Anodic Photocurrent Generation by Porphyrin-Terminated Helical

Mar 26, 2015 - Hirotaka Uji , Kazuyoshi Tanaka , and Shunsaku Kimura. The Journal of Physical Chemistry C 2016 120 (7), 3684-3689. Abstract | Full Tex...
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Anodic Photocurrent Generation by Porphyrin-Terminated Helical Peptide Monolayers on Gold Hirotaka Uji, Yuji Yatsunami, and Shunsaku Kimura* Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-Daigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: Photocurrent generation of porphyrin-terminated helical peptide selfassembled monolayers (SAMs) was studied in an aqueous solution. The anodic photocurrent was prevailing, but the cathodic photocurrent was observed with applying negative bias voltage on the working electrode. The bias dependence of the photocurrent was explained successfully by theoretical calculation with taking into account the redox potential shift by J-aggregate of porpyrins, the helix dipole, and photoenergy migration in the SAM. The dark current was insignificant even at the forward bias voltage.

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INTRODUCTION The artificial photoenergy conversion system is one of the most promising subjects of molecular electronics, and there have been attractive reports in the point of view of biomimetics.1−4 These reports rely on organic molecules, which are considered to have more potential with regard to device miniaturization than today’s silicon-based devices. In order to develop the organic molecular devices, however, it is imperative to understand the electron transfer reaction in relation to the molecular orbitals as typified by HOMO and LUMO instead of the way of thinking based on electron transmission or band theory.1 We have been focusing our attention on helical peptides as a key molecule to construct artificial molecular devices. Especially Aib-containing peptides adopt stable helical structures,5 and possess a good property about molecular self-assembly on gold to form regularly packed monolayers.6−8 In the electrical viewpoint, helical peptides are a good mediator for electron transport with a tunneling factor (β) of 0.66,9−14 and form a macrodipole moment along the helix axis, which can accelerate the electron transfer through the helical peptides,15−19 and also modify HOMO and LUMO levels of the π-conjugated systems nearby.20,21 In our previous studies, we have demonstrated that helical peptides on gold could be used for various functions including electron transfer, photoenergy harvesting, and rectification.16,19,22−25 In the most of these reports, we combined helical peptides with chromophores to achieve precise control of their geometrical arrangement in the selfassembled monolayers (SAMs). We could successfully explain the electron transfer reactions in the SAMs theoretically by the electron tunneling and the electron hopping mechanisms with taking consideration of the energy gaps between the HOMO © 2015 American Chemical Society

level or the LUMO level of the chromophore and the gold Fermi level including the influence of the surface potential due to the helix dipole.22,25 However, there still remains one factor to be solved, which is the effect of interchromophore interaction in the densely packed SAMs on the electron transfer reaction.26,27 In this study, we newly synthesized a porphyrin-terminated helical peptide and prepared its SAMs with varying the porphyrin concentration by mixing a porphyrin-free helical peptide (Figure 1). We used here tetraphenylporphyrin (TPP), whose molecular size is 1.8 nm and slightly larger than helical rod diameter of 1.2 nm. However, TPP at the molecular terminal of the helix rod can be accommodated in the well packed peptide SAMs when the helical peptides tilt from the surface normal to generate more space per one peptide molecule than the upstanding helices. The SAMs were

Figure 1. Chemical structures of SA16TPP and SA16M. Received: February 2, 2015 Revised: March 25, 2015 Published: March 26, 2015 8054

DOI: 10.1021/acs.jpcc.5b01100 J. Phys. Chem. C 2015, 119, 8054−8061

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The Journal of Physical Chemistry C characterized by infrared reflection−absorption spectroscopy (IRRAS), ellipsometry, cyclic voltammetry (CV), and absorption spectroscopy to reveal interactions among TPP in the SAMs. The photocurrent generations of the SAMs were measured in an aqueous solution containing triethanolamine (TEOA) as an electron donor and methylviologen (MV) as an electron acceptor. In order to understand in depth the electron transfer reactions, photocurrent generation was also observed with applying bias voltage on working electrode (I−V curve) to change the relative position of the HOMO or the LUMO level against the gold Fermi level. The I−V curves obtained with varying TPP concentrations in the SAMs were subjected to curve fitting with theoretically calculated curves to elucidate the contribution of intermolecular interaction among TPP to the electron transfer reaction in the SAMs.

