Nonlinear optical properties of complexes of π‐conjugated polymers

Abstract

Novel host–guest complexes of conjugated polyimines with gallium chloride have been proposed and studied as an approach to enhance the third‐order nonlinear optical properties of the host conjugated polymers. The third‐order nonlinear optical susceptibility χ(3) (–3ω;ω,ω,ω) of two conjugated polyimines and their GaCl3 complexes, measured by third harmonic generation spectroscopy in the wavelength range 0.9 ∼ 2.4 μm, shows that χ(3) is enhanced in the nonresonant region, whereas it is reduced in the resonant region due to complexation. These results are explained by complexation mediated modifiction of the electronic structure of the host polymers. Two theoretical models consisting of four electronic energy levels were found to give a good description of the frequency dispersion and resonances in the χ(3) (–3ω;ω,ω,ω) spectrum of the host–guest complexes. The resulting fitting parameters were used to predict the degenerate four wave mixing χ(3) (–ω;ω,–ω,ω) spectrum for the new materials. The results of the present study suggest that host–guest complexation is a promising approach for improving the third‐order nonli‐near optical properties of conjugated polymers.

Probing Structure−Property Relationships in Third-Order Nonlinear Optical Polymers: Third Harmonic Generation Spectroscopy and Theoretical Modeling of Systematically Derivatized Conjugated Aromatic Polyimines

“…The observed large bathochromic shift of the absorption spectrum of PHOPI compared to both PPI and PMOPI can be understood in terms of intramolecular hydrogen-bonding-induced conformation change, which facilitates a more coplanar structure compared to either PPI or PMOPI. It is well-known from the X-ray crystal structure of trans - N -benzylideneaniline, a model compound of PPI, that the N -phenyl ring is twisted 55° from the plane of the imine group (CHN) due to the conjugation between the nitrogen lone-pair electrons and the π electrons on the adjacent benzene ring. , Our prior studies of GaCl 3 and diaryl phosphate complexes of conjugated polyimines showed that complexation resulted in a large bathochromic shift of the absorption spectrum relative to that of the parent polymer. , The net effect of either hydrogen bonding to the imine nitrogens of PHOPI and MO−PHOPI or their complexation by Lewis acids or protonic acids is a more coplanar molecular structure which is favorable to greater electronic delocalization in the ground state. Although there is a small bathochromic shift in going from PHOPI to MO−PHOPI, it is the intramolecular hydrogen bonding in both polymers that dominates their ground-state electronic structures.…”

Recent developments in polymeric charge transfer complexes

“…Polymers with significant third-order susceptibilities are typically those that also have the ability to conduct electricity, but polymers with heterocyclic laddertype structures are also very good. One of the papers published in this issue deals in detail with thirdorder susceptibilities [47].…”