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.