Head-to-head type poly(3-(1-dodecynyl)thiophene-2,5-diyl), HH-P3(C CDec)Th, formed a well aligned structure in its cast film, and UV-vis absorption peak of the cast film showed a large shift by about I 10 nm to a longer wavelength from that (518 nm) observed in solution. Under high pressure HH-P3(C CDec)Th exhibited clear piezochromism; the UV-vis absorption peak at about 605 nm observed under atmospheric pressure shifted to 672 nm under 10.71 GPa, suggesting an increase in electronic interaction between the polymer molecules under high pressure. The pi-pi* transition energy decreased by 0.028 eV/GPa in the range of 0-5.2 GPa. (c) 2007 Elsevier Ltd. All rights reserved.