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 (Emax) 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(Emax) 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 Emax 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.