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

hCChh

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 violet760-630 nm 630-600 nm 600-570 nm 570-500 nm 500-450 nm 450-430 nm 430-400 nm

radio 3105 m3.9810-8 kJ mol-1

310-1 m3.9810-4 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

5 nm 239104 kJ mol-1

5 nm 1.20109 kJ mol-1

micro-wave

far-infrared

near-infrared

visible

ultra-violet

vacuum violet

X-ray

C

h

4) Interaction between light and media

refraction

transmission

absorption

Reflection Scattering

adxI

dI

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

)exp(0 axII

)]exp(1[00 axIIIIa

I- intensity of light, x the thickness of the medium, a the absorption coefficient.

adxI

dI

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.

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.

5) Photoexcitation:

Excitation between different electronic level

Process Transition Timescale (sec)

Light Absorption (Excitation) S0 → Sn ca. 10-15 (instantaneous)

Internal Conversion Sn → S1 10-14 to 10-11

Vibrational Relaxation Sn* → Sn 10-12 to 10-10

Intersystem Crossing S1 → T1 10-11 to 10-6

Fluorescence S1 → S0 10-9 to 10-6

Phosphorescence T1 → S0 10-3 to 100

Non-Radiative DecayS1 → S0

T1 → S0 10-7 to 10-5

10-3 to 100

Jablonsky diagram

Radiation-less decay

Fluorescent minerals emit visible light when exposed to ultraviolet light

Endothelial cells under the microscope with three separate channels marking specific cellular components

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

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 = 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 s-1 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

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.

For opposing reaction:

A B

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

At equilibrium [z] a

kI

k

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.

a

n r

I

For H2+ Cl2 2HCl = 104 ~ 106

For H2+ Br2 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:

6CO2 + 6H2O + nh C6H12O6 + 6O2 rGm = 2870 kJ mol-1

For formation of a glucose, 48 light quanta was needed.

%7.354.16748

2870

red light with wave-length of 700 nm

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

TiO2Ag

Photolysis of water

Photooxidation 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 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 carbo

n 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 an

d Norrish reactions.

(6) Many polymerizations are started by photoinitiator , which decompose

upon absorbing light to produce the free radicals for Radical polymerizati

on.

(7) In photoresist technology, used in the production of microelectronic c

omponents.

6.6 the way to produce light: Chemical laser and 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 transitionCF3I 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).

PPV+PEO+LiCF3SO3

*****

V V

MEH-PPV

V

glassITO

MEH-PPV

Ca

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

firefly

The firefly, belonging to the family Lampyridae, is one of a

number of bioluminescent insects capable of producing a c

hemically created, cold light.

http://yahooligans.yahoo.com/content/animals/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, 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,

Specialities of laser