A new organo-inorganic hybrid of poly(cyclotriphosphazene-4,4′-bipyridinium)chloride with a large electrochromic contrast

Abstract

A new organo-inorganic hybrid electrochromic material poly(cyclotriphosphazene-4,4′-bipyridinium)chloride salt was synthesized wherein each phosphorus atom in the triphosphazene core is linked by diquaternized 4,4′-bipyridyls (PPBP). The electrochrome was characterized by ³¹P nuclear magnetic resonance, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS confirmed the formation of P–N⁺ and P–N linkages in the PPBP and the conversion of covalently bound Cl to ionic Cl⁻ species. The transparent PPBP material undergoes three reversible one electron-transfer reactions to yield a radical cation insoluble film which first acquires a purple hue and then turns deep blue. Electrochromic devices were constructed with the PPBP hybrid dissolved in a highly conductive, thermally stable and electrochemically inert ionic liquid based gel electrolyte and a Prussian blue (PB) layer as the anode. The PPBP–PB device showed an extremely large coloration efficiency of 504 cm² C⁻¹, an exceptionally high transmission modulation of 70.5% at 590 nm, one among the highest reported contrasts in organic electrochromics, a large reflectance contrast of 59.2% at 545 nm and fast switching kinetics. The device was durable as it retained 96.2% of its original transmission after 1000 color-bleach cycles, thus exemplifying its use for both reflective and transmissive electrochromic applications. Electrochemical impedance studies revealed charge transfer to be a less resistive process during reduction relative to oxidation which cumulatively ensues in shorter coloration times. Our studies demonstrate the yet untapped potential of the inorganic (PNCl₂)₃ in steering the synthesis of new organo-inorganic hybrid materials, capable of undergoing facile redox phenomena, manifesting in unequalled functional properties and thereby offering opportunities to use this trimer for preparing a whole gamut of new high performance electroactive compounds.