Toward an Air-Stable Triradical with Strong Spin Coupling: Synthesis of Substituted Truxene-5,10,15-triyl

Abstract

With the aim to achieve air-stable polyradical species manifesting strong spin coupling, synthetic endeavors are made toward triradical molecules featuring a truxene-triyl skeleton. Commonly used steric-hindering side groups such as 2,4,6trichlorophenyl and 9-anthracenyl are both found to be incompetent at stabilizing the targeted truxene triradical, which appears to be elusive from isolation and characterization. Nonetheless, singlecrystal structures of adducts formed by relevant radicals are obtained, which strongly suggests the transient existence of the designed triradicals. Finally, a truxene triradical comprising 1-anthracenyl along with two 9-anthracenyl substituents is successfully isolated and found to exhibit decent stability in air. We propose that because of the smaller dihedral angle assumed by 1-anthracenyl with respect to the plane of truxene-triyl, more effective π-conjugation allows the spin density to be more widely delocalized and distributed to the anthracenyl side groups. Thus, higher stability is gained by the triradical molecule.

A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

“…By examining the non‐bonding singly occupied molecular orbitals (SOMOs) in the optimized quartet state, pronounced non‐disjoint characters are perceivable (i.e., with significant spatial overlapping emerging across the π‐conjugated polycyclic skeleton, Figure 5b), which clearly supports the FM exchange interactions among the electron spins. Compared to the prototype truxene triradical depicting extraordinarily high spin density on the small central benzene ring, [ 34 ] the expanded π‐framework of benzotrithiophene in current triradicals modifies the spin interactions by dispersing the spin density. While maintaining the favorable equilateral triangular FM coupling motif, the mitigated coupling strength apparently accounts for the enhanced chemo‐stability of TR2 .…”

A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

“…By examining the non‐bonding singly occupied molecular orbitals (SOMOs) in the optimized quartet state, pronounced non‐disjoint characters are perceivable (i.e., with significant spatial overlapping emerging across the π‐conjugated polycyclic skeleton, Figure 5b), which clearly supports the FM exchange interactions among the electron spins. Compared to the prototype truxene triradical depicting extraordinarily high spin density on the small central benzene ring, [ 34 ] the expanded π‐framework of benzotrithiophene in current triradicals modifies the spin interactions by dispersing the spin density. While maintaining the favorable equilateral triangular FM coupling motif, the mitigated coupling strength apparently accounts for the enhanced chemo‐stability of TR2 .…”

Triindenotriphenylenes: Spin‐Frustrated Triskelion Triradicals with Excellent Ambient Stability

“…Likewise, fractions of the triradical character inferred from natural orbital occupancies at doublet multiplicity were 0.79-0.83 for all diastereomers of 1 and 2. The doubletquartet gaps ΔE DQ calculated for 1 and 2 from UDFT energy differences [25,32] take small positive values (ca. 1 kcal/mol, Table S3), in contrast to experimental findings (see below).…”

“…The chemical robustness of 1 and 2 , which permits handling under ambient conditions on a timescale of days, is remarkable for electron‐rich open‐shell organics. In fact, truxene‐5,10,15‐triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups [25] …”

“…In fact, truxene-5,10,15triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups. [25] Single crystals of 1 and 2 were obtained by vapor diffusion of MeOH into solutions in toluene and benzene, respectively, and analyzed using X-ray diffraction. While the molecular structures of 1 and 2 are chiral, both compounds crystallized as racemates in the orthorhombic Pbca space group.…”

Benzo‐Extended Cyclohepta[def]fluorene Derivatives with Very Low‐Lying Triplet States

“…It is worth to mention that 1 – 3 can be easily decomposed by protic solvent, such as methanol and ethanol. For instance, by adding methanol to the CH 2 Cl 2 solution of 1 – 3 , the peak of product [P] + disappeared while a new peak of [P+MeO] + appeared (Figure S61–S63), as monitored by MALDI‐TOF MS, which provide an indirect proof for their open‐shell nature [24] …”

A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

“…By examining the non‐bonding singly occupied molecular orbitals (SOMOs) in the optimized quartet state, pronounced non‐disjoint characters are perceivable (i.e., with significant spatial overlapping emerging across the π‐conjugated polycyclic skeleton, Figure 5b), which clearly supports the FM exchange interactions among the electron spins. Compared to the prototype truxene triradical depicting extraordinarily high spin density on the small central benzene ring, [ 34 ] the expanded π‐framework of benzotrithiophene in current triradicals modifies the spin interactions by dispersing the spin density. While maintaining the favorable equilateral triangular FM coupling motif, the mitigated coupling strength apparently accounts for the enhanced chemo‐stability of TR2 .…”

