title: Quinoidal Oligo(9,10-anthryl)s with Chain-Length-Dependent Ground States: A Balance between Aromatic Stabilization and Steric Strain Release.
abstract: Quinoidal π-conjugated polycyclic hydrocarbons have attracted intensive research interest due to their unique optical/electronic properties and possible magnetic activity, which arises from a thermally excited triplet state. However, there is still lack of fundamental understanding on the factors that determine the electronic ground states. Herein, by using quinoidal oligo(9,10-anthryl)s, it is demonstrated that both aromatic stabilisation and steric strain release play balanced roles in determining the ground states. Oligomers with up to four anthryl units were synthesised and their ground states were investigated by electronic absorption and electron spin resonance (ESR) spectroscopy, assisted by density functional theory (DFT) calculations. The quinoidal 9,10-anthryl dimer 1 has a closed-shell ground state, whereas the tri- (2) and tetramers (3) both have an open-shell diradical ground state with a small singlet-triplet gap. Such a difference results from competition between two driving forces: the large steric repulsion between the anthryl/phenyl units in the closed-shell quinoidal form that drives the molecule to a flexible open-shell diradical structure, and aromatic stabilisation due to the gain of more aromatic sextet rings in the closed-shell form, which drives the molecule towards a contorted quinoidal structure. The ground states of these oligomers thus depend on the overall balance between these two driving forces and show chain-length dependence.
authors.affiliation: Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 (Singapore), Fax: (+65) 6779-1691.
authors.forename: Zhenglong
authors.initials: Z
authors.lastname: Lim
authors.affiliation: Division of Physical Sciences & Engineering and, KAUST Catalysis Center, King Abdullah University of, Science and Technology (KAUST), Thuwal 23955-6900, (Saudi Arabia).
authors.forename: Bin
authors.initials: B
authors.lastname: Zheng
authors.affiliation: Division of Physical Sciences & Engineering and, KAUST Catalysis Center, King Abdullah University of, Science and Technology (KAUST), Thuwal 23955-6900, (Saudi Arabia). kuowei.huang@kaust.edu.sa.
authors.forename: Kuo-Wei
authors.initials: KW
authors.lastname: Huang
authors.affiliation: Synthesis & Integration, Institute of Materials Research, and Engineering, 3 Research Link, 117602 (Singapore). ye-liu@imre.a-star.edu.sg.
authors.forename: Ye
authors.initials: Y
authors.lastname: Liu
authors.affiliation: Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 (Singapore), Fax: (+65) 6779-1691. chmwuj@nus.edu.sg.|Synthesis & Integration, Institute of Materials Research, and Engineering, 3 Research Link, 117602 (Singapore). chmwuj@nus.edu.sg.
authors.forename: Jishan
authors.initials: J
authors.lastname: Wu
keywords: aromaticity; electronic structure; hydrocarbons; radicals; strained molecules