Photoelectrochemical processes are investigated for chromoionophore I (ETH 5294) dissolved in 4-(3-phenylpropyl)pyridine (PPP) and deposited in the form of a microdroplet array (through evaporation deposition from a solution in acetonitrile) onto a 5 mm diameter basal plane pyrolytic graphite (BPPG) electrode. Stable biphasic dark voltammetric responses (two electron/two proton) are observed in phosphate buffer solution (from pH 2 to 12) with a switch in reactivity at pH 5 due to a biphasic protonation step. The photoelectrochemical activity at pH 2 is investigated further by phototransient amperometry. The protonated chromoionophore I is shown to be the photoactive component (supported by EPR data) and “hole transfer” at the liquid|liquid interface to aqueous oxalate is demonstrated. This interfacial hole-transfer process can be “switched off” by hydrophobic anions (PF6−), which compete for cationic liquid|liquid PPP surface binding sites. Implications for light harvesting and liquid semiconductor properties are discussed.
Chromoionphore I (or ETH 5294; Figure 1 A) has been used as a transmissive or fluorescent probe molecule in many types of hydrophobic sensor membranes, for example, for sensing H+,1, 2, 3 Li+,4 K+,5 nitrite,6 surfactants,7 or HgII.8 Chromoionophore I is usually employed in polyvinyl chloride (PVC) membranes, but has also been shown to respond to acids in ionic liquids.9 This versatile range of sensing applications is based on the optical response to proton or ion transfer, for example, for HgII detection. Corresponding detection methods based on microspheres10, 11 and micro-optical sensors12 have been developed. Chromoionophore I has been used in optode membranes for pH,13 and in optical wave-guide bio-sensor applications,14 for example, for urea determination. Sol–gel sensor structures with chromoionophore I15 have also been demonstrated. Chromoionophore I diffusion has been investigated by potential step16 and potentiometry methods17 at membranes. However, electrochemical or photoelectrochemical properties of chromoionophore I have previously not been reported. It is shown herein that chromoionophore I exhibits a well-defined biphasic redox transition and protonation (corresponding to the well-known optical transition used in sensing) with a shift in reversible potential, which indicates the biphasic protonation process.18 Furthermore, chromoionophore I dissolved in 4-(3-phenylpropyl)pyridine (PPP) is photoelectrochemically active. In the excited state, charge separation and liquid|liquid interfacial hole quenching by aqueous oxalate is reported.
Work on both triple phase boundary ion-transfer processes19, 20 and photoelectrochemical processes21, 22, 23 started with phenylenediamine derivatives. In previous reports, photoelectrochemical processes in microdroplet media (or under triple phase boundary conditions24) occurred within the organic phase (coupled to interfacial ion transfer) through photocomproportionation25 or with added duroquinol reagents26 (see Figure 1 B). However, to extract redox energy from photoelectrochemical processes, it is necessary for “hole transfer” to happen across the liquid|liquid phase boundary (see Figure 1 C). Herein, we provide initial evidence for liquid|liquid hole transfer from a photoexcited state within the oil phase to the aqueous oxalate quencher anion. This reaction is proposed to occur within the oil–aqueous interfacial double layer. This process is suppressed by interfacial cation site blocking, for example, by the addition of hydrophobic PF6− into the aqueous phase. Implications for future photoelectrochemistry and energy harvesting processes at “soft interfaces” are discussed.
2. Results and Discussion
Dark Voltammetry of Chromoionophore I in PPP
Chromoionophore I is a Nile Blue type dye that is made hydrophobic by an amide group with a hydrophobic tail (see Figure 1 A). The electrochemistry of amide-derivatised Nile Blues has been reported27 and the general mechanism for Nile Blue (a phenoxazine mediator) reduction has been shown to follow a two-electron pathway.28, 29 Chromoionophore I dissolved in PPP and immobilised on basal plane pyrolytic graphite (BPPG) in the form of microdroplets shows similar electrochemical characteristics when immersed into aqueous buffer solution. Figure 2 A shows typical voltammograms obtained at different scan rates in 0.5 M phosphate buffer at pH 2. The reduction at 0.1 V versus a saturated calomel electrode (SCE) is associated with a corresponding oxidation and indicates overall chemical reversibility. The voltammetric signal is stable over prolonged potential cycling due to insignificant losses of the hydrophobic dye into the aqueous phase.
