Thermally Activated Delayed Fluorescence Emitters: Molecular Design, Photophysics, and Sensing Application

Thermally activated delayed fluorescence (TADF) is a strategy for harvesting both singlet and triplet excitons in some purely organic emitters and is attracting increasing interest beyond their use in organic light-emitting diodes (OLEDs), including chemical sensing. In this thesis, coumarin-based donor-acceptor systems are investigated as a versatile platform for the development of TADF-active emitters and their integration into functional sensing materials. Following an overview of the photophysical principles governing fluorescence, intersystem crossing, reverse intersystem crossing, and TADF, molecular design strategies aimed at controlling charge-transfer (CT) character and the singlet–triplet energy splitting (ΔEST) are discussed. A family of carbazole-coumarin derivatives is synthesized and systematically characterized using steady-state and time-resolved photoluminescence spectroscopic techniques, supported by electrochemical analysis. Several derivatives exhibit TADF in rigid environments, enabling clear structure-property relationships to be established with respect to donor strength, molecular conformation, and excited-state energetics. Selected emitters were subsequently integrated into rigid polymeric matrices and paper-based substrates to assess their performance as optical sensors for volatile organic compounds (VOCs). The impact of matrix rigidity, polarity, and microenvironmental heterogeneity on emission colour, spectral shape, and delayed fluorescence was analysed, revealing substrate-dependent sensing responses. Advanced and exploratory systems involving acceptor modification and donor π-extension were also investigated using a combination of experimental and computational approaches, with the aim of assessing how these structural variations influence excited-state character and provide a broader perspective on the molecular design space of coumarin-based donor–acceptor emitters. Overall, this work demonstrates how systematic exploration of coumarin-based donor-acceptor architectures can be connected to their photophysics and how this can be exploited towards the development of functional luminescent sensing materials.