for room temperature phosphorescence but their potential toxicity and instability particularly in case of blue phosphors still remain to be solved. Room temperature phosphorescence from metal-free organic phosphors has attracted much attention in recent years since metal-free phosphors were presented as bright as organometallic compounds, quantum dots, and fluorescent molecules at room temperature. In previous study, our group focused on the development of host-guest systems to suppress vibration of phosphors such as doped crystal, amorphous rigid polymer and immobilization between polymer and phosphors via hydrogen bonding or covalent bonding. However, the quantum yield is still much lower than that of conventional fluorescent dyes. For example, the best quantum yield of room temperature phosphorescence is around 30% in polymer matrix. The low quantum yield prevents it from utilizing real phosphorescent applications. In this dissertation, we focused on how to improve further phosphorescence quantum yield by improving phosphorescence radiative decay as well as suppressing non-radiative decay. First, general molecular design principles to achieve bright room temperature phosphorescence from metal-free organic phosphors are thoroughly discussed. By synthesizing a series of molecules and analyzing their photophysical properties, we discovered how molecular structures affect the non-radiative decay pathway by intramolecular vibration, and both phosphorescent radiative decay and non-radiative decay by intermolecular vibration. Molecular design strategies to tune the phosphorescent color without significantly decreasing the phosphorescence quantum yield were also discussed. We also developed a new scheme to improve the phosphorescent radiative decay and consequently enhance the emission intensity by incorporating an external component such as plasmonic nanometals and adapting metal organic frameworks (MOFs). Through a systematic investigation on localized surface plasmon resonance (LSPR) based phosphorescence enhancement, we built insightful understanding on the LSRP based phosphorescence enhancement mechanism and revealed optimized conditions to achieve brightest phosphorescence. Finally, MOF was investigated with organic phosphor molecules as an organic ligand. MOF-polystyrene (PS) composite having a organic phosphor ligand produced bright room temperature phosphorescence by suppressing molecular vibrational dissipation and enhancing the phosphorescent radiative decay. A summary of future perspective of metal-free organic phosphors to overcome their limitations and to realize futuristic potential applications is provided in the last chapter of the dissertation.