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) hake 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 H-1 nuclear magnetic resonance (NMR) data, Fluorescence-free first hyperpolarizability beta 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 beta(0) were obtained by application of the two-state model. The HRS results indicate that the N-aryl chromophores in [2-4]PF6 have considerable larger beta(0) values than their N-methyl counterpart in [1]PF6, with a ca. 10-fold increase in beta(0) observed in moving from [1]PF6 to [4]PF6 (25 --> 230 x 10(-30) esu). Stark (electroabsorption) spectroscopic studies in butyronitrile glasses at 77 K allowed the deviation of dipole moment changes Deltamu(12) (10.9-14.8 D), which have been used to calculate beta(0) according to the two-state equation beta(0) = 3Deltamu(12)(mu(12))(2)/2(E-max)(2) (mu(12) = transition dipole moment). With the exception of [I]PF6. the Stark-derived beta(0) values are in reasonable agreement with those from HRS. However, the increase in beta(0) in moving from [1]PF6 to [4]PF6 is only 2-fold for the Stark data (90 --> 185 x 10(-30) esu). The observed trend of increasing 00 in the order [1]PF6 < [3]PF6 < [2]PF6 < [4]PF6 arises from a combination of decreasing E-max and increasing Delta mu(12), with only a slight increase in mu(12) between [1]PF6 and [4]PF6. It is likely that the beta(0) values for [3]PF6 are lower than expected due to the steric effect of the ortho-NO, group, which causes twisting of the DNPh ring out of the plane of the stilbozolium 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 hulk structure. [2]PF6 is essentially isostructural with the related Schiff base salt trans-4-[(4-dimethylaminopheny)iminomethyl]-N-hexafluorophosphate ([8]PF6). Second harmonic veneration (SHG) studies on [2]PF6 and [8]PF6 using a 1907 nm laser and sieved powdered samples (53-63 mu m) 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.