Abstract
Theoretical design of conjugated proton cranes, based on 7-hydroxyquinoline as a tautomeric sub-unit, has been attempted by using ground and excited state density functional theory (DFT) calculations in various environments. The proton crane action request existence of a single enol tautomer in ground state, which under excitation goes to the excited keto tautomer through a series of consecutive excited-state intramolecular proton transfer (ESIPT) steps with the participation of the crane sub-unit. A series of substituted pyridines was used as crane sub-units and the corresponding donor-acceptor interactions were evaluated. The results suggest that the introduction of strong electron donor substituents in the pyridine ring creates optimal conditions for 8-(pyridin-2-yl)quinolin-7-ols to act as proton cranes.
Highlights
Excited state proton transfer reactions involve proton transfer through a ground state pre-existing hydrogen bond, giving rise to another tautomer in the excited state [1,2,3,4,5].Due to dramatic structural change, the excited tautomer possesses photophysical properties, different from those of the ground state specie
Absorption and emission are observed from the E tautomer only in non-protic solvents at room temperature [73], whereas in aqueous solution, fluorescence was observed from the K form only [74]
Solvent-assisted excited-state proton transfer was proposed, based on the experimental data, in protic solvents [72,76,78], where the process is facilitated by the increased excited state acidity/basicity of the enol/imino groups [79]
Summary
Excited state proton transfer reactions involve proton transfer through a ground state pre-existing hydrogen bond (either intra- or intermolecular), giving rise to another tautomer in the excited state [1,2,3,4,5].Due to dramatic structural change, the excited tautomer possesses photophysical properties, different from those of the ground state specie. Proton cranes are switching systems where a proton is transferred over a long distance within the same molecule. In general they contain a tautomeric sub-unit with clearly defined proton donor and proton acceptor parts, which exchange, under external stimuli, the tautomeric (or pseudo tautomeric) proton using a crane sub-unit as a proton delivery system. Proton transfer occurs from the tautomeric to the crane sub-unit, leading to the unstable transfer state. Depending on the relative proton acceptor ability of the two competitive sites in the deprotonated tautomeric sub-unit, the protonated crane sub-unit might or might not rotate around the axle, leading, after release of the proton, to the end or to the standby state, correspondingly. The obtained end state can relax back to the standby either thermally or through applying another stimulus (another excitation wavelength for instance)
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