When investigated over a range of scan rates, a linear dependence of the peak current versus the scan rate is observed (Figure 2 B); this is indicative of rapid transport within small microdroplets. The effect of pH on the chemically reversible reduction of chromoionophore I was investigated next. A shift in the voltammetric response due to the involvement of protons in reduction was observed as well as a more subtle and complicated change in peak shape (see Figure 3 A).
A plot of the charge under the reduction or oxidation peak versus pH, or a plot of the corresponding number of electrons transferred per chromoionophore I molecule immobilised at the electrode surface (see Figure 3 B), is consistent with a two-electron mechanism over the entire pH range studied herein. Therefore, mechanism A (acidic range, see Figure 3 C) can be described by Scheme 1, in which the protonated form of the oxidised dye is reduced to the protonated leuco form of chromoionophore I. In contrast, under more alkaline conditions, mechanism B (basic range, see Figure 3 C) is operative, as shown in Scheme 2.
The transition from mechanism A to B occurs at approximately pH 5 (consistent with the colour change exploited in optical sensor applications, see above) in phosphate buffer, but the transition point is sensitive to the presence of more hydrophobic anions (similar to processes in optical sensor membranes12 and related biphasic protonation mechanisms for ferrocene redox systems18). The additional broadening of both reduction and oxidation peaks and the widening of the peak-to-peak separation at more alkaline pH have been attributed to a change at the PPP–water liquid|liquid interface, as described previously for the pentoxyresorufin dye system.26 Additional variability in the shape and magnitude of voltammetric responses (see Figure 3 B) is mainly due to the random nature of microdroplet formation by solvent evaporation.
Photovoltammetry of Chromoionophore I in PPP
Photoelectrochemical responses were studied initially with pulsed light from a halogen lamp (see the Experimental Section) applied during cyclic voltammetry. Figure 4 A shows typical data. The black curve in Figure 4 was obtained without pulsed light and the voltammogram in red shows the effect of light pulses. Photoresponses are weak, but occur at potentials positive of 0.1 V versus SCE. That is, the oxidised form of the chromoionophore I dye is photochemically active and the reduced leuco-chromoionophore I (with a filled LUMO) is photochemically inactive. The experiment was repeated after the addition of 10 mM KPF6 into the aqueous electrolyte to explore the effect of hydrophobic anions (not shown). Only insignificant changes in both the shape of the cyclic voltammogram and the photoresponses were observed (a decrease in the photoresponse occurs, see below). Therefore, anion transfer at the liquid|liquid interface does not appear to be important as part of both the overall dark or photoelectrochemical mechanisms. Additional survey experiments at pH 7 and 12 (not shown) revealed that the most clear photocurrent responses were observed at pH 2, which was therefore selected for further study.
Next, photocurrent transients were recorded in chronoamperometry mode at a fixed potential of 0.4 V versus SCE. Figure 4 B and C shows photocurrent transients reminiscent of those usually seen at semiconductor electrodes.30 The light-on transient is linked to a peak followed by current decay into a photostationary current signal (anodic). The light-off transient shows a cathodic peak followed by return to equilibrium current conditions. The peak transients are likely to be associated with charge-carrier mobility effects and the development of concentration gradients in the microdroplet phase (rather than charging of a static space-charge region as seen in solid bulk semiconductors30, 31). The initial peak in the light-on transient can be interpreted tentatively in terms of holes reaching the electrode surface first and causing an anodic peak30 consistent with a faster apparent “hole” diffusion relative to “electron” diffusion (Dhole>Delectron). The decay of current towards the photostationary state would then be linked to the establishment of electron and hole concentration gradients within microdroplets. Contributions to these apparent diffusion rates may arise here from “hole hopping”, for example, through collisions of chromoionophore I or PPP solvent molecules. The extent of the “active” reaction zone in microdroplets is currently not known. Also, the molecular mechanism and products for these photoprocesses in the absence of quenchers, such as oxalate (see below), are currently unknown.
