Gurmit S. Bahra

2,4-Bis[4-(N,N-dibutylamino)phenyl] squaraine: X-ray crystal structure of a centrosymmetric dye and the second-order non-linear optical properties of its non-centrosymmetric Langmuir–Blodgett films

Crystals of the title compound exist as a green monoclinic phase [space group P21/c with a= 9.046(1), b= 19.615(2), c= 9.055(1)A, β= 116.107(5)°, Z= 2] and a purple triclinic phase. The chromophore is both planar and centrosymmetric and its dimensions indicate a tendency towards a quinoidal structure with extensive delocalisation. The Langmuir–Blodgett (LB) films show two types of aggregation with absorption maxima at 655 nm and 540 nm. These monolayers also exhibit strong second harmonic generation (SHG) comparable to the intensity from films of hemicyanine dyes. The anomalous non-linear optical properties are attributed to a serendipitous non-centrosymmetric packing arrangement within the films and to an intermolecular charge transfer contribution to the bulk second-order susceptibility.

Crystal growth and hydration effects in morpholinium 4-hydroxybenzoate (M4HB)

The growth of a large single crystal of the new non-linear optical salt morpholinium 4-hydroxybenzoate (M4HB) is reported. Growth of crystals from dry ethanol, or 2-propanol, yields the non-centrosymmetric form belonging to space group Cc (No. 9). This is the desirable structure since it exhibits second harmonic generation (SHG). Attempts to improve solubility by using a water : ethanol co-solvent result in the total loss of SHG activity due to the formation of morpholinium 4-hydroxybenzoate dihydrate in the centrosymmetric space group P2 1 /c (No. 14).

‘Molecular zips’: evidence of the interdigitation of layers in Langmuir-Blodgett films of an optically non-linear dye and a compatible spacer

Second-harmonic generation (SHG) from alternate-layer Langmuir–Blodgett (LB) films of (E)-4-[2-(4-methoxynaphthyl)-ethenyl]-N-docosylquinolinium bromide (dye) and N-octadecyl-4-methylquinolinium bromide (spacer) increases quadratically with the number of bilayers with a susceptibility of χzzz(2)= 20 pm V–1 at 1.064 µm. The lower layers show significant variation of the second-harmonic intensity across the surface whereas the thicker films provide evidence of self-healing with a maximum variation of ± 10% across 90% of the area studied. The thickness and the real and imaginary parts of the dielectric permittivity from the surface plasmon resonance (SPR) at λ= 532 nm are l= 31.8 A, Iµr= 2.44 and Iµi= 0.23 for monolayer films of the dye and l= 42.9 A, Iµr= 2.85 and Iµi= 0.29 for the dye–spacer bilayer. The dye and spacer have compatible interlocking geometries and the diminished depth of the spacer layer, 11 A by difference, may be explained by interdigitation of the alkyl chains at the interface. The bilayer thickness of 42.9 A from SPR is in agreement with the d-spacing of 42 ±A from X-ray synchrotron diffraction.

Non-centrosymmetric alignment of optically nonlinear dyes

Abstract Langmuir-Blodgett (LB) film structures of the two-legged dye, (E)-4-[2- alkan oyloxyphenyl) ethenyl]- N -docosylpyridinium iodide, C n H 2 n +1 C(O)OC 6 H 4 CHCHC 5 H 4 N + C 22 H 45 I − , are dependent upon the alkanoyl group, there being a cross-over from a centrosymmetric Y-type arrangement to a non-centrosymmetric Z-type arrangement as n increases. The second-harmonic intensity from films comprising an even number of layers is negligible for 1 ⩽ n ⩽ 10, increases with the number of LB layers when n ⩾ 12 and is greatly reduced when n = 21. The two-legged molecules adopt a ‘stretched’ rather than a ‘U-shaped’ configuration and there is a tendency towards an antiparallel alignment within layers as the length of the alkanoyl group approaches that of the second alkyl chain. The stearoyloxy homologue ( n = 17) has a layer thickness, from ellipsometry, of 52 A and an in-plane area, from grazing incidence synchrotron diffraction, of 19.72 ± 0.04 A 2 molecule −1 .

Nonlinear absorption in carbon-60

This paper describes the investigation of nonlinear behavior in a solution of C60. A Q-switched Nd:YAG laser operating at 532 nm was used to look at the change of absorbance during the time of the pulse. The open literature has suggested that scattering may provide part of the nonlinear process found in C60 solutions and this problem is addressed in this paper. Nonlinear scattering in toluene, a carbon suspension in toluene, and C60 solution in toluene was investigated between 0 and 90 degrees. The C60 solution produced as much nonlinear scattering as the toluene on its own.

Improved Second Harmonic Generation from Langmuir-Blodgett Films of an Azacrown Ether Fullerene Derivative

The amphiphilic fullerene derivative (1-aza-18-crown-6)-hydrofullerene-60 (C 60 -1A18C6, 1) exhibits improved Langmuir and Langmuir-Blodgett (LB) film forming properties and the resulting LB films exhibit a nonlinear second order susceptibility, X (2) pp of 3.2 pm/V, the highest yet reported for a fullerene or fullerene derivative.