We report the synthesis of a Zn(II)-ions-switchable [2]rotaxane which can modulate an intramolecular reversible electronic energy transfer (REET) between two mechanically linked pyrene- and ruthenium complex-based chromophores. This system was designed to control this photophysical process through the mechanical bond by increasing/decreasing chromophores distance following addition/removal of metallic ions. NMR spectroscopy analyses showed it can exist as a dynamically locked Zn(II)-containing system, where both chromophores are far apart in space, and a metal-free co-conformers mixture in a biased dynamic equilibrium, where both chromophores are closer in space, by subsequent addition of Zn(II) ions and KCN. Additional spectroscopic analyses highlighted how ruthenium complex triplet excited state lifetime is highly dependent of chromophores distance with an approximate 1.8-fold increase of ruthenium complex 615 nm emission upon Zn(II) ions excess addition. More specifically, it revealed how the excitation energy is more distributed to the ruthenium complex upon Zn(II) ions addition and less toward the pyrene unit, which can stock part of this energy as a quasi-isoenergetic “reservoir” in metal-free rotaxane. This research work represents an advancement towards the development of new tools for lifetime-based chemosensing and photosensitizer.
Is ice always covered by a thin layer of water? This question has been discussed for over 150 years. Here we show that the apparent contact angle of a droplet of water on ice increases steeply with decreasing ice temperature, from around 12 degrees near the melting point, to close to 160 degrees at −100∘C. This indicates that ice is never completely wetted. We quantitatively model the temperature dependence of the apparent contact angle by assuming the droplet's contact line gets pinned due to the crystallization of a thin layer of ice on the cold surface. However close to the melting temperature, where the formation of the ice layer is slowest, surface energy considerations need to be included to explain the nonzero contact angle observed in experiments.
In many physical systems, degrees of freedom are coupled via hydrodynamic forces, even in the absence of Hamiltonian interactions. A particularly important and widespread example concerns the transport of microscopic particles in fluids near deformable boundaries. In such a situation, the influence of elastohydrodynamic couplings on Brownian motion remains to be understood. Unfortunately, the temporal and spatial scales associated with the thermal fluctuations of usual surfaces are often so small that their deformations are difficult to monitor experimentally, together with the much slower and larger particle motion at stake. Here, we propose a minimal model describing the hydrodynamic coupling of a colloidal particle to a fluctuating elastic mode, in presence of an external periodic potential. We demonstrate that the late-time diffusion coefficient of the particle increases with the compliance of the elastic mode. Our results reveal and quantify two features: first, spontaneous microscopic transport in complex environments can be affected by soft boundaries - a situation with numerous practical implications in nanoscale and biological physics; and second, the effects of fast and tiny surface deformations are imprinted in long-term and large-distance colloidal mobility, and are therefore measurable in practice.
Strong electric field annihilation by particle–antiparticle pair creation, also known as the Schwinger effect, is a non-perturbative prediction of quantum electrodynamics. Its experimental demonstration remains elusive, as threshold electric fields are extremely strong and beyond current reach. Here, we propose a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac fermions with an approximate electron–hole symmetry. Using transport measurements, we report on universal one-dimensional Schwinger conductance at the pinchoff of ballistic graphene transistors. Strong pinchoff electric fields are concentrated within approximately 1 μm of the transistor’s drain and induce Schwinger electron–hole pair creation at saturation. This effect precedes a collective instability towards an ohmic Zener regime, which is rejected at twice the pinchoff voltage in long devices. These observations advance our understanding of current saturation limits in ballistic graphene and provide a direction for further quantum electrodynamic experiments in the laboratory.
