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CHM 5175: Part 2.2
Introduction to Molecular Photophysics
Ken HansonMWF 9:00 – 9:50 am
Office Hours MWF 10:00-11:00
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RainbowsGlassesMirage
Refractometer
Moon LightButterfly Wings
Sea ShellsSoap Bubbles
Two-slit expHolograms
Shadow Blur
Sand in WaterSunsets
UV-VisFluorometry
TASolar Cells
Interaction of Light with Matter
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Narrowing Our Focus• Absorption/Transmission• Visible spectrum
Visible Light (hn)
Electronic Transitions- electrons excited from one energy level to another.
• Atomic• Molecular• MaterialsSample
Interaction of Light with Matter
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e-p+
Hydrogen Absorption
Higher ELower E
hn
e-
p+
Ground State Excited State
hn
Energy
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2 2
1 1H
l h
E Rn n
Hydrogen Absorption
“white” light source Hydrogen Sample Prism
Line Spectrum
H
Rydberg Formula
HH
H
H
H
H
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1 e-
80 e-
10 e-
Increasing Complexity
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N
N
N
N
N
PtCl250 e-
Atomic Transitions (movement of electrons)
+ Molecular Transitions
(movement of electron density)
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Transitions
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N
N
N
N
N
PtCl
e-p+
hn
e-
p+
Atomic Transitions
hn
N
N
N
N
N
PtCl
Molecular Transitions
hn
hn
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C O
R
R
C H
R
R
R
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Types of Molecular Transitions
σ - σ*max < 150 nm
p - p*max 200 - 800 nm
C C
R
R R
R
n - p*max 150 - 300 nm
C
O
H
HH
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C H
R
R
R
Types of Molecular Transitions
High energy photons
• methane = 125 nm
• ethane = 135 nm
Ground State Excited State
hn
Bonding
Antibonding
σ - σ*max < 150 nm
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Types of Molecular Transitions
p - p*max 200 - 800 nm
Visible photons
• benzene = 260 nm
• tetracene = 500 nm
hn
Bonding
Antibonding
C C
R
R R
R
Ground State Excited State
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C O
R
R
n
n
Types of Molecular Transitions
n - p*max 150 - 300 nm
Visible photons
• acetone = 280 nm
• pyridine = 270 nm
hn
Non-Bonding
Antibonding
Ground State Excited State
C OR
R
C OR
R
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Types of Molecular Transitions
σ - σ*max < 150 nm
p - p*max 200 - 800 nm
n - p*max 150 - 300 nm
C
O
H
HH
400300200 500100
p - p*
n - p*s - s*
Wavelength (nm)
Abso
rptio
n
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Types of Molecular Transitions
Metal Centered (MC)max 200 –800 nm
[Co(H2O)6]2+
MMCTmax 300 –800 nm
NN
NN
N
NRuII
MLCTmax 300 –1000 nm
LMCTmax 300 –1000 nm
MnO4-
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Types of Molecular Transitions
[CoCl4]2-
[Co(H2O)6]2+
Metal Centered (MC)
d-d transitionsmax 200 – 800 nm
M M + L
t2g
eg
• 3d and 4d transition metals (+ ligands)
• Relatively weak (0-1000 M−1cm−1)
• Early structural determination
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Types of Molecular Transitions
• Low-lying empty ligand orbital
• Low oxidation state metal (electron rich)
• High d orbital energy
NN
NN
N
NRuII hn
e-
Metal-to-Ligand Charge Transfer (MLCT)max 300 – 1000 nm
250 300 350 400 450 500 550 6000
2
4
6
8
10
(10
4 M
-1cm
-1)
Wavelength (nm)
MLCT
p - p* M M + L
t2g
eg
L
p*
NN
NN
N
NRuIII (-)
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Types of Molecular Transitions
• Ligand with high E lone pairs (S or Se)
• Metal with low-lying empty orbitals
Ligand-to-Metal Charge Transfer (LMCT)max 300 – 1000 nm
M M + L
t2g
eg
L
p
Mn-O4-
O2- (p) Mn7+
Purple
e-
Cd-SS2- (p) Cd2+
Yellow
e-
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Types of Molecular TransitionsMetal-to-Metal Charge Transfer (MMCT)
max 300 – 800 nm
MMCTe-
M1
M1 + L
M2
M2 + L
II
III
t2g
eg
t2g
eg
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C N Mn
O
O
L
L
Fe
N
N
L
L
Types of Molecular Transitions
t2g
eg
eg
t2g
p
p*
M1
M1 + M2 + L
M2
600500400 700300
MCLMCT
MLCT
Wavelength (nm)
Abso
rptio
n
MMCT
e- e-
e-
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Vo
V4
V3
V2V1
Complete Diagram
σ - p*p - p*
n - p*
MC
LMCT
MLCT
MMCT
σ - σ*
n - σ*
Transitions
Vo
V4
V3
V2V1
Vo
V4
V3
V2V1
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Energy
E0
E1
E2
Transitions
Electronic
Vibrational
Rotational
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Vo
V4
V3
V2V1
Vo
V4
V3
V2V1
Vo
V4
V3
V2V1
Complete Diagram
Energy
E0
E1
E2
S0
S1
S2
Jablonski Diagram
Transitions
Electronic
Vibrational
Rotational
Energy
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Complete Diagram
ExcitationInternal Conversion
Fluorescence
S0
S1
S2
Jablonski Diagram
Non-radiative decay
Energy
Ground State (S0)
First Excited State (S1)
Second Excited State (S2)
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22Singlet Triplet
Spin Flip
Complete Diagram
S0
S1
S2
Jablonski Diagram
Energy
hn
Ground StateS0
Singlet Excited StateS1
ExcitationInternal Conversion
FluorescenceNon-radiative decay
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Nicholas J. Turro, Principles of Molecular Photochemistry
Triplet/Singlet Excited States
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Triplet
Singlet
Lower Energy
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Ground State
Singlet Excited State
hspin-orbit coupling
Triplet Excited State
Ground State
h
Spin-Orbit Coupling
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Quantum Numbersn = Principal
l = Angular
ml = Magnetic
ms = Electron spin
Heavy Atoms Pt, Ir, I...
