Dr.wael elhelece photochemistry 431chem
Upcoming SlideShare
Loading in...5
×
 

Dr.wael elhelece photochemistry 431chem

on

  • 435 views

431chem course Aljouf university, college of science, chemistry department. ...

431chem course Aljouf university, college of science, chemistry department.
. Fates of Excited State Molecules.
• Absorption and emission of electromagnetic radiation.
• Einstein coefficients, absorption probabilities.
• Fluorescence and phosphorescence.
• Internal conversion and intersystem crossing.
• Photodissociation and predissociation.
• Jablonski diagram.
. Lasers.
• Requirements for laser action.
• Population inversions.
• Properties of laser radiation.
• Examples of lasers.
• Applications in spectroscopy and photochemistry.
Dr Wael A. Elhelece.

Statistics

Views

Total Views
435
Views on SlideShare
434
Embed Views
1

Actions

Likes
3
Downloads
9
Comments
0

1 Embed 1

http://www.slideee.com 1

Accessibility

Categories

Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

Dr.wael elhelece photochemistry 431chem Dr.wael elhelece photochemistry 431chem Presentation Transcript

  • Photochemistry Dr. Wael A. El-Helece
  • Dr. Wael A. El-Helece Electronic excitation of atoms and molecules. Excited states of polyatomic molecules. Kinetics of electronic excited state. Electronic energy transition. Chemical reactivity of excited electronic molecules. Photo-electronic and photo-ionic spectra. Diffraction of light in laboratory and outdoor (environment).
  • Contents Principles Spectral regions Applications Experimental set-up Excitation Organic photochemistry Inorganic and organometallic photochemistry Carbon nanotubes References Dr. Wael A. El-Helece
  • Introduction Heat Electricity Electromagnetic irradiation (light) Energy Dr. Wael A. El-Helece
  • Photochemistry Chemical reactions accompanied with light. • 1. Action of light → chemical change (light induced reactions) 2. Chemical reaction → light emission (chemiluminescence) Dr. Wael A. El-Helece
  • Photochemistry Study of chemical reactions that proceed with the absorption of light by atoms or molecules. Dr. Wael A. El-Helece
  • Principles Grotthuss–Draper law 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. Dr. Wael A. El-Helece
  • Spectral regions Ultraviolet: 100–400 nm Visible Light: 400–700 nm Near infrared: 700–2500 nm Dr. Wael A. El-Helece
  • 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. • • Photophysical 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. • Dr. Wael A. El-Helece
  • What is Photochemistry about? • Photochemistry is concerned with the changes in chemical and physical behaviour of molecules following absorption of one (or more) photons. • Primarily consider absorption of visible/UV although IR absorption may also change chemical behaviour *Mainly concerned with electronic excitation, usually accompanied by some vibrational excitation (and rotational in gas phase ) excitation. Dr. Wael A. El-Helece
  • Chemistry of excited states • Electronic excitation change of molecular orbital occupancy increased energy change of bonding characteristics and possibly geometry change of charge distribution Dr. Wael A. El-Helece
  • possible changes of resultant electron spin, orbital symmetry Change of Lifetime Electron donating/accepting ability Acid/base characteristics Symmetry or energetic constraints on reaction Dr. Wael A. El-Helece
  • Excited states of formaldehyde Resembles alkoxy radical No free radical properties Dr. Wael A. El-Helece
  • Fig 1Dr. Wael A. El-Helece
  • Fig 2: Jablonski Diagrams Dr. Wael A. El-Helece
  • Significance of photochemical processes • Atmospheric and astrophysical chemistry • Photosynthesis • Lasers • Solar energy • Semiconductor etching • Biological damage – skin cancer etc • Vision • New chemistry • Chemical Dynamics Dr. Wael A. El-Helece
  • Chemiluminescence: P4 (g) + O2 (g)+H2O (g) P4 O10 + hυ green Bioluminescence: - Mushrooms - insects - fishes Luminescence Dr. Wael A. El-Helece
  • Definitions and terms Light: electromagnetic field vibration spreading in quanta (photons) Photon: the smallest amount of light carrying energy Dr. Wael A. El-Helece
