ELECTRON TRANSPORT IN 4H-1,1-DIOXO-4-(DICYANOMETHYLIDENE)THIOPYRANS -INVESTIGATION OF X-RAY STRUCTURES OF NEUTRAL MOLECULES, ELECTROCHEMICAL REDUCTION TO THE ANION-RADICALS, AND ABSORPTION PROPERTIES AND EPR-SPECTRA OF THE ANION-RADICALS

A series of 4H-1,1-dioxo-4-(dicyanomethylidene)thiopyrans 1-15, which are useful as electron-transport materials in xerography, were prepare d by condensation of malononitrile with the corresponding 4H-1,1-dioxo thiopyran-4-one. X-ray crystal structures of 1-5 were indicative that the thiopyran ring is planar in 1, 2, and 5 and is boat shaped in the tert-butyl-substituted derivatives 3 and 4. The 2-thienyl substituent in 5 is rigorously coplanar with the thiopyran ring in 5. The anion ra dicals of 1-6, which were prepared by electrochemical or lithium napht halenide reduction, were examined by EPR and absorption spectroscopy. The experimental spin densities as determined at atomic centers by EPR spectroscopy are consistent with values determined by an ab initio ca lculation on the anion radical bf 9. The reduction potentials of 1-15 as measured by second-harmonic AC voltammetry: (SHACV) correlated with the sum of Hammett substituent constants at the 2- and 6-positions. C oulometric reduction and reoxidation was indicative of reversible one- electron processes in these systems. Parent compound 2 gave completely irreversible reduction even by SHACV. Electron mobilities of thin fil ms of 3, 4, or 15 molecularly dispersed in a polyester binder were fou nd to increase with increased planarity of the electron-transport mate rial and increased pi-delocalization of the unpaired spin.