Influence of Surface-Attachment Functionality on the Aggregation, Persistence, and Electron-Transfer Reactivity of Chalcogenorhodamine Dyes on TiO₂

Abstract

Chalcogenorhodamine dyes bearing phosphonic acids and carboxylic acids were compared as sensitizers of nanocrystalline TiO₂ 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 <b>1-E</b> (E = O, Se), 4-carboxyphenyl in dyes <b>2-E</b> (E = O, S), 5-phosphonothien-2-yl in dyes <b>3-E</b> (E = O, Se), and 4-phosphonophenyl in dyes <b>4-E</b> (E = O, Se). All dyes adsorbed to TiO₂ 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 <b>1-E</b> and <b>2-E</b> desorbed rapidly and completely from TiO₂ into acidified CH₃CN, but phosphonic acid-functionalized dyes <b>3-E</b> and <b>4-E</b> persisted on TiO₂ 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 <b>1-E</b> and <b>2-E</b>. 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 <b>1-E</b> and <b>2-E</b> 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–TiO₂ interfaces without greatly impacting the aggregation of dyes or the interfacial electron-transfer reactivity