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