Ground state
reactants
Ground state
reactants
hν
Excited state
reactants
Ground state
products
Reaction
Intermediates
Ground state
products
2.1
Ground state
reactants
Excited state
reactants
Reaction
Intermediates
Ground state
products
ISC
Singlet
States
Triplet
States
Internal
conversion
Fluorescence
Intersystem crossing from
the triplet to the ground state
Ground
Singlet
State
Phosphorescence
2.2
Transitions in formaldehyde
2
2
2
Ψ(CH =O) = (1s ) (1s ) (2s ) (σ
2
O
(π
C
2
CO
O
2
) (n ) (π* )
O
CH
2
2
) (σ' ) (σ
CH
CO
2
)
0
CO
Transitions at relatively low (UV/Vis) energies involve:
(π
2
2
0
) (n ) (π* )
CO
O
CO
π*
n
π
ground
state
n,π*
π,π*
standard abbreviations
2.3
Excited state properties determined by type of excitation
π*
n
π
n,π*
Half-filled orbital
localized on oxygen:
species resembles
an alkoxy radical
π,π*
Transitions involve
only the π system, no
free radical properties
expected. In aromatic
ketones the aryl π system
is usually involved
2.4
Basics of carbonyl photochemistry
T1 state
S1
Benzophenone: n,π*
T2
T1
p-MeO-ketones: π,π*
Benzophenone
phosphorescence
O
So
380
420
460
500
Wavelength, nm
540
2.5
Vibrational structure in benzophenone phosphorescence
0,2
0,0
380
420
460
500
Wavelength, nm
540
C=0 distance
2.6
Frank-Condon Principle
A mechanical analogue to the
Frank-Condon principle for
radiative transitions. The motion
of a point representing the
motion of the vibrating atoms is
shown as a sequence of arrows.
These arrows represent the motion
of a representative point on the
curve.
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
Electronic transitions occur sufficiently fast that only
"vertical" transitions are of importance. The nuclei are
frozen as the transition takes place.
2.7
Absorption and emission properties of anthracene
Which is the right set of potential
energy curves for this system?
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
2.8
Emission
Absorption
Wavelength, nm
The 0,0 emission band is
very small or totally absent.
Origin of the problem ?
Implications in terms of
Frank-Condon effects ?
2.9
Emission
Absorption
Wavelength, nm
Which is the right set of potential energy curves ?
2.10
Vibrational separation determined by type
of atoms and of bonds involved
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
2.11
Ground state
reactants
Excited state
reactants
small
Reaction
Intermediates
big
S
1
S
1
T
Ground state
products
1
T
1
S
S
0
0
n,π*
π,π*
2.12
Singlet-triplet splitting for states
with the same electronic configuration
E(S ) = 0
0
E(S ) = E(n,π *) + K(n,π *) + J(n,π * )
1
E(T ) = E(n,π *) + K(n,π *) - J(n,π *)
1
∆E
ST
= 2J(n,π *) > 0
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
2.13
Ground state
reactants
Excited state
reactants
Type
CH 2 =CH 2
CH 2 C=O
Ph 2 C=O
Reaction
Intermediates
Ground state
products
∆E(S-T)
π,π*
70
π,π*
40
π,π*
35
n,π*
n,π*
10
7
kcal/mol
2.14
Ground state
reactants
Excited state
reactants
Singlet
States
k
isc
Reaction
Intermediates
Ground state
products
Triplet
States
Intersystem crossing from the first
excited singlet state to the lowest
triplet state
2.15
Intersystem crossing
Assisted by spin-orbit coupling (SOC)
BEST WHEN involving px → py jump
localized on a single atom
El Sayed Rules
n,π* ↔ n,π*
Forbidden
n,π* ↔ π,π*
'Allowed'
π,π* ↔ π,π*
Forbidden
from N.J. Turro, “Modern Molecular Photochemistry”, 1978
2.16
More on benzophenone
77 K
S2 ( π, π*)
100 kcal
~1012 s -1
S1 (n, π*)
74 kcal
T 2 ( π, π*)
1011 s -1
100%
T 1 (n, π*)
69 kcal
<107
380
420
460
500
Wavelength, nm
540
6 -1
10 s
Room T
1.8 x 102 s -1 (90%)
S0
Jablonski diagram at 77 K
2.17
Aspects of an introduction to photochemistry
Ground state
reactants
Excited state
reactants
Orbital occupancy
Carbonyl photochemistry
Vibrational structure
Frank Condon
Anthracene
ST gaps
Intersystem crossing
Reaction
Intermediates
Ground state
products
Some kinetics
Delayed fluorescence
Excimers and exciplexes
Energy gap law
Fluorescence yields
Correlation diagrams
Energy transfer
Sensitization and quenching
2.18