A dichloro derivative of lucigenin has been synthesized and found to exhibit almost no fluorescence. Calculations using the graphical unitary group approach (GUGA), as implemented in the GAMESS program suite and using the full Brett-Pauli spin-orbit operator yields unusually large spin-orbit coupling (SOC) values. Comparison with experimental and theoretical results for similar systems identify the intramolecular heavy-atom effect (HAE) observed for chlorine in this system to be comparable to that observed for bromosubstituted polyaromatics in earlier studies. (C) 2009 Elsevier B. V. All rights reserved.