Two isomers of the closed-ring forms of 1,2-bis( 2,5-dimethyl-3-thienyl) perfluorocyclopentene (DMTF) having different S....S distances and dihedral angles between the two thiophene rings, are simultaneously observed in the FT-Raman spectrum by irradiation of UV light on microcrystals of the open-ring forms. Quantum chemistry calculations at the MP2/6-31G(d,p) level for several model clusters predict that an isomer having larger S....S distance is preferentially stabilized in crystal due to the electrostatic interaction between the two stacked thiophene rings of the closed-ring form and the open-ring form. (c) 2008 Published by Elsevier B. V.