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.

Photograph displays fluorescence under exposure to UV lamp (356 nm). ESPT is demonstrated by (A) aqueous solution of compound (1) at pH 4.5 and (B) same solution at pH 2.0. ESPT-assisted FRET is demonstrated by (C) mixture of compound (1) and Rh B at pH 4.5, (D) same solution at pH 2.0.Figure optionsDownload as PowerPoint slideHighlights
► Excited state proton transfer assisted FRET is observed with the inclusion complex of (N-6-deoxy-β-cyclodextrin-6-yl-1-aminocarbonyl)-3-(4-fluorobenzoyl)-7-pyrridin-4-yl indolizine, (1), with Rh B in the pH range 4.5–2.0.
► FRET to Rh B in the complex occurs much more efficiently from the diprotonated form than the monoprotonated form of (1).
► Compound (1) is a potential FRET based pH sensor.
► The fluorophore cap of the β-CD derivative opens up during protonation.