A Mechanism for Strong SHG by Centrosymmetric Anilinosquaraine Dyes

Abstract

Aggregation of centrosymmetric squaraines into acentric dimers, followed by intermolecular charge transfer (CT) between the centric monomers of the dimer, has been proposed in the past as the mechanism by which these molecules exhibit surprisingly high SHG when deposited as Langmuir–Blodgett monolayer films. Here an alternative mechanism is considered, in which the center of symmetry is destroyed by distortion of the charge distribution in one molecule by the highly polar molecular environment of its nearest neighbors, so that the large potential contributions from intramolecular CT to SHG become symmetry allowed.

Exciton Coupling Model for the Emergence of Second Harmonic Generation from Assemblies of Centrosymmetric Molecules

“… 3 − 7 The presence of SHG activity was attributed to symmetry breaking from interchromophore coupling. 8 An intermolecular charge transfer mechanism was proposed in the case of the squaraines, in which two monomers form a T-shaped dimer. However, the actual structures of the squaraine multimers are not known, given the challenges of obtaining high-resolution structures of single-monolayer organic films.…”