10. 4 Photochemistry

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10. 4 Photochemistry
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4.1 Brief introduction
1) photochemistry
The branch of chemistry which deals with the
study of chemical reaction initiated by light.
2) Energy of photon
The photon is quantized energy light
quantum
Where h is the Plank constant, C the velocity of
light in vacuum, ? the wave-length of the light,
and ? the wave number.
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3) Spectrum of visible light
Rainbow, the natural spectrum of visible light
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3?105 m3.98?10-8 kJ mol-1
radio
3?10-1 m3.98?10-4 kJ mol-1
micro-wave
6?10-4 m 1.99?10-1 kJ mol-1
far-infrared
3?10-5 m 3.99 kJ mol-1
near-infrared
800 nm 149.5 kJ mol-1
visible
400 nm 299.0 kJ mol-1
ultra-violet
150 nm 797.9 kJ mol-1
vacuum violet
5 nm 239?104 kJ mol-1
X-ray
5 nm 1.20?109 kJ mol-1
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4) Interaction between light and media
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I- intensity of light, x the thickness of the
medium, a the absorption coefficient.
Lamberts law when a beam of monochromatic
radiation passes through a homogeneous absorbing
medium, equal fraction of the incident radiation
are absorbed by successive layer of equal
thickness of the light absorbing substance
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Beers law
The equal fractions of the incident radiation
are absorbed by equal changes in concentration of
the absorbing substance in a path of constant
length.
? Is the molar extinction coefficient, C the
molar concentration.
Both Lamberts law and its modification are
strictly obeyed only for monochromatic light,
since the absorption coefficients are strong
function of the wave-length of the incident
light.
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5) Photoexcitation
Upon photoactivation, the molecules or atoms
can be excited to a higher electronic,
vibrational, or rotational states.
A h? ? A
The lifetime of the excited atom is of the
order of 10-8 s. Once excited, it decays at once.
Excitation between different electronic level
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Process Transition Timescale (sec)
Light Absorption (Excitation) S0 ? Sn ca. 10-15 (instantaneous)
Internal Conversion Sn ? S1 10-14 to 10-11
Vibrational Relaxation Sn ? Sn 10-12 to 10-10
Intersystem Crossing S1 ? T1 10-11 to 10-6
Fluorescence S1 ? S0 10-9 to 10-6
Phosphorescence T1 ? S0 10-3 to 100
Non-Radiative Decay S1 ? S0 T1 ? S0 10-7 to 10-510-3 to 100
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Radiation-less decay
Jablonsky diagram
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Fluorescent minerals emit visible light when
exposed to ultraviolet light
Endothelial cells under the microscope with three
separate channels marking specific cellular
components
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7) Decay of photoexcited molecules
Radiation transition
Fluorescence and phosphorescence
non-reactive decay
Vibrational cascade and thermal energy
Radiationless transition
decay
Reaction of excited molecule A ? P
reactive decay
Energy transfer A Q ? Q ? P
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5.2 Photochemistry
The first law of photochemistry Grotthuss
and Draper, 1818 light must be absorbed by a
chemical substance in order for a photochemical
reaction to take place.
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The second law of photochemistry / The law of
photochemical equivalence Einstein and Stark,
1912
The quantum of radiation absorbed by a molecule
activates one molecule in the primary step of
photochemical process.
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The activation of any molecule or atom is
induced by the absorption of single light
quantum.
one einstein
? Lh? 0.1196 ? J mol-1
Under high intensive radiation, absorption of
multi-proton may occur.
A h? ? A A h? ? A
Under ultra-high intensive radiation, SiF6 can
absorb 20 40 protons.
These multi-proton absorption occur only at I
1026 proton s-1 cm-3, life-time of the
photoexcited species gt 10-8 s. Commonly, I 1013
1018 proton s-1 cm-3, life-time of A lt 10-8 s.
the probability of multi-proton absorption is
rare.
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The primary photochemical process
A chemical reaction wherein the photon is one
of the reactant.
S h? ? S
Some primary photochemical process for molecules
AB C
Dissociation into radicals
AB- C
Ions Photoionization
ABC e-
ABC h?
photoionization
ABC
Activated molecules Photoexcitation
ACB
Intramolecular rearrangement Photoisomerization
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Secondary photochemical process
Energy transfer A Q ? Q
donor
acceptor
Q ? P (sensitization), Asensitizer
Q A (quenching), Qquencher
Photosensitization, photosensitizers,
photoinitiator
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6.3 kinetics and equilibrium of photochemical
reaction
For primary photochemical process
Zeroth-order reaction
