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 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.

Dye aggregation identified by vibrational coupling using 2D IR spectroscopy

“…The efficiency of charge transfer depends on many factors, including the photosensitizer electronic orbitals, conjugation of the linker, and the anchoring groups. [5][6][7][8][9][10][11][12][13][14][15] For example, differing levels of conjugation have been shown to inject electrons with different, sometimes multiexponential, kinetics, and different binding groups also show different efficiencies of charge transfer. 11,12 Dye orientation on the surface also has a large effect, as a dye molecule that is lying flat usually has different electron transfer rates than those perpendicular to the surface.…”

“…In principle, dye aggregation should shift or broaden the UV-Vis spectra, but UV-Vis absorptions of these dyes are so broad that it can be difficult to recognize aggregation or the size of aggregates as compared to other effects that shift and broaden the absorption spectrum. 6,29,30 As a result, for many samples, it is unclear what role aggregated dyes play in the overall kinetics. Infrared and two-dimensional infrared (2D IR) spectroscopies are another way to identify and measure aggregation, because vibrational coupling alters the vibrational frequencies of dye aggregates.…”

Modification and Potential Applications of Organic–Inorganic Non-Siliceous Hybrid Materials

“…Very recently, phosphonic acids were found to offer a promising alternative owing to their high affinity toward the surfaces of metal oxides and the relatively stronger binding than carboxylic acids [48,49], and they would thereby give better long-term stability of DSSCs. Mulhern et al [50] have analyzed the influence of the surface-attachment functions of the dyes on electron transfer at the dye-TiO 2 interface and long-term stability. Chalcogenorhodamine dyes were attached to the surface of nanocrystalline TiO 2 through phosphonic or carboxylic acid functions.…”

“…Nonetheless, in addition to the fact that phosphonic acids present more stable alternatives, the charge injection rates can be prohibited to some extent due to the tetrahedral phosphorus center and loss of conjugation [50,51]. Combining the superior binding stability of phosphonate and the good electron injection efficiency of carboxylate has resulted in a feasible method (Fig.…”

Plasmon-enhanced fluorescence of dyes on silica-coated silver nanoparticles: A single-nanoparticle spectroscopy study

“…RB is chosen as the fluorophore since it absorbs at the 532-nm laser excitation wavelength and somewhat overlaps with the LSPR spectrum of SCNPs. Moreover, rhodamines have been used as sensitizers in DSSCs [38,39], such that RB-doped SCNPs (RB/SCNPs) are a reasonable model system for understanding dye-metal interactions in plasmon-enhanced photovoltaics. Although plasmon-enhanced fluorescence is most significant for weak absorbers and poor emitters, high quantum yield molecules can also exhibit substantial fluorescence enhancements [33].…”