Cis → trans and trans → cis isomerizations of styrylcoumarins in the solid state. Importance of the location of free volume in crystal lattices

Abstract

We have examined the photobehavior of a set of isomers of 2-pyranone-annulated stilbenes (6-styrylcoumarin 1, 7-styrylcoumarin 2, 4-methyl-6-styrylcoumarin 3, and 4-methyl-7-styrylcoumarin, 4) in their crystalline phases. While the cis isomers of 1-3 undergo cis → trans photoisomerizations in the solid state, cis-4 and the trans isomers of 1-3 do not; the trans isomer of 4 undergoes photo-induced intermolecular reactions. Solution-state irradiations of the trans isomers of 1-4 lead to the cis isomers quite readily, as does cis-4 lead to trans-4, which suggests that the absence of geometric isomerization of the trans isomers and the lack of reactivity of cis-4 in the solid state are due to molecular packing effects. X-Ray crystal structural analyses of 1-4 reveal interesting conformational preferences for the styrenic moieties and differences in the total 'free' volumes within the lattices, but neither factor explains satisfactorily why some of the molecules undergo geometric isomerizations in their single crystals and others do not. Using the PLATON program, we have located the sizes and positions of 'void volumes' within the crystal lattices, and identified trajectories necessary for atomic motions to lead to geometric isomerizations to understand the reactivities of 1-4. The voids in the reactive cis isomers of 1-3 crystals are located along the trajectories needed for geometric isomerization. The relevant voids in the crystals of cis-4 and the trans isomers of 1 and 2 (the non-isomerizing molecules for which suitable crystals could be grown for X-ray analyses) are located along a trajectory that does not permit isomerization. We hypothesize that the classical momentum gained from the initial motions that are facilitated due to the voids in the crystals of the cis isomers of 1-3, as well as the heat dissipated to the local environment by internal conversions and vibronic cascade of the Franck-Condon states, helps to drive the system over potential energy barriers that would not be possible otherwise. Cis-4 and the trans isomers of 1 and 2, as well as other examples from the literature in which geometric isomerizations do or do not occur in the solid state, also follow the predictions based upon the PLATON analyses. On these bases, it is suggested that the methodology described may be generally applicable for predicting when geometric isomerizations (and possibly other reactive processes) in crystalline materials will occur.