SYNTHESIS, CRYSTAL-STRUCTURE, AND 2ND-ORDER NONLINEAR-OPTICAL PROPERTIES OF A NEW PHASE-MATCHABLE CYANINE DYE

A new chiral hemicarboxonium salt, (ephem)(BF4), was synthesized by ac tion of alpha-1-((methylamino)ethyl)benzyl alcohol, (-)-ephedrine, on a carboxonium tetrafluoroborate salt. The compound crystallizes in mon oclinic space group F2(1). a = 11.153(1) Angstrom, b = 8.187(1) Angstr om, c = 16.104(1) Angstrom, beta = 91.26(1)degrees, Z = 2. The molecul ar origin of the nonlinearity was calculated using the INDO/SCI-SOS ap proach. The static molecular hyperpolarizability (beta(0)) is equal to 10.7 x 10(-30) cm(5) esu(-1). The compound is phase matchable and exh ibits an efficiency around 20 times that of urea at 1.097 mu m. The or bital description of the electronic transitions reveals that the carbo n skeleton is mainly responsible for the NLO response, The possibility of growing large single crystals (>0.5 mm(3)) associated with a good transparency (lambda(max) = 420 nm) suggests that pentamethinium salts may be an interesting family of materials for second-order optical no nlinearities.