levine, pp. 800-804 photochemistry

§10. 6 Photochemistry

Out-class reading:

Levine, pp. 800-804 photochemistry

6.1 Brief introduction of light

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


4) Interaction between light and media

)exp(0 axII −=

adxI

dI=−

)]exp(1[00 axIIIIa −−=−=

I: intensity of light,

x: the thickness of the medium,

a: the absorption coefficient.


Beer’s law:

0 exp( )aI I cx= −

Lambert’s law


Light

beam

Upon photo irradiation, 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

(1) Photoexcitation:6.2 Physical processes of Excitation and decay


The Foundation of a typical Jablonski Diagram

(1) Photoexcitation:

6.2 Physical processes of Excitation and decay


A. Jablonski, Efficient of Anti-Stokes Fluorescence in Dyes,

Nature, 1933, Jun. 10, 839-840

Jablonski diagram



Absorbance

Vibrational Relaxation and Internal Conversion

Intersystem Crossing

Fluorescence

Phosphorescence

光子光激发基态激发态能级能带雅布

隆斯基（Jablonski）图激发态衰变振动弛

豫内转换系间穿越辐射跃迁无辐射跃迁

单线态三线态荧光磷光

Radiation-less decayWhich is which?



Transition Time Scale Radiative Process?

Internal Conversion 10-14

- 10-11

s no

Vibrational Relaxation 10-14

- 10-11

s no

Absorption 10-15

s yes

Phosphorescence 10-4

- 10-1

s yes

Intersystem Crossing 10-8

- 10-3

s no

Fluorescence 10-9

- 10-7

s yes



https://chem.libretexts.org/Textbook_Maps/Physical_and_Theoretical_Chemistry_Textbook_Maps/Suppl

emental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Jablo

nski_diagram

(2) 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 Physical processes of Excitation and decay


6.3 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

6.3 Photochemistry

= Lh = 0.1196 J mol-1

one einstein

E = h F-F


http://en.wikipedia.org/wiki/File:Johannes_Stark.jpg

http://en.wikipedia.org/wiki/File:Einstein_patentoffice.jpg

primary step of photochemical process:

A chemical reaction wherein the photon is one of the reactant.

S + h → S*

h

6.3 Photochemistry

What is the nature of activation energy of a photochemical reaction?


The primary photochemical process:

S + h → S*

Some primary photochemical process for molecules

6.3 Photochemistry


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 Photochemistry


http://en.wikipedia.org/w/index.php?title=Photoinitiatiors&action=edit&redlink=1



Under high intensive radiation, absorption of multi-photon may occur.

A + h →A*

A* + h →A**

Under ultra-high intensive radiation, SiF6 can absorb 20~ 40 protons.

These multi-photon 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.

About multi-photon absorption

6.3 Photochemistry


6.4 Kinetics and equilibrium of photochemical reaction

For primary photochemical process

2*R R PaI kh+ ⎯⎯→ ⎯⎯→


Secondary photochemical process

HI + h H + I

H + HI H2 + I

I + I → I2

Generally, the primary photochemical

reaction is the r. d. s.

⎯→⎯ 2k


ak⎯⎯→

Decomposition of HI. H-I = 298 kJ mol-1

Why do we write this as the step?


For opposing reaction with participation of photon:

At equilibrium

The composition of the equilibrium mixture is determined by radiation intensity.

Dark reaction and photochemical reaction. Application.



6.5 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: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 Quantum yield and energy efficiency


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

6.6 The way to harness solar energy—photosynthesis


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

6.6 The way to harness solar energy


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.


Solar ⎯→ heating:

Solar ⎯→ electricity: photovoltaic cell / photoelectrochemical cell

Solar ⎯→ chemical energy:



Photolysis of water/

Photooxidation of organic pollutant

Photochemical reaction—photocatalysts ??

S + h → S*

S* + R → S+ + R-

4S+ + 2H2O → 4S + 4H+ + O2

2R-+ 2H2O → 2R + 2OH-+ H2

S = Ru(bpy)32+



Photolysis of water based on semiconductors


TiO2 the most important photocatalyst.

Modification of TiO2.


6.7 The way to produce light:

Photoluminescence, Electroluminescence, Chemiluminescence,

Electrochemiluminescence, Light-emitting diode

Chemiluminescence


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 of

lampyridae, is one of a number of

bioluminescent insects capable of

producing a chemically created, cold

light.



MEH-PPV

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:

electrochemiluminescence (ECL).



6.8 Laser and laser chemistry:

1917, Einstein proposed the possibility of laser.

1954, laser is realized.

1960, laser is commercialized.

Population inversion

light amplification by stimulated

emission of radiation



(1) Chemical HF - HBr laser: 2.7 m

and 4.2 m

(2) A chemical non-chain DF laser 3.5

to 4.1 m

(3) Supersonic chemical oxygen-iodine

lasers

(4) Chemical HF laser

(5) N2O-laser

(6) Pulsed HF/DF lasers


(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-induced reaction

Laser Heating--absorption

Laser cooling—emission

Regulation of

molecular state

Laser cooling refers to a technique in which

atomic and molecular samples are cooled down

to near absolute zero through the interaction

with one or more laser fields.

All laser cooling techniques rely on the fact

that when an object (usually an atom) absorbs

and re-emits a photon, its momentum changes.


William Daniel Phillips

born Nov. 5, 1948

American physicist.

For Laser cooling

朱棣文(born Feb. 28, 1948)

American physicist

cooling and trapping of

atoms with laser light

Claude Cohen-Tannoudji

born 1 Apr. 1933

a French physicist.

He shared the 1997 Nobel Prize

in Physics.



Discussion

(1) Should photochemical processes obey thermodynamics?

(2) Can laser cooling break the second thermodynamic law?

(3) How can we increase the energy efficiency of TiO2 in photolysis of

water?

(4) Explain the principle by using electrooxidation and reduction to

produce light based on polymeric semiconductor.


levine, pp. 800-804 photochemistry

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