s-Indacene Revisited: Modular Synthesis and Modulation of Structures and Molecular Orbitals of Hexaaryl Derivatives

Abstract

Though s-indacene is an intriguing antiaromatic hydrocarbon of 12 π-electrons, it has been underrepresented due to the lack of efficient and versatile methods to prepare stable derivatives. Herein we report a concise and modular synthetic method for hexaaryl-s-indacene derivatives bearing electron-donating/-accepting groups at specific positions to furnish C 2h -, D 2h -, and C 2v -symmetric substitution patterns. We also report the effects of substituents on their molecular structures, frontier molecular orbital (MO) levels, and magnetically induced ring current tropicities. Both theoretical calculations and X-ray structure analyses indicate that the derivatives of the C 2h -substitution pattern adopt different C 2h structures with significant bond length alternation depending on the electronic property of the substituents. Due to the nonuniform distribution of the frontier MOs, their energy levels are selectively modulated by the electron-donating substituents. This leads to the inversion of the HOMO and HOMO–1 sequences with respect to those of the intrinsic s-indacene as theoretically predicted and experimentally proven by the absorption spectra at visible and near-infrared regions. The NICS values and the 1H NMR chemical shifts of the s-indacene derivatives indicate their weak antiaromaticity. The different tropicities are explained by the modulation of the HOMO and HOMO–1 levels. In addition, for the hexaxylyl derivative, weak fluorescence from the S2 excited state was detected due to the large energy gap between the S1 and S2 states. Notably, an organic field-effect transistor (OFET) fabricated using the hexaxylyl derivative exhibited moderate hole carrier mobility, a result which opens the door for optoelectronic applications of s-indacene derivatives.

Tetra‐tert‐butyl‐s‐indacene is a Bond‐Localized C2h Structure and a Challenge for Computational Chemistry

“…During the course of this study, Cheng and Tobe et al reported a series of substituted hexaaryl-s-indacenes with C 2h , D 2h , and C 2v symmetries, as evidenced by X-ray measurements and calculations at the B3LYP level. [26] We computed the symmetrically substituted hexaxylyl-s-indacene (compound 1 f in reference 26) and found that while hexaxylyl-s-indacene exhibits a D 2h s-indacene core at the B3LYP level, minimum geometries at the M06-2X, ωB97X-D, and M11 levels show a C 2h s-indacene core. Computed proton chemical shifts based on M11 geometries give the best match with experiment (Figure 2).…”

Substituent effects on paratropicity and diatropicity in π-extended hexapyrrolohexaazacoronene

“…To the best of our knowledge, no studies have examined the substituent effects in antiaromatic compounds. 11 In this study, we synthesized homoHPHAC monocations and trications with various substituents and investigated their effects on the optical, redox, and geometrical properties and para(dia)tropicity of a series of homoHPHAC mono(tri)cations. Friedel-Cras-type reactions were used to enable the introduction of various substituents.…”

Synthesis and Characterization of Dibenzothieno[a,f]pentalenes Enabling Large Antiaromaticity and Moderate Open-Shell Character through a Small Energy Barrier for Bond-Shift Valence Tautomerization

“…Since the first description by Breslow in 1967, antiaromaticity has been one of the most attractive concepts to describe the electronic features of cyclic π-conjugated molecules. − Unlike aromaticity, antiaromaticity gives rise to destabilization, π-bond localization, paratropic ring current, and a decrease in the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gap in the π-conjugated system . These electronic properties have attracted interest for their utility in advanced organic materials. − One interesting topic of study in antiaromatic molecules involves the interconversion of single and double bonds through a process called bond-shift valence tautomerization. An antiaromatic molecule distorts toward a lower-symmetric structure with pronounced bond length alternation (BLA) to avoid the electronic instability caused by π electron delocalization.…”

“…The presence of the accessible high-symmetric structure should fuel rich insights into the electronic properties driven by its pronounced antiaromaticity or open-shell character (Figure B). Incorporation of quinoidal subunits into the 4 n π-conjugated circuit provides a possible design guide for reducing the barrier for bond-shift valence tautomerization. ,, From this perspective, we have recently focused on diareno­[ a , f ]­pentalenes − and disclosed the interrelation between their antiaromatic and open-shell characteristics, , showing hole transport behavior . In a series of diareno­[ a , f ]­pentalenes 1 – 5 (Figure A), the following two principles that govern the interrelation between the antiaromatic and open-shell characteristics are derived: (1) the mode of the embedded o -quinoidal moiety and (2) the formal molecular symmetry of the ring fusion pattern.…”