Dynamic Covalent Properties of a Novel Indolo[3,2‐b]carbazole Diradical

Abstract

This work describes the synthesis and properties of ad icyanomethylene-substituted indolo[3,2-b]carbazole diradical ICz-CN. This quinoidal system dimerises almost completely to (ICz-CN) 2 ,w hich contains two long C(sp 3)À C(sp 3) s-bonds between the dicyanomethylene units. The minor open-shell ICz-CN component in the solid-state mixture was identified by EPR spectroscopy.C yclic voltammetry and UV-visible spectroelectrochemical data, as well as comparison with reference monomerI Cz-Br reveal that the nature of the one-electrono xidation of (ICz-CN) 2 at ambient temperature and ICz-CN at elevated temperature is very similar in all these compounds due to the prevailing localization of their HOMO on the ICz backbone. The peculiar cathodic behaviour reflects the coexistence of (ICz-CN) 2 and ICz-CN. The involvement of the dicyanomethylene groupss tabilizes the close-lying LUMO and LUMO + 1o f(ICz-CN) 2 and especially ICz-CN comparedt oI Cz-Br,r esulting in ad istinctive cathodic response at low overpotentials. Differently from neutralI Cz-CN, its radicala nion and dianion are remarkably stable under ambient conditions. The UV/Vis(-NIR) electronic transitions in parent (ICz-CN) 2 and ICz-CN and their different redox forms have been assigned convincingly with the aid of TD-DFT calculations. The s-bond in neutral(ICz-CN) 2 is cleaved in solution and in the solid-state upon soft external stimuli (temperature, pressure), showing as trong chromism from light yellow to blue-green. Notably,i nt he solid state, the monomeric diradical species is predominantly formed under high hydrostatic pressure (> 1GPa).

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals

“…Free radicals that exist in equilibrium with dimers are useful building blocks for stimuli-responsive materials, − spin crossover materials, , and dynamic covalent assemblies. ,,− Such metastable radicals dimerize to form either weak σ dimers or π dimers (pimers). − See Figure . While σ dimerization is a standard bonding motif, the radical–radical bond within π dimers features a multicenter covalent bonding pattern that results from head-on π orbital overlap that results in the atoms being closer than van der Waals distance but much longer than a traditional two-atom bond. − …”

Tuning the Diradical Character of Indolocarbazoles: Impact of Structural Isomerism and Substitution Position

“…To the best of our knowledge, our recent work on the 3,9-dicyanomethylene-indolo[3,2- b ]carbazole [named p -32b-ICz in Figure 1 (see also Figure S1 )] is the only study to date that has focused on ICz-based diradicaloids. 58 Due to the large diradical character and interesting electronic properties found for p -32b-ICz , here we extend our study to the influence of structural isomerism in ICz-based diradicals.…”

“…In recent works, the investigation of structural isomerism of carbon-based diradical systems has provided a reliable assessment of how the small geometric changes impact the properties of the system due to the alterations in the conjugation backbone. − However, nitrogen-centered singlet diradicals have seldom been explored because of synthetic challenges. − In fact, while different reports dealt with the investigation of indenofluorene-based diradicals, ,,, their nitrogen-centered indolocarbazole (ICz) analogues have been scarcely investigated. To the best of our knowledge, our recent work on the 3,9-dicyanomethylene-indolo­[3,2- b ]­carbazole [named p -32b-ICz in Figure (see also Figure S1)] is the only study to date that has focused on ICz-based diradicaloids . Due to the large diradical character and interesting electronic properties found for p -32b-ICz , here we extend our study to the influence of structural isomerism in ICz-based diradicals.…”

Electro-optical π-radicals: design advances, applications and future perspectives

“…97 Other dicyanomethyl substituted structures like carbazole and indolo[3,2-b]carbazole diradicals undergo reversible formation/dissociation of s-bonded cyclic aggregates (s-tetramers and s-dimers) in a similar fashion on cooling and heating, respectively. Their special feature is that the dissociation can be also achieved mechanically in the solid state by applying pressure in the few GPa range, as studied by Delgado et al [142][143][144] Otsuka et al 145 tethered tetraarylsuccinonitrile in a polystyrene chain to suppress the recombination of its dissociated radical form. Mechanical grinding of the solid sample generated a pair of dicyanomethyl radicals (dissociation ratio was approximately 5%) that were stable in air and showed unusually high-energy emission in the yellow spectral region.…”