A macrocyclic oligofuran: synthesis, solid state structure and electronic properties

Abstract

We report the first π-conjugated macrocyclic system with an oligofuran backbone. The calculated HOMO-LUMO gap is similar to that of the corresponding linear polymer, indicating a remarkable electron delocalization. The X-ray structure reveals a planar conformation, in contrast to the twisted conformation of macrocyclic oligothiophenes. The intermolecular π-π stacking distance is extremely small (3.17 Å), indicating very strong interactions. The macrocycle forms large π-aggregates in solution and shows a tendency toward highly ordered multilayer adsorption at the solid-liquid interface. The face-on orientation of molecules explains the higher hole mobility observed in the out-of-plane direction.

Chart 1. Structures of the linear (L) and macrocyclic (C) α,α′-oligofurans (nBFI) and α,α′-oligothiophenes (nT) discussed in this work. R′ = n-butyl, R = 2-octyldodecyl.

Fig. 1. Calculated (B3LYP/6-311G(d)) (a) strain energy per unit for C-nBFI; (b) HOMO–LUMO gap for L-nBFI (red) and C-nBFI (blue), where the black arrow indicates the tetramer; and (c) the HOMO and LUMO surfaces of C-4BFI.

Scheme 1. Synthesis of C-4BFI Conditions: (a) Br₂, FeCl₃, CH₂Cl₂, rt, dark; (b) Pd(PPh₃)₄, toluene, 90 °C; (c) Ni(COD)₂, 2,2′-bipyridine, THF, 50 °C, 48 h.

Fig. 2. X-ray structure of C-4BFI. (a) Ellipsoid representation and (b) packing in stick representation excluding the 2-octyldodecyl groups. Hydrogens and solvent molecules are omitted for clarity. (c) Electrostatic potential map of C-4BFI dimer, calculated at the B3LYP/6-311G(d) level. (d) Stick representation of C-4BFI column, including 2-octyldodecyl side groups, showing slip-stack packing arrangement.

Fig. 3. Normalized UV-Vis absorption (solid line) and emission (dashed line) spectra for L-4BFI (red), C-4BFI (blue) in chloroform, and C-4BFI film (green). Inset: photograph of solid C-4BFI fluorescing upon illumination with UV light (365 nm).

Fig. 4. Dynamic light scattering (DLS) of C-4BFI and L-4BFI in hexane. (a) c = 1 × 10^–5 M, and (b) c = 1 × 10^–4 M.

Fig. 5. (a) STM image of C-4BFI (5 × 10^–2 M) at the 1,2,4-trichlorobenzene–graphite interface. Image scale: 19.3 × 19.3 nm². Imaging conditions: I_set = 1 pA, and V_bias = –1.2 V. (b) Molecular modelling unit cell parameters: a = 1.7 ± 0.1 nm, b = 2.4 ± 0.1 nm and γ = 84 ± 1°. (c) STM image showing the multilayer. Image scale: 13.6 × 13.6 nm². Blue arrow shows the vacancy defect in the upper layer through which the STM contrast still shows a molecular structure (from the layer below). (d) Molecular model displaying CO···H hydrogen bonding between two adjacent macrocycles.

Fig. 6. (a) Hole-only diode SCLC characteristics of C-4BFI, where m is the slope; (b) transfer characteristics of C-4BFI films blended with nitrofluorene acceptor.