  • Energy of photons (A. Einstein) E = ch h= h = Planck’s constant (6.6 · 10-34 Js) c = speed of light (3 · 108 ms-1) l = wavelength n = frequency Dr. Wael A. El-Helece
  • Einstein’s Equivalency Principle One particle of a chemical substance can absorb onlyone photon from a light beam: ΔE = hn For one mole: ΔE = Nhn N = Avogadro’s number (6.02 x1023) Dr. Wael A. El-Helece
  • Chemical bond energies: from 100 – 1000 kJ/mol Light energies: 604 kJ/mol-1 302 151 200 nm 400 nm 800 nm ULTRAVIOLET VISIBLE INFRARED So UV – and VIS region is expected to induce chemical reactions. Dr. Wael A. El-Helece
  • Laws of Photochemistry 1. Only light that is absorbed can produce photochemical change (Grotthus, Draper) 2. A molecule absorbs a single quantum of light is becoming excited (Stark, Einstein) Dr. Wael A. El-Helece
  • Mechanisms of Light Absorption Excitation X2 h *X2 A bonding electron is lifted to a higher energy level (higher orbital) Dr. Wael A. El-Helece
  • Interaction of Light and Materials a) excess energy transferred to the surrounding. X2* → X2 + M* b) fluorescence or phosphorescence. X2* → X2 + hυ c) excess energy supplies the activation energy of the reaction. X2* + Y → chemical reactionDr. Wael A. El-Helece
  • h X2 X + X (photodissociation) (energy of the photon supplies the dissociation heat) Types of photochemical reactions a) Photodissociation b) Photosynthesis: when a larger molecule is formed from simple ones. c) Photosensitized reactions: when an excited molecule supplies activation energy for the reactants. Dr. Wael A. El-Helece
  • Photodissociation Photolysis of hydrogen bromide HBr h H + Br (photochemical reaction) H + HBr H2 + Br Br + Br Br2 (dark reactions) Overall: 2HBr h H2 + Br2 Dr. Wael A. El-Helece
  • Note: 1 photon absorbed, 2 molecules of HBr dissociated: QUANTUM YIELD = 2 1 = 2 number of molecules undergoing the process number of quanta absorbed = Dr. Wael A. El-Helece
  • Ozone formation in the atmosphere (at about 25 km altitude) O2 O + O (λ ˂240 nm) O2 +2O (+M*) 2O3 (+M*) Note: M absorbs energy released in the reaction M Quantum Yield = 2/1 = 2 hυ Dr. Wael A. El-Helece
  • Ozone formed in the reaction above absorbs UV light as well: O3 O2 + O (λ ˂340 nm) O3 +O 2O2 Notes: 1. Ozone shield protects the Earth surface from high energy UV radiation (of the Sun) 2. Air pollution (freons: fully halogenated hydrocarbons; nitrogen oxides emitted by aeroplanes etc.) may accelerate the decomposition of ozone  ozone hole hυ Dr. Wael A. El-Helece
  • Photosynthesis The photosynthesis of hydrogen chloride Overall reaction: Cl2 + H2 2HCl [no reaction in darkness] Dr. Wael A. El-Helece
  • Mechanism: h Cl2 < 500 nm 2Cl Photochem. initiation Cl + H2 HCl + H Dark reactions H + Cl2 HCl + Cl Chain reaction H + H + M H2 + M* Cl2 + M*Cl + Cl + M Recombination reactions (chain is terminated) Note: Quantum yield is about 106 (explosion) Dr. Wael A. El-Helece
  • Photosensitized reactions Photosynthesis in plants Overall reaction: 6CO2 + 6H2O C6H12O6+6O2 carbohydrate h ; chlorophyll several steps Dr. Wael A. El-Helece
  • Notes: 1.Chlorophyll acts as a catalyst absorbing and transferring the photon energy for reduction of carbon dioxide to carbohydrate 2. This reaction maintains the life on the Earth: sunlight carbohydrate CO2; H2O Fossile energy (coal, oil, natural gas) Food Dr. Wael A. El-Helece
  • Dr. Wael A. El-Helece Absorption The Beer-Lambert Law A beam of light (intensity I0) passes through a sample of Length (l) with concentration (c). I0lc The intensity, I, of light transmitted through the sample is given by the Beer-Lambert Law: A = log10 I/I0 = e˂(v)cl
  • Photography a)Photographic film: colloidal suspension of finely powdered silver halogenide in gelatine b) When exposed to light AgBr granuli become activated according to the intensity of light AgBr AgBr*h Dr. Wael A. El-Helece
