Direct Interfacial Electron Transfer from High-Potential Porphyrins into Semiconductor Surfaces: A Comparison of Linkers and Anchoring Groups

Abstract

This study probes a series of linkers and anchoring groups for direct interfacial electron transfer (IET) from high-potential porphyrins into semiconductor surfaces. Eight different linker–anchor combinations of CF3-substituted, high-potential porphyrins were designed, synthesized, and characterized. Specifically, a series of four anchors was examined (carboxylate, hydroxamate, phosphonate, and silatrane), along with two different linkers (phenylene and benzanilidylene), which differ in terms of their electronic conjugation and overall length. The electrochemical and photophysical properties of the porphyrins were evaluated by steady-state and transient spectroscopies in solution and on mesoporous SnO2 substrates for use as dye photosensitizers in aqueous photoelectrochemical cells. IET dynamics were measured using time-resolved terahertz (TRTS) and transient absorption spectroscopies. From TRTS measurements, injection yields were determined relative to a commonly used phosphonated ruthenium­(II) polypyridyl complex, which is reported to have near quantitative injection yield. We find that IET occurs through space rather than through the linkers, due to the tilted orientation of the adsorbed porphyrins in direct contact with the metal oxide surface. As a result, the anchoring groups have a less significant effect on IET dynamics than for adsorbates exhibiting through-linker injection. Experiments are supported by DFT calculations, including the analysis of different electron-injection pathways. Direct IET offers the advantage of the selection of anchoring groups based solely on chemical/photoelectrochemical stability and synthetic viability, irrespective of the electronic coupling of the anchoring group to the metal oxide surface.

Chemistry on the electrodes: post-functionalization and stability enhancement of anchored dyes on mesoporous metal oxide photoelectrochemical cells with copper-free Huisgen cycloaddition reaction

“…9 Immobilization via phosphonic acid derivatives is more robust, but still suffers from long-term instability. 10,11 Some recent studies present results, which show that alkoxysilane [12][13][14] and hydroxamic acid 15,16 derivatives may provide even higher stabilities than phosphonate linkages. However, for practical utility, a photoelectrochemical device for DSPECs or DSSCs has to be stable during very long periods of operation, even in aqueous solutions, and over an extended pH range.…”

A conductive metal–organic framework photoanode

“…71 As a result, OPTP has been widely applied to study dye-sensitized metal oxide systems. 15,32,[72][73][74][75] As indicated by our theoretical investigations, the carboxylate-bearing TTFTB ligand is energetically poised to photoinject electrons into TiO 2 . Therefore, TTFTB-sensitized samples were studied alongside Zn 2 TTFTB-TiO 2 samples to compare how injection and/or recombination dynamics differ in the presence of an extended structure of overlapped chromophores, such as in Zn 2 TTFTB.…”

“…26,[28][29][30][31] Another strategy is to focus on the localization of the hole within the photosensitizer component of the system. For example, recombination dynamics have been altered in molecular photosensitizers by modifying the anchoring group, 32,33 the linker conjugation, 34 or the linker length. 35 However, such systems can be difficult to synthesize and tune, especially when an additional level of complexity is added by the requirement of interfacing the photosensitizer with a catalyst to perform useful redox reactions.…”

Electronic structure and optical properties of isolated and TiO2‐grafted free base porphyrins for water oxidation: A challenging test case for DFT and TD‐DFT

“…Over the last decades, theoretical and computational modeling, essentially based on density functional theory (DFT) and time‐dependent DFT (TD‐DFT), has successfully assisted the experimental research in boosting the DSSCs technology, by providing accurate material's optical and redox properties prediction, atomistic insights into the surface sensitization mechanism, as well as reliable models for the simulation of interfacial injection and recombination processes . Concerning the spectroscopic properties of the fully organic dyes and the electronic structure of the related dye‐sensitized substrates, DFT and TD‐DFT methods have been shown to provide a noncoherent description, requiring the use of long‐range corrected functionals or hybrids with 40–50% of non‐local exchange for the accurate prediction of the optical properties and hybrids with 20–25% of non‐local exchange (B3LYP, PBE0, etc.)…”

“…to deliver a physical and quantitative energy levels lining up at the dye/semiconductor interface . Due to their general lower performances in DSSCs, less attention has been paid to free bases porphyrins and to the systematic evaluation of the performances of hybrid functionals in predicting the optical and redox properties of the stand‐alone and surface‐grafted sensitizers.…”