The self-assembly of a series of tetrathiafulvalene (TTF) derivatives at the interface between non-volatile organic solutions and the graphite surface has been studied by scanning tunnelling microscopy (STM). The TTFs have been prepared such that they bear none, one, two (in different constitutions) or four alkyl chains of different lengths and different functional groups. The STM images reveal that the packing of the TTF cores can effectively be controlled by changing the substitution pattern on the heterocycle. Several structures are seen at the interphase - parquet-type packing, single and double core tapes, and even isolated molecules - all of which have the TTF core essentially coplanar with the surface. Molecular modelling has shown that several orientations of the molecules are practically equal in energy on the graphite, which explains the polymorphous packing of some of the molecules. Solvent effects also play a role in determining the 2D structures.