Molecular Design Strategies for Enhancing Triplet Harvesting Efficiency in Organic Emitters and Aggregates via Thermally Activated Delayed Fluorescence (TADF) and Room Temperature Phosphorescence (RTP)

Abstract

This thesis focuses on the design and development of multifunctional organic luminogens with advanced photophysical properties, addressing key challenges in the field of optoelectronics and sensory applications. A significant emphasis is placed on overcoming aggregation-caused quenching (ACQ), a common limitation in organic solid-state emitters, and exploring mechanisms such as aggregation-induced emission (AIE), thermally activated delayed fluorescence (TADF), and room-temperature phosphorescence (RTP). Through systematic molecular modifications, the work aims to enhance luminescence efficiency, broaden functionality, and optimize performance in real-world applications. The research begins by establishing a foundational understanding of the principles governing light emission, focusing on triplet harvesting mechanisms essential for efficient solid-state emission. By systematically modifying molecular structures, the study achieves transformative outcomes, such as converting ACQ-dominated systems into AIE-active luminogens with additional functionalities like mechanochromism and RTP. For instance, directed modifications in acceptor cores and linker groups enable the emergence of multifunctional properties, exemplifying the potential of structural engineering to fine-tune photophysical behavior. A significant portion of the thesis explores regioisomeric donor-acceptor systems to understand their impact on TADF efficiency, spin-orbit coupling, and reverse intersystem crossing (RISC) rates. The findings reveal that subtle variations in molecular geometry can significantly alter energy gaps, packing arrangements, and emission characteristics, leading to materials with finely controlled luminescent properties. The ortho-, meta-, and para-isomers investigated showcase distinct photophysical profiles, with the para-isomer demonstrating the highest TADF efficiency and the ortho-isomer excelling in mechanochromic luminescence and second harmonic generation (SHG). The thesis also highlights innovations in donor unit design, such as the incorporation of phenoxazine and carbazole derivatives, to enhance TADF performance in molecular aggregates. These modifications lead to emitters with rapid RISC rates, diverse emission lifetimes, and external quantum efficiencies exceeding 20% in OLED applications. Moreover, the development of hyperfluorescence processes achieves pure yellow emission with high efficiency, demonstrating the materials' potential for practical device integration. In addition to optoelectronics, the thesis investigates two-photon absorption and anti-Stokes shifted photoluminescence in aggregated luminogens, showcasing their utility in advanced applications such as bioimaging and photocurrent generation. By combining experimental observations with theoretical insights, the work provides a comprehensive understanding of the factors governing solid-state emission, laying the groundwork for the design of next-generation multifunctional luminogens. Overall, this thesis makes significant contributions to the molecular design, photophysical understanding, and functional optimization of organic luminogens, advancing their application in optoelectronics, bioimaging, and sensory devices. The findings not only address longstanding challenges but also open new avenues for the development of highly efficient, versatile, and solution-processable materials for future technologies.