Triindenotriphenylenes: Spin‐Frustrated Triskelion Triradicals with Excellent Ambient Stability

“…Likewise, fractions of the triradical character inferred from natural orbital occupancies at doublet multiplicity were 0.79-0.83 for all diastereomers of 1 and 2. The doubletquartet gaps ΔE DQ calculated for 1 and 2 from UDFT energy differences [25,32] take small positive values (ca. 1 kcal/mol, Table S3), in contrast to experimental findings (see below).…”

“…The chemical robustness of 1 and 2 , which permits handling under ambient conditions on a timescale of days, is remarkable for electron‐rich open‐shell organics. In fact, truxene‐5,10,15‐triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups [25] …”

“…In fact, truxene-5,10,15triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups. [25] Single crystals of 1 and 2 were obtained by vapor diffusion of MeOH into solutions in toluene and benzene, respectively, and analyzed using X-ray diffraction. While the molecular structures of 1 and 2 are chiral, both compounds crystallized as racemates in the orthorhombic Pbca space group.…”

Benzo‐Extended Cyclohepta[def]fluorene Derivatives with Very Low‐Lying Triplet States

“…It is worth to mention that 1 – 3 can be easily decomposed by protic solvent, such as methanol and ethanol. For instance, by adding methanol to the CH 2 Cl 2 solution of 1 – 3 , the peak of product [P] + disappeared while a new peak of [P+MeO] + appeared (Figure S61–S63), as monitored by MALDI‐TOF MS, which provide an indirect proof for their open‐shell nature [24] …”

A Stable Triindenobenzotrithiophene Triradical with Equilateral Ferromagnetic Spin Coupling

“…By examining the non‐bonding singly occupied molecular orbitals (SOMOs) in the optimized quartet state, pronounced non‐disjoint characters are perceivable (i.e., with significant spatial overlapping emerging across the π‐conjugated polycyclic skeleton, Figure 5b), which clearly supports the FM exchange interactions among the electron spins. Compared to the prototype truxene triradical depicting extraordinarily high spin density on the small central benzene ring, [ 34 ] the expanded π‐framework of benzotrithiophene in current triradicals modifies the spin interactions by dispersing the spin density. While maintaining the favorable equilateral triangular FM coupling motif, the mitigated coupling strength apparently accounts for the enhanced chemo‐stability of TR2 .…”

Triindenotriphenylenes: Spin‐Frustrated Triskelion Triradicals with Excellent Ambient Stability

“…Likewise, fractions of the triradical character inferred from natural orbital occupancies at doublet multiplicity were 0.79-0.83 for all diastereomers of 1 and 2. The doubletquartet gaps ΔE DQ calculated for 1 and 2 from UDFT energy differences [25,32] take small positive values (ca. 1 kcal/mol, Table S3), in contrast to experimental findings (see below).…”

“…The chemical robustness of 1 and 2 , which permits handling under ambient conditions on a timescale of days, is remarkable for electron‐rich open‐shell organics. In fact, truxene‐5,10,15‐triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups [25] …”

“…In fact, truxene-5,10,15triyl, which can be seen as a lower benzologue of 1 (Scheme 1), was too unstable to be isolated even when substituted with bulky anthryl groups. [25] Single crystals of 1 and 2 were obtained by vapor diffusion of MeOH into solutions in toluene and benzene, respectively, and analyzed using X-ray diffraction. While the molecular structures of 1 and 2 are chiral, both compounds crystallized as racemates in the orthorhombic Pbca space group.…”

Benzo‐Extended Cyclohepta[def]fluorene Derivatives with Very Low‐Lying Triplet States

“…It is worth to mention that 1 – 3 can be easily decomposed by protic solvent, such as methanol and ethanol. For instance, by adding methanol to the CH 2 Cl 2 solution of 1 – 3 , the peak of product [P] + disappeared while a new peak of [P+MeO] + appeared (Figure S61–S63), as monitored by MALDI‐TOF MS, which provide an indirect proof for their open‐shell nature [24] …”