Introduction to Photochemistry and Electron Transfer Theory · Introduction to Photochemistry and...

Introduction to Photochemistry and Electron Transfer Theory

Tomoyasu ManiDepartment of ChemistryUniversity of Connecticut

10/30/2019 1

CT Japan

Photochemistry Workshop

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Color Changes with Complexations: Observation of Charge-Transfer

Absorption

https://mani.chem.uconn.edu/photochem-workshop/

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Department of Chemistry at the University of Connecticut

• @ Storrs, CT• 26 tenure‐track or tenured professors• Located in the Chemistry Building • 65 cutting‐edge research and teaching labs

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What is Photochemistry?

The chemistry concerned with the chemical effects of light. Generally, a chemical reaction is caused by using UV, visible, infrared light. 

700 nm 400 nm600 nm 500 nm

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Why Important?

Photosynthesis is Driven by Light! We can see “inside” by Light!

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Converting Light to Something Else.

Making Molecules with Light. Making Electricity from Light.

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What is Going On? General Jablonski Diagram

FluorescencePhotonAbsorption

Ene

rgy

S3

S2

S1

IC

Ground State

Singlet Manifold

IC = internal conversion

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Photoexcitation = Excess Energy

FluorescencePhotonAbsorption

Ene

rgy

S3

S2

S1

IC

Ground State

Singlet Manifold

IC = internal conversion

Excited

GroundState

9https://cen.acs.org/biological‐chemistry/biotechnology/Chemistry‐Pictures‐Laser‐activated/97/web/2019/08

Fluorescence Emission

BLUE Light

Photo Excitation

ElectronTransfer

Light Emission

10Wikimedia

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Photo‐induced Electron Transfer (ET)

S0A

h

S1A

S1A B

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Photo‐induced Electron Transfer (ET)

S0A

h

S1A ET

RP=radical pairor

CS = charge‐separated state

A+ B‐

SRP

Rudolph A. Marcus1992 Nobel Prize

A+ B‐

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Photo‐induced Electron Transfer (ET): Orbital View

S0A

h

S1A

S1A B

Donor Acceptor

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Photo‐induced Electron Transfer (ET): Orbital View

S0A

h

S1A ET

A+ B‐

RP=radical pairor

CS = charge‐separated state

A+ B‐

SRP

Donor Acceptor

Wikipedia

Electron Transfer is one of the Most Fundamental Chemical Reactions

Zn(s) → Zn2+(aq) + 2e‐ Cu2+ (aq) + 2e‐→ Zn2+(aq)

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Photo‐induced ET at the Heart of Sciences & Applications

RadicalIons

Charges (i.e. Solar Cells)

S0A

h

S1A ET

SRP

S1A B

RP=radical pairor

CS = charge‐separated state

A+ B‐ Catalysts (e.g. Solar Fuels)

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Solar Cells: Charge Separation and Transport are Critical Steps

+‐

Non-polar Environment

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Radical Pairs ( a Pair of e- & h+) : Critical intermediates

A+ B-Quantum biology

Magneto reception in European Robin1

Images from 1Peter Hore (Oxford Univ.)’s Website2Wikimedia Commons3Dewey Holten (Wash U)’s Website4Lakhwani, G.; Rao, A.; Friend, R. H., Annu. Rev. Phys. Chem. 2014. 65, 557

Organic Solar Cells4

Organic Light Emitting Diodes2

h

RPs Charge injection

S1

GS

Photosynthesis Reaction Center3

Quantum Information Science

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Thermal vs Optical Electron Transfer

S0A

h

S1A ET

S1A B

A+ B-

SRP

Thermal

S0A

h

S1A

SRP

Optical

Photon absorption comes with e- transfer

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Light Absorption: How Do We See Colors?

Solution Absorbs RED

We see BLUE!

700 nm400 nm 600 nm500 nm

Higher Energy Lower Energy

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Molecules that Absorb Different Wavelength have Different Colors

Higher EnergyLower Energy

N N

NNPd

N

N

+HNCl-

Light Absorption: How Do We See Colors?

700 nm400 nm 600 nm500 nmHigher Energy Lower Energy

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Light Absorption: Spectrophotometer to Quantify

Chemistry LibreTexts

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N N

NNPd N

N

+HNCl-

700 nm400 nm 600 nm500 nm

Higher Energy Lower Energy

Wavelength (nm)

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Thermal vs Optical Electron Transfer

S0A

h

S1A ET

S1A B

A+ B-

SRP

Thermal

S0A

h

S1A

SRP

Optical

Photon absorption comes with electron transfer

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Charge-Transfer (CT) Absorption (very rough approximation)

Donor Acceptor

No Interactions

Donor Acceptor

hν

hν

Their Own Absorption

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No Interactions of Donor and Acceptor

Absorption of DonorAbsorption of Acceptor

Wavelength (nm)

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Let them Interact … Optical Electron Transfer

hν

Donor Acceptor

IPD

EAA

Vacuum energy

Donor Acceptor

hν

IE = Ionization EnergyEA = Electron Affinity

28Donor Acceptor

hν

hν

Donor Acceptor

hν

CT Absorption: Orbital View

Normal (Local) CT

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Charge-Transfer (CT) Absorption: New Absorption Band!

Absorption of DonorAbsorption of Acceptor

CT absorption band!

Wavelength (nm)

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New Absorption Band Depends on Donor* = Color Change

Absorption of DonorAbsorption of Acceptor

CT absorption band!

Wavelength (nm)

*Keeping the same acceptor.

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Donor AcceptorComplex

Charge-Transfer (CT) Absorption (More Accurate Picture)

hν

ψG = a* ψ(D,A) + b*ψ(D+-A-)

ψ* ~ψ(D+-A-)

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Higher EnergyLower Energy

N N

NNPd

N

N

+HNCl-

700 nm400 nm 600 nm500 nmHigher Energy Lower Energy

Why is CT Absorption Significant?

In one molecule: Color change with significant structural change

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Why is CT Absorption Significant?

Color change by changing donor or acceptor, which can be very small structural change!

700 nm400 nm 600 nm500 nmHigher Energy Lower Energy

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Charge-Transfer (CT) Absorption: Importance

Applications in organic semiconductors;Implications in organic solar cells etc…

e-

+ -

Widely observed in inorganic molecules as metal-to-ligand charge-transfer (MLCT) absorption.

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Charge-Transfer (CT) Absorption: Experiment

Acceptor Donors

Goals:1.Understand CT absorption2.Observe CT absorption3.Analyze and “quantify” why color changes with different donors

+

Color?K

Color?K

Mes

T2

T3

KColor?

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Charge-Transfer (CT) Absorption: Experiment

Acceptor Donors

+

Color?K

Color?K

Mes

T2

T3

KColor?

Goals:1.Understand CT absorption2.Observe CT absorption3.Analyze and “quantify” why color changes with different donors4. (Optional) Obtain association constants from absorption

spectra (see section 6 in the handout).