Resonance Balance Shift in Stacks of Delocalized Singlet Biradicals

摘要

Recently we succeeded in the isolation of delocalized singlet biradicals utilizing the spin-delocalizing character of the phenalenyl radical. We demonstrated that the singlet biradical 1 has strong spin-spin interactions between molecules through the overlap of phenalenyl rings in the one-dimensional (1D) chain even though the closed-shell Kekule structure 1 can be drawn as a resonance contributor (Scheme 1). Huang and Kertesz gave further insight into the spin-spin interactions from a theroretical point of view and showed that the spin-spin interaction between the molecules was predicted to be stronger than that within the molecule. These experimental and theoretical findings are associated with very fundamental issues: Do delocalized singlet biradicals actually have open-shell character? Are the electrons coupled within a molecule involved in covalent bonding between molecules? In this study we will demonstrate that intra- and intermolecular spin-spin interactions strongly correlate and can be altered in magnitude by an applied external field. Our proposal is based on the experimentally determined molecular structure of 2, a temperature-dependent reflection spectrum of 2, and a pressure-dependent reflection spectrum of 1. Methyl groups at the 2- and 10-positions in 2, where the frontier molecular orbital has very small coefficients, are expected to alter the distance between the overlapping phenalenyl rings with respect to the analogous separation in 1, and as a result, the magnitude of the intermolecular spin-spin interaction should be affected.