Electron Density Analysis of the “O–O” Charge-Shift Bonding in Rubrene Endoperoxide

Rubrene endoperoxide p-xylene (1)
has been obtained in crystalline form from recrystallization and purification
of the organic semiconductor, rubrene, and for the first time characterized
by single-crystal X-ray diffraction methods. 1 is produced
by reaction of rubrene with molecular oxygen to create rubrene endoperoxide,
C42H28O2, in which an O2-bridge is joining the two phenyl-substituted C atoms opposite each
other in the second of the four aromatic rings in tetracene thereby
breaking the resonance along the tetracene moiety. The electron density
distribution of 1 reveals that the intramolecular O–O
bond is best characterized as charge-shift bonding with loss of electronic
concentration in the interatomic region evidenced by the Laplacian.
Likewise the ELI-D indicates little electron localization in this
region. Furthermore, source function and ELI-D analysis of 1 clearly quantifies the lack of electronic delocalization across
the six-membered ring that carries the peroxide-bridge.