Luminescent copper(i) halide and pseudohalide phenanthroline complexes revisited: simple structures, complicated excited state behavior

Abstract

We have synthesized a series of luminescent trigonal [CuX(dtbphen)] (X = I (1), Br (2), Cl (3), CN (4), dtbphen = 2,9-di-tert-butylphenanthroline) and tetrahedral [Cu₂(μ-I)₂(L)₂] (L = phenanthroline (5), 2,9-dimethylphenanthroline (6)) copper diimine complexes. Bearing in mind the chemical simplicity of this class of long-known Cu(I) phenanthroline compounds, it is surprising that they exhibit non-trivial photophysical properties, which have not been fully recognized. They display broad XMLCT absorption between ca. 450–600 nm, and the broad emission between ca. 550–850 nm in the solid state occurring with lifetimes on the μs timescale indicates phosphorescence, although the energetic overlap between excitation and emission suggests thermally activated delayed fluorescence (TADF) from S₁. In line with the latter assumption, low temperature measurements of 1–6 in the solid state show an energetic separation of emission and excitation. However, a counter-intuitive decrease of emission intensity and simultaneous increase of the emission lifetime at low temperatures are observed for 1, which indicates two triplet states also being involved. Our DFT and TD-DFT calculations show that emission from the lowest excited triplet state T₁ is of ³LMXCT nature, separated by only ca. 0.16 eV from S₁. Low temperature photophysical measurements at 77 K in a glassy matrix of 2 in 2-Me-THF and of 6 in the solid state are in agreement with the theoretical results, revealing in addition that π-interactions in the solid state also greatly influence the photophysical properties, making a clear conclusion towards TADF ambiguous. This study suggests that other related simple and long-known Cu(I) systems may exhibit a similar, more complex excited state behavior than previously appreciated, involving several emitting states and important intermolecular interactions.