Towards Magnetic Carbo‐meric Molecular Materials

Abstract

Numerous studies have underlined the putative diradical character of π-conjugated molecules that can be described by closed-shell Lewis structures, for instance, p-dimethylene p-n phenylenes, or long polyacenes. In the latter compounds, the only way to save the aromaticity of the six-membered rings is to give up the Lewis electron pairing in the singlet biradical ground state. The present work considers the possibility of doing the same by using the basic C2 units of carbo-meric architectures. A series of acyclic and cyclic carbo-meric architectures is studied by using UB3LYP DFT broken-symmetry calculations, including spin decontaminations and subsequent geometry optimization of the singlet diradical. The C2 units are shown to stabilize the singlet biradical by spin delocalization, two of them playing approximately the same role as one radical-insulating 1,4 phenylene moiety. The results are generalized to the investigation of open-shell polyradical singlet states of rigid hydrocarbon structures, the symmetry and rigidity of which can assist cooperativity and self spin polarization effect. Several synthesis targets with challenging magnetic/spin properties are suggested in the carbo-mer series.

Steric/π‐Electronic Insulation of the carbo‐Benzene Ring: Dramatic Effects of tert‐Butyl versus Phenyl Crowns on Geometric, Chromophoric, Redox, and Magnetic Properties

“…The 1 H NMR spectra of 6 – 8 in CDCl 3 exhibit classical features of other phenyl‐substituted carbo ‐benzenes, namely, a strong deshielding of the ortho ‐ 1 H nuclei at δ =9.5–9.6 ppm due to the diatropic ring current over the C 18 macrocycle . The t Bu 1 H nuclei of 5 – 8 also resonate at relatively low field ( δ ≈2.5 ppm; Figure ), with chemical shifts deshielded by 1 ppm with respect to those of 1,2,3,4‐ and 2,3,4,5‐tetra‐ tert ‐butylbenzene ( δ =1.27–1.49 ppm) .…”

“…Within the family of cumulene‐containing conjugated cyclic hydrocarbons, carbo ‐benzenes, display particular optical, electrical, magnetic, or mesogenic properties that can be a priori attributed to their common C 18 core, combining unique geometric and π‐electronic features. In spite of a 4 n +2 π z ‐electron count, however, the moderate energetic aromaticity of the C 18 ring (about one‐third of the C 6 ring of benzene) enables conjugation with π‐unsaturated substituents.…”

Mono‐ and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis‐1,4‐([3]Cumulene)‐p‐carboquinoid System

“…In contrast, in a recent computational study, derivatives that lacked substantial π‐accepting properties (e.g. carbene, R 2 C, R=alkyl, aryl, nitrile, acetylene) were found to form biradical species that maintained the non‐quinoidial, −C≡C−C 6 H 4 −C≡C− bonding pattern …”

“…In contrast, in arecent computational study,derivatives that lacked substantial p-accepting properties (e.g.carbene,R 2 C, R = alkyl, aryl, nitrile,acetylene) were found to form biradical species that maintained the nonquinoidial, ÀCCÀC 6 H 4 ÀCCÀ bonding pattern. [37] NHC 1 readily reacted with phenylethynyl bromide at room temperature in pentane to give the salt 2[Br] in reasonable yield (57 %) as at an-colored powder (Scheme 1). However, the reaction of NHC 1 with 1,4bis(bromoethynyl)benzenedid not go to completion at room temperature (Scheme 1).…”

Do Diradicals Behave Like Radicals?

“…Back in 1947, Coulson and co-workers estimated the singlet–triplet gap for this molecule to be around 8–9 kcal/mol . However, calculations performed in the context of the current study at a CASPT2/cc-pVTZ level (as well as by others) , lead to the much bigger gap cited. An experimental value is not available due to the extremely high reactivity of the lowest triplet state, which has inhibited its characterization up to today …”