  • Ago AgBr* developer reduction Unactivated granuli will be unaffected (but photosensitive!) d) Fixation: Unaffected (photosensitive) AgBr should be removed: AgBr + 2S2O3 2- [Ag(S2O3)2]3- + Br - c) Development: Treating the exposed film with a mild reducing agent the activated granuli will accelerate the reduction to metallic silver (black). Dr. Wael A. El-Helece
  • Applications *Photosynthesis. *The formation of vitamin D. *Photodegradation. *Many polymerizations are started by photoinitiatiors. Dr. Wael A. El-Helece
  • Process of photosynthesis 6CO 22 6H O+ Sunlight Chlorophyll C H O O6 6 12 6 + 2 The carbohydrates so formed have been forming the basis of life on earth. Dr. Wael A. El-Helece
  • 39 Dr. Wael A. El-Helece
  • 40 Dr. Wael A. El-Helece
  • 41 Dr. Wael A. El-Helece
  • Dr. Wael A. El-Helece42
  • 43 Dr. Wael A. El-Helece
  • 44 Dr. Wael A. El-Helece
  • Dr. Wael A. El-Helece45
  • Dr. Wael A. El-Helece46
  • Dr. Wael A. El-Helece47
  • Dr. Wael A. El-Helece48
  • So what are those funny symbols behind the O atoms and O2 molecules? Term Symbols. Spectroscopy: A Quick Qualitative Description Term symbols show the energy state of atoms and molecules, as described by the quantum numbers. Atomic Quantum Numbers: n – principal quantum number. Value: 1, 2, 3, .... Tells which shell of an atom the e- resides. The farther from the nucleus the higher the n. l the azimuthal quantum number. Value: 0 to n-1. Describes the orbital angular momentum of the shape of the orbital. s – the spin quantum number. Value: ±½. j – the total (spin plus azimuthal) quantum number. Important for heavier atoms. Dr. Wael A. El-Helece
  • Spectroscopy: A Quick Qualitative Description, cont. Energy states of Molecules: Molecular Quantum Numbers L – the azimuthal quantum number. Value: 0 to n-1. Orbital angular momentum s – the spin quantum number. Value: ±½. Same as in atoms. J – rotational quantum number. Value: 1, 2, 3, .... Tells which shell of an atom the e- resides. The farther from the nucleus the higher the n. n – vibrational quantum number. Value: 1, 2, 3, .... K – vertical component of the total angular momentum. This QN only exists for polyatomic molecules. g/u – gerade/ungerade; symmetry terms. Reflection through the center of symmetry of molecule. +/- – plus/minus; symmetry terms. Reflection through the plane of symmetry of molecule. Only for diatomics. Dr. Wael A. El-Helece
  • Sensitisation and Quenching Certain reactions are known which are not sensitive to light. These reactions can be made sensitive by adding a small amount of foreign material which can absorb light and stimulate the reaction without itself taking part in the reaction. Such an added material is known as sensitiser and the process is sensitisation. H H C C COOH COOH Maleic acid hv Br 2 H H C CHOOC COOH Fumaric acid Dr. Wael A. El-Helece
  • Quenching : - When a photochemical excited atom has a chance to undergo collision with another atom or a molecule before it fluoresces, the intensity of the fluorescent radiation may be diminished or stopped. This phenomenon is known as quenching. Quenching is a radiationless process involving two molecules. A collision between a molecule in its excited state and another chromophoric or reactive molecule is quenching, the collision-induced, radiationless relaxation of an excited state to the ground state. The quenching process implies an interesting kinetic competition, the treatment of which is referred to as a Stern-Volmer analysis. Dr. Wael A. El-Helece
  • A* A Lifetime of A* without Q = r = 1/k 1 11 Q A* A k + kq Lifetime of A* with Q = r2 ][ 1 ][ 1 1 1 2 Qk r Qkk r qq Stern-Volmer quenching kinetics Dr. Wael A. El-Helece
  • Fig 3: Dr. Wael A. El-Helece
  • Singlet and Triplet States and their Reactivity It is essential to define some terminology with the help of the following diagram Fig 1: Spin orientation on the absorption of a light photon Most molecules have an even number of electrons and thus in the ground state, all the electrons are spin paired. The quantity 2S + 1, where S is the total electron spin, is known as the spin multiplicity of a state. hvhv (a)(b) (c) Antibonding Orbital Bonding Orbital Dr. Wael A. El-Helece
