Thermochromic Polydiacetylene Nanotubes from Diketopiperazine-Containing Diacetylenes

Abstract

Polydiacetylenes (PDAs) are a family of conjugated polymers which can be readily prepared by polymerization of self-assembled diacetylenes (DAs). PDA is well known for its unique color and fluorescence change under wide range of external stimuli. 2,5-Diketopiperazine (DKP), also known as cyclo(Gly-Gly), is one of the most abundant subunits found in various natural products. This simplest class of cyclic peptide has been extensively studied in the field of biomedical and crystal engineering because its cis-amide functionalities allow hydrogen bonding and structural diversity can be achieved by introducing new substituents at six different positions and stereochemistry at four positions. In this study, we have prepared a series of DKP-containing diacetylenes DKP-DAs by coupling 10,12-pentacosadiynoic acid with 2,5-diketopiperazine. Self-assembled DKP-DAs afforded tubular structures and UV irradiation to the supramolecular aggregates resulted in the blue-colored PDA nanotubes. In addition, the PDA nanotubes displayed a reversible thermochromism. Origin of the colorimetric reversibility was studied using NMR and IR spectroscopic analyses. We were able to confirm that this formation of nanotube with reversible thermochromic property can be attributed to intermolecular hydrogen bonding between DKP moieties in the supramolecular PDA.