Quadratic optical nonlinearities of N-methyl and N-aryl pyridinium salts

Four series of dyes with dimethylamino electron donor groups and N-R-pyridinium (R = methyl Me, phenyl Ph, 2,4-dinitrophenyl 2,4-DNPh, or 2-pyrimidyl 2-Pym) electron acceptors are studied as their hexafluorophosphate salts. The intramolecular charge-transfer (ICT) energies (Emax) of these compounds decrease within each of the series in the order R = Me > Ph > 2,4-DNPh >2-Pym, as the electron-accepting ability of the pyridinium ring increases. Hyper-Rayleigh scattering with femtosecond 1300 or 800 nm lasers yields fluorescence-free first hyperpolarizabilities β, and static first hyperpolarizabilties β0[H] are obtained via the two-state model, Dipole moment changes Δμ12 for the ICT transitions obtained from Stark spectroscopy afford β0[S] values by using β0 = 3Δμ12(μ12)2/2(Emax) 2 (Emax)2 (μ12 transition dipole moment). The β0[S] data show that the combination of pyridyl N-arylation with conjugation extension affords large increases in β0. The β0[H] data generally agree with this conclusion, but resonance effects may explain some apparent anomalies. X-ray structural studies on various salts reveal that the use of tosylate onions is not a generally applicable approach to engineering noncentrosymmetric structures of pyridinium salts. However, trans-N-phenyl-4-(4-dimethylaminophenyl-4-buta-1,3-dienyl)pyridinium hexafluorophosphate adopts the polar space group Cc, and shows a very large powder second harmonic generation efficiency from a 1907 nm laser, which is similar to that of the well-studied material trans-4'-(dimethylamino)-N-methyl-4-stilbazolium tosylate (DAST).