Introduction to Photochemistry and Electron Transfer Theory · Introduction to Photochemistry and...
Transcript of 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
2
Color Changes with Complexations: Observation of Charge-Transfer
Absorption
https://mani.chem.uconn.edu/photochem-workshop/
3
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
4
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
5
Why Important?
Photosynthesis is Driven by Light! We can see “inside” by Light!
6
Converting Light to Something Else.
Making Molecules with Light. Making Electricity from Light.
7
What is Going On? General Jablonski Diagram
FluorescencePhotonAbsorption
Ene
rgy
S3
S2
S1
IC
Ground State
Singlet Manifold
IC = internal conversion
8
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
11
Photo‐induced Electron Transfer (ET)
S0A
h
S1A
S1A B
12
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‐
13
Photo‐induced Electron Transfer (ET): Orbital View
S0A
h
S1A
S1A B
Donor Acceptor
14
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)
15
16
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)
17
Solar Cells: Charge Separation and Transport are Critical Steps
+‐
Non-polar Environment
18
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
19
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
20
Light Absorption: How Do We See Colors?
Solution Absorbs RED
We see BLUE!
700 nm400 nm 600 nm500 nm
Higher Energy Lower Energy
21
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
22
Light Absorption: Spectrophotometer to Quantify
Chemistry LibreTexts
23
N N
NNPd N
N
+HNCl-
700 nm400 nm 600 nm500 nm
Higher Energy Lower Energy
Wavelength (nm)
24
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
25
Charge-Transfer (CT) Absorption (very rough approximation)
Donor Acceptor
No Interactions
Donor Acceptor
hν
hν
Their Own Absorption
26
No Interactions of Donor and Acceptor
Absorption of DonorAbsorption of Acceptor
Wavelength (nm)
27
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
29
Charge-Transfer (CT) Absorption: New Absorption Band!
Absorption of DonorAbsorption of Acceptor
CT absorption band!
Wavelength (nm)
30
New Absorption Band Depends on Donor* = Color Change
Absorption of DonorAbsorption of Acceptor
CT absorption band!
Wavelength (nm)
*Keeping the same acceptor.
31
Donor AcceptorComplex
Charge-Transfer (CT) Absorption (More Accurate Picture)
hν
ψG = a* ψ(D,A) + b*ψ(D+-A-)
ψ* ~ψ(D+-A-)
32
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
33
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
34
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.
35
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?
36
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).