Investigations of Supramolecular Host-Guest Systems by Fluorescence Spectroscopy

Our group is interested in the study of supramolecular systems using steady-state and time-resolved fluorescence techniques. In particular we are interested in host-guest inclusion complexes, in which an organic guest molecule becomes incorporated inside the internal cavity of a larger, hollow host molecule. This is the simplest example of a supramolecular system, an organization of two or more molecules held together only by intermolecular forces, such as van der Waals forces and hydrogen bonding. This simple complexation is illustrated in the picture below.

The host molecules of interest are all large, cage-like organic molecules, with well-defined nonpolar internal cavities; the specific hosts of interest in our group are described below. In order to study these complexes, we choose polarity sensitive fluorescence probes as guests. Such guests include the well-known anilinosulphonate fluorescent probes, such as 1,8-ANS and 2,6-ANS, amongst others. These probes in general show an extreme sensitivity of their fluorescence to the polarity of their local environment. In most cases, these probes are extremely fluorescent in nonpolar media, but nearly non-fluorescent in a polar medium. Since all of our work is done in the aqueous phase, and all of the hosts of interest contain relatively nonpolar internal cavities, the probes become much more fluorescent when incorporated into the host than when free in solution. Thus, a large fluorescence enhancement is observed upon formation of the host-guest complex. This fluorescence enhancement is easily measured by steady-state fluorescence spectroscopy, providing us with an accurate and sensitive method for studying these systems. A variety of important information can be obtained, such as the nature of the specific interactions, the complexation equilibrium constant K, and DG, DH, and DS for the complexation process. It is also possible to obtain properties of the host molecule itself, such as the polarity of the internal cavity.

In addition to the use of the observed fluorescence changes for the study of the complexation process, some of these changes have potential applications in the fields of molecular recognition and trace analysis. If the fluorescent guest is a molecule of environmental (or other) interest, such as PCBs or pesticides, then addition of an appropriate host molecule may increase the observed fluorescence, resulting in an improved sensitivity of the trace fluorescence detection of these compounds. Furthermore, such a fluorescent system could be used simply to detect the presence of specific molecules, and thus serve as a sensitive molecular sensor.

Our work has been focused on the investigation of the inclusion complexes of three families of host molecules, namely cyclodextrins, cucurbituril, and calixarenes. All of these hosts are large, hollow organic molecules, with well-defined internal cavities capable of including smaller guest molecules.

Cyclodextrins (CDs).

These large cyclic oligosaccharides have an overall bucket shape, into which organic species can become incorporated. There are three common CDs: a, b, and g, which contain 6, 7, or 8 monomers, respectively. These have cavity sizes of 5.7, 7.8, and 9.5 Å, allowing for differential complexation of molecules of different sizes. We are also interested in derivatized CDs, in which some of the hydroxyl hydrogens have been replaced by other groups, such as methyl or hydroxypropyl groups. These modified CDs have an even greater potential for fluorescence enhancement than their unmodified parents. In some cases, we have found that modified s-cyclodextrins can enhance the intensity of a fluorescence probe by a factor of 180 times! This has definite potential applications, in the use of fluorescence materials, and in the use of fluorescence as a detection technique. Publications # 28 and 30 deal with modified CDs.

Cucurbituril

Cucurbituril is a unique cage compound, consisting of a C, N s-framework. Cucurbituril is extremely rigid, with a well-defined internal cavity. The cavity is relatively small, with an internal diameter of 5.5 Å, accessible by openings of only 4.0 Å. This molecule thus has tremendous potential as a selective host. It has been shown to have an exceptional ability to encapsulate alkylammonium ions. More recently, it has also been shown to encapsulate neutral organic compounds. We have been working on studying host-guest complexes of cucurbituril by fluorescence spectroscopy. Publication # 31 describes a fluorescent solid containing cucurbituril and the fluorescent probe 1,8-ANS.

Calixarenes

Calix[n]arenes are macrocyclic oligomers of phenol, usually consisting of 4, 6, or 8 monomer units (n). They contain a large hydrophobic cavity, and are thus analogous to cyclodextrins, but with two major differences: 1) the internal cavity is lined by the p-electrons of the aromatic rings, in contrast to the purely s-framework of cyclodextrins; and 2) only one rim is lined by hydroxyl groups, and the rings are linked by methylene bridges, which results in a much greater rotational freedom of the individual rings. Thus, calixarenes have a very different type of internal cavity, and a much greater range of possible conformers (and therefore cavity size and shape), as compared with cyclodextrins. Their supramolecular host properties can therefore be expected to be distinctive. For example, two distinct stable conformations of a general calix[4]arene are shown above. We are now studying calixarene host-guest complexes using fluorescence spectroscopy.

Dr. Wagner's citations can be found on the Island Scholar website.