Nanostructured materials: basic concepts and microstructure☆

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

The Halogen Bond

Gold nanoparticles in delivery applications

Stimuli-reponsive polymers and their bioconjugates

The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

The role of hydrogen bonding in the formation or stabilization of liquid crystalline phases has only recently been appreciated. Following the first, wellestablished examples of liquid crystal formation from the dimerization of aromatic carboxylic acids, through hydrogen bonding, several classes of compounds have recently been synthesized, the liquid crystalline behavior of which is also dependent on intermolecular hydrogen bonds between similar or dissimilar molecules. In this review the main classes of compounds exhibiting liquid crystallinity due to hydrogen bonding are presented to show the diversity of organic compounds that can be used as building elements in liquid crystals. The molecules are either of the rigid-rod anisotropic or amphiphilic types such as molecules appropriately functionalized with pyridyl and carboxyl groups, whose interaction leads to the formation of liquid crystals; amphiphilic carbohydrates and amphiphilic and bolaamphiphilic compounds with multiple hydroxyl groups whose dimerization or association is indispensable for the formation of liquid crystals; and certain amphiphilic carboxylic acids with monomeric or polymeric mesogens and amphiphilic-type compounds bearing different moieties, whose interaction may lead to the formation of mesomorphic compounds. Associated with the macroscopic display of liquid crystalline phases is the supramolecular structure, and therefore rather extended discussion of these structures are included in this review.

Thermotropic liquid crystals formed by intermolecular hydrogen bonding interactions

The role of hydrogen-bonding interactions in the formation and/or stabilization of liquid crystalline phases has been recognized in recent years and significant work has been conducted. Following the first and well-established examples of liquid crystal formation through the dimerization of aromatic carboxylic acids, several classes of compounds have been prepared by the interaction of complementary molecules, the liquid crystalline behaviour of which is crucially dependent on the structure of the resulting supramolecular systems. In this review the main classes of liquid crystals prepared through hydrogen-bonding interactions are presented, with the aim of establishing, in the first place, the diversity of organic compounds that can be used as building elements in the process of liquid crystal formation. Rigid-rod anisotropic or amphiphilic-type molecules, appropriately functionalized with recognizable moieties, interact in the melt or in solution and lead to the formation of supramolecular complexes tha...

Supramolecular hydrogen-bonded liquid crystals

Molecular composites were prepared by solubilizing pyrene in di- aminobutane poly(propyleneimine) den- drimers having 32 or 64 primary amine end groups (DAB-32 or DAB-64). The dendrimer - pyrene binding constants were determined as Kpy/DAB-32a 16 725 200m ˇ1 and Kpy/DAB-64a 33 858 663m ˇ1 by fluorescence spec- troscopy. Fluorescence studies were also employed to probe the release of pyrene from the interior of dendrimers as a function of pH. When the pH value of the system was decreased from pH 11 by addition of HCl, the fluorescence inten- sity of the system was found to increase by approximately tenfold at pH 2 - 4. In addition, at pH 2, the ratio of the first to the third vibrational peak of pyrene (I1/I3) increased from 0.9, the value typical for pyrene solvated in dendrimer solution, to 1.60, the value characteristic of pyrene in water. Pyrene release from the interior of dendrimers was con- firmed by fluorescence quenching when sodium iodide was added, since NaI does not affect the emission of dendrim- er-solubilized pyrene. Finally, fluores- cence quenching was used to locate the solubilization sites of pyrene within the dendrimer microcavities. These sites are close to the core of the dendrimer, near the tertiary amino groups which are also responsible for quenching the fluores- cence of the dendrimer-bound pyrene.

/pdf/poly-propyleneimine-dendrimers-as-ph-sensitive-controlled-3c893sylj9.pdf

Poly(propyleneimine) Dendrimers as pH‐Sensitive Controlled‐Release Systems

Quaternized poly(propylene imine) dendrimers have been prepared and investigated for their ability to function as pH-responsive controlled-release systems. These functionalized dendrimers are able to solubilize pyrene due to charge transfer complexation between the probe and the tertiary amino groups of the dendrimeric branches. The introduction of the quaternary ammonium groups at the external surface of the dendrimers affects the protonation titration profile as compared to the parent compounds leading to the release of the entrapped pyrene within a narrow pH region and basically when the intermediate nitrogens are protonated. These properties render these materials promising candidates for pH-responsive controlled-release systems possibly including prospected drug delivery applications.

Quaternized Poly(propylene imine) Dendrimers as Novel pH-Sensitive Controlled-Release Systems

This review discusses the development of functional and multifunctional dendrimeric and hyperbranched polymers, collectively called dendritic polymers, with the objective of being applied as drug and gene delivery systems. In particular, using as starting materials known and well-characterized basic dendritic polymers, the review deals with the type of structural modifications to which these dendritic polymers were subjected for the development of drug carriers with low toxicity, high encapsulating capacity, a specificity for certain biological cells, and the ability to be transported through their membranes. Proceeding from functional to multifunctional dendritic polymers, one is able to prepare products that fulfill one or more of these requirements, which an effective drug carrier should exhibit. A common feature of the dendritic polymers is the exhibition of polyvalent interactions, while for multifunctional derivatives, a number of targeting ligands determine specificity, another type of group secures stability in biological milieu and prolonged circulation, while others facilitate their transport through cell membranes. Furthermore, dendritic polymers employed for gene delivery should be or become cationic in the biological environment for the formation of complexes with the negatively charged genetic material.

Constantinos M. Paleos

Papers

Thermotropic liquid crystals formed by intermolecular hydrogen bonding interactions

Supramolecular hydrogen-bonded liquid crystals

Poly(propyleneimine) Dendrimers as pH‐Sensitive Controlled‐Release Systems

Quaternized Poly(propylene imine) Dendrimers as Novel pH-Sensitive Controlled-Release Systems

Molecular Engineering of Dendritic Polymers and Their Application as Drug and Gene Delivery Systems