The concentration of the chromoionophore dye has a strong effect on the photocurrent, as shown in Figure 4 D. An almost linear increase is observed, which is indicative of a direct link of Beer–Lambert light absorption by the dye in the microdroplet to the resulting phototransient responses. Further evidence for the nature of the photoexcited species is obtained from “action spectrum” analysis. Figure 5 A shows UV/Vis absorption data for chromoionophore I in non-protonated and protonated PPP. These spectral data are in good agreement with UV/Vis data reported previously in other organic solvents or membrane systems.8
Action spectra were obtained with a set of LEDs with approximately equal intensity (calibrated with a photodiode) to cover the range of wavelengths. Figure 5 B shows a plot of the phototransient current response plotted versus the wavelength. A clear maximum is observed at λ≈580 nm, which is in excellent agreement with the absorption band of the protonated chromoionophore I (cf. Figure 5 A). Therefore, this has to be regarded as the main photoactive component under these conditions.
Additional evidence for the presence of long-lived photoexcited intermediates comes from EPR measurements. If a halogen light (see the Experimental Section) is switched on, a clear signal for a paramagnetic species is observed. Time transients for light-on/light-off transients (not shown) clearly link the signal to the presence of light. To better resolve spectral features, the temperature was increased to 313 K (see Figure 6 B), but only two hyperfine coupling constants estimated as 8 (N) and 2 G (H) were revealed. The chemical nature of the radical species is therefore currently not fully resolved.
Photovoltammetry of Chromoionophore I in PPP Immersed in Aqueous Oxalate
Although evidence for the photoelectrochemical chromoionophore I process is strong, there is so far no overall mechanism to identify the products of the photoanodic process. To clearly associate a hole-quenching process at the liquid|liquid interface with the photoprocess, further experiments were performed in the presence of oxalate anions, which are well-known sacrificial hole quenchers, in the aqueous phase.32 In acidic media, the process involves the irreversible formation of CO2. Oxalic acid has pKA values of 4.19 and 1.23 at 25 °C,33 and therefore, exists predominantly as the mono-anion in aqueous phosphate buffer at pH 2.
Typical photocurrent transients without and with 4 mM oxalate are shown in Figure 7 A and B, respectively. A significant increase in the photocurrent in the presence of oxalate suggests effective hole quenching at the liquid|liquid interface. Photocurrents increase significantly with oxalate concentration and then plateau at an aqueous oxalate concentration of approximately 40 mM. A step in the data at about 4 mM appears reproducibly and may be associated with localised CO2 gas-bubble formation at the triple phase boundary (see Figure 7 C). Most strikingly, the presence of a competing hydrophobic anion, such as PF6−, causes a complete loss of photoactivity (see Figure 7 C). This phenomenon could be interpreted as competition for liquid|liquid interfacial binding sites (protonated PPP), at which hydrophobic PF6− blocks the interaction of hydrophilic oxalate with the positively charged PPP oil–water interface26 and thereby stops the interfacial hole-quenching process. In addition, PF6− could cause depolarisation of the liquid|liquid interface. Therefore, efficient photocurrents appear to be strongly dependent on the binding interaction of the hole quencher with the interface. In future, appropriate molecular hole-quenching systems need to be developed to not only dissipate the energy (as with oxalate), but to transport redox energy across the aqueous phase into a suitable counter/collector electrode.
Chromoionophore I voltammetry in the dark and under biphasic (microdroplet) conditions was reported and the pH dependence revealed similar “switch” behaviour to that exploited in optical membrane sensing systems. Hydrophobic chromoionophore I provided a highly stable and reproducible two-electron/two-proton redox system in the oil phase with a midpoint potential indicative of the aqueous solution pH.
