Chemistry Reference
In-Depth Information
Y-axis
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
TCB
b =
22
×
10
−
30
esu
D
3
h
PCB
b =
8
×
10
−
30
esu
C
2
n
Figure 2.28
Schematic representation of prototypical non-centrosymmetric planar D
3h
and C
2v
symmetry
aromaticmolecules:TCB,1,3,5-trichlorobenzeneandPCB,pentachlorobenzene.
radicals. Moreover, the considerably large
values obtained point to an enhancement of the second order
NLO activities according to their open shell electronic states.
119
However, it is not possible to extract
an irrefutable conclusion about the influence of open shell character on NLO responses, since a closer
look at radicals
1
and
27-33
reveals that they are composed of three individual subunits - thee aryl
groups - each one having a NLO contribution. More precisely, 1,3,5-trichlorobenzene (TCB) and the pen-
tachlorobenzene (PCB) compounds have also octupolar components and belong, respectively, to
D
3
h
(pure
octupolar system) and
C
2
v
(with dipolar and octupolar contributions) group of symmetries (Figure 2.28).
For this reason, PTM radicals give access to individual pure octupolar molecules, constituting individual
octupolar subunits, for which the term of 'Super Octupolar' systems was coined. If an octupolar syn-
ergistic effect takes place, this would greatly contribute to enhance the NLO responses of this type of
materials.
Along this line, the comparison between purely octupolar radical compounds
1
and
27
may be very
interesting. Both systems have
D
3
symmetry and are composed of three individual building blocks with
NLO contributions based on PCB and TCB subunits, respectively. HRS measurements show that the
β
β
value of
1
is lower than that obtained for
27
. Interestingly, such values follow the trend found for
the molecular hyperpolarizabilities derived from each individual octupolar building block (Figure 2.28),
suggesting that a synergetic effect between individual building blocks may occur.
To give more insight to the relationship between the electronic configuration and the NLO responses of
PTM radicals, cyclic volatmmetry studies of radicals
1, 27-33
in dichloromethane, using 0.1 M
n
Bu
4
NPF
6
as electrolyte, were performed and analysed. Interestingly, the representation of the hyperpolarizability
β
value in front of the reduction/oxidation potentials of radicals
1, 27-33
yields a linear dependence
(Figure 2.29). Indeed, as the oxidation potential decreases, the hyperpolarizability
β
value increases
β
whereas, on the contrary, the easier to reduce the radicals the lower results the
values. This con-
firms a direct correlation between the energy of the electronic transition and the 'Super Octupolar' NLO
response.
120
2.2.3.2 Nonlinear optical properties of 'push-pull' systems
The PTM radicals discussed so far are octupolar systems that exhibit relatively high hyperpolarizabilities.
The incorporation of PTM radicals into 'push - pull' (dipolar) systems represents an alternative design
strategy for NLO active systems.
121
values of the series of radicals
shown in Figure 2.30 have been measured using HRS techniques in dichloromethane solution at room
With this aim, the hyperpolarizability
β
Search WWH ::
Custom Search