Construction of Pnictogen-Rich Cyclophosphazene-Based Manganese(II) Coordination Polymers: Structural Insights and Visible-Light Photocatalytic Activity

Abstract

Three novel manganese(II)-based coordination polymers, PCP-27, PCP-28, and PCP-29, were synthesized using a cyclotriphosphazene-derived tetracarboxylate ligand (H4L) and different N-donor ligands (2,2’-bpyridine (bpy), terpyridine (tpy), and phenanthroline (phen), respectively). SC-XRD revealed that the compounds exhibit distinct structural dimensionalities, including 1D chain structures (PCP-27 and PCP-28) and a 2D layered structure (PCP-29), which further assemble into 3D supramolecular network through π–π stacking and hydrogen bonding interactions. Spectroscopic and thermal analyses confirmed successful coordination and high structural stability, with PCP-29 maintaining integrity up to 370 °C. Optical measurements revealed that all PCPs possess semiconducting properties with suitable band gaps (1.98–2.45 eV) for visible-light photocatalysis. Photocatalytic degradation experiments using MB and RhB under visible light showed that PCP-29 exhibited the highest removal efficiency (>90%) and excellent reusability over five cycles. EIS and CV indicated superior charge transfer and redox properties for PCP-29, consistent with its photocatalytic performance. Radical scavenging studies revealed different dominant degradation pathways for each PCP: •OH for PCP-27, h+ for PCP-28, and O2•⁻ for PCP-29. The observed photocatalytic performance is strongly correlated with the structural features of the coordinatio polymers. In particular, the higher dimensional connectivity and enhanced π–π stacking interactions in PCP-29 facilitate more efficient charge transport and separation, leading to superior photocatalytic activity, whereas the lower-dimensional architectures of PCP-27 and PCP-28 impose limitations on charge mobility, resulting in comparatively reduced performance. These findings establish a clear structure-property-performance relationship and highlight the critical role of band alignment in conjunction with framework dimensionality and structural features in governing charge transfer and ultimately tuning the photocatalytic efficiency of Mn(II)-based coordination polymers for environmental remediation applications.