Triplet Energy of 2, 2‐Dimethylisoindene from Electron‐Energy‐Loss Spectroscopy and Photoinduced Triplet Energy Transfer

Abstract

AbstractThe excited electronic states of 2, 2‐dimethylisoindene (1) have been studied by electron‐energy‐loss spectroscopy. Its vertical gas‐phase triplet (13B2), and singlet (11B2) 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 21Ag state below the 11B2 state, but this lack of observation does not exclude its existence. The lowest triplet state T1(1) was further characterized by flash photolysis. T1(1) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m̈s in degassed hexane. The adiabatic excitation energy of T1(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.