Introduction to Photochemistry Classifications of ......Introduction to Photochemistry Classifications of Photochemical Reactions Application of Photochemistry in Organic Synthesis

Organic Photochemistry

Introduction to Photochemistry

Classifications of Photochemical Reactions

Application of Photochemistry in Organic

Synthesis

Energies

100 kcal/mol= 4.3 eV= 286 nm= 35000 /cm (near UV)nano= 10-9

286 kcal/mol= 12.4 eV= 100 nm= 100000 /cm (far UV)

Typical Bond Energies

C-H = 110 kcal/mol

C-C = 80

C=C = 150

C=O = 170

Uv light 150 -40 nm wavelength, so this is sufficient energy to break bonds –knock

electrons out of bonding orbitals (electronic excitation).

C h e m ic a l ly u s e fu l l ig h t is g e n e ra l ly in th e r a n g e o f 2 0 0 -4 0 0 n m

O fte n e m p lo y f i l te r s to r e g u la te th e w a v e le n g th o f th e r a d ia tio n

n

*

n

*

n

*

n

*

n

*

g ro u n d s ta te (S0) n - * (S

1) n - * (T

1) - * (S

1) - * (T

1)

A Jablonski diagram, named after the Polish physicist AleksanderJabłoński, is a

diagram that illustrates the electronic states of a molecule and the transitions

between them.

The states are arranged vertically by energy and grouped horizontally by spin

multiplicity.

Radiative transitions are indicated by straight arrows and nonradiative transitions

by squiggly arrows.

The vibrational ground states of each electronic state are indicated with thick

lines, the higher rotational states with thinner lines.

Physical Processes Undergone by Excited Molecules

• So + hv --- S1 Excitation

• S1v -- S1 + heat Vibrational Relaxation

• S1 ----- So + hv Fluorescence

• S1 ---- So + heat Internal Conversion

• S1 --- T1 Intersystem Crossing

• T1v -- T1 + heat Vibrational Relaxation

• T1v -- So + hv Phosphorescence

• T1 --- So + heat Intersystem Crossing

• S1 + A (So) --- So + A (S1) Singlet-Singlet Energy Transfer

• T1 + A (So) -- So + A (T1) Triplet-Triplet Energy Transfer

Why Use Photochemistry

• Overcome large kinetic barriers in a short amount of time

• Produce immense molecular complexity in a single step

• Form thermodynamically disfavored products

• Allows reactivity that would otherwise be inaccessible by almost any other synthetic method

• The reagent (light) is cheap, easily accessible, and renewable

• Drawback• Reactivity is often unpredictable

• Many substrates are not compatible

• Selectivity and conversion are sometimes low

Chemical Processes undergone by Excited Molecules

(A -B -C ) A -B.

+ C.

S im p le C le a v a g e

(A -B -C ) E + F D e c o m p o s it io n

(A -B -C ) A -C -B In tr a m o le c u la r R e a r ra n g e m e n t

(A -B -C ) A -B -C ' P h o to is o m e r iz a t io n

(A -B -C ) A -B -C -H + R.

H y d ro g e n A to m A b s tr a c tio nR H

(A -B -C ) (A B C )2 P h o to d im e r iz a t io n

(A -B -C ) A B C + A * P h o to s e n s i t iz a t io nA

• 1) α-Cleavage (Norrish type I reaction). In solution the radicals undergo further reactions to give products.

• 2) Hydrogen Abstraction followed by cleavage = Norrishtype II cleavage.The radicals can abstract a Hydrogen atom from a donor. The resulting radicals can then undergo further reactions.

• An intramolecular example:

O

R1

R2

R2 R

2R1

R1

O

O

R1

R2

+

h , (3 0 0 n m )

s o lv e n t

1 a – f

3 f 4 f2 a – f

a b c d

R1

R2

C O 2 E tH

C O 2 M e

C O 2 M e

P h H P h

P h

C O 2 E tH

HH

e f

+

O

R1 R2

R2

R2R1

O

O

R2

25a–f

28a–f

1a–f

3a–f

17a–f

20a–f

22a–f

DPMODPM

R1O

O

R2

R1

R1

R2

R1

O

5

8