Synthesis, Crystal Structure, and Second-Order Nonlinear Optical Properties of a New Phase-Matchable Cyanine Dye

A new chiral hemicarboxonium salt, (ephem)(BF4),
was synthesized by action of α-1-((methylamino)ethyl)benzyl alcohol, (−)-ephedrine, on a
carboxonium tetrafluoroborate salt.
The compound crystallizes in monoclinic space group
P21. a = 11.153(1) Å,
b = 8.187(1) Å,
c = 16.104(1) Å, β = 91.26(1)°,
Z = 2. The molecular origin of the nonlinearity was
calculated
using the INDO/SCI-SOS approach. The static molecular
hyperpolarizability (β0) is equal
to 10.7 × 10-30 cm5
esu-1. The compound is phase matchable
and exhibits an efficiency
around 20 times that of urea at 1.097 μm. The orbital
description of the electronic transitions
reveals that the carbon skeleton is mainly responsible for the NLO
response. The possibility
of growing large single crystals (>0.5 mm3) associated
with a good transparency (λmax = 420
nm) suggests that pentamethinium salts may be an interesting family of
materials for second-order optical nonlinearities.