Practical Applications of Excited State Proton Transfer

In the present work, excited state proton transfer (ESPT) was investigated for a variety of applications with two main areas of focus; its excited state super acidity and its uniquely large apparent Stokes shift. When a molecule absorbs a photon in enters an excited state in which the electron density of the molecule rearranges. In some molecules this electron density shift occurs away from a proton which causes the proton to become more labile and therefore more acidic. The increased acidity allows for proton transfer to occur either inter or intramolecularly. In the first chapter of this work intermolecular ESPT was used to enantioselectively protonate a silyl enol ether to generate a chiral cyclic ketone. Utilizing the axial chirality of Br-VANOL coupled with its ESPT capable aromatic alcohols, 1-phenyl-2-(trimethylsiloxy)cyclohexene was protonated to yield an enantiomeric excess (ee) of 35%. This property was extended to a variety of other silyl enol ethers to demonstrate the versatility of this method. During our investigations into this system we attempted to use BINOL, a compound with similar axial chirality to VANOL, but observed no ee under the same conditions. Later this was found to be the result of an excited state racemization process that occurs after BINOLs proton transfer. Further attempting to use ESPT to influence the chirality of atropisomeric compounds we coupled chiral auxiliary groups to BINOL then irradiated the molecule to induce the racemization event which was detrimental to our last system mentioned above. In the presence of a suitable proton acceptor irradiating BINOL causes ESPT to occur, and BINOL enters a planar excited state. From this planar excited state BINOL relaxes then rotates one direction or the other as it reenters the ground state. The addition of a chiral auxiliary group induces a preference for rotating in one direction over the other initially resulting in ee of 25% for our prototypical system. Perhaps the most interesting aspect of this work was that the ee of the molecule was found to be entirely dependent on the nature of the chiral auxiliary group, and that starting from any other ee of BINOL coupled to the same chiral auxiliary group yields the same ee. These results indicate the presence of a photostationary equilibrium ee. Changing the chiral auxiliary group to Boc protected phenylglycine yielded an ee of up to 63%. After the generation of the enantioenriched compound the chiral auxiliary group is easily cleaved by LiOH at room temperature to yield pure enantioenriched BINOL. The second half of this work is focused on the unique photophysical properties of ESIPT (excited state intramolecular proton transfer), most notably its potentially large apparent Stokes shift. When a fluorophore undergoes ESIPT the electron density shifts away from the proton containing moiety and towards a proton accepting moiety. The result of this reorganization is that the fluorophores tautomerizes into a “keto” form. From this keto state it may relax to the ground state by emitting a photon which can be red shifted by hundreds of nanometers which presents as an apparent stokes shift. Our first application of this feature was to utilize ESIPT dyes in a metal organic framework (MOF) to generate different colored emissive materials. By varying ratios of the R, G, and B linkers we were able to create a variety of different colored emissive materials including several shades of white with high color rendering index. The final chapter of this work utilizes this same ESIPT apparent stokes shift as a highly versatile sensor for detection of metal ions and hydrazine. By designing a fluorophore with a chelating moiety, we observed large spectral shifts when metal ions were successfully bound to the fluorophore. The changes to the photophysical properties was found to be dependent upon which MII ion was bound. Most notably ZnII was found to increase the quantum yield (QY) of this molecule to a maximum of 60%. When the ESIPT active positions were protected the emission was turned off, allowing it to then be turned back on with a suitable cleaving agent. This allowed for the rapid visual detection of hydrazine.