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

Olga Yu. Vassilyeva,Vladimir N. Kokozay,Rostyslav P. Linnik,Elena A. Buvaylo,Brian W. Skelton,Yevheniia V. Lobko

doi:10.1039/d0ra10787e

Abstract

The aim of this work is to apply organic–inorganic hybrid salts made of imidazo[1,5-a]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-a]pyridinium and [L′]+ is 2-methylimidazo[1,5-a]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 a-axis. The pseudo-layered structure of 2 is built of the organic and inorganic layers alternating along the a axis. The adjacent CdCl42− anions in the inorganic layer show no connectivity. The organic–inorganic hybrids 1 and 2 were immobilized in situ 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.

Highlights

The importance of luminescent materials in modern life can hardly be overestimated
Traditional small organic molecules, semiconductor quantum dots, metal nanoclusters or luminescent transition metal complexes are most widely used as carriers of luminescence
The substituted imidazo[1,5-a]pyridinium cation L+ results from the oxidative cyclocondensation between one molecule of methylamine and two molecules of 2-PCA in methanol in the metal-free conditions

Summary

Introduction

The importance of luminescent materials in modern life can hardly be overestimated. Their growing applications in organic light-emitting diodes (LEDs),[1,2,3] solar cells,[4,5] imaging,[6] and sensors[7,8] drive the development of new luminescent compounds. Of the transition metal complexes, those based on heavy second- or third-row transition metals, There are problems in the usage of synthetic luminescent compounds, such as luminescence quenching effect,[11,12] and the impossibility to form the components or coatings with proper forms and mechanical resistance. These drawbacks are eliminated by immobilization of such compounds in a polymer matrix. Polymer-based luminescent lms have received tremendous attention in a wide range of applications such as chemo-/bio-sensing,[13] solar concentrators,[14] and down-converters for blue LEDs.[15]

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