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Keywords:

hyper-rayleigh scattering, high-frequency demodulation, transition-metal complexes, charge-transfer states, electroabsorption spectroscopy, molecular hyperpolarizability, multiphoton fluorescence, organic salts, enhancement, chromophores, Science & Technology, Physical Sciences, Technology, Chemistry, Multidisciplinary, Chemistry, Physical, Nanoscience & Nanotechnology, Materials Science, Multidisciplinary, Physics, Applied, Physics, Condensed Matter, Chemistry, Science & Technology - Other Topics, Materials Science, Physics, HYPER-RAYLEIGH SCATTERING, HIGH-FREQUENCY DEMODULATION, CHARGE-TRANSFER STATES, ELECTROABSORPTION SPECTROSCOPY, MOLECULAR HYPERPOLARIZABILITY, MULTIPHOTON FLUORESCENCE, ORGANIC SALTS, COMPLEXES, ENHANCEMENT, POLARIZABILITIES, 02 Physical Sciences, 03 Chemical Sciences, 09 Engineering, Materials, 34 Chemical sciences, 40 Engineering, 51 Physical sciences

Abstract:

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 (E-max) 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 losers yields fluorescence-free first hyperpolarizabilities beta, and static first hyperpolarizabilities beta(0)[H] are obtained via the two-state model. Dipole moment changes Deltamu(12) for the ICT transitions obtained from Stark spectroscopy afford beta(0)[S] values by using beta(0) = 3Deltamu(12)(mu(12))(2)/2 (E-max)(2) (mu(12) = transition dipole moment). The beta(0)[S] data show that the combination of pyridyl N-arylation with conjugation extension affords large increases in beta(0). The beta(0)[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 anions 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).