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 Where h is the Plank constant, C the velocity of light in vacuum, the wave-length of the light, and the wave number.
3105 m 3.9810-8 kJ mol-1 310-1 m 3.9810-4 kJ mol-1 610-4 m 1.9910-1 kJ mol-1 310-5 m 3.99 kJ mol-1 800 nm 149.5 kJ mol-1 400 nm 299.0 kJ mol-1 150 nm 797.9 kJ mol-1 50 nm 239104 kJ mol-1 5 nm 1.20109 kJ mol-1 radio micro-wave Microwave chemistry far-infrared near-infrared visible photochemistry ultra-violet vacuum violet radiochemistry X-ray
400 nm 760 nm 760-630 nm 630-600 nm 600-570 nm 570-500 nm 500-450 nm 450-430 nm 430-400 nm orange green blue indigo violet red yellow 3) Spectrum of visible light
absorption transmission Reflection refraction Scattering 4) Interaction between light and media 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 dx I- intensity of light, x the thickness of the medium, a the absorption coefficient.
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.
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
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 7) Decay of photoexcited molecules
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. one einstein = Lh = 0.1196 J mol-1
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 AB· + C· Dissociation into radicals AB- + C+ Ions Photoionization ABC+ + e- ABC + h 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 Zeroth-order reaction
Secondary photochemical process HI + h H + I H + HI H2 + I I + I I2 Generally, the primary photochemical reaction is the r. d. s.
For opposing reaction: k+ A + hB k r- = k-[B] r+ = k+Ia At equilibrium The composition of the equilibrium mixture is determined by radiation intensity.
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, 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.
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: gap
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+
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 Jy-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.
h h pumping Chemical reaction? 6.6 The way to produce light: Chemiluminescence 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* firefly H + Cl2 HCl*+ Cl 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 V ***** V Ca PPV+PEO+LiCF3SO3 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). MEH-PPV S.-Y. ZHANG, et al.Functional Materials, 1999, 30(3):239-241
n’ level Radiationless transition Excitation / pump m upper level Radiation transition n lower level 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.
Specialities of laser • High power: emission interval: 10-9, 10-11, 10-15. 100 J sent out in 10-11s =1013 W； • temperature increase 100,000,000,000 oCs-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