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

Abstract

Open‐shell non‐alternant polycyclic hydrocarbons (PHs) are attracting increasing attention due to their promising applications in organic spintronics and quantum computing. Herein we report the synthesis of three cyclohepta[def]fluorene‐based diradicaloids (1–3), by fusion of benzo rings on its periphery for the thermodynamic stabilization, as evidenced by multiple characterization techniques. Remarkably, all of them display a very narrow optical energy gap (Egopt=0.52–0.69 eV) and persistent stability under ambient conditions (t1/2=11.7–33.3 h). More importantly, this new type of diradicaloids possess a low‐lying triplet state with an extremely small singlet–triplet energy gap, as low as 0.002 kcal mol−1, with a clear dependence on the molecular size. This family of compounds thus offers a new route to create non‐alternant open‐shell PHs with high‐spin ground states, and opens up novel possibilities and insights into understanding the structure–property relationships.

Synthesis of Defective Nanographenes Containing Joined Pentagons and Heptagons

“…MesMgBr: mesitylmagnesium bromide; Pd(PPh 3 ) 2 Cl 2 : bis(triphenylphosphine)palladium(II) chloride; (Bpin) 2 : bis(pinacolato)diboron; SPhos: 2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-biphenyl; K 3 PO 4 : potassium phosphate tribasic; Pd 2 (dba) 3 : tris(dibenzylideneacetone)dipalladium(0). [100] Scheme 20. Synthesis of nonalternant nanographenes containing azulene units by alkyne-based cyclization: a) compound 123, b) compounds 126 and 127, and c) compound 132.…”

“…Very recently, three benzo‐extended cyclohepta[ def ]fluorene derivatives ( 112 , 117 , and 121 in Scheme 19 ) exhibiting open‐shell singlet ground state were reported by Feng et al. [ 100 ] In this work, 5–7 pair was constructed in the latter stage through F—C alkylation followed by dehydrogenation. To this end, key dialdehyde intermediates ( 110 , 115 , and 119 ) were synthesized by Suzuki coupling in a yield of 81%, 34%, and 70%, respectively.…”

“…MesMgBr: mesitylmagnesium bromide; Pd(PPh 3 ) 2 Cl 2 : bis(triphenylphosphine)palladium(II) chloride; (Bpin) 2 : bis(pinacolato)diboron; SPhos: 2‐dicyclohexylphosphino‐2′,6′‐dimethoxy‐1,1′‐biphenyl; K 3 PO 4 : potassium phosphate tribasic; Pd 2 (dba) 3 : tris(dibenzylideneacetone)dipalladium(0). [ 100 ]…”

6,6′-Biindeno[1,2-b]anthracene: An Open-Shell Biaryl with High Diradical Character

“…30 Moreover, benzo-fused cyclohepta[def ]-fluorenes were separately reported in the same year, demonstrating singlet ground states with very low-lying triplet states. 31 On the other hand, diradical III consisting of [2.2]paracyclophane substituted with two fluorenyl radicals was synthesized, and an open-shell triplet ground state via through-space spin−spin coupling was proposed, while an analogous structure with orbital overlaps between fluorenyl radicals displayed a singlet ground state. 32 For further development of PCH diradicals, explorations of novel structural motifs is of considerable importance.…”

Synthesis and Isolation of a Kekulé Hydrocarbon with a Triplet Ground State

“…Similarly, Ma, Bogani, and Feng et al. reported the synthesis of benzo‐fused cyclohepta[ def ]fluorenes possessing a singlet ground state with a very low‐lying triplet excited state [21] . On the basis of these previous attempts, herein we describe the synthesis and isolation of a Kekulé hydrocarbon with an experimentally confirmed triplet ground state by designing a kinetically‐stabilized derivative of bisdibenzo[3,4 : 5,6]cyclohepta[1,2‐ a : 2,1‐ d ]benzene.…”