In the experiments detailed in this paper we have used 7,7,8,8-
tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF)
as model compounds in order that the radical products formed
following electron transfer at the water/ 1,2-dichloroethane (DCE)
interface could be monitored by in situ EPR spectroscopy so that
the performance of the liquid/ liquid EPR cell could be quantified.
TCNQ and TTF can be reduced or oxidized respectively by one
-•
+•
electron to form stable anion (TCNQ ) or cation (TTF ) radicals.
The behavior of quinones and semiquinones at the oil/ water
interface is of particular interest because their redox behavior is
extremely sensitive to their solution environment. In protic
solvents, quinones generally undergo a two- electron-two-proton
Fig u re 1 . Structures of the organic-phase molecules.
2
reduction to the associated hydroquinone (QH ), whereas two
successive one-electron reductions (to the semiquinone radical
anion, Q-• and the dianion, Q2-) are seen both in aprotic solvents
and in protic media under basic conditions (pH > 2).13 Quinones
are also of interest because of their widespread occurrence in a
variety of biological electron-transfer processes, for example, in
the mitochondrial chain.14 Spectroscopic methods, such as EPR15
and resonance Raman techniques,16 have been applied to ubiquino-
ne molecules in attempts to rationalize the redox chemistry in
terms of the molecular environment. For example, ubiquinones
in Rhodobacter sphaeroides photosynthetic reaction centers form
either the corresponding semiquinone radical anion or the
hydroquinone, depending on their position within the protein.17
However, the voltammetric behavior of a quinone dissolved
in an oil at the water interface is difficult to predict as it is possible
that two different reactions could occur: either (i) the semi-
quinone produced by one-electron reduction is immediately
protonated at the water interface or (ii) the semiquinone is not
protonated and is able to diffuse from the interface into the bulk
oil phase. Electrochemical and EPR experiments allow the
discrimination between the two mechanistic pathways by measur-
ing the number of electrons transferred during the reduction of
the quinone in the former case, and in the latter case, by
measuring the hyperfine structure and intensity of the EPR signal
of the reduced product(s). Unfortunately, many biologically
important quinones are reduced at potentials outside the range
accessible by liquid/ liquid electrochemistry due to the interfer-
ence of ion-transfer processes at very negative or very positive
potentials. Therefore, to mimic the liquid/ liquid redox behavior
of a biological quinone dissolved in a fat at the water interface,
we have chosen 2,3,5,6-tetrachloro-p-benzoquinone (TCBQ) and
2
,3,5,6-tetrafluoro-p-benzoquinone (TFBQ) as model compounds
dissolved in DCE. Both TCBQ and TFBQ are suitable for
electron-transfer studies at the ITIES because they partition
preferentially to the oil phase (DCE), are easily reduced, and form
stable radical anions. TCBQ has previously been used as an
oxidant of ascorbic acid at the ITIES,18 while its reduction by
potassium ferrocyanide in dimethyl sulfoxide/ water mixtures has
been investigated by UV-visible absorption spectroscopy.19
EXPERIMENTAL SECTION
Chemicals and Reagents. The organic phase reagents
7,7,8,8-tetracyanoquinodimethane (98%), tetrathiafulvalene (97%),
2
,3,5,6-tetrachloro-p-benzoquinone (99%), and 2,3,5,6-tetrafluoro-
p-benzoquinone (97%) were purchased from Aldrich and used as
received (molecular structures are given in Figure 1). The organic
phase supporting electrolyte used principally for this study, bis-
(
triphenylphosphoranylidene)ammonium tetrakis(4-chlorophenyl)-
+
-
borate (BTPPA TCPB ), was prepared from bis(triphenylphos-
phoranylidene) ammonium chloride (Aldrich, 97%) and potassium
tetrakis(4-chlorophenyl)borate (Fluka, 98%) according to a litera-
ture method.20 A similar procedure was employed in the syn-
(
11) (a) Goldberg, I. R.; Bard, A. J. J. Phys. Chem. 1 9 7 4 , 78, 290. (b) Goldberg,
I. R.; Boyd, D.; Hirasawa, R.; Bard, A. J. J. Phys. Chem. 1 9 7 4 , 78, 295. (c)
Waller, A. M.; Compton, R. G. Comp. Chem. Kinet. 1 9 8 9 , 29, 297. (d)
Compton, R. G.; Dryfe, R. A. W. Prog. React. Kinet. 1 9 9 5 , 20, 245. (e)
Allendoerfer, R. D.; Martinchek, G. A.; Brukenstein, S. Anal. Chem. 1 9 7 5 ,
thesis of tetraphenylarsonium tetrakis(4-chlorophenyl)borate
+
-
(TPhAs TCPB ), from tetraphenylarsonium chloride (Aldrich,
7%) and potassium tetrakis(4-chlorophenyl)borate. Tetrabutyl-
ammonium hexafluorophosphate (n-Bu NPF ; Fluka, electrochemi-
9
4
1
1
7, 890-894. (f) Compton, R. G.; Coles, B. A. J. Electroanal. Chem. 1 9 8 3 ,
44, 87. (g) Fiedler, D. A.; Koppenol, M.; Bond, A. M. J. Electrochem. Soc.
