Solvent Dependence of the Driving Force of Charge Transfer on the Size of

the Electron Donor and Acceptor

Benjamin G. Steyer, Antonio S. Contreras, Duoduo Bao, and Valentine I. Vullev

Department of BioengineeringUniversity of California, Riverside

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

Outline

Microfluidics Biosensing Surface Chemistry Charge Transfer

◦ Charge Transfer in Biomimetic and Bioinspired Systems.

Vullev Group

Photoinduced Electron Transfer

E LUMO

HOMO

DA

D*

Locally excited (LE)

state

et

E LUMO

HOMO

D+

A–

Charge transfer (CT)

stateRehm-Weller Equation

ΔG

Better understanding of fundamental principles of charge transfer estimation

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

Purpose

Wan, J. et al. Solvent dependence of the charge-transfer properties of a quaterthiophene-anthraquinone dyad. Journal of Photochemistry and Photobiology. Feb 8, 2008.

Rehm-Weller Equation

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.

•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.

Estimation of Charge Transfer Driving Force

Redox Properties of Ferrocene

Electolyte Concentration as a Source of Error

http://www.gamry.com/Products/DrBobsCell.htm

Cyclic voltammetry (CV) to determine the one-electron redox potentials of donor and acceptor species.

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

Procedure

Methods

Ferrocene

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.

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

Dependence of the half-wave oxidation potential of ferrocene, , on the concentration of the supporting electrolyte, CTBATFB, for three different solvents.

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

Use of Chromophore to Calculate Dielectric Constants

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 εA

Born Correction Term

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 ε

0 8.931 9.232 9.265 9.36

10 9.7720 10.550 12.4

100 14.1200 18.0

500 24.2

Dielectric constants, ε, of CH2Cl2solutions containing TBATFB with different concentrations of a CTBATFB

Dependence of the dielectric constant ofthe electrolyte solutions, on the electrolyteconcentration, 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 the half-wave oxidationpotential 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.

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.

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.

Solvent Dependence on Size of Redox Chromophore

D A

e–

D+. A–.

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

Synthesize chromophores with different sizes.◦ AIN-A◦ 6-Dimethylamino-2-phenyl-benzo[de]isoquinoline-1,3-dione (AIN-B)

AIN-B 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

Procedure

Synthesis of (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.

(e) N-phenyl-4-dimethylamino- 1,8-napthlimide (ANI-A)

N

O OO

+

NH2

Ar, reflux, 90 OC

+ DME, + EtCOOH

N

N OO

+ H2O

(a) 4-Bromo-1,8-napthalic anhydride

OO O

Br

N

CN

N

O OO

Br

CN

+ +Ar

175 OC

(c)(c)(d) Aniline(b) 3-Dimethyl-

aminopropanenitrile

Propionic acid

155°C

N OO

N

AIN-A

NMR in DMSO

Synthesis of AIN-B

OO O

Br

N

CN

N

O OO

Br

CN

+ +Ar

175 OC

N

O OO

+Ar, reflux, 90 OC

+ DME, + EtCOOH

N

N OO

C6H13

+ H2OC6H13NH2

(c) (e) hexylamine (AIN-B)

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.

(a) 4-Bromo-1,8-napthalic anhydride

(c)(b) 3-Dimethyl-

aminopropanenitrile

AIN-B

Results

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.

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

Conclusions

Dependence of the half-wave oxidation potential of ferrocene, , on the concentration of the supporting electrolyte, CTBATFB, for three different solvents.

The relationship between the half-wave reduction potential and the concentration of the supporting electrolyte, CTBATFB, for two different chromophores with different molecular sizes

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.

More data needs to be collected to examine the relationship between size and solvent dependency.

Synthesize larger chromophores

Implement knowledge in the engineering of novel redox chromophores with application in more efficient photovoltaic devices.

Future Directions

Perylene Derivatives

Acknowledgements Special thanks to Duoduo Bao, Antonio Contreras,

Alex Gerasimenko Dr. Vullev, as well as Jun Wang and the BRITE Program