Controlling Triplet-Harvesting Pathways and Nonlinear Optical Properties in Cu(I) Iodide-Based Polymers through Ligand Engineering

Organic–inorganic hybrid metal halides have become enormously important in optoelectronics, sensing, photosensitization, etc. In this study, we report a structural transition from a staircase configuration to a cubane configuration in Cu(I) iodide-based polymers influenced by the coordination behavior of two different π*-acceptor ligands. The staircase polymer structure, coordinated with 3-cyanopyridine, demonstrates efficient thermally activated delayed fluorescence from (metal+halide)-to-ligand charge transfer [^1/3(M+X)LCT] states, with a singlet–triplet energy splitting of ∼9 meV. Conversely, upon replacement of the cyano with an amino group at the same position, a one-dimensional polymeric structure of Cu₄I₄ cubane-type clusters is formed, which shows strong cluster-centered (³CC) orange emission at room temperature. Temperature-dependent photoluminescence studies indicate that the ³CC state behaves as a self-trapped excitonic state with significant exciton–phonon coupling having a Huang–Rhys factor of 58.6. Additionally, we report this cubane-type cluster polymer acts as an efficient nonlinear optical material showing third harmonic generation with a χ⁽³⁾ value of 1.32 × 10^–18 m² V^–2 and a laser-induced damage threshold of 25.87 GW/cm².