The air-tolerant singlet biradical 1,3-diphosphacyclobutane-2,4-diyl is a strong, reversibly oxidizable phosphorus-heterocyclic electron donor, and catenating two or three of these units through non-conjugative spacers yields bis- and tris(biradicals) whose split, stepwise oxidations reveal a through-space electrostatic interaction between the biradical chromophores. The 2013 study that established this (Ito, Kobayashi, Mikami, Org. Lett. 2013, 15, 3404) left the detailed nature of that interaction unresolved: the mixed-valent radical-cation intermediates implied by the two-step oxidations have never been isolated or fully characterized spectroscopically or structurally — the authors' own preliminary chemical-oxidation UV/Vis and spectroelectrochemical measurements on the bis(biradicals) were left unanalyzed; the origin of the anomalously strong interaction transmitted by the diphenyl ether-4,4'-diyl spacer of compound 6 (ΔEox = 134 mV, essentially equal to the more extended biphenyl spacer of compound 3 at 136 mV) is unexplained; and the linear tricatenated compound 12 shows only two of the expected three oxidation waves for reasons that were never clarified. No subsequent work (2013-2025) has resolved any of the three core questions. The problem — characterize the mixed-valent oxidized states, the spacer dependence, and the multi-site redox structure of catenated 1,3-diphosphacyclobutane-2,4-diyl biradicals, with electro-functional materials as the motivating application — therefore remains open.

Background

A biradical (diradical) is a molecule whose two frontier electrons occupy two nearly degenerate molecular orbitals; when the two radical centers coexist in one molecule with weak mutual interaction, the species is called a singlet biradical or biradicaloid. The 1,3-diphosphacyclobutane-2,4-diyl is a four-membered P2C2 ring whose radical centers sit at the two carbon atoms; with bulky 2,4,6-tri-tert-butylphenyl (Mes*) groups on the ring phosphorus atoms it is one of the rare organic-type singlet biradicals that is isolable and air-tolerant, and it behaves as a strong, reversibly oxidizable electron donor.

Catenation — covalently linking several of these redox-active units into one molecule through spacer groups — was first achieved by Ito, Miura, Morita, Yoshifuji and Arduengo (Angew. Chem. Int. Ed. 2008; Z. Anorg. Allg. Chem. 2009; Heteroat. Chem. 2010) with xylyl and benzyl-type spacers. In cyclic voltammetry (CV), two interacting, identical redox sites in one molecule are oxidized at two distinct potentials; the separation ΔEox between the first and second half-wave oxidation potentials converts through the Nernst relation into a comproportionation constant Kc that measures the thermodynamic stabilization of the one-electron-oxidized state. That state is a mixed-valent radical cation (in the Robin-Day classification, one unit oxidized to a cation while the other remains neutral, as in classical intervalence compounds, whose diagnostic signature is a low-energy charge-transfer absorption arising from photoinduced electron transfer between the two sites). The earlier xylyl-spaced systems showed stepwise oxidations, but their methylene (CH2) linkers are directly bonded to the ring phosphorus atoms, so the observed communication could not be cleanly separated from homoconjugation — through-space interaction mediated by the P-CH2 sigma bonds.

To isolate a genuinely through-space interaction, Ito, Kobayashi and Mikami (Org. Lett. 2013, 15, 3404; the source of this problem) catenated two 1,3-diphosphacyclobutane-2,4-diyl units across spacers that cannot sustain sigma conjugation: biphenyl (twisted about the central bond; compounds 3-5 for the 4,4'-, 3,3'- and 2,2'-diyl isomers), diphenyl ether (compounds 6 and 7) and diphenylmethane (compound 8), with one further bis(biradical) (compound 9) completing the series. X-ray crystallography and density functional theory (DFT) geometries gave the distances between the centroids of the two P2C2 rings, and CV in dichloromethane showed two reversible one-electron oxidations for compounds 3, 4, 6, 7 and 8 with ΔEox = 136, 96, 134, 102 and 84 mV (Kc = 200, 42, 185, 53 and 26), from which the authors concluded that the through-space electrostatic interaction between the electron-donating units is operational when the ring-centroid separation is within roughly 1 nm. Two linear tricatenated tris(biradicals), 11 (terphenyl spacer, negligible interaction) and 12 (a methyl-tolyloxy-benzene-based diaryl ether-type linear spacer), were also prepared; 12 showed only two oxidation waves (ΔEox = 78 mV).

