Linear Biazulene Isomers: Effects of Molecular and Packing Structure on Optoelectronic and Charge-Transport Properties

Herein
we describe the structure–property relationships
of three biazulene isomers (2,6′-biazulene (BAz1), 2,2′-biazulene
(BAz2), and 6,6′-biazulene (BAz3)). The unimolecular planarities
of these molecules follow the order BAz2 > BAz1 > BAz3, which
is ascribable
to the planar five-membered and the twisted seven-membered biaryl
rings. This order is the same as the conjugation-expansion order,
and the intramolecular reorganization energies show a similar trend.
Hole mobility follows the order BAz2 > BAz3 > BAz1, where the
lowest
mobility of BAz1 is attributable to its asymmetric molecular orbital
distribution. 2,2′-Binaphthalene (BNp), a structural isomer
of biazulene, shows no field-effect transistor characteristics. Transfer
integrals clearly support the observed superiority and inferiority
of these hole-transport properties. This study demonstrates the crucial
importance of both molecular structure and molecular orbital symmetries
for charge transfer. The dimer approach involving the 2,6-positions
of azulene generates linear structures due to the structural features
of its five-membered and seven-membered rings. Hence, azulenes, which
are relatively asymmetric from an aromatic perspective, exhibit high
symmetries as their biazulene isomers. This is in contrast to the
loss of symmetry when naphthalene is dimerized through its 2-position.