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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

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chemistry 2

Chemistry 2

Lecture 13

Everything

learning outcomes from lecture 12
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

Assumed knowledge

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.

extraterrestrial solar spectrum
Extraterrestrial solar spectrum

6000 K thermal spectrum

atmospheric absorption
Atmospheric absorption

Absorption by water,

Carbon dioxide and ozone. Scattering too!

O3

H2O

H2O & CO2

overtones!

absorption of light by the earth
Absorption of light by the earth

ISC

ISC

ISC

IC

IC

Absorption

Fluorescence

Chlorophyll fluorescence

in satellite image

S2

T2

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…

S1

T1

Phosphorescence

S0

greenhouse
Greenhouse

(average)

absorption and re emission of infrared radiation by atmospheric molecules
Absorption and re-emission of infrared radiation by atmospheric molecules

E

E/2

E/4

E

ATMOSPHERE

E/2

E

E/4

E

E/2

2E

EARTH

the greenhouse effect is due to ir absorption
The greenhouse effect is due to IR absorption

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

E

E=168

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

E

2E

EARTH

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

greenhouse gases
Greenhouse gases

If 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 plants
Green plants

ISC

ISC

S2

T2

Absorption

Fluorescence

ISC

IC

IC

S1

T1

Phosphorescence

S0

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

S2-S0

S1-S0

Can be modelled as a particle on a ring system.

light harvesting
Light harvesting

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

chlorophylls

S2-S0

S1-S0

energy transfer is internal conversion in bichromophoric molecule
Energy transfer is internal conversion in bichromophoric molecule

IC

IC

Absorption

Fluorescence

*

S2-S0

S2

T2

S1

hn

T1

S0

hn

photodynamic cancer therapy
Photodynamic cancer therapy

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

ISC

S1

T1

hn

S1

energy transfer

T0

S0

O2

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.

triplet triplet annihilation tta
triplet-triplet annihilation (TTA)

TTA

sensitizer

sensitizer

S1

S1

S1

S1

ISC

ISC

T1

T1

T1

T1

S0

S0

S0

S0

emitter

emitter

spin states of two triplets
spin states of two triplets

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

requirements for tta upconversion
requirements for TTA upconversion

TTA

sensitizer

S1

Step down by >>kBT

S1

ISC

More than half S1

T1

T1

S0

emitters

single threshold solar cells
single threshold solar cells

unoccupied

energy

levels

IC!

~32% max

V

electrons

up conversion
Up-conversion

Limiting efficiency of an

Upconversion cell is about 50%

cell

Up-conversion unit

good luck
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.
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