Ammonium and borate substituents polarize π-electron systems, which then exhibit internal CT bands in the UV/Vis spectrum. These systems (for example 1) are distinctly more transparent than analogous push–pull-substituted compounds. Since their first-order hyperpolarizabilities are nevertheless similar to push–pull compounds, they are better suited to applications for second harmonic generation.
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Lambert et al. (1996) studied this question.
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