N-Phenylated N-Heteroacenes: Synthesis, Structures, and Properties

N,N-Diphenyl Dihydrophenazines: Using π-Extension to Access Dicationic Multifunctional Materials

Dihydrophenazines are receiving increasing attention due to applications in numerous fields of chemistry, from light emission to organo-photocatalysis. Despite this growing interest and numerous works involving the preparation of radical cations based on this scaffold, the isolation and study of the aromatic dications obtained by 2 electron oxidation of dihydrophenazines is still mostly unexplored. From this point of view, along with the substitution at the N atoms generally used to tune dihydrophenazine properties, the π-extension of the phenazine core could play a crucial role in making dicationic states accessible. This could result in an extension of the knowledge on these elusive dications and in potentially highly interesting applications ranging from material science to molecular actuators.

Potential Building Blocks for 1,4-Dihydro-N-heteroacenes

Studies have been performed aimed at the synthesis of N-heteroacenes via substitution reactions of 4,5-difluoro-1,2-dinitrobenzene with a diamine. The fluorine atoms are displaced first, followed by an activated nitro group. Two intermediates have been characterised by X-ray single-crystal structure determinations. Their intermolecular interactions were examined by Hirshfeld surfaces to assess their suitability for organic molecular electronics. The high reactivity of the phenazine, which is prone to oxidise and rearrange, as are displacement products prepared from it, is explained by the formation of a cis-aci-nitro form from the secondary amine of the phenazine and a nitro group.

A Potential Iterative Approach to 1,4-Dihydro-N-Heteroacene Arrays

A new method for the synthesis of substituted 1,4-dihydrophenazines is reported and the structure of N-butyl-5-methyl-3-nitro-5,10-dihydrophenazine is proven by an X-ray single-crystal structure determination.

A Multiple Excited-State Engineering of Boron-Functionalized Diazapentacene Via a Tuning of the Molecular Orbital Coupling

Harvesting high-energy excited-state energy is still challenging in organic chromophores. An introduction of boron atoms along the short axis of the diazapentacene backbone induces multiple emission characteristics. Our studies reveal that the weak molecular orbital (MO) coupling of the S₃-S₁ transition is responsible for the slow internal conversion rates. Such MO coupling-regulated anti-Kasha emission is different from the large band gap-induced anti-Kasha emission character of classical azulene derivatives. Theoretical studies reveal that a strong MO coupling of the S₃-S₀ transition is responsible for the higher photoluminescence quantum yield of the anti-Kasha emission in a more polar solution (tetrahydrofuran: 11%; cyclohexane: 0%). Such an MO coupling factor is generally overlooked in anti-Kasha emitters reported previously. Furthermore, the multiple emission can be regulated by solvent polarity, solvent temperature, and fluoride anion binding. As a proof of concept of harvesting high-energy emission, the multiple emission character has allowed us to design single-molecule white-light-emitting materials.

5,11-Diazadibenzo[hi,qr]tetracene: Synthesis, Properties, and Reactivity toward Nucleophilic Reagents

5,11-Diazadibenzo[hi,qr]tetracene was synthesized as a new nitrogen-substituted polycyclic heteroaromatic compound by Pd-catalyzed cycloisomerization of an alkyne precursor followed by oxidative cyclization with bis(trifluoroacetoxy)iodobenzene. The substitution of imine-type nitrogen atoms significantly enhanced its electron-accepting character and facilitated the direct nucleophilic addition of arylamines under strongly basic conditions to afford the desired amino-substituted products. The introduction of amino groups induced a remarkable red-shift in their absorption spectra; the tetrasubstituted product exhibited intense near-infrared absorbing property. Furthermore, the π-electronic system, which includes a redox-active 1,4-diazabutadiene moiety, underwent reversible interconversion to its corresponding reduced form upon reduction with NaBH₄ and aerobic oxidation.

Tuning J-aggregate Formation and Emission Efficiency in Cationic Diazapentacenium Dyes

Enhancement of the luminescence efficiency of two new diazapentacenium salts (D1 and D2) of more than 55 for D1 and 22 times for D2) in poor solvents, acetonitrile and/or dichloromethane, was observed and rationalized as formation of emissive J-aggregates. Both compounds displaying 4-n-decylphenyl substituents at the 7,14-carbons and phenyl (D1) or 2,6-difluorophenyl (D2) substituents at the quaternary nitrogen atoms in 5,12-positions have been synthetized in a two-step procedure involving a two-fold Buchwald-Hartwig-type CN cross-coupling and an electrophilic Friedel-Crafts-type cyclization. The optical properties of the dicationic diazapentacenium salts in various solvents and in thin films have been investigated by steady-state and time-resolved absorption and photoluminescence spectroscopies. In thin films and in good solvents, isolated molecules coexist with aggregates. Nonetheless, D1 is seven times more emissive than D2, reflecting a higher J-aggregate contribution in the former.

Synthesis and Electronic Properties of Directly Linked Dihydrodiazatetracene Dimers

5,12-Dihydro-5,12-diazatetracene (DHDAT) dimers with different substitution patterns are synthesized: a symmetric one with a C-C bond between the monomer units (1) and two asymmetric ones with a C-N bond between the monomer units (2 and 3). The DHDAT units are planar in the C-C linked dimer 1 but perpendicularly oriented in the C-N linked dimers 2 and 3 (from X-ray analysis). The electronic ground-state interaction between the two units is large in 1 and small in 2 and 3. The emission behavior of 3 is different from that of other dimers and its monomer; it displays positive solvatochromism, characteristic for electron donor-acceptor molecules, despite its donor-donor type structure. Compound 3 exhibits a unique multi-step thermochromic emission behavior. The emission behavior is attributed to the asymmetric distribution of the HOMO and LUMO of DHDAT.

Stable Radical Cations of N,N'-Diarylated Dihydrodiazapentacenes

A series of quinoidal N,N'-diaryldiaza-N,N'-dihydropentacenes (Quino) was prepared in a two-step reaction, starting from quinacridone. Oxidation of Quino furnishes stable radical cations, isoelectronic to the radical anions of the azaacenes, whereas the dicationic species are isoelectronic to neutral azapentacenes. The spectroscopic properties of the diaryldiazapentacenes and their oxidized mono- and dications are equivalent to that of the dianion of tetraazapentacene (TAP), its radical anion and the neutral TAP.

Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.

Conjugation between a 3-D icosahedral carborane and a fused 2-D π-ring system is ambiguous. To address this issue, we prepared several carborane-fused carbo- and heterocycles. Detailed studies on their molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of (1) considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and (2) considerable conjugation between a 3-D carborane and a fused 2-D π-ring system. These results will shed some light on the design of new carborane-based materials.