Dithienothiophene-phenylene cooligomers with n-hexyloxy or n-dodecyloxy substituents have been synthesized and compared to the previously reported unsubstituted parent compound. The effect of substituents on the thermal, electronic, optical. thin film structure and field-effect transistor (OFET) properties was investigated. Structural phase transitions from highly-ordered nanocrystalline to liquid crystalline were observed at 241 and 213 degrees C for n-hexyloxy- and n-dodecyloxy-substituted compounds respectively, different from the parent compound. For the alkoxy-substituted compounds, the absorption spectra in thin film blue shift 50 nm, while the fluorescence spectra in thin film red shift 88-100 nm compared to those in solution. The OFET devices based on the alkoxy-substituted compounds exhibit mobilities as high as ca 0.02 cm(2)V(-1)s(-1) and their performance is sensitive to the alkoxy substituents and substrate temperatures. (C) 2008 Elsevier B.V. All rights reserved.