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 studied by electron‐energy‐loss spectroscopy. Its vertical gas‐phase triplet (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 states of (all‐ E )‐octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 2 1 A g state below the 1 1 B 2 state, but this lack of observation does not exclude its existence. The lowest triplet state T 1 ( 1 ) was further characterized by flash photolysis. T 1 ( 1 ) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m̈s in degassed hexane. The adiabatic excitation energy of T 1 ( 1 ) was bracketed to the range of 1.1 ± 0.1 eV by energy‐transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of 1 and the lowest excited doublet state of its radical cation \documentclass{article}\pagestyle{empty}\begin{document}${1}^{+\kern0pt {.}}$\end{document} – essentially a non‐ Koopmans ' state – are discussed.