Space‐demanding intramolecular isomerizations in the solid state

The thermal isomerizations of meso‐ and rac‐3,4‐dibromo‐1,6‐diphenyl‐1,6‐bis(p‐tolyl)‐1,2,4,5‐hexatetraene (1) to give stereospecifically the 3,4‐bis(phenyl‐p‐tolylmethylene)‐1,2‐dibromocyclobutenes 3 and 5 + 6 were studied in the solid state using atomic force microscopy (AFM) and interpreted on the basis of known crystal structural data. These isomerizations run to completion in the bulk and include highly space‐demanding internal rotations around the central bond. Far‐reaching anisotropic molecular movements are detected on the major faces that align the surface features along cleavage planes in the initial phase rebuilding stage. Only one of three identified cleavage planes of meso‐1 is successful, owing to closer interactions of the bromine substituents in the non‐used cleavage planes. Thus, very fine details can be correlated and predicted for the occurrence of internal rotations and molecular movements in the crystal lattice. The second stage in these intramolecular isomerizations, the phase transformation, produces very high features up to 100 nm and still parallel to the preferred cleavage plane of meso‐1 but in the µm range without relation to the initial crystal structure in the case of rac‐1.