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.

Cyclohepta[def]fluorene as a bistable molecule: first principles studies on its electronic structure and the effects of benzo-extension, substitution and solvation

“…Successful synthesis and isolation of kinetically protected derivatives of 1 was accomplished by Horii et al and the ground states were reported as singlet states in all cases that were examined. 21,22 Benzo-extended derivatives of 1 were also reported 23,24 with the work of Lombardi et al 23 showing that the considered derivatives have singlet ground states and very low-lying triplet states with singlet triplet gaps as small as 0.002 kcal mol À1 .…”

Towards the Tetrabenzo-Fused Circumazulene via In-Solution and On-Surface Synthesis

“…As shown in Scheme 1a , 4-chlorophenanthrene ( 2 ) was first synthesized through multi-step procedures in our previous work. 32 , 33 Subsequently, Suzuki coupling of compound 2 with 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)isophthalaldehyde gave 2-(phenanthren-4-yl)isophthalaldehyde ( 3 ) in 34% yield. Then, the oxidation of dialdehyde 3 by potassium permanganate (KMnO 4 ) gave the 2-(phenanthren-4-yl)isophthalic acid ( 4 ) in a quantitative yield.…”

“…27 – 32 Our synthetic approaches include both solution-based and on-surface synthesis routes, utilizing predesigned precursors based on the indeno[6,7,1,2- ghij ]pleiadene core. 33 , 34 In the context of in-solution synthesis, we successfully achieved a structure in an isomeric form with the correct molecular weight. Unfortunately, this isomeric structure, while potentially containing our target, poses challenges in terms of purification.…”