Covalently Linked 5,6,11,12-Tetraazanaphthacene Dimer and Its Triptycene-Capped Derivatives as Electron Acceptors

The development of electron transport and n-type materials is still largely dominated by a limited number of organic semiconductors, with fullerenes at the forefront. In contrast, substantial progress has been made in developing hole transport and p-type materials. Therefore, expanding the range of electron acceptors, making them solution-processable, and elucidating their structural arrangement by X-ray crystallography is essential. We synthesised 2,2'-bi-(5,6,11,12-tetraazanaphthacene) (bi-TANC) and its triptycene end-capped derivative, 2,2'-bi(8,13-dihydro-8,13-[1,2]benzenonaphtho-5,6,15,16-tetraazanaphthacene) (bi-TpTANC), as electron acceptors. Bi-TANC exhibits a herringbone-like crystal packing with intermolecular π-π overlap, which is observed in typical organic n-type semiconductors. However, it showed poor solubility, similar to larger acenes. In contrast, bi-TpTANC exhibited favourable solubility, and its electrochemistry in solution was investigated. In the cyclic voltammogram of bi-TpTANC, reversible redox waves corresponding to 3-step/4-electron transfer were observed at -0.795 V (1e-), -0.927 V (1e-), and -1.44 V (2e-) as half-wave potentials. The redox wave associated with the two-electron transfer on the negative low-potential side indicates the presence of through-bond charge delocalisation in the monoanionic state. Furthermore, the LUMO level of bi-TpTANC is -4.1 eV, which indicates its potential as a promising air-stable n-type material.

Overcrowded 14,14'-Bidibenzo[a,j]anthracenes: Challenges in Syntheses and Atypical Property of Lacking Symmetry-Breaking Charge Transfer (SBCT)

14,14'-Bidibenzo[a,j]anthracenes (BDBAs) were prepared by iridium-catalyzed annulation of 5,5'-biterphenylene with alkynes. The molecular geometries of overcrowded BDBAs were verified by X-ray crystallography. The two dibenzo[a,j]anthryl moieties are connected through the sterically hindered 14 positions, resulting in highly distorted molecular halves. The conformation with a small twist angle between two molecular halves can minimize steric conflicts between the substituents at 1 and 13 positions and the carbon atoms of the central axis, as well as steric clashes between those substituents. One such example is octafluoro-substituted BDBA, where the interplanar angle between two anthryl moieties is approximately 31° (currently the lowest reported value, cf. 81° in 9,9'-bianthracene). The intramolecular interactions and electronic couplings between two molecular halves resulted in upfield 1H NMR signals, redshifted absorption and emission bands, and a reduced HOMO-LUMO gap. Photodynamic investigations on BDBAs indicated that the formation of the conventional symmetry-breaking charge transfer (SBCT) state was suspended by restricted rocking around the central C-C bond. Such a mechanism associated with this highly constrained conformation was examined for the first time.

Structure and Chiroptical Properties of Anthra[1,2-a]anthracene-1-yl Dimers as New Biaryls

Dimers of anthra[1,2-a]anthracene-1-yl units and its mesityl derivative were synthesized by Ni(0)-mediated coupling of the corresponding chloro derivatives as new biaryls. The X-ray analysis and DFT calculations revealed that two polycyclic aromatic units with nonplanar deformations took a twisted conformation about the single bond as a chiral axis. Enantiomers of the nonsubstituted compound were resolved by chiral HPLC, and the enantiopure samples showed intense Cotton effects at 321 nm in the circular dichroism (CD) spectra and emission bands at 449 nm in the circularly polarized luminescence (CPL) spectra with dissymmetry factor of |glum| 3.6×10-3. The absolute stereochemistry of this biaryl was determined by the theoretical calculation of CD spectrum by the time-dependent DFT method. The barrier to enantiomerization was determined to be 108 kJ mol-1 at 298 K. The dynamic process proceeded via a stepwise mechanism involving the helical inversion of each aromatic unit and the rotation about the biaryl axis as analyzed by the DFT calculations.

A Stable Hexaazaoctacene Cruciform σ-Dimer

Buchwald-Hartwig coupling of a triisopropylsilyl (TIPS)-ethynylated dibromo-N,N'-dihydrotetraazapentacene with 1,4-bis(TIPS-ethynyl)-2,3-diaminonaphthalene furnishes a dihydrohexaazaoctacene. Its oxidation with MnO₂ results in a 7,7'-bi(hexaazaoctacenyl). In addition to eight TIPS-ethynyl groups, the bioctacene motif protects the azaoctacene subunits. The biazaoctacenyl displays a τ_1/2 of > 5 d in dilute solution under ambient conditions. In the crystalline state it is persistent for > 10 months.