Molecular crystals transition moment directions

A particularly useful aspect of dichroic measurements is the chance to probe orientational order using more than one electronic transition in a molecule. Thus optical order parameters can be determined for different absorption bands, and if the transition moment directions are known, it is possible to determine both order parameters 5 and D. If it is assumed that there is a relationship between the uniaxial and biaxial order parameters, as given in Fig. 5 of Sec. 1 in this chapter, then it is possible to obtain both order parameters from the polarized spectra from a single absorption band [25]. This method has been applied [26] to the determination of order parameters of rigid aromatic probes, such as azulene, phenan-threne, and anthracene and related compounds. Dichroism measurements on impurity molecules in liquid crystal solvents have also been used [27, 28] to study inter-molecular interactions, and their influence on electronic absorption bands. Polarization effects of the type described above for sim-... 

The direction of the transition moment is of practical consequence in dyes used in liquid crystal display systems. It is important for such applications that the direction of the transition moment is aligned with the molecular axis of the dye. Since this is the case with azo dye 15f, the dye would appear to be a reasonable candidate as a liquid crystal display dye (see Chapter 10 for further discussion of this application of dyes). 

In contrast to polyenes the aromatic molecules exhibit not only the Ti — So absorption, but also the longlived T — So emission, which gives rise to phosphorescence phenomena of rigid solvents and crystals. This is another important field of applications of spin-orbit quadratic response theory. Such calculations refer to lifetimes, transition moments, oscillator strengths and polarization directions for the radiative decay of molecular triplet states. These quantities may either be averaged over the triplet levels or refer to specific triplet spin sublevels depending on the conditions for the relevant experimental measurements. 

The orientation of transition moments in the molecular framework can be predicted by quantum mechanical calculations (Sections 2.1.4 and 4.4). Experimentally, the direction of transition moments can be assessed by studying the absorption of linearly polarized light by samples in which the molecules are preferentially oriented. Complete orientation is available in single crystals. However, optical measurements on single crystals are demanding and rarely practicable, not least because extremely thin crystals are required for absorption studies. 

The excitonic bandstructure is constructed upon the interactions of the transla-tionally equivalent molecules (/n) and the non-translationally equivalent molecules (7i2> 7i3, /h). The observable excitonic optical transitions have to be taken at = 0 and thus the differences in the transition energies (the so called Davydov-splitting (ii)) differ due to the non-translationally equivalent interactions. Since the different components correspond to electronic states of different crystal symmetry, the polarization properties for the transitions are different as well (iii). It is worth noting that in a molecular crystal the transition moments of the individual molecules are no longer the principal directions for the optical transitions, but the crystal symmetry leads to a polarization with respect to the symmetry axes of the crystal. 

For example, the 815 cm-l peak shifted to 825 cm i and the doublet at 1730/1750 cm-l, indicative of crystal-like structures has disappeared at the highest shear rate. The spectral changes near 815 cm-1 of Fig. 15a could conceivably correspond to those of Fig. 13 which contained the spectra at different temperatures around the discotic/ isotropic liquid transition. On the other hand, no disappearance of the carbonyl structures occurred with simple phase transition. TTie disappearance of the carbonyl bands at 1730/1750 cm at the highest shear rate must therefore corresponds to a molecular realignment in which the dipole moment changes causing the infixed bands have been reoriented along the direction of observation. 

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