Photoregeneration of Biomimetic Nicotinamide Adenine Dinucleotide

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Photoregeneration of Biomimetic Nicotinamide Adenine Dinucleotide Analogues via a Dye-Sensitized Approach George N. Hargenrader,†,‡,§ Ravindra B. Weerasooriya,†,‡,§ Stefan Ilic,†,‡ Jens Niklas,‡ Oleg G. Poluektov,‡ and Ksenija D. Glusac*,†,‡ †

Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States Chemical Sciences and Engineering, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, United States

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ACS Appl. Energy Mater. Downloaded from pubs.acs.org by 91.200.81.24 on 12/01/18. For personal use only.

S Supporting Information *

ABSTRACT: Two-step photochemical reduction of an acridinium-based cation 2O+ to the corresponding anion 2O− was investigated using a dye-sensitized approach involving 2O+−COOH attached to the surface of a wide-bandgap semiconductor, p-NiO. The cation 2O+ and corresponding one-electron reduced radical form, 2O• were synthesized and characterized using steady-state UV/vis and electron paramagnectic resonance spectroscopy. The thermodynamics for the photoinduced hole injection from 2O+ and 2O• were evaluated and found to be favorable. Subsequent femtosecond transient absorption spectroscopy was utilized to evaluate the photoinduced hole injection into NiO, starting from 2O+−COOH/NiO and 2O•− COOH/NiO samples. The excitation of 2O+−COOH at 620 nm initiated fast (2.8 ps) hole injection into NiO. However, 90% of the charge-separated population recombined within ∼40 ps, while ∼10% of the charge-separated population exhibited lifetimes longer than the time scale of our instrument (1.6 ns). In the case of 2O•−COOH/NiO, the light absorption occurs predominantly by NiO (2O•−COOH absorbs at 310 nm) and is associated with the electron transfer from the conduction band of NiO to the radical. The charge-separated state in this case appears to be long-lived, based on the slow (ns) growth of the trapped carriers formed on the NiO surface. The results of this work indicate that the photochemical reduction of 2O+ to the corresponding hydride form (2OH) can be achieved, opening the possibility of using such a dye-sensitized approach for regeneration of nicotinamide adenine dinucleotide analogues in enzymatic and chemical catalysis. KEYWORDS: photochemistry, NiO, NADH, dye sensitization, stable radical, multiple charge accumulation

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donor from oxidized NAD+ analogues. In natural systems, the regeneration of NADH occurs by a hydride transfer from appropriate substrates, such as glucose, fatty acids, and alcohols. The hydride transfer is catalyzed by dehydrogenases that activate NAD+ by orienting it into a hydride-accepting boat geometry.6 Chemical methods for regeneration of NADH also employ a hydride transfer process from metal hydrides using molecular hydrogen7,8 or formate ions9 as terminal

INTRODUCTION In nature, hydride transfer reactions are often achieved by nicotinamide adenine dinucleotide (NADH) or its phosphate analogue, NADPH. Almost 90% of known oxidoreductases utilize one of these two co-factors to perform substrate reductions with high specificity and stereoselectivity.1 Some of these biocatalytic reactions, such as enantioselective reduction of carbonyl groups in the synthetic pathway for the drug Lipitor, are applied in pharmaceutical industry for synthesis of chiral drug precursors.2 Parallel scientific efforts explore synthetic NADH analogues, such as Hantzsch esters3 and other hydride donors,4,5 as stoichiometric reagents for reductions involving CO, CC, CN, and other bonds.3 The increased demand for NADH and its analogues has led to the need for methods that efficiently regenerate the hydride © XXXX American Chemical Society

Special Issue: New Chemistry to Advance the Quest for Sustainable Solar Fuels Received: September 17, 2018 Accepted: November 9, 2018

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DOI: 10.1021/acsaem.8b01574 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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Scheme 1. (A) Schematic Representation of 2O+−COOH/ NiO Film Samples. (B) Catalytic Cycle Showing Photochemical Reduction of 2O+a

