Quadratic Nonlinear Optical Properties of N-Aryl Stilbazolium Dyes

摘要

The new salts trans-4-p-(dimethylamino)-N-R-4-pyridinium chloride (R = methyl (R1), 4-methoxyphenyl (R2), 3-methylphenyl (R3), phenyl (R4), 2,4-dinitrophenyl (R5)), with same conjugated systerm but different substitute, have been synthesied based on "push-core-pull-core-push/pull" structure. Fluorescence-free first hyperpolarizability P values of compounds Rl to R5 were measured by using hyper-Rayleigh scattering (HRS) technique in methanol solutions with an Nd:YAG nanosecond laser. Here fluorescence spectrometer (Edinburgh Instruments ltd, model FLS920) was used to eliminate background of two-photon fluorescence. The embedding phenyl into N-alkyl stilbazolium salts makes the planarity broken. So the two-photon fluorescence of compounds R2 to R5 decreases intensively compares with compound Rl's. The HRS results indicate that the N-aryl chromophores in compounds R3 to R5 has considerably larger (3 values than their N-methyl counterpart in compound R1. On one hand, the broken of planarity reduces their two-photon fluorescence and perhaps increases their β_0 On the other hand, compound R5 with increasing electronegativity substitutes makes its β_0 increases due to the greater electron-withdrawing abilities of N-aryl- with respect to N-methyl pyridinium groups. An analogous molecular engineering strategy should also enhance the quadratic NLO activities of purely organic pyridinium dyes such as stilbazolium salts.