Photochemistry

Photochemistry is the study of the interaction of electromagnetic radiation with matter resulting into a physical change or into a chemical reaction.

It is mainly concerned with rates and mechanisms of reactions resulting from the exposure of reactants to light radiations. It is defined as the study of chemical effect produced by light radiations ranging
from 2000 to 8000 A0 wavelength. The photochemical reactions are induced by light radiation and are influenced by intensity of light radiations.

On other hand the reactions which are caused by heat and in absence of light are called thermal or dark reactions. The thermal or dark reactions are influenced by temperature, concentration, presence of
catalyst etc.

Nature of light: -
Light is a form of electromagnetic radiations. All electromagnetic radiations have wave characteristic and travels at the same speed of light i.e. 3 x 1010 𝑐𝑚/𝑠 , but their wave length is different. The unit of wave length is nanometer i.e. 1 𝑛𝑚 = 10−9 𝑚.
𝛾𝑟𝑎𝑦 𝑎𝑛𝑑 𝑋𝑟𝑎𝑦𝑠 have very small wave length i.e. less than 10−11𝑚. while radio waves are in the order of 104𝑐𝑚.
The visible light ranges from the violet region at about 380 nm to red region at 750 nm.

Dark or Thermal reactions: -
These are the ordinary chemical reactions which are influenced by temperature, concentration of reactants, presence of catalyst etc. except light radiations.

 Photochemical reactions: -
A photochemical reaction may be defined as any reaction which is induced or influenced by the action of light on the system

 Processes of photochemical reactions

Primary Processes

  • One molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes.
  • Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or react with another molecule.
  • Photo physical processes include radiative transitions in which the excited molecule emits light in the form of fluorescence or phosphorescence and returns to the ground state, and intramolecular non-radiative transitions in which some or all of the energy of the absorbed photon is ultimately converted to heat.

Secondary process:

Activated species undergoes chemical reaction.

Does not involve the absorption of light.

Eg., Photochemical combination of Cl2 and H2 (It is chain mechanism).

Lambert’s Law:

This law states that decrease in the intensity of monochromatic light with the thickness of the absorbing medium is proportional to the intensity of incident light.

on integration changes to

Where , I0 = intensity of incident light. I=intensity of transmitted light. K= absorption coefficient.

 Beer’s Law :

It states that decrease in the intensity of monochromatic light with the thickness of the solution is not only proportional to the intensity of the incident light but also to the concentration ‘c’ of the solution.

Where, Є = molar absorption coefficient or molar extinction coefficient.

Beer-Lambert Law Statement

The Beer-Lambert law states that:

For a given material sample path length and concentration of the sample are directly proportional to the absorbance of the light.

The Beer-Lambert law is expressed as:

A = εLc 

where,

A is the amount of light absorbed for a particular wavelength by the sample

ε is the molar extinction coefficient

L is the distance covered by the light through the solution

c is the concentration of the absorbing species

The absorbance is related to the ratio of the intensity of light that enters the sample and leaves the sample.

A = log10 (I0/I)

I0 = Incident Light-Intensity of light before sample

I = Transmitted Light – Intensity of light after sample

 

Beer-Lambert Law Graph

Laws of photochemistry:

GrotthuSs-Draper Law (First Law of Photochemistry):

Only the light which is absorbed by a molecule can be effective in producing photochemical changes in the molecule.

Limitations: -
1. It does not show the relationship between the quantity of light absorbed by a substance and the
molecules reacted.
2. It is only applicable to the primary photochemical process and fails to the secondary process.
3. All the absorbed radiations do not cause photochemical reactions

 Stark-Einstein’s Law (Second Law of Photochemistry):

It states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction.

The energy absorbed by one mole of the reacting molecules is

The quantity of energy (𝐸 = 𝑁ℎ𝜗) absorbed per mole is called as Einstein. Einstein is the quantum of energy (𝐸 = 𝑁ℎ𝜗) absorbed per mole of the substance.

 

Quantum yield or Quantum efficiency (): −

The extent of photochemical reaction is given in terms of quantum yield or quantum efficiency. It is defined as the ratio of number of molecules reacting chemically in a given time to the total number of quanta
absorbed in the same time. On other way the quantum efficiency is defined as the number of molecules reacting per quantum or per Einstein.

The quantum yield is depending up on the intensity of light. If the law of photochemical
equivalence is correct. The quantum yield should be unity. Quantum yield may be varying from 0
𝑡𝑜 106

Factors affecting quantum yield:-

1. All primary photochemical process is endothermic. Hence, quantum yield increases with temperature.

2. We know energy absorbed by molecule is inversely proportional to wavelength. Hence, quantum yield will be higher at the lower wavelength and vice versa.

3. As speed of photochemical reaction is proportional to intensity of light. Hence, quantum yield increases with intensity and vice versa.

4. The addition of inert gas in photochemical reaction the quantum yield.

 

Photosensitization

Photosensitized reactions:An electronically excited molecule can transfer its energy to a second species which then undergoes a photochemical process even though it was not itself directly excited.

Luminescence

The glow produced in the body by methods other than action of heat i.e. the production of cold light is called Luminescence.

It is of three types,

Chemiluminescence

The emission of light in chemical reaction at ordinary temperature is called Chemiluminescence

e.g. The light emitted by glow-worms

Fluorescence

Certain substances when exposed to light or certain other radiations absorb the energy and then immediately start re-emitting the energy.

Such substances are called fluorescent substances and the phenomenon is called fluorescence .

e.g Organic dyes such as eosin,fluorescein etc.

 vapour of sodium,mercury,iodine etc.

Phosphorescence

There are certain substances which continue to glow for some time even after the external light is cut off.

Thus, phosphorescence is a slow fluorescence.

Fluorescence and phosphorescence in terms of excitation of electrons

Jablonski diagram

A Jablonski diagram is a diagram that illustrates the electronic states of a molecule and the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity.

Nonradiative transitions are indicated by squiggly arrows and radiative transitions by straight arrows. The vibrational ground states of each electronic state are indicated with thick lines, the higher vibrational states with thinner lines.The diagram is named after the Polish physicist Aleksander Jabłoński

Radiative transitions involve the absorption, if the transition occurs to a higher energy level, or the emission, in the reverse case, of a photon. Nonradiative transitions arise through several different mechanisms, all differently labeled in the diagram. Relaxation of the excited state to its lowest vibrational level is called Vibrational relaxation.

This process involves the dissipation of energy from the molecule to its surroundings, and thus it cannot occur for isolated molecules. A second type of nonradiative transition is internal conversion (IC), which occurs when a vibrational state of an electronically excited state can couple to a vibrational state of a lower electronic state. A third type is intersystem crossing (ISC); this is a transition to a state with a different spin multiplicity. In molecules with large spin-orbit coupling, intersystem crossing is much more important than in molecules that exhibit only small spin-orbit coupling. This type of nonradiative transition can give rise to phosphorescence.