Observations on the Photochemical Behavior of Coumarins and Related Systems in the Crystalline State
I. INTRODUCTION The occurrence of photoreactivity in crystals was recognized toward the end of the last century. However, vigorous research activity following the first report began only about 30 years ago. The rotational and transitional molecules being arrested in the solid state as against the situations in the gaseous and liquid states of mat ter are responsible for the regiospecific and stereospecific products obtained from crystals. The generation of photopolymorphs [1,2] and chiral compounds from crystals containing achiral molecules [3-5] has been achieved via photoirradiation of crystal under certain conditions of molecular organization. Progress in the area of solid-state photochemistry depends on the developments in experimental tech niques such as automated x-ray diffraction and many very efficient structure-solv ing packages using the intensity data. This chapter is not a history of solid-state organic photochemistry, as there is a large number of excellent reviews on this subject available [6 - 8 ] that the reader would find profitable to read. We discuss here only some recent results obtained during the course of the studies on the pho todimerization of coumarins and resulted systems. Also, we touch upon topics such as the effect of temperature and wavelength on the progress of reactions in the crystalline state.