The Journal of Physical Chemistry B
Article
subsequent formation of phenoxyl radical and/or cation.36
Based on the experimental data and the literature precedence,
the proposed mechanism for the formation of TTBDQ from
compound 2 under electrochemical oxidation condition may
involve phenoxyl radical formation, as in Scheme 1.
ACKNOWLEDGMENTS
■
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund for support of this
research.
Compound 2 may form a phenoxyl radical, 2•, either via
radical cation intermediate, 2•+, in a one-electron oxidation and
proton loss, or via phenolate anion, 2−, via one-electron
oxidation (Scheme 1). Both species, parent phenol or
phenolate anion, may produce phenoxyl radicals which may
undergo further dimerization reactions via 4′-site of their
respective resonance contributor to produce dimerized product
TTBDQ. Both species, the phenolate anion and phenol, were
observed in CV of 2, and they both contributed to the
formation of TTBDQ, as evidenced by the potential range-
dependent measurements. A similar mechanism, involving a
phenoxyl radical, was proposed for the chemical formation of
TTBDQ from 2 under alkaline conditions in the presence of
O2.1,37 This transformation was ascribed to the free-radical
mechanism under thermal decomposition of nitrosophenol.
Compounds 1 and 2 reportedly also formed stable benzyl
radicals and underwent dimerization, upon chemical oxidation,
into stilbenequinone and diphenoquinone, respectively.5−7 The
formation of substituted benzyl radicals from substituted
phenols has been previously reported.5−7 It has been shown
that phenoxyl radicals, generated from phenolate anions, may
dimerize or undergo disproportionation, among other reac-
tions.10 The ionic mechanism, which would include the
condensation reaction, was not the likely route in formation
of TTBDQ.1 The electrochemical and spectroscopic data
indicated that the number of bulky substitutents and their
location play a critical role in reactivity of this class of
compounds. Subsequently, the tunability of chemical structure
and electronic effects may further expand their functional
utility.
ABBREVIATIONS
■
CV, cyclic voltammetry; SWV, square-wave voltammetry;
TBAP, tetrabutylammonium perchlorate.
REFERENCES
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4. CONCLUSIONS
The electrochemical and spectroscopic evaluation of tert-butyl-
substituted phenols revealed that most phenols were not easily
oxidized under electrochemical conditions to generate an
electrochromic product. Selective electrochemical oxidation of
2 to produce TTBDQ was associated with stability of the
phenoxyl radical intermediate and a lack of steric hindrance at
the 4′-site. By comparison, the chemical oxidation of all phenols
resulted in the significant product formation. The data indicate
that the utility of this class of compounds may be extended by
carefully tuning the structural parameters and experimental
conditions. Hence, a complete understanding of the selectivity
of electrochromism may aid in designing new functional
molecules.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
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S
(16) Chen, S.; Peng, H. M.; Webster, R. D. Infrared and UV-vis
spectra of phenoxonium cations produced during the oxidation of
phenols with structures similar to vitamin E. Electrochim. Acta 2010,
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CVs, SWVs, and UV−vis spectra (PDF)
(17) Tumer, M.; Aslantas,̧ M.; Şahin, E.; Deligonul, N. Structural
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characterization properties of the 3,3′-5,5′-tetra-tert-butyl-4,4′-diphe-
noquinone. Spectrochim. Acta, Part A 2008, 70, 477−481.
(18) Khan, M. A.; Osman, A.; Tuck, D. G. The electrochemical
synthesis and structure determination of 3,3′,5,5′-tetra-tert-butyl-1,1′-
biphenylidene-4,4′-quinone. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1986, 42, 1399−1402.
AUTHOR INFORMATION
Corresponding Author
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Notes
The authors declare no competing financial interest.
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J. Phys. Chem. B 2016, 120, 8914−8924