Chemistry Reference
In-Depth Information
0
SQ
exp
Dexter(exchange) short - range:
k
SQ
ðRÞ¼k
ðaRÞ
The nomenclature that was developed in connection with energy and charge transfer processes
is as follows, an
exime
is a transient dimer formed by the combination of an excited (usually
aromatic) molecule and a second similar (usually unexcited) molecule. Such a dimer bonds only in
the excited state and promptly dissociates in losing its excitation energy. The term
exiplex
, explained
by Birks [
92
], describes a complex between two molecules, one a donor and the other one, an
acceptor, which subsequently dissociate in a deactivation process. One of the components of the
exiplex
, either the donor or the acceptor, is in excited state while the counterpart, acceptor or donor,
is in the ground state. An
eximer
is then just a special case in which the two constituent molecules
are identical. While numerous charge-transfer complexes can form between certain molecules in
the ground state, a number of compounds can form only charge-transfer complexes when either
the donor or the acceptor is in an excited state. Formation of eximers was observed in a number
of aromatic polymers, such as polystyrene, poly(vinyl naphthalenes), poly(vinyl toluene), and
others [
93
].
An
exterplex
is composed of three molecules and often takes an important role in photophysical
and photochemical processes. Polymers with pendant aromatic chromophores and dimeric
compounds often show efficient
exterplex
formation due to high local chromophore concentration
in their structure. It was observed that
exiplex
emission spectra from a chromophore is usually
broad, structureless, and red-shifted to the corresponding monomer fluorescence. The extend of
such a shift is a function of the distance between the two components of the complex. It is also
strongly affected by the polarity of the media. Martic et al. [
95
] obtained emission spectra of
the exiplexes of anthracene and
r
-toluidine in toluene and in polystyrene. While
the maximum band of the emission spectra in toluene at 30
C is at 616 nm, in polystyrene it is
shifted to 400 nm. The exiplex emission spectra in a copolymer of styrene with 4-
N
,
N
, dimethyl-
p
-diaminostyrene
is at 480 nm. The maxima of the emission spectra are temperature-dependent. The maxima shifts in
toluene solution to shorter wavelength and in polystyrene it is the opposite, and it shifts to longer
wavelength with an increase in temperature. The maxima approaches common value at the glass
transition temperature of polystyrene. Similar results were reported by Farid et al. [
96
] who studied
formation of
exiplexes
of 4-(1-pyrenyl)butyrate in different solvents and in polymers.
Chemical and physical changes take place in molecules when they absorb energy and reach an
excited state. This is particularly true of carbonyl compounds. There is a change, as already stated, in
the dipole moments of the molecules. This is due to the fact that dipole moments depend upon
the distribution of the electrons. In carbonyl compounds, this change is particularly large. Also the
geometry of the molecule changes from the ground to the excited states. In addition, the chemical
properties of the molecules change. Thus, phenol, for instance, is a weak acid, but in the excited state
it is a strong acid. This can be attributed to the
N
,
N
p ! p
* transition where one of the pair of
p
electrons
is promoted to an antibonding orbital.
By the same reason, the acid strength of benzoic acid is less than in the excited state because the
charge in this case is transferred to carbonyl group. The excited states of both phenol and benzoic acid
can be illustrated follows [
93
]:
OH
OH
+
light
