Polychlorotriphenylmethyl Radicals: Towards Multifunctional Molecular Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 66

unidirectional excitations. The main advantage of octupolar molecules belonging to either planar T d , D 3 h
or C 3 v or non-planar D 3 or C 2 symmetry groups, when compared to dipolar 'push - pull' systems, lies
on the looser structural requirements, like centrosymmetrical molecular arrangements, as well as in the
improvement of the balance between transparency and second order NLO signals. 116
In this section, the capability of PTM radicals to generate NLO optical responses, either as octupolar
materials or as a component of a 'push - pull' system, is presented and the corresponding hyperpolarizability
values is provided. 117

2.2.3.1 Nonlinear optical properties of octupolar systems

Molecular nonlinear optical coefficients of PTM radicals 1 and 27-33 were measured, using the Hyper-
Rayleigh Scattering (HRS) technique, in dichloromethane solution at room temperature irradiating with
a laser light at 1064 nm. The molecular quadratic hyperpolarizability (

β ) values for all these radicals

10 − 30 esu) used as solvent. Data reported in
Figure 2.27 show that radicals 1 and 27-33 exhibit relatively high NLO responses with

were calibrated against pure dichloromethane ( β = 0 . 43 ×

β

values ranging

10 − 30 esu. This fact is not surprising, since this family of radicals is struc-
turally very similar to crystal violet, which also exhibits a large

10 − 30

× ×

from 118 to 755

value due to its octupolar symmetry. 118
In fact, all these compounds have the general formula Ar 1 Ar 2 Ar 3 Z, where Z is an sp 2 hybridized car-
bon atom - a C + in crystal violet and C • in PTM radicals - and Ar i denotes an aromatic group with
polarizable substitutents - a diethylamine (NEt 2

β

) substituent in crystal violet and chlorine atoms in PTM

Cl

Cl H Cl Cl

H Cl Cl

Cl

Cl Cl Cl Cl

Cl Cl Cl

Cl Cl

Cl Cl Cl

Cl Cl Cl

Cl Cl

H H Cl Cl Cl Cl

Cl

Cl Cl Cl Cl

Cl Cl Cl

Cl

Cl Cl Cl Cl

Cl Cl Cl

Cl

Cl H Cl Cl

H Cl Cl

Cl

Cl H CH 3

28
363 × 10 − 30 esu

27
755 × 10 − 30 esu

29
118 × 10 − 30 esu

1

10 − 30 esu 362 ×

Cl

Cl Cl

Cl H

Cl H H

Cl H

H

Cl H Cl

Cl Cl Cl

Cl

Cl Cl Cl

Cl

Cl Cl

Cl Cl

Cl Cl

H H

H H

Cl

Cl

Cl

Cl Cl

Cl Cl

Cl

Cl

Cl Cl

Cl Cl

Cl Cl

Cl

H

H Cl

Cl Cl

Cl H

H

Cl

H

Cl

NH 2

31 32 33

30
407 × 10 − 30 esu

10 − 30 esu 10 − 30 esu 10 − 30 esu

237 593 262

× × ×

Figure2.27 Hyperpolarizability β valuesofPTMradicals 1,27-33 measuredwithHRStechnique.