11. 11
UV Radiation is defined as "that portion of the electromagnetic spectrum between x
rays and visible light.“ UV Radiation, like all other electromagnetic radiation, is
composed of energy particles called photons. The below image of the electromagnetic
spectrum demonstrates the size and frequency of each energy.
What is Ultraviolet Radiation?
12. 12
consequence of organic molecules absorbing a quantum of visible or UV
light is to promote an electron to a vacant orbital of higher energy
The energy of visible or UV light is of the same order as that of electronic
transitions within atoms or molecules
13. 13
Once the molecule has absorbed this energy, it is said to be in an excited state from
which various reaction possibilities arise.
For an electron to be promoted from an energy level E1 to a higher energy level E2, the
incident radiation must have a frequency which gives a value E greater than or equal to
(E2-E1). Thus, E(E2-E1) = h
The energy of the incident radiation is proportional to the frequency of the radiation
E= h
14. Energy is re-emitted
from the singlet state
at a longer
wavelength (S1 to S0
“flourescence”).
Energy is re-emitted
from the triplet state
at an even longer
wavelength (T1 to S0
“phosphorescence”).
The singlet or triplet state
undergoes a radiationless
conversion to electronic
ground state and gives off
the energy in the form of
heat (excited vibrational
states). However, the
molecule in its excited
vibrational state can undergo
a degradation reaction.
Energy is transferred
to another molecule
which then dissipates
the energy by the
above three
photophysical
processes.
14
certain functional groups, such as a carbonyl group, can absorb the UV radiation and be excited to higher
energy state. The excited electron usually returns ‘unchanged’ to the electronic ground state because the
energy is dissipated by one of the following radiative and macroradiative photophysical routes:
15. ?
Photosensitizer
compound that absorbs
light and transfers energy
to a second compound
and, in so doing, undergoes
no net change. The second
compound proceeds to
produce radicals by photo
cleavage or H- abstraction
reactions.
15
UV light provides lower energy levels than other sources of ionizing radiation, it is
impossible for direct cleavage of C–C or C–H bonds of starch molecules to occur for
the formation of free radicals. Hence there is a need for a photosensitizer
(photoinitiator) that can absorb a low-energy photon and become activated,
leading to the formation of free radicals.
18. Under UV light excitation, the photoinitiator (I) is first promoted to its
excited singlet state (1Ix). Then, via fast intersystem crossing, it converts
into triplet state (3Ix). This transient state can undergo either direct
cleavage of the molecule or hydrogen abstraction with amine compounds
or hydrogen donors (DH). The triplet transient state can be quenched by
the monomer by an energy transfer process that does not undergo any
chain initiation. Accordingly, this process must be regarded as a dead loss
pathway. The longer the life time of the triplet state, the more efficient
the quenching process becomes. Thus,
18
22. Photo-induced modification of starch
Degradation
(in absence of
oxygen or air)
Oxidation
(in presence of
oxygen or air)
Grafting (in
presence of
monomer)
Crosslinking
(in solid or in
solution)
22
23. All the energy coming from the Sun, from which the earth receives 1.5x1018
kWh per year, approximately 28000 times the consumption of all the world in
that period.
Solar radiation as source of light
23
24. Solar collectors
No concentration or
low temperature, up
to 150o C
Medium concentration
or medium
temperature, from
150o C to 400o C
High concentration
and high temperature,
over 400o C.
24
what is important in photocatalysis
is not only the amount of radiation
collected, but its wavelength
28. 28
Future work can be done on the
basis of using photo initiators/UV
system by establishing a “solar
energy collector” to collect UV
from sun light and use it instead
of UV from the mercury lamp
29. 29
I am greatly
thankful to
Prof. James T.
Guthrie
Colour Chemistry
Department,
University of
Leeds, England
For gifting me
the UV lamp and
related
accessories
