Electron donor−π−acceptor chromophores 5, 9, 11, 18−20, 21, 22, 27, 28a, 28c, 31, 32, 34−36, 38a−c, 41a, 41c, and 42 have been synthesised. The donor units are 1,3-dithiole and ferrocene; conjugated ethylenic, phenyl, phenylenevinylene, thienyl, bithienyl, terthienyl, or thienylenevinylene linkers act as a central π-electron relay unit, and dicyanomethylene and polynitrofluorene groups as the acceptor unit. The electronic absorption spectra display a broad low-energy intramolecular charge transfer band in the visible region (500−700 nm) the energy (hν ICT ഠ 1.7−2.5 eV) and intensity (ε ഠ 5000−50000 m −1 cm −1 ) of which depend substantially on the nature of both D and A moieties and on the structure of the linker unit. Nonlinear optical properties have been evaluated using the EFISH technique: the highest µβ (0) values are observed for 38b [(900±300)×10 −48 esu] and 42

We study 2-(p-N-hexadecyl-N-methylamino)benzylidene-1,3-indandione (1a) as a model compound for the preparation of nonlinear optically (NLO) active Langmuir-Blodgett (LB) layers. The pressure-area (π-A) isotherm of (1a) at the air-water interface is investigated. A limiting area of 0.60 ( 0.02 nm 2 per molecule is found. The alternate-layer LB deposition of compound (1a) and an inert spacer, cadmium stearate, at different surface pressures is performed. The effects of temperature and delay time between spreading of the Langmuir film and the deposition of the LB films are studied. An increase of collapse pressure from 20 to 30 mN/m is observed as the delay time increases. UV-vis spectra indicate a uniform transfer of (1a) and show compressioninduced changes as a function of the deposition conditions. A split of the absorption maximum is observed for LB films deposited at higher pressures and lower temperatures. The two bands can result from two different molecular electronic transitions, affected by the aligning influence of the surface, and the different environments in the film compared to solution. Alternatively, these two bands can be associated with (at least) two different conformers. The orientation of the transition moments is evaluated on the basis of polarized UV-vis spectra at different angles of incidence, and found to be 40-44°. Significant second harmonic (SH) generation by the LB films is observed. Analysis of the SH response gives a tilt angle of 44°, in agreement with the finding from UV absorbance.

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