ELECTRON-ENERGY-LOSS SPECTROSCOPY OF 6,6'-DIMETHYLFULVENE - FIRST DETECTION OF THE TRIPLET-STATE

Low-energy electron-energy-loss spectra of 6,6'-dimethylfulyene deposi ted on a thin film of solid argon are measured at a temperature of 16 K. The spectra make it possible to locate the lowest triplet state wit h an onset of the transition band at 1.9 eV and a vertical transition energy of approximately 2.3 eV. This is the first observation of a tri plet state in a compound with a pentafulvene pi-electron system. Semie mpirical calculations, as well as ab initio multiconfiguration self-co nsistent-held calculations for fulvene using the 4-31G basis set, lead to an assignment of the observed triplet state as I B-3(2). Both 1 B- 3(2) and the lowest excited singlet state I B-1(2) mainly arise from t he excitation from the highest occupied molecular orbital to the lowes t unoccupied molecular orbital. A singlet-triplet splitting of 1.05 eV is derived for these states. This is about 2.5 times smaller than the corresponding value in an open-chain conjugated 6 pi-system represent ed by 1,3,5-trans-hexatriene. The difference expresses the smaller exc hange interaction between the highest occupied and lowest unoccupied m olecular orbital in fulvene and related compounds. Properties of the l owest tripler state of the parent compound fulvene are calculated by u se of the multiconfiguration self-consistent-held method. According to these results the fulvene rr-electron system is expected to be planar in this electronic state. The stabilization with respect to a structu re twisted by 90 degrees around the exocyclic carbon-carbon bond amoun ts to 0.62 eV. (C) 1995 American Institute of Physics.