Intermolecular Charge Separation in Aggregated Rhodamine Dyes Used in Solar Hydrogen Production

Various modern solar light-harvesting systems, including those used in photovoltaics and solar fuel production, depend on efficient electron transfer from a surface-bound molecular dye to nanoscopic semiconductor particles. However, the productive electron transfer competes with a variety of other relaxation pathways for the dye, and the dominant pathway can change dramatically depending on its environment. A new sulfur-substituted thiorhodamine dye was synthesized having exceptional light-harvesting qualities for solar energy applications and for solar hydrogen production in particular. The dye was created with a thiophene spacer bearing a phosphonate-ester (<b>1-Ester</b>) or phosphonic-acid (<b>1-Acid</b>) allowing for excellent solubility in MeCN or the ability to functionalize metal oxide semiconductor nanoparticles such as TiO₂. While <b>1-Ester</b> is found to be fully monomeric in MeCN, <b>1-Acid</b> readily forms H-aggregated dimers which, upon photoexcitation, undergo charge separation to an ion pair (IP) in 1.5 ps. For <b>1-Acid</b> dimers, the stabilization of the IP causes an increase in lifetime to 270 ps compared to the 75 ps lifetime of the monomer. When <b>1-Acid</b> is attached to TiO₂, the inhomogeneous surface creates a distribution of chromophore packing structures where a range of transition dipole coupling environments is present such that both excimers and IPs can form. In a variety of solvent environments, ultrafast electron injection was found to occur in <300 fs from the dye to the semiconductor while IP formation occurs in 2–4 ps. For all aggregates studied, the photophysics was the same whether pumped at 620 nm, exciting to the 0–0 absorption band, or at 565 nm to the 0–1 transition that is dramatically enhanced by transition-dipole coupling in the H-aggregate. Surprisingly, the long-time, >2 ns, persistent formation of the charge-separated state following charge injection to TiO₂ only accounts for ∼10% of the photoexcited population, with the dominant relaxation pathways being IP and excimer formation. IP and excimer formation lower the total energy of the aggregate below the conduction band edge of TiO₂, deactivating the electron transfer process. The implications of IP and excimer formation in systems for solar light harvesting are discussed

Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance

A library of six selenorhodamine dyes (<b>4-Se</b>–<b>9-Se</b>) were synthesized, characterized, and evaluated as photosensitizers of TiO₂ in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis­(julolidyl)- or bis­(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO₂. Absorption bands of solvated dyes <b>4-Se</b>–<b>9-Se</b> were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO₂ as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes <b>4-Se</b> and <b>7-Se</b> initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65–80% under monochromatic illumination, but the dyes desorbed rapidly from TiO₂ into solutions of HCl (0.1 M) in a CH₃CN:H₂O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes <b>5-Se</b>, <b>6-Se</b>, <b>8-Se</b>, and <b>9-Se</b> exhibited lower IPCEs (49–65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO₂, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis­(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis­(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO₂ in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media

Organotellurium Fluorescence Probes for Redox Reactions: 9‑Aryl-3,6-diamino­telluro­xanthylium Dyes and Their Telluroxides

Several 9-aryl-3,6-diamino­telluro­xanthylium dyes with phenyl, 2-methylphenyl, and 2,4,6-trimethylphenyl substituents at the 9-position were prepared. The characterization of these compounds included determination of ¹²⁵Te NMR spectra, fluorescence quantum yields (Φ_F), and quantum yields for the generation of singlet oxygen [Φ­(¹O₂)]. While these compounds were essentially nonfluorescent (Φ_F < 0.005), they produce ¹O₂ with Φ­(¹O₂) between 0.43 and 0.90. The tellurorosamines were oxidized with ¹O₂ via self-photosensitization to the corresponding telluroxides, which allowed their preparation free of excess oxidant. Telluroxides with a 9-(2-methylphenyl) or 9-(2,4,6-trimethylphenyl) substituent were fluorescent with quantum yields for fluorescence between 0.20 and 0.31. Steric bulk at the 9-position of the resulting telluroxides impacted rates of inter- and intramolecular attack of nucleophiles and stability of the telluroxide in aqueous media near physiological pH. The yield of reduction of the telluroxide with glutathione was also dependent on the steric bulk of the 9-aryl substituent. The structure of products from oxidation of the 9-(4-bromophenyl) tellurorosamine was determined by X-ray crystallography and indicated the addition of oxygen nucleophiles to the 9-position of the telluroxide oxidation state of the tellurorosamine

Synthesis and Properties of Heavy Chalcogen Analogues of the Texas Reds and Related Rhodamines

Analogues of Texas red incorporating the heavy chalcogens S, Se, and Te atoms in the xanthylium core were prepared from the addition of aryl Grignard reagents to appropriate chalcogenoxanthone precursors. The xanthones were prepared via directed metalation of amide precursors, addition of dichalcogenide electrophiles, and electrophilic cyclization of the resulting chalcogenides with phosphorus oxychloride and triethylamine. The Texas red analogues incorporate two fused julolidine rings containing the rhodamine nitrogen atoms. Analogues containing two “half-julolidine” groups (a trimethyltetrahydroquinoline) and one julolidine and one “half-julolidine” were also prepared. The photophysics of the Texas red analogues were examined. The S-analogues were highly fluorescent, the Se-analogues generated single oxygen (¹O₂) efficiently upon irradiation, and the Te-analogues were easily oxidized to rhodamines with the telluroxide oxidation state. The tellurorhodamine telluroxides absorb at wavelengths ≥690 nm and emit with fluorescence maxima >720 nm. A mesityl-substituted tellurorhodamine derivative localized in the mitochondria of Colo-26 cells (a murine colon carcinoma cell line) and was oxidized <i>in vitro</i> to the fluorescent telluroxide