Chromophore Orientation Techniques

Linear-dichroic spectra of SA monolayers prepared from mixtures of OTS and a cyanine-dye surfactant established the absence of dimerization and the orientation of the chromophore parallel to the substrate [183]. In contrast, the same cyanine dye underwent sandwich-type dimer formation in LB films and had its chromophore oriented perpendicular to the water surface [192]. These results highlight an important difference between LB and SA monolayers. Parameters which determine monolayer formation on an aqueous subphase are also responsible for the orientation and organization of the surfactants therein. Furthermore, the configuration of the surfactants is retained regardless of the structure of the substrate to which the floating monolayer was subsequently transferred to by the LB technique. Conversely, in SA monolayers, surfactant organization is primarily dependent upon the nature of the substrate [183]. 

In all techniques the chromophore orientation is frozen by cooling the poled film to the room temperature under the applied external field (Fig. 24) or by thermal or photocrosslinking during the poling procedure. In the case of thermal crosslinking it is important to tightly control the poling temperature and to in-... 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

Pure and doped crystals allow investigation of chromophore interactions and exciton motion. Polarized spectroscopy can be uniquely useful for assigning spectral features and determining orientations of intermediates or products in single crystals, but birefringence often makes the technique less versatile than might have been imagined (Section II.D.4). 

Tg = 125-140°C, and was stable in a N2 atmosphere to 220°C. The 13C NMR spectrum revealed the presence of 7t-interaction between those chromophoric groups that are in isotactic relationship to each other ( 30-35% of total). The orientation of the chromophores in a PPNA sample (Tg = 125°C) was achieved in a thin film by the corona poling technique at temperatures above Tg. The subsequent freezing process resulted in a polymeric film that exhibited an initial high second-order nonlinear coefficient, d33=31 pm/V, as measured by Maker-fringe technique with 1.06 im fundamental. 

Detailed information about the components of the second-order susceptibility y2)(-2w to, w) can be obtained from second harmonic measurements on well-defined samples such as single crystals or oriented thin films, the latter obtained by procedures such as the asymmetric Langmuir-Blodgett deposition technique or electric-field poling of NLO chromophore-doped polymers.31 In the case of single-crystal samples, the second harmonic is... 

As for Raman spectroscopy one may expect resonance Raman effect and/or surface-enhanced Raman scattering (SERS). By using these effects, the Raman spectrum of a monolayer film may be enhanced by 103-106. Resonance Raman spectroscopy is useful for exploring the electronic structure of monolayers with a chromophore and SERS technique is applied to study structure, orientation, and interactions of monolayers on a silver or gold surface. 

Since the CD of oriented molecules are extremely sensitive to the interaction of chromophores, such data could be very useful in the investigation of exciton phenomena. The technique also allows one to determine the degree of order and orientation of chromophores in a macromolecule. Thus, CD studies on oriented visual pigments, and possibly on some selected visual pigment analogs, could be very informative in clarifying the role played by induced optical activity of the chromo-phore and/or exciton coupling. 

Clearly, situations intermediate between perfect order and random distributions occur in arrays of absorbing chromophores, and a treatment is required that allows expression of the orientational distribution of structural units such as crystallites or segments which may be fluorescent in a bulk sample having uniaxial or biaxial symmetry. A complete mathematical approach using a herical harmonic expansion technique has been developed which expresses the distribution as spherical harmonics of various orders in terms of the Euler angles which specify the orientation of the coordinate system in a fixed structural unit with respect to the coordinate system in the bulk sample This is of use in solid systems, where time dependence is not observed. 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

---