Reversible luminescent reaction of amines with copper(I) cyanidew

One of the significant on-going challenges in chemistry is the specific and convenient detection of compounds of interest in the environment. Detection of volatile organic compounds (VOCs) via chemical ‘‘sniffing’’ requires an interaction between the VOC and a detector substrate, resulting in a measurable change. Crystal inclusion of VOCs into various d and d metal complexes has been found to affect intermetallic or other weak interactions, yielding visible color changes. In contrast, direct and reversible VOC–metal bonding is very unusual. Bonding of various nucleophiles to metal centers can serve to modulate substrate photophysics, leading to a wider variety of responses. Herein we report that reversible surface bonding of amines to CuCN gives rise to a remarkable range of visible photoluminescent responses. We have recently reported the structures and luminescence behavior of copper(I) cyanide networks with bridging diamine ligands. Luminescence emission of CuCN–amine adducts occurs in the visible region, representing a bathochromic shift from the 392 nm emission of CuCN. A DFT examination of [Cun(CN)n+1] polymeric chains (as models for the infinite chains found in CuCN) showed that the favored excitations are of p-symmetry, corresponding roughly to dCu/pCN pCu/p*CN. 5 Evidence was found for a low-energy triplet excited state with bent geometry at copper. Since bending at copper is an outcome of ligand coordination, this finding is consistent with the lower energy emission associated with CuCN–amine complexes compared to that of CuCN itself. The observation that CuCN luminescence wavelength is modulated by various incoming ligands suggests that this system might provide the foundation for an amine vaporsensing device. We have now found that direct addition of amine (L) liquid or diffusion of amine vapor to solid CuCN at room temperature produces a variety of visible colored luminescence responses (see Fig. 1, Pipd = piperidine, Cy = cyclohexyl, Morph = morpholine, Pipz = piperazine, Py = pyridine). While CuCN produces barely visible luminescence emission centered at 392 nm, CuCN samples exposed to many liquid or vapor amines luminesce at wavelengths shifted well into the visible region. To probe the nature of the interaction between CuCN and the amines, CuCN suspensions in various neat amines were heated at 70 1C.z In most cases the CuCN did not dissolve in the amine, transforming directly to the solid (CuCN)Ln complex. Thermogravimetry (TGA) of the (CuCN)Ln products revealed clean, quantitative loss of amine between 25 and 250 1C, depending on amine volatility. The following complexes were prepared: L = Py (n= 2, 1a; 0.8, 1b), 2-MePy (n = 1.5, 2a; 1, 2b), 3-MePy (n = 1.5, 3a; 1, 3b), 4-MePy (n = 1.5, 4), 2-EtPy (n = 1, 5), 3-EtPy (n = 1.5, 6a; 1, 6b), 4-EtPy (n = 1, 7), 4-BuPy (n = 1.5, 8), Pipd (n = 1.25, 9), N-MePipd (n = 1, 10), N-EtPipd (n = 0.75, 11), MeMorph (n = 1, 12), and Me2NCy (n = 1, 13). For those amines that yielded more than one product (1a/b, 2a/b, 3a/b, 6a/b) the more amine-rich species was produced directly, and overnight vacuum treatment converted it cleanly to the less amine-rich species. These conversions are also evident in the TGA traces Fig. 1 Luminescence of CuCN + liquid L under 254 nm light at room temperature. A: Pipd, B: N-MePipd, C: N-Et-Pipd, D: NMePyrrolidine, E: Me2NCy, F: NEt3, G: N-MeMorph, H: N-MePipz, I: N,N-Me2Pipz, J: Py, K: 2-MePy, L: 3-MePy, M: 4-MePy, N: 2-EtPy, O: 3-EtPy, P: 4-EtPy, Q: 4-BuPy.