In the quest for organic second-order nonlinear materials, self-assembled structures based on octupolar molecules have
shown to be promising candidates. The presented work focuses on an octupolar substituted 1,3,5-trisalkynylbenzene that
forms crystals that show significant second-harmonic generation (SHG). Continuous polarization SHG measurements
were performed to elucidate the tensorial nature of this response. Unfortunately the model at hand was unable to fit the
polarization patterns due to depolarization of the second harmonic beam caused by multiple scattering in the sample.
Although the chromophore is clearly achiral, these patterns revealed a different response for left and right circularly
polarized light, also known as SHG circular difference. This implies that these molecules spontaneously assemble in
chiral superstructures. These surprising results were confirmed by circular dichroism spectroscopy, revealing a
monosignate CD band that must be attributed to a supramolecular chiral organization.
Hyper-Rayleigh scattering (HRS) is the only second-order nonlinear optical (NLO) technique allowed in an isotropic solution. Being an even-order technique, it is sensitive to (non-centro)symmetry. Therefore, HRS is a unique tool to determine solution-phase structure, both in a qualitative and a quantitative way. Examples are presented of the elucidation of the actual D2d octopolar structure of highly symmetric, yet non-centrosymmetric chromophores and of the
determination of the value for the opening angle of a chromophore-functionalized alkynyl-calix[4]arene cavity.
In recent years there has been significant interest in the ability to switch the second-order nonlinear optical (NLO)
response at the molecular level. A compound can be considered as an NLO switch when the response can be turned to
an 'on 'and 'off' state. Several switching schemes at the molecular level have been envisioned. Earlier schemes used
isomerisation and tautomerisation, causing changes in the nature and/or degree of conjugation between electron donor
and acceptor. An alternative approach is based on lowering the electron donating capacity of the electron donor or the
withdrawing capacity of the acceptor group. Here we will present results based on protonation/deprotonation and
oxidation/reduction of the donor group.
Modulating the electronic, magnetic and optical properties at the molecular level using an external trigger has been
extended to the field of nonlinear optics. The switching molecule is designed to have a complexation site or a redox
active unit which can guarantee electronic communication over the whole conjugated system. The alteration of the
nonlinear property is then induced by the modulation of the internal charge-transfer due to the response of the molecule
to the external stimulus. This can be achieved by cation binding, protonation/deprotonation or electrochemically.
The last little more than five years have seen an outstanding advance in the design of NLO-active organic molecules with octopolar symmetry, despite the often pessimistic predictions for the practical application potential of organic molecular materials. This contribution aims to review the latest developments, and, with special emphasis on efficient bulk materials based on functional molecules, tries to point out promising developments and strategies in order to promote organic molecular materials as useful candidates in modern nonlinear optics.
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