Synthesis, linear & non linear optical (NLO) properties of some indoline based chromophores

Abstract

A series of indoline-π-acceptor chromophores has been synthesized and the members first hyperpolarizabilities measured to investigate the effect of conjugation length, the use of various substituents and configurational locking on the nonlinear optical response. While increasing the length of the conjugated interconnect enhances the optical nonlinearity, ring locking was found to have little effect, although this is thought to be due to the electronic properties of the substituent groups, rather than structural factors. Nonetheless, all of the compounds were found to have high molecular hyperpolarizabilities with values of up to 1485 × 10⁻³⁰ esu when measured in chloroform at 1300 nm, findings which confirm that these chromophores are excellent candidates for further study. X-ray crystallographic studies were performed on three of the compounds and bond length alternation values obtained; these were found to correlate well with the observed molecular responses.

Introduction

Organic nonlinear optical (NLO) chromophores containing donor(D)–π–acceptor(A) units have been widely reported in the literature as the enabling materials for a range of photonic devices [1], [2], [3], [4]. The fundamental measure of a molecule’s NLO response, and therefore potential for incorporation into NLO materials, is its figure-of-merit, μ.β, where μ is the molecular dipole moment, and β is the first hyperpolarizability. In order to be considered a viable candidate for further study a molecule should have a first hyperpolarizability greater than approximately 500 × 10⁻³⁰ esu and an overall figure-of-merit greater than 5000 × 10⁻⁴⁸ esu. Furthermore, in order to obtain an optimal and usable electro-optic response, it is necessary to incorporate a high weight percent of a given chromophore into either a host polymer matrix, or by covalently attaching it to a suitable polymer substrate [5], [6], [7], [8], [9], [10], [11], [12]. In addition, for a macroscopic NLO response to be observed it is necessary for the molecular dipoles of the embedded chromophores to be aligned acentrically, and this is usually achieved via either contact or corona poling [3]. However, due to the tendency of the dipoles of the active chromophores to relax back to an isotropic state (thereby reducing the observed NLO response) it is often necessary to employ techniques such as cross-linking in order to preserve the requisite non-centrosymmetry [6].

Chromophores with highly polar (i.e., zwitterionic) ground states often exhibit poor solubilities as well as a tendency to readily form aggregates. This is clearly problematic when considering their use in NLO materials as this means they cannot be incorporated into host polymers at high loadings. Furthermore, the presence of significant aggregation will lower the overall poling efficiency of the final NLO material as well as increase the propensity for deleterious post-poling relaxation of the aligned dipoles. As a result, further structural modifications are often required to the active chromophores to minimise aggregation and the inclusion of bulky, “arene-rich” substituents has been shown to be particularly effective in achieving this, thereby greatly increasing the observed macroscopic response in NLO materials [8].

We have previously reported a synthetic methodology [13], [14], [15], [16], [17] that allows entry to a number of high figure-of-merit NLO chromophores with aromatisable donors, (e.g., 1,4-dihydropyridinylidene, 1,4-dihydroquinolinylidene), and containing the powerful acceptor 4,5,5-trimethyl-3-cyano-2(5H)-furanylidenepropane dinitrile (TCF) e.g., compounds 1 and 2. While this approach allowed for ease of synthesis and for a controlled increase in the extent of conjugation in the molecules, the resultant “parent” merocyanines are prone to significant amounts of aggregation [16], and this is due to their highly polar, zwitterionic ground states. Evidence for H-aggregation in these compounds was routinely observed via the appearance of a high energy shoulder when UV–Vis absorption spectra were obtained in solvents of low polarity such as dioxane and chloroform. Given that the dielectric constants of these solvents resemble those found for polymer systems, viz. ɛ <10, this is clearly problematic as it suggests these molecules will resist alignment and be prone to relaxation after poling.

As a continuation of this work, we describe here further developments to our synthetic methodology, and extend the series to include chromophores with a non-aromatisable indoline donor group. Furthermore, we will also discuss structural modifications that are designed to increase their NLO figure-of-merit – these include ring locking of the π-conjugated system along with the incorporation of bulky substituents, e.g., diphenyl groups, onto this central ring system. In addition, we have substituted a halogen atom in middle of the conjugated interconnect with a variety of substituents in order to assess the impact from both a steric and electronic point of view.