Heteroatom functionalization of phenalenyl: synthesis, structures, and properties of hexa-substituted phenalenyliums

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Design and Synthesis of a C3 Symmetrical Phenalenyl Derivative with Three Oxo Groups by Regioselective Deoxygenation/Oxygenation

Tri-tert-butylated 4,7-dihydroxyphenalenone was designed and synthesized from a corresponding 4,9-dimethoxyphenalenone derivative by regioselective deoxygenation/oxygenation. The 4,7-dihydroxyphenalenone derivative showed a chromic behavior accompanied by protonation and deprotonation, giving monocation and dianion species, respectively, and their C₃ symmetric electronic structures were elucidated by experimental and theoretical methods.

Design, Synthesis and Photochemical Properties of a Phenalenone-Based pH Sensor: Switchable pH Sensing in Four Detectable Channels

The synthesis and pH-sensing property of a novel phenalenone-based compound, 9-(4-hydroxyphenylamino)-1-oxo-phenalenone (HPAP), is reported. The newly synthesized compound is capable of functioning as a pH sensor in the region of pH 7 to 12. The sensor can be used as a colorimetric indicator in the transition from pH 10 to pH 11. The sensor is able to function in four detectable channels. All four channels (UV, emission, colorimetric/visible and photoluminescence) have been shown to be reversible, thus implying the reuse of this single-molecule sensor and indicator for several experiments. Mechanistic investigations have been performed by UV, NMR and DFT studies which indicate that a photoinduced electron transfer (PET) based mechanism could be operative. Straightforward and cost-effective application of the sensor in thin-layer chromatography has also been established.

Synthesis of tetrachalcogenide-substituted phenalenyl derivatives: preparation and solid-state characterization of bis(3,4,6,7-tetrathioalkyl-phenalenyl)boron radicals

We report the synthesis and properties of a series of spiro-bis(3,4,6,7-tetrachalcogenide-substituted-phenalenyl)boron salts and two of the corresponding tetrathioalkyl-substituted spiro-bis(phenalenyl)boron radicals [tetrathiomethyl (10) and tetrathioethyl (11)] in which all of the active positions of the phenalenyl (PLY) nucleus are functionalized. In the solid state, radicals 10 and 11 exist as a weak π-dimers due to the steric congestion of the thioalkyl groups in the superimposed PLY units. As a result, the spins are localized in the isolated (nonsuperimposed) PLY rings, and the structure, magnetic susceptibility measurements, and band structure calculations confirm that these PLY units are unable to undergo strong intermolecular interaction as a result of the orientation of the thioalkyl groups.