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.