2,6-BIS(PYREN-1-OYL)PYRIDINE - A NEW FLUORESCENT-PROBE WITH A HIGH-SENSITIVITY TO HYDROGEN-BONDING SOLVENTS FOR THE DEVELOPMENT OF SELECTIVE SENSORS

New probes 2,6-bis(pyren-1-oyl)pyridine (1) and 1,3-bis(pyren-1-oyl)benzene (2), with pyrenoyl moieties attached to a six-membered ring spacer, were synthesized. The presence of a single nitrogen atom in the spacer of 1 seems to be responsible for the sensitivity of its fluorescence towards hydrogen-bonding solvents and quenchers. The fluorescence emission maxima of I and 2 decrease dramatically with solvent polarity parameter E(T)(30), but their absorption spectra show no sensitivity. In the case of 1, the color of fluorescence changes from blue in cyclohexane to orange-red in acetonitrile. Although the fluorescence energies of 1 and 2 decrease with increased solvent polarity, their relative fluorescence yields show a more complex dependence on solvent polarity. The fluorescence yield of 1 decreases monotonically with polarity, while 2 has a maximum value in anisole. In addition, hydrogen-bonding solvents quench the emission from 1 very strongly, whereas 2 shows no such sensitivity for protic solvents. Aniline, phenol and water quench the emission from 1 with quenching constants of 12, 4.6 and 2.2 M(-1) respectively, whereas the corresponding quenching constants for 2 are 9.5, 3.5 and 0.27 M(-1). A large deuterium isotope effect was observed for the fluorescence quenching of I by water (KSVH2O/KSVD2O=1.7). In non-polar solvents, hydrogen-bonding quenchers, such a phenol and aniline, quench the emission of I much more efficiently than anisole, a non-hydrogen-bonding quencher. From a comparison of the fluorescence quenching characteristics of 1 and 2 by hydrogen-bonding and non-hydrogen-bonding quenchers, it can be concluded that hydrogen bonding plays a key role in the quenching of the excited state of 1. In contrast, the carbon analog 2 shows no such discrimination between hydrogen-bonding and non-hydrogen-bonding quenchers. Therefore, the single nitrogen atom present in 1 is important in determining the photophysical properties of 1 and perhaps provides a hydrogen-bonding acceptor site. The high sensitivity of the fluorescence of 1 towards solvent polarity, as well as the hydrogen-bonding ability of the solvents, makes 1 an attractive candidate for the design of highly selective fluorescence sensors.