Levine, pp. 800-804 photochemistry
Transcript of Levine, pp. 800-804 photochemistry
6.1 Brief introduction of light
The branch of chemistry which deals with the study of chemical reaction
initiated by light.
1) Photochemistry
The photon is quantized energy: light quantum
hCC
hh ===
Where h is the Plank constant, C the velocity of light in vacuum, the
wave-length of the light, and the wave number.
2) Energy of photon
§10. 6 Photochemistry
4) Interaction between light and media
)exp(0 axII −=
adxI
dI=−
)]exp(1[00 axIIIIa −−=−=
I: intensity of light,
x: the thickness of the medium,
a: the absorption coefficient.
6.1 Brief introduction of light
Beer’s law:
0 exp( )aI I cx= −
Lambert’s law
§10. 6 Photochemistry
Light
beam
Upon photo irradiation, 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.
IR spectrum
(1) Photoexcitation:6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
The Foundation of a typical Jablonski Diagram
(1) Photoexcitation:
6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
A. Jablonski, Efficient of Anti-Stokes Fluorescence in Dyes,
Nature, 1933, Jun. 10, 839-840
Jablonski diagram
(1) Photoexcitation:6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
Absorbance
Vibrational Relaxation and Internal Conversion
Intersystem Crossing
Fluorescence
Phosphorescence
光子 光激发 基态 激发态 能级 能带 雅布
隆斯基(Jablonski)图 激发态衰变 振动弛
豫 内转换 系间穿越 辐射跃迁 无辐射跃迁
单线态 三线态 荧光 磷光
Radiation-less decayWhich is which?
(1) Photoexcitation:6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
Transition Time Scale Radiative Process?
Internal Conversion 10-14
- 10-11
s no
Vibrational Relaxation 10-14
- 10-11
s no
Absorption 10-15
s yes
Phosphorescence 10-4
- 10-1
s yes
Intersystem Crossing 10-8
- 10-3
s no
Fluorescence 10-9
- 10-7
s yes
(1) Photoexcitation:6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
https://chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_Textbook_Maps/Suppl
emental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Jablo
nski_diagram
(2) Decay of photoexcited molecules
decay
non-reactive
decay
reactive decay
Radiation
transition
Radiationless
transition
Fluorescence and phosphorescence
Vibrational cascade and thermal
energy
Reaction of excited molecule
A* → P
Energy transfer:
A* + Q → Q* → P
6.2 Physical processes of Excitation and decay
§10. 6 Photochemistry
6.3 Photochemistry
(1) The first law of photochemistry:
Grotthuss and Draper, 1818:
Light must be absorbed by a chemical
substance in order to initiate a
photochemical reaction.
§10. 6 Photochemistry
(2) The second law of photochemistry / The law of photochemical equivalence
One quantum of radiation absorbed by a
molecule activates one molecule in the
primary step of photochemical process.
Einstein and Stark, 1912
6.3 Photochemistry
= Lh = 0.1196 J mol-1
one einstein
E = h F-F
§10. 6 Photochemistry
primary step of photochemical process:
A chemical reaction wherein the photon is one of the reactant.
S + h → S*
h
6.3 Photochemistry
What is the nature of activation energy of a photochemical reaction?
§10. 6 Photochemistry
The primary photochemical process:
S + h → S*
Some primary photochemical process for molecules
6.3 Photochemistry
§10. 6 Photochemistry
Energy transfer: A* + Q → Q*
Q* +A (quenching), Q:quencher
Q* → P (sensitization), A*:sensitizer
Secondary photochemical process
donor acceptor
Photosensitization, photosensitizers, photoinitiator
6.3 Photochemistry
§10. 6 Photochemistry
Under high intensive radiation, absorption of multi-photon may occur.
A + h →A*
A* + h →A**
Under ultra-high intensive radiation, SiF6 can absorb 20~ 40 protons.
These multi-photon absorption occur only at I = 1026 photon s-1 cm-3, life-time of
the photoexcited species > 10-8 s.
Commonly, I = 1013 ~ 1018 photon s-1 cm-3, life-time of A* < 10-8 s. The probability
of multi-photon absorption is rare.
About multi-photon absorption
6.3 Photochemistry
§10. 6 Photochemistry
6.4 Kinetics and equilibrium of photochemical reaction
For primary photochemical process
2*R R PaI kh+ ⎯⎯→ ⎯⎯→
§10. 6 Photochemistry
Secondary photochemical process
HI + h H + I
H + HI H2 + I
I + I → I2
Generally, the primary photochemical
reaction is the r. d. s.
⎯→⎯ 2k
6.4 Kinetics and equilibrium of photochemical reaction
ak⎯⎯→
Decomposition of HI. H-I = 298 kJ mol-1
Why do we write this as the step?
§10. 6 Photochemistry
For opposing reaction with participation of photon:
At equilibrium
The composition of the equilibrium mixture is determined by radiation intensity.
Dark reaction and photochemical reaction. Application.
6.4 Kinetics and equilibrium of photochemical reaction
§10. 6 Photochemistry
6.5 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.
a
n r
I
= =
For H2+ Cl2→ 2HCl = 104 ~ 106
For H2+ Br2→ 2HBr = 0.01
> 1, initiate chain reaction.