hydride sources. Another approach toward the regeneration of NADH analogues employs a two-electron, proton-coupled reduction of NAD+ analogues, mediated by electrochemical or chemical1 methods. While electrochemical reduction represents the most direct regeneration pathway that can be readily coupled with renewable energy sources, the process is not efficient due to the chemical instability of the one-electron reduced NAD radical and its tendency to dimerize. Photochemical methods were also utilized for the regeneration of NADH analogues. A number of organic and inorganic molecular chromophores were used as light-harvesting elements, such as eosin Y, BODIPY,10 porphyrins, and Ru complexes.1,11 Furthermore, semiconducting light absorbers were employed, including carbon nitride,12 silicon nanowires,13 and nanocrystals of CdSe, CdTe, CdS, and ZnS.14−17 In the absence of an appropriate mediator, photochemical NADH regeneration is hindered by rapid dimerization of the NAD radical formed upon the first photochemical event,1,17 as was observed during electrochemical regeneration. To avoid this, the photochemical regeneration is often conducted in the presence of an electron mediator that accumulates two electrons from the light-harvesting unit and then reduces the NAD+ analogue directly to the NADH form via hydride transfer. The most commonly used electron mediator is a rhodium-based organometallic compound that can undergo a photochemical proton-coupled two-electron reduction to form the Rh−hydride complex.1,10,12,13,16 Other organometallic compounds, based on Ru and Co, were also applied as successful electron mediators.11 Additionally, photochemical NADH recovery was achieved by coupling light-absorbing CdSe nanocrystals with a flavoenzyme, where the flavin cofactor served as an electron mediator and hydride donor.14 Recently, we reported a number of acridinium-based hydride acceptors (NAD+ analogues) that exhibit reversible reduction behavior, indicating that the one-electron reduced radicals are chemically stable.5 In addition, some of the reported NAD+ analogues were shown to exhibit strong absorption in the visible range, making them potential chromophores for light harvesting.18 This preliminary work has prompted us to investigate whether a two-step photochemical reduction of NAD+ analogues can be achieved using a dye-sensitization approach involving NAD+ analogues attached to the surface of a wide-bandgap p-type semiconductor. For this purpose, we report here an investigation of one such NAD+ analogue, namely, 2O+−COOH, attached to the surface of nanostructured p-NiO (Scheme 1). The steady-state and timeresolved spectroscopies were utilized to investigate the thermodynamics and kinetics of the photochemical reduction of 2O+ to the corresponding anion 2O−. It is anticipated that such a photochemical sequence, followed by the protonation of 2O−, can be readily utilized in biomimetic catalytic reduction of relevant substrates, as presented in Scheme 1B. At a more general level, the sequence of photoinduced charge separation events and subsequent charge accumulation at the catalytic site is a signature of most photocatalytic systems and is critical for efficient utilization of solar energy. However, with several notable exceptions,19−22 these sequential photochemical processes are rarely investigated experimentally, due to difficulties associated with the isolation of, often reactive, intermediates involved in the charge accumulation sequence. The results of our study show that the photochemical two-step reduction of 2O+ can be achieved efficiently using NiO as a hole acceptor, demonstrating that,

Two model compounds were used: 2O+−COOH, which contains carboxylate groups for attachment to NiO, and 2O+, where -R = -CH3 (used for solution-based studies). a

once the radical state (2O•) is chemically stabilized, the regeneration of the hydride form is feasible. Our study also revealed two challenges associated with the photochemical sequence, namely, the rapid charge recombination at the dye/ NiO interface and the poor absorption characteristics of 2O•.

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EXPERIMENTAL SECTION

General Method. Absorption spectra were measured with either a Cary 300 (Agilent), DU 800 (Beckman Coulter), or Ocean FX (Ocean Optics) spectrophotometer. 1H and 13C NMR spectra were collected on an Avance 300 MHz system and Avance III 500 MHz instruments (Bruker). The XRD measurements were performed on a Miniflex (II) desktop (Rigaku) X-ray diffractometer. The SEM measurements were performed on a S-2700 SEM (Hitachi) scanning electron microscope. The fluorescence experiments were collected on a FluoroMax 4 (Horiba) spectrofluorometer in a 1 cm quartz cell. Electron impact ionization (EI) and matrix-assisted laser desorption ionization (MALDI) mass spectra were measured on a QP5050A (Shimadzu) and a Daltonics Omniflex (Bruker) mass spectrometer. Synperonic triblock copolymer F-108 and other starting materials were purchased from Sigma-Aldrich and used without further purification. The synthesis of 6O+, 4O+, and 2O+ were reported previously.18 4O+−COOH Synthesis. A solution of 4-aminobenzoic acid (304 mg, 2.22 mmol, 1.4 equiv) and 2,6-lutidine (15 mL, 535 mmol) in 13 mL of acetonitrile was heated to reflux. Then, a solution of 6O+ (800 mg, 1.53 mmol, 1 equiv) was slowly added, and the resulting mixture was refluxed overnight. The reaction mixture was then poured in 15 mL of acidified (50% aqueous HClO4) ice-cooled water, followed by extraction with dichloromethane. Organic layers were combined and dried over anhydrous sodium sulfate, and the solvent was evaporated under reduced vacuum. The crude product was purified using gravitational SiO2 column (CH2Cl2/CH3OH = 9/1, 1% formic acid) to yield 250 mg of red product (28%). MS-MALDI: m/z 496 [M+ calculated for C30H26NO6+]. 1H NMR (CD3CN, 300 MHz): δ 7.98 (dd, 2H), 7.94−7.86 (m, 2H), 7.67−7.57 (m, 2H), 7.50 (t, 1H), 7.10 (d, 2H), 6.93 (d, 2H), 6.84 (d, 2H), 3.64 (s, 6H), 3.59 (s, 6H). 2O+−COOH Synthesis. To a solution of 4O+−COOH (100 mg, 0.17 mmol, 1 equiv) in 1 mL of N-methyl-2-pyrrolidone was added 1 mL of methylamine solution in ethanol (33%, 4.42 mmol, 26 equiv), and the mixture was stirred at 90 °C for 8 h under ambient conditions. The resulting dark green reaction mixture was allowed to cool to room temperature and poured in acidified (50% aqueous HClO4) ice-cooled water. The precipitate was filtered and washed with a copious amount of ether. The crude precipitate was dissolved B


