Small change, big red shift: syntheses, structure and photoluminescence of Cu2Br2(Ph3P)2py2 (py = pyridine, 4-vinylpyridine)

The search for efficient, luminescent metal complexes for optoelectronic devices raised the interest for emissive Cu(I) compounds. Cu(I)-complexes of the type [Cu2X2(PR3)2(py)2] (py = pyridine derivative, X = Cl, Br, I) usually are efficient, blue-emitting compounds that can potentially be used as new emitter materials in organic light-emitting diodes. Aside from photophysical requirements, the introduction of functional groups is often necessary to enable processing and stabilization, e.g. by cross-linking or polymerization. Using 4-vinylpyridine instead of pyridine to enable crosslinking during device fabrication, a structurally similar complex with different photophysical properties is obtained: a bathochromic shift of over 100 nm occurs, while the photoluminescence quantum efficiency is drastically decreased. DFT-calculations, X-ray-diffraction and photoluminescence spectroscopy were used to compare Cu2Br2(Ph3P)2(pyridine)2 and Cu2Br2(Ph3P)2(4-vinylpyridine)2. The structural differences regarding the core unit, Cu2Br2P2N2, are only small, but the π-system of the pyridine ligand is enlarged by the vinyl group leading to a significant red-shift of the emission wavelength.