Synthesis and Luminescence of Optical Memory Active Tetramethylammonium Cyanocuprate(I) 3D Networks

The structures of three tetramethylammonium cyanocuprate(I) 3D networks [NMe₄]₂[Cu(CN)₂]₂•0.25H₂O (1), [NMe₄][Cu₃(CN)₄] (2), and [NMe₄][Cu₂(CN)₃] (3), (Me₄N = tetramethylammonium), and the photophysics of 1 and 2 are reported. These complexes are prepared by combining aqueous solutions of the simple salts tetramethylammonium chloride and potassium dicyanocuprate. Single-crystal X-ray diffraction analysis of complex 1 reveals {Cu₂(CN)₂(μ₂-CN)₄} rhomboids crosslinked by cyano ligands and D_3h {Cu(CN)₃} metal clusters into a 3D coordination polymer, while 2 features independent 2D layers of fused hexagonal {Cu₈(CN)₈} rings where two Cu(I) centers reside in a linear C_∞v coordination sphere. Metallophilic interactions are observed in 1 as close Cu⋯Cu distances, but are noticeably absent in 2. Complex 3 is a simple honeycomb sheet composed of trigonal planar Cu(I) centers with no Cu^…Cu interactions. Temperature and time-dependent luminescence of 1 and 2 have been performed between 298 K and 78 K and demonstrate that 1 is a dual singlet/triplet emitter at low temperatures while 2 is a triplet-only emitter. DFT and TD-DFT calculations were used to help interpret the experimental findings. Optical memory experiments show that 1 and 2 are both optical memory active. These complexes undergo a reduction of emission intensity upon laser irradiation at 255 nm although this loss is much faster in 2. The loss of emission intensity is reversible in both cases by applying heat to the sample. We propose a light-induced electron transfer mechanism for the optical memory behavior observed.