Chemistry Reference
In-Depth Information
incident and scattered photons have slightly differ-
ing energies, whereas vibrational energy is trans-
ferred either from the light beam to the scattering
molecule or
vice versa
. Whether it is proper to regard
the scattered and incident photons as the same or
different, however, is a debate outside the scope of
this chapter.)
If we can regard a single photon as comparable
to a reagent molecule, we can similarly regard
Avogadro's number of photons as being equivalent
to a mole of photons. This quantity of light is known
as the
einstein
.
5.2 Absorption of light by molecules
Fig. 1
8
.14
A typical Jab
´
on´ ski diagram, showing
photophysical processes involving a singlet electronic ground
state (S
0
) and both singlet (S
1
) and triplet (T
1
) electronic
excited states. Processes not accompanied by the absorption
or emission of a photon are indicated by means of dashed
arrows. Key:
A
, absorption of a photon;
F
, fluorescence
(emission);
IC
, internal conversion;
ISC
, intersystem crossing;
P
, phosphorescence (emission);
T
, thermal activation
(heating);
VR
, vibrational relaxation.
Excited states
When a molecule absorbs a photon it is promoted to
a higher energy state—an
excited state
—that may
have excess electronic, vibrational or rotational
energy, depending on the energy of the photon. The
absorption process can occur only when the photon
energy precisely matches the difference in energy D
E
between this higher level state and the original state
of the molecule:
orbital theory, when a photon is absorbed by a
singlet ground-state molecule an electron is pro-
moted from a filled orbital into a higher energy
vacant orbital, thus generating an electronic excited
state with two unpaired electrons (Fig. 18.15). If
these unpaired electrons retain their antiparallel
spins, the excited state produced is also a singlet
(denoted S
1
for the lowest excited singlet state, and
S
2
, S
3
, etc. for higher singlet states) (cf. Fig. 18.15(b)).
If the unpaired electron spins become parallel,
however, a paramagnetic
triplet excited state
results
(denoted T
1
for the lowest excited triplet, and T
2
, T
3
,
etc. for higher triplets) (cf. Fig. 18.15(c)). There is no
triplet state corresponding to the singlet ground
state. The two complete sets of singlet and triplet
electronic states are referred to as the singlet and
triplet
manifolds
, respectively. Each triplet has a lower
energy than the corresponding singlet, as indicated
for T
1
and S
1
in Fig. 18.14. (This is a consequence of
Hund's rule of maximum multiplicity.) Molecules
with unpaired electrons in their ground states, e.g.
free-radical species, O
2
and many transition metal
complexes, will have additional types of electronic
states at relatively low energies (doublets, quadru-
plets, quintuplets, etc.) and frequently will have
D
E
=
E
p
=
h
n
For electronic excitation, the most common case
occurs when a molecule in its electronic
ground state
is promoted to an electronic excited state. This is
illustrated diagrammatically by the upward arrow
A
in Fig. 18.14. For most molecules, transition from the
electronic ground state to the lowest energy excited
state requires energy corresponding to light in the
UV-visible region of the spectrum. Therefore, the
UV-visible absorption spectrum of a molecule yields
information about the energies of its electronic
excited states. The energy gaps between these
excited states and the ground state are typically so
large that, at equilibrium at normal temperatures,
only a negligible proportion of molecules will exist
in an electronic excited state.
Energy diagrams in which the various electronic
and vibrational energy levels are indicated by short
horizontal lines are known as
Jab
´
on´ ski diagrams
.
Figure 18.14 is a Jab
´
on´ ski diagram for a molecule
with no unpaired electrons in the electronic ground
state, i.e. a non-paramagnetic
singlet state
(denoted
S
0
). The majority of organic and many inorganic
molecules are of this type. In the familiar molecular
