Synthesis of Rhodamine Analogues for Applications in P-glycoprotein Inhibition, Dye-Sensitized Solar Cells, and Solar Hydrogen Evolution
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Novel chalcogenorhodamine dyes have been made in the Detty group and found to be effective in applications for P-glycoprotein (P-gp) inhibition, solar hydrogen evolution, and dye-sensitized solar cells. Based on the promising results from recent work regarding amide/thioamide dyes and their effects on P-gp activity, the synthesis of a new library of compounds to probe further the P-gp binding pocket has been completed. As part of this library, a route toward the synthesis of a new rhodamine precursor, the half-julolidyl xanthone ( hjx ), was developed, which incorporates added rigidity and lipophilicity into the xanthone core compared with bis(dimethylamino) xanthones, while also introducing possible added steric interactions with P-gp through the gem-dimethyl group. The synthesis of hjx proceeded with an overall yield of 20% and has been used successfully to synthesize several new rhodamine dyes in >30% yield. An overall library of twelve compounds, four of which incorporate the half-julolidyl core, have been screened for P-gp inhibition/modulation and were found to act as modulators of P-gp, with half-julolidyl compound I exhibiting one of the greatest potencies (9.3 ± 1.1 μM) and being transported the slowest (34 ± 22 nm s -1 ). In general, thioamides were found to be transported more slowly than amides, in agreement with previous work. We have also made and tested various dyes for their ability to act as photosensitizers (PS) for the photo-generation of hydrogen in both homogeneous and heterogeneous aqueous systems. The ortho- carboxyphenyl rhodamine derivatives 3a and 3c, as well as the 2,3-carboxythienyl rhodamine derivative 7a were made in yields of 30%, 28%, and 24%, respectively. These, as well as many other compounds, were tested for their ability to generate hydrogen, and while many dyes facilitated hydrogen production, dye 3c was the most successful, generating hydrogen with a turnover frequency (TOF) of 5500 moles of H 2 /moles PS/hour over a period of 2 h in a homogeneous system containing a cobalt catalyst. The dyes also showed promising results when tested in a heterogeneous system containing a platinum catalyst on TiO 2 , where 3c was again the most successful, yielding 900 TON (moles of H 2 /moles PS) over a period of 5 h. Lastly, a new series of rhodamine dyes have been synthesized that replace the traditional carboxylic acid attachment group with a phosphonate group, which was expected to bind more tightly to the TiO 2 semiconductor surface in dye-sensitized solar cells. These compounds were tested in collaboration with the Watson group at the University at Buffalo and were indeed found to bind much more tightly than their carboxylic acid counterparts, which almost completely desorb from the surface in less than half an hour. These new cells exhibit incident photon to current efficiencies (IPCEs) of greater than 90% for extended periods of time (more than 24 h), and the dyes have been found to bind ∼100 times more tightly than their carboxylic acid counterparts. In addition, the selenium phosphonic acid analogues have been found to generate hydrogen better than any other derivative over extended periods in the heterogeneous aqueous system containing platinized titania, showing that electron injection is facilitated by attachment to the TiO 2 surface.