10. 4 Photochemistry

4.1 Brief introduction The branch of chemistry which deals with the study of chemical reaction initiated by light. 1) photochemistry 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. 2) Energy of photon

3) Spectrum of visible light Rainbow, the natural spectrum of visible light 400 nm760 nm red orange yellow green blue indigo violet nm nm nm nm nm nm nm

radio 3  10 5 m3.98  kJ mol -1 3  m3.98  kJ mol -1 6  m 1.99  kJ mol -1 3  m 3.99 kJ mol nm kJ mol nm kJ mol nm kJ mol -1 5 nm 239  10 4 kJ mol -1 5 nm 1.20  10 9 kJ mol -1 micro-wave far-infrared near-infrared visible ultra-violet vacuum violet X-ray

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

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 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.

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 s. Once excited, it decays at once. 5) Photoexcitation: Excitation between different electronic level

ProcessTransition Timescale (sec) Light Absorption (Excitation)S 0 → S n ca (instantaneous) Internal ConversionS n → S to Vibrational RelaxationS n * → S n to Intersystem CrossingS 1 → T to FluorescenceS 1 → S to PhosphorescenceT 1 → S to 100 Non-Radiative Decay S 1 → S 0 T 1 → S to to 100

Jablonsky diagram Radiation-less decay

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

7) Decay of photoexcited molecules decay non-reactive decay reactive decay Radiation transition Radiationless transition Fluorescence and phosphorescence Vibrational cascade and thermal energy Reaction of excited molecule A *  P Energy transfer: A * + Q  Q *  P

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.

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  = 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, SiF 6 can absorb 20~ 40 protons. These multi-proton absorption occur only at I = proton s -1 cm -3, life-time of the photoexcited species > s. Commonly, I = ~ proton s -1 cm -3, life-time of A * < 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 ABC + h  AB· + C· Dissociation into radicals AB - + C + Ions Photoionization ABC + + e - photoionization ABC * Activated molecules Photoexcitation ACB Intramolecular rearrangement Photoisomerization

Energy transfer: A * + Q  Q * Q * +A (quenching), Q:quencher Q *  P (sensitization), A * :sensitizer Secondary photochemical process donoracceptor 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  H 2 + I I + I  I 2 Generally, the primary photochemical reaction is the r. d. s.

For opposing reaction: A B r + = k + I a r - = k - [B] 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 H 2 + Cl 2  2HCl  = 10 4 ~ 10 6 For H 2 + Br 2  2HBr  = 0.01  < 1, the physical deactivation is dominant  = 1, product is produced in primary photochemical process  > 1, initiate chain reaction.

Energy efficiency:  = ————————— Light energy preserved Total light energy Photosynthesis: 6CO 2 + 6H 2 O + nh   C 6 H 12 O 6 + 6O 2  r G m = 2870 kJ mol -1 For formation of a glucose, 48 light quanta was needed. red light with wave-length of 700 nm

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

TiO 2 Ag Photolysis of water Photooxidation of organic pollutant Photochemical reaction: S + h   S * S * + R  S + + R - 4S + + 2H 2 O  4S + 4H + + O 2 2R - + 2H 2 O  2R + 2OH - + H 2 S = Ru(bpy) 3 2+

Photosensitive reaction Reaction initiated by photosensitizer. 6CO 2 + 6H 2 O + nh   C 6 H 12 O 6 + 6O 2 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. 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. Fd is a protein with low molecular weight 4Fd ADP P 2-  4Fd ATP 4- + O 2 + H 2 O + H + 3ATP Fd 2+ + CO 2 + H 2 O + H + 3P 2-  (CH 2 O) + 3ADP P Fd 3+ 8h8h

All the energy on the global surface comes from the sun. The total solar energy reached the global surface is 3  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.

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.

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

The reverse process of photochemistry A + BC  AB * + C High pressure: collision deactivation Low pressure: radiation transition CF 3 I  CF 3 + I * H + Cl 2  HCl * + Cl A + + A -  A 2 * 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).

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

firefly The firefly, belonging to the family Lampyridae, is one of a number of bioluminescent insects capable of producing a chemically created, cold light. als/photo/9807.html

Moon jelly

Laser: light amplification by stimulated emission of radiation 1917, Einstein proposed the possibility of laser. 1954, laser is realized. 1960, laser is commercialized. Population inversion Excitation / pump n lower level n’ level m upper level Radiationless transition Radiation transition

1)High power: emission interval: 10 -9, , J sent out in s =10 13 W. temperature increase 100,000,000,000 o C s -1 2) Small spreading angle: 0.1 o 3) High intensity: 10 9 times that of the sun. 4) High monochromatic: Ke light:  = nm, for laser:  = nm, Specialities of laser