The quadratic (∝E
2) nonlinear optical (NLO) properties of pseudosymmetric organic cations are
investigated. These chromophores belong to a family of highly polarizable streptocyanine dyes built in
two steps by amination of their related carboxonium salts. They exhibit a nearly perfect optical transparency
in the 200−1100 nm range, except around 700 nm, where a single but narrow band is present. Some of
these dyes crystallize in noncentrosymmetric space groups, which, despite a symmetrical π-electronic
structure, leads to symmetry lowering and restores a push−pull character to the optical transition and
hence a sizable molecular hyperpolarizability. One of these molecules is optimized for a solid-state NLO
effect and exhibits an efficiency 120 times that of urea in second harmonic generation at 1.907 μm.
Seven new nonacarbon chain carboxonium salts were synthesized from variously substituted arylethanones, except a few which were too deactivated to react. By the action of several amines, thirteen new nonacarbon chain streptocyanine dyes were synthesized. Photophysical properties of these new dyes were evaluated to highlight that they all absorb around 700 nm and emit fluorescence above 720 nm. As the
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