Polymerized thin films based on polydiacetylenes (PDAs) undergo distin
ct color transitions that lend themselves to applications in biosensin
g, 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 F
ourier transform infrared spectroscopy. The thermochromic transition t
emperature is found to be between 70 and 90 degrees C. At the molecula
r level, the ordering of the film increases at the thermochromic trans
ition and remains ordered up to temperatures well above the transition
(e.g., 130 degrees C). No evidence for previously suggested entanglem
ent or disordering of the alkyl side chains is observed. The pendant s
ide chains rearrange from a partially disordered configuration charact
eristic of the blue film, to a well-ordered close-packed hexagonal arr
angement in the red form. The rearrangment of the pendant side chains
is linked to the formation of the red phase PDA.