dr.wael elhelece photochemistry 431chem

Dr. Wael A. El-Helece1

Photochemistry

الضوئية الكيمياء


Electronic excitation of atoms and molecules.. والجزيئات للذرات االلكترونية اإلثارة

Excited states of polyatomic molecules. . الذرات عديدة للجزيئات المثارة الحالة

Kinetics of electronic excited state.. المثارة الحالة حركية

Electronic energy transition.االلكترونات انتقال طاقة

Chemical reactivity of excited electronic molecules.. المثارة للجزيئات الكيميائي النشاط

Photo-electronic and photo-ionic spectra.. وااليونى االلكتروني الطيف

Diffraction of light in laboratory and outdoor (environment). والبيئة المعمل فى الضوء انكسارالخارجية


Contents المحتوياتPrinciples المبادئ

Spectral regions الضوء مناطق

Applications تطبيقات

Experimental set-up معملية تطبيقات

Excitation االستثارة

Organic photochemistry العضوية الضوئية الكيمياء

Inorganic and organometallic photochemistry

Carbon nanotubes الكربونية النانومترية األنابيب

References المراجع


Introductionتقديم

Heat حرارة Electricityكهرباء Electromagnetic اشعاع irradiation (light)

كهرومغناطيسي

Energyالطاقة


Photochemistryالضوئية الكيمياء

Chemical reactions accompanied with light.بطاقة • المصحوبة الكيميائية التفاعالت

الضوء1. Action of light → chemical change (light induced reactions)

( ضوئيا ( محثة تفاعالت كيميائي تغير الضوء تأثير

2. Chemical reaction → light emission (chemiluminescence)

انبعاث ( ) ضوء انبعاث الكيميائى التفاعل


Photochemistry الضوئية الكيمياء

Study of chemical reactions that proceed with

the absorption of light by atoms or molecules.

تحت تحدث والتي الكيميائية التفاعالت دراسة

للضوء والجزيئات الذرات امتصاص تأثير


Principles مبادئ

Grotthuss–Draper law درابر جاتس قانون

light must be absorbed by a chemical substance in order for a

photochemical reaction to take place.

كيميائي تفاعل يحدث لكي كيميائية مادة بواسطة يمتص أن يجب الضوء

ضوئي.

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.

من أكثر وينشط يتأثر وال كيميائي نظام بواسطة يمتص الضوء من جزء لكل

. بالكوانتم يعرف فيما واحد جزئ


Spectral regions

الضوء مناطق

Ultraviolet: 100–400 nm فوق الضوء منطقة

البنفسجية

Visible Light: 400–700 nm المرئي الضوء منطقة

Near infrared: 700–2500 nm تحت الضوء منطقة

الحمراء


Primary Processesاألولية العمليات

• One molecule is excited into an electronically excited state by absorption of a photon, it can undergo a number of different primary processes.

على • قادرا يصبح وبالتالي الكترونيا مثار إلى ويتحول الفوتون طاقة الجزيء يمتص . مبدئية تحول عمليات عدة حدوث

• Photochemical processes are those in which the excited species dissociates, isomerizes, rearranges, or react with another molecule.

ويعاد : ضوءكيميائىتغير • وتتجمع تتكسر المثارة الجزيئات خاللها من التي العمليات. أخرى جزيئات مع تتفاعل أو ذراتها ترتيب

• 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.

لتعود: ضوءفيزيائيةتغيرات • مختلفة صور في للضوء انبعاث إعادة خاللها يحدث. المستقرة إلى المثارة الحالة من الجزيئات


What is Photochemistry about?الضوئية؟ الكيمياء علم يدور عما

• Photochemistry is concerned with the changes in chemical and physical

behaviour of molecules following absorption of one (or more) photons.

المتصاص نتيجة تحدث التي والفيزيائية الكيميائية بالتغيرات الضوئية الكيمياء علم يهتم

. الضوء الجزىئ

• Primarily consider absorption of visible/UV although IR absorption may also

change chemical behaviour

. كيميائي بتغير مصحوبا يكون المختلفة المناطق من الضوء امتصاص أن لنفترض مبدئيا

*Mainly concerned with electronic excitation, usually accompanied by some

vibrational excitation (and rotational in gas phase ) excitation.

