1.3 Photochemistry Versus Thermal Chemistry
7
of A must be excited at least once during one hour, a fraction τ/3600 of them will
be in the excited state at every instant during the experiment. With τ = 10
−9 s,
the excited fraction would be about 3 · 10
−13 . The population of an excited state
at thermal equilibrium is proportional to the factor exp(−ΔE/K B T ), where K B is
Boltzmann’s constant and ΔE is the energy difference with respect to the ground
state (lowest energy level). With ΔE = 400 kJ/mol, again a reasonable value, in
order to reach the same population of excited molecules as in the photochemical
experiment we must raise the temperature to T = 1670 K. This would destroy most
molecular samples.
Where is the difference? Heating the whole sample pours energy in all the translational, rotational, and vibrational modes of all molecules, many of which will
certainly react in unwanted ways. On the contrary, the optical excitation puts energy
only in the few happy molecules that absorb a photon and get electronically excited.
This allows to promote reactions that would not occur in thermal chemistry, in the
first place endothermic reactions that would be thermodynamically nonviable. Of
course, the excited molecules dispose of the excess energy by transferring it to the
surrounding molecules, by emitting radiation or by storing it as chemical energy
in the reaction products: every single molecule can channel its surplus energy in a
different way.
Besides the energetic aspects, we shall see that excitation of molecules with photons in the above energy range changes the electronic wavefunction. This means that
some bonds are weakened, others may be strengthened, and the molecular equilibrium geometry and the charge distribution are altered as well as the interactions with
surrounding molecules. All these changes contribute to differentiate the photoreactivity from the ground state thermal chemistry.
1.4 An Overview of Photochemical and Photophysical
Processes
In this section we shall list the main photochemical and photophysical phenomena
that can occur after photoexcitation, with some comments about energy disposal
which is one of the main differences between thermal and photochemical reactivity.
We shall also anticipate some data about typical timescales, since they determine the
competition between the energetically viable primary processes.
The usual ways to indicate the excitation of molecule A are
A + hν → A
∗
(1.22)
or
A
hν
−→ A
∗
(1.23)
7
of A must be excited at least once during one hour, a fraction τ/3600 of them will
be in the excited state at every instant during the experiment. With τ = 10
−9 s,
the excited fraction would be about 3 · 10
−13 . The population of an excited state
at thermal equilibrium is proportional to the factor exp(−ΔE/K B T ), where K B is
Boltzmann’s constant and ΔE is the energy difference with respect to the ground
state (lowest energy level). With ΔE = 400 kJ/mol, again a reasonable value, in
order to reach the same population of excited molecules as in the photochemical
experiment we must raise the temperature to T = 1670 K. This would destroy most
molecular samples.
Where is the difference? Heating the whole sample pours energy in all the translational, rotational, and vibrational modes of all molecules, many of which will
certainly react in unwanted ways. On the contrary, the optical excitation puts energy
only in the few happy molecules that absorb a photon and get electronically excited.
This allows to promote reactions that would not occur in thermal chemistry, in the
first place endothermic reactions that would be thermodynamically nonviable. Of
course, the excited molecules dispose of the excess energy by transferring it to the
surrounding molecules, by emitting radiation or by storing it as chemical energy
in the reaction products: every single molecule can channel its surplus energy in a
different way.
Besides the energetic aspects, we shall see that excitation of molecules with photons in the above energy range changes the electronic wavefunction. This means that
some bonds are weakened, others may be strengthened, and the molecular equilibrium geometry and the charge distribution are altered as well as the interactions with
surrounding molecules. All these changes contribute to differentiate the photoreactivity from the ground state thermal chemistry.
1.4 An Overview of Photochemical and Photophysical
Processes
In this section we shall list the main photochemical and photophysical phenomena
that can occur after photoexcitation, with some comments about energy disposal
which is one of the main differences between thermal and photochemical reactivity.
We shall also anticipate some data about typical timescales, since they determine the
competition between the energetically viable primary processes.
The usual ways to indicate the excitation of molecule A are
A + hν → A
∗
(1.22)
or
A
hν
−→ A
∗
(1.23)