Nuclearity Control for Efficient Thermally Activated Delayed Fluorescence in a Cu^I Complex and its Halogen-Bridged Dimer

We report on the thermally activated delayed fluorescence (TADF) properties of a novel iodine-bridged CuI dimeric complex and its structurally related monomer. The chemical environment around the copper centers is identical in both complexes, providing a clean comparison to understand the effect of nuclearity in CuI emitters. Efficient room-temperature TADF (≈80% of the total emission) is observed in both compounds. Similar singlet–triplet splittings were found for the monomer and the dimer (554 and 583 cm–1, respectively), while the dimer triplet lifetime (90.0 μs) was longer than that of the monomer (46.0 μs). Experimental findings were rationalized by time-dependent density functional theory and complete active space self-consistent field calculations, identifying key structural factors determining TADF properties such as the key role of iodine in spin–orbit coupling mixing and the importance of near degeneracies in donor and acceptor orbitals for promoting state mixing. Unavoidable modifications associated with a change in nuclearity (e.g. intermolecular interactions, molecular charge, or modification of some binding motifs) can be also designed to promote TADF performance.