Molecular self-assembly is currently utilized by many groups in efforts towards the nanofabrication of materials, molecular photonics, and crystal engineering. Self-assembly involves the spontaneous aggregation of small molecules or salts into specific structures using a variety of intermolecular interactions and shape-complementarity. In doing so, this process accomplishes the most difficult steps in ordering molecular components in 1- 2- and 3-dimensions to form supramolecular structures with designed characteristics. However if self-assembly is to provide a means to create useful thin films, polymers, and crystals where the photonic molecules are in precise, predictable geometries, then the self-assembled structures must be stable enough to manipulate, such as deposit on surfaces.
Porphyrins may be self-assembled by: (i) hydrogen-bond molecular recognition groups grafted directly onto the porphyrin macrocycle, (ii) coordination of exocyclic ligands to appropriate metal ions, (iii) axial ligation of the metalloporphyrin by multidentate ligands, (iv) disulfide bond formation, and (v) hybrids of the above. We [1-3] have demonstrated that simple supramolecular arrays of porphyrins may be formed by all of these separate interactions. We present data showing that large arrays, tapes and stacks of porphyrins with precise molecular orientations may be made by several of the above means, and that some of them are robust enough to deposit on surfaces.
We have recently pushed the methodology into the next level of complexity by the design and formation of a discrete supramolecular array of nine porphyrins. Self-assembly is accomplished by coordination of exocyclic pyridyl groups on three different porphyrin derivatives to 12 palladium(II) dichlorides 4 different types of molecules self-assembling to a 21 member, 25 nm2 array. The next step is to address the following questions.
How do pre self-assembled supramolecular species deposit on surfaces?
Once there, do they have the same structure and orientation?
Do they have the same function?
Can they be further manipulated?
Extensive experiments with the kinetics and thermodynamics of the formation of the nonamers and their nano crystals in solution, and comparing this data to the observed structures on various surfaces, allows for the predictable and controllable deposition of nano scaled structures on these surfaces. The time the solution is deposited on the surface after the nonamer is formed largely dictates the size of the crystal; however, the relationship is complex, and dynamic. The mechanism for this behavior probably lies in the fact that there are two equilibria and several thermodynamic minima in the formation of the nano crystals. The equilibrium between the component parts of the nonamer, and the equilibrium between the nonamer and the nano scaled crystals is obvious. But there seems to be several local minima in the energy surface for the formation of the crystalline material whereby large globular structures form first, and these in turn aggregate, then the 5x5x5 crystalline material forms.
Acknowledgement. N.S.F CAREER 9732950, PSC-CUNY-30, CUNY Collaborative Award.
Drain, C.M.; Lehn; J.-M.. J. Chem. Soc., Chem. Commun.1994, 2313.