Dyad Sensitizer of Chlorophyll with Indoline Dye for Panchromatic

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Dyad Sensitizer of Chlorophyll with Indoline Dye for Panchromatic Photocatalytic Hydrogen Evolution Yuan Sun, Yuliang Sun, Chunxiang Dall’Agnese, Xiao-Feng Wang, Gang Chen, Osamu Kitao, Hitoshi Tamiaki, Kotowa Sakai, Toshitaka Ikeuchi, and Shin-ichi Sasaki ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00380 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 6, 2018

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ACS Applied Energy Materials

Dyad Sensitizer of Chlorophyll with Indoline Dye for

Panchromatic

Photocatalytic

Hydrogen

Evolution Yuan Sun,† Yuliang Sun,† Chunxiang Dall’Agnese,† Xiao-Feng Wang, †* Gang Chen,† Osamu Kitao, ‡ Hitoshi Tamiaki, ∥Kotowa Sakai,⊥ Toshitaka Ikeuchi,⊥ Shin-ichi Sasaki∥, ⊥

†

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education),

College of Physics, Jilin University, Changchun 130012, PR China. ‡

Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science

and Technology (AIST), Tsukuba Central 5, Ibaraki, 305-8565, Japan. ∥

Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan.

⊥

Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan.

To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Photocatalytic H2 evolution under wide visible spectral range is a challenging issue. In this work, we report the synthesis of a novel organic dyad consisting of an indoline-rhodanine πconjugation with a strong absorption in green region and a chlorophyll derivative with intense absorbance in purple and red regions. This synthetic dyad (Dyad) was employed as a photosensitizer in the TiO2-based system for photocatalytic H2 evolution with Pt as the assisting catalyst and ascorbic acid (AA) as the sacrificial reagent. The Dyad-sensitized Pt/TiO2 photocatalyst exhibited a maximum turnover number (TON) of 1044 after 6 h of continuous light irradiation (λ > 400 nm). In comparison, under the same conditions, the TONs of photocatalytic systems based on sole counterpart chlorophyll (Chl) or indoline dye (Ind) were merely 267 or 301, respectively. Moreover, the apparent quantum yield (AQY) of Dyad/Pt/TiO2 (1.27%) is much higher than that of Chl/Pt/TiO2 (0.37%) and Ind/Pt/TiO2 (0.20%) under 420 nm monochromatic light irradiation. The interfacial charge transfer and recombination processes between TiO2 and Dyad, Chl, or Ind were evaluated with photocurrent responses and electrochemical impedance spectroscopy. The DFT calculation was in accordance with the observed charge recombination processes. The high photocatalytic activity of Dyad was attributed to not only an excellent light absorption ability over the whole visible range, but also an efficient electron transfer and a balanced charge recombination processes between TiO2 and Dyad. In addition, the sustained activities of the H2 evolution systems were 78%, 84%, and 38% for Dyad, Chl, and Ind, respectively, after three 6-h illumination, indicating a similarly high reusability of both Dyad and Chl dyes as compared to Ind dye. This study simultaneously solves the problems of insufficient visible spectral response, poor stability, and high cost in photocatalytic water-splitting H2 evolution.


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KEYWORDS:

Chlorophyll,

Indoline,

Dyad

Sensitizer,

Panchromatic

Sensitization,

Photocatalysis, Hydrogen Evolution Reaction. 1. INTRODUCTION Energy is of great importance for human survival and development. Solar energy is clean, infinite, renewable, and available everywhere on the surface of the Earth. Hydrogen produced from solar energy gives an ideal way to solve the energy and environmental problems.1-4 Nowadays, extensive studies on photocatalytic hydrogen evolution (PHE) through water splitting have been reported. Especially, TiO2-based PHE has attracted tremendous attention because of its low toxicity, abundance, and high stability.5-7 Nevertheless, pure TiO2 only exhibits photocatalytic activity under illumination of ultraviolet light, due to the large band gap of TiO2. Modification of TiO2 surface with low band gap organic/inorganic sensitizers extends the photoresponse of TiO2-based catalysis into visible region.8-11 Moreover, an ideal dye-sensitizer should hold some other characteristics, such as low toxicity, cost-effectivity, and easy production. Chlorophylls are the most abundant natural pigments and widely found in green plants and photosynthetic bacteria.12-13 Chlorophylls with a chlorin macrocycle typically exhibit two types of intrinsic absorption peaks, i.e., the Soret band at 350-450 nm and the Q bands at 550-700 nm.14 Chlorophylls and their derivatives are potential candidates of the sensitizers for PHE, because they are abundant, readily available, environmental-friendly, and tunable with their photochemical and photophysical properties.15-16 Therefore, we recently reported some chlorophyll dyes with a carboxyl anchoring group at a different peripheral position of the chlorin macrocycle to sensitize TiO2 for PHE.17 We found that the PHE performance was highly dependent on both the position and the number of the carboxyl groups. Unfortunately, these chlorins absorb weakly the blue-green light, where the sunlight intensity is high. In natural