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ACS Applied Energy Materials Scheme 2. Synthetic Pathways for Preparation of 2O+ and 2O+−COOH

(TBAP) were used. Spectroelectrochemical measurements were performed using Pt mesh working electrode (EF-1365, Bioanalytical Systems), nonaqueous Ag/Ag+ reference, and Pt wire as an auxiliary electrode. A solution of 1 mM 2O+ in acetonitrile containing 0.1 M TBAP was purged with argon prior to the experiment. Changes in the absorption were monitored on an Ocean FX Miniature spectrometer (Ocean Optics) in 10 s intervals after applying a potential of −1.1 V vs NHE. Electron Paramagnetic Resonance Measurements. Continuous wave (CW) X-band (9−10 GHz) electron paramagnetic resonance (EPR) experiments were carried out with a Bruker ELEXSYS E500 II EPR spectrometer (Bruker Biospin,), equipped with a rectangular resonator (TE102, ER4102ST) and a helium gas flow cryostat (ICE Oxford). Data processing was done using Xepr (Bruker BioSpin) and MATLAB 7.11.2 (MathWorks) environment. Simulations of the EPR spectra were performed using the EasySpin software package (version 5.2.20). Density Functional Theory Calculations. The geometry optimizations of 2O• were carried out using the B3LYP functional,25 with the def2-TZVP26 basis set. The implicit solvent model CPCM27 for diethyl ether was used. After the minima were confirmed by the absence of imaginary frequencies, the spectroscopic parameters were obtained via single point DFT calculations, performed with the B3LYP functional and EPRII basis set.28 For these calculations the program package ORCA (4.0) was employed.29 The geometry optimization and spectroscopic parameters of 2O+− COOH were calculated in a manner similar to that of 2O•, using the wb97xd functional, 6-311+g basic set,30,31 and the implicit solvent model CPCM27 for ethanol was used. The electronic transition frequencies were obtained using time-dependent DFT using the same functional, basis set, and solvation model as the ground-state optimization. For these calculations Gaussian 09 was employed.32 The coordinates of the optimized geometries of 2O+−COOH and 2O• are provided in Tables S2 and S3 (Supporting Information, SI). Femtosecond Pump Probe Measurements. The 800 nm laser pulses were produced at a 1 kHz repetition rate by a mode-locked Ti:sapphire laser and regenerative amplifier (Astrella, Coherent Inc.). The output from the Astrella was split into pump and probe beams. The pump beam was sent into an optical parametric amplifier (OPerA Solo, Coherent Inc.) to obtain the desired excitation wavelengths of 620 and 310 nm. The energy of the pump beam was 600 nJ/pulse for 2O+ in ethanol, 80 nJ/pulse for 2O• in diethyl ether, and 200 nJ/pulse for all other measurements. The probe beam was focused into a 4 mm CaF2 crystal that was continuously translated with a linear stage (Newport MFA-CC), to generate the white light continuum between 350 and 750 nm. Solution samples were measured in a 2 mm cuvette and stirred with a magnetic stir rod to prevent sample degradation. The solutions were made to have the absorption in the 0.1−0.3 range at the excitation wavelength. Thin film samples were placed in a nitrogen-purged 1 cm cuvette. The relative polarization of pump and

in dichloromethane and selectively precipitated by addition of ether until the filtrate was no longer red, yielding 45 mg of green product (47%). MS-MALDI: m/z 463 (M+ calculated for C29H23N2O4+). 1H NMR (CD3CN, 500 MHz): δ 8.42 (t, 1H), 7.97 (t, 2H), 7.67 (d, 2H), 7.52 (d, 2H), 7.49 (d, 2H), 6.99 (d, 2H), 6.629 (d, 2H), 4.15 (s, 6H), 3.78 (s, 3H). 2O +−COOH/NiO Nanoparticle Film Preparation. NiCl2 solution was prepared using a sol−gel procedure similar to that described in the literature.23 Nickel chloride (1.0 g) and F-108 polymer (0.9 g) were ground into a fine tan powder using a mortar and pestle. Then, the powder was added to deionized water (2.5 mL) and ethanol (5.0 mL). The tan slurry was sonicated until it became a clear green solution. The solution was heated at 30 °C for 3 days in the dark. Finally, the white precipitate was centrifuged, and the green supernatant was collected. Fluorine-doped tin oxide (FTO) glass was cleaned by 5 min sonication cycles of soapy water, deionized water, and isopropanol. The glass slides were dried at 100 °C and cooled to room temperature. Using label paper with the labels removed, Scotch tape was adhered to the label paper, and a hole punch was used to punch 0.36 cm2 holes in the tape. The tape with holes was removed and adhered to the glass slide. A 50 μL drop of the NiCl2 solution was placed above the hole, and the solution was doctor-bladed across the hole with a microscope glass slide. After 20 min at room temperature, the tape was removed and the films were annealed in air at 450 °C for 30 min with a ramp rate of 2 °C/min. Thicker films were made by repeating the doctor blade and annealing processes. NiO thin films, while still warm (80 °C), were immersed in an ethanol solution of 0.3 mM 2O+−COOH for 24 h. The films were washed with ethanol and dried under hot air flow. 2O• Solution Preparation. A solution containing 0.3 mg of 2O+ in an appropriate amount of deionized water was sonicated until 2O+ completely dissolved. The solution was purged with nitrogen for 30 min; samarium(II) iodide (0.4 mg, 1.2 equiv) was then added and the mixture stirred for 15 min. Under inert atmosphere, the reaction mixture was extracted with distilled diethyl ether and the organic layer was washed with water. 2O•−COOH/NiO Film Preparation. The 2O+−COOH/NiO thin films were kept overnight in nitrogen-purged 0.3 mM SmI2 in anhydrous acetonitrile. The films were then washed with acetonitrile and dried under nitrogen flow. Electrochemical Measurements. Cyclic voltammetry was performed using a BASi epsilon potentiostat in a VC-2 voltammetry cell (Bioanalytical Systems) using platinum working electrode (1.6 mm diameter, MF-2013, Bioanalytical Systems), a nonaqueous Ag/ Ag+ reference electrode (MF-2062, Bioanalytical Systems), and a platinum wire (MW-4130, Bioanalytical Systems) as a counter electrode. Electrochemical potentials were referenced to NHE by adding 0.548 V to the experimental potentials.24 Spectroscopic grade solvent (DMSO) and the electrolyte tetrabutylammonium perchlorate C


