Vapoluminescent thin-film with unsaturated copper(i) complex for rapid light-on sensing of N-heteroaromatic vapour

Abstract

A light-on sensor that could detect N-heteroaromatic vapour by luminescence was fabricated using a coordinatively unsaturated Cu(I) complex [CuI(xantphos)] (xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene, Cu-x). The vapoluminescent thin film, Cu-x@PVP was prepared using Cu-x and polyvinylpyrrolidone (PVP) via spin-coating. When the hardly emissive film was exposed to pyridine (py) vapour, it emitted bright blue-green luminescence immediately (≦15 s), while on exposure to 2-methylpyrazine (Mepyz), yellow emission occurred. By taking up vapour molecules, four-coordinate Cu(I) complexes with intense luminescence originating from thermally activated delayed fluorescence (TADF) are rapidly produced, as evidenced from both spectroscopy and computational studies. The unsaturated structure of Cu-x enabled a rapid response to vapour based on the ligand addition reaction compared to previous vapochromic Cu(I) complex systems based on the ligand-exchange reaction. Cu-x@PVP was easily recovered by the release of vapour molecules in the air, indicating a reversible vapochromic behaviour. X-ray diffraction measurements indicated a reversible transformation from the triangular Cu-x to the tetrahedral species [CuI(L)(xantphos)] (Cu-x–L; L = py, Mepyz) by the coordination/dissociation of the N-heteroaromatic vapour molecules. The efficient vapour response for the thin film was confirmed compared with that of the bulk powder: The apparent rate constant to the vapochromic response, k_obs, was estimated to be 3 × 10⁻² s⁻¹ for Cu-x@PVP based on the emission intensity evolution by the Mepyz vapour, which was approximately six times larger than that of the Cu-x powder sample (k_obs = 5 × 10⁻³ s⁻¹). The results demonstrate that the utilisation of the coordinatively unsaturated Cu(I) complex provides a promising strategy for the easy sensing of toxic vapours, such as N-heteroaromatics.