Organic photochemistry involves chemical changes caused by light in the visible or ultraviolet range. Absorption of a photon excites a molecule to an electronically excited state, which may have different properties than the ground state. This excited state is more reactive and can lead to product formation through subsequent reaction steps. The quantum yield measures the efficiency of light absorption causing a specified change, defined as the number of product molecules formed per absorbed photon.

1. Organic Photochemistry
 Definition: Any chemical change bought about by
light/electromagnetic radiation
◦ Chemical Change : Any event in the molecular level after absorbing a
photon.
No need for an overall chemical change
◦ Wavelength range of e.m. radiation : 700-100nm (visible or uv)
 Examples: Photosynthesis, vision etc..
Photosynthesis

2. Basic Steps in Photochemistry
 Absorption of photon (with uv or light frequency) by a molecule
takes it to an electronically excited state, which is the starting
point for the subsequent reaction steps
Ground state: Electrons try to occupy the lowest possible
orbitals in pairs
Excited state: One electron occupy a higher energy orbital
than the lowest energy level available for it
(overall energy of the molecule also increases)
 An electronically excited species (finite life time only) may have
different physical and chemical properties than the ground state.
 An electronically excited state is more energetic than the ground
state and leads to more possibilities for the reaction
 The electronic configuration of excited state is more favorable for
product formation

3. Quantum Yield
 A property relevant to most photo physical and
photochemical processes
 A measure of efficiency with which absorbed radiation
causes the molecule to undergo a specified change
 It is the number of product molecules formed for each
quantum of light absorbed
no: of photons emitted
Quantum yield for fluoresence =
no:of photons absorbed