4. The Characterisation of Dendrimers
The determination of the dendritic structures and properties can be approached from the perspective of either a synthetic chemist or a polymer chemist. The size of the molecules involved means that methods which are familiar to synthetic chemists are stretched to their limits, while the methods used by polymer chemists often lack precision.
4.1. Structural Elucidation
The ultimate proof of molecular structure is often reached by the determination of a crystal structure In the case of dendrimers, however, this technique is of little use. The polymeric nature of dendrimers generally leaves their solid-state structure without any long-range order.[1] Even when some degree of structure is present for instance in the case of very dense-surfaced spherical dendrimers there is such a degree of disorder within the inner generations that a crystal structure cannot be determined. There are a few examples of crystalline[2] dendrimers and powder and single crystal[37,3] X-ray studies of dendrimers, but these are confined to low generations and rigid, hindered molecules. Nuclear Magnetic Resonance (NMR) spectroscopy and mass spectrometry have both been invaluable techniques in the characterisation of dendrimers. Indeed, 1H and 13C NMR spectra of dendrimers can be surprisingly simple and contain a great deal of information about defects and impurities in their structures. The mass spectrometry of dendrimers has benefited from the soft ionisation techniques developed for the study of large biomolecules. Many results have been obtained by the use of Matrix Assisted Laser Desorption / Ionisation - Time of Flight (MALDI-TOF)[4] and ElectroSpray (ES)[5] mass spectrometric methods. Other methods, familiar to polymer chemists, have been used extensively in the characterisation of dendrimers. Electron microscopy has been of great use in the visualisation of dendrimers[6] and their aggregates,[17,18,89] and Gel Permeation Chromatography (GPC) has been used for the calculation of radii of gyration, hydrodynamic radii[7] and polydispersities. Low Angle Laser Light Scattering (LALLS), Small Angle Neutron (SANS)[8] and X-ray (SAXS)[9] Scattering techniques have met with limited use.
4.2. Physical Properties of Dendrimers
Dendrimers are of interest for their unusual physical properties. Much work has been carried out in areas such as melt viscosity,[10] glass transition temperature,[11] rheological properties[12] and even on the photoinduced electron transfer to C60.[13] The most exciting physical property of dendrimers is the variation of their intrinsic viscosities with molecular weight. It is found that, when the generation increases beyond a certain point, the intrinsic viscosity begins to decline, contrary to the behaviour of linear polymers (Figure 10). This effect is believed to be a consequence of the globular shapes of high generation dendrimers leaving them unable to 'tangle' with one another after the manner of linear polymers.
4.3. Characterisation of the Dendritic Microenvironment
Many investigators have made use of functional probes in order to study dendritic microenvironments. These probes can either be attached covalently to the dendritic structure, like the photochemical,[35] chiral,[69,80] and solvatochromic[67] moieties that have already been discussed, or they can be introduced as guest species.[14] Spectroscopic methods have also produced information about dendritic microenvironments. PAMAM dendrimers have been shown20 to have decreased 13C relaxation times for internal generations, suggesting that these moieties are less mobile than the surface groups.[15] Rotational-Echo Double Resonance (REDOR) solid-state NMR[16] spectroscopy has been used[17] to examine the shape of the Fréchet polyethers, and Electron Spin Resonance (ESR)[18] spectroscopy of complexed PAMAMs have been examined. Computer modelling[21,39,77] of dendrimers has been used extensively for the purposes of visualisation and dynamics experiments. A great deal of insight into dendrimer behaviour can be obtained. However, care must be exercised in the interpretation of the results.