Hydrogen-bonding environment suppresses thermally activated delayed fluorescence.

Thermally activated delayed fluorescence (TADF) is a promising innovation in display technology where the nonemissive triplet excitons can be thermally converted back into emissive singlet excitons through reverse intersystem crossing. Organic TADF emitters often feature donor-acceptor (D-A) architectures, whose conformations critically influence emission dynamics and efficiency. Introducing intramolecular hydrogen-bonding between D and A moieties is an emerging strategy to rigidify the structure and improve TADF emission. However the influence of environmental factors on such hydrogen-bonding interactions remains unclear. Here we investigate the impact of the hydrogen-bonding medium on TADF emission using steady-state and time-resolved emission spectroscopy. Protic solvents universally quench TADF emission, correlating with reduced prompt emission lifetimes, while delayed lifetimes remain largely unchanged. A clear kinetic isotope effect unequivocally confirms that solvent protons directly participate in hydrogen-bonding interactions with the photoexcited emitter, thereby perturbing its excited-state energetics. Ultrafast spectroscopy reveals a picosecond D-to-A intramolecular charge transfer event that slows in viscous media indicating a D-A torsional relaxation. The relaxation time further slows in a protic environment highlighting the role of solvent-emitter hydrogen-bonding interactions resulting in unfavourable excited state D-A conformations and diminished emission. These findings underscore the importance of microenvironment control in designing efficient TADF emitters for display applications and photocatalysis.