title: Higher Order π-Conjugated Polycyclic Hydrocarbons with Open-Shell Singlet Ground State: Nonazethrene versus Nonacene.
abstract: Higher order acenes (i.e., acenes longer than pentacene) and extended zethrenes (i.e., zethrenes longer than zethrene) are theoretically predicted to have an open-shell singlet ground state, and the radical character is supposed to increase with extension of molecular size. The increasing radical character makes the synthesis of long zethrenes and acenes very challenging, and so far, the longest reported zethrene and acene derivatives are octazethrene and nonacene, respectively. In addition, there is a lack of fundamental understanding of the differences between these two closely related open-shell singlet systems. In this work, we report the first synthesis of a challenging nonazethrene derivative, HR-NZ, and its full structural and physical characterizations including variable temperature NMR, ESR, SQUID, UV-vis-NIR absorption and electrochemical measurements. Compound HR-NZ has an open-shell singlet ground state with a moderate diradical character (y0 = 0.48 based on UCAM-B3LYP calculation) and a small singlet-triplet gap (ΔES-T = -5.2 kcal/mol based on SQUID data), thus showing magnetic activity at room temperature. It also shows amphoteric redox behavior, with a small electrochemical energy gap (1.33 eV). Its electronic structure and physical properties are compared with those of Anthony's nonacene derivative JA-NA and other zethrene derivatives. A more general comparison between higher order acenes and extended zethrenes was also conducted on the basis of ab initio electronic structure calculations, and it was found that zethrenes and acenes have very different spatial localization of the unpaired electrons. As a result, a faster decrease of singlet-triplet energy gap and a faster increase of radical character with increase of the number of benzenoid rings were observed in zethrene series. Our studies reveal that spatial localization of the frontier molecular orbitals play a very important role on the nature of radical character as well as the excitation energy.
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.|Key Laboratory of Advanced Material, Department of Chemistry, East China University of Science and Technology , 130 Meilong Road, Shanghai, 200237, China.
authors.forename: Rui
authors.initials: R
authors.lastname: Huang
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.
authors.forename: Hoa
authors.initials: H
authors.lastname: Phan
authors.affiliation: Department of Materials Science and Engineering, National University of Singapore , 119260, Singapore.
authors.forename: Tun Seng
authors.initials: TS
authors.lastname: Herng
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.
authors.forename: Pan
authors.initials: P
authors.lastname: Hu
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.
authors.forename: Wangdong
authors.initials: W
authors.lastname: Zeng
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.
authors.forename: Shao-Qiang
authors.initials: SQ
authors.lastname: Dong
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.
authors.forename: Soumyajit
authors.initials: S
authors.lastname: Das
authors.affiliation: Key Laboratory of Advanced Material, Department of Chemistry, East China University of Science and Technology , 130 Meilong Road, Shanghai, 200237, China.
authors.forename: Yongjia
authors.initials: Y
authors.lastname: Shen
authors.affiliation: Department of Materials Science and Engineering, National University of Singapore , 119260, Singapore.
authors.forename: Jun
authors.initials: J
authors.lastname: Ding
authors.affiliation: IKERBASQUE - Basque Foundation for Science & Donostia International Physics Center & Kimika Fakultatea, Euskal Herriko Unibertsitatea , Paseo Manuel de Lardizabal, 4, 20018 Dgonostia-San Sebastián, Euskadi, Spain.
authors.forename: David
authors.initials: D
authors.lastname: Casanova
authors.affiliation: Department of Chemistry, National University of Singapore , Science Drive 3, 117543, Singapore.|Institute of Materials Research and Engineering, A*STAR , 2 Fusionopolis Way, Innovis #08-03, 138634, Singapore.
authors.forename: Jishan
authors.initials: J
authors.lastname: Wu