We are interested in the design and syntheses of mainly two classes of macromolecules, viz., dendrimers and polymers. The design of these macromolecules is inspired by specific applications that our group is interested in and sometimes by the sheer beauty of the structures that one can generate using organic syntheses as a tool box. The structures that we imagine are often inspired by the intricacies of the nature's nature macromolecules such as proteins and nucleic acids.
Dendrimers, derived from the Greek word dendron (tree), are highly branched molecules that are built using iterative synthetic strategies. One of the most attractive aspects of dendrimers is that these molecules can be built as a pure single molecule even at very high molecular weights. Another interesting aspect of dendrimer is that they become globular at high molecular weights and therefore are useful as supramolecular hosts. We have developed a new design strategy to functionalize the interiors of these globular dendrimers to enhance the versatility of these molecules as hosts. In addition, we have also reported three different synthetic strategies to introduce sequences in dendrimers, i.e. these methodologies afford the capability to vary each monomer unit within a dendron or dendrimer. We are now working on several projects that demonstrate the capabilities of dendrimers in host-guest binding, catalysis, drug delivery, and other biological applications
The versatility of proteins as functional materials in biological systems makes it desirable to understand and develop ways to control functional group presentation in synthetic macromolecules. The advantage of synthetic custom-designed synthetic systems is that these molecules should be more robust and therefore could be used for a variety of applications. Also, functionalities beyond those available in the twenty amino acids could be incorporated. In addition to the fundamental interests in synthesizing such molecules, these molecules could find use in numerous applications. Examples of applications that are of interest to our group include: controlled release of drug molecules to maintain constant drug levels in the body, tissue engineering to accelerate wound-healing process, and catalysis in the aqueous phase to perform organic synthesis under environmentally benign conditions.
We are using custom-designed amphiphilic dendrimers and polymers as potential biomimetic materials. A dendrimer with diverse set of functional groups displayed on its surface is shown above schematically. Such functional group diversities in macromolecules are rare. We have developed two different synthetic methodologies to achieve such structures.
Among the different strategies that are being taken to cure cancer, chemotherapy is one of the most widely used approaches. However, chemotherapy patients often experience harmful side effects, which are caused by the non-specific toxicities of the drug molecules; i.e. drug molecules indiscriminately attack healthy cells as well as tumor cells. Therefore, the need for drug delivery systems that will reduce such non-specific toxicities is obvious. Two promising approaches are possible to improve the patientsí quality of life. The first possibility involves developing methods of selectively delivering the drug molecules to tumor cells. The second possibility involves delivering chemoprotective agents that will provide relief from the harmful side effects of chemotherapy.
In our group, we are interested in the first approach. We are developing new strategies to selectively deliver chemotherapeutic drug molecules to cancer cells.
Controlling the vectorial transport of energy and electrons in organized molecular assemblies has been of current interest due to the potential applications of these architectures in artificial photosynthesis, photovoltaic cells, materials for optical data storage, thin-film transistors, and electroluminescent materials. The required functional group organizations have been approached using linear arrays of porphyrins and diimides or other molecular assemblies such as liquid crystals, zeolites, polymers, peptides, and amphiphiles. The unique architecture of dendrimers provides a new dimension for the precise placement of charge-transport units in a single molecule. Since dendrimers are highly branched macromolecules, they offer control over the placement of functional groups in higher dimensions than that offered by linear arrays.
Dendrimers should also be advantageous for the funneling of charges from the core to the periphery, because the number of functional units doubles with each layer. Several photophysical studies based on dendriticarchitectures have been reported over the past few years. However, in most of these cases, the light harvesting functional groups are incorporated as the core unit or as the peripheral units, perhaps largely due to the high synthetic facility with which these arrays can be obtained. To exploit all the architectural features offered by dendrimers, it is advantageous to build them where these functional groups are incorporated as the repeating units.
The system that provides fast charge separation but slow charge recombination would be ideal for electron transfer materials. Recently, we have found that meta-conjuagated polyacetylene (PA) can meet these requirements. Based on time-resolved measurements, we have showed that para- and meta-conjugated PA have similar forward but very different backward electron transfer. The explanation for these differences could be originated from the fact that meta linkage blocks electronic communication in the ground state but acts like a wire in the excited state. As a result, the forward electron transfer in meta PA is comparable to that in para PA while the backward electron transfer undergo with much slower rate.
Currently, to construct novel efficient electron transfer compounds, we have been putting an effort to combine advantages provided by both dendrimers and meta-linked molecules. Dendrimers containing meta-conjugated PA are considered promising for efficient electron transfer and now under our interest.