Persistent peri-Heptacene: Synthesis and In Situ Characterization

Abstract

n-peri-Acenes (n-PAs) have gained interest as model systems of zigzag-edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n-PAs larger than peri-tetracene remains challenging because of their intrinsic open-shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n-PA, that is, peri-heptacene (7-PA), in which the reactive zigzag edges are kinetically protected with eight 4-tBu-C₆ H₄ groups. The formation of 7-PA is validated by high-resolution mass spectrometry and in situ FT-Raman spectroscopy. 7-PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t_1/2 ≈25 min) under inert conditions. Moreover, electron-spin resonance measurements and theoretical studies reveal that 7-PA exhibits an open-shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next-generation (3 N+1)-PAs (where N≥3).

Keywords: Scholl reaction; acenes; graphene; nanostructures; radicals.

Figure 1

Clar's sextet (cyan‐colored benzenoid ring) depictions of 4‐PA, 5‐PA and 7‐PA, and the calculated (CASSCF (8,8)/6‐31G**) diradical (y ₀) and tetraradical (y ₁) indices. The substituents in 4‐PA and 7‐PA are omitted in the resonance structures for clarity.

Scheme 1

Synthesis of peri‐heptacene (7‐PA). Reagents and conditions: (i) Pd(PPh₃)₂Cl₂, CuI, Et₃N, THF, RT, 24 h, 87 %; (ii) Pd(PPh₃)₄, K₂CO₃, toluene/EtOH/H₂O, 85 °C, 24 h, 50 %; (iii) Pd(PPh₃)₄, K₂CO₃, toluene/EtOH/H₂O, 85 °C, 24 h, 41 %; (iv) ICl, dichloromethane, −78 °C, 3 h, 29 %; (v) 1. n‐BuLi, 10, toluene, −10 °C, 2 h; 2. BF₃⋅OEt₂, dichloromethane, 25 °C, 30 min, 40 % (over two steps); (vi) DDQ, toluene, 25 °C, 10 min.

Figure 2

a) HR MALDI‐TOF mass spectra of 3 and 7‐PA in dithranol matrix. The 7‐PA was in situ generated by mixing toluene solution of 3 (8×10⁻⁴ M) with DDQ. b) Experimental and simulated Raman spectra of precursor 3 and 7‐PA. c) Representation of the D and G nuclear displacement patterns of graphene molecules. d) The normal modes of 3 corresponding to the D and G Raman transitions. e) The D and G Raman transitions of 7‐PA. Red arrows indicate nuclear displacements, whereas green and blue segments indicate stretching and shrinking bonds, respectively.

Figure 3

UV‐vis‐NIR spectra of the tetrahydro‐precursor 3 (green) (3×10⁻⁵ M) and the time‐dependent absorption changes of the 7‐PA (red) in dry toluene at RT under Ar (in the absence of light). The insets show the magnified view of NIR region and the absorbance (at 812 nm) of the 7‐PA (under argon) at different time intervals. The 7‐PA was in situ generated by mixing toluene solution of 3 with 2.5 equiv DDQ.

Figure 4

a) Calculated (UCAM‐B3LYP/6–311G**) density of the unpaired electrons of the singlet diradical forms of 4‐PA, 5‐PA and 7‐PA in gas‐phase. b) Side views of the relaxed structure of 7‐PA (to reduce the calculation time tert‐butyl groups on the phenyl rings are ignored). c) Calculated (UB3LYP/6–311G**) NICS(1) and ACID plots of 7‐PA.