Guanidine Substitutions in Naphthyl Systems to Allow a Controlled Excited-State Intermolecular Proton Transfer: Tuning Photophysical Properties in Aqueous Solution

The excited-state intermolecular proton transfer process (ESPT) in aqueous solution is achieved and controlled by the incorporation of guanidine groups in a fluorescent structure. The bisguanidine under investigation exhibits a dual fluorescence emission with a very high Stokes shifts in water, approximate to 86 (7890) and 210 (14 500) nm (cm(-1)), and an excited-stated deprotonation coupled to an intramolecular charge transfer (ICT) process contributes to this emission. The study demonstrates that the emission properties of the different protonation states are strongly dependent on the solvent environment, which also allows luminescence of the molecule to be tuned. The results of this work show the potential utility of guanidine substitution for the stabilization of ESPT-ICT processes in water and allow the subsequent logical design of new stimulus-responsive fluorophores.