Chemistry Reference
In-Depth Information
ways that the excited state becomes deactivated. The return to the ground state from the triplet one
requires again an inversion of the spin. In Fig.
10.4
,
a
0
represent the energies of light absorbed,
a
and
b
,
h
, and
I
the energies of internal conversion,
c
represents return to the ground state by way of
fluorescence, and
return by way of phosphorescence.
The Franck and Condon principle states that during an electronic transition the various nuclei in
the molecule do not change their position or their moment [
90
]. What it means is that electronic
transitions are much more rapid (l0
─
15
s) than nuclear motions (10
─
12
s) so that immediately after the
transitions the nuclei have nearly the same relative positions and velocities that they had just before
the transitions. The energy of various bonding and antibonding orbitals increases for most molecules
in the following order,
d
s< p<
n
< p
< s
In molecules with heteroatoms, such as oxygen or nitrogen, however, the highest filled orbitals in the
ground state are generally nonbonding, essentially atomic, n orbitals. This, for instance, is a case with
ketones and aldehydes. These molecules possess electrons that are associated with oxygen and are not
involved in the bonding of the molecules. The n electrons in formaldehyde can be illustrated as follows:
n-electrons
H
O
H
As explained above, in the triplet state the spin of the excited electron becomes reversed. This
results in both electrons having the same spin. From purely theoretical approach, such an electronic
configuration is not allowed. Due to the fact that the excited electron cannot take up its original
position in the ground state until it assumes the original spin, the triplet state is relative long-lived. For
instance, in benzophenone at 77
C the lifetime can be 4.7
10
─
3
s. Orchin and Jaffe wrote [
88
] that
the triplet state has a lifetime of 10
─
3
s. By comparison, the lifetime of a singlet state is about 10
─
8
to
10
─
7
s. Also, in the triplet state the molecule behaves as a free-radical and is very reactive. The carbon
atom has a higher electron density in the excited state than in the ground state. This results in a higher
localized site for photochemical activity at the orbital of the oxygen. Because the carbonyl oxygen in
the excited state is electron-deficient, it reacts similarly to an electrophilic alkoxy radical. It can, for
instance, react with another molecule by abstracting hydrogen.
At higher frequencies (shorter wavelength) of light, if the light energy is sufficiently high,
p ! p
*
transitions can also take place. All aromatic compounds and all conjugated diene structures possess
delocalized
*.
In general, the excited states of molecule are more polar than the ground states. Polar solvents,
therefore, tend to stabilize the excited state more than the ground state. As shown in Fig.
10.4
, the
triplet state is lower in energy than the corresponding singlet state. This is due to the fact that the
electron-electron repulsion is minimized, because they do not share each other's orbitals according to
the Pauli exclusion principle. Thus, less energy is required for the triplet state.
The chemical reactivity of organic molecules is determined principally by the electron distribution
in that molecule. When the electron distribution changes, due to absorption of light and subsequent
transitions, photochemical reactions take place while the molecule is in an electronically excited
state. The phenomenon of light absorption, formation of the excited states, and subsequent reactions
obey four laws of organic photochemistry, as was outlined by Turro [
87
]:
p
systems. Because there are no n electrons, all transitions in these systems are
p ! p
1. Photochemical changes take place only as a result of light being absorbed by the molecules.
2. Only one molecule is activated by one photon or by one quantum of light.
3. Each quantum or photon which is absorbed by a molecule has a given probability of populating
either the singlet state or the lowest triplet state.
4. In solution, the lowest excited singlet and triplet states are the starting points for the photochemical
process.
