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Bond Ellipticity Alternation: An Accurate Descriptor of the Nonlinear Optical Properties of π‑Conjugated Chromophores

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journal contribution
posted on 02.03.2018, 00:00 by Thiago O. Lopes, Daniel F. Scalabrini Machado, Chad Risko, Jean-Luc Brédas, Heibbe C. B. de Oliveira
Well-defined structure–property relationships offer a conceptual basis to afford a priori design principles to develop novel π-conjugated molecular and polymer materials for nonlinear optical (NLO) applications. Here, we introduce the bond ellipticity alternation (BEA) as a robust parameter to assess the NLO characteristics of organic chromophores and illustrate its effectiveness in the case of streptocyanines. BEA is based on the symmetry of the electron density, a physical observable that can be determined from experimental X-ray electron densities or from quantum-chemical calculations. Through comparisons to the well-established bond-length alternation and π-bond order alternation parameters, we demonstrate the generality of BEA to foreshadow NLO characteristics and underline that, in the case of large electric fields, BEA is a more reliable descriptor. Hence, this study introduces BEA as a prominent descriptor of organic chromophores of interest for NLO applications.