Transcript Solvent Dependence of the Driving Force of Charge Transfer

Benjamin G. Steyer, Antonio S. Contreras, Duoduo Bao,
and Valentine I. Vullev
Department of Bioengineering
University of California, Riverside
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Introduction to Vullev Group
Photoinduced charge transfer and its
importance in photovoltaic devices
Charge transfer estimation and possible
sources of error in its calculation
Isolate and investigate of two sources of error
in the calculation of charge transfer driving
force
Discuss the results of our experiments and
future directions for our work
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Microfluidics
Biosensing
Surface Chemistry
Charge Transfer
◦ Charge Transfer in Biomimetic and Bioinspired
Systems.
E
LUMO
E
ΔG
D*
D
Locally excited
(LE) state
et
A
HOMO
Rehm-Weller Equation
LUMO
D+

A–
HOMO
Charge
transfer (CT)
state
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Better understanding of fundamental principles of
charge transfer estimation
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Isolation of several factors that may cause significant
error in the estimation of charge transfer driving force.
◦ Solvent dependence (Wan Jiandi, et al)
Supporting electolyte concentration in determination of
standard oxidation and reduction potentials (CV
measurements)
Solvent dependence with respect to size of redox
chromophore
Wan, J. et al. Solvent dependence of the charge-transfer properties of a quaterthiopheneanthraquinone dyad. Journal of Photochemistry and Photobiology. Feb 8, 2008.
Estimation of Charge Transfer Driving Force
Rehm-Weller Equation
•Where
and
are the standard oxidation and reduction
potentials for the donor and the acceptor.
•Eis the zero-to-zero energy of the principal chromophore.
•ΔGs and W are, respectively, the Born and Coulombic correction terms.
Born Correction Term
•εD and εA are the dielectric constants of the solutions in which donor and
acceptor redox potentials were measured.
•ε is the dielectric constant of the media for which ΔGet is calculated and the
spectroscopic measurements are conducted.
Redox Properties of Ferrocene
Methods
Cyclic voltammetry (CV) to determine the oneelectron redox potentials of donor and acceptor
species.
Procedure
Ferrocene was chosen as a redox probe because of
its well defined one-electron oxidation to a
ferrocenium ion, and the relative stability of the ion.
Three organic solvents with different polarities were
chosen (dichloromethane, acetonitrile,
dimethylformamide)
CV measurements were taken of ferrocene in the
three solvent media with supporting electrolyte
concentrations of 1mM to 500mM
http://www.gamry.com/Products/DrBobsCell.htm
Ferrocene
Results
Cyclic voltammograms for ferrocene (5 mM) in the presence
of various concentrations of supporting electrolyte, TBATFB,
for different solvents: (a) dichloromethane, (b) acetonitrile
and (c) dimethylformamide.
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Ferrocene’s oxidation potential can be reliably approximated to its
half-wave potential, defined as the midpoint between the values
of the potentials corresponding to the anodic and the cathodic
peak in the cyclic voltammograms.
For each of the solvent media, an increase in the concentration of
the electrolyte from 1 mM to 500 mM resulted in considerable
shifts of the anodic peaks to less positive values.
Results
 For all three solvent media, the increase in the
TBATFB concentration shifted the oxidation
potential toward more negative values.
 This electrolyte-induced effect was most
pronounced for the least-polar of the three
solvent, CH2Cl2
Dependence of the half-wave oxidation potential of
ferrocene,
, on the concentration of the
supporting electrolyte, CTBATFB, for three different
solvents.
N-phenyl-4-dimethylamino-1,8-napthlimide (ANI-A) was used to estimate
the dielectric constants of the dichloromethane solutions of the supporting
electrolyte (TBATFB).
εD
Born Correction Term
εA
Results
Solvatochromism of AIN-A. Normalized
fluorescence spectra of Ph-ANI for different
solvents (10 μM Ph-ANI, ex = 410 nm).
Dependence of the fluorescence maximum on the
dielectric constant of the solvent: chloroform
(CHCl3), dichloromethane (CH2Cl2), benzonitrile
(PhCN), acetonitrile (MeCN) and dimethylsulfoxide
(DMSO).
Dielectric Properties of CH2Cl2 Electrolyte Solutions
CTBATFB /
mM
0
1
2
5
10
20
50
100
200
500
ε
8.93
9.23
9.26
9.36
9.77
10.5
12.4
14.1
18.0
24.2
Dielectric constants, ε, of CH2Cl2
solutions containing TBATFB with different
concentrations of a CTBATFB
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Dependence of the dielectric constant of
the electrolyte solutions, on the electrolyte
concentration, CTBATFB, presented
logarithmically.
