Mechanistic insights into excited state intramolecular proton transfer in isolated and metal chelated supramolecular chemosensors.

We report a computational study of excited state intramolecular proton transfer in a series of related and progressively more complex organic chromophores ranging from 2-(2'-hydroxyphenyl)-benzoxazole (HBO) through to the 5-(benzo[d]oxazol-2-yl)-2-(4-((bis(pyridin-2-ylmethyl)amino)methyl)benzo[d]oxazol-2-yl)-4 hydroxyphenolate (HDBO') anion. The latter chelates group 12 metal cations (X = Zn2+, Cd2+ and Hg2+), and can serve as a fluorescence-based sensor for such metals. Initial π* ←π excitation of the ground (S0) state enol-tautomer induces charge separation in the first excited singlet (S1) state and drives the subsequent proton transfer (i.e. enol→keto tautomerism). The keto-tautomer constitutes a local minimum on the S1 PES, and is responsible for highly Stokes shifted fluorescent emission; S1(enol) → S0 fluorescence is proposed to account for the shorter wavelength emission from the X-HDBO' complexes. Derivatives of HDBO' that should retain the favourable visible absorption and heavily Stokes shifted emission properties but, additionally, offer higher fluorescence quantum yields (i.e. enhanced metal sensing capability) are proposed.