photochemistry ppt

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ppt

• TOPIC ORGANIC
  PHOTOCHEMISTRY
• 
  Presented by-
• 
  attul naji

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THEORY OF PHOTOCHEMISTRY

• Photochemistry, a
  sub-discipline of chemistry, is the study of
  chemical reactions that proceed with absorption
  of light by atoms or molecules.
• Light is a type of
  electromagnetic radiation, a source of energy.
  The light must be absorbed by a chemical
  substance in order for a photochemical reaction
  to take place.
• For each photon of light absorbed by a
  chemical system, no more than one molecule is
  activated for a photochemical reaction, as
  defined by the quantum yield.

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• Chemical reactions occur only
  when a molecule is provided the necessary
  activation energy. In case of photochemical
  reactions light provides the activation energy.
• Photochemical reactions involve
  electronic reorganization, initiated by
  electromagnetic radiation. The reaction are
  several orders of magnitude faster than thermal
  reaction.

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PHOTOPHYSICAL PROCESSES

• If the absorbed radiation is not used to
  cause a chemical change, it is re-emitted light
  of larger wavelength. The three such
  photophysical processes which can occur are
  -
• a) Fluorescence
• b) Phosphorescence
• c) Chemiluminescence

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FLUORESENCE

• Certain molecules (or
  atoms) when exposed to light radiation of short
  wavelength, emits light of longer wavelength.
• EXPLANATION When a molecule absorb high energy
  radiation, it is excited to higher energy states.
  Then it emits excess energy through several
  transitions to the ground state. Thus the excited
  molecule emits light of longer frequency. The
  colour of fluorescence depends on the wavelength
  of light emitted.

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Ex A solution of chlorophyll in ether shows
blood red fluorescence.
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PHOSPHORESCENCE

• When a substance absorbs radiation
  of high frequency and emits light even after the
  incident radiation is cut off, the process is
  called phosphorescence.
• Phosphorescence is chiefly caused by
  ultraviolet and visible light. It is generally
  shown by solids.

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Ex Phosphorescent powder under visible light,
UV and total darkness.
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CHEMILUMINESCENCE

• Some chemical
  reactions are accompanied by the emission of
  visible light at ordinary temperature. The
  emission of light as a result of chemical action
  is called chemiluminescence. The light emitted
  in a chemiluminescence reaction is also called
  cold light because it is produced at ordinary
  temperature.

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• EXPLANATION In a chemiluminescence reaction, the
  energy released in the reaction makes the product
  molecule electronically excited. The excited
  molecule then gives up its excess energy as
  visible light while reverting to ground state.

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Ex The glow of fireflies due to the aerial
oxidation of luciferin in presence of enzyme
luciferase.
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ELECTRONIC EXCITATION

• In the case of visible or UV light, the energy of
  a photon is roughly in the region that would be
  appropriate to promote an electron to a higher
  energy level. Different wavelengths would be able
  to promote different electrons, depending upon
  the energy difference between an occupied
  electronic energy level and an unoccupied one.

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JABLONSKI DIAGRAM

• A Jablonski diagram is a diagram that illustrate
  the electronic stats of a molecule and the
  transitions between them. The states are arranged
  vertically by spin multiplicity. Nonradioactive
  transition are indicated by squiggly arrows and
  radioactive transitions by straight arrows. The
  vibrational ground states of each electronic
  state are indicated with thick lines, the higher
  vibrational states with thinner lines.

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• It is basically an energy diagram, arranged with
  energy on a vertical axis. The energy levels can
  be quantitatively denoted, but most of these
  diagrams use energy levels schematically. The
  rest of the diagram is arranged into columns.
  Every column usually represents a specific spin
  multiplicity for a particular species

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The diagram is named after the polish physicists
Aleksander Jablonski.
S1

• S0

T1
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FRANCK-CONDON PRINCIPLE

• The Franck-condon principle describes the
  intensities of vibronic transitions, or the
  absorption or emission of a photon.
• It states that when a molecule is undergoing an
  electronic transition such as ionization, the
  nuclear configuration of the molecule experiences
  no significant change.

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• This is due in fact that nuclei are much more
  massive than electrons and the electronic
  transition takes place faster than the nuclei can
  respond. When the nucleus realigns itself with
  the new electronic configuration, the theory
  states that it must undergo a vibration.
• The nucleus in a molecule has coulombic forces
  acting on it from the electrons and other nuclei
  of the system once a molecule undergoes the
  electronic transition, the resulting coulombic
  forces serve to change the energy of the
  molecule. This change brings it from the ground
  state to an excited state and results in the
  nuclei changing its vibrational state. The
  vibrational structure shows that the absorption
  spectrum consist of many lines instead of a
  single sharp electronic absorption line.

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PHOTOSENTIZERS

• In many photochemical reactions the reactant
  molecule does not absorbs the radiation required
  for the reaction. Hence the reaction is not
  possible. In such cases the reaction may still
  occur if a foreign species such as mercury vapour
  is present. The mercury atom absorbs the incident
  radiation and subsequently transfer its energy to
  the reactant molecule which is activated. Thus
  the reaction occurs.

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• A species which can both absorb and transfer
  radiant energy for activation of the reactant
  molecule, is called a photosenitizers.

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EINSTEINS LAW OF PHOTOCHEMICAL EQUIVALENCE

• A fundamental law of photochemistry that
  establishes that every photon absorbed cause one
  elementary reaction.
• In a photochemical reaction, each molecule of the
  reacting substance absorbs a single photon of
  radiation causing the reaction and is activated
  to form the products.

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• In terms of Quantum efficiency
• Quantum Efficiency ?
  
• No. of molecules reacting in a given
  time
• No. of quantas of light absorbed in
  the
• same time
• Experimentally,
• ? rate of chemical reaction
• quanta absorbed per second.

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• A hv ---------gt A
• A ---------gt B
• Overall A hv -----gt B
• A molecule A absorbs a photon of radiation
  and gets activated. The activated molecule (A)
  then decomposes to yield B.

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.

• The number N of reacted molecules is related to
  the energy E absorbed by the system by the
  equation
• If 1 mole of a A absorbs 1 mole of photons or one
  einstein of energy, E. The value of E can be
  calculated by using the expression given below
• E 2.859/ ? x 105 Kcalmol?¹

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