A non-topotactical thermal rearrangement without change of the macroscopic crystal shape

The solid-state thermal transformation of rac-3.8-di(tert-butyl)- 1,5,6,10-tetraphenyldeca-3,4,6,7-tetraene-1,9-diyne (1) into 4,5,9,10-tetraphenyltricyclo[6.2.0.0(2.6)]deca-1,3,5,7,9-pentaene (3) was investigated by x-ray crystallography and atomic force microscopy (AFM) on two different faces. The enantiopure crystals of I that were picked from the conglomerate were monoclinic in the chiral space group C2 (No. 5) and were refined to an R factor of 0,0375. The monolayers stack in a straight manner without displacement leaving (001) cleavage planes. The layered structure is seen in AFM traces at certain stages of the reaction. The overall shape of single crystals of 1 does not change even though they turn deeply blue-green at 140-150 degreesC. However, the initial roughness in the micrometer region is flattened out or thoroughly rearranged by long-range molecular movements giving phase rebuilding and phase transformation without detachment of the product from the parent crystal. Thus. the chemical reaction is limited to the surface region and to internal cracks of the crystal. A topotactical transformation is experimentally excluded and cannot be modelled at the given crystal structure. The chemical yield depends on the crystal size and is significant. The thermochrome 3 reverts to I but only in the outer surface region at anaerobic storage, whereas the known solid-state oxidations occur in air. The topographic differences of these processes were traced with AFM. The release of strain in the surface region is evidenced by anisotropic crack formation that initially follows the crystal packing of 1 and could be imaged and compared with an ordinary microscope after the thermal reversion or oxidation. Copyright (C) 2001 John Wiley & Sons, Ltd.