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1.50 + 0.00i. The thickness was obtained as the average on more than five different regions of the monolayer. IRRAS Measurements. The IRRAS spectra of the peptide monolayers on a gold substrate were recorded on a Nicolet Magna 850 Fourier transform infrared spectrometer with a Harrick RMA-1DG/VRA reflection attachment. The incident light angle was set at 84° from the surface normal. The number of interferogram accumulations was 1000. The molecular orientation of the helical peptide was determined from of the amide I/amide II absorbance ratio in the IRRAS spectrum according to the following formula under the assumption of uniform orientation of the helix axis around the surface normal.6,28 I1/I2 = 1.5

EXPERIMENTAL SECTION

(3cos2 γ − 1)(3cos2 θ1 − 1) + 2 (3cos2 γ − 1)(3cos2 θ2 − 1) + 2

where Ii, γ, and θi (i =1 or 2 corresponding to amide I or amide II) represent the observed absorbance, the tilt angle of helical axis from the surface normal, and the angle between the transition moment and the helix axis, respectively. The values of the θ1 and θ2 were taken to be 39° and 75°, respectively, for αhelical conformation.29 Cyclic Voltammetry. Cyclic voltammograms were obtained with using a BAS model 604 voltammetric analyzer. For the evaluation of the membrane packing, a standard threeelectrode setup was used with the monolayer-modified substrate as the working electrode, Ag/AgCl in a 3 M NaCl aqueous solution as the reference electrode, and a platinum wire as the counter electrode in a glass vessel capped with a silicon rubber. The solution was 1 mM K4[Fe(CN)6] containing in 1 M KCl aqueous solution prepared with MilliQ water, which was deaerated with argon gas for 15 min prior to the experiments. The applied potentials of the working electrode reported here were with respect to the reference electrode. The area of the working electrode exposed to the electrolyte solution was 0.9−1.1 cm2. The porphyrin redox potentials were determined by cyclic voltammograms with a standard three-electrode setup under nonaqueous conditions with the monolayer-modified substrate as the working electrode, an Ag/Ag+ in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) acetonitrile solution as the reference electrode, and a platinum wire as the counter electrode in a glass vessel capped with a silicon rubber. The solution was a 0.1 M TBAPF6 in dichloromethane solution, and it was deaerated with argon gas for 15 min prior to the experiments. After each measurement, the reference electrode was checked with Fc compound. The applied potentials of the working electrode reported here were with respect to SCE using the literature value of Fc/Fc+ in TBAPF6 dichloromethane to be 0.41 V vs SCE. The area of the working electrode exposed to the electrolyte solution was 0.9−1.1 cm2. Photocurrent Generation Experiments. Photocurrent measurements were carried out using a standard three-electrode setup with the monolayer-modified substrate as the working electrode, Ag/AgCl in a 3 M NaCl aqueous solution as the reference electrode, and a platinum wire as the counter electrode in a glass vessel capped with a silicon-rubber. The solution was a 0.05 M TEOA, 0.05 M MV, and 0.1 M Na2SO4 aqueous solution prepared with Milli-Q water, which was deaerated with argon gas for 15 min prior to the experiments. The applied potentials of the working electrode reported here were with respect to the reference electrode. The monolayer-

