CH-442. Photochemistry I. Prof. Jacques-E. Moser.

Transcription

1 CH-442 Photochemistry I Prof. Jacques-E. Moser

2 Content PHOTOCHEMISTRY I 1. Basic principles 1.1 Introduction 1.2 Laws of light absorption 1.3 Radiation and molecular orbitals 1.4 Selection rules 1.5 Light absorption by solids 2. Molecular photophysics 2.1 Excited state's deactivation pathways 2.2 Kinetics of photochemical processes 2.3 Intermolecular energy transfer 3. Photochemical reactions 3.1 Photodissociation 3.2 Light-induced electron transfer 3 Content (contd) 4. Synthetic organic reactions 4.1 Reactions of ethenes and aromatic compounds 4.2 Photochemistry of carbonyl chromophore 4.3 Photo-oxygenation reactions 5. Polymer photochemistry 5.1 Photo-polymerization and cross-linking 5.2 Photodegradation and stabilization of polymers 6. Natural photochemical processes 6.1 Atmospheric reactions 6.2 Photochemistry of waters and soils 6.3 Natural photosynthesis 6.4 Mechanisms of vision 4

3 1. Basic principles

4 1.1 Introduction 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 = kj mol 1 imply absorption of photons that should individually carry an equivalent amount of energy. Ultraviolet Ultra-violet Visible Infrared Infra-rouge UV A B C Blue bleu Green vert Yellow jaune Red rouge NIR λ / nm ν / cm -1 E / ev E / kj mol 1 7 Bond energies Bond ΔH [kj mol 1 ] λ [nm] Bond ΔH [kj mol 1 ] λ [nm] H H N N C H N=N N H N N P H N O C C N P C O O H C N O S C-Cl O Cl C=C O O C=O C F O=O C S

5 Types of photochemical reactions 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" Example: H 2 + Cl 2 2 HCl b) ΔGr > 0 (endergonic, non spontaneous) Energy required by the reaction is brought by light. Light energy is (partially) converted into chemical energy. Example: Natural photosynthesis hν CO 2 + H 2 O 1 6 C 6H 12 O 6 + O 2 ΔG = 496 kj mol 1 chloroplastes 9 Functions associated with light A hν B (± ΔG) 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) 10

6 Fundamental laws of photochemistry Grotthuss-Draper law (1812, 1842) Light must be absorbed by a chemical substance in order for a photochemical reaction to take place. Stark-Einstein law ( ) 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. ΔG molecule = N A hν = N A hc λ Theodor von Grotthuss ( ) John W. Draper ( ) 1 Einstein = 1 mol of photons = NA photons Johannes Stark ( ) Albert Einstein ( ) 11

7 1.2 Laws of light absorption Phenomenological (macroscopic) law of absorption IR I T = I A I R I0 x0 l xl IT I R T = I T transmittance reflectance I A A = log ( I T ) = logt absorbance absorptance Lambert's law I(x) = exp( αx) Johann H. Lambert ( ) ln I(x) = αx ln I T α = absorption constant [cm 1 ] = lnt = αl Link with the medium's complex refractive index: n = n iκ [ ] κ = absorption coefficient [-] α = 4π κ λ 0 (imaginary part of the refractive index) 13 Beer-Lambert Law I0 c l IT A = log I T = logt = ε c l [-] c molar concentration [mol l 1 ] l optical pathlength [cm] ε molar decadic extinction coefficient Example: c = 10 3 M, ε = 10 4 mol 1 L cm 1 T = 0.01, A = 2 99% of the light is absorbed within the first 2 mm of the solution August Beer ( ) Superimposition of absorbing systems Transmittance is multiplicative: Absorbance is additive: T tot = A tot = i i T i A i 14

8 Justification of Beer-Lambert law S I(x) σ Initial assumptions - individual molecules totally block light within a characteristic cross-section σ - monochromatic light dx I(x+dx) - molecules do not cast any shadow on each other (only conceivable if the concentration c is very low) Absorptance of a solution volume S dx containing n molecules : di I(x + dx) I(x) = = n σ = c S N A dx σ = c σ N A dx I(x) I(x) S S 1 I(x) di = c σ N A dx l ln I 0 = c σ l N A By defining : ε = σ N A log(e) = σ N A log I = A = ε c l 15 Absorption by non-continuous media Absorption and reflexion by a specular (mirror-like) surface = I R + I A + I T R s = I R / Fresnel law specular reflectance R s = I R = (n 1)2 + n 2 κ 2 at ϕ = 0 (n + 1) 2 + n 2 κ 2 n0 = 1 n 1 ϕ ϕ I T I R Rs κ [ ] n = Augustin Fresnel ( ) 16

9 Absorption by a scattering medium Diffuse reflectance = I Rd + I A + I T Schuster-Kubelka-Munk theory x I0 IRd I0 IT l I(x) dx I0 IRd 0 I(x+dx) J Phenomenological extinction constants: k [cm 1 ] absorption k s 0 = 1 dx ln di { di = ki 2dx si 2dx + sj 2dx dj = } kj 2dx sj 2dx + si 2dx IT 17 I Kubelka and Kubelka-Munk equations Kubelka's hyperbolic solutions R = 1 R g (a b coth (b 2s l)) a + b coth (b 2s l) R g T = b a sinh (b 2s l) + b cosh (b 2s l) with: R g = background reflectance a = 1+ k s b = a 2 1 Kubelka-Munk simplified solution l F(R ) = k s = (1 R )2 2R Absorber homogeneously dispersed in a scattering medium (powder) k [cm 1 ] = ln(10) ε [mol 1 L cm 1 ] c [mol L 1 ] F(R ) = (1 R ) 2 2R = ε c ln(10) s 18 I

10 Integrating sphere for diffuse reflectance spectroscopy Specular light trap Specular white plate A. Diffuse reflectance B. Total reflectance 19