We are interested in the interactions of small molecules (such as NO) and ions (such as NO+) with electro-active (organic) receptors with a view toward elucidating the factors responsible for strong and specific binding as well as the usage of these receptors for the construction of practical molecular sensing devices for various analytes such as NO, NO2, NOCl, NOBr, O2, O3, CO, CO2, SO2, SO3, RCO2-, etc. The ultimate endeavor is to fabricate these molecular sensors into nano-scale machines for practical usage as devices to detect and quantify various analytes in vivo for biomedical applications as well as in vitro to monitor the atmospheric pollution. Our work in this area is presently focuses on the preparation of molecular receptors for nitric oxide (NO) - an important biological messenger. We have recently observed that the stilbenoid hydrocarbon BCO and various conformers of calixarene ethers capture NO with unprecedented efficiency upon oxidative activation (e.g. eq. 1).
These novel organic receptors show dramatic color changes upon binding of NO, which penetrates deep within the cavities formed by cofacial aryl groups, as shown by the X-ray structures in Figure 1-3. Moreover, the strong noncovalent (charge-transfer) interaction of NO with the cofacial aromatic receptors is easily switched on and off by oxidation and reduction, respectively. Thus, the important finding that receptors containing cofacial aryl groups bind NO extremely efficiently is now being exploited for the development of efficient molecular sensors for practical applications for monitoring of nitric oxide.