‘Non‐Koopmans’ States in Stilbene Cations and a Remarkable Example to the «SDT‐Equation»: Indeno [2,1‐a]indene

Abstract

AbstractThe radical cations of indeno [2, 1‐a]indene (1), stilbene (2) and 3, 5, 3', 5'‐tetramethylstilbene (3) were prepared by γ‐irradiation of the neutral precursors in an electron‐scavenging matrix at 77 K. Their electronic spectra were recorded and compared to the photoelectron spectra (PE.) of the neutral precursors. The results show that either the fourth or the fifth excited doublet state of the cations is of «Non‐Koopmans» type, with specific doublet energy (D) D (2Bg)=2.74 eV (1+), =2.59 eV (2+), =2.49 eV (3+). Remarkably, 1+ possesses two electronic states in the 2.7‐2.8 eV energy range: 2Au («Koopmans»‐type) and 2Bg («Non ‐Koopmans»‐type). The «SDT»‐equation \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm D} = \sqrt {{\rm S} \cdot {\rm T}} $\end{document} approximately connecting excited singlet (S) and triplet (T) states of a neutral alternant system with the excited doublet (D) states of its radical cation ‐ provided e‐promotion occurs For all three excited states between the same (paired) orbitals‐is satisfyingly exemplified by 1: S1 = 3.92 eV and T1= 2.06 eV for 1, D4 or 5=2.74 eV for 1+.