Photochemistry Reactions involving photons'

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PhotochemistryReactions involving photons.
http//oktatas.ch.bme.hu/oktatas/konyvek/fizkem/fi
zkem2/fotokemia
(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
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Electromagnetic spectrum important for
photochemistry
E
VUV
200 nm
UV
400 nm
E h? hc/?
700 nm
IR
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Excited states and related bond strength
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Multiplicity

• Name redived from 2S 1
• S 0 singlet
• S ½ doublet
• S 1 triplet

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The Jablonski diagram
E
singlet triplet splitting
involving a photon
without photons
S
S1
S2
T1
T2
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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)

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Absorption
E
S
S1
S2
T1
T2
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Lambert Beer law

• I I0 10-ecl
• e decadic absorption coefficient
• unit dm3mol-1cm-1
• T I/I0 T() 100 I/I0
• A -lg T lg (1/T) lg I0/I ecl

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Typical absorptions
n ? p carbonyls, tiocarbonyls, nitro-, azo-
and imino- group containing compounds p ?
p alkenes, alkynes, aromatics n ? s amines,
alcohols, haloalkanes s ? s alkanes
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Absorption
S
S1
S2
T1
T2
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Vibrational relaxation
E
S
S1
S2
T1
T2
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Deactivation channels of the singlet state
E
?
S
S1
S2
T1
T2
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Fluorescence emissionwithout change of spin
state
E
S
S1
S2
T1
T2
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IC internal conversion
E
S
S1
S2
T1
T2
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ISC intersystem crossing (spinváltó átmenet)
E
S
S1
S2
T1
T2
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Phosphorescence emissionwith change of spin
state
E
S
S1
S2
T1
T2
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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.

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Deactivation channels of the excited singlet state
M hn kfl
M kIC
3M kISC
1M
M ( Q or Q) kq
Q
Miso or M M kmr
A
MA or M A- kbr
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Deactivation channels of the triplet state
M hn kph
M kISC
M ( Q or Q) kq
Q
3M
Miso or M M kmr
A
MA or M A- kbr
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Quantum efficiency
F number (rate) of chosen process number
(rate) of photons absorbed
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Quantum efficiency
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M hn kfl
M kIC
3M kISC
1M
M ( Q or Q) kq
Q
Miso or M M kmr
A
MA or M A- kbr
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Stern-Volmer plot
I0/I
1
Q
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Energy transfer

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

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Trivial energy transfer

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

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Long-range dielectric interaction
The rate is proportional to the -6th power of the
distance between donor and acceptor
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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
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Triplet-triplket energy transferPHOTOSENSITIZATIO
N