االلكترونية . االستثارة عملية علي االساسى التركيز سيكون


Chemistry of excited statesالمثارة الحالة كيمياء

• Electronic excitation . االلكترونية اإلثارة

Þ change of molecular orbital occupancy.. الجزيئية المدارات ملئ في تغير

Þ increased energy.. الطاقة في زيادة

Þ change of bonding characteristics and possibly geometry.

. الفراغي التركيب وكذا الترابط مواصفات في تغيرÞ change of charge distribution.

. الشحنات توزيع في تغير


Þpossible changes of resultant electron spin, orbital

symmetry

في التماثل وكذا االلكترونات حركة فى محتمل تغير

المدارات.

ÞChange of في تغير

Lifetime الحياة فترة

Electron donating/accepting ability وجذب فقد على المقدرة

االلكترونات

Acid/base characteristics والقلوية الحموضة خاصية

Symmetry or energetic constraints on reaction أو التماثل

التفاعالت طاقة .التأثيرات


Excited states of formaldehydeللفورمالدهيد المثارة الحالة

Resembles alkoxy radical شق تمثلااللكوكسى

No free radical properties ال

حر شق خواص

الحالة األرضية

Dr. Wael A. El-Helece14Fig 1


Fig 2: Jablonski Diagrams


Significance of photochemical processesالضوئية الكيمياء تطبيقات

• Atmospheric and astrophysical chemistry والفضاء الجوى الغالف .كيمياء

• Photosynthesis الضوئى البناء عمليات• Lasers الليزر • Solar energy الشمسية الطاقة• Semiconductor etching الموصالت أشباه• Biological damage – skin cancer etc الجلد وسرطان البيولوجى التحلل• Vision الرؤية• New chemistry الجديدة الكيمياء• Chemical Dynamics الكيميائية الميكانيكية


۞Chemiluminescence: الكيميائي الضوئي االنبعاث

P4 (g) + O2 (g)+H2O (g) P4 O10 + hυ green

۞Bioluminescence: الحيوى الضوئي االنبعاث

- Mushrooms الغراب عش

- insects الحشرات

- fishes األسماك

Luminescence الكيميائي الضوئي االنبعاث


Definitions and termsومصطلحات تعريفات

Light: electromagnetic field vibration مجال اهتزاز

كهرومغناطيسي

spreading in quanta الكمية انتشار

(photons) الفوتونات

Photon: the smallest amount of light الضوء من كمية اصغر

carrying energy طاقة تحمل


Energy of photons (A. Einstein)الفوتون )اينشتاين(طاقة

E = c

h h=

h = Planck’s constant (6.6 · 10-34 Js) بالنك ثابت

c = speed of light (3 · 108 ms-1) الضوء سرعة

l = wavelength الموجى الطول

n = frequency التردد


Einstein’s Equivalency Principleايناشتاين مبدأ

One particle of a chemical substance can absorb onlyone photon

from a light beam:

واحد فوتون يمتص أن يستطيع الكيميائية المادة من واحد جزيئ

الضوء شعاع من

ΔE = hn

For one mole: ΔE = Nhn

N = Avogadro’s number (6.02 x1023)


Chemical bond energies: الكيميائية الروابط طاقات

from 100 – 1000 kJ/mol

Light energies: الضوء طاقات

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.

مع تتداخل البنفسجي فوق وكذا المرئي الضوء موجات لذا

. الكيميائية التفاعالت


Laws of Photochemistryالضوئية الكيمياء قوانين

1. Only light that is absorbed can produce photochemical change

(Grotthus, Draper)

. ضوءكيميائى تغير ينتج الذي هو الممتص الضوء درابر (فقط )جروتس

2. A molecule absorbs a single quantum of light is becoming

excited (Stark, Einstein)

المثار هو الضوء من واحد كوانتم الممتص وايناشتا (الجزيء )ينستارك


Mechanisms of Light Absorptionالضوء امتصاص ميكانيكية

Excitation اإلثارة

X2h *X2

A bonding electron is lifted to a higher energy level (higher orbital).

. الطاقة مستويات من اعلي مستوى إلى يزاح ترابط الكترون


Interaction of Light and Materials

والضوء المادة بين التداخل

a) excess energy transferred to the surrounding.

. المحيط الى تنتقل زيادة طاقة

X2* → X2 + M*

b) fluorescence or phosphorescence. أو فلوروسنس

فسفوروسنس

X2* → X2 + hυ

c) excess energy supplies the activation energy of the reaction.

