Organic Room-Temperature Phosphorescence Materials: from Fundamental Triplet Exciton Modulations to Novel Applications in Biosensing and Bioimaging

Abstract

Metal-free purely organic phosphors are an emerging class of room-temperature phosphorescence (RTP) materials and have attracted great attention over the past decade, owning to their potentials in a variety of advanced photofunctional applications, including organic light emitting diodes (OLEDs), chemical sensing, bioimaging, phototheranostics, and digital security. Organic phosphorescence refers to photons emitted from triplet excited states (excitons), which is associated with spin-forbidden transitions. The weak spin-orbit coupling (SOC) due to the lack of heavy atoms in purely organic materials leads to long-lived triplet excitons that are highly susceptible to oxygen quenching and molecular vibration deactivation. Therefore, organic phosphorescence is typically only observed at cryogenic temperatures and under inert conditions. This presents both opportunities and challenges in triplet exciton utilization at room temperature.

The first part of this dissertation highlights the technologies we have developed leveraging the unique oxygen quenching aspect of organic RTP for in vivo tissue hypoxia imaging and in vitro phosphorimetric biodetection. Ischemia-induced hypoxia is a common complication associated with numerous retinopathies affecting more than 160 million people worldwide. Early detection and long-term visualization of retinal tissue hypoxia is essential to understand the pathophysiology and treatment of ischemic retinopathies, however, no effective solution exists to this problem. We have demonstrated a versatile lipid-polymer hybrid organic RTP nanoparticle (NP) platform that optically visualizes chorioretinal tissue hypoxia in real-time and with high signal-to-noise ratio. This represents the first non-destructive detection method. Additionally, we have extended the application of these organic RTP NPs to a highly sensitive phosphorimetric assay through the integration with a signal-amplifying enzymatic oxygen scavenging reaction and a sandwich-DNA hybridization assay on 96-well plates. The phosphorimetric sensor demonstrates sequence-specific detection of a cell-free cancer biomarker with a 0.5 pM detection limit. By re-programming the detection probe, our methodology can be adapted to a broad range of biosensor applications for biomarkers of great clinical importance yet difficult to detect due to their low abundance in vivo.

In the second and third parts of this dissertation, we focus on fundamental studies of photophysical characteristics of triplet excitons and provide deeper insight into material design strategies for modulating triplet energy and accelerating light extraction more efficiently. Despite the controversial opinions on the matter of spin angular momentum conservation during energy transfer, we have shown efficient intermolecular triplet-to-singlet energy transfer (T-S ET) in purely organic luminescent materials through a pseudo-Förster resonance energy transfer pathway. The T-S ET process quenches the organic RTP emission from donor triplet excitons and sensitizes acceptor singlet excitons, which ultimately induces ambient delayed fluorescence, with the ET efficiency as high as 75.5%. Finally, we aim to address the weak SOC and molecular vibration issues simultaneously in a single material system—metal-organic framework (MOF). Organic phosphors are incorporated into highly ordered and rigid MOFs via coordination bonding to zirconium (Zr)- and hafnium (Hf)-metal clusters. The obtained MOFs are isostructural to UiO-67, and exhibit strong long-lived RTP and stable high-temperature phosphorescence (HTP) that persists up to 400K. The heavier UiO-67 (Hf) framework is found to be more effective than its Zr counterpart at suppressing molecular motions and facilitating phosphorescence emission. This work has identified critical material design parameters and structural basis for organic phosphorescence of high brightness and desirable stability, hence holds the promise to solve the long-standing efficiency roll-off issue in phosphorescence OLEDs.