Dominating Antiaromatic Character of as-Indacene Decides Overall Properties of a Formally Aromatic Dicyclopenta[c]fluorenothiophene

Abstract

Dicyclopenta[c]fluorenothiophene 5 was synthesized as the isoelectronic polycyclic heteroarene analogue of an as-indacenodifluorene with a (4n + 2)π-electron perimeter. Single-crystal and 1 H NMR analyses indicated a quinoidal ground state for 5, which was supported by theoretical calculations while suggesting a degree of antiaromaticity of the as-indacene subunit greater than that for sindacenodifluorene 3. The dominant antiaromaticity for 5 was evidenced by the broad weakly intense absorption tail reaching the near-IR region, four-stage redox amphotericity, and small HOMO− LUMO energy gap.

A Thiophenoradialene-Embedded Polycyclic Heteroterphenoquinone Exhibiting Dominant Antiaromatic Traits

“…The mesityl groups are nearly orthogonal to the heptacyclic π-backbone, with an average torsional angle of 69.7°. The pentafulvene subunit of 7 exhibited significant C−C bond length alternation (BLA), as shown in Figure 2a (the full bond length analysis, along with esd values, is displayed in Figure S10a) with C�C bonds a−c (see the labeling in Figure 23,24 DFT optimization of 7 at the B3LYP/6-31G(d,p) level of theory suggested a singlet closed-shell ground state with a large singlet−triplet energy gap (18.8 kcal/mol). The HOMA 18 analyses of the optimized closed-shell structure of 7 provided further support for the substantial BLA for rings C (−0.38) and B (−0.36), while insignificant BLA for terminal A rings (0.93) suggested its aromaticity.…”

“…Compound 7 showed an intense absorption in the visible region at λ max = 504 nm [ε = 50 150 M −1 cm −1 (Figure 4b)], which is accompanied by a low-energy shoulder at ∼635 nm, despite the shortening of the linear πconjugation path in comparison to that of 6. Such a weakly intense shoulder band with its absorption tail extending to 800 nm (Figure 4b, inset) is reminiscent of strong ground state antiaromatic character, 23 which may be attributed to the dominant antiaromaticity of the pentafulvene subunit for 7, as supported by NICS/ACID calculations. According to TDDFT, the absorption at 504 nm of 7 was an admixture of HOMO → LUMO+1, HOMO−1 → LUMO, and HOMO−5 → LUMO (501 nm; f = 0.47) transitions, while the weakly intense absorption tail in the range of 535−800 nm resulted due to the combinations of several weak and apparently forbidden transitions [f < 0.021 (Table S6)], including the forbidden HOMO → LUMO transition (TDDFT; 685 nm; f = 0.006), implying ground state antiaromatic character.…”

“…The reversible reduction events signify the strong antiaromaticity of the pentafulvene subunit of 7, which resulted in facile electron injections to form the 4n + 2 cyclopentadienyl anion. 23,27 Notably, thia-hybrid [5]radialene 3 was reported to be an electron donor 7h and could not be reduced, while thia-hybrid [5]radialene-embedded 7 could act as an electron acceptor and donor. The electrochemical HOMO and LUMO energy levels of 6 were −5.52 and −3.82 eV, respectively, as estimated from eV.…”

“…29 The antiaromaticity was demonstrated experimentally by the weakly intense absorption band in the range of 535−800 nm owing to a forbidden HOMO → LUMO transition, facile twostage electron reductions with good reversibility, and nonemissive properties. 23,26,27 Our current synthetic approach has the potential to construct other heteroatom (i.e., selenium) 30 counterparts of 7 (and 6) with tunable solid state ordering and HOMO−LUMO energy gaps for potential application as ambipolar organic semiconductors, 31 which is underway.…”

“…Compound 7 displayed redox amphotericity with two reversible reductions at half-wave potentials of −1.02 V ( E 1/2 red1 ) and −1.40 V ( E 1/2 red2 ) and a quasi-reversible oxidation at 1.05 V ( E peak ox ). The reversible reduction events signify the strong antiaromaticity of the pentafulvene subunit of 7 , which resulted in facile electron injections to form the 4 n + 2 cyclopentadienyl anion. , Notably, thia-hybrid [5]­radialene 3 was reported to be an electron donor and could not be reduced, while thia-hybrid [5]­radialene-embedded 7 could act as an electron acceptor and donor. The electrochemical HOMO and LUMO energy levels of 6 were −5.52 and −3.82 eV, respectively, as estimated from the onset potentials, affording an energy gap of 1.72 eV.…”