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Secondary photochemical process
HI h? ? H I
H HI ? H2 I
I I ? I2
Generally, the primary photochemical reaction
is the r. d. s.
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For opposing reaction
A B
r kIa
r- k-B
At equilibrium
The composition of the equilibrium mixture is
determined by radiation intensity.
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6.4 quantum yield and energy efficiency
Quantum yield or quantum efficiency (?)
The ratio between the number of moles of
reactant consumed or product formed for each
einstein of absorbed radiation.
For H2 Cl2? 2HCl ? 104 106 For H2 Br2?
2HBr ? 0.01
? lt 1, the physical deactivation is dominant ?
1, product is produced in primary photochemical
process ? gt 1, initiate chain reaction.
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Energy efficiency
Light energy preserved
?
Total light energy
Photosynthesis
6CO2 6H2O nh? ? C6H12O6 6O2
?rGm 2870 kJ mol-1
For formation of a glucose, 48 light quanta was
needed.
red light with wave-length of 700 nm
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6.5 The way to harness solar energy
Solar ? heating Solar ? electricity
photovoltaic cell photoelectrochemical cell Solar
? chemical energy
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Photolysis of water
Photooxidation of organic pollutant
TiO2
Ag
Photochemical reaction
S h? ? S S R ? S R- 4S 2H2O ? 4S
4H O2 2R- 2H2O ? 2R 2OH- H2
S Ru(bpy)32
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Photosensitive reaction
Porphyrin complex with magnesium
Reaction initiated by photosensitizer.
When reactants themselves do not absorb light
energy, photoensitizer can be used to initiate
the reaction by conversion of the light energy to
the reactants.
6CO2 6H2O nh? ? C6H12O6 6O2
Chlorophyll A, B, C, and D
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Light reaction the energy content of the light
quanta is converted into chemical energy. Dark
reaction the chemical energy was used to form
glucose.
8h?
4Fd3 3ADP3- 3P2- ??
4Fd2 3ATP4- O2 H2O H
Fd is a protein with low molecular weight
3ATP3- 4Fd2 CO2 H2O H 3P2-
? (CH2O) 3ADP3-
3P2- 4Fd3
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All the energy on the global surface comes
from the sun. The total solar energy reached
the global surface is 3 ? 1024 J y-1, is 10,000
times larger than that consumed by human being.
only 12 of the total incident energy is
recovered for a field of corn.
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Examples of photochemical reactions (1)
photosynthesis, in which most plants use solar
energy to convert carbon dioxide and water into
glucose, disposing of oxygen as a side-product.
(2) Humans rely on photochemistry for the
formation of vitamin D. (3) Vision is initiated
by a photochemical reaction of rhodopsin (4) In
fireflies, an enzyme in the abdomen catalyzes a
reaction that results in bioluminescence (5) In
organic reactions are electrocyclic reactions,
photoisomerization and Norrish reactions. (6)
Many polymerizations are started by
photoinitiator , which decompose upon absorbing
light to produce the free radicals for Radical
polymerization. (7) In photoresist technology,
used in the production of microelectronic
components.
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6.6 the way to produce light Chemical laser
and chemiluminescence
Photoluminescence, Electroluminescence,
Chemiluminescence, Electrochemiluminescence,
Light-emitting diode
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The reverse process of photochemistry
A BC ? AB C
High pressure collision deactivation
Low pressure radiation transition
CF3I ? CF3 I
H Cl2 ? HCl Cl
A A- ? A2
Emission of light from excited-state dye
molecules can be driven by the electron transfer
between electrochemically generated anion and
cation radicals a process known as
electrochemi-luminescence (ECL).
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S.-Y. ZHANG, et al. Functional Materials, 1999,
30(3)239-241
MEH-PPV
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firefly
The firefly, belonging to the family Lampyridae,
is one of a number of bioluminescent insects
capable of producing a chemically created, cold
light.
http//yahooligans.yahoo.com/content/animals/photo
/9807.html
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Moon jelly
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Laser light amplification by stimulated emission
of radiation
Population inversion
1917, Einstein proposed the possibility of
laser. 1954, laser is realized. 1960, laser is
commercialized.
Radiationless transition
n level
m upper level
Excitation / pump
Radiation transition
n lower level
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Specialities of laser

• High power emission interval 10-9, 10-11,
  10-15. 100 J sent out in 10-11s 1013 W.
  temperature increase 100,000,000,000oC s-1
• 2) Small spreading angle 0.1 o
• 3) High intensity 109 times that of the sun.
• 4) High monochromatic Ke light ?? 0.047 nm,
  for laser ?? 10-8 nm,

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