Synthesis. All chemicals were purchased from commercial suppliers and used without further purification. SA16M and SA16TPP were synthesized by a conventional liquid-phase method (synthetic section in the Supporting Information). All the intermediates were identified by 1H NMR spectroscopy (Bruker DPX-400), and the final products were further confirmed by MALDI mass spectrometry (Bruker ultraflexIIIKE). The purity of the intermediates was checked by thin-layer chromatography, and the purity of the final compounds was checked by HPLC (TOSOH System 8020). Preparation of Self-Assembled Monolayers (SAMs). Gold glass substrates for infrared reflection−absorption spectroscopy (IRRAS) and photocurrent generation experiments were prepared by the following procedure. A glass slide (76 mm × 5 mm) was immersed in sulfuric acid for at least 3 h, thoroughly washed with distilled water three times and methanol, and then dried in vacuum for 20 min. A gold glass substrate was prepared by vapor deposition of chromium (300 Å) and then of gold (99.99%, 2000 Å) onto the cleaned glass slide by a vacuum deposition system (Osaka Vacuum NKS350). The metal layers thickness was monitored by a quartz oscillator (INFICON XTM/2 Deposition Monitor). The prepared gold glass substrates were immediately used for selfassembling of peptides. The peptide solutions in ethanol were adjusted to 0.1 mM. The gold glass substrates were then incubated in the peptide solution for 24 h. The substrate was rinsed with ethanol and ethanol/chloroform (1/1 v/v) in this order, and dried under a nitrogen stream and in vacuum for 20 min. Gold mica substrates for cyclic voltammetry were deposited on the surface of freshly cleaved mica by thermal evaporation of gold (1000 Å) and then were annealed just before preparation of the SAMs. Gold quartz substrates for absorption and fluorescence spectroscopy were deposited on the surface of a quartz slide (40 mm × 12 mm) by thermal evaporation of gold (20 Å). These freshly prepared gold substrates were immersed in peptide solutions by the same procedure for samples of a gold glass substrate. Ellipsometry. The thickness of the peptide monolayers on gold glass substrates was determined by an auto ellipsometer (MIZOJIRI DHA-OLX/S). A helium−neon laser of 632.8 nm was used as the incident light, and the incident angle was set at 65°. The thickness of the monolayer was calculated automatically by using an equipped program. In the calculation, the complex optical constant of the monolayer was assumed to be 8055

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The Journal of Physical Chemistry C modified substrate was photoirradiated with an Asahi Spectra MAX-302 with band-pass filters. The light intensities were measured by a Light Meter (DeltaOHM HD 2302.0). The irradiation area of the electrode was ca. 0.9−1.1 cm2. Spectroscopy. The absorption spectra of the peptide SAMs were recorded on a Shimadzu UV- 2450PC spectrometer.

