Molecular dichroic ratios

The external reflection of infrared radiation can be used to characterize the thickness and orientation of adsorbates on metal surfaces. Buontempo and Rice [153-155] have recently extended this technique to molecules at dielectric surfaces, including Langmuir monolayers at the air-water interface. Analysis of the dichroic ratio, the ratio of reflectivity parallel to the plane of incidence (p-polarization) to that perpendicular to it (.r-polarization) allows evaluation of the molecular orientation in terms of a tilt angle and rotation around the backbone [153]. An example of the p-polarized reflection spectrum for stearyl alcohol is shown in Fig. IV-13. Unfortunately, quantitative analysis of the experimental measurements of the antisymmetric CH2 stretch for heneicosanol [153,155] stearly alcohol [154] and tetracosanoic [156] monolayers is made difflcult by the scatter in the IR peak heights. 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

It has been shown (Beer (72)] that under certain circumstances it is permissible to consider a polymer specimen as made up of a fraction,/, of fully oriented chains (i. e with molecular axis exactly parallel with the fiber axis) and the remainder, 1 — /, of randomly oriented chains. The necessary condition is satisfied by axially oriented polymers, regardless of the specific nature of the distribution of chains. A convenient relationship between /, the observed dichroic ratio R, and the dichroic ratio of perfectly oriented chains R0= is given as follows (Fraser (59)]. In an axially oriented specimen the extinction coefficient in a given direction for the unoriented portion is simply related to en and ea for the oriented portion. Since the total extinction coefficient for the latter is given by e - 2 F,a, the extinction coefficient for the unoriented portion in a given direction is... 

In order to evaluate quantitatively the orientation of vibrational modes from the dichroic ratio in molecular films, we assume a uniaxial distribution of transition dipole moments in respect to the surface normal, (z-axis in Figure 1). This assumption is reasonable for a crystalline-like, regularly ordered monolayer assembly. An alternative, although more complex model is to assume uniaxial symmetry of transition dipole moments about the molecular axis, which itself is tilted (and uniaxially symmetric) with respect to the z-axis. As monolayers become more liquid-like, this may become a progressively more valid model (8,9). We define < > as the angle between the transition dipole moment M and the surface normal (note that 0° electric field of the evenescent wave (2,10), in the ATR experiment are given by equations 3-5 (8). 

Molecular structure and absorption spectrum of an organic dye with a positive dichroic ratio. 

Similar lifetime issues were encountered for a large variety of dyes with an elongated molecular structure with an extended conjugated core similar to that of the azo dyes, such as the merocyanine dyes (163-165 n = 1-3), shown in Table 3.19, as well as azomethines and methine dyes. However, most of these exhibit much lower order parameters and dichroic ratios than conventional azo dyes prepared earlier. 

The tetralins (181-183) collated in Table 3.23 are red liquid crystals with a transition moment orthogonal to the molecular long axis, see Figure 3.13. Since they are liquid crystalline themselves they are very soluble in nematic host mixtures. Such mixtures exhibit high negative dichroic ratios and can be used in White and Taylor GH-LCDs with positive contrast. Furthermore, mixtures containing an additional anthraquinone dye of positive dichroic ratio can be used to switch from one colour to another under the action of an electric field. 

A system of such particles is considered in which the particle axes are distributed in the electric field according to the Boltzmann distribution, and absorbance of the polarized llj t is calculated, neglecting both polarization of the polypeptide molecule by such a low external field (0-300 V/cm) and molecular fields for such a long particle 23). The dichroic ratio of the band in questicm is given in the form ... 

The transition moment directions of some of the absorption bands of PBLG in liquid crystalline state have been investigated in a CaF2 cell by means of the infra-red dichroic ratio (56). All the transition moment directions measured at 1 °C are very similar to those observed 32) on mechanically oriented films of PBLG (Table 2), verifying that all the polymer molecules present a parallel or nearly parallel orientation in the molecular cluster and that the main chain and the side chains of the polymer molecule are fixed as in the solid film. No significant difference is observed between the systems of solution testing. The orientation of the solvent molecules (methylene moletules) is detected in an electric field in accordance with the NMR observation (56). 

When the direction of the external field is reversed, the scattering contours dis-fday a striking time dependent change accompanying the turn around of the molecular clusters. New equatorial scattering rqipears in both the Vh and the H, patterns. The intensity of the Vh transmission varies with time having two maxima (and one minimum), and these three limited intensities occur while the infra-red dichroic ratio at 3300 cm decreases and stays around its minimum value of about one in the cluster reversal process. It is true to say that when the direction of the molecular... 

The influence could be minimized by evaluating the dichroic ratios related to two spectrally neighboured bands the transition moments of which are parallel to one of the molecular transverse axes each Due to the uniaxiality of the pha.se these bands should be similarly influenced, so that reliable information on D results (Korte, 1983 Dampen, 1991). To be sure about the transition moments being. strictly parallel to one of the three molecular axes, the symmetry of the molecules should be C2, or D21, (Michl and Thulstrup, 1986), what in strict sense is hardly found with mesogenic molecules. 

The uniaxial PF2/6 alignment can be quantified in terms of the mean-field theory discussed previously. The degree of alignment in equilibrium (f2) is a function of the number-averaged molecular weight (M ) as described by Eq. 6. This prediction has been studied by photoabsorption in [55] and there it has been shown that the solid angle T2 is expressed in terms of the dichroic ratio in absorption (R) as... 

This polymer layer functioned not only as a hole injection layer but also as a molecular alignment layer. A high dichroic ratio exceeding 20 was observed in three color pixels [106]. 

Apart from the difference in dichroic ratio, the total absorption of the NH and C=0 stretching bands of the contracted film is less than that of the stretched film. This change is almost certainly due to a closer parallelism of the molecular chains with the plane of the film surface in the stretched specimen. It has been noted above that the contracted films appear to be thicker as judged from the amount of absorption at the CH bands, and this would most likely be due to chains becoming less nearly parallel with the plane of the surface. 

The steady-state dichroic ratio of liquid crystalline solutions of PBLG (Fig. 3) increases with external field strength and the a mptotic value is 4.5—4.7, regardless of the polymer concentration for completely birefringent solutions 23). It may be safe to say that all the polymer molecules are parallel or neady parallel within molecular aggregates 31). Therefore, the value of 7 for the particle is tentatively assumed... 

The development of H-bonded complexes is now considered. An influence of UV irradiation on optical properties of low-molecular-weight azobenzene-con-taining material (Fig. 2.4b) has been investigated (Aoki et ah, 2000) on the basis of such interactions. The first observation of photoinduced optical anisotropy in H-bonded complexes of azobenzene dyes and copolymers (Fig. 2.4b) has been recently demonstrated (Medvedev et ah, 2005). In this case, the induced anisotropy was stable, and the maximum dichroic ratio of 2 has been observed. A kinetics of the induction of birefringence (maximum value of ca. 0.01) in one of these complexes is shown in Fig. 2.5. An influence of H-bonding on the mesomorphic and photoorientation properties was recently demonstrated (Cui and Zhao, 2004). In this approach, the amorphous azopyridine side-chain polymer was converted into liquid crystalline polymers through self-assembly with a series of commercially available, aliphatic, and aromatic carboxylic acids (Fig. 2.4d). 

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