Cyano disubstituted tetrabenzoindeno[2,1-a]fluorene: open-shell or closed-shell?

Organic diradicaloids have lately emerged as potential spintronic materials. We report the unprecedented synthesis of a near-IR absorbing indeno[2,1-a]fluorene derivative that displays remarkably low LUMO (-4.15 eV) and a small HOMO-LUMO gap (0.85 eV). NMR/EPR studies indicated its open-shell diradical property, which was supported by DFT calculations while suggesting a 30% diradical character and a small singlet (S)-triplet (T) gap (-2.52 kcal mol-1). A large bond length alternation of the as-indacene core for its single-crystals indicated a quinoidal contribution with greater antiaromaticity, which is in line with the small diradical character despite showing a small S-T gap.

A Thiophenoradialene-Embedded Polycyclic Heteroterphenoquinone Exhibiting Dominant Antiaromatic Traits.

A thiophenoradialene-embedded polycyclic heteroterphenoquinone (PHTPQ) derivative of diindeno[1,2-b:2',1'-d]thiophene-2,8-dione, with antiaromatic characteristics, was synthesized by dehydrogenating its fluorescent dihydro PHTPQ precursor. The antiaromatic character was evidenced by the visible absorption band with a weakly intense tail extending to 800 nm in the near-infrared region (forbidden HOMO → LUMO transition) and non-emissive and amphoteric redox properties. Single-crystal and (anti)aromaticity analyses found a non-aromatic thiophene core while suggesting antiaromaticity/paratropicity of the pentafulvene subunits dominating the overall ground state properties.

Constructing 1-Ethoxyphenanthro[9,10-e]acephenanthrylene for the Synthesis of a Polyaromatic Hydrocarbon Containing a Formal Azulene Unit.

peri-Acenoacenes are attractive synthetic targets, but their non-benzenoid isomeric counterparts were unnoticed. 1-Ethoxyphenanthro[9,10-e]acephenanthrylene 8 was synthesized and converted to azulene-embedded 9, which is a tribenzo-fused non-alternant isomeric motif of peri-anthracenoanthracene. Aromaticity and single-crystal analyses suggested a formal azulene core for 9, which showed a smaller highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap with a charge-transfer absorption band and brighter fluorescence than 8 (quantum yield (Φ): 9 = 41.8%, 8 = 8.9%). The reduction potentials of 8 and 9 were nearly identical, and the observations were further supported by density functional theory (DFT) calculations.

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

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 1H 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 s-indacenodifluorene 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.

Unveiling a Quinoidal 2,3:10,11-Dibenzoheptazethrene.

Parent 2,3:10,11-dibenzoheptazethrene is a singlet diradicaloid polycyclic hydrocarbon in the ground state that did not change its diradical character upon substitution (methyl and triisopropylsilylethynyl). Described herein are the synthesis and characterization of an ethoxy/3,5-(CF3)2C6H3-substituted 2,3:10,11-dibenzoheptazethrene 3 that prefers to retain its p-quinoidal core and shows zero diradical character, as determined by single-crystal analysis and density functional theory calculations. Negative solvatochromism, π-π interactions, Csp2-H···O hydrogen bonding, intramolecular charge transfer, redox amphotericity, and a narrow HOMO-LUMO energy gap make 3 a potential candidate for application in optoelectronics.

Revisiting indeno[2,1-c]fluorene synthesis while exploring the fully conjugated s-indaceno[2,1-c:6,5-c']difluorene.

Described herein is an alternative synthetic approach for conjugated indeno[2,1-c]fluorene, including the experimental and theoretical investigations of a tetraradicaloid s-indaceno[2,1-c:6,5-c']difluorene that belongs to the rarely explored indacenodifluorene family containing 4n + 2 monocyclic conjugated π-electrons. Expedient synthesis, broad absorption reaching 1150 nm, and small HOMO-LUMO energy gap make [2,1-c:6,5-c']s-IDF a promising candidate for optoelectronic applications.