9 9 5 , 142, 862. (h) Webster, R. D.; Bond, A. M.; Coles, B. A.; Compton, R.
4
6
cal grade) was used in several experiments to avoid problems
associated with the photoinduced oxidation of tetraphenylborate
derivatives. HPLC-grade 1,2-dichloroethane (Sigma-Aldrich), and
UHQ-grade water of a resistivity not less than 18 MΩ‚cm obtained
from an Elgastat system (High Wycombe, Bucks, U.K.), were
mutually saturated before use. Analytical-grade potassium ferro-
cyanide and potassium ferricyanide were supplied by BDH
chemicals.
G. J. Electroanal. Chem. 1 9 9 6 , 404, 303-308. (i) Webster, R. D.; Bond, A.
M.; Schmidt, T. J. Chem. Soc., Perkin Trans. 2 1 9 9 5 , 1365. (j) Compton, R.
G.; Waller, A. M. In Spectroelectrochemistry: Theory and Practice; Gale, R.
J., Ed.; Plenum Press: New York, 1988; Chapter 7.
(
(
12) Dryfe, R. A. W.; Webster, R. D.; Coles B. A.; Compton, R. G. J. Chem. Soc.,
Chem. Commun. 1 9 9 7 , 779.
13) Chambers, J. Q. In The Chemistry of Quinoid Compounds; Patai, S., Ed.;
Wiley: New York, 1974; Vol. 1, Chapter 14, p 737. Chambers, J. Q. In The
Chemistry of Quinoid Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New
York, 1988, Vol. 2, Chapter 12, p 719. Gupta, N.; Linschitz, H. J. Am. Chem.
Soc. 1 9 9 7 , 119, 6384-6391.
P rocedures for Liquid/ Liquid Interfacial Measurements.
The cell used to perform the interfacial polarization EPR experi-
(
(
(
(
14) Tyler, D. D. The Mitochondrion in Health and Disease; VCH: New York,
1
992; p 284.
15) Joela, H.; Kasa, S.; Lehtovuori, P.; Bech, M. Acta Chem. Scand. 1 9 9 7 , 51,
33.
16) Zhao, X.; Ogura, T.; Okamura, M.; Kitagawa, T. J. Am. Chem. Soc. 1 9 9 7 ,
19, 5263.
(18) Suzuki, M.; Umetani, S.; Matsui, M.; Kihara, S. J. Electroanal. Chem. 1 9 9 7 ,
2
420, 119.
(19) Girgis, M. M.; Osman, A. H.; Arifien, A. E. Indian J. Chem. 1 9 9 1 , 30A,
1
235.
17) Paddock, M. L.; McPherson, P. L.; Feher, P. M.; Okamura, M. Y. Proc. Natl.
Acad. Sci. U.S.A. 1 9 9 0 , 87, 6803.
(20) Osborne, M. D.; Shao, Y.; Pereira, C. M.; Girault, H. H. J. Electroanal. Chem.
1 9 9 4 , 364, 155.
Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 793