Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co(III)(dmgH)(2)(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H(2)/mol PS/h for O, S, and Se derivatives, respectively (Φ(H(2)) = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (∼2.6 ns) and shorter for the Se analog (∼0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS(-) that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.

Chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids were compared as sensitizers of nanocrystalline TiO(2) in dye-sensitized solar cells (DSSCs). The dyes were constructed around a 3,6-bis(dimethylamino)chalcogenoxanthylium core and varied in the 9 substituent: 5-carboxythien-2-yl in dyes 1-E (E = O, Se), 4-carboxyphenyl in dyes 2-E (E = O, S), 5-phosphonothien-2-yl in dyes 3-E (E = O, Se), and 4-phosphonophenyl in dyes 4-E (E = O, Se). All dyes adsorbed to TiO(2) as mixtures of H aggregates and monomers, which exhibited broadened absorption spectra relative to those of purely amorphous monolayers. Surface coverages of dyes and the extent of H aggregation varied minimally with the surface-attachment functionality, the structure of the 9-aryl group, and the identity of the chalcogen heteroatom. Carboxylic acid-functionalized dyes 1-E and 2-E desorbed rapidly and completely from TiO(2) into acidified CH(3)CN, but phosphonic acid-functionalized dyes 3-E and 4-E persisted on TiO(2) for days. Short-circuit photocurrent action spectra of DSSCs corresponded closely to the absorptance spectra of dye-functionalized films; thus, H aggregation did not decrease the electron-injection yield or charge-collection efficiency. Maximum monochromatic incident photon-to-current efficiencies (IPCEs) of DSSCs ranged from 53 to 95% and were slightly higher for carboxylic acid-functionalized dyes 1-E and 2-E. Power-conversion efficiencies of DSSCs under white-light illumination were low (<1%), suggesting that dye regeneration was inefficient at high light intensities. The photoelectrochemical performance (under monochromatic or white-light illumination) of 1-E and 2-E decayed significantly within 20-80 min of the assembly of DSSCs, primarily because of the desorption of the dyes. In contrast, the performance of phosphonic acid-functionalized dyes remained stable or improved slightly on similar timescales. Thus, replacing carboxylic acids with phosphonic acids increased the inertness of chalcogenorhodamine-TiO(2) interfaces without greatly impacting the aggregation of dyes or the interfacial electron-transfer reactivity.

A series of chalcogenorhodamine dyes with oxygen, sulfur, and selenium atoms in the xanthylium core was synthesized and used as chromophores for solar hydrogen production with a platinized TiO2 catalyst. Solutions containing the selenorhodamine dye generate more hydrogen [181 turnover numbers (TONs) with respect to chromophore] than its sulfur (30 TONs) and oxygen (20 TONs) counterparts. This differs from previous work incorporating these dyes into dye-sensitized solar cells (DSSCs), where the oxygen- and selenium-containing species perform similarly. Ultrafast transient absorption spectroscopy revealed an ultrafast electron transfer under conditions for dye-sensitized solar cells and a slower electron transfer under conditions for hydrogen production, making the chromophore's triplet yield an important parameter. The selenium-containing species is the only dye for which triplet state population is significant, which explains its superior activity in hydrogen evolution. The discrepancy in rates of electron transfer appears to be caused by the presence or absence of aggregation in the system, altering the coupling between the dye and TiO2. This finding demonstrates the importance of understanding the differences between, as well as the effects of the conditions for DSSCs and solar hydrogen production.

Twelve thiorhodamine derivatives have been examined for their ability to stimulate the ATPase activity of purified human P-glycoprotein (P-gp)-His10, to promote uptake of calcein AM and vinblastine into multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells, and for their rates of transport in monolayers of multidrug-resistant, P-gp-overexpressing MDCKII-MDR1 cells. The thiorhodamine derivatives have structural diversity from amide and thioamide functionality (N,N-diethyl and N-piperidyl) at the 5-position of a 2-thienyl substituent on the thiorhodamine core and from diversity at the 3-amino substituent with N,N-dimethylamino, fused azadecalin (julolidyl), and fused N-methylcyclohexylamine (half-julolidyl) substituents. The julolidyl and half-julolidyl derivatives were more effective inhibitors of P-gp than the dimethylamino analogues. Amide-containing derivatives were transported much more rapidly than thioamide-containing derivatives.