Evidence for Significant Through‐Space and Through‐Bond Electronic Coupling in the 1,4‐Diphenylcyclohexane‐1,4‐diyl Radical Cation Gained by Absorption Spectroscopy and DFT Calculations

Abstract

Photoinduced single-electron-transfer promoted oxidation of 2,5-diphenyl-1,5-hexadiene by using N-methylquinolinium tetrafluoroborate/biphenyl co-sensitization takes place with the formation of an intense electronic absorption band at 476 nm, which is attributed to the 1,4-diphenylcyclohexane-1,4-diyl radical cation. The absorption maximum (lambda(ob)) of this transient occurs at a longer wavelength than is expected for either the cumyl radical or the cumyl cation components. Substitution at the para positions of the phenyl groups in this radical cation by CH(3)O, CH(3), F, Cl, and Br leads to an increasingly larger redshift of lambda(ob). A comparison of the rho value, which was obtained from a Hammett plot of the electronic transition energies of the radical cations versus sigma(+), with that for the cumyl cation shows that the substituent effects on the transition energies for the 1,4-diarylcyclohexane-1,4-diyl radical cations are approximately one half of the substituent effects on the transition energies of the cumyl cation. The observed substituent-induced redshifts of lambda(ob) and the reduced sensitivity of lambda(ob) to substituent changes are in accordance with the proposal that significant through-space and -bond electronic interactions exist between the cumyl radical and the cumyl cation moieties of the 1,4-diphenylcyclohexane-1,4-diyl radical cation. This proposal gains strong support from the results of density functional theory (DFT) calculations. Moreover, the results of time-dependent DFT calculations indicate that the absorption band at 476 nm for the 1,4-diphenylcyclohexane-1,4-diyl radical cation corresponds to a SOMO-3 --> SOMO transition.

Acid Is Key to the Radical-Trapping Antioxidant Activity of Nitroxides

“…Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise indicated. TEMPO + BF 4 – , azocumene, bis­(nitroxide) 1 , phosphine 2 , and BODIPY 3 were synthesized by literature precedent. Oxidizable substrates (cumene, styrene, paraffin oil) were carefully purified from stabilizers and hydroperoxides before use and were stored refrigerated under nitrogen.…”

Directed Heterodimerization: Stereocontrolled Assembly via Solvent-Caged Unsymmetrical Diazene Fragmentation

“…The use of the mixed sulfamide 6 as the precursor to the unsymmetrical diazene 5 would provide a platform for the directed assembly of the two monomeric amines, 7 and 7′ . Implementation of this strategy in complex synthesis would require: (a) synthesis of cyclotryptamine-based mixed sulfamides, (b) mild conditions for their conversion to the corresponding unsymmetrical diazenes followed by fragmentation, (c) solvent-cage-controlled radical pair combination, and (d) minimization of out-of-cage coupling (homodimerization) and disproportionation …”

Luminescent Radicals

“…Noticeably, the transition band of 3 Ph 2 HD was red-shifted relative to that of the 2-phenylpropyl radical possessing half of the Ph 2 HD structure . The red shift could reasonably be understood in terms of efficient through-space and through-bond interactions between the two radical units in Ph 2 CHD, as similarly observed in one-electron oxidized Ph 2 CHD . The TL wavelength depended on substituents on the phenyl rings.…”