From a simple push–pull dye to a broad-band absorbing and photoconducting material by assembly with CuBr†

Abstract

A thermally and electrochemically stable 2D coordination polymer (CP) of formula [Cu₂Br₂L₂]_n (UDS-6) has been designed using the push–pull chromophore ligand 2-(9H-fluorenylidene)malononitrile (L) and the modest and yet non-innocent CuBr salt. Its X-ray structure reveals a series of piled 2D-layers separated by ∼3.55 Å, inside which π-stacked L₂ pairs are placed in a head-to-tail conformation (interplanar L⋯L distance is ∼3.41 Å) and assembled by (CN)₂Cu(μ-Br)2Cu(NC)₂ rhomboids as secondary building units (SBUs) where each nitrile fragment links different L's. UDS-6 exhibits several interesting photonic properties such as a large absorption spectrum extending to ∼1700 nm, a near-infrared (NIR) and anti-Kasha emission (λ_em ∼ 1000 nm), exciton migration across the solid and photoconductivity, all of which drastically differ from those of L in the solid state (absorption extends to ∼600 nm, λ_em ∼ 700 nm, and L is not a photoconductor). Density functional theory (DFT) computations indicate that the lowest energy excited states are metal-halide-to-ligand charge transfer (MXLCT) states where the electron rich Cu₂Br₂ units and the electron withdrawing L act as the electron density donor and acceptor, respectively. UDS-6 is a photoconductor on its own and a mechanism study reveals the presence of photo-induced electron transfer (ET) in a 1 : 1 blend composed of tetraphenylporphyrinzinc(II), ZnTPP, a well-known electron donor, and L within the structure of UDS-6 with a rate, k_ET, of 5.4 × 10⁷ s⁻¹, which assigns L as the electron acceptor and consequently the charge carrier in UDS-6.