Intermolecular interaction nonlinear optical properties

Nonlinear optical (NLO) properties are usually considered to depend on the intrinsic features of the molecule and on the arrangement of a material. An intermediate level of complexity should also be taken into account, that of the formation of well-defined supermolecules, resulting from the association of two or more complementary components held together by a specific array of intermolecular interactions (1). Such intermolecular bonding may yield more or less pronounced NLO effects in a variety of supramolecular species (2). Thus, three levels of nonlinear optical properties may be distinguished the molecule, the supermolecule and the material. The molecular and supramolecular levels involve respectively - intramolecular effects and structures, -... 

In this paper, we make use of molecular modelling techniques, particularly the AMI semiempirical molecular orbital method, to study the intermolecular interactions that are important for determining the manner in which crystal formation takes place. We are particularly inter ested in compounds that can potentially exhibit nonlinear optical properties. The calculational techniques are directed towards providing insight into the manner in which the desired nonlinear optical properties can be op timized in the macromolecular crystal state.(1)... 

Among all the experimental investigations performed in anthraquinone-doped nematic liquid crystals, two results are worth being noted here as they highlight some important features of the microscopic intermolecular interactions involved in this phenomenon. In the first experiment, we have found that slight changes in the molecular structure of relatively simple anthraquinone dyes may lead to dramatic modifications of the macroscopic nonlinear optical properties of the liquid-crystalline host [22]. This is shown in Fig. 5.2... 

The linear polarizability, a, describes the first-order response of the dipole moment with respect to external electric fields. The polarizability of a solute can be related to the dielectric constant of the solution through Debye s equation and molar refractivity through the Clausius-Mosotti equation [1], Together with the dipole moment, a dominates the intermolecular forces such as the van der Waals interactions, while its variations upon vibration determine the Raman activities. Although a corresponds to the linear response of the dipole moment, it is the first quantity of interest in nonlinear optics (NLO) and particularly for the deduction of stracture-property relationships and for the design of new... 

The theory of electric polarizability is of fundamental importance to the rational approach and interpretation of large classes of phenomena [1]. In particular, these properties are of fundamental importance to intermolecular interaction studies [2], nonlinear optics [3], collision-induced spectroscopy [4], and the simulation of fluids [5,6]. They are also routinely associated with general molecular characteristics as hardness [7], softness [8], hypersoftness [9], stiffness [10], and compressibility [11]. Understandably, polarizability is also linked to reactivity [12]. Another important field is QSAR, QSPR studies, and the understanding of pharmacological activity [13, 14]. 

Several methods have been developed in order to determine the macroscopic optical properties [63], of which the simplest is the oriented gas model due to Chemla et al. [64, 65] In that method, the hnear and nonlinear susceptibilities (Eq. (8.2) are calculated from simple tensor sums of the (hyper)polarizabihties of the molecules constituting the elementary unit cell. Corrective factors can subsequently be added to account for the effects of local electric fields. The relevance of this method is ensured provided the intermolecular interactions are weak, while the macroscopic responses are strongly dependent on the values of local field factors. More sophisticated schemes take into account the intermolecular interactions. They include the supermolecule model [66-69], where an aggregate of... 

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