The two lowest singlet excitation energies of 26 2-imidazolyl-2-thiazolylazo compounds have been investigated by ab initio methods within the second-order polarization propagator approximation (SOPPA). Various combinations of 4- and 5-substituents at both the imidazole and thiazole units have been studied for obtaining pi --> pi* excitation energies corresponding to an absorption maximum in the red or infrared regions. In many cases, pi --> pi excitation energies below 2 eV (corresponding to wavelengths longer than 600 run) are found for molecules that possibly may be synthesized. For the molecules included in this study, the longest wavelength was calculated to be 1049 nm. (C) 2001 Elsevier Science B.V. All rights reserved.