Crystal engineering of molecular materials: design, synthesis and solid-state investigation of photoreactive and thermo/photochromic solids.

Abstract

This project was focused on the design, by means of crystal engineering, of organic molecular solids and on their interactions with light. My research project can be divided into two main themes: i)[2+2] and [4+4] photoreactions in the solid state; ii)Thermo- and photochromic phenomena in salicylidene aniline crystals. A knowledge of how molecules are located in the crystal packing is essential to understand the structure-properties relationships. My investigation was focused on how is possible to tune some properties in crystalline materials by making changes in the crystal packing. I then looked at the relationship between the crystal packing and the effect of the stimulation by light. In both points (i) and (ii) a suitable alignment of molecules in the crystal packing was required. In order to favour photoreactions in the solid-state, it was essential for adjacent double bonds to be parallel and with a distance of maximum 4.2A. In order for thermo- and photochromism in crystals of salicylidene anilines to occur, the main conditions required were “close” and “open” packings, respectively. Crystal packing modifications have been achieved by the use of co-crystallization, salts formation and formation of coordination compounds, both in solution and via mechanochemical methods. Different crystallization methods were used, e.g. liquid diffusion, solvent evaporation, vapour diffusion, crystallization from melt, crystal grow from gel. In some cases, no single crystals have been obtained, thus some compounds have been structurally characterized using powder diffraction data. The solid-state characterization of all the compounds was performed by means of the following techniques: Single Crystal X-Ray Diffraction (SCXRD); Powder X-Ray Diffraction (PXRD); Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC); Infrared (FT-IR and ATR-FTIR) spectroscopy; Solid state absorption and emission spectroscopy (at ISOF-CNR); Scanning Electron Microscopy (SEM) and Raman spectroscopy (in collaboration with UniBO), and NMR spectroscopy (in collaboration with UniTO).