Excited state proton transfer (ESPT) and fluorescence resonance energy transfer (FRET) have been linearly coupled, leading to an efficient pH-sensitive energy transfer from carbazole to a potentially bioactive ketocyanine dye, 2-[3-(N-methyl-N-phenylamino)-2-propenylidene] indanone (MPAPI). The prototropic product produced exclusively from the photoexcited carbazole in the presence of alkali serves as the energy donor. The efficiency of the energy transfer process has been enhanced through the introduction of cyclodextrins of suitable dimensions. The imperative feature of the present model system involving the linear coupling of ESPT and FRET processes lies in its simplicity for designing pH sensitive molecular switches. (C) 2009 Elsevier B. V. All rights reserved.