N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

Abstract

Dynamic covalent bonds and their chemistry have been of particular interest both from a fundamental and materials science aspect. Demonstrated herein is that triphenylamine (TPA) and carbazole (Cz), substituted with a dicyanomethyl radical, are useful motifs for dynamic covalent chemistry as they have the appropriate bond strength between monomer units as well as high stability and synthetic simplicity. TPA and Cz units substituted by two dicyanomethyl radicals formed macrocyclic oligomers classified as novel types of azacyclophanes, and in particular, the TPA-based diradical gave a cyclic dimer in almost quantitative yield. The cyclic oligomers exhibited thermo- and mechanochromic behavior resulting from the generation of radical species by intermonomer C-C bond cleavage.

N‐Substituted Dicyanomethylphenyl Radicals: Dynamic Covalent Properties and Formation of Stimuli‐Responsive Cyclophanes by Self‐Assembly

“…By X-ray single-crystal analysis, this compound was shown to be a cyclic dimer (3 2 ; Figure 2). [15] The two CÀC bonds between monomers were also longer (1.634 and 1.643 ) than typical C(sp 3 ) À C(sp 3 ) bonds, thus suggesting the weakness of these bonds and the possibility of the bond cleavage by an external stimuli. The phenyl rings incorporated in the macrocycle were largely twisted around the central nitrogen atoms to reduce the strain derived from its macrocyclic structure.…”

“…The TPA and Cz dimers, 1 2 and 2 2 , respectively, were prepared by the oxidation of the precursors of the corresponding radicals [5] (Schemes 1 b and c; see Scheme S1 in the Supporting Information), and their structures were unambiguously determined by X-ray single crystal analysis [15] (see Figure S3). The C À C bond between monomers were significantly longer (1.616 for 1 2 and 1.626 for 2 2 ) than usual C(sp 3 )ÀC(sp 3 ) bonds (1.54 ).…”

“…The structure of 4 3 was unequivocally determined to be a cyclic trimer by single-crystal X-ray analysis (Figure 3). [15] Interestingly, there exists two enantiomer in a crystal. One has a right-handed helicity and is regarded as a P isomer, and the other has a left-handed helicity and is regarded as a M isomer.…”

Formation of a Macrocycles‐in‐a‐Macrocycle Superstructure with All‐gauche Conformation by Reversible Radical Association

“…[6,9] Thus the TPAc ore is used here to facilitate reversible association and dissociation of radicals.Alow dissociation enthalpy of about 55 kJ mol À1 is expected for the corresponding C(sp 3 ) À C(sp 3 )b onds,a sd etermined from ad imer model compound, [6] enabling reversible conversion of the C(sp 3 )ÀC(sp 3 )b ond to radicals at room temperature. Para electron-donating substitution was found to be crucial for stabilizing aryldicyanomethyl radicals and weakening C(sp 3 )ÀC(sp 3 )bonds formed between them.…”

“…To confirm that the hexameric macrocycle 2 was not the kinetic product, the precursor 4 was treated with DDQ at low temperature (À40 8 8C), which led to precipitation of ad ark green powder (inset in Figure 1b)i na cetone.T he dark powder showed an intense ESR signal (g e = 2.0029) at room temperature that could be attributed to across-linked kinetic polymer formed by random radical-radical association, which resulted in the trapping of al arge amount of unreacted radicals,t hus the dark color.T he two samples prepared at different temperatures exhibited different temperature dependence of their ESR spectra. [6] Thesolution also show weak ESR signal with g e = 2.0031 (Figure 1d). This observation can be explained by thermal dissociation of the macrocycle 2 leading to unpaired radical species.S imilar behavior was also observed by measuring its temperature-dependent absorption spectra in the solid state (Supporting Information, Figure S3).…”