A Bioinspired Strategy for Directional Charge Propagation in Photoelectrochemical Devices Using Supramolecular Machinery

Molecular photoelectrochemical (PEC) devices are hampered by electron–hole recombination after photoinduced electron transfer (PET), causing losses in power conversion eciency (PCE). Inspired by natural photosynthesis, we demonstrate the use of molecular machinery as a strategy to inhibit recombination, through organization of molecular components and unbinding of the nal electron acceptor after reduction. We show that preorganization of the macrocyclic 3-NDI-ring electron acceptor to the P STATION dye forming the P STATION :3-NDI-ring pseudorotaxane, enables a “ring launching” event, upon PET from P STATION to 3-NDI-ring releasing 3-NDI-ring •− . Implementing P STATION :3-NDI-ring into p-type dye-sensitized solar cells (p-DSSCs) revealed a vefold increase in PCE compared to benchmark dye P1, unable to facilitate pseudorotaxane formation. This active repulsion of anionic 3-NDI-ring •− with concomitant reformation P STATION :3-NDI-ring circumvents recombination at semiconductor–dye interface, affording a twofold enhancement in hole lifetime. We envision this concept of supramolecular-directed charge-propagation will encourage further integration of molecular machinery into PEC devices.


Introduction
Arti cial photosynthesis aims to create photoelectrochemical (PEC) devices for the conversion of solar energy into fuels, using the natural photosynthetic process as a blueprint. 1 One of the challenges in PEC devices is e cient charge separation with concomitant suppression of competing charge recombination, 2 required for the generation of both photocurrent and redox potential to drive energetically uphill, fuel-forming reactions. The natural photosynthetic apparatus promotes effective charge separation through the organization of pigments and electron acceptors in speci c geometries via supramolecular interactions (Fig. 1a). Photosystem II (PSII) uses the plastoquinone/hydroquinone (Q B /QH 2 , Fig. 1a) redox couple to spatially remove electrons after photoinduced charge separation at the reaction center. The terminal electron accepting Q B is hydrogen bound within the PSII protein, close to the plastoquinone A (Q A , Fig. 1a). 3,4 After two consecutive proton coupled electron transfer events, Q B is reduced to hydroquinone QH 2 , and the a nity for the binding pocket of PSII is lost. 5 The liberated QH 2 diffuses away to participate in subsequent redox chemistry (at Cytochrome b 6 f, Fig. 1a), and the PSII binding pocket is occupied by another Q B for the subsequent photocycle. Mimicking this photoinduced supramolecular control of docking-and-release events of a redox mediator may represent a viable strategy for reducing charge recombination improving power conversion e ciencies (PCEs) in PEC devices.
Dye-sensitized solar cells (DSSCs) are PEC devices with operational principles that parallel natural photosynthesis; a) light absorption is achieved by molecular components (Fig. 1c) DSSC (p-DSSC) are typically 1-2 orders of magnitude lower. This disparity precludes e ciency improvements in tandem DSSCs and arti cial photosynthetic PEC devices. The origins of PCE differences between nand p-DSSCs is attributed to the charge carrier characteristics of the semiconductor, with very slow (4 × 10 − 8 cm 2 s − 1 ) 7 charge (hole) diffusion in NiO compared to that in TiO 2 (electron diffusion 10 − 4 cm 2 s − 1 ). 8 As a result of this slow hole transport, charge recombination at the semiconductor-electrolyte interface is a much larger issue in NiO-based p-DSSC (Fig. 1c, Pathway 5 & 6). 9,10 The natural photosystem circumvents undesirable recombination pathways by preorganizing the redox mediator Q B at the electron-accepting docking site, followed by unbinding of the reduction product QH 2 , effectively separating the charges spatially. In contrast, the DSSC relies on collisional electron transfer under diffusional control, making the restrictive process mass transfer of the reduced redox mediator species away from the semiconductor surface to the counter electrode for regeneration. Former studies imply that dye-mediator interactions between dye and redox mediator could have a favorable effect for the overall PCE of both nand p-DSSC. [11][12][13][14] The stimulated binding and unbinding events found in the natural photosystem are of central importance in the eld of arti cial molecular machines. These include extraordinary examples of functional architectures including molecular pumps 15,16 , propellors, 17,18 robotic arms 19 , molecular muscles 20,21 and a nanocar. 22 Essential for the function of these molecular machines is the reversible bond, whose dynamic nature allows for molecular motion upon a chemical, electrochemical or photochemical stimulus. 23,24 An example relevant to the work at hand is the photoelectrochemical trigger that leads to the reduced a nity of a macrocycle for a binding site in (pseudo)rotaxane structures, resulting in molecular ring launching or shuttling events. 25,26 The question that we address in this paper is if pseudorotaxane motifs can be used as molecular machinery, engendering the preorganization and launching of redox mediators in a p-DSSC, effectively emulating the docking and active replacement of the electron carrying Q B /QH 2 redox couple in PSII. For this we utilize macrocyclic redox mediator (3-NDI-ring) that threads onto the dye P STATION to form the P STATION :3-NDI-ring pseudorotaxane (Fig. 1b). Directional electron transport in the PEC device is established by the inbuilt free-energy impetus that actively shuttles reduced redox mediator 3-NDI-ring •− away from the thread, promoting movement away from the semiconductor-dye interface. Finally, the thread favorably binds the next neutral 3-NDI-ring molecule to reform the pseudorotaxane. Application of this concept in a p-DSSC results in PCE increases by a factor 5, attributable to reduced interfacial charge recombination phenomena (Fig. 1b-c).

