introduction to photochemistry and electron transfer theory · introduction to photochemistry and...
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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).