Crystal Engineering of Acentric Styryl Quinolinium Crystals with Strongly Hydrogen-Bonded Phenolic Anions

We
report on new acentric styryl quinolinium crystals with phenolic sulfonate
counteranions and investigate their supramolecular interactions that
affect their quadratic nonlinear optical properties. The phenolic
group acting as an electron-donor as well as hydrogen-bond donor site
is located at one end of the anion, while the sulfonate group acting
as an electron-acceptor as well as hydrogen-bond acceptor site is
located at the opposite end of the anion. New styryl quinolinium crystals
with 4-hydroxybenzenesulfonate and 6-hydroxynaphthalene-2-sulfonate
counteranions exhibit a large macroscopic optical nonlinearity with
very efficient second harmonic generation (SHG) efficiency. In styryl
quinolinium 4-hydroxybenzenesulfonate crystals, the styryl quinolinium
cation chromophores exhibit an acentric ordering with a high order
parameter close to 1.0, which is optimal for electro-optic applications
or THz-wave generation. The 4-hydroxybenzenesulfonate counteranions
form strong head-to-tail hydrogen bonds, and they are also packed
in acentric layers. The direction of the polar axes in cation and
anion layers is practically identical. Therefore, the introducing
phenolic group acting as an electron-donor as well as hydrogen-bond
donor to the sulfonate counteranion is a potential technique for crystal
engineering to tailor molecular ordering as well as the physical properties
of salt-type quinolinium derivatives.