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
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.
Energy is the most important thing
Extraterrestrial solar spectrum
6000 K thermal spectrum
Atmospheric absorption
Absorption by water,
Carbon dioxide and ozone.
Scattering too!
O3
H2O
H2O & CO2
overtones!
Blue ice is due to overtone absorption
overtones!
Absorption of light by the earth
Chlorophyll fluorescence
in satellite image
ISC
IC
S2
T2
ISC
IC
S0
Absorption
Fluorescence
S1
ISC
T1
Phosphorescence
While the earth fluoresces a
little, the majority of
incoming energy is internally
converted into heat, and reradiated in vibrational
infrared transitions of water,
rocks, asphalt…
Greenhouse
(average)
Absorption and re-emission of infrared
radiation by atmospheric molecules
E
E/2
E/4
E
ATMOSPHERE
E
E/2
E/4
E/2
EARTH
E
2E
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
TE  4 0.6  T  0.88T
E
E=168
E
2E
EARTH
But, with single layer blackbody
atmosphere absorbing outgoing radiation,
Earth heats to irradiate twice the incoming
energy.
TE  4 1.2  T  1.05T
But the atmosphere does not absorb all
outgoing radiation…. And is best
represented as a multlayer. Clouds and
weather complicate matters.
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 absorb CO2 and synthesize sugars using light energy from the sun.
S2-S0
S1-S0
ISC
IC
Can be modelled as a particle on a ring system.
S0
Absorption
IC
Fluorescence
S2
ISC
T
2
S1
ISC
T1
Phosphorescence
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
*
S2-S0
IC
S2
T
2
IC
hn
S0
Absorption
Fluorescence
S1
T1
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
hn
T1
S1
energy transfer
T0
S0
triplet sensitizer
O2
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?
triplet-triplet annihilation (TTA)
sensitizer
S1
S1
ISC
ISC
T1
S0
S1
sensitizer
TTA
T1
emitter
T1
T1
S0
S0
emitter
S1
S0
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
sensitizer
S1
Step down by >>kBT
S1
ISC
T1
More than half S1
TTA
T1
S0
emitters
single threshold solar cells
unoccupied
energy
levels
IC!
~32% max
V
electrons
Up-conversion
Limiting efficiency of an
Upconversion cell is about 50%
cell
Up-conversion unit
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.