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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 OR

RC H

R

R

R

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Types of Molecular Transitions

σ - σ*max < 150 nm

p - p*max 200 - 800 nm

C CR

R R

R

n - p*max 150 - 300 nm

C

O

H

HH

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C HR

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

p

p

p

p

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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 CR

R R

R

Ground State Excited State

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C OR

R

n

p

n

p

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 ORR

C ORR

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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* - *

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 + Lt2g

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

Lp

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 MnO

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

MCLMCTMLCT

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

Energyp

p

p

p

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

hnspin-orbit coupling

Triplet Excited State

Ground State

hn

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

+ hn

*

Twist + CT*

+ hn

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

OHN

OHN O

NH

ON

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

p

p

p

p

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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OHNN

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?