A durable and fast-responsive photochromic and switchable luminescent polyviologen–polyoxometalate hybrid

Abstract

A novel photochromic and luminescent switchable hybrid of [(AV²⁺)(p-AV)(EuW₁₀O₃₆)]_n·2nH₂O (1; AV²⁺ = N,N′-bisaminopropyl-4,4′-bipyridinium bromide hydrobromide; p-AV = poly(1-ethyl-1′-butyl-4,4′-bipyridinium)) has been synthesized by combining a fluorescent POM with monomeric and in situ generated polymerized viologen molecules. Photochromic transformation of 1 from colorless to dark blue under UV-irradiation was observed due to its in situ generated polymeric viologen units, in which the coloration and decoloration time were both lower than 10 minutes. Especially, the decoloration process can be easily realized by exposure of 1 to humid air (95% relative humidity) within 5 minutes, which allows 1 to be used as a potential humidity sensitive material. The reversible coloration and decoloration process can be repeated for more than 40 cycles, which makes 1 an intriguing stable inorganic-viologen-based photochromic hybrid. Intermolecular fluorescence resonance energy transfer (FRET) from inorganic polyoxoanions to the organic photochromic component was observed and was responsible for the modulation of the red emission of 1 to its quenching state when it was under UV-irradiation. The photochromism and photomodulated fluorescence of 1 opens a new avenue for developing durable and quick-responsive smart optical materials based on polyoxometalate and viologen.