Synergistic regulation of nonbinary molecular switches by protonation and light

We report a molecular switching ensemble whose states may be regulated in synergistic fashion by both protonation and photoirradiation. This allows hierarchical control in both a kinetic and thermodynamic sense. These pseudorotaxane-based molecular devices exploit the so-called Texas-sized molecular box (cyclo[2]-(2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene); 14+, studied as its tetrakis-PF6− salt) as the wheel component. Anions of azobenzene-4,4′-dicarboxylic acid (2H+•2) or 4,4′-stilbenedicarboxylic acid (2H+•3) serve as the threading rod elements. The various forms of 2 and 3 (neutral, monoprotonated, and diprotonated) interact differently with 14+, as do the photoinduced cis or trans forms of these classic photoactive guests. The net result is a multimodal molecular switch that can be regulated in synergistic fashion through protonation/deprotonation and photoirradiation. The degree of guest protonation is the dominating control factor, with light acting as a secondary regulatory stimulus. The present dual input strategy provides a complement to more traditional orthogonal stimulus-based approaches to molecular switching and allows for the creation of nonbinary stimulus-responsive functional materials.

Dynamical limits by downsizing for the photo-induced switching in a molecular material revealed by time-resolved X-ray diffraction

Cooperative molecular switching at the solid state is exemplified by spin crossover phenomenon in crystals of transition metal complexes. Time-resolved studies with temporal resolutions that separate molecular level dynamics from macroscopic changes, afford clear distinction between the time scales of the different degrees of freedom involved. In this work we use 100 ps X-ray diffraction to follow simultaneously the molecular spin state and the structure of the lattice during the photoinduced low spin to high spin transition in microcrystals of [FeIII(3-MeO-SalEen)2]PF6. We show the existence of a delay between the crystalline volume increase driven by the propagation of collective volumic strain waves, and the cooperative macroscopic switching of molecular state. Such behaviour is different from the expectation that phase transformation only requires atomic displacements in the unit cell, that can occur simultaneously with propagation of a volumic strain. Model simulations and discussions of the physical picture explain the phenomenon with thermally activated kinetics governed by local energy barriers separating the molecular states.

Twistable dipolar aryl rings as electric field actuated conformational molecular switches

The electric field induced conformational response of a range of twistable dipolar aryl ring systems is studied using density functional theory based calculations. We assess which factors are most important for efficient molecular switching.

β-Amyloid peptides tailor switching behaviors of Donor-Acceptor Stenhouse Adducts

Molecular switching plays a critical role in biological and displaying systems. Here we demonstrate the first use of peptides to operate molecular switches of donor-acceptor Stenhouse adducts (DASAs), a series of negative photochromes that are highly promising for applications ranging from smart material to biological systems. Fluorescence imaging proved Aβ40 species could make SHA-2 more stable in the linear configuration than without peptide and decrease the rate of molecular switching. According to molecular dynamics simulation, SHA-2 bound to protein resulted in substantial changes in the tertiary structure of Aβ40 monomer with the region of Glu22-Ala30 partially unfolded and being more exposed to water. This structural change is likely to impede the aggregation of Aβ40, as evidenced by fluorescence and ProteoStat® aggresome detection experiments. SHA-2 is able to inhibit the aggregation of Aβ40 by producing the off-pathway structures. These results open ample opportunities for optically addressable potential widely apply DASAs in the biological system based on this peptides-tailor process.