  • 1. Cis-Trans Isomerizations When irradiated with uv-light olefins usually undergo cis-trans isomerization. The transformation can be carried out either by direct irradiation of the olefins or by sensitized irradiation. It may either occur through a singlet or a triplet excited species. It has been reported that isomerization in the triplet state has a lower barrier to rotation around the carbon-carbon bond. Dr. Wael A. El-Helece
  • Photoisomerization of Stilbenest Direct irradiation of solutions of either cis or trans-stilbene gives rise to a constant mixture having 93 % cis-stilbene and 7 % trans-stilbene. Initial absorption of light by either of these isomers has been found to be rapidly followed by intersystem crossing to the corresponding triplet states. Photoisomerization then takes place via inter conversion or probably via a common triplet intermediate. C H CH6 5 CHC H6 5 hv C HH C65 C H C65 H + C HH C65 C H C65 H Cis-Stibene 93 % Trans-Stibene 7 % Dr. Wael A. El-Helece
  • Dr. Wael A. El-Helece Spectroscopy and Photochemistry Take Home Messages 1. The spectra of atoms and molecules are related to their ability to interact with electromagnetic radiation, and to their shape and structure. 2. We use the observed spectra to determine the energy levels and geometry of atoms and molecules. 3. Extraterrestrial radiation is absorbed by the atmosphere except in window regions such as the visible and IR near 10 mm. 4. Transitions and reactions are influenced by selection rules, esp. spin conservation. 5. The energy and lifetime set the natural line shape: a. Rotations are slow, low energy, and very sharp. b. Vibrations are intermediate. c. Electronic transitions are very fast, high energy, and broad.
  • Dr. Wael A. El-Helece Spectroscopy and Photochemistry Take Home Messages, cont. 1. Oxygen: Schumann Runge Continuum <175 nm strong allowed. Schumann Runge Bands < 200 nm Herzberg Continuum < 242 nm forbidden weak. 2. Ozone: Hartley ~250 nm, allowed, strong. Huggins < forbidden, weaker ~330 nm Chappuis ~ 600 nm Forbidden, weak. 3. The production of OH and thus all of atmospheric chemistry depends strongly on the wavelength dependent absorption of UV radiation.
  • Organic Photochemistry Photochemical Process [Gurdeep.R.Chatwal, Reaction Mechanism and Reagents in Organic Chemistry, Himalaya Publications, 2005, p 932] Chapmann definition: - “It is the science which has been arising from the application of photochemical methods to organic chemistry and organic chemical methods to photochemistry”. Process of photosynthesis The carbohydrates so formed have been forming the basis of life on earth. 6CO 22 6H O+ Sunlight Chlorophyll C H O O6 6 12 6 + 2
  • Fig 1
  • Fig 2: Jablonski Diagrams
  • Sensitisation and Quenching Certain reactions are known which are not sensitive to light. These reactions can be made sensitive by adding a small amount of foreign material which can absorb light and stimulate the reaction without itself taking part in the reaction. Such an added material is known as sensitiser and the process is sensitisation. H H C C COOH COOH Maleic acid hv Br 2 H H C CHOOC COOH Fumaric acid
  • Quenching : - When a photochemical excited atom has a chance to undergo collision with another atom or a molecule before it fluoresces, the intensity of the fluorescent radiation may be diminished or stopped. This phenomenon is known as quenching. Quenching is a radiationless process involving two molecules. A collision between a molecule in its excited state and another chromophoric or reactive molecule is quenching, the collision-induced, radiationless relaxation of an excited state to the ground state. The quenching process implies an interesting kinetic competition, the treatment of which is referred to as a Stern-Volmer analysis.
  • A* A Lifetime of A* without Q = r = 1/k 1 11 Q A* A k + kq Lifetime of A* with Q = r2 ][ 1 ][ 1 1 1 2 Qk r Qkk r qq Stern-Volmer quenching kinetics
  • Fig 3:
