Synthesis, Spectroscopic Characterizations and DFT Calculations of Two-Dimensional Monoclinic and Orthorhombic Phases of Metal-Organic Frameworks Based on BTC-2,2′-Bipyridine and Cu(II) Ions

Abstract

This work reports the synthesis, structural, spectroscopic, and computational characterization of two metal-organic framework (MOF) materials obtained by solvothermal methods from 1,3,5-benzenetricarboxylic acid (BTC), 2,2′-bipyridine, and Cu(II) ions. X-ray diffraction (XRD) analyses revealed the formation of two crystalline phases: a monoclinic phase (MOF-2,2′-bipy-Mono) and an orthorhombic phase (MOF-2,2′-bipy-Orto). Infrared and Raman spectra confirmed the coordination of the BTC carboxylate groups, showing characteristic asymmetric and symmetric stretching modes νas (C=O) at ~ 1600 cm⁻¹ and ν s (C=O) at ~ 1353 cm⁻¹, respectively. Additionally, a band at ~ 650 cm⁻¹ corresponding to the ν(N-Cu) stretching vibration of coordinated bipyridine nitrogen atoms was observed, along with a signal at ~ 600 cm⁻¹ attributed to the ν(HO-Cu) vibration of the hydroxyl group bridging the copper centers. X-band EPR spectra exhibited axial signals typical of Cu(II) centers in distorted square-pyramidal geometries, with values of g∥ = 2.23 and g⊥ = 2.05 for MOF-2,2′-bipy-Mono, and g∥ = 2.16 and g⊥ = 2.03 for MOF-2,2′-bipy-Orto. UV-Vis spectra revealed characteristic metal-to-ligand charge transfer (MLCT) and π→π * transitions in the range of 300-500 nm. Density functional theory (DFT) calculations indicated that the valence band is dominated by Cu(II) and oxygen bands from BTC, while the conduction band primarily involves carbon and nitrogen band from 2,2′-bipyridine. Time-Dependent DFT (TD-DFT) studies confirmed that the main electronic transitions correspond to metal-ligand (Cu-BTC)-ligand (2,2′-bipy) charge transfer (ML-LCT) processes. Compared to the orthorhombic phase, the monoclinic phase exhibited higher sensitivity to external radiation stimuli. The results obtained show that the MOFs studied could potentially be used as optoelectronic devices.