Spectroscopic Investigation of Diarylethene Photochromes Linked to Silica Nanoparticles

The spectroscopic properties of silica nanoparticles decorated with fluorescent photochromic molecules at their surface were studied by combining UV−visible spectroscopy with fluorescence correlation spectros-copy 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. ■ INTRODUCTION During the past decade, molecular and nanoscale switches have attracted a great interest owing to their broad application fields such as molecular electronics or photonic devices. 1−4 More precisely, molecular photoswitches opened the way to new developments such as opto(bio)electronic materials, super resolution imaging, data storage, and photocatalysis. 5−11 In fact, using light as an external trigger allows a more precise spatial and temporal control between the two states of the molecular switch. Over the properties that can be switched, fluorescence is very attractive because its detection can be very sensitive and also because it plays a major role in sensing and imaging applications. 12−14 Some photochromic molecules offer a good opportunity to switch fluorescence when they undergo a reversible modification of their chemical structure under light irradiation. One of the most promising photochromic families is the diarylethene one (DAE) because these molecules show very efficient bistability and high fatigue resistance. 15 For these derivatives, the photochromic reaction consists of a photo-cyclization from an open form (OF) to a closed form (CF) under UV light irradiation and a backward photocycloreversion under visible light irradiation, respectively. 16 A wide variety of DAE photochromic compounds with fluorescence properties have been reported in the literature. A first type of molecular design consists of a chemical association between a DAE unit and a fluorescent moiety: 17−19 the emission of the fluorophore can be switched off by intramolecular resonance energy transfer when the absorption spectrum of the DAE in its CF overlaps the emission spectrum of the fluorophore. In a second type of association, the photochromic DAE and the fluorescent units are gathered together without direct chemical linkage, and fluorescence switching results from intermolecular resonance energy transfer, as is the case for particles with fluorescent and photochromic species grafted at the surface, for example. 20,21 A last situation corresponds to DAE molecules showing intrinsic fluorescence properties, either in the OF, in the CF, or in both