Positional Isomers of Tetramethoxypyrene-based Mono- and Biradicals

The positional isomers of <i>tert</i>-butylnitroxide (NO) substituted 4,5,9,10-tetramethoxypyrene-based mono- and biradical are synthesized. While the biradical 2,7-TMPNO in which two NO radical moieties are attached at the nodal plane of pyrene adopts a semiquinoid structure, the 1,6- and 1,8-isomers of the same exist in biradical form. The tuning of the antiferromagnetic exchange interactions is achieved by synthesizing the positional isomers of the biradical while maintaining the same radical moiety as well as the π spacer

Tetramethoxypyrene-Based Biradical Donors with Tunable Physical and Magnetic Properties

Synthesis of 2,7-disubstituted tetramethoxypyrene-based neutral biradical donors is reported. The biradicals were characterized by EPR, UV–vis, CV, SQUID, and single-crystal X-ray diffraction, and their optical, electrochemical, and structural properties were compared and discussed. The experimental results are well supported by DFT calculations. Systematic tuning of magnetic exchange interactions was achieved by varying the radical moieties

Positive Magneto-LC Effect in Conjugated Spin-Bearing Hexabenzocoronene

The first neutral spin carrying hexabenzocoronene (HBC) derivative is described. The conjugated phenyl nitroxide substituted HBC with five alkyl chains exhibits a positive magneto-LC effect in columnar hexagonal liquid crystalline phase as probed by differential scanning calorimetry and electron paramagnetic resonance spectroscopy. Surprisingly, at 140 K the Δ<i>M</i>_S = 2 transition can be observed indicating a thermally accessible triplet state between the neighboring molecules in the columnar arrangements

Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets

The tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor and acceptor functional groups were synthesized. The magnetic measurements and the DFT calculations were performed to ascertain the influence of the functional groups on inter- and intramolecular magnetic exchange interactions. While upon functionalizing the tolane bridge the intramolecular exchange interactions remained nearly unchanged, the fine-tuning of intermolecular exchange interactions could be achieved by employing the crystal engineering approach

Crystal Engineering of Tolane Bridged Nitronyl Nitroxide Biradicals: Candidates for Quantum Magnets

The tolane bridged nitronyl nitroxide biradicals with hydrogen bond donor and acceptor functional groups were synthesized. The magnetic measurements and the DFT calculations were performed to ascertain the influence of the functional groups on inter- and intramolecular magnetic exchange interactions. While upon functionalizing the tolane bridge the intramolecular exchange interactions remained nearly unchanged, the fine-tuning of intermolecular exchange interactions could be achieved by employing the crystal engineering approach

Equivalence of Ethylene and Azo-Bridges in the Modular Design of Molecular Complexes: Role of Weak Interactions

Structural equivalence is a general tool applied in crystal engineering for the predictable construction of molecular assemblies. In the present contribution we analyzed the equivalence of azo (−NN−) and ethylene (−CC−) bridges in the modular design of organic assemblies by studying 22 molecular complexes of 4,4′-azopyridine and 1,2-bis­(4-pyridyl)­ethene, of which 12 are novel. Unit cell similarity index (Π), as a numerical descriptor, was used to rationalize the observed equivalence/variance in the crystal packing of related complexes. A combined structural chemistry, database analysis and computational methods unveil the fact that the identity of the primary synthons alone does not ensure isostructurality; instead a concurrent effect of the contributions from both strong and weak/dispersive forces determines the structural equivalence. A statistical analysis based on a Cambridge Structural Database survey features an apparent inverse correlation that exist between N···I and I–I bond distances; a group of data points, however, deviate from this linear relation and was accounted on the basis of electrostatic potential distribution and interaction types

