Fluorinated Thiophene-Phenylene Co-Oligomers for Optoelectronic Devices

Organic
optoelectronics requires materials combining bright luminescence
and efficient ambipolar charge transport. Thiophene-phenylene co-oligomers
(TPCOs) are promising highly emissive materials with decent charge-carrier
mobility; however, they typically show poor electron injection in
devices, which is usually assigned to high energies of their lowest
unoccupied molecular orbitals (LUMOs). A widely used approach to lower
the frontier orbitals energy levels of a conjugated molecule is its
fluorination. In this study, we synthesized three new fluorinated
derivatives of one of the most popular TPCOs, 2,2′-(1,4-phenylene)­bis­[5-phenylthiophene]
(PTPTP) and studied them by cyclic voltammetry, absorption, photoluminescence,
and Raman spectroscopies. The obtained data reveal a positive effect
of fluorination on the optoelectronic properties of PTPTP: LUMO levels
are finely tuned, and photoluminescence quantum yield and absorbance
are increased. We then grew crystals from fluorinated PTPTPs, resolved
their structures, and showed that fluorination dramatically affects
the packing motif and facilitates π-stacking. Finally, we fabricated
thin-film organic field-effect transistors (OFETs) and demonstrated
a strong impact of fluorination on charge injection/transport for
both types of charge carriers, namely, electrons and holes. Specifically,
balanced ambipolar charge transport and electroluminescence were observed
only in the OFET active channel based on the partially fluorinated
PTPTP. The obtained results can be extended to other families of conjugated
oligomers and highlight the efficiency of fluorination for rational
design of organic semiconductors for optoelectronic devices.