Chemistry Reference
In-Depth Information
CHAPTER 3
SINGLET OPEN-SHELL CHARACTER OF POLYPERIACENES
Akihiro Shimizu, Akihito Konishi, Yasukazu Hirao
*
and Takashi Kubo
†
DEPARTMENT OF CHEMISTRY, GRADUATE SCHOOL OF SCIENCE, OSAKA UNIVERSITY,
MACHIKANEYAMA 1-1, TOYONAKA, OSAKA 560-0043, JAPAN
*
Y-HIRAO@CHEM.SCI.OSAKA-U.AC.JP;
†
KUBO@CHEM.SCI.OSAKA-U.AC.JP
Singlet open-shell character of polyperiacenes has been elucidated by
theoretical analysis based on the Clar's aromatic sextet valence bond model and
quantum chemical calculations. Anti-ferromagnetic ground state of large
polyperiacenes was relevant to the sufficient stabilization energy derived from
aromatic sextet formation to overcome the energetic penalty associated with
Π-bond cleavages. Experimental study on the smallest potential biradicaloid,
2,5,9,12-tetra-
TERT
-butylbisanthene
1
, demonstrated no appreciable biradical
character in physical measurements, whereas
1
showed biradicaloid behavior
toward oxygen.
1. INTRODUCTION
Graphene, which is a two dimensional sheet consisting of fused six
membered rings of carbon atoms, has zigzag and armchair edges.
STM/STS measurements have revealed the edge-localized electrons at
the zigzag edge,
1
where a large magnetic moment is suggested by
the tight binding band calculations.
2
Magnetic and related electronic
properties originating the edge spins are extensively studied theoretically
and experimentally for realization of carbon-based nanoelectronics.
3
In
this context, nanographenes as small components of the graphene sheet
have attracted considerable attention recently from synthetic
4
and
theoretical points of view,
5
and the theoretical studies using symmetry-
broken DFT calculations on nanographene molecules with zigzag edges
figure out anti-parallel spin orientation between the two edges in the
45
