Electron-energy-loss spectroscopy of 6,6′-dimethylfulvene: First detection of the triplet state

Abstract

Low-energy electron-energy-loss spectra of 6,6′-dimethylfulvene deposited 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 with 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 triplet state in a compound with a pentafulvene π-electron system. Semiempirical calculations, as well as ab initio multiconfiguration self-consistent-field calculations for fulvene using the 4-31G basis set, lead to an assignment of the observed triplet state as 1 3B2. Both 1 3B2 and the lowest excited singlet state 1 1B2 mainly arise from the excitation from the highest occupied molecular orbital to the lowest 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 6π-system represented by 1,3,5-trans-hexatriene. The difference expresses the smaller exchange interaction between the highest occupied and lowest unoccupied molecular orbital in fulvene and related compounds. Properties of the lowest triplet state of the parent compound fulvene are calculated by use of the multiconfiguration self-consistent-field method. According to these results the fulvene π-electron system is expected to be planar in this electronic state. The stabilization with respect to a structure twisted by 90° around the exocyclic carbon–carbon bond amounts to 0.62 eV.