Extremely π-Delocalized Nature of Dianthrylcarbene-Based High Spin Systems as Studied by CW and Pulsed 2D Electron Spin Transient Nutation Spectroscopy

Abstract

Electronic and molecular structures of bis(9,9′-dianthrylcarbene) and tris(9,9′-dianthrylcarbene) coupled by 1,4-diethynylbenzene and 1,3,5-triethynylbenzene, respectively, have been investigated by cw and pulsed ESR spectroscopy. Their spin multiplicities were identified by two-dimensional electron spin transient nutation (2D-ESTN) method based on the pulsed ESR technique. The fine-structure ESR spectra observed after UV-irradiation of the corresponding diazo precursor were analyzed with the help of a hybrid eigenfield method. The biscarbene is spin-singlet in the ground state and a thermally accessible triplet state with an axial symmetry was detected, showing that the biscarbene undergoes a linear and perpendicular conformation of the two 9,9′-dianthrylcarbene moieties. Such a conformation gives rise to an extremely large amount of π-spin delocalization in ethynylbenzene-based 9,9′-dianthrylcarbene derivatives. The electronic and molecular structures of tris(9,9′-dianthrylcarbene) in the septet ground state are also discussed in this context. The oligocarbenes under study are a novel series of Skell-type high-spin carbenes characterized by extremely π-delocalized nature.