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RESULTS AND DISCUSSION

Characterization. CD measurements of SA16TPP and SA16M in methanol showed a negative double-minimum pattern with comparative intensities at 208 and 222 nm (Figure S1, Supporting Information), indicating that the peptides took predominantly α-helical conformation similarly to other Aibcontaining peptides.30 SA16TPP was self-assembled on gold to form three kinds of monolayers with varying the surface density of porphyrin. These SAMs are designated as TPPM10, TPPM11, and TPPM12 where SA16TPP and SA16M were mixed in the feed solutions at molar ratios of 1:0, 1:1, and 1:2, respectively. The SAMs were analyzed by IRRAS showing amide I absorption at 1675 cm−1 with stronger intensity than amide II absorption at 1540 cm−1 (Figure 2A). The helical peptides therefore took vertical orientation on gold substrate.31 The tilt angles of the helix axis from the surface normal were calculated, based on the absorbance ratios of amide I and amide II, to be 32.8 ± 1.0°, 34.3 ± 4.7°, and 36.6 ± 1.3° for TPPM10, TPPM11, and TPPM12, respectively. The molecular length of SA16TPP is estimated to be 50 Å for the optimized structure by calculation using the Molecular Mechanics program 2 (MM2) method. The tilt angle of 32.8° leads to the monolayer thickness of 42 Å, which is found to be slightly thicker than that evaluated by ellipsometry of 33.1 ± 0.1 Å. It is considered that the porphyrin moiety should bend slightly toward gold surface from the helix axis. The membrane thicknesses of TPPM11 and TPPM12 were further thinner to be 28.0 ± 0.6 and 25.1 ± 0.1 Å, respectively, by ellipsometry due to the mixing of SA16M which lacks the porphyrin moiety. The structural defects in these peptide SAMs were evaluated by cyclic voltammetry in the presence of K4[Fe(CN)6] in aqueous solution (Figure 2B). The oxidation peaks of K4[Fe(CN)6] appeared insignificant with the SAMs of TPPM10 and TPPM11 showing that the peptides in these SAMs were densely packed. On the other hand, TPPM12 could not block the oxidation reaction, because of not only the larger tilt angle of TPPM12 than other SAMs, but also the higher concentration of SA16M having less steric hindrance of the methyl ester group than SA16TPP of the porphyrin group (Figure S2B). The SAM compositions on gold substrates were analyzed by the absorption spectroscopy to be 57/43 and 36/64 for TPPM11 and TPPM12, respectively (Figure S3), which are close to the compositions in the feed solutions. Figure 3A shows the normalized absorption spectra of peptide SAMs and SA16TPP in chloroform. The Soret band of the peptide SAMs are red-shifted by about 10 nm and broadened compared to the spectra in solution. The red-shifted absorption suggests that the porphyrin moieties in the SAMs should form the J-aggregates (head-to-tail aggregates), where one of the transition dipole moment aligns parallel to the aggregation axis.32 The other dipole moment orients vertically to the aggregation axis, resulting in blue-shifted peak around 390−410 nm. These features are the most significant in TPPM10 than the others,

Figure 2. (A) IRRAS spectra of TPPM10, TPPM11, and TPPM12. (B) Cyclic voltammograms of peptide SAMs and bare Au substrate in a 1 mM K4[Fe(CN)6] and 1 M KCl aqueous solution at a scan rate of 0.1 V/s.

showing that the porphyrins should associate tightly in TPPM10 due to the high porphyrin surface concentration. The electrochemical properties of the peptide SAMs were analyzed by cyclic voltammetry with using the peptide SAMs as working electrode in dichloromethane (Figure 3B). Irreversible oxidation peaks were observed at 1.01, 1.00, and 0.99 V for TPPM10, TPPM11, and TPPM12, respectively. These oxidation peaks are identified as TPP/TPP+ of SA16TPP on gold substrates. At smaller potentials than these peaks, it is noteworthy that minor oxidation peaks appeared at 0.72 V for TPPM10 and at 0.75 V for TPPM11. These minor peaks were not observed in the voltammograms of TPP and SA16TPP dissolved in dichloromethane solution (Figure S6), where there were two oxidation peaks which are assigned to TPP/TPP+ (0.95 V for TPP) and TPP+/TPP2+ (1.31 V for TPP). Accordingly, the minor peaks in the SAMs should be ascribed to the associated condition of TPP moieties as reported that aggregated molecules in general provide more complicated profiles than isolated molecules in the electrochemistry.33,34 8056

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Figure 3. (A) Normalized UV−vis absorption spectra of TPPM10, TPPM11, TPPM12 and of SA16TPP in chloroform. (B) Cyclic voltammograms of TPPM10, TPPM11, and TPPM12 in a 0.1 M TBAPF6 dichloromethane solution at a scan rate of 0.5 V/s.

Figure 4. (A) Time profile of photocurrent generation of TPPM10 in a 0.05 M TEOA and 0.05 M MV and 0.1 M Na2SO4 aqueous solution with a 430 nm light photoirradiation. (B) Action spectra of TPPM10 (red solid line) in a 0.05 M TEOA and 0.05 M MV and 0.1 M Na2SO4 aqueous solution with 380−450 nm light photoirradiation and UV−vis absorption spectra (red dashed line) of TPPM10.

