A thorough photophysical study of a photochrome-fluorophore dyad (3), combining a fluorescent laser dye (DCM-type, , Φ(1) = 0.27) and a photochromic diarylethene (2), obtained by click chemistry, is presented. In addition to photochromism, the open form (OF) of 2 exhibits fluorescence (Φ(-OF) = 0.016), whereas the closed form (CF) does not. Fluorescence is switched upon alternate UV/visible irradiation of 2. The emission band of 2-OF matches the absorption band of 1 (400-550 nm), whereas the emission band of 1 overlaps the absorption band of 2-CF (550-700 nm). Therefore, a photoreversible two-way excitation energy transfer (EET), controlled by the state of the photochromic moiety, is obtained in the dyad 3. Their efficiencies are quantified as Φ(EET)(OF→F) = 85% and as Φ(EET)(F→CF) ~ 100% from the comparison of emission and excitation spectra between 1, 2, and 3. These results are fully compatible with the shortening of fluorescence lifetimes (from τ(-OF) = 70 ps and 170 ps essentially to τ(-OF) < 10 ps) and to the values of Förster radii determined for 3 (R(0)(OF → F) = 29 Å and R(0)(F → CF) = 71 Å), evidencing a Förster-type resonance energy transfer mechanism (FRET). An important outcome of this two-way FRET is the possibility to quench 49% of the fluorescence in 3 at PSS upon UV irradiation, corresponding to the conversion extent of the photochromic reaction, which is different from 2 (α(CF) = 91%). This is a clear example of a situation where the presence of FRET between the photochromic unit and the fluorophore affects noticeably the photochromic properties of the dyad molecule 3.

International audienceHere, we show that the residual presence of reaction products markedly influences the growth mode of 3D superlattices made of silver nanocrystals. The silver nanocrystals (5 or 6 nm in diameter) are synthesized via the reduction of a silver triphenylphosphine complex in the presence of thiols. The presence of triphenylphosphine, coming from the decomposition of the silver precursor, is shown to change the superlattice growth mode in hexane from heterogeneous to homogeneous. In toluene, it remains homogeneous. This is in agreement with simulations based on a Flory-type model, which shows that the interaction potential between the silver nanocrystals in the presence of triphenylphosphine changes from repulsive to attractive in hexane and becomes even more attractive in toluene. Ex situ investigation of the different superlattices following solvent evaporation by scanning electron microscopy reveals different morphologies depending on the growth mode. A transition from films to individual hexagonal-shaped superlattices is observed when the growth mode changes from heterogeneous to homogeneous. Direct in situ observations of the 3D superlattice growth by liquid cell transmission electron microscopy follow in real time the formation process and movement of the superlattices in both hexane and toluene solvents

Silica-coated gold nanorods functionalized with grafted fluorescent and photochromic derivatives were synthesized and characterized. Spectroscopic investigations demonstrated that cross-coupled interactions between plasmonic, photochromic, and fluorescence properties play a major role in such nanosystems, depending on the thickness of the silica spacer, leading to multi-signal photoswitchability.