Light and pH tunable luminescence in a photochromic bisdiarylethene

In this work the luminescence of a bisdiarylethene, containing a benzobis(imidazole) core substituted with two aniline moieties, has been investigated. In previous research, it was found that both acidification and irradiation reversibly triggered colour changes of this compound, thus generating a multi-responsive acidichromic and photochromic system. Intense fluorescence emission, which was detected in several organic solvents, can be an additional light driven signal. In a dioxane/water (1 : 1, v/v) mixture, intensity and spectral position of luminescence have been found to drastically depend on the pH/H0 values of the solutions (pH 5/H0 −2 range) due to subsequent protonations (four steps) as the acidity of the solution changes. Alternated irradiations with UV and visible light lead to a decrease and increase, respectively, of the fluorescence intensity, due to the photochromic reaction producing a non-fluorescent compound. Quantum yields and lifetimes of fluorescence were determined as a function of the acidity. The results indicate that protonation shifts the emission to the red and decreases its intensity. The possibility of tuning the colour and intensity of luminescence by both acidification and irradiation generates a multi-switchable “fluorochromic” material.