With the support of the Organic and Macromolecular Chemistry Program, Professor Uwe Bunz has proposed a new type of cross-shaped chromophore (XF, cruciform, 1,4-bis(aryl-ethynyl)-2,5-distyrylbenzenes) in which the frontier molecular orbitals are localized on the two different axes. The Highest Occupied Molecular Orbital (HOMO) is localized on the distyrylbenzene axis, while the Lowest Unoccupied Molecular Orbital (LUMO) is localized on the bis(arylethynyl) transverse. This orbital arrangement translates electronic properties into spatial patterns; the XFs will contain metal coordinating electron-donating substituents on the distyrylbenzene axis and metal coordinating acceptors on the bis(arylethynyl) transverse achieving two-stage sensing of different metal cations. The addition of two analytes with the resulting fluorescence changes will be used to construct a Logic Table. The analytes are the input and the fluorescence change the output. They plan to deposit the XFs with and without metal salts onto solid supports to obtain sensor arrays for volatile organic compounds. The combination of different fluorophores and the use of the same fluorophore complexed with different metals will lead to changes in emissive properties when exposed to analytes such as volatile amines due to reversible decomplexation/protonation or deprotonation. Broader Impact: This chemistry will potentially allow detecting toxic metal ions in water and toxic volatile amines in the atmosphere using simple test strips impregnated with different XF-type fluorophores. The research will develop a molecular logic based upon recognition of different analytes with XFs for molecular computing. Educational Impact: The PI has an active outreach program with an inner city high school (Tri-Cities High). He has had and will have high school students in his laboratory working on the sensing of model analytes using either solutions of XFs and/or XFs immobilized on solid supports such as highly silanized silica gel to attract bright students into chemistry.
We have been able to prepare novel fluorophores by a simple two step reaction. These cross shaped molecular fluorophores have attractive responsive properties. Upon exposure to metal cations, to anions and to acids and bases they change color. The changes are somewhat selective depending upon the chemical structure of the analyte that is offered. Suitable substitution makes these fluorophores competent for different classes of analytes. Typically dialkylamines and pyridine units are used as functional groups for the reaction with metal cations and acids, while hydroxyl groups are useful for detection of bases. The specific molecular design allows for the analytes to induce larger shifts in fluorescence color, much more so than one would expect ordinarily. The trick is the spatial separation of the frontier molecular orbitals that allows for analytes to interact with parts of the cruciform that either only changes the HOMO or the LUMO position. A specific cross-shaped fluorophore containing hydroxyl groups on the four traverses can be used to detect and identify amines when used in different solvents. This approach is potentially useful for the detection and identification of fouling processes in food stuffs, but should also allow the discrimination of amines in environmental samples as long as they are sufficiently enriched.
