Tuning charge transport from unipolar (n-type) to ambipolar in bis(naphthalene diimide) derivatives by introducing π-conjugated heterocyclic bridging moieties

A series of acceptor-donor-acceptor (A-D-A) conjugated molecules based on naphthalene diimide dimers bridged with different pi-conjugated heterocyclic units (NDI-pi-NDI) have been designed and synthesized. By an ingenious design strategy, the LUMO (the lowest unoccupied molecular orbital) of the NDI-based small molecules is well controlled to a relatively constant value of -3.8 to -3.9 eV, whereas their HOMO (the highest occupied molecular orbital) could be tuned over a wide range, from -6.5 eV (compound 1) to -5.5 eV (compound 5), leading to varied band gaps from 2.6 eV to 1.5 eV. Organic field-effect transistor (OFET) characterization of these NDI-pi-NDI molecules shows that compounds 1, 2, and 3 have good n-type semiconducting properties in a N-2 atmosphere with the maximum electron mobilities up to 0.15 cm(2) V-1 s(-1), 0.46 cm(2) V-1 s(-1) and 0.57 cm(2) V-1 s(-1), respectively. Compounds 4 and 5, due to the high-lying HOMO levels and reduced energy band gaps, have ambipolar semiconducting properties and OFETs based on 5 show the highest electron and hole mobilities up to 1.23 cm(2) V-1 s(-1) and 0.0074 cm(2) V-1 s(-1), respectively. Moreover, the performances are enhanced under thermal treatment because of the increased crystallinity as evidenced by X-ray diffraction (XRD) and atomic force microscopy (AFM). The easily tunable electronic energy levels make the NDI-based semiconductors promising n-channel and ambipolar components in organic devices.