Excited state proton transfer assisted fluorescence resonance energy transfer in an inclusion complex of a β-CD derivative

The inclusion complex of Rhodamin B (Rh B) with (N-6-deoxy-β-cyclodextrin-6-yl-1-aminocarbonyl)-3-(4-fluorobenzoyl)-7-pyridin-4-yl indolizine [designated as compound (1)] is shown to be a potential pH sensor based on fluorescence resonance energy transfer (FRET). By steady state and time-resolved fluorimetry it has been established that (i) an excited state proton transfer (ESPT) occurs between the two prototropic forms, (1·H+) and (1·H2+2) of (1), (ii) a FRET occurs in the inclusion complex whereby energy is transferred to Rh B and (iii) (1·H2+2)* is much more efficient than (1·H+)* in the FRET. Hence this FRET is ESPT-assisted. Donor–acceptor distance from the FRET result shows that the (4-fluorobenzoyl)-7-pyridin-4-yl indolizine cap of (1) opens up in the (1·H2+2) form.