The syntheses, structures and nonlinear optical and related properties of salts with julolidinyl electron donor groups

Twelve, dipolar cations with 2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl (julolidinyl, Jd) electron donor groups and N-methyl/arylpyridinium, N-methylquinolinium or N-methylbenzothiazolium acceptor groups were synthesised. The chromophores were characterised as their PF6− salts using various techniques including electronic absorption spectroscopy and cyclic voltammetry; single crystal X-ray structures were determined for a large number of salts, all of which form nonpolar materials. A broad correlation between the electron acceptor strength and the degree of bond length alternation is evident. A key focus of this work is detailed comparisons of the Jd-containing chromophores with their previously studied 4-(dimethylamino)phenyl (Dap) analogues. Molecular, quadratic, nonlinear optical responses were determined using femtosecond hyper-Rayleigh scattering at both 1300 and 800 nm, as well as using Stark (electroabsorption) spectroscopic studies of intense, visible π → π∗ intramolecular charge-transfer bands. Substantial red shifts in the latter transitions are observed on replacing Dap with Jd which indicate that the latter acts as a more effective π-electron donor. With few exceptions, both hyper-Rayleigh scattering and Stark data generally show that using Jd instead of Dap increases the static first hyperpolarizability β0, in accord with expectations based on the two-state model. β0 values as high as 6 times larger than that of the chromophore in the technologically important material E-4′-(dimethylamino)-N-methyl-4-stilbazolium tosylate are observed.