Delayed Luminescent Materials: Development and Exploration of Applications

Abstract

Delayed luminescence (DL) refers to light emission that persists after the excitation source is removed. Differing from conventional fluorescence, DL has a considerably longer lifetime attributable to the slow exciton decay. It can be broadly classified into phosphorescence (Ph), thermally activated delayed fluorescence (TADF) and triplet-triplet annihilation (TTA), providing alternative channels for harvesting triplet excitons. DL materials, such as metal complexes, purely organic materials, and lanthanide-based systems, have revolutionized the display technology, marking one of the most successful applications of DL. Besides, they hold tremendous application potential in background-free bioimaging and data encryption technologies. The thesis work predominantly focuses on lanthanide-based and purely organic room temperature phosphorescent (ORTP) materials. The first part of the thesis explores the applications of lanthanide-based soft materials, culminating in a paper-based point-of-care (POC) diagnostic protocol for antibiotic-resistant bacterial infections. Integrated with an affordable imaging device, it significantly reduces the time-to-diagnosis to 2 h as compared to 1-3 days taken by the “Gold standard” bacterial culture method. This diagnostic platform, validated with simulated UTI samples, shows potential for adoption in pathological laboratories, supporting antimicrobial stewardship in resource-limited areas. This strategy was subsequently extended for developing a sensor for hydrogen peroxide with a detection limit of 700 nM. A real-life application of this sensor was demonstrated by analyzing H2O2 in commercial hand sanitizers, which were extensively used during the COVID-19 pandemic, revealing an inconsistency in peroxide concentration in two out of five commercial brands. The second part of the thesis chronicles how a strenuous analysis of a trace impurity in a mixture unveiled a dopant-based host/guest material with remarkable RTP features. The mechanism of RTP was elucidated through comprehensive experimental and theoretical investigations. Moreover, tunable RTP color was generated by synergistically controlling the ground and excited state geometries of a single guest molecule using different host matrices. The thesis ends with demonstrating an intriguing functional behavior of the isolated impurity. It exhibited stimuli responsive reversible RTP-to-RTP switching which could be used for designing smart optical material with potential applications in stress sensing and anticounterfeiting technologies. Given the challenges associated with ORTP, this study marks a significant achievement in this field.