Cooperativity in Photofoldamer Chloride Double Helices Turned On with Sequences and Solvents, Around with Guests, and Off with Light.

Photofoldamers are sequence-defined receptors capable of switching guest binding on and off. When two foldamer strands wrap around the guest into 2:1 double helical complexes, cooperativity emerges, and with it comes the possibility to switch cooperativity with light and other stimuli. We use lessons from nonswitchable sequence isomers of aryl-triazole foldamers to guide how to vary the sequence location of azobenzenes from the end (FEND) to the interior (FIN) and report their impact on the cooperative formation of 2:1 complexes with Cl-. This sequence change produces a 125-fold increase from anti-cooperative (α = 0.008) for FEND to non-cooperative with FIN (α = 1.0). Density functional theory (DFT) studies show greater H-bonding and a more relaxed double helix for FIN. The solvent and guest complement the synthetic designs. Use of acetonitrile to enhance solvophobicity further enhances cooperativity in FIN (α = 126) but lowers the difference in cooperativity between sequences. Surprisingly, the impact of the sequence on cooperativity is inverted when the guest size is increased from Cl- (3.4 Å) to BF4- (4.1 Å). While photoconversion of interior azobenzenes was poor, the cis-cis isomer forms 1:1 complexes around chloride consistent with switching cooperativity. The effect of the guest, solvent, and light on the double-helix cooperativity depends on the sequence.

High-fidelity Recognition of Organotrifluoroborate Anions (R-BF3 - ) as Designer Guest Molecules.

The recognition of boron compounds is well developed as boronic acids but untapped as organotrifluoroborate anions (R-BF3 - ). We are exploring the development of these and other designer anions as anion-recognition motifs by considering them as substituted versions of the parent inorganic ion. To this end, we demonstrate strong and reliable binding of organic trifluoroborates, R-BF3 - , by cyanostar macrocycles that are size-complementary to the inorganic BF4 - progenitors. We find that recognition is modulated by the substituent's sterics and that the affinities are retained using the common K+ salts of R-BF3 - anions.

Electrospinning Hetero-Nanofibers of Fe3 C-Mo2 C/Nitrogen-Doped-Carbon as Efficient Electrocatalysts for Hydrogen Evolution.

Heterostructured electrocatalysts with multiple active components are expected to synchronously address the two elementary steps in the hydrogen evolution reaction (HER), which require varied hydrogen-binding strength on the catalyst surface. Herein, electrospinning followed by a pyrolysis is introduced to design Fe3 C-Mo2 C/nitrogen-doped carbon (Fe3 C-Mo2 C/NC) hetero-nanofibers (HNFs) with tunable composition, leading to abundant Fe3 C-Mo2 C hetero-interfaces for synergy in electrocatalysis. Owing to the strong hydrogen binding on Mo2 C and the relatively weak one on Fe3 C, the hetero-interfaces of Fe3 C-Mo2 C remarkably promote HER kinetics and intrinsic activity. Additionally, the loose and porous N-doped carbon matrix, as a result of Fe-catalyzed carbonization, ensures the fast transport of electrolytes and electrons, thus minimizing diffusion limitation. As expected, the optimized Fe3 C-Mo2 C/NC HNFs afforded a low overpotential of 116 mV at a current density of -10 mA cm-2 and striking kinetics metrics (onset overpotential: 42 mV, Tafel slope: 43 mV dec-1 ) in 0.5 m H2 SO4 , outperforming most recently reported noble-metal-free electrocatalysts.