A new synthetic procedure toward substituted dithiophthalides, 5,6-dialkyloxydithiophthalide and 5,6-dithioalkyldithiophthalide, is presented. 5,6-Dithiooctyldithiophthalide was obtained from 4,5-dichlorophthalic acid in an eight-step reaction with an overall yield of 26%. 5,6-Dioctyloxydithiophthalide was obtained from 4,5-dihydroxyphthalic acid dimethyl ester in a seven-step reaction (overall yield 15%). Both monomers were polymerized by a thermal and nonoxidative polymerization that resulted in soluble poly(isothianaphthene) derivatives with a band gap of about 1.2 eV. Photoinduced absorption measurements revealed the existence of charged excitations upon illumination. The photoinduced charge generation, combined with the extensive light-harvesting properties and the easy processability, makes these materials quite promising for photovoltaic applications. (C) 2003 Wiley Periodicals, Inc.