Formation of Cyclophane Macrocycles in Carbazole-Based Biradicaloids: Impact of the Dicyanomethylene Substitution Position

Abstract

We have recently demonstrated that carbazole-based biradicaloids are promising building blocks in dynamic covalent chemistry. To elucidate their intriguing dynamic covalent chemical properties, it is necessary to understand the physical origin of their biradical nature. To this end, here we focus on two quinoid carbazole systems substituted with dicyanomethylene (DCM) groups via para ( p -Cz-alkyl) or meta positions ( m -Cz-ph), which are able to form cyclophane macrocycles by the formation of long C–C bonds between the bridgehead carbon atoms linked to the DCM groups. We aim at exploring the following questions: (i) How is the biradicaloid character of a quinoid carbazole affected by the substitution position of the DCM groups? (ii) How is the stability of the resulted cyclophane aggregate attained? (iii) How is the dynamic interconversion between the carbazole-based monomers and cyclophane aggregates affected by this subtle change in the substitution pattern position? Density functional theory-based calculations reveal that both p -Cz-alkyl and m -Cz-ph are open-shell biradicals in the ground electronic state, with the DCM substitution in the meta position resulting in a more pronounced biradical character. In contrast, the derivatization via the nitrogen of the carbazole unit is not predicted to affect the biradicaloid character. The spontaneous nature of the cyclophane-based macrocycle formation (i.e., the cyclic tetramer in p -Cz-alkyl and the cyclic trimer and the tetramer in m -Cz-ph) is supported by the negative relative Gibbs free energies calculated at 298 K. Interestingly, cyclic oligomers in which the DCM groups are inserted in the meta position tend to adopt folded conformations with attractive π–π interactions resulting in more stable aggregates; in contrast, note that an extended ring-shaped conformation is acquired for ( p -Cz-alkyl)4. In addition, the larger spin density on the bridgehead carbon atom in the meta-substituted system strengthens the bridging C–C bond in the aggregate forms, hampering its dissociation. In fact, the C–C bond dissociation of ( m -Cz-ph)4 and ( m -Cz-ph)3 was suppressed in solution state, although it was achieved in solid state in response to soft external stimuli (i.e., temperature and grinding). In summary, we report a very comprehensive study aiming at elucidating the challenging chemical properties of carbazole-based biradicaloid systems.

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

“…How-ever,a na dditional, weak n(CN) feature appears after grinding at around 2224cm À1 which is attributed to the formationo f ICz-CN, as supported by DFT calculations (Figure 10 b), indicating the presence of isolated monomers. In contrast, the s-aggregates formed by the analogous carbazole-based Cz-CN systems dissociate much easier in the solid state [6,7] ;t his difference is ascribed to the longer conjugated ICz backbones enabling stronger p-p interactions between the co-facially overlapped cores in the pancake (ICz-CN) 2 aggregates. Also, note that an additional, very weak n(CN) band appearsa ta round 2189 cm À1 that might be relatedt ot he presence of open linear staircased imers, (ICz-CN) 2 -op, as revealed by DFT calculations ( Figure S19).…”

“…This is in good analogy with the well-known trend in diradical systems to form weak and long CÀCs ingle bonds in the process of cofacial coupling between the radical centres, giving rise to the formation of s-aggregates. [5,7,51] With the aim of predicting the most favourable intermolecular arrangement of the s-dimer aggregate, we performed DFT calculations for different dimer conformations [52] (Figure 2a): 1) an antiparallel arrangement of the two ICz-CN chains;a nd 2) ap arallel orientation with co-facially superimposed units, therebym aximizing the orbitalo verlap between the p-conjugated cores. [53] The calculated free energiesr evealed that ICz-CN is able to form ap ancake dimer structure resulting from attractive p-p interactions between two indolocarbazole backbones (Figures 2a and S5).…”

“…The broken equilibrium caused by the electron transfer converts the cyclophane dimer readily to the monomer anion that undergoes the second reversible reduction.R eversely,t he reoxidation of [ICz-CN] À to the neutral diradical restores the equilibrium with the dominant cyclophane dimer.I tw ould have been even more conclusive to conduct the reversible reductiono fi solated diradical ICz-CN in o-dichlorobenzene at 393 K( FiguresS12 and S13) but the experimentals etup for this experiment was not at disposal. used IR and Raman spectroscopies to probe structural effects causing any mechanochromic changes [6,7,54,58] and for the investigation of the diradical character of quinoidal molecules. [59,60] Upon application of mechanical stimuli, that is, grinding of (ICz-CN) 2 powder with am ortar to prepareaKBr pellet for IR spectroscopy (estimated maximum pressure/stressc a.…”

“…Chromo-activem aterials have attracted much interest of the scientificc ommunity in different fields as they are able to reversibly change colourw hen 1) exposed to certainl ight energies (photochromic switches), [1,2] 2) slightly deformed with small compressives train [3,4] or 3) lightly heated/cooled. [5][6][7] Reversible homolytic covalent bond cleavage/formation involving radicals peciesh as been recently demonstrated to be an efficient strategy to obtain multi-responsivec hromic soft materials. [5][6][7][8][9][10][11][12][13][14][15] Owing to this property,o rganic mono-and diradicals have emerged as essential building blocks in dynamic covalent chemistry (DCC).…”

“…Owing to this property, organic mono‐ and diradicals have emerged as essential building blocks in dynamic covalent chemistry (DCC) [16–18] . The unique feature of the open‐shell systems to form weak self‐assembled molecular complexes has earmarked organic radicals as prospective for a great number of applications such as stimuli‐responsive soft materials, [5–7] spintronics, [19] spin‐crossover materials [20, 21] or molecular self‐assembly [22] …”