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ACS Applied Energy Materials probe beams was set at the magic angle. After passing through the sample, the probe continuum was coupled into an optical fiber and input into a CCD spectrograph (Ocean Optics, Flame-S−UV−vis− ES). The data acquisition was achieved using in-house LabVIEW (National Instruments) software routines. The group velocity dispersion of the probing pulse was determined using nonresonant optical Kerr effect measurements.33 Global and target analysis was performed using CarpetView (Light Conversion).

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RESULTS AND DISCUSSION Model System. As a model NAD+ analogue for this study, we selected an acridinium derivative 2O+ for several reasons. First, the absorption spectrum of 2O+ exhibits two bands in the visible range, with maxima at 433 and 622 nm.18 Furthermore, we have previously shown that 2O+ can efficiently sensitize hole injection into p-GaP,18 indicating a favorable first photoreduction step to form 2O• (Scheme 1B). Second, the one-electron reduction of 2O+ is reversible on the electrochemical time scales,5 suggesting that 2O• is sufficiently stable for the two-electron photoreduction presented in Scheme 1B. Finally, we recently determined that the hydride form (2O−H) has a similar hydride donor ability to BNAH, a close relative of natural NAD(P)H cofactors.5 Synthesis of 2O+ has been reported before and includes a sequential aromatic nucleophilic substitution (SNAr) of methoxy-substituted carbenium ion (6O+) with the primary amine CH3NH2 (Scheme 2).34,35 The substitution of methoxy groups was facilitated by the irreversible nitrogen-bridged ring closure that leads to a formation of the more stable acridinium species 4O+. Due to the improved stability of 4O+, the subsequent SNAr to form 2O+ was achieved under elevated temperatures. To facilitate the covalent attachment of 2O+ to the surface of NiO nanoparticles, the derivative 2O+−COOH was synthesized following the slightly modified methodology shown in Scheme 2, using 4-aminobenzoic acid to introduce the carboxylic linker. Because of the decreased nucleophilic character of aromatic amines, the initial SNAr substitution to form 4O+−COOH was achieved only under elevated temperature and in the presence of a base (lutidine), as a nucleophile activator. Finally, the formation of 2O+ −COOH was performed using CH3NH2 in the same manner as that for the dye without linker (2O+). The one-electron reduction of 2O+ in TBAP/acetonitrile occurs at E° = −0.5 V vs NHE and is reversible at the time scale used in our cyclic voltammetry experiments (Figure 1a). To investigate the electronic structure of 2O•, spectroelectrochemical measurements were performed under the applied potential of −1.1 V vs NHE (Figure 1b). The reduction of 2O+ is monitored as a decrease in absorption bands at 433 and 622 nm, and a concomitant increase of the featureless absorption at λ < 340 nm was observed and assigned to the 2O•. A similar absorption band was observed from 2O• in diethyl ether obtained through chemical reduction of 2O+, using SmI2 as a reducing agent (E°(SmI2) = −0.59 V vs NHE36), as shown in Figure 1c. In the presence of oxygen, absorption feature at λ < 340 nm disappears and the partial recovery of 2O+ absorption bands is observed (Figure S1, Supporting Information). This oxidation process is consistent with our assignment of the λ < 340 nm feature to 2O•. Aggregation behavior of 2O• was observed in the concentration-dependent fluorescence experiments (Figure S2); and the assembled 2O• aggregate emits at 480 nm. A brief explanation for the difference in solvents is in order. Due to their different solubilities, spectroscopy of 2O+

