Charge-Transfer States in Thermally Activated Delayed Fluorescence Molecules: Dynamics and Mechanisms

Advanced optical spectroscopy is used to gain a deep understanding of the excited state dynamics of Thermally Activated Delayed Fluorescence (TADF) emitters and the key parameters that influence their performance for potential application in organic light-emitting diodes (OLEDs). Throughout this thesis, the structural and dynamical origins of Charge Transfer (CT) states are explored, focusing on how molecular architecture and environmental conditions dictate these systems’ behaviour. Specifically, this work explores the optimization of emission mechanisms by tailoring the interactions within different types of CT molecules, exploiting their high structural tunability to efficiently harvest non-emissive triplet excitons.

Initially, bridged D-D’-A systems are investigated, revealing the competitive nature of through-bond (TBCT) and through-space (TSCT) charge transfer mechanisms. The findings indicate that while strong donor-acceptor pairs and optimal co-facial overlap stabilize TSCT, the introduction of phenyl spacers or rigid environmental matrices can disrupt this alignment, shifting the dominant pathway, and vice versa. Further investigation into TSCT systems with the triptycene-bridged molecule TpAT-tFFO demonstrates that high efficiency in this molecule can be driven by ground-state conformational interchanging between nearly degenerate isomers. This dynamic equilibrium, combined with low vibrational motions in the excited state, allows the molecule to adapt its geometry to the host environment, maximising its emissive and triplet harvesting properties.

To address the kinetic limitations of multiresonant (MR)-TADF emitters, specifically their slow reverse intersystem crossing rates (krISC), a peripheral heavy-atom effect (HAE) strategy is presented. The halogenation of the MR-core is found to significantly accelerate the rate of reverse intersystem crossing. Furthermore, investigations into guest-host systems reveal a high propensity for intermolecular interactions, but rather than being detrimental, these interactions are shown to further increase the efficiency of the molecule, amplifying the rate of reverse intersystem crossing by an order of magnitude in films.

Finally, this study examines the effects of rigidifying traditional D-A systems into co-planar structures, a concept previously thought to be unachievable for efficient CT generation. Remarkably, these rigidified systems provide stable, dispersion-free CT emission and robust triplet-harvesting properties. Importantly, the molecules studied show a change in their conjugation due to their specific bonding sites, leading to distinct bonding and non-bonding interactions that break excited-state conjugation, and in this manner modulating the donor strength.