Upload
ria-english
View
23
Download
2
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
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.