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Photochemistry Iphotochemistry.epfl.ch/PC/PC1_Lesson_1.pdf · 7 Photochemistry (light-induced...
Transcript of Photochemistry Iphotochemistry.epfl.ch/PC/PC1_Lesson_1.pdf · 7 Photochemistry (light-induced...
Prof. Jacques-E. Moser
http://photochemistry.epfl.ch/PC.html
Photochemistry I
CH-442
Content
PHOTOCHEMISTRY I
1. Basic principles1.1 Introduction1.2 Laws of light absorption1.3 Radiation and molecular orbitals1.4 Selection rules1.5 Light absorption by solids
2. Molecular photophysics2.1 Excited state's deactivation pathways2.2 Kinetics of photochemical processes2.3 Intermolecular energy transfer
3. Photochemical reactions3.1 Photodissociation3.2 Light-induced electron transfer
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Content (contd)
4. Synthetic organic reactions4.1 Reactions of ethenes and aromatic compounds4.2 Photochemistry of carbonyl chromophore4.3 Photo-oxygenation reactions
5. Polymer photochemistry5.1 Photo-polymerization and cross-linking5.2 Photodegradation and stabilization of polymers
6. Natural photochemical processes6.1 Atmospheric reactions6.2 Photochemistry of waters and soils6.3 Natural photosynthesis6.4 Mechanisms of vision
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1. Basic principles
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Photochemistry (light-induced chemistry)
Chemistry: forming or breaking of chemical bonds and charge transfer within or between molecules.
Photochemical reactions are processes during which the energy required for their activation (ΔU‡ ) or their development (ΔGr ) is provided by an electromagnetic radiation.
Activation energies of the order of ΔU‡ = 100 kJ·mol–1 and bond energies of the order of ΔG = 200-400 kJ·mol–1 imply absorption of photons that should individually carry an equivalent amount of energy.
100 200 300 400 500 600 700 800 900 1000
1000015000200003000050000100000
10.0 5.0 4.0 3.0 2.5 2.0 1.7 1.5 1.3
λ / nm
�ν / cm -1
E / eV
UV A B C bleu vert
jaun
e
roug
e
NIR
Ultra-violet Visible Infra-rouge
Ultraviolet Infrared
Blue
Blue
Gre
en
Yello
w
Red
E / kJ·mol–1800 400 100200300 150
1.1 Introduction
Bond ΔH [kJ mol–1] λ [nm] Bond ΔH [kJ mol–1] λ [nm]
H–H 436 274 N–N 160 748
C–H 413 290 N=N 631 190
N–H 393 304 N�N 941 127
P–H 297 403 N–O 201 595
C–C 347 345 N–P 297 403
C–O 358 334 O–H 464 258
C–N 305 392 O–S 265 451
C-Cl 397 301 O–Cl 269 445
C=C 607 197 O–O 204 586
C=O 805 149 C–F 552 216
O=O 498 240 C–S 259 461
Bond energies
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CO2 + H2O hνchloroplastes⎯ →⎯⎯⎯ 1
6 C6H12O6 + O2 ΔG = 496 kJ ⋅mol–1
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a) ΔGr < 0 (exergonic reaction, spontaneous) Light enable for overcoming the activation barrier or to lower it by acting as a catalyst.
Such reactions are called "photocatalytic"
b) ΔGr > 0 (endergonic, non spontaneous)
Energy required by the reaction is brought by light. Light energy is (partially) converted into chemical energy.
Example: H2 + Cl 2 → 2 HCl
Example: Natural photosynthesis
∆∆
∆
Types of photochemical reactions
a) Light as a reactant- synthesis of B - reaction inhibition (photo-stabilization of A)
b) Light as an energy vector
- endergonic formation of B - energy storage
c) Light as information vector
- optical absorption profile (photography, information storage) - charge density profile (xerography) - 3D material profile (photolithography)
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Functions associated with light
A hν⎯ →⎯ B (± ΔG)
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Theodor von Grotthuss(1785-1822)
John W. Draper(1811-1882)
Grotthuss-Draper law (1812, 1842)
Light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
Johannes Stark(1874-1957)
Albert Einstein(1879-1955)
Stark-Einstein law (1908-1913)
Also known as the "photo-equivalence law"For each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction.
