The head-to-head-type poly(3-alkynylthiophene-2,5-diyl) HH-M(C CR)Th has been shown to exhibit a strong tendency to self-assemble. The UV-vis spectrum of HH-P3(C CDec)Th (Dec = decyl) in 1,2-dichlorobenzene at 131 degrees C showed a peak at lambda(max) = 520 nm, which shifted to lambda(max) = 553 nm at 25 degrees C, with a shoulder peak at 608 nm. However, the UV-vis spectrum of an HH-P3(C CDec)Th film showed a more pronounced shift in its UV-vis absorption peak to a longer wavelength. These phenomena are the characteristics of the self-assembly of the polymer molecule. The HH-P3(C CDec)Th film had a large third-order nonlinear optical susceptibility chi((3)) of 3.6 x 10(-11) esu. (c) 2007 Elsevier B.V. All rights reserved.