Quadratic nonlinear optical properties of N-aryl stilbazolium dyes

The new salts trans-4′-(dimethylamino)-N-R-4-stilbazolium hexafluorophosphate (R = methyl, Me 1, phenyl, Ph 2, 2,4-dinitrophenyl, DNPh 3, 2-pyrimidyl, Pym 4, Scheme 1) have been prepared. Their electronic absorption spectra show intense, visible intramolecular charge-transfer bands, the energy (E_max) of which decreases in the order R = Me > Ph > DNPh > Pym. This trend arises from the steadily increasing electron deficiency of the pyridinium ring, a phenomenon also observed in cyclic voltammetric and ¹H nuclear magnetic resonance (NMR) data. Fluorescence-free first hyperpolarizability β values of [1-4]PF₆ were measured by using femtosecond hyper-Rayleigh scattering (HRS) with acetonitrile solutions and a 1300 nm laser, and static first hyperpolarizabilities β₀ were obtained by application of the two-state model. The HRS results indicate that the N-aryl chromophores in [2-4]PF₆ have considerably larger β₀ values than their N-methyl counterpart in [1]PF₆, with a ca. 10-fold increase in β₀ observed in moving from [1]PF₆ to [4]PF₆ (25 → 230 × 10^-30 esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the derivation of dipole moment changes Δμ₁₂ (10.9-14.8 D), which have been used to calculate β₀ according to the two-state equation β₀ = 3Δμ₁₂(μ₁₂)²/2(E_max) ² (μ₁₂ = transition dipole moment). With the exception of [1]PF₆, the Stark-derived β₀ values are in reasonable agreement with those from HRS. However, the increase in β₀ in moving from [1]PF₆ to [4]PF₆ is only 2-fold for the Stark data (90 → 185 × 10^-30 esu). The observed trend of increasing β₀ in the order [1]PF₆ < [3]PF₆ < [2]PF₆ < [4]PF₆ arises from a combination of decreasing E_max and increasing Δμ₁₂, with only a slight increase in μ₁₂ between [1]PF₆ and [4]PF₆. It is likely that the β₀ values for [3]PF₆ are lower than expected due to the steric effect of the ortho-NO₂ group, which causes twisting of the DNPh ring out of the plane of the stilbazolium unit. A single crystal X-ray structure shows that [2]PF₆ crystallizes in the space group Cc, with head-to-tail alignment and almost parallel stacking of the pseudo-planar stilbazolium portions of the cations to form polar sheets within a polar bulk structure. [2]PF₆ is essentially isostructural with the related Schiff base salt trans-4-[(4-dimethylaminophenyl)iminomethyl]-N-phenylpyridinium hexafluoro-phosphate ([8]PF₆). Second harmonic generation (SHG) studies on [2]PF₆ and [8]PF₆ using a 1907 nm laser and sieved powdered samples (53-63 μm) afforded efficiencies of 470 and 240 times that of urea, respectively. Under the same conditions, the well-studied compound [1]p-MeC₆H₄SO₃ gave an SHG efficiency of 550 times that of urea.