Figure 1. Chemical and electrochemical reduction of 2O+ to 2O•: (a) cyclic voltammetry of 1.0 mM 2O+ in 0.1 M tetrabutyl ammonium perchlorate (TBAP)/acetonitrile (Pt working electrode, scan rate 100 mV/s); (b) UV/vis spectroelectrochemical measurements of 1.0 mM 2O+ in 0.1 M TBAP/acetonitrile solution under the applied potential of −1.1 V vs NHE; (c) UV/vis absorption spectra of 2O• prepared via electrochemical (blue) and chemical (purple) methods. Chemical reduction was achieved using SmI2, as described in the Experimental Section. Electrochemical trace has a 0.2 offset. (d) Experimental EPR spectrum of ca. 100 mM 2O• in diethyl ether (green) prepared chemically using SmI2, recorded at T = 295 K, modulation amplitude 80 mG, and power 18 dB. Simulated spectrum (red) as described in the Experimental Section.

and 2O+−COOH was performed in ethanol, while 2O• was studied in diethyl ether. Additional characterization of 2O• was performed using EPR spectroscopy (Figure 1d, green trace). This spectrum can be easily saturated with increase of microwave power and at low power consists of a large number of narrow lines with 80 mG width and ca. 100 mG separation. This behavior is typical for highly conjugated organic radicals.37 In order to prove that this EPR spectrum is indeed 2O•, the geometry optimized at the DFT level was used to reconstruct the EPR spectrum (Figure 1d, red trace) by using 21 proton and 2 nitrogen hyperfine parameters shown in Table S3. Experimental and theoretical EPR spectra are in very good agreement, both having similar width and the same splitting of the lines. While more detailed D


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ACS Applied Energy Materials analysis of the EPR spectrum is not feasible, owing to the poor signal-to-noise ratio of the experimental spectrum, the overall agreement is a solid conformation that the experimental spectrum is due to 2O•. The photoinduced electron transfer sequence presented in Scheme 1b requires two sensitizers, 2O+ and 2O•. Thus, the photophysical behavior of these two compounds was investigated in more detail, and the steady-state absorption and emission spectra are presented in Figure 2. The cationic

Figure 3. UV−vis transmittance of 2O+−COOH in ethanol (dashed), UV−vis absorption of bare NiO (black), 2O+−COOH/NiO (red), 2O•−COOH/NiO (blue), and recovery of 2O+−COOH/NiO after exposure to atmosphere (green), on a quartz slide.

is achieved by spatially separating the charge carriers formed during the initial electron transfer, thus reducing their Coulombic attraction. In the case of hole injection into pNiO, the ideal orientation of the dye involves placing a donor moiety near the carboxylate linker and the acceptor moiety far away from the interface.39−42,45 The absorption spectrum of 2O• appears in the UV region, below 345 nm, and is featureless (Figure 2b). The fluorescence spectrum appears at 344 nm, and the excitation spectrum does not match exactly the absorption spectrum of 2O• (λex‑max = 319 nm), possibly due to other photophysical or photochemical processes that take place from higher electronic states of 2O•. The observed lack of absorption in the visible region is consistent with previous reports on electronic properties of simple triarylmethyl radicals.37,46 Unfortunately, the insufficient absorption in the visible range by 2O• is not ideal for the dye-sensitization sequence presented in Scheme 1. Our future studies will focus on designing mixed valence derivatives of NAD+ analogues, since it was shown that the triaryl radicals with mixed valence character exhibit intervalence chargetransfer bands in the visible and near-IR regions.47 Nickel oxide was selected as a transparent oxide for photosensitization studies, because it exhibits p-type conductivity due to the presence of Ni vacancies in oxygen-rich conditions.48 In addition, methods for fabrication of nanoporous NiO films have been developed49 and utilized in fabrication of dye-sensitized solar cells.50,51 NiO nanoparticle films were prepared using a sol−gel technique reported previously and characterized using SEM and XRD (Figures S3 and S4). The UV/vis absorption spectrum of NiO (Figure 3) is dominated by strong absorption at λ < 360 nm, which is consistent with the previously reported bandgap of 3.55 eV.52,53 Additional weak absorption features appear in the visible range and are assigned to oxidized Ni(III) species at the NiO surface formed during the preparation of nanostructured films.53 The chemisorption of 2O+−COOH onto NiO films resulted in the appearance of absorption bands in the visible range that are similar to the absorption of 2O+−COOH in ethanol (Figure 3). The absorption maximum of the lowest energy band of 2O+−COOH adsorbed on NiO (626 nm) is slightly

Figure 2. Normalized UV−vis absorption, emission, and excitation spectra of (a) 2O+ in ethanol (λex = 620 nm, λem = 660 nm), and (b) 2O• in diethyl ether (λex = 310 nm, λem = 348 nm). Panel a also shows the electronic transition energies and contour plots of HOMO and LUMO calculated using TD-DFT (wb97xd/6-311+g, CPCM ethanol).

