TRIPLET ENERGY OF 2,2-DIMETHYLISOINDENE FROM ELECTRON-ENERGY-LOSS SPECTROSCOPY AND PHOTOINDUCED TRIPLET ENERGY-TRANSFER

The excited electronic states of 2,2-dimethylisoindene (1) have been s tudied by electron-energy-loss spectroscopy. Its vertical gas-phase tr iplet (1(3)B(2)), and singlet (1(1)B(2)) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding stat es of (all-E)-octatetraene, which serves as a reference compound. Thes e shifts are partially reproduced by ZINDO calculations. The spectra g ive no evidence for a 2(1)A(g) state below the 1(1)B(2) state, but thi s lack of observation does not exclude its existence. The lowest tripl et state T-1(1) was further characterized by flash photolysis. T-1(1) was observed as a transient intermediate, lambda less than or equal to 350 nm, with a lifetime of 8 mu s in degassed hexane. The adiabatic e xcitation energy of T-1(1) was bracketed to the range of 1.1 +/- 0.1 e V by energy-transfer experiments. Relationships between the energies o f the lowest excited singlet and triplet states of 1 and the lowest ex cited doublet state of its radical cation 1(+) -essentially a non-Koop mans' state-are discussed.