Multiswitchable Acidichromic and Photochromic Bisdiarylethene. An Experimental and Theoretical Study

A recently synthesized and structurally characterized bisdiarylethene containing a benzobis(imidazole) core substituted with two aniline moieties has been investigated from spectroscopic, thermodynamic, and photochemical points of view, using both experimental and theoretical tools. Due to the presence of four basic centers and of two photochromic diarylethenes in the molecular structure, the solutions of this compound change color upon acid addition and/or UV irradiation. Neutralization and/or visible irradiation lead to bleaching. In a water/dioxane (1/1, v/v) mixture, pK_a values and absorption spectra of the open and closed neutral and protonated species, as well as quantum yields of the photochromic cyclization and cycloreversion processes, have been determined at selected pH/H₀ values by spectrophotometric and photostationary spectrokinetic methods. Time-dependent density functional theory simulations have been carried out and provide insight into the protonation sites and photochromic properties. The results show that the four acidity constants overlap within a restricted pH/H₀ interval (4.5/–1.5) generating spectra of the colorless and colored forms that progressively shift toward the red. The possibility of continuously tuning the color of the solutions by external stimuli, such as irradiation and acidification, generates a multiswitchable acidichromic and photochromic material.