§10. 6 Photochemistry

6.1 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

radio micro-wave far-infrared near-infrared visible ultra-violet vacuum violet 3  10 5 m 3.98  kJ mol -1 3  m 3.98  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 nm 239  10 4 kJ mol -1 5 nm 1.20  10 9 kJ mol -1 X-ray photochemistry radiochemistry Microwave chemistry

3) Spectrum of visible light 400 nm 760 nm red orange yellow green blueindigoviolet nm nm nm nm nm nm nm

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

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. IR spectrum 5) Photoexcitation:

Jablonsky diagram Radiation-less decay Which is which?

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

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 One quantum of radiation absorbed by a molecule activates one molecule in the primary step of photochemical process. Einstein and Stark, 1912 The activation of any molecule or atom is induced by the absorption of single light quantum.  = Lh  = J mol -1 one einstein A chemical reaction wherein the photon is one of the reactant. S + h   S *

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 = photon s -1 cm -3, life-time of the photoexcited species > s. Commonly, I = ~ photon s -1 cm -3, life-time of A * < s. The probability of multi-photon absorption is rare. These multi-proton absorption occur only at I = photon s -1 cm -3, life-time of the photoexcited species > s. Commonly, I = ~ photon s -1 cm -3, life-time of A * < s. The probability of multi-photon absorption is rare. absorption of multi-proton

(3) The primary photochemical process: 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 + h B r + = k + I a r - = k - [B] At equilibrium The composition of the equilibrium mixture is determined by radiation intensity. k+k+ kk

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, 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: 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.

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 gap

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.

6.6 The way to produce light: 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. firefly The firefly, belonging to the family Lampyridae, is one of a number of bioluminescent insects capable of producing a chemically created, cold light.

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

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

Laser Heating Laser cooling