Two Isomeric Azulene Decorated Naphthodithiophene Diimide (NDTI)-Based Triads: Molecular Orbital Distribution Controls Polarity Change of OFETs through Connection Position.

Altering the charge carrier transport polarities of organic semiconductors by the molecular orbital distribution is of great interesting. Herein, we report two isomeric azulene decorated naphthodithiophene diimide (NDTI)-based triads (e. g. NDTI-B2Az and NDTI-B6Az), in where two azulene units were connected with NDTI at 2-position of the azulene ring in NDTI-B2Az, whereas two azulene unitd were incorporated with NDTI at 6-position of the azulene ring in NDTI-B6Az, respectively. The two isomeric triads were excellently soluble in common organic solvents. DFT calculations on the molecular orbital distributions of the triads reveal that the LUMOs are completely delocalized over the entire molecule for both NDTI-B2Az and NDTI-B6Az, indicating a great potential for n-type transport behavior, whereas the HOMOs are mainly delocalized over the entire molecule for NDTI-B2Az or only localized at the terminal two azulene units for NDTI-B6Az, respectively, implying a great potential for p-type transport behavior for the former and a disadvantage of hole carrier transport for the latter. Under ambient conditions, solution-processed bottom-gate top-contact (BGTC) transistors based on NDTI-B2Az showed ambipolar FET characteristics with high electron and hole mobilities of 0.32 (effective electron mobility approximately 0.14 cm(2) V(-1) s(-1) according to a reliability factor of 43%) and 0.03 cm(2) V(-1) s(-1) (effective hole mobility approximately 0.01 cm(2) V(-1) s(-1) according to a reliability factor of 33%), respectively, whereas typically unipolar n-channel behavior is found for a film of NDTI-B6Az with a high electron mobility up to 0.13 cm(2) V(-1) s(-1) (effective electron mobility approximately 0.06 cm(2) V((-1)) s(-1) according to a reliability factor of 43%). The results indicate that the polarity change of OFETs based on the two isomeric triads could be controlled by the molecular orbital distributions through the connection position between azulene unit and NDTI.