  • Singlet and Triplet States and their Reactivity It is essential to define some terminology with the help of the following diagram Fig 1: Spin orientation on the absorption of a light photon Most molecules have an even number of electrons and thus in the ground state, all the electrons are spin paired. The quantity 2S + 1, where S is the total electron spin, is known as the spin multiplicity of a state. hvhv (a)(b) (c) Antibonding Orbital Bonding Orbital
  • 1. Cis-Trans Isomerizations When irradiated with uv-light olefins usually undergo cis-trans isomerization. The transformation can be carried out either by direct irradiation of the olefins or by sensitized irradiation. It may either occur through a singlet or a triplet excited species. It has been reported that isomerization in the triplet state has a lower barrier to rotation around the carbon-carbon bond.
  • Photoisomerization of Stilbenes Direct irradiation of solutions of either cis or trans- stilbene gives rise to a constant mixture having 93 % cis-stilbene and 7 % trans-stilbene. Initial absorption of light by either of these isomers has been found to be rapidly followed by intersystem crossing to the corresponding triplet states. Photoisomerization then takes place via inter conversion or probably via a common triplet intermediate. C H CH6 5 CHC H6 5 hv C HH C65 C H C65 H + C HH C65 C H C65 H Cis-Stibene 93 % Trans-Stibene 7 %
  • o When the spins are paired { } as shown in Fig (a), the upward orientation of the electron spin is cancelled by the downward orientation so that S=0. This is illustrated below: s1 = ½ ; s2 = – ½ so that S= s1+s2 = ½ – ½ = 0 o Hence, 2S + 1 = 1. Thus, the spin multiplicity of the molecule is 1. We express it by saying that the molecule is in the singlet ground state. o When the absorption of a photon of a suitable energy h , one of the paired electrons goes to a higher energy level (excited state), the spin orientation of the two singlet electrons may be either parallel { } or antiparallel, { }, as shown in Fig (b) and (c) respectively.
  •  If the spins are parallel, as shown in Fig (b), then, S= s1+s2 = ½ + ½ =1 so that 2S+1=3.  Thus, the spin multiplicity of the molecule is 3. This is expressed by saying that the molecule is in the triplet excited state.  If however, the spins are antiparallel, as shown in Fig (c), then, S= s1+s2 = ½ – ½ = 0 so that 2S+1=1. Thus, the spin multiplicity of the molecule is 1. This is expressed by saying that the molecule is in the singlet excited state.  Since the electron can jump to any of the higher electronic states depending upon the energy of the photon absorbed, we get a series of the singlet excited states, Sn where n=1, 2, 3, 4 ……and a series of triplet excited states, Tn where n=1, 2, 3, 4 ……
  • Thus, S1, S2, S3………. are known as the first singlet excited state, second singlet excited state, third singlet excite state……..etc. Similarly, T1, T2, T3……….. are called the first triplet excited state, second triplet excited state, third triplet excited state….etc. It has been shown quantum mechanically that a singlet excited state has higher energy than the corresponding triplet excited state. Accordingly, the energy sequence is as shown below. and so on332211 TSTSTS EE;EE;EE
  • On absorption of light photon, the electron of the absorbing molecule may jump form S0 to S1,S2 or S3 singlet excited state depending upon the energy of the photon absorbed as shown in Jablonski diagram [Fig: 2]. For each singlet excited state (S1, S2, S3………. ), there is a corresponding triplet excited state (T1, T2, T3……….. )’ The molecule, whether in singlet or triplet excited state, is said to be activated. Thus; where A0 is the molecule in the ground state and A* is the molecule in the excited state. The activated molecule returns to the ground state by dissipating its energy through the non- radiative and radiative transition process. A*+A0 hv
  • Photoreactions of Carbonyl Compounds; Enes, Dienes & Arens [Gurdeep.R.Chatwal, Reaction Mechanism and Reagents in Organic Chemistry, Himalaya Publications, 2005, p 959-961] Only two types of electronic excitations is possible in the photochemistry of enes; to * Promotion of an electron from to * needs and to *.more energy (available only from the light of wavelength lower than 150 nm). Therefore, it is difficult to take place under usual experimental conditions. to * excitation has been experimentally accessible because it needs the absorption of the light of about 180-210nm for nonconjugated olefins and of about 220 nm or more for conjugated olefins.
