Ratiometric Recognition of Protons by a Multiple Tagged Designer Fluorescent Chemosensor.

Molecular architecture with different fluorophoric units can offer improved and effective recognition of biologically important analytes. We present here a new strategy for the design of ratiometric chemosensors that operate by photoinduced electron transfer (PET). This ratiometric sensor endowed with tryptophan and anthracene exhibits high sensitivity, excellent selectivity and remarkable reversibility towards recognition of H+ in methanol. This "Turn-On" type behaviour is crafted into the molecule by incorporation of bispidine entity. Effective quenching of the fluorescence of the anthracene by the adjacent amine groups of the bispidine results in negligible fluorescence from the anthracene group leading to highly sensitive recognition of protons by the compound as H+ protonate the amine functionalities giving rise to the emergence of the fluorescence from the anthracene group. This, combined with the reduction in the fluorescence from the Trp group by H+, results in highly sensitive ratiometric nature of the response especially at low [H+].