Advanced spectroscopy studies of highly-efficient emitter molecules

Abstract

This thesis aims to use advanced optical spectroscopy to gain deep understanding of the properties of highly-efficient emitter molecules and the key parameters that strongly influence their performance for potential application in organic light-emitting diodes (OLEDs). Through this thesis, significant factors that impact the thermally activated delayed fluorescence (TADF) molecules and their mechanisms are addressed. These molecules exhibit many factors which can affect their performance, including electronic coupling, environmental conditions, conformational arrangements, and design aspects. Initially, these techniques are employed to uncover the highly complex photophyscis properties of a specific molecule, 10-phenyl-10H,10′H-spiro[acridine9,9′-anthracen]-10′-one, known as ACRSA. As previously reported in a study involving a detailed computation chemistry, this molecule has complex behaviour due to its highly decoupled nature of the donor and acceptor units. The finding from this thesis revealed that ACRSA behaves as three different subsystems. While each these subsystems follows Kasha’s rules individually, the molecule as a whole completely disregards them. Exploring the environment conditions, time-resolved optical spectroscopy studies indicate that the rigid bridge present in ACRSA prevents variations in the rates of the reverse intersystem crossing (rISC). This effect, typically observed in TADF in solid host, have significant implications for the performance of OLEDs. Apart from the photoluminescence techniques, this thesis also explores the use of pump-probe techniques as a powerful tool to analyse the role of the dark excited states in the TADF molecules. This technique combined with the optical spectroscopy techniques enables the mapping of the electronic excited states, their transition, and their nature. Finally, time-resolved optical spectroscopy was employed to study the alternative design of TADF molecule, known as through-space charge transfer (TSCT). The results obtained provided the first experimental evidence of TSCT. This study also significantly contributes to design rules for TADF molecules, demonstrating the role of the bridge on these molecules and importance of the molecular reorganisation in the TSCT formation.