Ru(II)-Bpy Complexes Bound to Nanocrystalline TiO₂ Films through Phenyleneethynylene (OPE) Linkers:  Effect of the Linkers Length on Electron Injection Rates

Abstract

Three rigid-rod Ru-bpy sensitizers with linkers made of an oligo(phenyleneethynylene) (OPE) bridge varying in length (9.6-23.3 Å) and ending with an isophthalic acid anchroing unit were bound to TiO 2 (anatase) nanoparticle films, and the photoinduced electron injection rates for all three compounds were studied by femtosecond laser spectroscopy. The sensitized films were excited at 530 nm, in the MLCT region, and the injection dynamics was probed in the 600-1000 nm range. The excited state for all three compounds (τ ∼ 2 µs in acetonitrile solution) was readily quenched upon binding to TiO 2 . All injection traces could be fitted with bi-exponential decays on a measured timescale of 130 ps. Plots of the fast or slow component of the electron injection time constants probed at 670 nm vs the spacer length yielded a very low value for the attenuation factor ( ∼ 0.04 Å -1 ). Although the dependence was weak, the length of the linker clearly resulted in slower injection rates. The weak and monotonic linker length dependence had not been observed with other rigid-rod models, demonstrating the importance of anchoring groups in the design of model compounds for electron transfer studies at the semiconductor interfaces.Ru(II)-polypyridine complexes are among the most efficient photosensitizing dyes that have been employed in nanocrystalline TiO 2 solar cells (Grätzel cells or DSSCs). 1 Photoexcitation of the Ru dyes, which are covalently bound to the TiO 2 nanoparticles surface through anchoring groups, is followed by electron injection into the conduction band of the semiconductor. The electron injection is usually an ultrafast process, complete within a few tenths of femtoseconds. 2 Because of the potential applications in the field of solar energy conversion and the fundamental interest in interfacial electron-transfer processes, the charge injection step is investigated by many groups worldwide. 2,3 To date, however, there are only a few experimental studies addressing the influence of the spacer length on the rates of electron transfer at dye-semiconductor nanoparticles interfaces. 4,5 To study this effect, we developed a variety of rigid linkers for binding dyes to the semiconductor. 5-7 Here we report the femtosecond laser spectroscopy study of electron injection for three rigid-rod Ru sensitizers with linkers varying in length, 1, 2, and 3 in Figure 1, 7,8 bound to TiO 2 thin films. Each linker is made of an oligo(phenyleneethynylene) (OPE) bridge that ends with an isophthalic acid (Ipa) unit. The two anchoring COOH groups on the Ipa unit ensure fewer degrees of freedom and surface orientations, when compared to structurally similar model dyes with one COOH group that have been described by others. 4c However, the angle at which 1-3 bind to the surface is not known, and for this reason in this study, we refer to the length of the linker (C to N) rather than to a dye-to-surface distance. The FT-IR-ATR spectra of the rods bound to TiO 2 displayed a broad carboxylate band at ∼1600 cm -1 , which is most consistent w...

Grafting Copper and Gallium Corroles onto Zinc Oxide Nanoparticles

“…This synthetic pathway provides an effective monodentate anchoring group and increases the distance of the corrole macrocycle from the metal surface. It has been already proven that the length of the linker results in slower injection rates and that the presence of a cyanoacrylic acid moiety enhances the substrate absorption into thin films if compared with the only carboxylic acid counterpart …”

“…It has been already proven that the length of the linker results in slower injection rates and that the presence of a cyanoacrylic acid moiety enhances the substrate absorption into thin films if compared with the only carboxylic acid counterpart. [23][24][25] To test the effectiveness of the binding event, two different corrole-coated ZnO nanoparticles were prepared. The hybrid materials were achieved with a one-pot technique, where the corrole was directly added to the precursor solution for the hydrothermal growth of ZnO nanorods.…”

Modular Synthesis of Elongated Phosphonate Bipyridines

“…In a recently published article on hole injection dynamics of ruthenium polypyridyl complexes into NiO, it was suggested, that the -I effect of the carboxylic anchoring groups (without linkers between chromophore and NiO) might hinder hole injection. [26,27] They suggest that direct contacts of the Ru complex in the sponge-like environment of the mesoporous TiO 2 films may act as "short-circuits", highlighting the importance of anchoring group design. [20] It is noteworthy that phosphonic esters can also be used to modify TiO 2 or other inorganic substrates.…”

“…Concerning charge carrier transport there was only weak linker-length dependency. [26,27] They suggest that direct contacts of the Ru complex in the sponge-like environment of the mesoporous TiO 2 films may act as "short-circuits", highlighting the importance of anchoring group design. However, structure-property relationship studies for Ru-[(N ∧ N) 2 (C ∧ N)]+ derivatives, where N ∧ N represents 2,2Ј-bipyridine, and C ∧ N represents bidentate phenylpyridine derivatives with carboxylate anchoring groups, were performed by Wu et al for their application on NiO in p-type DSSCs.…”

Efficient Preparation of Photoswitchable Dithienylethene‐Linker‐Conjugates by Palladium‐Catalyzed Coupling Reactions of Terminal Alkynes with Thienyl Chlorides and Other Aryl Halides

“…[1][2][3][4] A detailed understanding of the elementary processes on the surfaces, and their photoinduced dynamics requires a rational molecular design of model compounds. [5][6][7] It has been demonstrated that many properties of molecular adsorbate systems already depend upon the surface-immobilization strategy. [4,8] For example, the formation of aggregates strongly depends upon the orientation of single dye molecules and their lateral interaction on the metal or semiconductor surface.…”