Two-dimensional molecular porous networks constructed by surface assembling

In this review, we present an up-to-date account of various two-dimensional molecular porous networks that possess periodically arranged voids of different symmetries. These Porous networks may be used as secondary templates to host guest ensembles. The size of the periodic voids and inter-void distance can be well below 10 nm that is hard to achieve with any other known techniques employed in nanoscience and nanotechnology. Moreover, the assembled voids may have various functional groups that can be chemically and biologically sensitive to specific ensembles. This makes the porous networks useful in preparing chemical and biological sensors. As the void size may be stepwise tuned at the scale that is comparable with the Fermi length of electrons, the confined space within the voids could serve as a spatial cavity for new chemical reactions. All these potential applications have driven people to develop various novel molecular porous networks. The tough challenges in all these efforts are associated with the controllability, predictability, stability and applicability of these porous networks. Numerous studies have led to the conclusion that the most efficient approach to fabricating these molecular porous networks is probably surface assembling of various functional organic molecules that form either uni-molecular or bimolecular systems. The assembling process can be carried out either under ultrahigh vacuum conditions or in liquid environments. The final assembling structures are actually balanced products of various interactions among the assembling molecules. substrates and/or solvents. In order to control and tune the porous network structures. one has to finely tweak these interactions. Among all the interactions, the inter-molecule interaction might be the most important. During surface assembling, non-covalent weak interactions are normally made use of to hold the molecules together. These weak interactions are typically fragile to external energy perturbations, which makes it extremely difficult to control and predict the assembled porous networks. In this context, we focus on the van der Waals force, dipole-dipole interaction. hydrogen bond and metal-ligand coordination and mainly describe various strategies utilized to tune these weak interactions so that specific molecular porous networks can be achieved in a controlled manner. Besides the inter-molecule interactions. we will also discuss effects of the substrates, solvents and external fields that could be used to realize the molecular porous networks. In addition to the non-covalent bonds, chemical bonds can be used to fabricate chemically stable porous networks too. After having discussed the issues described above, we will move on to introduce some applications of these porous networks. Finally, we will make summaries of the strategies and approaches so far developed to tune and control the molecular porous networks at the surfaces, and our personal perspectives on future challenges and applications of the molecular porous networks. (C) 2009 Elsevier B.V. All rights reserved.