Photoswitching Using Visible Light: A New Class of Organic Photochromic Molecules

A versatile new class of organic photochromic molecules that offers an unprecedented combination of physical properties including tunable photoswitching using visible light, excellent fatigue resistance, and large polarity changes is described. These unique features offer significant opportunities in diverse fields ranging from biosensors to targeted delivery systems while also allowing non-experts ready synthetic access to these materials

o-Fluoroazobenzenes as readily synthesized photoswitches offering nearly quantitative two-way isomerization with visible light

Azobenzene functionalized with ortho-fluorine atoms has a lower energy of the n-orbital of the Z-isomer, resulting in a sepn. of the E and Z isomers' n→π* absorption bands. Introducing para-substituents allows for further tuning of the absorption spectra of o-fluoroazobenzenes. In particular, electron-withdrawing ester groups give rise to a 50 nm sepn. of the n→π* transitions. Green and blue light can therefore be used to induce E→Z and Z→E isomerizations, resp. The o-fluoroazobenzene scaffold is readily synthesized and can be inserted into larger structures via its aryl termini. These new azobenzene derivs. can be switched in both ways with high photoconversions, and their Z-isomers display a remarkably long thermal half-life

Area Increase and Budding in Giant Vesicles Triggered by Light: Behind the Scene

Biomembranes are constantly remodeled and in cells, these processes are controlled and modulated by an assortment of membrane proteins. Here, we show that such remodeling can also be induced by photo-responsive molecules. We demonstrate the morphological control of giant vesicles in the presence of a water-soluble orthotetrafluoroazobenzene photoswitch (F-azo) and show that the shape transformations are based on an increase in membrane area and generation of spontaneous curvature. The vesicles exhibit budding and the buds can be retracted by using light of a different wavelength. In the presence of F-azo, the membrane area can increase by more than 5% as assessed from vesicle electrodeformation. To elucidate the underlying molecular mechanism and the partitioning of F-azo in the membrane, we used molecular dynamics simulations. Comparison with theoretically calculated shapes reveals that the area difference between the two leaflets of the vesicle membrane is not constrained during the budding process and that the experimentally 2 observed shapes are well described by the spontaneous curvature model. Our results show that exo- and endocytotic events can be controlled by light and that these photo-induced processes provide an attractive method to change membrane area and morphology

ortho-​Fluoroazobenzenes: Visible Light Switches with Very Long-​Lived Z Isomers

Improving the photochemical properties of molecular photoswitches is crucial for the development of light-​responsive systems in materials and life sciences. ortho-​Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-​electron-​withdrawing F atoms ortho to the N = N unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E​/Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-​electron-​withdrawing groups (EWGs) work in concert with ortho-​F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-​fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations

First-principle-based MD description of azobenzene molecular rods

Extensive density functional theory (DFT) calculations have been performed to develop a force field for the classical molecular dynamics (MD) simulations of various azobenzene derivatives. Besides azobenzene, we focused on a thiolated azobenzene’s molecular rod (4′-{[(1,1′-biphenyl)-4-yl]diazenyl}-(1,1′-biphenyl)-4-thiol) that has been previously demonstrated to photoisomerize from trans to cis with high yields on surfaces. The developed force field is an extension of OPLS All Atoms, and key bonding parameters are parameterized to reproduce the potential energy profiles calculated by DFT. For each of the parameterized molecule, we propose three sets of parameters: one best suited for the trans configuration, one for the cis configuration, and finally, a set able to describe both at a satisfactory degree. The quality of the derived parameters is evaluated by comparing with structural and vibrational experimental data. The developed force field opens the way to the classical MD simulations of self-assembled monolayers (SAMs) of azobenzene’s molecular rods, as well as to the quantum mechanics/molecular mechanics study of photoisomerization in SAMs