Molecular imaging of thermochromic carbohydrate-modified polydiacetylene thin films

Polymerized thin films based on polydiacetylenes (PDAs) undergo distinct color transitions that lend themselves to applications in biosensing, surface modification, nonlinear optics, and molecular electronics. The mechanism of the thermochromic blue to red color transition of PDA thin films was investigated at the molecular level using atomic force microscopy and at the macroscopic level with visible absorption and Fourier transform infrared spectroscopy. The thermochromic transition temperature is found to be between 70 and 90°C. At the molecular level, the ordering of the film increases at the thermochromic transition and remains ordered up to temperatures well above the transition (e.g., 130°C). No evidence for previously suggested entanglement or disordering of the alkyl side chains is observed. The pendant side chains rearrange from a partially disordered configuration characteristic of the blue film, to a well-ordered close-packed hexagonal arrangement in the red form. The rearrangment of the pendant side chains is linked to the formation of the red phase PDA.