Cu(II)-Based Pyridyl Bridging Coordination Polymer and Its Fe Composite: Structure, Sensing, Schottky Device, and Hydrogen Evolution Reaction

[Cu(3-bph)(PABA)₂(H₂O)] (<b>CP1</b>) (3-bph = <i>N</i>,<i>N</i>'-bis(3-pyridylmethylene)hydrazine and PABA^- = <i>p</i>-amino benzoate) is a pyridyl-N bridging Cu^II coordination polymer, and PABA^- acts as a carboxylate-O donor forming a square pyramidal CuN₂O₃ motif following a zigzag one-dimensional (1D) lattice. The <b>CP1</b> shows weak antiferromagnetic coupling (<i>J</i> = -0.196(1) cm^-1), and emission appears at 352 nm (λ_ex = 293 nm), which is selectively quenched by Fe³⁺ via the FRET mechanism. The limit of detection (LOD) is 0.081 μM, and the Stern-Volmer constant (<i>K</i>_sv) is 1.52 × 10⁵ M^-1. The <b>CP1</b> shows electrocatalytic HER activity with an overpotential of 628 mV at a current density of 10 mA cm^-2 (η₁₀) in 0.5 M H₂SO₄ with a Tafel slope of 129.8 mV dec^-1. Because of the selectivity of <b>CP1</b> to Fe³⁺, a composite, <b>Fe(III)@CP1</b>, is used to examine the current density, but it is not impressive. <b>Fe(II)@CP1</b> shows better HER activity. The composite <b>Fe(II)-n@CP1</b> (mole ratio of Fe(II)/CP1; 1:1, 1:2, 1:3) shows better HER activity, and <b>Fe(II)-2@CP1</b> shows the lowest overpotential (η₁₀ = 430 mV) at a current density of 10 mA cm^-2 with a Tafel slope of 92.4 mV dec^-1. The optical band gap of <b>CP1</b> (3.72 eV; theoretical, 3.47 eV) suggests a semiconducting behavior and forms an ITO/<b>CP1</b>/Al electrode for the Schottky diode with an electrical conductivity of 7.47 × 10^-5 S cm^-1.