Results And Discussion
Design and synthesis. The p-DSSCs in this study are based on the well documented P1 dye (Fig. 1e), and as such, the design of the molecular machinery started with this molecular scaffold. The dye P STATION is an analogue of P1 where the terminal (dicyano)vinyl electron acceptors are replaced with cyanoacrylate esters to facilitate introduction of a glycol-tethered 1,5-dioxynaphthalene (DNP). The DNP unit acts as binding station for electron-de cient molecular rings through the formation of pseudorotaxane suprastructures. The naphthalene diimide-based macrocycle 3-NDI-ring ( Fig. 1e) binds to the DNP recognition sites of P STATION , and was designed in an analogous fashion to NDI-based macrocycles previously reported to form pseudorotaxanes with DNP recognition sites at the surface-liquid interface. 27,28 As the 3-NDI-ring functions as redox mediator in the envisioned p-DSSC, its redox properties are of key importance, and these compare favorable to those of the typically used I − /I 3 − (vide infra). Thus the proposed 3-NDI-ring:P STATION pseudorotaxane photosensitizer (Fig. 1e) is anticipated to improve the PCE of the DSSC device in two ways. 14 Firstly, the 3-NDI-ring as redox mediator is preorganized close to the dye by the DNP recognition sites of P STATION at the surface-electrolyte interface, favoring charge propagation (Fig. 1b, Step 3) over recombination (Fig. 1c, Pathway 6). Secondly, upon photoexcitation ( Fig. 1b, Step 1) and subsequent hole injection into NiO (Fig. 1b, Step 2), the resulting P STATION *− :3-NDIring species transfers an electron to the 3-NDI-ring within the pseudorotaxane (Fig. 1b, Step 3), yielding P STATION :3-NDI-ring •− . Upon reduction the 3-NDI-ring •− loses its a nity for the thread of P STATION and is replaced by a neutral 3-NDI-ring from the bulk electrolyte. The reduced 3-NDI-ring •− is thus actively repelled from the NiO-dye interface (launching effect (2) Binding of 3-NDI-ring to the DNP recognition site within P STATION were prohibited by limited solubility of the dye, therefore the recognition site moiety DNP-thread (Fig. 2d) was used to analyze pseudorotaxane formation by 1 H NMR titration. A typical up eld shift (0.5 ppm) in the 1 H NMR spectra for the aromatic protons of the 3-NDI-ring in CD 2 Cl 2 was observed ( Supplementary Fig. 6). 27 (Fig. 2b). 30 The K a ascertained from 1 H NMR was complemented by spectrophotometry by probing the formation of the DNP-thread:3-NDI-ring complex by UV-Vis titration (Fig. 2d). The spectral overlap of the P STATION ICT absorption (λ = 455 nm) precluded observation of the CT evolving from pseudorotaxane formation (λ = 460 nm) over the course of the titration.
Monitoring the absorption at λ = 460 nm UV-vis at 460 nm and tting to a 1:1 binding model afforded a K a = 160 M − 1 for the DNP-thread:3-NDI-ring pseudorotaxane (Fig. 2c, Table 1). The differences between K a derived from NMR and UV-Vis were rationalized by the difference in solvent polarity (CD 2 Cl 2 vs. valeronitrile/MeCN (15:85) respectively). Immobilization of P STATION onto NiO electrodes (vide infra, Supplementary Fig. 13) and immersion into a 3-NDI-ring solution (20 µM in MeCN) led to a decrease in 3-NDI-ring absorption intensity at λ = 378 nm. Given that the control experiment with P1 in place of P STATION ( Supplementary Fig. 14) experienced no absorption drop at 378 nm, we could ascribe the absorption decreases to the binding of 3-NDI-ring to the DNP recognition sites of the P STATION -NiO.