. الالزمة التنشيط بطاقة التفاعل تمد زيادة طاقة

X2* + Y → chemical reaction


hX2 X + X (photodissociation)

(energy of the photon supplies the dissociation heat)للتكسير الالزمة الحرارة تمد الفوتون طاقة

Types of photochemical reactions التفاعالت أنواعالكيمياءضوئية

a) Photodissociation الضوء بواسطة تكسر تفاعالت

b) Photosynthesis: الضوئي البناءwhen a larger molecule is formed from simple ones.

بسيطة جزيئات من كبير جزيء يتكون عندما

c) Photosensitized reactions: ضوئيا مستحثة تفاعالت when an excited molecule supplies activation energy for the reactants.

. الكيميائي للتفاعل الالزمة التنشيط طاقة مثار جزيء يمد عندما


Photodissociation الضوئي التكسير

Photolysis of hydrogen bromide بواسطة الهيدروجين بروميد تكسير

الضوء

HBr hH + Br (photochemical reaction)

H + HBr H2 + Br

Br + Br Br2

(dark reactions)

Overall: الكلى التفاعل

2HBr h H2 + Br2


Note: الحظ

1 photon absorbed, 2 molecules of HBr dissociated:

( الهيدروجين ( بروميد من يتكسر جزيئان يثار يمتص واحد فوتون

QUANTUM YIELD = 21 = 2

number of molecules undergoing the processnumber of quanta absorbed=


Ozone formation in the atmosphere (at about 25 km altitude)

الجوي الغالف في األوزون تكوين عملية

O2 O + O (λ ˂240 nm)

O2 +2O (+M*) 2O3 (+M*)

Note: M absorbs energy released in the reaction

التفاعل) Mمالحظة: ( من تخرج التي الطاقة تمتص

Quantum Yield = 2/1 = 2

hυ


Ozone formed in the reaction above absorbs UV light as well:

. البنفسجية فوق األشعة يمتص التفاعل في المتكون األزون

O3 O2 + O (λ ˂340 nm)

O3 +O 2O2

Notes: مالحظات

1. Ozone shield protects the Earth surface from high energy UV

radiation (of the Sun) االوزون األشعة طبقة من األرض سطح تحمى

البنفسجية فوق

2. Air pollution (freons: fully halogenated hydrocarbons; nitrogen oxides

emitted by aeroplanes etc.) may accelerate the decomposition of ozone ozone

hole0

الهواء آكاسيد تلوث الهلجنة كاملة الهيدروكربونات الفريونات

. األوزون تآكل الى تؤدى عوامل كلها الطائرات من النيتروجين

hυ


Photosynthesis الضوء بواسطة التحضير

The photosynthesis of hydrogen chloride كلوريد تحضير

الهيدروجين

Overall reaction: الكلى التفاعل

Cl2 + H2 2HCl [no reaction in darkness]


Mechanism: حدوث طريقةالتفاعل

hCl2 < 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 (chainis terminated)

Note: الحظ

Quantum yield is about 106 (explosion) بمقدار يكون الكمي الناتججدا كبير


Photosensitized reactions ضوئيا المستحثة التفاعالت

Photosynthesis in plants النبات في الضوئي البناء

Overall reaction: الكلي التفاعل

6CO2 + 6H2O C6H12O6+6O2

carbohydrate

h; chlorophyllseveral steps


Notes: مالحظات

1.Chlorophyll acts as a catalyst absorbing and transferring the photon

energy for reduction of carbon dioxide to carbohydrate

ثاني الختزال طاقة إلى ويحوله الضوء يمتص كحافز يعمل الكلوروفيل

كربوهيدرات إلى وتحويله الكربون .أكسيد

2. This reaction maintains the life on the Earth:

األرض على الحياة أساس التفاعل .وهذا

sunlight carbohydrateCO2; H2O

Fossile energy(coal, oil, natural gas) Food


Absorption االمتصاص The Beer-Lambert Law المبرت بيير قانونA beam of light (intensity I0) passes through a sample of Length (l) with concentration (c).

قوته ضوئي طول I0شعاع خالل عينة عبر وتركيز lيمر .cممر

The intensity, I, of light transmitted through the sample is given by the Beer-Lambert Law:

A = log10 I/I0 = eᵋ(v)cl


Photography الضوئي التصويرa)Photographic film: colloidal suspension of finely

powdered silver halogenide in gelatine.

الضوئي التصوير من: فيلم ناعم مسحوق من غروي معلق

. الجالتين فى الفضة بروميد

b) When exposed to light AgBr granuli become activated

according to the intensity of light.