The increase in the electrolyte concentration causes close to
a three-fold increase in the dielectric constant of the CH2Cl2
solutions.
Conclusions
 Dependence of redox potentials on
the concentration of supporting
electrolyte is significant for solutions
composed of non-polar solutions
(i.e. dichloromethane).
 This contributes a significant source
of error in the overall calculation of
the overall charge transfer driving
force.
Dependence of the half-wave oxidation
potential of ferrocene on the concentration of
the supporting electrolyte. The exponential
data fits were performed for the concentration
region between 20 mM and 500 mM TBATFB.
 Redox measurements conducted in
polar solvents (i.e. acetonitrile and
dimethylformamide), using
approximations of the dielectric
constants as those of the neat
solvents do not contribute a large
source of error to the calculation of
the charge transfer driving force.
D
A
e–
We predict that a smaller size
chromophore will have less
dependence on media polarity
because there will be less surface
area for solvent molecules to
impede charge transfer
D+.
A–.
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Synthesize chromophores with different sizes.
◦ AIN-A
◦ 6-Dimethylamino-2-phenyl-benzo[de]isoquinoline-1,3-dione (AIN-B)
AIN-B
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AIN-A
Use cyclic voltammetry to determine the solvent
dependence of oxidation potentials on the size of
chromophores.
◦ CV of AIN-A and AIN-B taken at 50, 100, 200, and 500mM
TBATFB concentrations
Synthesis of (ANI-A)
Br
N
N
N
N
Br
Ar
+
NH2
O
155°C
Ar, reflux, 90
C
+
175 OC
+
+
Propionic acid
+ DME, + EtCOOH
O
O
O
CN
(b) 3-Dimethylaminopropanenitrile
(a) 4-Bromo-1,8napthalic anhydride
CN
O
O
(c)
O
O
O
O
(c)
O
N
O
(d) Aniline
(e) N-phenyl-4-dimethylamino1,8-napthlimide (ANI-A)
Synthesis of (ANI-A) was done using a two step reaction. The first
portion of the reaction requires reaction of compound (a) in solvent (b)
for 3 hours under argon atmosphere and water flux at 175°C. The
second part of the reaction requires a 1:6 molar ratio of intermediate
product (c) with compound (d) in propionic acid under argon and water
reflux at 155°C for 48 hours.
H2 O
N
O
N
AIN-A
NMR in DMSO
O
Br
N
O
CN
O
O
(b) 3-Dimethylaminopropanenitrile
(a) 4-Bromo-1,8napthalic anhydride
O
(c)
O
+
+ C6H13NH2
+
175 OC
O
Ar, reflux, 90 OC
Br
Ar
+
N
N
N
+ DME, + EtCOOH
CN O
O
O
O
N
C6H13
(c)
(e) hexylamine
Synthesis of (ANI-B) was also completed using a two
step reaction. The first portion of the reaction requires
reaction of compound (a) in solvent (b) for 3 hours under
argon atmosphere and water flux at 175°C. The second
part of the reaction requires a 1:6 molar ratio of
intermediate product (c) with solvent (e) in DME at 90°C
under argon and water reflux for 12 hours.
(AIN-B)
O
H2 O
AIN-B
Data shows expected trends
Increase in TBATFB concentration
causes anodic peaks to move
towards more positive values and
cathodic peaks to move toward
more negative values
CV taken of AIN-A at supporting electrolyte
concentrations of 50,100,200,500mM.
CV taken of AIN-B at supporting electrolyte
concentrations of 50,100,200,500mM.
The relationship between the half-wave reduction
potential and the concentration of the supporting
electrolyte, CTBATFB, for two different
chromophores with different molecular sizes
Dependence of the half-wave oxidation potential of
ferrocene,
, on the concentration of the
supporting electrolyte, CTBATFB, for three different
solvents.
 For all three solvent media, the increase in the TBATFB concentration
shifted the oxidation potential toward more negative values.
 The size difference between the two chromophores shows that AIN-B, the
smaller of the chromophores, has less dependence on salt on changes in
salt concentration. More data is needed to confirm this result.
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More data needs to be collected to examine the relationship
between size and solvent dependency.
Synthesize larger chromophores
Perylene Derivatives
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Implement knowledge in the engineering of novel redox
chromophores with application in more efficient photovoltaic
devices.
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Special thanks to Duoduo Bao, Antonio Contreras,
Alex Gerasimenko Dr. Vullev, as well as Jun Wang
and the BRITE Program