Engineering of Acentric Stilbazolium Salts with Large Second-Order Optical Nonlinearity and Enhanced Environmental Stability

A series of organic nonlinear optical (NLO) materials
based on
the stilbazolium derivatives of 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium
tosylate (DAST) were synthesized, and the single crystals were grown
from the solutions. Single-crystal structure determinations and Kurtz
powder tests reveal that replacement of the p-toluenesulfonate
anion in the DAST crystal, forming 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium
4-aminotoluene-3-sulfonate (1) and 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium p-chlorobenzenesulfonate (2), constructs new
acentric crystals with quite a large powder second-harmonic generation
(SHG) of more than 3 orders of magnitude larger than that of urea
around 2.0 μm, whereas the other two complexes show their centrosymmetric
structures without any SHG effects. A possible estimation of the molecular
first hyperpolarizabilities βijk performed on 2 further confirms its origin of its enhanced
NLO merits. Furthermore, both 1 and 2 exhibit
even higher environmental stabilities, including the antiwater performance
and the formation of anhydrous phases in water-containing solutions.
We believe that the findings of the further counterion modification
will bring new stilbazolium derivatives with improved optical nonlinearities
and the physicochemical performances.