ΔGmolecule = NA ⋅hν = NA ⋅hcλ
1 Einstein = 1 mol of photons = NA photons
Fundamental laws of photochemistry
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Johann H. Lambert(1728-1777)
Lambert's law
Phenomenological (macroscopic) law of absorption
α = absorption constant [cm–1]
x0
I0 IT
IR
xl
l
IT = I0 − IA − IR
I(x) = I0 ⋅ exp(−αx)
ln I(x)I0
= −αx ln ITI0
= lnT = –αl
T =ITI0
transmittance IAI0
absorptanceIRI0
reflectance
A = − log ( ITI0
) = – logT absorbance
Link with the medium's complex refractive index:
⌢n = n − iκ [−]
α =4π ⋅κλ0
κ = absorption coefficient [-]
(imaginary part of the refractive index)
1.2 Laws of light absorption
Example: c = 10–3 M, = 104 mol–1 · L · cm–1
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molar decadic extinction coefficient
I0 IT
l
c A = − log ITI0
= − logT = ε ⋅ c ⋅ l [-]
ε
c molar concentration [mol l–1]l optical pathlength [cm]
Superimposition of absorbing systems
Beer-Lambert Law
Transmittance is multiplicative:
Absorbance is additive:
ε
⇒ T = 0.01, A = 2 ⇒ 99% of the light is absorbed within the first 2 mm of the solution
Ttot = Tii∏
Atot = Aii∑
August Beer(1825-1863)
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Justification of Beer-Lambert law
Initial assumptions
- individual molecules totally block light within a characteristic cross-section σ- monochromatic light
- molecules do not cast any shadow on each other (only conceivable if the concentration c is very low)
dx
σS
I(x+dx)
I(x)
Absorptance of a solution volume S·dx containing n molecules :
−dII(x)
= I(x + dx) − I(x)I(x)
= n ⋅σS = c ⋅S ⋅NA ⋅dx ⋅σ
S = c ⋅σ ⋅NA ⋅dx
−1I(x)
dI = c ⋅σ ⋅NA ⋅dx
By defining : ε = σ ⋅NA ⋅ log(e) = σ ⋅NA
2.303⇒
0
l
∫⎯ →⎯ − ln I
I0 = c ⋅σ ⋅ l ⋅NA
− log II0 = A = ε ⋅ c ⋅ l
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Absorption by non-continuous media
n0 = 1
n ≠ 1
I0 = IR + IA + IT
Rs = IR / I0 specular reflectance
Absorption and reflexion by a specular (mirror-like) surface
Fresnel law
Augustin Fresnel(1788-1827)
Rs =IRI0
=(n −1)2 + n2 ⋅κ 2
(n +1)2 + n2 ⋅κ 2ϕ = 0at
0 2 4 6 8 10 12 14 160
0.2
0.4
0.6
0.8
1.0
κ [−]
n = 1.414
R s
ϕI0 IR
IT
ϕ
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Absorption by a scattering medium
I0 = IRd + IA + ITDiffuse reflectance
Schuster-Kubelka-Munk theory
IT
I0
IT
IRd
I0
IT
IRd
dx
x
l
0
I(x)
I(x+dx) J
I0
Phenomenological extinction constants:
k [cm−1] absorption ks→0 = −1dx
⋅ ln dII0 { }−dI = −kI ⋅2dx − sI ⋅2dx + sJ ⋅2dx
dJ = −kJ ⋅2dx − sJ ⋅2dx + sI ⋅2dx
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Kubelka and Kubelka-Munk equations
Kubelka's hyperbolic solutions
Kubelka-Munk simplified solution
IT
a = 1+ ks
b = a2 −1
Rg = background reflectancewith:
l→∞ ⇒ F(R∞ ) = ks = (1− R∞ )
2
2R∞
Absorber homogeneously dispersed in a scattering medium (powder)
F(R∞ ) = (1− R∞ )
2
2R∞
= ε ⋅ c ⋅ ln(10)s
T =b
a ⋅ sinh (b ⋅2s ⋅ l) + b ⋅ cosh (b ⋅2s ⋅ l)
R =1− Rg (a − b ⋅ coth (b ⋅2s ⋅ l))a + b ⋅ coth (b ⋅2s ⋅ l) − Rg
k [cm–1] = ln(10) ⋅ε [mol–1 L cm–1]⋅c [mol L–1]
Integrating sphere for diffuse reflectance spectroscopy
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B. Total reflectance
Specular white plate
A. Diffuse reflectance
Specular light trap