An improved efficiency/transparency trade-off for second-harmonic generation by extending the π-electron bridge of an optically nonlinear dye

The second-harmonic intensity from Langmuir-Blodgett films of 5-{4-[2-(4-dimethylaminophenyl)vinyl]benzylidene}-2-octadecyl-5,6,7,8-tetrahydroisoquinolinium octadecyl sulfate(1), interleaved with poly(tert-butyl methacrylate), increases quadratically with thickness to more than 100 bilayers. The susceptibility, molecular hyperpolarisability, thickness, refractive indices and chromophore tilt angle, relative to the substrate normal, are chi ((2))(zzz) = 45 +/- 5 pm V(-1) at 1064 nm, beta = 8 x 10(-38) m(4) V(-1), l = 3.65 +/- 0.12 nm bilayer(-) (1), n(omega) = 1.53 +/- 0.01, n(2 omega) = 1.56 +/- 0.01 and phi = 44 +/- 2 degrees. The moderately high susceptibility arises from an optimised transparency/efficiency trade-off, the alternate-layer LB film being transparent at 1064 nm and having a very slight absorbance of 3.5 x 10(-4) bilayer(-1) at 532 nm. In contrast, alternate-layer films of the corresponding 7-diethylamino-2-oxo-2H-chromen-3-ylmethylene (2) and 3-(dimethylaminophenyl)prop-2-enylidene (3) derivatives, also reported, have absorbances of 1.1 x 10(-3) and 1.5 x 10(-3) bilayer(-1) at 532 nm and resonantly enhanced susceptibilities of 60 and 95 pm V(-1) respectively.