The paper left the interaction's detailed nature unresolved. The mixed-valent radical cations implied by the two-step oxidations were never isolated or fully characterized: the authors performed preliminary chemical-oxidation UV/Vis and spectroelectrochemical measurements on the bis(biradicals), but their own grant report records that "detailed analysis is a future task" and that under electrode oxidation almost no interaction-dependent spectral signature was observed, so features arising from the small mixed-valent stabilization energy were hard to detect. The only EPR (electron paramagnetic resonance) observation of an oxidized species in this family remains the study of a monomeric cation radical in solution by Yoshifuji, Arduengo, Konovalova, Kispert, Kikuchi and Ito (Chem. Lett. 2006), with no structural characterization and no isolated catenated (mixed-valent) example. The origin of the unexpectedly strong interaction through the diphenyl ether spacer of compound 6 — whose ΔEox (134 mV) nearly equals that of the geometrically more extended biphenyl compound 3 (136 mV) despite the different spacer — is not understood. The electronic structure of the tricatenated 12, in which the two terminal units apparently merge into a single redox process rather than giving three distinct waves, is unclarified. Finally, the application of these catenated biradical systems in electro-functional materials (for example as p-type semiconductors in organic field-effect transistors, OFETs) remained unexplored.

Problem Statement

Determine the detailed electronic structure and the mechanism of the through-space electrostatic interaction between catenated 1,3-diphosphacyclobutane-2,4-diyl units, as left unresolved by Ito, Kobayashi and Mikami (Org. Lett. 2013, 15, 3404; DOI 10.1021/ol401456h):

  1. Mixed-valent intermediates: generate the one-electron-oxidized (mixed-valent) radical cations of at least the bis(biradicals) 3 and 6 and the tris(biradical) 12 of that paper and characterize them spectroscopically (EPR; UV/Vis-NIR including any intervalence charge-transfer band) and/or structurally (X-ray diffraction of crystalline radical-cation salts), establishing the localization of the oxidation site, the spin distribution, and the magnitude of inter-unit electronic coupling. This goes beyond the authors' own preliminary chemical-oxidation UV/Vis and spectroelectrochemical measurements on the bis(biradicals), which their FY2012 grant report records as leaving "detailed analysis" a future task, with almost no interaction-dependent spectral signature observed under electrode oxidation.

  2. Spacer dependence: explain the origin of the unexpectedly strong interaction transmitted by the diphenyl ether-4,4'-diyl spacer of compound 6, whose ΔEox (134 mV, Kc = 185) is essentially equal to that of the geometrically more extended biphenyl-4,4'-diyl spacer of compound 3 (136 mV, Kc = 200) - that is, provide a validated experimental and/or computational account of whether the interaction is purely electrostatic or contains orbital/exchange contributions, and of which spacer features (inter-chromophore distance, conformation, oxygen lone-pair polarization) control its magnitude across the paper's bis(biradical) series (compounds 3-9).

  3. Multi-site redox structure: clarify the electronic structure of the linear tricatenated tris(biradical) 12, in which only two oxidation potentials are observed rather than three - specifically whether the central unit and the pair of terminal units behave as the two distinct redox sites, and why the two terminal oxidation potentials merge.

  4. Application: on the basis of (1)-(3), demonstrate the use of such catenated biradical systems in electro-functional materials (for example as p-type semiconductors or hole-transfer materials in organic field-effect devices).

Items (1)-(3) jointly constitute the interaction problem and are mutually dependent: each asks for the same interaction's consequence at a different level (oxidized-state structure, spacer trend across the neutral series, multi-site pattern). Item (4) builds on (1)-(3), addressing the application of such catenated biradical systems in electro-functional materials.

References

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  20. Shigekazu Ito (FY2010-2012). Final research report, KAKENHI Grant 22350058, "Synthesis of unusual conducting nano-ribbons by catenating non-planar singlet biradicals"; output list includes Org. Lett. 2013, DOI 10.1021/ol401456h. https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-22350058/ (report PDF: https://kaken.nii.ac.jp/en/file/KAKENHI-PROJECT-22350058/22350058seika.pdf)
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