Modulating luminescence via conformational control of nonradiative processes: A new approach to chemosensor design

Luminescent chemosensors based on conformational restriction offer an alternative signal transduction mechanism wherein the detection of analyte does not require direct electronic communication between the receptor and fluorophore. Consequently, the two can be spatially separated, greatly expanding the scope of receptors and guests that are compatible with the specific lumophore employed. It was hypothesized that this modular design could be easily realized in bichromophoric systems where the relative orientation of two π systems could be manipulated by an external force (i.e., analyte).; A systematic endeavour was undertaken to test the idea of signaling via conformational restriction in a series of biphenyl and tolan fluorophores equipped with crown-ether binding domains. In both systems, the quantum yields for fluorescence proved to be a very sensitive function of the degree of flexibility about the coannular bond. Noncovalent rigidification of the biphenyl or tolan framework was accomplished by the interaction of the crown-ether cavity with various alkali and alkaline earth metals, producing fluorescence enhancements exceeding 5-fold. In the case of the biphenyls, hypsofluoric shifts were also observed for certain complexes. The source of these enhancements was determined to be a reduction in the rate of nonradiative decay rather than any fundamental change in the radiative decay rate. Time-resolved fluorescence measurements were used to elucidate the specific nonradiative processes subject to conformational control. It was found that restricted rotation in biphenyls alters the degree of intersystem crossing while similar conformational constraints in tolans alters the degree of internal conversion.; The knowledge obtained from studies of these organic fluorophores was then applied to ruthenium(II) polypyridyl complexes containing modified crown-ether bipyridine ligands in an effort to control nonradiative processes for signaling purposes in transition metal complexes. Several members of the ruthenium series responded positively to various metal ions and exhibited bathofluoric shifts in emission energies in some cases. Time-resolved measurements showed that suppression of internal conversion was most likely responsible for these observations. These studies show that conformational restriction in both organic systems and transition metal complexes is a viable signal transduction mechanism for luminescent chemosensing. Moreover, they emphasize the importance of conformation in determining the rates of nonradiative decay.