A novel bipyridine-based fluorescent host for diphenyl phosphate: affinity, photo-response and mechanism

Abstract

6,6′-Bis(hexylamino)-2,2′-bipyridine 1 exhibits an efficient blue fluorescence and the properties of 1 as a fluorescent host have been studied in various organic solutions. The host 1 shows high affinity for diphenyl phosphate 5 (K >10⁴ dm³ mol^-1), but only low affinity for hexanoic acid. On association with 5, 1 exists in a protonated form and forms an ion-pair type complex. In cyclohexane, blue fluorescence diminishes on association with 5, and simultaneous formation of two hydrogen bonds between amino hydrogens of 1 and phosphate oxygens contribute to stabilize the host–guest complex. In contrast, in polar solvents such as acetonitrile, a decrease in the blue fluorescence and a concomitant increase in green fluorescence from the complex is observed, and solvation to the phosphate anion stabilizes the polar ion-pair complex. These results confirm that the host 1 exhibits high affinity and selectivity with a sensitive fluorescence-response for the guest 5 both in nonpolar and polar organic solvents.