Stimuli-Responsive Molecular Duplexes Displaying Duplex-to-Duplex Switching

Synthetic duplexes with high stabilities have promising potential for mimicking biomolecular functions and developing supramolecular smart materials. Herein, we describe the synthesis and stimuli-responsive properties of molecular duplexes derived from indolocarbazole-pyridine (I-P) oligomers. These duplexes adopt nonclassical helical structures, stabilized by I-P hydrogen-bonding pairs in anhydrous chlorinated solvents. Notably, the longest duplex 6₂ (11-mer)₂ displays remarkable stability, forming twenty hydrogen bonds; its exchange energy barrier was determined to be ΔG^≠=22.0 kcal ⋅ mol^-1 at 75 °C in anhydrous (CDCl₂)₂. Upon the addition of water, a hydrated duplex 6₂ (11-mer)₂⊃10H₂O was formed, with one water molecule inserted between each I-P hydrogen-bonding pair. The Hill coefficient (n) for this process is 6.1, demonstrating extremely positive cooperativity. Conversely, the hydrated duplex 6₂ (11-mer)₂⊃10H₂O was completely converted into the original anhydrous duplex 6₂ (11-mer)₂ when the temperature was increased. Interconversion between these two distinct duplexes can be repeatedly carried out by varying the temperature. Furthermore, reversible switching between hetero-duplexes and homo-duplexes was also demonstrated by controlling the temperature, with concomitant changes in the characteristic emission signals.