Chemistry Reference
In-Depth Information
where
is the wavelength of the interacting radiation. All reactions that are photochemical in nature
involve electronically excited states at one time or other. Each one of these states has a definite energy,
lifetime, and structure. The property of each state may differ from one to another and the excited states
are different chemical entities from the ground state and behave differently. The return to the ground
state from the excited state, shown in Fig.
10.4
, can take place by one of three processes [
85
]:
l
1. The molecule returns directly to the ground state. This process is accompanied by emission of light
of a different wavelength in the form of fluorescence.
2. An intersystem conversion process takes place to the T
1
state, where the electron reverses its spin.
The slower decay of excitation from the triplet state to the ground state is accompanied by
emission of phosphorescence.
3. The molecule uses the energy of excitation to undergo a chemical reaction
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 n 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 a hydrogen.
At higher frequencies (shorter wavelength) of light, if the light energy is sufficiently high,
*
transitions can also take place. All aromatic compounds and all conjugated diene structures possess
delocalized
p ! p
*.
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 that 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 as stated by
the Pauli exclusion principle Thus, less energy is required for the triplet state.
This dissipation of the excitation energy can also be illustrated as follows:
p
systems. Because there are no n electrons, all transitions in these systems are
p ! p
A
0
+ fluorescence
A
0
+ light
A*
A
0
+ heat
chemical reaction
where A
o
represents any organic molecule and A* represents the same molecule in an excited state.
In the process of energy dissipation from the singlet and return to the ground states, the light
emission by fluorescence is at a different wavelength than that of the light that was absorbed in the
excitation. This is because some energy is lost in this process of the electron returning from its lowest
excited state to the ground state. The energy, however, may also, depending upon the structure of the
molecule, be dissipated in the form of heat, as shown above. And, also, a third form of energy
dissipation can occur when the molecule undergoes a chemical reaction. Depending, again on the
molecular structure, the chemical reactions can be rearrangement, isomerization, dimerization (or
coupling), fragmentation, or attack on another [
90
-
92
] molecule. Some examples of such reactions are:
hν
