Aromaticity and π-bond covalency: prominent intermolecular covalent bonding interaction of a Kekulé hydrocarbon with very significant singlet biradical character

Abstract

An anthracene-linked bisphenalenyl Kekulé molecule with very significant singlet biradical character has shown a prominent covalent bonding interaction between molecules in a molecular aggregate. High aromatic stabilization energy in the anthracene linker is responsible for the significant singlet biradical character.

Recent Progress in Quinoidal Singlet Biradical Molecules

“…Tetraphenyl derivatives 12 and 13 of 10 and 11 were prepared by multistep syntheses. 35,36 X-ray analysis revealed that 12 and 13 form 1D stacks in a slipped stacking arrangement with a superimposed phenalenyl overlap, which was almost identical to that of 8 and 9 ( Figure 8). Very short ππ contacts (3.170 ¡ for 12, 3.122 ¡ for 13) were also observed.…”

Phenalenyl‐Based Open‐Shell Polycyclic Aromatic Hydrocarbons

“…Intermolecular covalent bonding interactions were investigated with single crystals of tetraphenyl derivatives 19 and 20 . X‐ray analysis revealed that 19 and 20 afforded 1D chains in a slipped stacking arrangement with the superimposed phenalenyl moieties overlapping, which were almost identical to that of 13 (Figure ) . The very short π–π contacts (3.16 Å for 19 , 3.15 Å for 20 ) indicate the adequate covalent bonding interaction between molecules.…”

“…Intramolecular covalent bonding interactions of the two unpaired electrons in 15 and 16 were assessed with di‐ tert ‐butyl tetraphenyl derivatives 17 and 18 . The SQUID measurements of powdered 17 and 18 revealed smaller singlet–triplet energy gaps (16 kJ/mol for 17 , ∼8 kJ/mol for 18 ) than that of 10 .…”

“…The stronger interaction between molecules than within a molecule could be directly observed by polarized reflection measurements on a crystal of 20 . We carefully investigated the intensities of the lowest‐energy band (1025 nm) by rotating polarized light in steps of 30° on the (010) surface.…”

Why 1,2-quinone derivatives are more stable than their 2,3-analogues?

“…It should be noted that Clar structure shown in Scheme 5b is reinforced by all applied aromaticity indices (see Table S1 in Supporting Information). Summarizing, in 2,3-quinones for n > 2, the diradical singlet situation is favored, as in the case of the acenes [25,59,61,63] and in other polycyclic aromatic hydrocarbons [26][27][28][29][30][64][65][66][67][68][69] and graphene nanoflakes [70,71]. Therefore, for 2,3-isomers, only the results of the ground states (singlet closed shell for n ≤ 2 and singlet open shell for n ≥ 3) are presented below.…”