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 () to the interior () 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 to non-cooperative with (α = 1.0). Density functional theory (DFT) studies show greater H-bonding and a more relaxed double helix for . The solvent and guest complement the synthetic designs. Use of acetonitrile to enhance solvophobicity further enhances cooperativity in (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 BF (4.1 Å). While photoconversion of interior azobenzenes was poor, the - 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.