Taken together with the results of Figure 3, a part of TPPM10 and TPPM11 is composed of J-aggregates, and these aggregated porphyrins are the origin of the oxidation peak around 0.7 V. Photocurrent Generation. Photocurrent generation of these peptide SAMs were studied by irradiation with a 430 nm light in an aqueous solution containing TEOA as an electron donor and MV as an electron acceptor. Figure 4A shows the current profile of TPPM10 with on and off of the light irradiation at the applied voltage of 0 V. TPPM10 generated 548 ± 72 nA anodic photocurrent in response to photoirradiation. The action spectrum of TPPM10 is agreeable with the absorption spectrum in the region of the Soret band (Figure 4B). The electron transfer scheme is considered as follows: (1) the porphyrin moiety is photoexcited by a 430 nm light, TPP*, (2) TEOA donates an electron to the hole of TPP* HOMO level to generate an anion radical, and (3) an electron is transferred from TPP* LUMO level to the gold Fermi level.

TPPM11 and TPPM12 also generated anodic photocurrents of 213 ± 71 and 91.5 ± 45 nA, respectively. These photocurrents correspond to the quantum yields of 1.1% for TPPM10, 0.76% for TPPM11, and 0.51% for TPPM12. Since the quantum yield increases with raising the fraction of SA16TPP, the electron transfer reaction should be promoted by the presence of the associated porphyrin moieties in the peptide SAM. The dependence of photocurrent generation on the applied bias voltage was examined and summarized in Figure 5A. The anodic photocurrent observed at applied bias of 0 V decreased the intensity with loading minus voltage on the working electrode, and eventually the cathodic photocurrent was admitted at applying the bias voltage over −0.3 V about TPPM10. Other peptide SAMs also changed the photocurrent direction with applying negative bias on the SAMs. It is 8057

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Figure 5. (A) Dependence of the quantum efficiency of photocurrent generation on the applied potentials: experimental results of the peptide SAMs (TPPM10, TPPM11, and TPPM12) in a 0.05 M TEOA and 0.05 M MV and 0.1 M Na2SO4 aqueous solution with 430 nm light photoirradiation, and the theoretically calculated curves including entry 1 and entry 2 (Table 1). (B) Schematic illustration of theoretical calculations, where kTPP‑Au, kAu‑TPP, and kdTPP represent electron transfer rate constants from porphyrin to gold (anodic photocurrent), electron transfer rate constant from gold to porphyrin (cathodic photocurrent), and deactivation rate constant of the excited porphyrin moiety. (C) Formulas of electron transfer rate constants where ρ, HDA, h, λ, kB, T, e, EApp, ETPPox, ETPPred, and Edipole represent effective density of electronic states in a gold near the Fermi level, electronic coupling between porphyrin moiety and gold, Planck’s constant, reorganization energy, Boltzmann constant, absolute temperature, electron charge, applied potential, redox potential of TPP+/TPP or TPP*/TPP−, redox potential of TPP+/TPP* or TPP/TPP−, and surface potential of the SAM induced by helical peptide dipole moments, respectively.1

other if the electron transfer rates were determined just by the driving forces respectively of TPP LUMO level/gold Fermi level and gold Fermi level/TPP HOMO level in Figure 5B. However, the experimental value of Ei0 was −0.15 V, which is significantly lower than 0.19 V described above, indicating the preference of the anodic photocurrent in the peptide SAMs. Theoretical calculations were carried out for in depth analyses of the electron transfer reactions. Photocurrents were calculated on the basis of the electron transfer rate constants for nonadiabatic electron transfer between a gold substrate and chromophore.1,22