Cyclic voltammetry (CV, Supplementary Fig. 8) of 3-NDI-ring revealed four reductions, attributed to two independent reduction events at the NDI and two at the pyromellitic moieties of the 3-NDI-ring. The redox events were fully reversible, demonstrating electrochemical stability, which is an important requirement for redox mediators in DSSCs. The rst reduction potential of the 3-NDI-ring (-0.35 V vs. NHE) is 0.55 V lower than that of P STATION * − (Fig. 1d), facilitating exergonic electron transfer from P STATION * − to 3-NDIring. CV of the DNP-thread:3-NDI-ring complex shows that binding of the DNP-thread to the 3-NDI-ring has a small effect on the reduction potential (40 mV). Importantly, scan rate dependent CV experiments demonstrate that reduction of 3-NDI-ring in the model DNP-thread:3-NDI-ring prompts a loss of a nity and unbinding from the DNP-thread ( Supplementary Fig. 9). 31 This "ring launching" effect re ected by the 40 mV reduction potential decrease, is only observed for the rst reduction event of the 3-NDI-ring when bound to the DNP-thread (-0.39 V vs. NHE compared to -0.35 V vs. NHE for the free 3-NDI-ring (Supplementary Table 2). The absence of this typical shift in the three subsequent reduction events show that the mono-reduced ring 3-NDI-ring •− unbinds DNP-thread after the rst reduction.   Chopped light amperometry experiments were performed, where the light is switched on and off in periods of 10 seconds with an increasing illumination density starting from 5 mW cm − 2 to 50 mW cm − 2 at short current conditions. For both solar cells the J SC increases with the light intensity, as expected for these DSSCs (Fig. 3c), and the larger increase of the P STATION :3-NDI-ring based cell is in line with the better performance. Interestingly, the shape of the photocurrent response for the P1 shows tailing behavior that increases with light intensity. This indicates mass transfer limitations of the redox mediator through the mesoporous electrode (Fig. 3c inset), which can be expected for large molecules like 3-NDI-ring at low concentrations. 32,33 This tailing behavior is not observed for P STATION based DSSCs in line with preorganization of the redox mediator and e cient replacement of reduced 3-NDI-ring •− for neutral 3-NDIring 0 , leading to high local concentrations of 3-NDI-ring at the dye-electrolyte interface even at very low (25 mM) concentrations of redox mediator.
Differences in solar cell performance originating from pseudorotaxane formation were further probed by electrochemical impedance spectroscopy (EIS). Performing EIS under varying light intensities affords insight into electron-hole recombination at the semiconductor-dye interface through determination of the hole lifetime (τ h ) as a function of V OC (Fig. 3d). 34 At any given V OC the hole lifetime for P STATION (624 ms at 0.1 V) is two times longer than P1 (324 ms at 0.1 V), implying that less recombination occurs in the pseudorotaxane system. This could either arise from a difference in recombination resistance (R REC ) or from a change in chemical capacitance (C µ ), originating from a valence band shift. 35 The C µ ( Supplementary Fig. 25) shows no dependency on the applied voltage and a minimal shift between the P1 and P STATION DSSCs, expected given the similarity of the systems, thus cannot be the reason for V OC enhancements in P STATION :3-NDI-ring p-DSSCs. The measured R REC for the P STATION system (3.20 × 10 5 Ω cm − 2 at 0.1 V) is higher than for P1 (2.93 × 10 5 Ω cm − 2 at 0.1 V) meaning that the difference in hole lifetime originates from lower recombination at the semiconductor-electrolyte interface. This effect further supports the active charge recti cation bestowed P STATION :3-NDI-ring p-DSSCs by introducing molecular machinery to the in uence the preorganization and replacement of the redox mediator in the solar cell.

Conclusion
Charge recombination is one of the key issues to solve in the area of p-DSSCs, and inspired by binding/unbinding events of redox mediators in natural photosynthesis, we studied if molecular machinery can be implemented in p-DSSC to facilitate directional electron transport to reduce charge recombination. The P STATION dye based on the P1 benchmark system is equipped with a docking station for preorganization of ring-shaped redox mediator (3-NDI-ring) that form pseudorotaxanes. Reduction of 3-NDI-ring by electron transfer in P STATION * − :3-NDI-ring pseudorotaxane prompts disassembly of the supramolecular complex resulting in ring launching of the reduced mediator, making space for a new neutral redox mediator to bind. The p-DSSCs based on P STATION :3-NDI-ring pseudorotaxanes exhibit enhanced performance across all photovoltaic parameters in comparison to the P1, which does not facilitate preorganization of the 3-NDI-ring mediator. Chopped light amperometry and EIS under varying light intensities showed that both preorganization and ring launching contributes to lowering recombination and a twofold extension to hole lifetimes, leading to a higher V OC and 5 times increase in PCE in p-DSSC. We envision that this bio-inspired approach to integrate arti cial molecular machinery in p-DSSCs for supramolecular charge-transfer recti cation is a strategy that could be expanded to other PEC devices for solar energy conversion technologies.