While the term ‘dendritic’ implies architecture, the simple prefix ‘nano’ defines size or dimension. Based on the recent proliferation of publications and patents in each of these fields, it is appropriate to examine some of the key driving forces. It is also worthwhile to consider expectations for the materials properties, technology, and commercial space at the confluence of these two very active frontiers.
Philip W. Anderson proposed an attractive list of rewards, consequences, and possibilities for society whenever scientists are successful at “breaking through boundaries in the hierarchical complexity of matter” [Science (1972) 177, 393]. This is a rich premise. As a synthetic/physical organic chemist, this has special meaning. Historically, the introduction of new quantized, building blocks into synthesis strategies has produced major revolutions. Among the significant ‘breakthroughs’ are the following: ‘the atom hypothesis’ (Lavoisier, 1789); ‘the molecular hypothesis’ (Dalton, 1808); ‘organic chemistry’ (Wohler, 1828); ‘architectural isomerism’ (Berzelius, 1832); and ‘the macromolecular hypothesis’ (Staudinger, 1926). In each advance, new complexity yields novel materials with behaviors that cannot be understood by simple extrapolation of the building block properties. In essence, “new complexity is not only different, but always more than the linear summation of its components”.
In 1808, Dalton described his New System of Chemical Philosophy. Based on the atom modules (bricks) he envisioned, and their propensity to form bonds (electronic mortar), literally millions of small inorganic and organic structures of incalculable value have been synthesized. These small molecules bear no similarity to their bricks or mortar, exhibit different properties, and adhere to different rules. The importance of architecture, even within the same complexity level, was demonstrated by Berzelius nearly 170 years ago in the simple rearrangement of identical elemental compositions into new isomers and allotropes. Barely 80 years ago, Staudinger broke another ‘complexity barrier’ when he demonstrated his ‘macromolecular hypothesis’. This allowed the catenation (polymerization) of small, monomer building blocks into covalent structures (polymers) of nanoscale proportions, albeit with broad molecular weight distributions. The three major polymer architectures that constitute traditional plastics technology (linear, crosslinked, and branched) have vastly different properties and the ‘plastics revolution’ has given society many important enhancements in life.
The ‘dendritic state’ is a new, fourth class of polymer architecture with four subclasses: random hyperbranched polymers, dendrigrafts, dendrons, and dendrimers. The monodisperse nature of dendrons and dendrimers makes them important for nanoscientists. They are unlike traditional polymers in that critical nanoscale parameters, such as size, shape, and functionality, can be precisely controlled through their architecture, i.e. their cores, interiors, and surfaces.
The core may be thought of as the molecular information center. It determines the size, shape, directionality, and multiplicity of surface functionality. Within the interior, the branch cell amplification region is found. This defines the volume and type of containment space enclosed by the terminal groups, offering a variety of possible ‘guest-host relationships’. Finally, the surface consists of reactive or passive terminal groups. These may serve as polyvalent nano-scaffolding, upon which new generations of dendrimers can be covalently attached for further growth. Alternatively, the surface groups may function as control gates for the entry and departure of guest molecules from the interior.
The core, interior, and surface determine all the properties of dendrimers. With the exception of biological polymers, or perhaps fullerenes, no other covalent structure offers such ‘bottom-up’ control. These quantized, designable modules have been used as both nanoscale building blocks and single molecule devices.
The term ‘dendritic effect’ has become a new and pervasive expression. It is used to describe surprising and unexpected properties not found in traditional polymeric materials. Ordinary, monomeric building blocks are used in the construction of dendritic materials, so one cannot claim that these new properties were predictable by simple extrapolation of the building block properties. Could it be that we are in fact enjoying a new, contemporary ‘complexity enhancement’ event? One that coincidentally resides in the dynamic nanotechnology domain? This will only be determined by further work, additional progress, and a deeper understanding. If so, we should all take great comfort in Anderson’s predictions and look forward to exciting new dendritic/nano concepts, rules, properties, and products that will enhance and improve the human condition in this world.
Donald A. Tomalia is director of the National Center for Dendrimer-Based Nanotechnology, Central Michigan University and president and chief technical officer of Dendritic NanoTechnologies, Inc.