Based on the theoretical finding that the geometry of 1:1 complexes formed between two molecules containing planar
aromatic rings, governs the relative contribution of the molecular first hyperpolarizability tensor elements to the total
hyperpolarizability (β), we have demonstrated how the geometry of such complexes in solution can be described.
Hyper-Rayleigh scattering (HRS) technique has been used to measure the second harmonic (SH) scattered light from the
complexes in a polarization resolved manner along two mutually perpendicular directions (X and Z) in the laboratory
frame (XY being the plane of linear and circular polarization and Z being the direction of propagation of the polarized
incident light beam). The macroscopic depolarization ratios, D and D' obtained from the polarization resolved HRS
measurements are analyzed to obtain the geometry of two 1:1 electron donor-acceptor complexes between p-xylene and
tetrachloro-p-benzoquinone, and durene and
dichlorodicyano-p-benzoquinone. The typical values of the two
depolarization ratios, D and D', for a planar molecule with C2v symmetry are 0.2 and 0.33, respectively. The D and D'
values obtained for the complexes are higher at ~0.24 and ~0.4, respectively. This implies a deviation of the complex
geometry from the C2v symmetry that is expected from a perfect cofacial or T-shape structures. Perhaps, a twisted V
shape with tilt and twist between the two molecular planes would explain the geometry of the complexes.
While structure-properties relationships are quite actively and successfully investigated at the molecular level of
engineering of optical nonlinear responses, supramolecular
structure-property relationships are an appealing field. The
realization that interchromophoric interactions between strongly polar/polarizable NLO chromophores can significantly
affect the NLO response of each chromophoric unit as well as promote associations has opened new dimensions for
molecular design. Several elegant routes have been implemented to hinder or counterbalance dipole-dipole interactions
between dipolar NLO chromophores for the elaboration of second-order materials (for SHG or electro-optical
modulation). At opposite, we have implemented a reverse strategy by confining discrete numbers of NLO push-pull
chromophores in close proximity within covalent organic nanoclusters with the aim to exploit interchromophoric
interactions in order to achieve enhanced NLO responses. As a proof of concept, we present here the investigation of
two-series of multichromophoric covalent assemblies built from NLO push-pull chromophores showing that cooperative
enhancement can be achieved both for second-order optical responses (first hyperpolarizabilities) or third-order
responses (two-photon absorption cross-sections).
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