Solvatochromism and Photo-Induced Intramolecular Electron TransferKatelyn J. Billings; Bret R. Findley1

1Department of Chemistry and Physics, Saint Michael’s College, Colchester, VT 05439

CALCULATING THE CHANGE IN DIPOLE MOMENT1:

ABSTRACTOur proposed research is intended to develop a laboratory exercise for publication, suited for a physical chemistry class. The exercise centers around the study of solvatochromism, which affects both emission and absorption in intramolecular photo-induced electron transfer. The method itself involves steady state absorption and steady state fluorescence measurements and allows for the determination of the change in dipole moment between the excited and ground states of the covalently linked electron donor and acceptor. It also provides for an experimental measurement of polarity for solvents and solvent mixtures.

BACKGROUND

A —D

hυ1

A* —D Electron

Transfer A- —D+

Solvent Relaxation

A- —D+

hυ2

A —D

E

EXPERIMENTAL PROCEDURE•Take fluorescence measurements for solvatochromic dyes in a variety of solvents (with different polarities).

•Plot , the maximum emission frequency in cm-1, versus Δf, a solvent polarity parameter.

•Slope of this line ultimately yields Δμ, the difference in excited and ground state dipole moment of the solute.

O

N

3

Coumarin 153

4-amino-N-methylphthalimide

Reichardt’s Dye

Plot of vct vs D f from Steady State Fluorescence

y = -8464.7x + 22022

R2 = 0.8905

0.0015

5000.0015

10000.0015

15000.0015

20000.0015

25000.0015

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45

Df

v ct

Our Experimental Δμ : 7.05DGiven that: ρ = 3.9Ǻ

Literature values1:Theoretical Δμ = 3.9DExperimental Δμ = 6.0D

Fluorescence Spectra for Coumarin 153

**Note the Bathochromic Shift

FUTURE PLANS•Calculate the change in dipole moment for Reichardt’s Dye, as it does NOT have a non-polar ground state.•Reevaluate our data—Check Fluorimeter calibration/correction for accuracy.•Test lab and submit for publishing.

~ ~2

2

1 1

2 1 4 2

nf

n

D

2

30

2(0)

(4 )ct ct fhc

D D

Where:

•vct = frequency of maximum emission in cm-1

•vct (0)= frequency of maximum emission in the gaseous phase in cm-1 •Δμ = difference in excited and ground state dipole moment•ρ = the radius of the solute cavity•4πε0 = gas permittivity constant

•Δf = the solvent factor (change in the reaction field)

•ε = the solvent dielectric constant

•n = the refractive index

•h = Planck’s constant

•c = speed of light

~

~

COMPOUNDS STUDIED SAMPLE RESULTS

SAMPLE DATA

As one alters the polarity of the solvent, the fluorescence produced undergoes a wavelength (or color) change in a process known as solvatochromism.

•Bathochromic shift: the wavelength of emission shifts towards the red end of the spectrum (lower photon energy),

•Hypsochromic shift: the wavelength shifts towards the blue end of the spectrum (higher photon energy).

For solutes with non-polar ground states:• Changes with the solvent polarity stabilize the excited A-—D+ complex

•This lowers the energy gap between excited A-—D+ and ground A-D states • emission spectra for a particular D-A complex is indicative of the polarity of the solvent and the dipole moment of the excited A-—D+ complex.

1.3416335.94--Acetonitrile

1.47933 (20˚C)46.45 (20˚C)--Dimethyl Sulfoxide

1.421158.93--Dichloromethane

1.521855.621--Chlorobenzene

----7.84Reichardt's Dye

----3.334-ANMP

----3.92Cu153

η (@ 25˚C)5ε (@ 25˚C)5ρ (Ǻ)Compound

TABLE OF CONSTANTS

When dealing with intramolecular photo-induced electron transfer (PET),

•An electron donor and acceptor are covalently linked together (A-D complex)

•The donor or acceptor is locally excited by a photon (h1)

•The complex undergoes electron transfer •As a result the positively charged donor cation is bonded to the negatively charged acceptor anion.

•This A--D+ excited state complex follows one of two potential pathways whereby it returns to the ground state D-A complex

•releasing either a photon via fluorescence (h2) OR•heat

A - D A* - D A- - D+

A-D + h2

A - D + ΔHh1

kETk -ET, non-rad

k-ET, rad

REFERENCES(1)Hermant, R. M.; Bakker, N. A. C.; Scherer, T.; Krijnen, B.; Verhoeven, J. W. J. Am.

Chem. Soc, 1990, 112, 1214-1221.

(2) Maroncelli M., Fleming, G. R. J. Chem. Phys, 1987, 86, 6221-6239.

(3) Chapman, C. F.; Fee, R. S.; Maroncelli, M. J. Phys. Chem., 1995, 99, 4811-4819.

(4) Mente, S. R.; Maroncelli, M. J. Phys. Chem B., 1999, 103, 7704-7719.

(5) Riddick, J. A; Bunger, W. B.; Sakano, T. K. Organic Solvents: Physical Properties and Methods of Purification; Wiley Interscience: New York, 1986; Vol. 2.

LAB SETUP & CONCLUSIONS

Students will:

•Run steady state absorption and fluorescence spectra for:•Reichardt’s Dye &•Coumarin 153 OR•4-amino-N-methylphthalimide

Which will be dissolved in Chlorobenzene, DCM, DMSO, & Acetonitrile•Graph the results and calculate Δμ.

In conclusion this lab will help students as: It gives them a good introduction to photo-induced intramolecular electron transfer, which has applications to photovoltaic and solar cells, nanotechnology and many chemical and biological processes. It provides them with a quantitative method for analyzing solvatochromism.• It deepens their understanding of basic physical chemistry topics.

ACKNOWLEDGEMENTS

The authors would like to thank the NASA Vermont Space Grant initiative for funding this research project.

ct

(for Coumarin 153)