Reactive Intermediates
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Want to see time development of excited states
and free radicals
Excited states and free radicals act as individual
chemical species during their existence.
They are species of particular interest because of
their high energy content.
If you can capture their energy content, you can
do chemistry that you cannot do in ground
states.
How to Utilize the Energy Content?
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If excited states channel their energy into specific
bonds, then photochemistry can occur.
If scavengers or quenchers can find the excited
state or free radical in time, then the electronic
or chemical energy can be captured by the,
ordinarily, stable scavenger or quencher.
Different Actions of Scavengers
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Direct capture of free radicals.
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Repair of damage caused by radicals.
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This second mechanism is important for the
repair of damage by free radicals in biological
systems.
Motivations
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Oxidative stress
• Alzheimer’s disease
• Biological aging
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Basic issues
• Neighboring-group effects
• Details of oxidative scheme
Radical Repair
and
Antioxidants
R-S-H
1
H
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N-Acetyl-L-Methionine Amide
STABILIZATION OF SULFUR RADICAL CATIONS VIA
INTRAMOLECULAR SULFUR-NITROGEN AND SULFUR-OXYGEN BOND
FORMATION
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Met-Gly
Gly-Met
Characterizing Excited States
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Excited states are not stationary states when
consideration is made of the electromagnetic
field.
Therefore, excited-state processes are of
primary significance.
What happens to the energy when matter
absorbs sunlight or UV?
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Gin and tonic glows under “black light” fluorescence
Things heat up when sitting in the sun –
radiationless transitions
Some objects, like TV screens, glow after use or
after the light is turned off – phosphorescence
Objects appeared colored under visible light –
differential absorption
Excited-State Processes (Intramolecular)
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Fluorescence (fast radiative process)
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Phosphorescence (slow radiative process)
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Radiationless Transitions
• Internal Conversion (transition with no spin flip)
• Intersystem Crossing (transition with spin flip)
• Vibrational Relaxation (heat produced)
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Photochemistry (bonds broken)
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Photoionization (no bonds broken, e- ejected)
Photochemistry
Jabłonski - diagram
ISC
S1
IC
excited
singlet state
singlet
ground state
T1
fluorescence
vibrational
relaxation
S1
ISC
S0
(heat)
phosphorescence
S0
excited
triplet state
T1
State Picture
Orbital Picture
Radiationless Transitions
Showing Nuclear Contributions
“Stokes” shift
Absorption vs Emission
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E = hc / 
Lifetimes & Quantum Yields
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Triplet states have much longer lifetimes than
singlet states
In solutions, singlets live on the order of
nanoseconds or 10’s of nanoseconds
Triplets in solution live on the order of 10’ or
100’s of microseconds
Triplets rarely phosphoresce in solution
(competitive kinetics)
Competitive Kinetics
Intramolecular decay channels
isc
T
p
S0
Intermolecular decay channels
T + Q  S0 + Q’
d [T ]
 kisc[T]  k p [T]  kq [Q][T]
dt
[T]  [T]0 exp  k isc  k p  k q [Q]t
Intermolecular Excited-State Reactions
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Energy Transfer
A* + B  A + B*
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Electron Transfer
D* + A  D+ + A
D + A*  D+ + A
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Hydrogen Abstractions
Note:
Have to have excited
states that live long enough
to find quenching partner
by diffusion
Absorpcja przejściowa – diagram Jabłońskiego.
Transient Absorption
ISC
S2
Tn
IC
TA
IC
TA
ISC
S1
T1
A
IC
F
P
C
IS
S0
Important Types of
Organic Excited States
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,* states, particularly in aromatics and
polyenes
n,* states, particular in carbonyls
S2
1,*
S1
1n,*
S0
isc
T2
3,*
T1
3n,*
Example:
Lowest electronic states
of Benzophenone
Weak Spin Interactions
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Triplet states, spin interactions are weak so
excited triplet states can live for some time
But they have lower energy than their
corresponding singlet states with the same orbital
configurations
By Pauli Exclusion principle - no two electrons in
the same system can have the same quantum
numbers
By Pauli Exclusion principle like spins avoid each
other – correlation hole around each electron
Photochemistry of Triplet States
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3n,*
states are particularly good in Habstractions: they act like free radicals
Molecules in excited states are generally more
reactive in electron-transfer reactions than are
their ground states
Excited-State Electron Transfer
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Because of the “hole” in the HOMO of the ground
state, the excited states have a low-lying orbital
available for accepting electrons
D
e
LUMO
HOMO
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Because of the electron in the highly excited
orbital, e.g the LUMO of the ground state, the
excited state is a good donor for elect. transfer
e
LUMO
A
HOMO
Why Triplet Quantum Yield is high in
Benzophenone?
S2
1,*
S1
1n,*
isc
T2
3,*
T1
3n,*
Lowest electronic states
of Benzophenone
S0
(1)1n,* states have small krad because of small orbital overlap
(2) kisc is large because of low-lying 3,* and El-Sayed’s Rule
Selection Rules for ISC
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El-Sayed’s Rule
Intersystem crossing between states of like
orbital character is slower than ISC between
states of different orbital character.
Energy Gap Law
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The rate of radiationless transitions goes as the
exponential of the energy gap between the 0-0
vibronic levels of the two electronically excited
states
Rational of Energy Gap Law
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Related to the probability of undergoing
muliphononic events which gets more difficult as
the number of phonons increases
Franck-Condon factors get smaller as the
difference in nuclear excitations between
electronically excited states increase
Classical Franck-Condon Factor
Demonstrated for
absorption
Quantum Mechanical
Franck-Condon Factors
Demonstrated for
absorption
Deuterium Effect
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Radiationless transitions, e.g. intersystem
crossing, slow down with deuterium substitution
Franck-Condon Factors again are involved
Relevance to Kasha’s Rule
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Tend to go to lowest levels in each multiplicity
Relatively small gaps between higher singlet
states, so IC is fast
Within a particular electronic manifold, the
vibrational relaxation is fast – here you can go
one phonon at a time and still almost conserve
energy within the molecule
Robinson-Frosch Formula
Radiationless Transitions
dP(nm)/dt = (42/h) |Vmn|2 FC (Em)
Probability per unit time of making a radiationless
transition from the initial electronic state n, to a
set of vibronic levels in the electronic state m is
equal to the product of three factors
(1) The square of the electronic coupling element
(2) The Franck-Condon factor between the vibrational
levels of the initial electronic state and the final one
(3) The density of final vibronic states
Implications of Robinson-Frosch
dP(nm)/dt = (42/h) |Vmn|2 FC (Em)
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Electronic matrix elements are usually a factor of
a thousand less for transitions that change spin
Franck-Condon factors favor small changes in
vibrational quantum numbers
Higher density of states favors faster transitions