therefore concluded that the helical peptide can transfer electrons in both directions from N-terminus to C-terminus and from C-terminus to N-terminus. Notably, electron transfer schemes about the anodic photocurrent and the cathodic photocurrent are different with each other as shown in Figure 5B. The most different point between them is the electron transfer from TPP− LUMO level to the gold Fermi level virtually via LUMO levels of amide bonds for the anodic photocurrent, while the electron transfer from the gold Fermi level to TPP+ HOMO level virtually via HOMO levels of amide bonds for the cathodic photocurrent. As shown in Figure 5A, there are zero-photocurrent potentials (Ei0) where anodic current and cathodic current are canceled out. The zerophotocurrent potentials were −0.25, −0.21, and −0.15 V for TPPM10, TPPM11, and TPPM12, respectively. Imahori and co-workers studied photocurrent generation in the porphyrin-terminated alkanethiol SAMs on gold substrates, and found that the cathodic photocurrent was prevailing in their systems.26,27 In contrast to the alkanethiol SAMs, the peptide SAMs therefore prefer the anodic photocurrent. This point is also characterized by Ei0 from the following consideration. The redox potentials of TPP are 1.19 V for TPP+/TPP and −0.81 V for TPP/TPP−.35 When the applied potential would put the gold Fermi level just at the center of these two redox potentials (bias voltage of 0.19 V), the photocurrent should become zero due to the cancellation of the anodic photocurrent and the cathodic photocurrent with each

i = N (kAu−TPP − k TPP−Au)/(kAu−TPP + k TPP−Au + kdTPP)

where i, N, kAu−TPP, kTPP−Au, and kdTPP represent photocurrent at the applied potential, the number of photons absorbed by terminal porphyrin moieties per unit time, electron transfer rate constant from gold to porphyrin (cathodic photocurrent), electron transfer rate constant from porphyrin to gold (anodic photocurrent), and deactivation rate constant of the excited porphyrin moiety (Figure 5B), respectively. Rate constants of electron transfer kAu−TPP and kTPP−Au at any given applied potentials were calculated by the equations shown in Figure 5C, where ρ is an effective density of electronic states in a gold near the Fermi level assumed to be 0.3 eV−1, HDA is an electronic coupling between porphyrin and gold, h is Planck’s constant, λ is reorganization energy, kB is the Boltzmann constant, T is absolute temperature, e is electron charge, EApp is applied 8058

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The Journal of Physical Chemistry C Table 1. Summary of Conditions for Theoretical Calculations redox potential (V) ETPPox

ETPPred

surface potential (V)

kdTPP (s−1)

λ (eV)

8

108 108 108 107

theoretical photocurrents

entry 1 entry 2

1.19 1.19

−0.81 −0.81

0 −0.15

4.0 × 10 4.0 × 108

0.90 0.90

experimental curves

TPPM12 TPPM11 TPPM10 SA16TPP SAM of Figure 6

0.95 0.90 0.85 0.85

−0.95 −1.00 −1.05 −1.05

−0.15 −0.15 −0.15 −0.15

4.0 3.3 2.1 6.0

× × × ×

0.90 0.80 0.75 0.70

potential, ETPPox is the redox potential of TPP+/TPP or TPP*/ TPP−, ETPPred is the redox potential of TPP+/TPP* or TPP/ TPP−, and Edipole is the surface potential of the SAM induced helical peptide dipole moments. The HDA value in Figure 5C was calculated as 3.8 × 10−5 eV using following equation.36

There is still one controversial point if taking the tunneling factor of β value for the anodic photocurrent differently from the cathodic photocurrent is irrelevant discussion. In the present calculations, β of helical peptides was commonly taken to be 0.66 A−1 despite that the virtually used molecular orbitals are the LUMO level of peptide bonds for the anodic photocurrent or HOMO level for the cathodic photocurrent. This issue remains to be discussed even with the successful curve fittings here with using the common tunneling factor. Finally, we have tried to optimize the photocurrent generation with the SA16TPP SAM (TPPM10) by examining the preparation conditions. When the SAM was prepared from a mixed solution of ethanol and chloroform (Figure S7), the anodic photocurrent increased up to 3.9 μA/cm2 (Figure 6).

