Chemistry 2

Lecture 13 Everything

Learning outcomes from lecture 12• Be able to explain Kasha’s law by describing internal conversion• Be able to define fluorescence quantum yield• Be able to describe intersystem crossing and how it leads to

phosphoresence• Be able to explain why the phosphorescence occurs at lower

energy (“red-shifted”) and is slower than fluorescence

The sequence of events that can occur after absorption, including emission, fluorescence, phosphorescence, non-radiative decay, internal conversion and intersystem crossing. The use of Jablonski diagrams to describe these processes.

Assumed knowledge

Energy is the most important thing

Extraterrestrial solar spectrum

6000 K thermal spectrum

Atmospheric absorption

Absorption by water,Carbon dioxide and ozone. Scattering too!

H2O & CO2H2OO3

overtones!

Blue ice is due to overtone absorption

overtones!

Absorption of light by the earthChlorophyll fluorescencein satellite image

S2

S1

S0

IC

IC

T1

T2

Absorption

Fluorescence

ISC

ISC

ISC

Phosphorescence

While the earth fluoresces a little, the majority of incoming energy is internally converted into heat, and re-radiated in vibrational infrared transitions of water, rocks, asphalt…

Greenhouse

(average)

Absorption and re-emission of infrared radiation by atmospheric molecules

EARTH

E

ATMOSPHERE

E

E/2

E/2

E/2

E/4

E/4

2E

E

E

The greenhouse effect is due to IR absorption

EARTH

E=168

2E

E

E

But, with single layer blackbody atmosphere absorbing outgoing radiation, Earth heats to irradiate twice the incoming energy.

TTTE 88.06.04

With no atmosphere, average temperature on earth is T0. If earth was blackbody with albedo of 0.40, then

TTTE 05.12.14

But the atmosphere does not absorb all outgoing radiation…. And is best represented as a multlayer. Clouds and weather complicate matters.

Greenhouse gasesIf atmosphere was purely N2 and O2, all surface-emitted radiation would escape into space. Gases which have oscillators that overlap the emission spectrum of earth’s 300K blackbody convey blackbody behaviour to the atmosphere (statistical emission/absorption). Peak emission of 300K object is 590cm-1.

Green plantsGreen plants absorb CO2 and synthesize sugars using light energy from the sun.

Can be modelled as a particle on a ring system.

S1-S0

S2-S0

S2

S1

S0

IC

IC

T1

T

2

Absorption

Fluorescence

ISC

ISC

ISC

Phosphorescence

Light harvestingThe energy in photosynthesis is transferred from chlorophyll to chlorophyll and funnelled into the reaction centre.

S1-S0

S2-S0

chlorophylls

Energy transfer is internal conversion in bichromophoric molecule

S2-S0

hn

*S2

S1

S0

IC

IC

T1

T2

Absorption

Fluorescence

hn

Photodynamic cancer therapy

S0

S1

T1

ISC

hn

triplet sensitizer

energy transfer

O2

T0

S1

Cytotoxic singlet oxygen is produced from energy transfer after intersystem crossing. Specific tissues can be targeted by laser irradiation of triplet sensitizer.

Skin transmits red and near infrared light effectively (800nm is great), but absorbs most visible and UV. Much effort goes into finding near IR absorbing triplet sensitizers for cancer therapy.

what is wrong with this picture?

T1

triplet-triplet annihilation (TTA)

S1

emitter

S0

emitter

sensitizer

S0

S1

T1

sensitizer

S0

S1

T1T1

S0

TTA

S1

ISCISC

spin states of two triplets

S T Q

T T Q

Q Q Q

1/9 collisions statistically gives singlet which can decay into excited S1 of one chromophore, and S0 opf the other.

requirements for TTA upconversion

S0

S1

T1

sensitizer

T1TTA

S1

ISC

emitters

More than half S1

Step down by >>kBT

single threshold solar cells

~32% max

electrons

unoccupiedenergy levels

V

IC!

Up-conversion

cell

Up-conversion unit

Limiting efficiency of an Upconversion cell is about 50%

Nothing wrong with this picture!

Good Luck!

Week 13 homework• Electronic spectroscopy worksheet in the tutorials• Complete the practice problems at the end of the lectures

• Note: ALL of the relevant past exam problems have been used as practice problems (either on the worksheets or as ‘end of lecture problems’. Other questions on past papers include parts which are no longer part of the course.