Importance of the curvature in electronic, structural and charge transport properties: oligomers of N-pyridine carbazole

Abstract

The recent development of π-conjugated nanohoops in organic electronics has opened new perspectives for this family of curved materials. Addressing the difference arising from curved vs. non-curved materials is an important concern in order to highlight the specific properties of curved materials. Herein, we discuss the significant differences arising between [4]-cyclo-N-pyridine-2,7-carbazole [4]C-Py-Cbz and its linear counterpart, N-pyridine-2,7-tetracarbazole [4]L-Py-Cbz. This study gathers optical, electrochemical, structural, morphological and charge transport properties and highlights the key role played by the curvature. We notably show how the electronic effects of the pending pyridines differently affect the molecular orbitals energies. We have also shown how structural rearrangements between ground and first excited states can be at the origin of the peculiar emission of nanohoops. Finally, the huge difference in terms of charge transport properties was discussed. Nanohoop [4]C-Py-Cbz displays a field-effect mobility in the saturated regime μ_FEsat of 3.4 × 10⁻⁶ cm² V⁻¹ s⁻¹, six times higher than that of its linear analogue [4]L-Py-Cbz (5.7 × 10⁻⁷ cm² V⁻¹ s⁻¹). However, the detailed study of the charge transport properties shows that the linear analogue forms a semi-conducting layer with a lower trap density, more favourable to the charge hopping processes. Theoretical calculations have provided clues on the origin of these differences, revealing that despite the intensity of electronic couplings is significantly in favor of [4]L-Py-Cbz, their highly anisotropic character renders them more sensitive to defects and disorder. This is consistent with the fact that [4]C-Py-Cbz displays experimentally better charge transport properties than [4]L-Py-Cbz. Such a type of structure–property relationship study provides fundamental findings of interest for the future design of high-performance nanohoops.