Photochromic molecular structure influence

During the past decade, theoretical calculations of hyperpolarizabilities have been performed to help synthetic chemists design optimum NLO structures. Although extremely accurate calculations are still out of reach, it is now possible to predict the influence of structural changes on the NLO coefficients. In the case of photochromes, theoretical calculations may be useful for predicting 3 values of thermally unstable colored forms. The theoretical methods generally employed to calculate molecular hyperpolarizabilities are of two types those in which the electric field is explicitly included in the Haniiltonian, frequently labeled as Finite Field (FF) and those which use standard time dependent perturbation theory, labeled Sum Over State (SOS) method. 

Liquid-crystalline phases with two-dimensional order (Table 1) influence photochromic processes because their molecular orientation is reversibly altered by photochromic reactions owing to the marked structural changes of the photoactive molecules.3 Intensive interest has been focused on the photoinduced alteration of mesophases admixed with photochromic compounds, since photochromism can be transformed to the reversible modification of the optical anisotropy of liquid-crystalline states. This phenomenon is applicable to optical storage of information as nondestructive readout can be performed with light of wavelengths far from those causing the photochromic reactions. 

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