derivative (2O+) absorbs well in the visible region, with the lowest energy band at 622 nm. The fluorescence spectrum occurs with a small Stokes shift, at 660 nm, and its excitation spectrum correlates well with the absorption features. With the addition of the linker, 2O+−COOH showed a slight blue shift in the absorption spectrum with peaks at 430 and 614 nm (see transmittance spectrum in Figure 3). The lowest electronic transition energy calculated using time-dependent DFT (vertical lines in Figure 2a) overestimates the experimentally observed transition by 0.68 eV, which is somewhat higher than the usual 0.5 eV error associated with DFT calculations.38 Based on these calculations, the lowest energy band arises due to a π−π* HOMO−LUMO transition lacking substantial charge transfer characteristic (see the orbital plots of Figure 2a). The observed lack of significant dipole moment in the excited state is not ideal for efficient charge separation at the dye−semiconductor interface. Previous studies have shown that the chromophores composed of appropriately oriented donor−acceptor moieties exhibit higher rates of injection of charge carriers into the semiconductor as well as longer lived lifetimes for charge-separated states.39−45 This beneficial effect E


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ACS Applied Energy Materials red-shifted relative the corresponding maximum of 2O+− COOH in solution (614 nm). Similar red shift in dye absorption upon binding to the oxide surfaces has been observed previously54−57 and is attributed to the perturbation of molecular orbital energies due to the interaction of the dye with the oxide.54,55 The 2O+−COOH/NiO sample was reduced chemically using SmI2, and the resulting film exhibited a loss of 2O+−COOH absorption bands (Figure 3), while the radical bands expected at λ < 345 nm were not observed due to the overlap with the absorption of NiO. To confirm the radical was formed, the film was exposed to air, causing reoxidation of the surface bound radical back to the cationic form. The resulting UV/vis absorption (Figure 3) shows that 54% of the absorption at 626 nm was recovered, confirming that the chemical reduction by SmI2 can be efficiently performed on the adsorbed NAD+ analogues. Experimental absorption spectra and cyclic voltammograms were used to construct the energy diagram presented in Scheme 3, which indicates thermodynamically feasible hole

accurately reproduced using triexponential function, with associated lifetimes of 12 ps, 197 ps, and 4.2 ns. The 12 ps component is associated with the blue shift of the 380 nm band and is assigned to solvent relaxation, in agreement with the previous report.59 Solvation is known to be comprised of several components including electronic, vibrational, rotational, and translational relaxation dynamics.60 The 12 ps component is likely only due to one of these relaxation processes, with the others not giving a strong enough signal to rise above the signal-to-noise ratio of our instrument. The origin of the 197 ps component was further investigated using variable pump fluence experiments in the 60−600 nJ/pulse range (Table S1, Figure S6). Since the transient kinetics and amplitudes did not vary with pump fluence, we conclude that the 197 ps component is not associated with the multiphotonic process. The concentration dependence of the transient signal was further investigated in the 12.5−100 μM range (Table S1, Figure S6). The data show that the amplitude of the 197 ps component increases with concentration, while the 4.2 ns component decreases. Based on these results, we assign the 197 ps component to the excited-state decay of aggregated 2O+, while the 4.2 ns component is assigned to the excitedstate decay of the monomeric 2O+. The aggregation of 2O+ at high concentrations was also observed in the ground-state absorption spectra (Figure S5), which show that the relative intensities of 433 and 622 nm bands change with concentration. Photoinduced charge separation and recombination kinetics were investigated in 2O+−COOH/NiO thin films (Figure 5). Transient signal observed at early times is spectrally identical to that observed for 2O+ in solution, and we assign it to the S1 state of the cation. The observed kinetics are, however, faster in the 2O+−COOH/NiO film, which is consistent with the photoinduced electron transfer from NiO to the excited 2O+− COOH. Unfortunately, the markers for the charge-separated states are not within the spectral window that was probed in our experiments: 2O•−COOH absorbs below 350 nm (Figure 2b), while holes in NiO undergo intraband transitions in the IR range.61 Thus, the kinetics of charge separation and recombination were probed indirectly, by comparing the kinetics of excited-state absorption, stimulated emission, and ground-state bleach bands. For example, most of the groundstate bleach signal observed at 585 nm (Figure 5b) decays within a few tens of picoseconds, which is consistent with rapid charge recombination process. A small fraction (∼10%) of the bleach signal persists for a long time (almost no decay was observed within the range of our delay line) and is associated with the weak absorption throughout the visible range (see the 1 ns spectrum in Figure 5a). Similar absorption in the visible range was observed upon anodic oxidation of NiO and was assigned to the formation of trapped holes on the NiO surface, in the form of Ni3+ and Ni4+ species.53,62 Thus, the long-lived component observed in our experiment is due to trapped holes formed either directly by hole injection from 2O+−COOH or indirectly by trapping the free holes from the valence band of NiO. Transient absorption data were modeled using target analysis using the model shown in Scheme 4. The laser pulse generates the S1-state 2O+•−COOH/NiO, which was assumed to undergo the same excited-state decay kinetics as 2O+ in ethanol (τS1 = 4.2 ns, Figure 4). When bound to NiO, an additional deactivation channel is introduced that is associated with the electron transfer from the valence band of NiO to

Scheme 3. Energy Diagram Showing Band Edge Potentials for NiO,52,58 along with the Ground- and Excited-State Reduction Potentials of 2O+ and 2O•a

a

The ground-state reduction potentials were measured by cyclic voltammetry, while the excited-state reduction potentials were calculated using experimental ground-state potentials and UV/vis absorption peaks.

