Hexamethoxyphenalenyl as a Possible Quantum Spin Simulator: An Electronically Stabilized Neutral π Radical with Novel Quantum Coherence Owing to Extremely High Nuclear Spin Degeneracy

Abstract

In recent years, organic open-shell molecules have drawn increasing attention from both fundamental and applied aspects, because they show unique structural electronic features and spin-nature-based functionalities that are intrinsically different from closed-shell molecules. [1][2][3][4] The latest trends are exemplified in chemical entities affording dynamic nuclear polarization to give rise to enormous sensitivity enhancement in biological NMR spectroscopy [3] and the use of molecular electron spins as synthetic spin quantum bits (qubits) for quantum information science and quantum computers (QCs). [4] The latest progress in molecular spin science has been underlain by the design and synthesis of airstable organic open-shell molecules. [1] Phenalenyl (PLY in Scheme 1) is an odd-alternant carboncentered neutral p radical with a high thermodynamic stability arising from delocalization of an unpaired electron over the planar D 3h -symmetric 13p-electron system. However, owing to a kinetic instability, in solution, PLY gradually decomposes to peropyrene, a closed-shell polycyclic hydrocarbon, through s dimerization even under oxygen-free conditions. [5] Introduction of tert-butyl groups into the phenalenyl skeleton allowed us to isolate, for the first time, TBPLY (Scheme 1) in its crystalline state. [6] Since then, by using the steric protection effect of bulky substituents, several kinds of air-stable phenalenyl derivatives were synthesized and isolated, all of which have exotic structures and properties owing to the extensively spin-delocalized nature of the phenalenyl system. [7][8][9] Efforts have been made to synthesize electronically stabilized phenalenyl systems by introducing heteroatom functional groups, such as alkoxy, [10a-c, 11a] amino, [10b-d] thio, [11] and cyano [10a, 12] substituents at the peripheral carbon atoms. These phenalenyl radicals are important to understand electronic perturbation effects and intermolecular interactions attributable to the introduced heteroatom substituents in solids. However, detailed investigations in this context, particularly relevant to the electronic-spin properties, have rarely been reported except for thio-or cyanosubstituted radicals, [11a, 12] owing to the intrinsic kinetic instability of the neutral radical species. [13] Here we report the synthesis, electronic-spin structure, and novel spin-based quantum coherence of the hexamethoxylphenalenyl neutral p radical 1C (Scheme 1), in which the coherence is related to a large number of equivalent nuclear spins. Interestingly, the symmetrical introduction of six methoxy groups into the phenalenyl skeleton played a crucial role not only to electronically stabilize the radical species 1C by the resonance effect, [14] but also to provide extremely wellresolved ESR hyperfine splittings with a very small linewidth in solution associated with temperature-dependent dynamic behavior of spin angular momenta in molecular frames. This unique hyperfine spectroscopic feature allowed us to investigate spin-based quantum na...

Synthesis, Characterization, and Functionalization of 1‐Boraphenalenes

“…6c reacts with MesMgBr to form two species in ac irca 9:1r atio (Scheme 5, top). Thes olid-state structure of 12 revealed ap lanar 1-boraphenalene unit and effectively orthogonal aryl groups.T he bond metrics in the boracycle were closely comparable to those found in 6a and 6c,i ncluding as hort C11ÀC12 distance of 1.360 (4) . Theminor product (12) is functionalised at boron, leaving the vinyl bromide group intact.…”

“…Despite repeated attempts,only compound 12 was amenable to crystallisation in our hands. Thes olid-state structure of 12 revealed ap lanar 1-boraphenalene unit and effectively orthogonal aryl groups.T he bond metrics in the boracycle were closely comparable to those found in 6a and 6c,i ncluding as hort C11ÀC12 distance of 1.360 (4) .…”

“…[1] Since Haddonss eminal report in 1975, [2] 1,a nd derivatives,h ave been of considerable interest for studying fundamental bonding phenomena (multi-centre bonding/s Vs. p dimerisation), [2,3] and for ar ange of applications (organic semiconductors,s pin memory,e lectrode materials). [4] Then onbonding SOMO of phenalenyl is key to its unique properties ( Figure 1), and phenalenyls display amphoteric redox behaviour,w ith oxidation furnishing a1 2p electron cation and reduction a14p electron anion. [1] Thekey properties of 1 can be modulated by functionalisation of the periphery or by incorporation of heteroatoms.…”

Exploring Closed-Shell Cationic Phenalenyl: From Catalysis to Spin Electronics

“…7 Furthermore, they synthesized an electronically stabilized phenalenyl radical (Chart 2, IV) with six methoxy groups introduced at all the α-carbon atoms functioning as a novel quantum spin simulator. 8 Nakasuji and coworkers designed a tribenzodecacyclenyl (Chart 2, V) with extended conjugation containing three phenalenyl units, which behaved as a six-stage amphoteric redox compound. 9 Kim, Wu, and co-workers reported a Ni-phenalenyl-fused porphyrinbiradical (Chart 2, VI).…”

Transition-Metal-Free Catalytic Carboalkoxylation of Styrenes at Room Temperature

“…In recent years, we adopted a strategy to avoid transitionmetal catalysts utilizing an odd alternant hydrocarbon, phenalenyl, which takes advantage of a readily accessible nonbonding molecular orbital. 17 Earlier, relatively stable phenalenyl radicals with singly occupied molecular orbital (SOMO) generated by the reduction of phenalenyl cation have been utilized to design various smart materials by research groups of Haddon, 18,19 Takui, 20 Kubo, 21 Morita, 22 and coworkers. The major advantage of phenalenyl-based radical as a catalyst has been argued and demonstrated in a few synthetically important chemical transformations, and the topic of using phenalenyl radical in catalysis has been recently reviewed.…”