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44891-B3
Preparation, Characterization, Film Fabrication, and Photoluminescence of Metal-Organic Networks of Copper(I) Cyanide

Robert D. Pike, College of William and Mary

The major finding from last year that copper(I) cyanide forms diamine and diimine networks has been further confirmed and expanded. The networks of CuCN with N-substituted piperazines (e.g., Me₂Pip = N,N′-dimethylpiperazine, MePip = N-methylpiperazine, EtPip = N-ethylpiperazine, Ph₂CHPip = N-diphenylmethylpiperazine) and hexamethylenetetramine (HMTA), among others, have been studied. These diamine and tetramine ligands (B) produce stable, highly luminescent (CuCN)_n(B) networks self-assembled in single-step, high-yield, aqueous reflux reactions of CuCN and B in the presence of KCN.

A common structural theme is the formation of (CuCN)₂B hexagonal honeycomb 6³ networks, which consist of tiled Cu₆(CN)₄B₂ units as a result of bridging B and mu₂-cyano groups. This motif is very common with piperazine networks of CuCN, such as (CuCN)₂(Me₂Pip), Figure 1A. The 6³ network pattern is also found threaded with additional CuCN chains in relatively copper-rich phases, such as (CuCN)₇(B)₂ = 2[(CuCN)₂B]•3CuCN (B = MePip, EtPip). In Figure 1B the 6³ network is viewed edge-on and is illustrated with its Cu atoms colored orange; the threaded CuCN chains are shown with Cu atoms colored green. (In all X-ray figures organic ligands are shown as wireframes and all cyano C/N atoms are colored grey to reflect site disorder.) CuCN-threaded networks are also noted for (CuCN)₄(B) = [(CuCN)₂(MePip)]•2CuCN (B = Me₂Pip, Et₂Pip). Non 6³ network motifs are seen for (CuCN)_n(HMTA) (n = 1.5, 2.5, 5) and (CuCN)₂(Ph₂CHPip), as illustrated in Figures 1C and 1D. The HMTA compounds show extensive mu₃-cyano bridging and the Ph₂CHPip compound reveals a CuCN chain uniquely decorated with -CN-B units.

The new copper(I) cyanide networks show interesting spectroscopic behavior. CuCN itself is colorless, absorbing in the UV (see Figure 2A), but luminesces intensely at the border of the UV and visible. The new diamine and tetramine networks are more-or-less white but in many cases are highly luminescent. This phenomenon is currently being investigated collaboratively by the P.I. with Prof. Howard Patterson (University of Maine, variable temperature spectroscopy) and Prof. Craig Bayse (Old Dominion University, computational analysis). The major findings are:

1)      Density Field Theory (DFT) calculations suggest that excitation in CuCN involves p MOs combining metal (3d to 4p) and cyano (pi to pi*) contributions. Excitation is thus an admixture of metal-centered (MC) and metal-to-ligand charge transfer (MLCT) transitions. Hence, the oft-made distinction between MC and MLCT is probably not meaningful for CuCN networks.

2)      CuCN-amine emission is red shifted and broadened with respect to that of CuCN (Figure 2). Additionally, fine structure is sometimes apparent in CuCN-amine excitation and emission (see Figure 2C). In threaded networks (e.g., see Figure 1B) the emission fine structure appears to result from a combination of CuCN (wavelength = 412 nm) and CuCN-amine (wavelength = 450 nm) components.

3)      An additional low-energy, thermochromic emission band in the yellow-orange region is sometimes present (see Figures 1A, C, D and 2D). The origin of this band is not yet understood.

4)      Time-Dependent DFT results suggest that the triplet state is lowered in energy via bending of the CuCN at copper. Since this effect opens a potential coordination site, it may ease reaction with incoming nucleophiles, facilitating exciplex formation. Conversely, bent CuCN-amine complexes should be expected show lower energy emission than does CuCN, as is experimentally observed.

Research has been initiated on the luminescence of monoamine (L) adducts of CuCN. It has been found that addition of small amounts of liquid or vapor-phase amine to solid CuCN causes luminescence emission to shift from the near UV value of 392 nm characteristic of CuCN to wavelengths well into the visible. Figure 3 reveals the varied effects of a variety of liquid amines on the luminescence emission of CuCN. As shown in Figure 3, slight chemical changes can produce the significantly different emission colors, e.g., green for CuCN/piperidine (Figure 3G), yellow for CuCN/N-methylpiperidine (3H) and pink for CuCN/N-ethylpiperidine (3I). The results of thermogravimetry and X-ray powder diffraction have indicated the following facts:

1)      Exposure of CuCN to amine liquid or vapor produces a surface coating of a new luminescent chemical phase that corresponds to only a few mass percent, with the bulk remaining CuCN.

2)      In each case, the new luminescent phase matches the authentic CuCN-L structure (when the latter is known).

3)      Eight CuCN-L phases are known, two of which have been solved in our lab, see Figure 4. Each consists of infinite CuCN chains decorated with 1-2 amines/copper, resulting in stoichiometries of (CuCN)(L)_q (q = 1.0, 1.33, 1.5, 2.0).

            These findings are of interest with regard to potential chemical sensors for volatile organics in the environment. Based on the above results, a putative “sniffing” device based on CuCN would be able to distinguish closely related N-alkylpiperidines. It would also be able to distinguish 2-, 3-, and 4-methylpyridine.

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