Bifunctional Charge Transfer Operated Fluorescent Probes with Acceptor and Donor Receptors. 1. Biphenyl-Type Sensor Molecules with Protonation-Induced Anti-Energy Gap Rule Behavior

On the basis of biphenyl (b) type molecules bpb-R substituted with a 2,2':6',2' '-terpyridine acceptor (bp) and either amino-type donor receptors (R = dimethylamino (DMA), A15C5 = monoaza-15-crown-5) or nonbinding substituents (R = CF(3), H, OMe) of various donor strengths, we developed a family of charge transfer (CT) operated monofunctional and bifunctional fluorescent sensors for protons and metal ions. These molecules are designed to communicate the interaction of an analyte with the acceptor and the donor receptor differing in basicity and cation selectivity by clearly distinguishable spectral shifts and intensity modulations in absorption and in emission as well as in fluorescence lifetime. From the dependence of the fluorescence spectra, fluorescence quantum yields, and fluorescence lifetimes of bpb-R on solvent polarity and proton concentration, the photophysics of bpb-R and their protonated analogues can be shown to be governed by the relaxation to a CT state of forbidden nature and by the switching between anti-energy and energy gap law type behaviors. This provides the basis for analytically favorable red shifted emission spectra in combination with comparatively high fluorescence quantum yields. Accordingly, bpb-H and bpb-OMe are capable of ratiometric emission signaling of protons. bpb-DMA reveals a protonation-induced ON-OFF-ON switching of its emission.