Chapter XI Mechanism of Complex Reactions --photochemistry

Chapter XI Mechanism of Complex Reactions 12.6 Photochemistry 12.6.1 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) Spectrum of visible light 400 nm 800 nm red orange yellow green blue indigo violet 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 310-5 m 3.99 kJ mol-1 far-infrared near-infrared 800 nm 149.5 kJ mol-1 400 nm 299.0 kJ mol-1 150 nm 797.9 kJ mol-1 visible ultra-violet vacuum violet 5 nm 239104 kJ mol-1 X-ray 5 nm 1.20109 kJ mol-1 4) Absorption of light beam absorption transmission dI   adx I I  I 0 exp( ax) I a  I 0  I  I 0 [1  exp( ax)] refraction Reflection Scattering Lambert’s law: I- intensity of light, x the thickness of the medium, a the absorption coefficient. 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 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. Excitation between different electronic level Radiation-less decay Jablonsky diagram 7) Decay of photoexcited molecules Radiation transition Fluorescence and phosphorescence non-reactive decay Radiationless transition decay Reaction of excited molecule A*  P reactive decay Energy transfer: A* + Q  Q*  P Vibrational cascade and thermal energy 8) Spectroscopy and radiochemistry IR and UV-Vis spectrum Infrared spectrum excitation between different vibrational levels, no reaction. Phonon ~ heat. radiochemistry X-rays, electron beam, neutron beam, -, -, - radiation, and cosmic radiation 12.6.2 Photochemistry The first law of photochemistry: Grotthuss and Draper, 1818: Only the light that is absorbed by a substance is effective in producing a photochemical change. 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. The activation of any molecule or atom is induced by the absorption of single light quantum.  = Lh = 0.1196  J mol-1 one einstein 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 s1 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. 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· AB- + C+ ABC + h ABC+ + eABC* Dissociation into radicals ions photoionization Activated molecules Intramolecular rearrangement ACB Secondary photochemical process Energy transfer: A* + Q  Q* donor acceptor Q*  P (sensitization), A*:sensitizer Q* +A (quenching), Q:quencher 12.6.3 kinetics and equilibrium of photochemical reaction For primary photochemical process R + h  Ia 2 *  R k P r  kI a Zeroth-order reaction Secondary photochemical process HI + h  H + I H + HI  H2 + I I + I  I2 d [ HI ] d[ H ]   kI a  k 2 [ H ][ HI ]   kI a  k2 [ H ][ HI ]  0 dt 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 r + = k +I a r- = k-[B] k [ B]  Ia k B At equilibrium The composition of the equilibrium mixture is determined by radiation intensity. 12.5.4 quantum yield and energy efficiency Quantum yield or quantum efficiency (): 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 For H2+ Br2 2HBr n  = 104 ~ 106  = 0.01  < 1, the physical deactivation is dominant  = 1, product is produced in primary photochemical process  > 1, initiate chain reaction. Energy efficiency:  = ————————— Total light energy Light energy preserved 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 red light with wave-length of 700 nm 12.6.5 The way to harness solar energy Solar  heating: Solar  electricity: photovoltaic cell photoelectrochemical cell Solar  chemical energy: Ag Conducting band electron p-Si hole Valence band Photoelectrochemistry and Photolysis Photolysis of water Photooxidation of organic pollutant Ag TiO2 Photochemical reaction: S + h   S* S* +R S+ + R- S = Ru(bpy)32+ 4S+ + 2H2O  4S + 4H+ + O2 2R-+ 2H2O  2R + 2OH-+ H2 Photosensitive reaction 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 Porphyrin complex with magnesium 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. 12.6.6 the way to produce light: Chemical laser and chemiluminescence h h Chemical reaction? pumping 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 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). V V * * * * * PPV+PEO +LiCF3SO3 V Ca MEH-PPV ITO glass S.-Y. ZHANG, et al. Functional Materials, 1999, 30(3):239-241 MEH-PPV frfy Luciferase: can send out chemiluminescence in 10-13 ~ 10-15 mol dm3 ATP. Moon jelly Laser: light amplification by stimulated emission of radiation Population inversion n’ level Radiationless transition m upper level Excitation / pump Radiation transition 1917, Einstein proposed the possibility of laser. 1954, laser is realized. 1960, laser is commercialized. n lower level 1) High power: emission interval: 10-9, 10-11, 10-15. 100 J sent out in 10-11s =1013 W. temperature increase 100,000,000,000oC 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, Exercises: p. 880, ex. 16