Nicholas J. Turro, Principles of Molecular Photochemistry
Spin-Orbit Coupling
Rotating Chair and Bicycle Wheel
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Jablonski Diagram
ExcitationInternal Conversion
FluorescenceNon-radiative decayIntersystem Crossing
PhosphorescenceS0
S1
S2
Energy T1
T2
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27Nicholas J. Turro, Principles of Molecular Photochemistry
Jablonski Diagram of Anthracene
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• Electron transfer• TICT• ESIPT• Photochemical
ReactionsExcitationInternal Conversion
FluorescenceNon-radiative decayIntersystem Crossing
Phosphorescence
Other Processes
S0
S1
S2
Energy T1
T2
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Excited State Electron Transfer
NN
NN
N
NRuII
hn
NN
NN
N
NRuIII (-)
e-e-
+ A RuIII(bpy)3 + A-
+hn
e-
+
RuII(bpy)3 [RuII(bpy)3]* A RuIII(bpy)3 A-
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Photosynthesis
Excited State Electron Transfer
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31Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77-80.
Photocatalytic α-alkylation of aldehydes
Excited State Electron Transfer
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Excited State Structural Change
+ h
*
Twist + CT*
+ h
Twisted Intramolecular Charge Transfer
N
CN
e-
e-
Pratt et al. J. Chem. Phys. 2005, 122, 084309
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ESIPT
reverse proton transfer
absorption emission
OH
N
OH
N O
N
H
O
N
H
Excited State Structural ChangeExcited State Proton Transfer
NH
N
N
N
N
OH
OH
NH
N
HN
N
N
OH
O
Hanson et al. Org. Lett. 2011, 13, 1598
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Photochemical ReactionsPhotopolymerization
Peachy Printer ($100)
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Photochemical ReactionsPhotolithography
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Photochemical ReactionsPhotoisomerization
hn
Ground State Excited State
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Photochemical ReactionsPhotoswitches
J. Am. Chem. Soc., 2013, 135 (16), pp 5974–5977
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ProcessesExcitation
FluorescencePhosphorescence
Non-radiative decayInternal conversion
Intersystem crossingPhotochemistry
S0
S1
S2
E T1
T2
“Complete” Jablonski Diagram
Product
Product
Measurement Technique
Absorption Spectroscopy
Fluorescence Spectroscopy
Transient Absorption Spectroscopy
Solar Cell Testing
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Side Note: Other ExcitationsThermal Excitation
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Side Note: Other ExcitationsChemical Excitation
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Side Note: Other ExcitationsSonoluminescence
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Side Note: Other Excitations
Nature 2008, 455, 1089–1092.
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Tribo/Fractoluminescence
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Side Note: Other ExcitationsElectroluminescence
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OH
N
N
NO2
Para Red
NN
NH2
H2N
Fast Brown
NNO3S
HO
SO3
Sunset Yellow (Food Yellow 3)
Side Note: Dye Structure
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Bright Blue Common Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing.
Royal Blue Common Food Uses Baked goods, cereals, snack foods, ice-cream, confections, cherries.
Orange-red Common Food Uses Gelatins, puddings, dairy products, confections, beverages, condiments.
Lemon-yellow Common Food Uses Custards, beverages, ice-cream, confections, preserves, cereals.
Orange Common Food Uses Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections
Side Note: Dye Structure
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Molecular Photophysics End
Any Questions?