لشدة نتيجة نشطة تصبح الفضة بروميد للضوء تعرضها عند

AgBrالضوء. AgBr*h


AgoAgBr* developerreduction

Unactivated granuli will be unaffected (but photosensitive!)

. متأثرة غير ستكون المثارة غير الحبيبة

d) Fixation: Unaffected (photosensitive) AgBr should be removed:

.التصحيح تحذف: ان يجيب المتأثرة غير الفضة بروميد

AgBr + 2S2O32- [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).

سوف : التقدم المثارة والحبيبة ضعيف مختزل بعامل الفيلم معاملة.( سمراء ( العنصرية الفضة الى االختزال تسرع


Applications تطبيقات

*Photosynthesis. الضوئي البناء

*The formation of vitamin D. فيتامين دتكوين

*Photodegradation. الضوئي التكسر

*Many polymerizations are started by photoinitiatiors.

. ضوئية باستثارة تبدأ البللمرة عمليات من كثير

http://en.wikipedia.org/wiki/File:Norrish2.png


Process of photosynthesis البناء عمليةالضوئى

6CO 22 6H O+Sunlight

ChlorophyllC H O O6

6 12 6 + 2

The carbohydrates so formed have been forming the

basis of life on earth.

الحياة أساس العملية هذه في الكربوهيدرات تكون

. األرض على


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.


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.


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

Br2

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

+k

q

Lifetime of A* with Q = r 2

][1

][1

11

2

Qkr

Qkkr 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)

AntibondingOrbital

BondingOrbital


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 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 5hv

C

HH C65

C

H C65

H

+C

HH C65

C

H C65

H

Cis-Stibene 93 % Trans-Stibene 7 %


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.


Spectroscopy and Photochemistry Take Home Messages, cont.

1. Oxygen: Schumann Runge Continuum <175 nm strong allowed.Schumann Runge Bands < 200 nmHerzberg 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 PhotochemistryPhotochemical 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

ChlorophyllC H O O6

6 12 6 + 2

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

Br2

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

+k

q

Lifetime of A* with Q = r 2

][1

][1

11

2

Qkr

Qkkr 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)

AntibondingOrbital

BondingOrbital

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

HH C65

C

H C65

H

+C

HH C65

C

H C65

H


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*+A 0 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

HH C65

C

H C65

H

+C

HH C65

C

H C65

H


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 OCH33CH O CH3 3

(I)

Photochemistry of butadieneButadiene 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..

hvand +

(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

Ohv

CH CCH33

O

CH 3

.+ CH C

O

3.

O

3.CH C2 CH C C CH3

OO

3

O

3.CH C

23 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

RR

O

RR

R

R

Barton reactionThe 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 rearrangementPhoto-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

http://en.wikipedia.org/wiki/Diels-Alder

http://en.wikipedia.org/wiki/Cycloaddition

http://en.wikipedia.org/wiki/Ene_reaction

http://en.wikipedia.org/wiki/Ene_reaction

An example is an oxygenation of citronellol

Singlet_Oxygenation_Citronellol

http://en.wikipedia.org/wiki/Citronellol

Applications of photoreactions in synthesisMany 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.

http://en.wikipedia.org/wiki/Photosynthesis

http://en.wikipedia.org/wiki/Carbon_dioxide

http://en.wikipedia.org/wiki/Glucose

http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Fireflies

http://en.wikipedia.org/wiki/Enzyme

http://en.wikipedia.org/wiki/Bioluminescence

http://en.wikipedia.org/wiki/Tetraphenylporphyrin

http://en.wikipedia.org/wiki/Methylene_blue

http://en.wikipedia.org/wiki/Methylene_blue

http://en.wikipedia.org/wiki/Photodynamic_therapy

http://en.wikipedia.org/w/index.php?title=Photoinitiatiors&action=edit&redlink=1

http://en.wikipedia.org/wiki/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

http://en.wikipedia.org/wiki/Polyvinyl_chloride

http://en.wikipedia.org/wiki/Cyclopentadienyliron_dicarbonyl_dimer

http://en.wikipedia.org/wiki/Photoresist

http://en.wikipedia.org/wiki/Microelectronic

http://en.wikipedia.org/wiki/Visual_perception

http://en.wikipedia.org/wiki/Rhodopsin

dr.wael elhelece photochemistry 431chem

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