Widely used metals, semiconductors or insulators contain well defined elements or compounds like semiconductors, Si or GaAs, where all chemical bonds between individual atoms are the same. The covalent bonds between silicon atoms in silicon crystal or covalent bonds between gallium and arsenic atoms in GaAs crystal make such single crystal similar to one very large molecule. In many substances, not all bonds are the same. In some, additionally to the covalent bonds, the weak Van der Waals bonds appear and may be nice oriented in parallel layers. These types of crystals are oft labeled as two dimensional crystals. As examples, graphene and transition metal dichalcogenides can be named. Specific 2D properties and new defects emerge and specify behavior of low dimensional structures. If the weak bonds are oriented in two non-parallel layers, the strong covalent bonds prevail only in one direction and one-dimensional crystals are formed. Needles made from NbSe₃ may be an example of such 1D crystal. However, the largest group of crystals, with weak bonds, is this where elements or molecules are weak bonded like in crystals made from noble gases, or crystals made from molecules. In the last the group, atoms forming molecule are bond by strong intra-molecular covalent bonds and molecules with each other are bonded by weak Van der Waals bonds. Millions of organic molecular crystals show such behavior. If the molecules in molecular crystals are not all the same, assume two different types of molecules, one type may behave as a charge acceptor or the other as a charge donator than new type of compounds, resembling binary compounds, may be formed. One of such group is classified as organic binary compounds or charge transfer salts. Whereas inorganic binary compounds are well represented in binary phase diagrams, binary phase diagrams composed of organic weak bonded compounds are practically non-existing, in spite that individual binary compounds are well defined and intensively studied. For example, Tetrathiafulvalene and 7,7,8,8-Tetracyanoquinodimethane (TTF-TCNQ) in the relation 1:1 are famous for excellent electrical conductivity reaching those of metals. Β’-(BEDT-TTF)-ICl2 in stoichiometry 1:1:2 is an organic superconductor with critical temperature as high as 14.2 K. One may ask, what are the properties of other compounds with stoichiometry 1:2, 2:3, 3:2 or do such compounds exist? Because the phase diagrams are not known this question cannot be simple answered and the computer simulations deliver only vague indications. Also the questions how to crystallize binary weak bonded compounds when even the existence of compounds is guessed is ambiguous. A weak bonded binary system, where phase diagram is not known but two binary compounds are already detected, is the system perylene –TCNQ . Compounds P1T1 and P3T1 have been reported in literature. One more, P2T1 has been found by authors and reported in this talk. The phase relation, crystal growth of binary compounds between perylene and TCNQ in relation to the general problem of crystallization and structure – properties relations in weak bonded systems is discussed. Single crystal of organic binary compounds of (perylene)1-TCNQ1 (P1T1), (perylene)2-TCNQ1 (P2T1) and (perylene)3-TCNQ1 (P3T1) were synthesized and grown in one process. Due to the weak bonds, both the solution and the physical vapor transport methods have been applied. Needle and plate-like single crystals were used for X-ray structure determination and for electrical measurement. Depends on stoichiometry, n-type, p-type and ambipolar behavior have ben measuring. This example illustrate that forming weak bonded binary or more complex compounds leads to new properties unavailable in simple monocomponent crystals.

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