Systematic study of the structure-property relationship of a series of near-infrared absorbing push-pull heptamethine chromophores for electro-optics

This study reports the facile synthesis, characterization and quantitative structure-property relationship analysis of molecular and material properties of tricyanofuran-based (TCF) dipolar heptamethines with different electron donors of indoline (F1), benzo[e]-indoline (F2), benz[cd]indoline (F3), and Michler’s base derivatives (M1 and M2). The linear and nonlinear optical (NLO) properties of these chromophores have been thoroughly investigated, and the relationship between molecular and bulk response has been analyzed and compared with dipolar tetraene AJLZ53 as one of the best chromophores for electro-optic (EO) devices. In particular, we provide responsible data collection and analysis of optical and EO properties for poled thin films using a widely-recognized and accredited methodology of prism coupling system with the help of rigid oriented gas model. We found that these push-pull heptamethines with synthetic efficacy exhibit high near-infrared absorption, excellent chemical stability and large hyperpolarizabilities (ß) varied from 1,023×10⁻³⁰ esu for F1, 3,047×10⁻³⁰ esu for M1, and 3,547×10⁻³⁰ esu for F3 at 1,304 nm in poled films, respectively. The ß values of these molecules are among the highest ones for TCF-based dipolar chromophores, and also agree well with reported analytical results in the solutions. In poled polymers with a modest chromophoric loading density of 1.3×10²⁰ cm⁻³, M1 and M2 give a high poled-induced noncentrosymmetric order and relatively large r₃₃ values around 40 pm V⁻¹ at 1,304 nm, indicative of large µß values and suitable structural modification for high poling efficiency. Furthermore, a binary EO polymer based on the co-loading of M1 and AJLZ53 achieve a large r₃₃ value of 143.3 pm V⁻¹ at 1,304 nm. Our studies suggest that concise synthesis and molecular design of push pull polymethines can be well guided by the tabulation of their linear and NLO properties in bulk materials, and streamline future development of high performance organic EO materials for photonic applications.