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