Energetics of Charge Separation in Medium Polarity Solvents
Brian Albert1, Juan Carlos Alicea2, Andrew R. Cook3, Kate Dorst2, John R. Miller3, Lori Zaikowski2
1Columbia University, 2Dowling College, 3Brookhaven National Lab, Chemistry Department
Introduction
Energetics of charge separation in highly polar media containing
electrolyte are accurately determined from the differences of readily measured
redox potentials. However, energetics change in less polar media due to
changes in solvation energies, and can be crudely estimated using the Born
equation or computational chemistry techniques, but are not readily measured.
In order to provide accurate energies for charge separation reactions in specific
systems and calibrations for computational chemistry techniques, this project
examined the reduction of quinones in the presence and absence of an
electrolyte and as a function of solvent polarity. Reactions were studied via
UV/VIS spectroscopy and conductivity.
Conductivity Results
E1/2 (mV/SCE)a, b
BQ = -400
F4BQ = +100
Cl4BQ = +140
Quinone (A)
Metallocene (D)
Electron acceptor (A) and electron donor (D)
K1
O
O
O
Cl
O
O
1
2
O
O
O
4
3
(A-, D+)
A- + D+
neutral molecules
ion pair
free ions
N
CN
O
Spectroscopic Results
2.0
N
O
A+D
5
CN
O
K2
Quinone anion absorbances shown as a function
of cobaltocene to quinone ratio. MeCN solvent.
Fluorenone and phenazine spectra indicated
charge transfer complexes but no anion formation.
BQ and ClBQ showed a decrease in anion
absorbance after 1 equivalent.
Absorbance
O
O
Cocp2 = -900
Fecp*2 = -80
BQ- + Cocp2+
BQ + Cocp2
MeCN
1.5
BQ-
1.0
Dissociation of quinone-metallocene ion pairs was measured by
continuous conductivity and comparison with spectra. With a decrease in
solvent polarity, lower conductivity was observed than expected based on
spectroscopic anion absorbance.
Conductivity of BQ, F4BQ, and Cl4BQ titrated with Cocp2 and Fecp*2 in
acetonitrile indicates complete ion dissociation. However, in THF the
reactions yielded only a fraction of free ions.
Molar conductivity of 167 uM (F4BQ) with 1 eq. Cocp2 indicates:
in THF: K1 for formation of ion pairs = 4.26 x 104 (∆Gº = -26.4 kJ/mol)
and K2 for dissociation into free ions = 2.7 x 10 -7 (11 kJ/mol). Hence
75% of reactants form ion pairs, but only 3.28% separate into free ions.
O
F
F
F
F
O
O
Cl
Cl
Cl
Cl
O
0.5
0.0
6
7
8
300
350
400
450
Benzoquinone 1 (BQ), chlorobenzoquinone 2 (ClBQ), tetramethylbenzoquinone
3 (Me4BQ), fluorenone 4, anthraquinone 5 (AQ), ethylanthraquinone 6 (EtAQ),
dicyanoanthracene 7 An(CN)2, phenazine 8.
Quinone
Eight quinones (Fig. 1) were titrated with cobaltocene Cocp2 as a reducing
agent in acetonitrile (e = 38) or THF (e = 7.6) with and without the electrolyte
tetrabutylammonium tetrafluoro-borate (TBABF4).
100 uM quinone solutions were titrated in a dry Argon atmosphere in 1 cm
path length quartz cuvettes with Cocp2 solutions of 3-10mM. Continuous wave
spectra from 200-900 nm were recorded with an OceanOptics spectrometer.
Anion peaks were identified by comparision with literature spectra of the
anions obtained electrochemically.
Conductivity Methods
Acknowledgments
Dr. Sean McIlroy for his advice and help around the lab.
Staff of Office of Educational Programs and BNL Chemistry Dept.
NSF Award #03-35799 and NSF Supplemental Funding for Faculty and
Student Team JCA and LZ. DOE DE-AC02-98-CH10886 supported AC, KD, JM.
Electrolyte
1mM
TBABF4
Spectroscopic Methods
To calculate K2 the ratio of free ions to ion pairs was determined by
conductivity with a Scientifica 645 meter. While ion pairs and anions have
similar absorption spectra, only free ions conduct.
20, 60, and 200 uM solutions of quinone in acetonitrile or THF were
titrated with solutions of Cocp2 and decamethylferrocene (FeCp*2)
Energetics
Wavelength (nm)
Conclusions
For moderately unfavorable reactions - AQ, EtAQ, and AN(CN)2 - electrolyte reduces the energy
of forming ions in acetonitrile. For highly unfavorable reactions – fluorenone and phenazine - and favorable reactions – BQ, ClBQ, and Me4BQ – electrolyte does not affect the energy of forming ions.
Charge transfer complexes are observed, but only when no ions form.
Decrease in conductivity at lower solvent polarity indicates that fewer free ions are formed. This
may be due to fewer ions being formed and/or that ion pairs are favored.
Future investigations include spectroscopic experiments on these molecules in less polar solvents
such as THF and butyl ether, and expanding the conductivity experiments to include the molecules
studied spectroscopically.
Examination of energetics of electron transfer in medium and low polarity solvents for quinonemetallocene redox pairs should enable generalizations to be made about such energies for other
molecules as a function of solvent polarity.
max
nm
Keq, calc
ecalc
10-3
cm-1M-1
5.4
5.1
3.8
3.7
5.2
5.1
7.7
6.9
8.9
7.8
4.9
3.0
11.1
11.1
10.8
10.8
7.7
7.7
8.5
8.5
Gcalc
-Eº vs
SCEa
Gd
mV
meV
meV
450
> 200
< -136
No
423
> 200
< -136
400b
-500
BQ
450
> 150
< -128
Yes
423
> 150
<-128
447
> 200
< -136
No
423
> 200
< -136
ClBQ
230e
-670
424
>5
< - 41
Yes
448
> 15
< -70
No
435
10.0
-59.3
Me4BQ
738b
-162
Yes
435
9.78
-58.6
No
550
< 1.6 x 10-3
> 166
Fluorenone
1220c
320
-3
Yes
549
< 1.6 x 10
> 166
No
546
0.28
32.7
AQ
816b, 800 c -84, -100
Yes
545
0.75
7.39
No
542
3.3 x 10-2
87.6
b
EtAQ
829
-71
Yes
543
6.6 x 10-1
10.4
No
704
7.2 x 10-2
67.7
An(CN)2
1015c
115
-1
Yes
705
3.6 x 10
26.1
No
543
< 1.1 x 10-4
> 234
Phenazine
1090c
190
-4
Yes
542
< 3.2 x 10
> 207
a Measured in DMF with TBABF
4
b Prince, R.C., Gunner, M.R., and Dutton, P.L. 1982. Quinones of Value to Electron-Transfer Studies:
Oxidation-Reduction Potentials of the First Reduction Step in an Aprotic Solvent. In Function of Quinones
in Energy Conserving Systems (B.L. Trumpower, ed.) Academic Press, New York, 29-33.
c Pedersen S. U.; Christensen T. B.; Thomasen T.; Daasbjerg K. 1998. J. Electroanal. Chem. 454, 123.
d Calculated from Eº(Cocp ) vs SCE = -900 mV. Bard AJ; Garcia E. 1993. Inorg. Chem. 32, 3528.
2
e Peover, M.E. and Davies, J.D. 1964. Trans. Faraday Soc. 60, 476.
Extinction coefficients e determined from fits of anion absorbance data.