0 2 |HDA|2 = |HDA | exp( −βl)

where H0DA is a preexponential factor of electronic coupling to be 0.76 eV according to the literature,36 β is a tunneling factor of helical peptide to be 0.66 A−1,10 and l is molecular bridge length using the thickness of monolayer determined by ellipsometry. ETPPox and ETPPred can be estimated from the cyclic voltamgrams showing the smaller values by about 0.3 V in the SAMs than those of single molecules (Figure 3B). In the calculations, these ETPPox and ETPPred values were therefore taken as variable but to be the reduced values from the literature values by 0.4 V at most. Edipole was previously estimated to be −0.15 V.7 kdTPP and λ values were other parameters for the curve fittings. The values employed for the calculations are listed in Table 1. Curve fittings were successfully carried out as shown in Figure 5A between the theoretically determined photocurrents and the experimentally observed photocurrents by taking kdTPP to be 2.1 × 108 s−1 for TPPM10, 3.3 × 108 s−1 for TPPM11, and 4.0 × 108 s−1 for TPPM12. These values are reasonable for deactivation rate constants when compared with the reported value for the singlet excited TPP of 8.3 × 107 s−1.37 In Figure 5A, there are other two theoretical curves, one is taking the TPP redox potentials from the literature35 (theoretical current of entry 1) and the other further considers the effect of the helix dipole to generate surface potential of −0.15 V (theoretical current of entry 2). The Ei0 value of this entry 1 curve is around 0.19 V, which is consistent with the result of the alkanethiol SAMs reported by Imahori and co-workers.26,27 The entry 2 curve is shifted to preference of the anodic photocurrent (Ei0 value is around 0.04 V). But, both curves could not fit the experimental curves. In order to obtain the successful curve fittings, we had to use the lower redox potentials by 0.25−0.35 V from the literature values,35 which are supported by the cyclic voltamgrams of the peptide SAMs (Figure 3B) showing that a part of porpyrins has a lower redox potential. The quantum efficiencies increased with raising the fraction of SA16TPP in the SAMs, and the highest value of 1.1% was obtained with TPPM10. As shown by the curve fittings, kdTPP values became the smallest with TPPM10, suggesting that the deactivation rate constant of the singlet excited TPP should be retarded because the activated photoenergy should migrate efficiently among porphyrins probably in the associated regions in TPPM10.

Figure 6. Dependences of the photocurrent and dark current on the applied potentials in the optimized SA16TPP SAM in a 0.05 M TEOA and 0.05 M MV and 0.1 M Na2SO4 aqueous solution with a 430 nm light photoirradiation.

This photocurrent value corresponds to 6.8% quantum efficiency. On the other hand, the dark current was suppressed significantly at a low value of several tens of nA/cm2. In general, photodiodes are used under the reverse bias voltage to suppress the dark current. However, in the present peptide SAM, the dark current was suppressed significantly to allow the use under the forward bias voltage, because the peptide SAM was composed of a well-packed monolayer to prevent leak current.

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CONCLUSION We have investigated on photocurrent generation about the porphyrin-terminated helical peptide SAMs. Anodic photo8059

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The Journal of Physical Chemistry C

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currents were prevailing in these peptide SAMs, which makes a contrast with the cathodic photocurrent in the porphyrinterminated alkane SAMs. The dependence of photocurrent generation on the applied bias voltage was explained successfully by the theoretical calculations. Accordingly, the preference of the anodic photocurrent is attributed to the factors that a part of the porphyrins associate together to form J-aggregates, allowing efficient photoenergy migration in the SAM, and lowering the redox potentials of the associated porphyrins. We have demonstrated 6.8% quantum efficiency and achieved a good ratio of photocurrent and dark current with the optimized peptide SAM.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of helical peptides, circular dichroism spectroscopy, characterization of SA16M and SA16TPP SAMs, absorption and fluorescence spectra, cyclic voltammetry in a solution, and porphyrin Q-band absorption spectra and action spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported partially by JSPS Fellows (24 5908). H.U. acknowledges the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.

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REFERENCES

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