Thermally Activated Delayed Fluorescence (TADF) and Enhancing Photoluminescence Quantum Yields of [Cu^I(diimine)(diphosphine)]⁺ ComplexesPhotophysical, Structural, and Computational Studies

Abstract

The complexes [Cu­(I)­(POP)­(dmbpy)]­[BF₄] (<b>1</b>) and [Cu­(I)­(POP)­(tmbpy)]­[BF₄] (<b>2</b>) (dmbpy = 4,4′-dimethyl-2,2′-bipyridyl; tmbpy = 4,4′,6,6′-tetramethyl-2,2′-bipyridyl; POP = bis­[2-(diphenylphosphino)-phenyl]­ether) have been studied in a wide temperature range by steady-state and time-resolved emission spectroscopy in fluid solution, frozen solution, and as solid powders. Emission quantum yields of up to 74% were observed for <b>2</b> in a rigid matrix (powder), substantially higher than for <b>1</b> of around 9% under the same conditions. Importantly, it was found that the emission of <b>2</b> at ambient temperature represents a thermally activated delayed fluorescence (TADF) which renders the compound to be a good candidate for singlet harvesting in OLEDs. The role of steric constraints within the complexes, in particular their influences on the emission quantum yields, were investigated by hybrid-DFT calculations for the excited triplet state of <b>1</b> and <b>2</b> while manipulating the torsion angle between the bipyridyl and POP ligands. Both complexes showed similar flexibility within a ±10° range of the torsion angle; however, <b>2</b> appeared limited to this range, whereas <b>1</b> could be further twisted with little energy demand. It is concluded that a restricted flexibility leads to a reduction of nonradiative deactivation and thus an increase of emission quantum yield