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.