The Brønsted acidity or basicity of molecules can change upon electronic excitation, these compounds are known as photoacids or photobases. The proton transfer process that modifies the molecular properties can be intramolecular or intermolecular. We report on a conjugated Schiff base molecule that features dual emission in organic protic solvents. These emission bands are ascribed to the non-protonated and protonated forms after intermolecular proton transfer (PT) from the solvent to the electronically excited photobase. Using Förster’s equation, we calculate an increase of 14 pKa units upon excitation which makes the pK*a of the photobase in ethanol about 28. We classify photobases in the excited state having a pK*a &gt; 20 as “super” photobases. Transient absorption shows that the non-protonated form decays with a 15 ps time constant- a time-scale that is similar to the average dielectric relaxation time of the ethanol solvent and points to the initiation of an ultrafast PT event which is limited by the finite dielectric relaxation time of the solvent. The non-protonated form decay is accompanied with a rise in stimulated emission signal that red shifts in energy on a 50-70 ps timescale. We ascribed this longer time-scale to the formation of a fully separated ion-pair following the PT reaction from the solvent to the Schiff base. We have also carried out experiments in deuterated ethanol. An isotope effect of 1.5 was observed in the short-time component of our PT event. This observation implies that the first decay component ascribed to ethanol dielectric relaxation is coupled with a full PT process. We attribute the solvent-relaxation limited part of the spectral shift to the formation of a partial proton-transferred (highly ploarized H-bonding) complex between the photobase and the solvent prior to the full PT event which creates the solvent separated ion-pair on a 50-70 ps timescale. We note that an isotope effect of about 2 is observed for the second full charge separation step that involves bond breaking and making.

Excited state intermolecular proton transfer of a “super” photobase

256th National Meeting (2018)

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