Chalcogenated polyacenes including organic semiconductors such as tetrathiotetracene, tetraselenotetracene, and hexathiopentacene are produced by reaction of a polyacene with elemental sulfur, selenium, and tellurium in the presence of nitrogen-containing hot solvents, preferably alkylated amides. N, N-dimethylformamide is the preferred solvent. The disclosed method of synthesis produces substituted polyacenes of high yield and purity in considerably shorter reaction times than achievable by prior art methods.