Structural modification of functionalized carbazoles: from Z' = 1 to Z' = 6.

Crystal structures with asymmetric units comprising large numbers of molecules (Z' > 4) remain rare. Their study provides a unique opportunity to further the understanding of the physical basis for crystal formation, supporting the advancement of crystal structure prediction, crystal engineering and the study of polymorphism. Here the crystal structures of a series of three related 2,3,9,9a-tetrahydro-1H-carbazoles are discussed, one of which exhibits an asymmetric unit comprising six crystallographically independent molecules (Z' = 6). Careful analysis of this structure reveals approximate Fdd2 symmetry and strong evidence for this being linked to a symmetry-reducing, low-temperature phase transition. Comparison of the structures reveals that the variation in Z' values is linked to conformational changes related to the lack of classical hydrogen bonding in the high-Z' structure.