Dimeric tetrabromo- p-quinodimethanes: synthesis and structural/electronic properties

Abstract

Despite their great potential as molecular building blocks for organic synthesis, tetrabromo-p-quinodimethanes (TBQs) are a relatively unknown family of compounds. Herein, we showcase a series of five derivatives incorporating two tetrabromo-anthraquinodimethane (TBAQ) units linked by π-conjugated spacers of different nature and length. The resulting dimers TBQ1-5 are fully characterised by means of thorough spectroscopic measurements and theoretical calculations. Interestingly, owing to the steric hindrance imposed by the four bulky bromine atoms, the TBAQ fragments adopt a characteristically warped geometry, somehow resemblant of a butterfly, and the novel dimers show a complex NMR pattern with signal splittings. To ascertain whether dynamic processes regarding fluxional inversion of the butterfly configurations are involved, first-principles calculations assessing the interconversion energy barriers are performed. Three possible stereoisomers are predicted involving two diastereomers, thus accounting for the observed NMR spectra. The rotational freedom of the TBAQ units around the π-conjugated linker influences the structural and electronic properties of TBQ1-5 and modulates the electronic communication between the terminal TBAQ moieties. The role of the linker on the electronic properties is investigated by Raman and UV-vis spectroscopies, theoretical calculations and UV-vis measurements at low temperature. TBQ1-5 are of interest as less-explored structural building precursors for a variety of scientific areas. Finally, the sublimation, self-assembly and reactivity on Au(111) of TBQ3 is assessed.

Scheme 1. Synthetic route to the dimeric tetrabromo-p-quinodimethanes TBQ1–5.

Fig. 1. Relative electronic energy (a) and molecular structure (b) of the different conformations localized at the BMK/6-31G(d,p) level on the potential energy surface of TBAQ. In (a), the number of imaginary frequencies of each structure is indicated within parentheses, and the arrows indicate the evolution of the structure upon deformation along the imaginary modes. In (b), side (left) and top (right) views are displayed. Color coding: H in white, C in gray and Br in yellow.

Fig. 2. Comparison of the ¹H-NMR spectra of TBQ2 and TBQ5 showing the splitting of the peri hydrogen signal of the former, consistent with the presence of two isomeric species.

Fig. 3. Rotational energy barrier profile calculated for TBQ2 at the BMK/6-31(d,p)+PCM(CH₂Cl₂) level along the torsion of the dihedral angle θ (in red) defining the relative orientation of the TBAQ moieties.

Fig. 4. (a) Raman spectra of the compounds in solid state at 298 K, employing a 785 nm laser (blue-shaded bands are those mainly discussed in the text). (b) BMK/6-31G(d,p) theoretical vibrational ν(CC) modes (wavenumbers in cm⁻¹) of TBQ2 showing the main CC displacements and related to the modes 1, 3 and 8a of benzene. (c) Wavenumber evolution of the ν(CC) vibrational modes (mode 1–3 of benzene) from the TBAQ units (in red) upon vibrational coupling with the central ν(CC) acetylene spacer (in purple) and with the ν(CC) of the single connecting bonds (in yellow). Vibrational splittings are qualitative. (d) Vibrational splitting of the 1–3 and 8a modes simultaneously from TBAQ to TBQ2. Experimental values are shown in bold (blue-shaded bands are those equally denoted in the experimental spectra).

Fig. 5. Comparison between the 785 nm solid-state laser Raman (pointing up) and the infrared (pointing down) spectra of TBAQ (below) and TBQ2 (above) at 298 K, normalized to the strongest Raman and infrared bands.

Fig. 6. Optical properties of TBQ1–5 in CH₂Cl₂ solution at 25 °C. The blue plot corresponding to the normalised absorption and the orange to the normalised emission.

Fig. 7. Variable-temperature UV-vis electronic absorption spectra of TBQ2 in 2-methyltetrahydrofurane upon cooling from 298 K to 80 K. The dotted grey line corresponds to the spectrum recorded at room temperature after cooling.

Fig. 8. On-surface deposition and reactivity of TBQ3 species on Au(111). (a) Overview STM image of the sublimation of a submonolayer coverage of TBQ3 on a pristine Au(111) surface kept at room temperature. V_b = 1 V, I_t = 100 pA, scale bar: 4.0 nm. (b) Left: Zoomed-in image of the blue rectangle indicated in panel (a) showing an intact molecule with two consecutive pairs of elongated bright protrusions. Right: Superposition of the chemical model. (c) Long-range STM image after the annealing of (a) at 150 °C. V_b = 0.5 V, I_t = 50 pA, scale bar: 4 nm. (d) Left: Zoomed-in image of the green highlighted area in panel (c). Right: Superposition of the chemical model.