Chromoionophore I was also photoelectrochemically active in PPP–aqueous biphasic environments. Weak, but clear, photoresponses were observed and shown to be associated with the protonated chromoionophore I. In the presence of oxalate hole quencher in the aqueous phase, clear evidence for interfacial hole transfer was obtained and new insights into the conditions during the hole transfer were obtained. The role of a competing hydrophobic anion, herein PF6−, in particular, showed that the interaction of the quencher with the liquid|liquid interface was crucial for energy harvesting. Important questions such as those concerning the type of species involved in the photoprocess and the concentration profiles for “electrons” and “holes” in microdroplets, as well as liquid|liquid interfacial reactivity,34 remain open. Further comparison with literature reports on known photoprocesses in polymer structures35 and in novel graphene-based materials36 is necessary. In future, liquid|liquid interface processes mimicking biological photosynthesis could be experimentally accessible through triple phase boundary photovoltammetry and a wider range of systems, including cases of interfacial hydrogen and oxygen formation, will be studied or screened to reveal the kinetic and thermodynamic features that are important in these light-energy harvesting processes.
Chemicals and Reagents
Chromoionophore I (Aldrich), PPP (Aldrich, 97 %), acetonitrile (Aldrich, Reagent Plus), sodium hydroxide (Aldrich,>97 %), orthophosphoric acid (Fischer Scientific, 85 wt %), sodium hexafluorophosphate (Aldrich, 98 %), oxalic acid (Aldrich, 98 %) and perchloric acid (Aldrich, 70 %) were obtained commercially and used without further purification. A Thermo Scientific purification device was used to provide filtered, demineralised water with a resistivity of >18.2 MΩ cm. The laboratory temperature was (20±2) °C.
A μAUTOLAB Type II potentiostat (Metrohm, Netherlands) was used for all voltammetric measurements. Photoelectrochemical action spectra were obtained with a set of LEDs (Thorlabs, UK) and an Ivium Technologies Compactstat potentiostat. A standard three-electrode electrochemical glass cell was used with a flat-bottomed 100 cm3 flask to allow light to be directed up through the cell onto the electrode. The working electrode used was a BPPG electrode with a diameter of 5 mm, mounted in a Teflon sheath, coated with oil microdroplets, and submerged into the aqueous phosphate buffer solution. The counter electrode was a platinum wire and the reference electrode was an SCE electrode. The phosphate buffer pH was measured with a JENWAY 3505 pH meter.
The light source used for photochemical investigations was a Fiber-Lite high-intensity halogen bulb (MI-150; Dollan-Jenner Industries), with an intensity of approximately 16.5 mW cm−2 at the electrode surface. Light intensity measurements were conducted with a Thorlabs optical power meter (PM100A). Chopping of the light was conducted manually. For variable wavelength experiments, the following Thorlabs mounted high power LED light sources were used: M405L2, M455L2, M530L2, M590L2, M625L2, M660L3, M735L3. LED sources were maintained at a constant intensity of approximately 50 mW cm−2 and pulsed by using a Function/Arbitrary generator TG4001. UV/Vis spectroscopy was conducted by using a PerkinElmer Lambda 40 spectrometer in 1 mm path length quartz cells. EPR spectra were measured on a Bruker EMXmicro spectrometer (X band) with nitrogen gas flow temperature control and sample illumination with a Fiber-Lite high-intensity halogen bulb (MI-150).
Electrode Preparation and Photoelectrochemical Measurements
The chromoionophore I/PPP microdroplet array on a BPPG electrode was achieved by deposition of a stock solution [10 cm3 acetonitrile, 80 mg PPP, and the required mass of chromoionophore I, plus 10 μL HClO4 (70 wt %)] and evaporation. Typically, an aliquot (5 μL) of this solution was deposited by pipette onto the electrode surface to provide a 40 nL volume final deposit of randomly arranged microdroplets. The electrode was cleaned after each set of experiments by rinsing with acetonitrile and re-polishing with fine silicon carbide paper.
S.A. thanks HEC Pakistan for financial support. F.M. thanks the EPSRC and the NSF for supporting the project “Microphase Photo-Electrochemistry: Light Driven Liquid–Liquid Ion Transfer Processes and Two-Phase Micro-Photovoltaic Systems” (EP/G002614/1). We thank the EPSRC UK National Electron Paramagnetic Resonance Service at The University of Manchester.