Terazulene Isomers: Polarity Change of OFETs through Molecular Orbital Distribution Contrast

Intermolecular orbital
coupling is fundamentally important to organic
semiconductor performance. Recently, we reported that 2,6′:2′,6″-terazulene
(TAz1) exhibited excellent performance as an n-type organic
field-effect transistor (OFET) via molecular orbital distribution
control. To validate and develop this concept, here we present three
other terazulene regioisomers, which have three azulene molecules
connected at the 2- or 6-position along the long axis of the azulene,
thus constructing a linear expanded π-conjugation system: 2,2′:6′,2″-terazulene
(TAz2), 2,2′:6′,6″-terazulene (TAz3), and 6,2′:6′,6″-terazulene (TAz4). TAz2 and TAz3 exhibit ambipolar
characteristics; TAz4 exhibits clear n-type transistor
behavior as an OFET. The lowest unoccupied molecular orbitals (LUMOs)
of all terazulenes are fully delocalized over the entire molecule.
In contrast, the highest occupied molecular orbitals (HOMOs) of TAz2 and TAz3 are delocalized over the 2,2′-biazulene
units; the HOMOs of TAz4 are localized at one end of
the azulene unit. These findings confirm that terazulene isomers which
are simple hydrocarbon compounds are versatile materials with a tunable-polarity
FET characteristic that depends on the direction of the azulene unit
and the related contrast of the molecular orbital distribution in
the terazulene backbone.