Synthesis, Structures, and Photophysical Properties of a Series of Rare Near-IR Emitting Copper(I) Complexes

Herein, we report on the synthesis and structural characterization of a series of trigonal and tetrahedral cationic copper(I) complexes, bearing phosphine or N-heterocyclic carbene ligands as donors, with benzthiazol-2-pyridine (pybt) and benzthiazol-2-quinoline (qybt) acting as π-chromophores. The compounds are highly colored due to their ¹MLCT (MLCT = metal-to-ligand charge transfer) states absorbing between ca. λ_abs = 400-500 nm, with ¹ILCT (ILCT = intraligand charge transfer) states in the UV region. The relative shifts of the S₀→S₁ absorption correlate with the computed highest occupied molecular orbital-lowest unoccupied molecular orbital gaps, the qybt complexes generally being lower in energy than the pybt ones due to the larger conjugation of the quinoline-based ligand. The compounds exhibit, for Cu^I complexes, rare intense long-lived near-IR emission with λ_max ranging from 593 to 757 nm, quantum yields of up to Φ = 0.11, and lifetimes τ of several microseconds in the solid state as well as in poly(methyl methacrylate) films. Although a bathochromic shift of the emission is observed with λ_max ranging from 639 to 812 nm and the lifetimes are greatly increased at 77 K, no clear indication for thermally activated delayed fluorescence was found, leaving us to assign the emission to originate from a ³(Cu→pybt/qybt)MLCT state. The red to near-IR emission is a result of incorporation of the sulfur into the chromophore ligand, as related nitrogen analogues emit in the green to orange region of the electromagnetic spectrum. The photophysical results and conclusions have further been corroborated with density functional theory (DFT)/time-dependent DFT calculations, confirming the nature of the excited states and also the trends of the redox potentials.