Preferences of interaction sites for proton-induced photophysical signalling with a derivatized rhodamine probe

Abstract

Chemosensors for selective and sensitive detection of analytes were developed following various structural, functional and operational design methodologies. The interaction between probe and analyte has been vital in chemosensor-based detection technologies. A pertinent question in such a system is to be convincingly addressed, that if the probes incorporating spatially-positioned multiple interaction sites offer preferences towards analyte binding for a favourable and optimal interaction. The derivatized Rhodamine based probe 1 exhibiting proton-induced dual mode signalling here contained several protonation sites. The absorption and fluorescence signal enhancement on protonation of the probe attributed to its proton-mediated spiro-ring opening. Out of several plausible conformations of protonated probe [1.H+], their DFT calculations revealed the proton binding preferentially to the N-donor atom on spiro-ring of 1 instead at other probable sites of protonation such as other hetero- donor atoms on the xanthene core or the carbonyl O- atom on the spiro-ring as the energetically favourable conformation. It was verified with the protonated probe complex's X-ray diffracted single crystal structure.