  • The initial excitation ( to *) usually takes place with no change in multiplicity and so a singlet excited state is formed. Unlike n to * transitions of ketones, this transition has been symmetry-allowed and thus results in a strong absorption band. The singlet excited state of olefins possesses less tendency to intersystem crossing and they themselves could initiate many photochemical reactions. However, the T1 states of olefins have been readily formed by intermolecular energy transfer from triplet donor to an olefin molecule. The photochemistry of singlet excited state of an olefin is appreciably different from that of its triplet state.
  • 1.Cis-Trans Isomerization of Stilbene Olefins usually undergo cis-trans isomerizations when irradiated with uv-light. The transformation can be carried out either by direct irradiation of the olefins or sensitized irradiation. It may either occur through a singlet or a triplet excited species. It has been reported that isomerization in the triplet state has a lower barrier to rotation around the carbon-carbon bond because simple olefins absorb light at about 200 nm.
  • • The photoisomerization of the stilbenes has been probably the direct irradiation of solutions of either cis or trans-stilbene gives rise to a constant mixture having 93 % cis-stilbene and 7 % trabs-stilbene. • Initial absorption of light by either of these isomers has been found to be rapidly followed by intersystem crossing to the corresponding triplet state. • Photoisomerization then takes place via inter- conversion or probably via a common triplet intermediate. C H CH6 5 CHC H6 5 hv C HH C65 C H C65 H + C HH C65 C H C65 H Cis-Stibene 93 % Trans-Stibene 7 %
  • 2.Dimerization Reaction o In this process there occurs the generation of an excited triplet molecule which subsequently reacts with a ground state molecule. o A well-known example involves the acetone- sensitized photodimerization of norbornene. o There may occur an intramolecular reaction between two properly situated double bonds in a molecule forming an isomeric substance. hv Acetone
  • 3. Addition reaction of cyclic olefins Cyclic olefins are also known to undergo addition reactions, on irradiation in methanol. The reaction of (I) with methanol has been reported to be sensitized by xylene. + CH OH3 hv Xylene H C OCH33 CH O CH3 3 (I)
  • Photochemistry of butadiene Butadiene is known to exist in solution as a mixture of S-trans (95 %) and S-cis (5 %) conformers. In the irradiation of butadiene, an electron gets promoted from 2 to 3 ( to * transition) which gives rise to the increased bonding between C2 and C3 at the expense of C1------C2 and C3------C4. Hence, conformational character of butadiene gets retained in the excited states. 95 % trans 5 % cis
  • Direct irradiation of butadiene gives rise to cyclobutene (I) and bicyclo butane (II). The formation of these products directly from the S1 state of the butadiene. The conformational characters of butadiene get retained in the S1 state, it is quite reasonable to speculate the S-cis butadiene has been the precursor of cyclobutene whereas the excited state resembling S-trans butadiene yields bucyclobutane. . .hv hv .. hv and + (I) (II)
  • Norrish reactions of acyclic ketones Photochemical excitation of ketones usually causes the homolytic fission of the -carbon- carbon bonds. This process is called -cleavage or Norrish type I reaction. Acetone which gets photolyzed in the vapour phase as well as in the liquid phase. Abaorption of light gives rise to the formation of an n to * excited state of acetone which undergoes a carbon-carbon cleavage to form a methyl radical and an acetyl radical.