injection from photoexcited 2O+• and 2O•* to NiO. The 2O− anion formed upon the second photoreduction step is expected to be readily protonated to form 2OH. This argument is consistent with the pKa value for 2OH of 27 in DMSO.5 While 2O+ exhibits good absorption in the visible range, the absorption of 2O• is unfortunately limited to the UV region. Thus, photochemical reduction of 2O+ to 2OH is feasible but requires the use of both visible and UV photons. Time-Resolved Studies. Femtosecond transient absorption spectroscopy was utilized to investigate the kinetics of interfacial charge transfer in 2O+−COOH/NiO films. Initial measurements were performed on 2O+ in ethanol (Figure 4). Transient spectra of 2O+ are composed of two excited-state absorption bands with maxima at 378 and 513 nm and a negative signal at wavelengths above 550 nm that are assigned to the ground-state bleach and stimulated emission signals (Figure 4a). The kinetic traces at selected wavelengths are F


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Figure 4. Transient absorption spectra (a) and kinetics (b) of 12.5 μM 2O+ in ethanol. λpump = 620 nm. Inset of panel a: Species associated spectra from global analysis providing three components with lifetimes of 12 ps, 197 ps, and 4.2 ns.

Figure 5. Transient absorption spectra (a) and kinetics (b) of 2O+−COOH/NiO film. λpump = 620 nm. Time constants fitted using target analysis are listed in Scheme 4.

holes with absorption in the visible range (τESC). The escaped charge carriers live longer than the time scale of our instrument. Lifetimes obtained using this kinetic model (Scheme 4) show that the charge separation occurs with τCS = 2.8 ps, indicating that photoexcited 2O+-COOH can efficiently inject holes into NiO. However, the charge recombination kinetics are also fast, with τCR = 31 ps. Similar behavior was observed by others42,58,63−65 and is likely associated with the presence of Ni3+ surface trap states62 and the fact that the thermodynamics for the charge recombination lie in the normal Marcus region.63 Several approaches have been utilized to increase the lifetime of charge-separated states. For example, the modification of the NiO surface is performed to reduce the number of Ni3+ trap states via chemical reduction using either NaBH4 or heat.62 The charge recombination kinetics was shown to slow down when a layer of Al2O3 is deposited between NiO and the dye.66 Alternatively, Li doping was used to improve the hole conductivity in NiO.67 Another approach toward long-lived charge-separated states involves the use of dyes that facilitate the migration of the photogenerated

Scheme 4. Kinetic Model Used To Analyze Photoinduced Kinetics of 2O+−COOH/NiO

photoexcited 2O+−COOH (τCS). Most of the chargeseparated state population undergoes the charge recombination (τCR), while a small portion of the charge carriers escape the recombination, as observed by the formation of trapped G


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Figure 6. Transient absorption spectra (a) and kinetics (b) of 2O• in diethyl ether. λpump = 310 nm.

Figure 7. Transient absorption spectra (a) and kinetics (b) of bare untreated NiO thin film on FTO glass. λpump = 310 nm. Inset of panel a: Component spectra from global analysis with three lifetimes of 0.63 ps, 90 ps, and Inf. Pump scatter from a small amount of an unfiltered 620 nm beam was removed.

electron away from the interface.40,41,45,68 In most cases, the dye is constructed by placing the electron-donating moiety (such as the aryl−amine group) immediately next to the carboxylate linker, followed by the light-absorbing unit and the acceptor moiety (such as dicyanovinylene group) placed away from the carboxylate linker.45 Additional improvement in the charge-separated state lifetimes is achieved with push−pull chromophores that contain thiophene or phenyl spacers that position the acceptor moiety several nanometers away from the semiconductor surface and this approach leads to record incident photon-to-electron conversion efficiencies.40 In the case of 2O+−COOH, the S1 state does not exhibit any chargetransfer characteristic (see orbital contour plots presented in Figure 2A), which does not enable the spatial separation of photogenerated charges and explains the rapid charge recombination observed here. In our future studies, the new generation of NAD+ analogues with push−pull characteristics will be investigated. The second reduction step from excited 2O radical was also investigated. The preliminary experiment involved a study of excited-state behavior of 2O• in solution (Figure 6). The

initially formed transient is assigned to the excited doublet D1 state of 2O• and consists of absorption bands at 400 and 660 nm. The signal decays with the 640 ps lifetime and is accompanied by the rise of a new transient with absorption at 550 nm. This absorption does not match the ground-state absorption spectra of 2O+ (Figure 2A) or 2O− (Figure S7) and is thus not associated with the photoinduced electron transfer to or from excited radical. Also, the 550 nm transient is not associated with the photochemical process, since no sample degradation was observed during the course of the experiment. It is most likely that the 550 nm transient arises due to the excimer formation, particularly since the aggregation effects were also observed in the steady-state absorption and emission spectra (Figure S2). Given that 2O• and NiO both absorb in the UV region, transient behavior of bare NiO was also investigated (Figure 7). The NiO transient signal appears with strong absorption at 350 nm and weak and broad signal throughout the visible range. Based on the previous study reported by Hammarström and co-workers,57 we assign the observed transient bands to one of the following species: free electrons that absorb in the H


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Figure 8. Transient absorption spectra (a) and kinetics (b) of 2O•−COOH/NiO prepared by chemical reduction of 2O+−COOH/NiO with SmI2. λpump = 310 nm. Inset of panel a: Component spectra from global fitting with four exponentials. The negative signal of the component with 4.5 ns lifetime is due to a growing component.