= 1, product is produced in primary
photochemical process
< 1, the physical deactivation is
dominant
§10. 6 Photochemistry
Energy efficiency:Photosynthesis:
6CO2 + 6H2O + nh → C6H12O6 + 6O2
rGm = 2870 kJ mol-1
For formation of a glucose, 48 light quanta
was needed.
%7.354.16748
2870=
=
6.5 Quantum yield and energy efficiency
§10. 6 Photochemistry
Photosensitive reaction
Reaction initiated by photosensitizer.
6CO2 + 6H2O + nh → C6H12O6 + 6O2
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.
Chlorophyll A, B, C, and D
Porphyrin complex with magnesium
6.6 The way to harness solar energy—photosynthesis
§10. 6 Photochemistry
Light reaction: the energy content of the light quanta is converted into chemical energy.
Dark reaction: the chemical energy was used to form glucose.
Fd is a protein with low molecular weight
4Fd3+ + 3ADP3- + 3P2- ⎯→
4Fd2+ + 3ATP4- + O2 + H2O + H+
3ATP3-+ 4Fd2++ CO2+ H2O + H+ 3P2-
→ (CH2O) + 3ADP3- + 3P2- + 4Fd3+
8h
6.6 The way to harness solar energy
§10. 6 Photochemistry
All the energy on the global surface comes from the sun.
The total solar energy reached the global surface is 3 1024 Jy-1, is 10,000 times
larger than that consumed by human being.
6.6 The way to harness solar energy
Only 1~2% of the total incident energy
is recovered for a field of corn.
§10. 6 Photochemistry
Solar ⎯→ heating:
Solar ⎯→ electricity: photovoltaic cell / photoelectrochemical cell
Solar ⎯→ chemical energy:
6.6 The way to harness solar energy
§10. 6 Photochemistry
Photolysis of water/
Photooxidation of organic pollutant
Photochemical reaction—photocatalysts ??
S + h → S*
S* + R → S+ + R-
4S+ + 2H2O → 4S + 4H+ + O2
2R-+ 2H2O → 2R + 2OH-+ H2
S = Ru(bpy)32+
6.6 The way to harness solar energy
§10. 6 Photochemistry
Photolysis of water based on semiconductors
6.6 The way to harness solar energy
TiO2 the most important photocatalyst.
Modification of TiO2.
§10. 6 Photochemistry
6.7 The way to produce light:
Photoluminescence, Electroluminescence, Chemiluminescence,
Electrochemiluminescence, Light-emitting diode
Chemiluminescence
§10. 6 Photochemistry
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.
firefly
The firefly, belonging to the family of
lampyridae, is one of a number of
bioluminescent insects capable of
producing a chemically created, cold
light.
6.7 The way to produce light:
§10. 6 Photochemistry
MEH-PPV
S.-Y. ZHANG, et al. Functional Materials, 1999, 30(3):239-241
Emission of light from excited-state dye molecules can be driven by the electron
transfer between electrochemically generated anion and cation radicals:
electrochemiluminescence (ECL).
6.7 The way to produce light:
§10. 6 Photochemistry
6.8 Laser and laser chemistry:
1917, Einstein proposed the possibility of laser.
1954, laser is realized.
1960, laser is commercialized.
Population inversion
light amplification by stimulated
emission of radiation
§10. 6 Photochemistry
6.8 Laser and laser chemistry:
(1) Chemical HF - HBr laser: 2.7 m
and 4.2 m
(2) A chemical non-chain DF laser 3.5
to 4.1 m
(3) Supersonic chemical oxygen-iodine
lasers
(4) Chemical HF laser
(5) N2O-laser
(6) Pulsed HF/DF lasers
§10. 6 Photochemistry
(1) High power: emission interval: 10-9, 10-11, 10-15.
100 J sent out in 10-11s =1013 W;
temperature increase 100,000,000,000 oCs-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,
Specialities of laser
6.8 Laser and laser chemistry:
§10. 6 Photochemistry
6.8 Laser and laser chemistry:
Laser-induced reaction
Laser Heating--absorption
Laser cooling—emission
Regulation of
molecular state
Laser cooling refers to a technique in which
atomic and molecular samples are cooled down
to near absolute zero through the interaction
with one or more laser fields.
All laser cooling techniques rely on the fact
that when an object (usually an atom) absorbs
and re-emits a photon, its momentum changes.
§10. 6 Photochemistry
William Daniel Phillips
born Nov. 5, 1948
American physicist.
For Laser cooling
朱棣文(born Feb. 28, 1948)
American physicist
cooling and trapping of
atoms with laser light
Claude Cohen-Tannoudji
born 1 Apr. 1933
a French physicist.
He shared the 1997 Nobel Prize
in Physics.
6.8 Laser and laser chemistry:
§10. 6 Photochemistry
Discussion
(1) Should photochemical processes obey thermodynamics?
(2) Can laser cooling break the second thermodynamic law?
(3) How can we increase the energy efficiency of TiO2 in photolysis of
water?
(4) Explain the principle by using electrooxidation and reduction to
produce light based on polymeric semiconductor.
§10. 6 Photochemistry