Grafting Electron-Accepting Fragments on [4]cyclo-2,7-carbazole Scaffold: Tuning the Structural and Electronic Properties of Nanohoops

Abstract

Since the first applications of nanohoops in organic electronics appear promising, the time has come to go deeper into their rational design in order to reach high-efficiency materials. To do so, systematic studies dealing with the incorporation of electron-rich and/or electron-poor functional units on nanohoops have to be performed. Herein, the synthesis, the electrochemical, photophysical, thermal, and structural properties of two [4]cyclo-2,7-carbazoles, [4]C-Py-Cbz, and [4]C-Pm-Cbz, possessing electron-withdrawing units on their nitrogen atoms (pyridine or pyrimidine) are reported. The synthesis of these nanohoops is first optimized and a high yield above 50% is reached. Through a structure-properties relationship study, it is shown that the substituent has a significant impact on some physicochemical properties (eg HOMO/LUMO levels) while others are kept unchanged (eg fluorescence). Incorporation in electronic devices shows that the most electrically efficient Organic Field-Effect transistors are obtained with [4]C-Py-Cbz although this compound does not present the best-organized semiconductor layer. These experimental data are finally confronted with the electronic couplings between the nanohoops determined at the DFT level and have highlighted the origin in the difference of charge transport properties. [4]C-Py-Cbz has the advantage of a more 2D-like transport character than [4]C-Pm-Cbz, which alleviates the impact of defects and structural organization.

Keywords: bridged cyclo‐oligophenylenes; charge transport; nanohoops; organic electronics; organic semiconductors.

Scheme 1

Synthesis of the targeted nanohoops [4]C‐Ph‐Cbz, [4]C‐Py‐Cbz, and [4]C‐Pm‐Cbz.

Figure 1

¹H‐NMR spectrum of [4]C‐Pm‐Cbz (purple lines), [4]C‐Py‐Cbz (blue lines), [4]C‐Ph‐Cbz (green lines), and [4]C‐Bu‐Cbz (red lines) in CD₂Cl₂.

Figure 2

Different views of single‐crystal structures of [4]C‐Py‐Cbz (Left) and [4]C‐Pm‐Cbz (Molecule A, middle and Molecule B, right). For the sake of clarity, hydrogens and solvents are not shown.

Figure 3

Intramolecular N‐H interactions for [4]C‐Py‐Cbz and [4]C‐Pm‐Cbz.

Figure 4

Views of the single‐crystal structure of the nanohoops: [4]C‐Pm‐Cbz, from left to right, views along the b‐axis, the c‐axis (molecular slices in the foreground at z = 0.0, in the background and drawn in black at z = 0.5) and the normal to (bc)‐plane (molecular slices at x = 0.0 in the foreground, in the background and drawn in black at x = 0.5); [4]C‐Py‐Cbz, from left to right, views along the normal to (ab)‐plane (molecular slices in the foreground at z = 0.0, in the background and drawn in black at z = 0.5), the b‐axis (molecular slices in the foreground with centroids at y = 0.0 and y = 0.2, in the background and drawn in black with centroids at y = 0.5 and y = 0.7) and the a‐axis (molecular slices in the foreground at x = 0.0, in the background and drawn in black at x = 0.5). For the sake of clarity, hydrogens are not shown and solvent molecules are only displayed for the views on the left.

Figure 5

[4]C‐Py‐Cbz (green lines), [4]C‐Pm‐Cbz (red lines), and [4]C‐Ph‐Cbz (blue lines). Absorption in dichloromethane (top left) and in spin‐coated thin film (bottom left). Emission in dichloromethane (top right, λ_exc = 350 nm) and in spin‐coated thin films (bottom right, λ_exc = 340 nm).

Figure 6

Representation of the energy levels and the molecular orbitals involved in main the electronic transitions of a) [4]C‐Ph‐Cbz, b) [4]C‐Py‐Cbz, and c) [4]C‐Pm‐Cbz (TD‐DFT, B3LYP/6‐311+g(d,p)), orbitals shown with an isovalue of 0.02 (e.bohr^‐3)^−1/2.

Figure 7

Normalized cyclic voltammograms of [4]C‐Py‐Cbz (green lines), [4]C‐Pm‐Cbz (red lines), and [4]C‐Ph‐Cbz (blue lines). Left: cathodic range, in DMF (0.1 M Bu₄NPF₆), middle and right: anodic range in CH₂Cl₂ (0.2 M Bu₄NPF₆). Sweep‐rate of 100 mV s⁻¹, platinum disk (diameter 1 mm) working electrode.

Figure 8

[4]C‐Py‐Cbz and [4]C‐Pm‐Cbz‐based OFETs. Top. Transfer Characteristics (Left), linear field‐effect mobility activation energy (Middle), and time‐dependent variations in V_TH under gate‐bias stress (Right). Bottom. AFM measurements.

Figure 9

a) Transfer integrals in the experimental crystal structure of [4]C‐Py‐Cbz. The HOMO‐HOMO couplings between the colored molecules are 19 meV, while the values for all other dimers do not exceed 7 meV. b) Transfer integrals in the experimental crystal structure of [4]C‐Pm‐Cbz. The HOMO‐HOMO coupling between the red or blue nanohoops is ≈13 meV but is close to zero across.