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10. 4 Photochemistry

10. 4 Photochemistry. 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.

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10. 4 Photochemistry

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  1. 10. 4 Photochemistry

  2. 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.

  3. 760 nm 400 nm 450-430 nm 760-630 nm 630-600 nm 600-570 nm 570-500 nm 430-400 nm 500-450 nm orange red green yellow indigo blue violet 3) Spectrum of visible light Rainbow, the natural spectrum of visible light

  4. 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

  5. absorption transmission Reflection refraction Scattering 4) Interaction between light and media

  6. 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

  7. 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.  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.

  8. 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

  9. Radiation-less decay Jablonsky diagram

  10. Fluorescent minerals emit visible light when exposed to ultraviolet light Endothelial cells under the microscope with three separate channels marking specific cellular components

  11. 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

  12. 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.

  13. 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.

  14. 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 > 10-8 s. Commonly, I = 1013 ~ 1018 proton s-1 cm-3, life-time of A* < 10-8 s. the probability of multi-proton absorption is rare.

  15. 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

  16. Secondary photochemical process Energy transfer: A* + Q  Q* donor acceptor Q*  P (sensitization), A*:sensitizer Q* +A (quenching),Q:quencher Photosensitization, photosensitizers, photoinitiator

  17. 6.3 kinetics and equilibrium of photochemical reaction For primary photochemical process Zeroth-order reaction

  18. Secondary photochemical process HI + h  H + I H + HI H2 + I I + I I2 Generally, the primary photochemical reaction is the r. d. s.

  19. For opposing reaction: A B r+ = k+Ia r- = k-[B] At equilibrium The composition of the equilibrium mixture is determined by radiation intensity.

  20. 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  < 1, the physical deactivation is dominant = 1, product is produced in primary photochemical process  > 1, initiate chain reaction.

  21. 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

  22. Ag p-Si Conducting band electron hole Photoelectrochemistry and Photolysis Valence band 6.5 The way to harness solar energy Solar  heating: Solar  electricity: photovoltaic cell photoelectrochemical cell Solar  chemical energy:

  23. Photolysis of water Photooxidation of organic pollutant Ag TiO2 Photochemical reaction: S + h  S* S* + R  S+ + R- 4S+ + 2H2O  4S + 4H+ + O2 2R-+ 2H2O  2R + 2OH-+ H2 S = Ru(bpy)32+

  24. 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

  25. 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+

  26. 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.

  27. 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.

  28. h h pumping Chemical reaction? 6.6 the way to produce light: Chemical laser and chemiluminescence Photoluminescence, Electroluminescence, Chemiluminescence, Electrochemiluminescence, Light-emitting diode

  29. 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).

  30. V V ***** V Ca PPV+PEO+LiCF3SO3 MEH-PPV ITO glass S.-Y. ZHANG, et al.Functional Materials, 1999, 30(3):239-241 MEH-PPV

  31. 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

  32. Moon jelly

  33. 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

  34. 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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