Transcript 幻灯片 1 - Shandong University

§10. 6 Photochemistry
6.1 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
  h  h
C

 hC 
Where h is the Plank constant, C the velocity of light in
vacuum,  the wave-length of the light, and  the wave
number.
3105 m 3.9810-8 kJ mol-1
310-1 m 3.9810-4 kJ mol-1
radio
 h
C

micro-wave
610-4 m 1.9910-1 kJ mol-1
far-infrared
Microwave
chemistry
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
photochemistry
ultra-violet
150 nm 797.9 kJ mol-1
vacuum violet
50 nm 239104 kJ mol-1
5 nm 1.20109 kJ mol-1
radiochemistry
X-ray
3) Spectrum of visible light
400 nm
760 nm
760-630 nm 630-600 nm
red
orange
600-570 nm
yellow
570-500 nm
green
500-450 nm
blue
450-430 nm
430-400 nm
indigo
violet
4) Interaction between light and media
dI

 adx
I
absorption
I  I 0 exp(ax)
transmission
I a  I 0  I  I 0[1  exp(ax)]
Reflection
Scattering
refraction
dx
I- intensity of light, x the thickness
of the medium, a the absorption
coefficient.
Lambert’s 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
I  I 0 exp(ax)
Beer’s 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.
I a  I 0 exp( cx)
 Is the molar extinction coefficient, C the molar concentration.
Both Lambert’s 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.
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.
IR spectrum
Radiation-less decay
Which is which?
Jablonsky diagram
7) Decay of photoexcited molecules
Radiation
transition
Fluorescence and
phosphorescence
Radiationless
transition
Vibrational cascade
and thermal energy
non-reactive
decay
decay
Reaction of excited
molecule A*  P
reactive decay
Energy transfer: A*
+ Q  Q*  P
6.2 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.
(2) The second law of photochemistry / The law of photochemical
equivalence
Einstein and Stark, 1912
One
quantum
of
radiation
absorbed by a molecule activates
one molecule in the primary step of
photochemical process.
A chemical reaction wherein the photon is one of the reactant.
S + h  S*
The activation of any molecule or atom is induced by the
absorption of single light quantum.
 = Lh = 0.1196  J mol-1
one einstein
absorption of multi-proton
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 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.
(3) The primary photochemical process:
S + h  S*
Some primary photochemical process for molecules
ABC + h
AB· + C·
Dissociation into radicals
AB- + C+
Ions Photoionization
ABC+ + e-
photoionization
ABC*
Activated molecules Photoexcitation
ACB
Intramolecular rearrangement
Photoisomerization
Secondary photochemical process
Energy transfer: A* + Q  Q*
donor
acceptor
Q*  P (sensitization), A*:sensitizer
Q* +A (quenching),
Q:quencher
Photosensitization, photosensitizers, photoinitiator
6.3 Kinetics and equilibrium of photochemical reaction
For primary photochemical process
R  h  R  P
Ia
r  kIa
*
k2
Zeroth-order reaction
Secondary photochemical process
k
HI + h 
H+I

k2
H + HI 
H2 + I
I + I  I2
d [HI]

 kI a  k2 [H][HI]
dt
d [H]

 kI a  k2 [H][HI]  0
dt
d [HI]

 kI a  k2 [H][HI]  2kI a
dt
Generally, the primary photochemical reaction is the r. d. s.
For opposing reaction:
A + h
k+
k
r- = k-[B]
r + = k +I a
At equilibrium
B
k
[B] 
Ia
k
The composition of the equilibrium mixture is determined
by radiation intensity.
6.4 Quantum yield and energy efficiency
Quantum yield or quantum efficiency ():
n
r
  
 Ia
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
 > 1, initiate chain reaction.
 = 1, product is produced in primary photochemical process
 < 1, the physical deactivation is dominant
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.
2870

 35.7%
48 167 .4
6.5 The way to harness solar energy
Solar  heating:
Solar  electricity: photovoltaic cell photoelectrochemical cell
Solar  chemical energy:
Ag
p-Si
Conducting band
electron
gap
hole
Photoelectrochemistry and Photolysis
Valence band
Photolysis of water
Photooxidation of organic pollutant
Ag
TiO2
Photochemical reaction:
S + h  S*
S* + R  S+ + R4S+ + 2H2O  4S + 4H+ + O2
2R-+ 2H2O  2R + 2OH-+ H2
S = Ru(bpy)32+
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
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+
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 1~2% of the total incident energy is recovered for a field of corn.
6.6 The way to produce light:
Chemiluminescence
h
h
Chemical
pumping
reaction?
Photoluminescence, Electroluminescence, Chemiluminescence,
Electrochemiluminescence, Light-emitting diode
The reverse process of photochemistry
A + BC  AB* + C
High pressure: collision deactivation
Low pressure: radiation transition
CF3I  CF3 + I*
H + Cl2 
HCl*
+ Cl
firefly
A+ + A-  A2*
Emission of light from
excited-state dye.
The firefly, belonging to the family Lampyridae, is
one of a number of bioluminescent insects capable
of producing a chemically created, cold light.
V
Ca
MEH-PPV
ITO
glass
Emission of light from excited-state dye
molecules can be driven by the electron
transfer
between
electrochemically
generated anion and cation radicals:
electrochemi-luminescence (ECL).
V
MEH-PPV
*
*
*
*
* 30(3):239-241
Functional Materials, 1999,
PPV+PE
O+LiCF
3SO3
S.-Y. ZHANG, et al.
V
Laser: light amplification by stimulated emission of radiation
Population inversion
n’ level
Radiationless transition
1954, laser is realized.
Excitation
/ pump
1917, Einstein proposed
the possibility of laser.
m upper level
Radiation transition
n lower level
1960, laser is
commercialized.
Specialities of laser
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 oC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,
Laser Heating
Laser cooling