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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

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)

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

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-

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)

22Singlet Triplet

Spin Flip

Complete Diagram

S0

S1

S2

Jablonski Diagram

Energy

hn

Ground StateS0

Singlet Excited StateS1

ExcitationInternal Conversion

FluorescenceNon-radiative decay

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

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

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

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

Molecular Photophysics End

Any Questions?