A photoinduced chromatic transition from blue to red for the polydiacetylene 10,12-pentacosadiynoic acid has been studied. This transition produces an obvious change in the ultraviolet-visible absorption and fluorescence emission spectra. A two-dimensional micropattern has been realized on the basis of this change and imaged with a confocal fluorescence microscope. Detailed information on the mechanism of the chromatic transition has been obtained by the application of resonance Raman and Fourier transform infrared methods. The results indicate that the conformational change of alkyl side chains constricted by hydrogen-bonded head groups imposes strain on the polymer backbone and finally leads to a drastic decrease in the pi-electron-conjugation length. (c) 2006 Wiley Periodicals, Inc.