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Photochemistry
Reactions 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
Electromagnetic spectrum important for
E
photochemistry
VUV
~200 nm
UV
~400 nm
E = hν = hc/λ
~700 nm
IR
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
singlet – triplet splitting
involving a photon
without photons
S
S1
S2
T1
T2
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 → p*
carbonyls, tiocarbonyls,
nitro-, azo- and iminogroup containing
compounds
p → p*
alkenes, alkynes, aromatics
n → s*
amines, alcohols,
haloalkanes
s → s*
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: emission
without 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: emission
with 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
+Q
+A
M + hn`
M
3M
M (+ Q or Q*)
Miso or M` + M``
MA or M+ + A-
kfl
kIC
kISC
kq
kmr
kbr
 
d [1 M ]
1
 k fl  k IC  k ISC  kq Q  kmr  kbr A M
dt
Deactivation channels of the
triplet state
3M
+Q
+A
M + hn``
M
kph
kISC`
M (+ Q or Q*)
kq
Miso or M` + M``
kmr
MA or M+ + A-
kbr
 
d [3 M ]
3
 k ph  k ISC `  kq Q  kmr  kbr A M
dt
Quantum efficiency
F
=
number (rate) of chosen process
number (rate) of photons absorbed
 
k fl  M
1
F fl 
 

 1
ki   M


 i deact.channels
Quantum efficiency
F fl 
k fl
k
i
i  deact . channels
F ph  F ISC 
F
1
i
i  deact . channels
k ph
k
j
j triplet. deact. channels
1M
+Q
+A
M + hn`
M
3M
M (+ Q or Q*)
Miso or M` + M``
MA or M+ + A-
F fl 
k fl
k
i
i  deact . channels
kfl
kIC
kISC
kq
kmr
kbr
Stern-Volmer plot
I0/I
I fl    F fl 
k fl
k  k q Q 
k fl k  k q Q 
k q Q 


 1
I fl k
k fl
k
I 0fl
1
[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
-r/l 2,
The rate is proportional to
r: the distance between donor and acceptor,
l: van derWaals distance
(e )
Triplet-triplket energy transfer
PHOTOSENSITIZATION