A Single Crystal Hybrid Ligand Framework of Copper(II) with Stable Intrinsic Blue-Light Luminescence in Aqueous Solution

Abstract

Single-crystal solid-liquid dual-phase hybrid organic-inorganic ligand frameworks with reversible sensing response facilitated by external stimuli have received significant attention in recent years. This report presents a significant leap in designing electronic structures that display reversible dual-phase photoluminescence properties from single-crystal hybrid ligand frameworks. Three-dimensional Cu(C₃N₂H₄)₄Cl₂ complex frameworks were formed through the intermolecular hydrogen bonding and π⋯π stacking supramolecular interactions. The absorption band peaks at 627 nm were assigned to d-d transition showing 10Dq = 15,949 cm^-1 and crystal field stabilization energy (CFSE) = 0.6 × 10Dq = 114.4 kJmol^-1, while the ligand-to-metal charge transfer (LMCT) of complexes was displayed at 292 nm. The intense luminescence band results from LMCT present at 397 nm. Considering its structure, air stability, framework forming and stable luminescence in aqueous solution, the Cu(C₃N₂H₄)₄Cl₂ complex shows potential for luminescence Cu-based sensors using emission intensity to detect heavy metal ion species.

Keywords: 3D network complexes; copper coordination complexes; crystal structure; electronic structure; luminescence.

Figure 1

Coordination environment about the Cu(II) metal center of square pyramidal [Cu(II)(C₃N₂H₄)₄Cl]Cl coordination complex showing hydrogen bonding as dashed line and its optical image.

Figure 2

A continuous 3D of copper hybrid organic–inorganic ligand frameworks showing the intermolecular interactions of hydrogen bonding and π⋯π stacking.

Figure 3

Simulated and experimental XRD patterns of copper(II) coordination complexes (left) and hydrogen bond interaction between coordination complexes and water molecules (right).

Scheme 1

The molecular orbital diagram showing ground state and excited state (*) (a), the Cu²⁺ electronic configuration 3d⁹ (t_2g⁶ e_g³) (b) and UV−Vis spectra (c) of copper hybrid organic−inorganic ligand frameworks.

Figure 4

XPS spectra of Cu(C₃N₂H₄)₄Cl₂ showing wide spectrum and binding energy states in (a) wide scan, (b) Cu 2p, (c) C1s, (d) N1s and (e) Cl2p orbitals.

Figure 5

(a) FTIR spectra showing functional group characteristic peaks at different wavenumbers, and (b) TGA thermogram of Cu(C₃N₂H₄)₄Cl₂ coordinate complexes.

Figure 6

Emission spectra of copper coordinate complexes at each excitation wavelength.