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red region of the visible range; trapped electrons and holes that absorb throughout the visible range; and heated NiO that absorbs in the UV region. The global analysis of our data revealed three components with similar spectral characteristics and with the following lifetimes: 0.63 ps, 90 ps, and a very long-lived transient, reflecting heterogeneity of time scales for charge trapping and recombination in nanostructured NiO films. The excitation of 2O•−COOH/NiO sample (Figure 8) gives rise to a transient signal that does not resemble the excited state of 2O• (Figure 6), but that of NiO (Figure 7), showing that the light is predominately absorbed by the semiconductor. The three kinetic components that were observed in bare NiO were also detected in the case of 2O•−COOH/NiO and are assigned to trapped charge carriers in NiO. However, the 2O•−COOH/NiO samples exhibit an additional growth component, as can be seen by monitoring the kinetics probed at 348 nm (Figure 8b). Since the free charge carriers in NiO and reduced 2O−−COOH do not absorb in the visible range, the rate of photoinduced charge separation cannot be determined from our transient absorption data. However, the charge recombination process is likely slow: the growth of the component at 348 nm occurs at the nanosecond time scale and is likely associated with the trapping of electrons during the charge recombination from 2O−−COOH. The fact that the growth of this component appears with a nanosecond time indicates that the chargeseparated state formed by photoexciting 2O•−COOH/NiO is significantly longer lived (∼4.5 ns) than that observed in the case of 2O+−COOH/NiO (31 ps). The exact time constant associated with this growth could only be estimated due to the time range of the instrument (1600 ps). The observed difference in lifetimes is likely due to the differences in the thermodynamic driving forces for the two charge recombination processes (ΔG = −1 eV for the 2O+−COOH/NiO sample and ΔG = −1.9 eV for 2O•−COOH/NiO, Scheme 3). The larger driving force associated with the charge recombination in 2O•−COOH/NiO likely places the process into the Marcus inverted region, thus reducing the rate of the process.

CONCLUSIONS

The results of our work show that the stepwise two-electron photochemical reduction of NAD+ analogues attached to the surface of NiO nanoparticles can, in principle, be achieved to form anionic NAD− analogue. The NAD+ analogue studied here, acridinium-based 2O+, was found to have a strong absorption in the visible range, showing that it can be used as an efficient chromophore for light-harvesting applications. Importantly, the one-electron reduced 2O radical was sufficiently inert chemically, allowing for the second photochemical reduction step to take place. This is in contrast to the natural NAD+ co-factor, which is known to undergo a dimerization process upon one-electron reduction.69 Both the cation and the radical were shown to efficiently sensitize NiO, providing encouragement for the feasibility of the photochemical regeneration of NADH analogues using the dye-sensitized approach. However, several challenges were identified during this investigation. First, the charge-separated species formed upon the photoexcitation of 2O+−COOH/NiO are short-lived, with most of the recombination taking place with 31 ps lifetime. These findings are consistent with previous reports and are associated with the poor electrical conductivity of NiO. We anticipate that these kinetics can be improved in the future by designing NAD+ analogues that facilitate charge separation at larger distances using donor−acceptor moieties, as described previously by others. The second challenge is associated with the fact that neutral 2O• radical does not exhibit sufficient absorption in the visible range, preventing it from acting as a chromophore for NiO sensitization. This problem can be circumvented if semiconductors with smaller bandgap are employed, such as p-GaP,70 where, by utilizing a dual absorber system (NAD+ analogu and the semiconductor), efficient coverage of the solar spectrum could be achieved. These challenges need to be addressed before the proposed photochemical regeneration of NADH can be utilized in enzymatic and chemical catalysis. I


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b01574.

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UV−vis spectra, concentration-dependent UV−vis and emission spectra, SEM images, XRD spectra, TA comparison under various conditions, coordinates for optimized structures, and parameters for EPR simulation (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stefan Ilic: 0000-0002-6305-4001 Jens Niklas: 0000-0002-6462-2680 Oleg G. Poluektov: 0000-0003-3067-9272 Ksenija D. Glusac: 0000-0002-2734-057X Author Contributions §

G.N.H. and R.B.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.D.G. thanks ACS Petroleum Research Fund (PRF; Grant 54436-ND4) for financial support and the Advanced Cyberinfrastructure for Education and Research (ACER) at the University of Illinois at Chicago for computations support. We thank Prof. Yiying Wu and Dr. Kevin A. Click for help with NiO nanoparticle film preparation. O.G.P. and J.N. thank the U.S. Department of Energy (Grant DE-AC02-06CH11357) and Argonne National Laboratory for their financial and computational support.

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