Spectroscopic Investigation of Diarylethene Photochromes Linked to Silica Nanoparticles

摘要

The spectroscopic properties of silica nanoparticles decorated with fluorescent photochromic molecules at their surface were studied by combining UV-visible spectroscopy with fluorescence correlation spectroscopy on diluted samples of particles dispersed in dimethyl sulfoxide (DMSO). We have first studied the properties of the fluorescent photochromic diarylethene molecules in DMSO using two-photon excitation at 720 nm so as to induce the cyclization reaction of the photochromic molecules from the open to the closed form. This diarylethene compound is fluorescent in its open form and nonfluorescent in its closed form, so the photochromic reaction offers the opportunity of modulating the fluorescence intensity between a bright ON state and a dark OFF state. The analysis of the lifetime measurements indicates that at least three conformers contribute to the emission of fluorescence. These results are in good agreement with our density functional theory calculations, which help us to assign the conformers to the photoreactive antiparallel conformer, the photoinactive parallel conformation, and a minor proportion of another photoinactive conformer, respectively. The evolution of the correlation profiles as excitation power increases suggests that another reaction occurs between the conformers beyond the photochromic reaction. The long spatial diffusion time of the decorated silica particle allows us to probe the dynamics of the attached fluorescent photochromic molecule on a longer time window than that allowed by the short diffusion time of the free molecule. The results can be satisfyingly interpreted by assuming that the photoinactive parallel conformer can turn into the photoactive antiparallel one which can in turn undergo the photochromic reaction. We have derived from the experimental results an order of magnitude of 30 GM for the two-photon absorption cross section of the photoactive antiparallel conformer.