§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

hCC

hh

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

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

X-ray

C

h

photochemistry

radiochemistry

Microwave chemistry

3) Spectrum of visible light400 nm760 nm

red orange yellow green blue indigo violet

760-630 nm 630-600 nm 600-570 nm 570-500 nm 500-450 nm 450-430 nm 430-400 nm

4) Interaction between light and media

refraction

transmission

absorption

Reflection Scattering

dx

)exp(0 axII

adxI

dI

)]exp(1[00 axIIIIa

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.

0 exp( )aI I 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.

)exp(0 axII

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

5) Photoexcitation:

Jablonsky diagram

Radiation-less decay

Which is which?

7) Decay of photoexcited molecules

decaydecay

non-reactive decay

non-reactive decay

reactive decay reactive decay

Radiation transition

Radiation transition

Radiationless transition

Radiationless transition

Fluorescence and phosphorescence

Fluorescence and phosphorescence

Vibrational cascade and thermal energy

Vibrational cascade and thermal energy

Reaction of excited molecule A* P

Reaction of excited molecule A* P

Energy transfer: A* + Q Q* 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 = 0.1196 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, 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.

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.

absorption of multi-proton

(3) The primary photochemical process:

S + h S*

Some primary photochemical process for molecules

ABC + hABC + h

AB· + C· AB· + C· Dissociation into radicals

AB- + C+ AB- + C+ Ions Photoionization

ABC+ + e- ABC+ + e- photoionization

ABC*ABC* Activated molecules Photoexcitation

ACBACB Intramolecular rearrangement

Photoisomerization

Energy transfer: A* + Q Q*

Q* +A (quenching), Q:quencher

Q* P (sensitization), A*:sensitizer

Secondary photochemical process

donor acceptor

Photosensitization, photosensitizers, photoinitiator

6.3 Kinetics and equilibrium of photochemical reaction

For primary photochemical process

akIr Zeroth-order reaction

2*R R PaI kh

Secondary photochemical process

HI + h H + I

H + HI H2 + I

I + I I2 2

[HI][H][HI]a

dkI k

dt

2

[H][H][HI] 0a

dkI k

dt

2

[HI][H][HI] 2a a

dkI k kI

dt

Generally, the primary photochemical reaction is the r. d. s.

2k

k

For opposing reaction:

A + h B

r+ = k+Iar- = k-[B]

At equilibrium [B] a

kI

k

The composition of the equilibrium mixture is determined

by radiation intensity.

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.

a

n r

I

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.

%7.354.16748

2870

6.5 The way to harness solar energySolar heating:

Solar electricity: photovoltaic cell photoelectrochemical cell

Solar chemical energy:

Valence band

Conducting band

electron

hole

p-SiAg

Photoelectrochemistry and Photolysis

gap

TiO2Ag

Photolysis of waterPhotooxidation of organic pollutant

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

Reaction initiated by photosensitizer.

6CO2 + 6H2O + nh C6H12O6 + 6O2

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

4Fd3+ + 3ADP3- + 3P2-

4Fd2+ + 3ATP4- + O2 + H2O + H+

3ATP3-+ 4Fd2++ CO2+ H2O + H+ 3P2-

(CH2O) + 3ADP3- + 3P2- + 4Fd3+

8h

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.

6.6 The way to produce light:

Chemiluminescence

h

Chemical

reaction?

pumping

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

CF3I CF3 + I*

H + Cl2 HCl* + Cl

A+ + A- A2*

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+PEO+LiCF

3SO3

*****

V V

MEH-PPV

V

glassITO

MEH-PPV

Ca

S.-Y. ZHANG, et al. Functional Materials, 1999, 30(3):239-241

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, 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,

Specialities of laser

Laser Heating

Laser cooling