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through <i>p-ter</i>-phenyl (PPP), <i>ter</i>-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of <b>3</b>, <b>5</b>, <b>6</b>, <b>PPP-NNSi, PTP-NNSi</b>, and final biradicals of <b>TTT-NN</b>, <b>TPT-NN</b>, and <b>PPP-NN</b>, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in <b>TTT-NN</b> and <b>TPT-NN</b> than for NN and phenyl units (∼29°) in <b>PPP-NN</b> were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals <b>TTT-NN</b>, <b>TPT-NN</b>, <b>PTP-NN</b>, and <b>PPP-NN</b> were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for <b>TTT-NN</b> and <b>TPT-NN</b>. The intramolecular exchange interactions for <b>TPT-NN</b> and <b>TTT-NN</b> were found in good agreement with the ones calculated by broken symmetry DFT calculations

Mixed Phenyl and Thiophene Oligomers for Bridging Nitronyl Nitroxides

The synthesis of four nitronyl nitroxide (NN) biradicals is described which are conjugatively linked through <i>p-ter</i>-phenyl (PPP), <i>ter</i>-thiophene (TTT) and alternating phenylene (P) and thiophene (T) units as PTP and TPT. We first utilized Suzuki and Stille coupling reactions through protection and deprotection protocols to synthesize these (NN) biradicals. Single crystals were efficiently grown for radical precursors of <b>3</b>, <b>5</b>, <b>6</b>, <b>PPP-NNSi, PTP-NNSi</b>, and final biradicals of <b>TTT-NN</b>, <b>TPT-NN</b>, and <b>PPP-NN</b>, whose structures and molecular packing were examined by X-ray diffraction studies. As a result, much smaller torsions between the NN and thiophene units (∼10°) in <b>TTT-NN</b> and <b>TPT-NN</b> than for NN and phenyl units (∼29°) in <b>PPP-NN</b> were observed due to smaller hindrance for a five vs a six membered ring. All four biradicals <b>TTT-NN</b>, <b>TPT-NN</b>, <b>PTP-NN</b>, and <b>PPP-NN</b> were investigated by EPR and optical spectroscopy combined with DFT calculations. The magnetic susceptibility was studied by SQUID measurements for <b>TTT-NN</b> and <b>TPT-NN</b>. The intramolecular exchange interactions for <b>TPT-NN</b> and <b>TTT-NN</b> were found in good agreement with the ones calculated by broken symmetry DFT calculations

π‑Extended and Curved Antiaromatic Polycyclic Hydrocarbons

Synthesis of antiaromatic polycyclic hydrocarbons (PHs) is challenging because the high energy of their highest occupied molecular orbital and low energy of their lowest unoccupied molecular orbital cause them to be reactive and unstable. In this work, two large antiaromatic acene analogues, namely, cyclopenta­[<i>pqr</i>]­indeno­[2,1,7-<i>ijk</i>]­tetraphene (CIT, <b>1a</b>) and cyclopenta­[<i>pqr</i>]­indeno­[7,1,2-<i>cde</i>]­picene (CIP, <b>1b</b>), as well as a curved antiaromatic molecule with 48 π-electrons, dibenzo­[<i>a</i>,<i>c</i>]­diindeno­[7,1,2-<i>fgh</i>:7′,1′,2′-<i>mno</i>]­phenanthro­[9,10-<i>k</i>]­tetraphene (DPT, <b>1c</b>), are synthesized on the basis of the corona of indeno­[1,2-<i>b</i>]­fluorene. These three antiaromatic PHs possess a narrow energy gap down to 1.55 eV and exhibit high kinetic stability under ambient conditions. Moreover, these compounds display reversible electron transfer processes in both the cathodic and anodic regimes. Their cation and anion radicals are characterized by in situ vis–NIR absorption and electron paramagnetic resonance spectroelectrochemistry. The X-ray crystallographic analysis confirms that while CIP and CIT manifest planar structures, DPT shows a curved π-conjugated carbon skeleton. The synthetic strategy starting from <i>ortho</i>-substituted benzene units to construct five-membered rings in this work provides a unique entry to novel pentagon-embedding or curved antiaromatic polycyclic hydrocarbons. In addition, besides the detailed chemical and physical investigations, microscale single-crystal fiber field-effect transistors were also fabricated