Structural Studies by X-Ray Diffraction

Abstract

7.1. INTRODUCTION During the past several years, the Institute of Chemical Physics in Cherno-golvoka of the Russian Academy of Science has been involved in intense research in the field of crystal chemical "engineering" of photochromic systems, studying the possibility and the ways of controlling the photochemical transformations in such systems via their crystalline and molecular structure. These investigations include a thorough study of the structures of the initial compound and the resulting photoproducts, and, if possible, of intermediates, using different physicochemical and computational methods [X-ray analysis, infrared (IR) and ultraviolet (UV) spectroscopy, and quantum-chemical methods] and elucidation of the nature of structural conversion and of the correlations between structure and photochemical properties. When photochemically active compounds are studied, the above problem is particularly complicated because the structure is being solved for the ground electronic state, while chemical transformations occur in the excited state, which may be characterized by an essentially different structure of the molecule. The nature of structural changes in the electronically excited state can be studied by quantum-chemical methods which are based on the molecular structure in the ground state. Though the problem is difficult, the recent results of crystal chemical "engineering" of photochromic systems testify to the vast potentialities of this approach. In this chapter, we will consider the structures of photochromic compounds of various classes, and we will discuss the structural factors that have been determined to affect the molecules' reactivity and ways of modifying the structure