No abstract

OQX(Z)-a-cyano-~-(N-hexadecylquinolin-4-ylium)-4-styryldicyanomethanide, C16H3,-Q3CNQ, has two Langmuir-Blodgett (LB) phases that differ in thickness, tilt angle and second-order susceptibility. For phase I, /=2.2 nm layer-', 4 = 8 " and ~( ~) = 1 8 0 pm V-' at 1.064 pm, whereas for phase II, /=1.6 nm layer-', #=24" and ~( ~) = 1 0 0 pm V-'. The susceptibility of phase I is the highest so far reported for an LB multilayer; the structure is non-centrosymmetric (Ztype) and, as predicted by theory, the second-harmonic intensity varies as /2w(N)=/2w(,)N2 as N increases from 1 to 200 layers. In contrast, C10H2,-Q3CNQ has a low susceptibility (x,z(~)= 6 pm V-') and neutron reflectivity studies have indicated an antiparallel alignment of chromophores at the air/water interface.

The linear and nonlinear optical properties of 2,4-bis[4-(N-methyl-N-alkylamino)-2,6-dihydroxyphenyl]squaraine (I), 2,4-bis[4-(N,N-dialkylamino)-2-hydroxyphenyl]squaraine (II), and 2,4-bis(8-hydroxy-9julolidinyl)squaraine (III) are reported. Electrospray ionization mass spectrometry (ESI-MS) has provided evidence of association in dilute solution (ca. 2 × 10 -5 mol dm -3 ) with m/z values which may be assigned to dimeric aggregates for dye II. Solid solutions of this dye in poly(vinyl acetate) show second-harmonic generation (SHG), albeit very weak, and therefore indicate that the dimeric aggregate is noncentrosymmetric. In contrast, dyes I and III form heptameric aggregates with their solid solutions being SHG-inactive. There is some correlation between the type of aggregate from ESI-MS and the Langmuir film spectra, but van der Waals interactions between the hydrophobic groups play a more significant role in determining the structure at the air-water interface. The absorption maxima of the Langmuir and deposited Langmuir-Blodgett films vary from 530 nm to the near-infrared, but only those films with a maximum at 650-700 nm, or a significant shoulder in this range, are SHG-active. The second-order behavior is not an inherent property of the molecule itself but, instead, is a result of intermolecular effects and is dependent upon the noncentrosymmetry of the aggregate and the film.

Langmuir-Blodgett ( LB) multilayers of (E)-N-alkyl-4-[ 24 4-docosyloxyphenyl)ethenyl] pyridinium bromide, C,H2,+ -Py+ -CH=CH-C6H,-O-C22H,,Br-(dye I), are centrosymmetric (Y-type) for n 6 6 and non-centrosymmetric (Z-type) for 8 6 n < 20. The areas obtained from grazing incidence synchrotron X-ray diffraction indicate that the molecules adopt a 'stretched' rather than a 'U-shaped' configuration and, unlike conventional amphiphilic materials, the long chain homologues do not invert during deposition. The second-harmonic intensity from films of the higher homologues (n 2 10) increases quadratically with the number of LB layers (120(N) = 120(l)N2) whereas for the lower homologues (n < 6) the intensity is negligible for even numbers of layers. The nonlinear optical properties of the sulfur (dye 11), selenium (dye 111) and quinolinium (dye IV) analogues are also reported and, in each case, quadratic enhancement of the second-harmonic intensity has been observed for suitable combinations of alkyl chain lengths. The second-order susceptibility, x(2)zzz, of a 100 layer LB film of (E)-N-dodecyl-4-[ 2-(4docosyloxyphenyl)ethenyl] quinolinium bromide (dye IV) is 20 pm V-' .

Second harmonic generation (SHG) from Langmuir-Blodgett (LB) films of the iodide (I) and octadecylsulfate (11) salts of E-Noctadecyl-4-[ 2-(4-dibutylaminophenyl)ethenyl] quinolinium and from films of a related pyridinium dye (111) increases with the number of layers. The LB film structures are non-centrosymmetric (Z-type) and have high second-order susceptibilities, x(')zzz, of 120 pm V-' for dye 1(20 layers), 80 pm V-' for dye I1 (100 layers) and 30 pm V-' for dye I11 (160 layers) at Am= 1.064 pm. The values are resonantly enhanced but for dye I11 the absorbance is very weak at the harmonic wavelength. The real and imaginary parts of the dielectric permittivity of dye I, derived from the surface plasmon resonance (SPR), are 3.14 f 0.06 and 0.66 f 0.03, respectively, at 532 nm and, with the exception of the monolayer, the thickness is 3.0 f 0.3 nm layer-'. Waveguiding overlays of dye I, evanescently coupled to side-polished optical fibres, have resulted in useful in-line wavelength-selective elements. The LB technique allows the necessary control of film thickness for such devices.Langmuir-Blodgett (LB) films have been extensively studied for second harmonic generation (SHG) in thr~ugh-planel-~ and guided-wave configuration^.'-^ Pitt and Walpita' first Paper 5/05257B;