Structural investigations of photochromic and thermochromic compounds in the solid state and some aspects of crystal engineering

Abstract

The thesis entitled “Structural Investigations of Photochromic and Thermochromic Compounds in the Solid State and Some Aspects of Crystal Engineering” comprises two parts. Photochromism occurring in three different classes of compounds-namely alkylisophthalaldehydes, fulgides, and cinnamaldehyde semicarbazones-in the solid state, as well as thermochromism exhibited by the crystals of the methyl iodide adduct of benzothiadiazole, are described in Part I of the thesis. Part II deals with studies in crystal engineering where the steering ability of fluorine is examined on two different organic systems: one planar (e.g., coumarin) and the other non planar (e.g., benzylidene DL piperitone). Chapter 1 presents a brief review of the phenomena of photochromism and thermochromism. Beginning with a brief review of the historical development of these areas, various classes of compounds exhibiting these properties are described. Representative examples are provided to highlight different mechanisms responsible for these phenomena. Potential applications of these compounds in a wide range of areas are also discussed briefly. The structure-reactivity correlation of photochromism of alkylisophthalaldehydes (Scheme 1) in the solid state is discussed in Chapter 2. The crystal and molecular structures of the colorless forms of the photochromic compounds 2,5 dimethylisophthalaldehyde (1) and 5 isopropyl 2 methylisophthalaldehyde (2) were determined by single crystal X ray diffraction studies. The intramolecular hydrogen abstraction process involved in the photoenolization of these compounds in the solid state is rationalized in light of relevant geometrical parameters. By combining these results with those reported in the literature on the nanosecond flash photolysis study of 2 methylacetophenone and related compounds, a complete reaction sequence (Scheme 2) of the reversible photoenolization reaction is proposed. Fulgides-the derivatives of dimethylenesuccinic anhydride-are among the well known and widely investigated photochromic compounds. A study of the photochromism (Scheme 3) of a thermally stable fulgide, (E)-2 isopropylidene 3 ( naphthylmethylene)succinic anhydride (3), in the crystalline state is described in Chapter 3. The crystal and molecular structures of the yellow form of compound 3 were determined by single crystal X ray analysis. The photochemical electrocyclic ring closure process is discussed in terms of relevant geometrical parameters of the molecule in the crystal. These analyses, along with lattice energy calculations and X ray powder diffraction studies of the yellow and red forms of 3, indicate that the electrocyclic reaction responsible for the photochromism of 3 may occur only at defect sites or crystal surfaces. Cinnamaldehyde semicarbazone (4) is known to exhibit photochromism in the solid state (Scheme 4). Detailed spectral and structural investigations of the three forms were undertaken to gain insight into the mechanism of photochromism of 4, and these studies are presented in Chapter 4. From the single crystal X ray structural investigation of the colorless form A and the yellow form B, as well as from supporting evidence based on solid state UV-Vis, IR, and NMR spectroscopy, it is concluded that photochromism in crystals of 4 does not occur in the bulk but is confined to the surface. A probable mechanism is shown in Scheme 5. Studies on the thermochromism of the methyl iodide adduct of benzothiadiazole (5) in the solid state are discussed in Chapter 5. The crystals of 5, orange at room temperature, turn red upon heating to approximately 100°C, and the process is reversible. A mechanism (Scheme 6) proposed in the literature suggested the involvement of sulfonium species. However, single crystal X ray analysis of the orange form of 5 revealed that it is not a sulfonium salt, as previously reported, but is in fact a quaternary ammonium salt. Based on crystal structure analysis, variable temperature powder diffraction, and spectral evidence, it is proposed that the thermochromic behavior involves minor structural changes upon heating. The reversible transformation (Scheme 7) involves interconversion between quaternary ammonium and sulfonium forms depending on the extent of interaction of iodide with the nitrogen and sulfur atoms. Studies in Crystal Engineering, which form Part II of the thesis, comprise two chapters. Chapter 6 begins with a review of various aspects of solid state photochemistry. The use of steering groups for crystal engineering is described. The steering capability of fluorine is demonstrated for the first time. In contrast to the packing and photobehavior of unsubstituted coumarin, the molecules of both 6 fluorocoumarin (6) and 7 fluorocoumarin (7) adopt a packing mode, enabling them to undergo [2+2] photodimerization in the solid state, stereospecifically forming a mirror symmetric dimer (Scheme 8). Having established that fluorine can be an efficient packing promoter in the case of planar coumarin molecules, its ability was next examined in a non planar system. Chapter 7 concerns the crystal packing and solid state photobehavior of p fluorobenzylidene DL piperitone (8). In this case, packing is not achieved, and molecular topology appears to override the steering ability of fluorine, resulting in packing. The formation of a centrosymmetric product (Scheme 9) upon irradiation of crystals of 8 is rationalized based on the observed packing. In the crystals of 6, 7, and 8, C-H···F interactions have been identified.