Papers on which I am a lead author are distinguished in the list below by the inclusion of their full abstract.

Electron spin resonance resolves intermediate triplet states in delayed fluorescence

Published in Nature Communications, 2021

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.

Selenium Substitution Enhances Reverse Intersystem Crossing in a Delayed Fluorescence Emitter

Published in Journal of Physical Chemistry C, 2020

Organic emitters exhibiting delayed fluorescence (DF) are promising luminescent materials for next-generation organic light-emitting diodes (OLEDs). Faster intersystem crossing rates and shorter emission lifetimes can be achieved in luminescent molecules through the incorporation of heavy atoms, which enhance spin–orbit coupling and promote intersystem crossing between singlet and triplet states. DF molecules often contain a sulfur atom, and reports of selenium-containing DF OLEDs also exist. However, the literature lacks a direct exploration of the effect of spin–orbit coupling on reverse intersystem crossing in a delayed fluorescence emitter by the substitution of selenium for sulfur. Here we show that substitution of selenium for sulfur in a modified thioxanthenone-triphenylamine analogue increases the rate of forward intersystem crossing by a factor of over 250 and the rate of reverse intersystem by a factor of 22. We attribute the increased rates to enhanced spin–orbit coupling from heavy atom substitution, and computational and electron spin resonance studies support this. This work provides an insight into future molecular design strategies for heavy-atom-containing, DF emitters.

A Simple Molecular Design Strategy for Delayed Fluorescence toward 1000 nm

Published in Journal of the American Chemical Society, 2019

Harnessing the near-infrared (NIR) region of the electromagnetic spectrum is exceedingly important for photovoltaics, telecommunications, and the biomedical sciences. While thermally activated delayed fluorescent (TADF) materials have attracted much interest due to their intense luminescence and narrow exchange energies, they are still greatly inferior to conventional fluorescent dyes in the NIR, which precludes their application. This is because securing a sufficiently strong donor−acceptor interaction for NIR emission alongside the narrow exchange energy required for TADF is highly challenging. Here, we demonstrate that by abandoning the common polydonor model in favor of a donor−acceptor dyad structure, a sufficiently strong donor−acceptor interaction can be obtained to realize a TADF emitter capable of photoluminescence close to 1000 nm. Electroluminescence at a peak wavelength of 904 nm is also reported. This strategy is both conceptually and synthetically simple and offers a new approach to the development of future NIR TADF materials.