  • At room temperature, two acetyl radicals undergo combination to form biacetyl. At temperature above 100oC, acetyl radicals get decarbonylated with the ultimate formation of ethane and carbon monoxide. CH CCH33 O hv CH CCH33 O CH3 . + CH C O 3 . O 3 .CH C2 CH C C CH3 OO 3 O 3 .CH C 2 3 3 CH . 3 + CO CH . 3 CHCH
  • The Paterno-Buchi Reaction Carbonyl compounds on irradiation in the presence of olefins yield oxetanes. This photocycloaddition is generally known as the Paterno-Buchi Reaction. The addition is carried out by irradiation with the light of wavelength absorbed only by the carbonyl group. The light energy needed for the n to * transition is able to initiate the reaction in simple cabonly compounds. O +C RR C C R R R R hv R R O R R R R
  • Barton reaction The Barton Reaction involves the photolysis of a nitrite to form a δ-nitroso alcohol. The mechanism is believed to involve a homolytic RO–NO cleavage, followed by δ-hydrogen abstraction and free radical recombination.
  • Photo-Fries rearrangement Photo-Fries rearrangement involves a radical reaction mechanism. This reaction is also possible with deactivating substituents on the aromatic group. Because the yields are low this procedure is not used in commercial production. However, photo-Fries rearrangement may occur naturally particular to UV light at a wavelength of about 310 nm.
  • Di- methane rearrangement The Di- methane rearrangement is a photochemical reaction of a molecular entity comprising two - systems, separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement reaction formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and bond formation between the lateral carbons of the non- migrating moiety
  • Photochemical conversion of Ergosterol to Vitamin D2 Ergosterol is a biological precursor (a provitamin) to vitamin D2. It is turned into viosterol by ultraviolet light, and is then converted into ergocalciferol, a form of vitamin D also known as D2 .  For this reason, when yeast (such as brewer's yeast) and fungi (such as mushrooms), are exposed to ultraviolet light, significant amounts of vitamin D2 are produced. Ergosta-5,7,22-trien-3β-ol
  • Singlet Oxygen Generation and Reaction • The lowest excited singlet state of O2 lies by only 94 kJ mol-1 above the triplet ground state. This 1Dg state is commonly populated by electronic energy transfer from photoexcited sensitizers. • Due to its excitation energy of 94 kJ mol-1 singlet oxygen is chemically extraordinary reactive. • The chemistry of singlet oxygen is different from that of ground state oxygen. For example, singlet oxygen can participate in Diels-Alder [4+2] and [2+2] cycloaddition reactions, ene reactions
  • citronellolAn example is an oxygenation of Singlet_Oxygenation_Citronellol
  • Applications of photoreactions in synthesis Many important processes involve photochemistry. The premier example is photosynthesis, in which most plants use solar energy to convert carbon dioxide and water into glucose, disposing of oxygen as a side-product. Humans rely on photochemistry for the formation of vitamin D. In fireflies, an enzyme in the abdomen catalyzes a reaction that results in bioluminescence. Photochemistry can also be highly destructive. Medicine bottles are often made with darkened glass to prevent the drugs from photodegradation. A pervasive reaction is the generation of singlet oxygen by photosensitized reactions of triplet oxygen. Typical photosensitizers include tetraphenylporphyrin and methylene blue. The resulting singlet oxygen is an aggressive oxidant, capable of converting C-H bonds into C-OH groups. In photodynamic therapy, light is used to destroy tumors by the action of singlet oxygen. Many polymerizations are started by photoinitiatiors, which decompose upon absorbing light to produce the free radicals for Radical polymerization.
  • • Photochemical reactions are not only very useful but also can be a serious nuisance, as in the photodegradation of many materials, e.g. polyvinyl chloride and Fp. • A large-scale application of photochemistry is photoresist technology, used in the production of microelectronic components. • Vision is initiated by a photochemical reaction of rhodopsin