Organic–inorganic hybrid tetrachlorocadmates as promising fluorescent agents for cross-linked polyurethanes: synthesis, crystal structures and extended performance analysis

Abstract

The aim of this work is to apply organic-inorganic hybrid salts made of imidazo[1,5-<i>a</i>]pyridinium-based cations and halometallate anions as fluorescent agents to modify cross-linked polyurethane (CPU) for the creation of flexible photoluminescent films. The use of ionic compounds ensures excellent dispersion of the luminescent components in the polymer matrix and prevents solid-state quenching. The absence of phase segregation makes it possible to fabricate uniformly luminescent films with a large area. To this, new tetrachlorocadmate salts [L]₂[CdCl₄] (1) and [L']₂[CdCl₄] (2), where L⁺ is 2-methyl-3-(pyridin-2-yl)imidazo[1,5-<i>a</i>]pyridinium and [L']⁺ is 2-methylimidazo[1,5-<i>a</i>]pyridinium cations, have been prepared and characterized by IR, NMR, UV-Vis spectroscopy and single crystal X-ray diffraction. The organic cations resulted from the oxidative cyclization-condensation involving CH₃NH₂·HCl and 2-pyridinecarbaldehyde in methanol (1), and formaldehyde, CH₃NH₂·HCl and 2-pyridinecarbaldehyde in an aqueous media (2). In the crystal of 1, loosely packed tetrahedral cations and π-π stacked anions are arranged in separate columns parallel to the <i>a</i>-axis. The pseudo-layered structure of 2 is built of the organic and inorganic layers alternating along the <i>a</i> axis. The adjacent CdCl₄ ^2- anions in the inorganic layer show no connectivity. The organic-inorganic hybrids 1 and 2 were immobilized <i>in situ</i> in the cross-linked polyurethane in low content (1 wt%). The photoluminescent properties of 1 and 2 in the solid state and in the polymer films were investigated. The semi-transparent CPU films, that remain stable for months, retain the photoluminescent ability of both hybrids in the blue region with a prominent red shift in their emission.