PhotochemistryReactions involving photons.

(Radiation-induced chemical processes: chemical transformations induced by high energy photons.

Radiochemistry (nuclear chemistry): processes in the nuclei of atoms.)

Tamás Vidóczy Institute of Structural Chemistry Chemical Research Center, HAS

http://oktatas.ch.bme.hu/oktatas/konyvek/fizkem/fizkem2/fotokemia

Electromagnetic spectrum important for photochemistry

IR

E

UV

~700 nm

~400 nm

E = hν = hc/λ

~200 nmVUV

Excited states and related bond strength

Multiplicity

Name redived from: 2S + 1

S = 0 singlet

S = ½ doublet

S = 1 triplet

The Jablonski diagram

E

S S1 S2 T1 T2

involving a photon

without photons

singlet – triplet splitting

The basic law of photochemistry: only absorbed radiation can cause

chemical change

spectroscopic transitions are quantized - line spectra (in gas phase at low pressure), band spectra (in condensed phases)

Absorption

E

S S1 S2 T1 T2

Lambert – Beer law

I = I0 10-εcl

ε: decadic absorption coefficient

unit: dm3mol-1cm-1

T = I/I0 T(%) = 100 I/I0

A = -lg T = lg (1/T) = lg I0/I = εcl

Typical absorptions

n → * carbonyls, tiocarbonyls, nitro-, azo- and imino- group containing compounds

→ * alkenes, alkynes, aromatics

n → * amines, alcohols, haloalkanes

→ * alkanes

Absorption

S S1 S2 T1 T2

Vibrational relaxation

E

S S1 S2 T1 T2

Deactivation channels of the singlet state

E

S S1 S2 T1 T2

?

Fluorescence: emissionwithout change of spin state

E

S S1 S2 T1 T2

IC: internal conversion

E

S S1 S2 T1 T2

ISC: intersystem crossing(spinváltó átmenet)

E

S S1 S2 T1 T2

Phosphorescence: emissionwith change of spin state

E

S S1 S2 T1 T2

Quenching

Deactivation of an excited state with the help of another species. We investigate the process from the point of view of the excited species, the state of the quencher is irrelevant.

Deactivation channels of the excited singlet state

1M

M + h` kfl

M kIC3M kISC

M (+ Q or Q*) kq+Q

Miso or M` + M`` kmr

MA or M+ + A- kbr

+A

MAQ]M[ 1

1

brmrqISCICfl kkkkkkdt

d

3M

M + h`` kph

M kISC`

M (+ Q or Q*) kq+Q

Miso or M` + M`` kmr

MA or M+ + A- kbr

+A

MAQ]M[ 3

`

3

brmrqISCph kkkkkdt

d

Deactivation channels of the triplet state

Quantum efficiency

= number (rate) of chosen process

number (rate) of photons absorbed Mk

Mk

channelsdeactii

flfl

1

.

1

channelsdeacti

i.

1

channelsdeacttripletjj

phISCph k

k

..

channelsdeactii

flfl k

k

.

Quantum efficiency

1M

M + h` kfl

M kIC3M kISC

M (+ Q or Q*) kq+Q

Miso or M` + M`` kmr

MA or M+ + A- kbr

+A

channelsdeactii

flfl k

k

.

Stern-Volmer plot

k

Qk

k

Qkk

k

k

I

I

Qkk

k

q

fl

qfl

fl

fl

q

flfl

1

I

0

fl

1

I0/I

[Q]

Energy transfer

• Through radiation (trivial)

• Without radiation– long-range, coulomb-interaction (Förster)– short-range, electron-exchange (Dexter)

Trivial energy transfer

Condition: the emission spectrum of the donor and absorption spectrum of the acceptor must overlap.

Long-range dielectric interaction

The rate is proportional to the -6th power of the distance between donor and acceptor

Short-range, electron exchange interaction

The rate is proportional to (e-r/l)2, r: the distance between donor and acceptor, l: van derWaals distance

Triplet-triplket energy transferPHOTOSENSITIZATION