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photosynthesis, this wavelength region is usually covered by bilin and carotenoid molecules with different π-conjugation lengths. However, the accessory pigments are unstable in vitro undergoing fast degradation in ambient environment. Thus, any alternatives should be considered to sufficiently absorb the above light for efficient solar H2 production. Organic indoline dyes have been extensively employed as a light absorber in dye-sensitized and organic photovoltaic devices.18-21 Particularly, indoline dyes have very strong absorption in the bluegreen region.22 Herein, we synthesized a novel dyad molecule (Dyad) by connecting a chlorin sensitizer, methyl trans-32-carboxypyropheophorbide a with a commercially available indoline dye, D102, through the ester bonding. Synthetic Dyad is employed as a sensitizer to modify the surface of TiO2 via C32 carboxylate for panchromatic PHE. Compared the results with control samples of either chlorophyll or indoline dye (Chl or Ind) or their combination obtained either by coadsorption or physical mix, the Dyad-based photocatalyst gave a much higher H2 production rate of up to 4176 µmol g-1 and turnover number (TON) of 1044, after 6-h irradiation without UV light. In order to figure out what was behind such a significant enhancement in the photocatalytic activity, the UV-vis absorption spectra, photocurrent responses, cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were employed for each Dyad, Chl, or Ind dye. 2. EXPERIMENTAL SECTION 2.1. Synthesis of Dyad Pheophorbide-d ethylene glycol ester23 and a commercially available indoline dye D102 was connected by 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC·HCl) and 4dimethylaminopyridine (DMAP) to give the ester linkage dyad in 78% yield. Wittig reaction of the formyl group on chlorin ring with (tert-butoxycarbonylmethylene)triphenylphosphorane and


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the following cleavage of the tert-butyl ester in CF3COOH were done as previously reported24 to give the corresponding carboxylic acid Dyad in 90% yield for two steps (for synthetic details, see Supporting Information). 2.2. Synthesis of dye/TiO2/Pt photocatalyst Pt/TiO2 particles (containing 1wt% Pt) were prepared by a conventional photochemical deposition method.25 TiO2 (1 g, Degussa P25) was added to an aqueous solution (100 ml) of H2PtCl6 (0.25 ml, 8wt% aqueous solution) and MeOH (25 ml), and then irradiated by a 350 W xenon lamp (without an optical filter) with stirring for 2 h. The prepared Pt/TiO2 nanoparticles were centrifuged and washed with MeOH and dried under vacuum at 60 °C. Single dyesensitized Pt/TiO2 powders were prepared by a conventional impregnation method: The Pt/TiO2 nanoparticles (0.1 g) were dispersed at a acetonitrile/tert-butyl alcohol (1:1, v/v) solution of dyes (0.08 mM) with stirring in the dark for 20 h. The co-adsorbed mixture was prepared by immersing 0.1 g Pt/TiO2 nanoparticles at a acetonitrile/tert-butyl alcohol (1:1, v/v) solution containing both Chl and Ind dyes (0.04 mM for each dye, n:n=1:1) and stirring in the dark for 20 h. After immersion, the solvent was removed by centrifugation and filtration. The obtained powder of dye-adsorbed Pt/TiO2 photocatalyst was then washed with the mixed solvent and dried under vacuum at 60 °C. 2.3. Characterization UV-vis absorption was determined with a Shimadzu UV-3100 spectrophotometer (Japan). The CV, photocurrent responses and EIS were obtained on a VSP multichannel potentiostatic– galvanostatic system (Bio-Logic SAS, France). 2.4. Photoelectrochemical measurements


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The dye-sensitized TiO2 (Degussa P25) films electrodes were prepared by the conventional doctor blade technique and then sintered at 500 °C for 30 min under air. They were immersed in a acetonitrile/tert-butyl alcohol solution (1:1, v/v) of 0.01 mM carboxylated dyes in the dark for 12 h, then rinsed with the mixed solvent and dried. Photocurrent responses and EIS measurements were carried out on a standard three electrode cell with an aqueous solution of Na2SO4 (0.5 M) as the electrolyte and ascorbic acid (AA) (2 g L1

) as the sacrificial agent. The dye-sensitized TiO2 films coated on FTO glass were used as the

working electrodes, with a Pt flake as the counter electrode and Ag/AgCl as the reference electrode. EIS were recorded with an amplitude of 10 mV over the frequency range of 0.01 Hz to 105 Hz under illumination, and the analysis was performed by Z-view software. CV measurements in dry CH2Cl2 were also performed in three electrode cell using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte. Working and counter electrodes were platinum wires, and Ag/AgCl was used as the reference electrode. The measured potentials were calibrated with a ferrocene/ferrocenium (Fc/Fc+) redox couple. 2.5. Computational methods All calculations were done by Gaussian 0926 with CAM-B3LYP exchange-correlations functional and the 6-31G(d,p) basis set with PCM(water) as described before.27-29 2.6. H2 production performance The PHE experiments were carried out in a closed 6 ml photoreactor under a 350 W xenon lamp irradiation through a cut-off filter (λ > 400 or 600 nm). The dye/TiO2/Pt photocatalyst (2.5 mg) was suspended in 3 ml of an aqueous solution containing 50 mM AA (pH = 2.8) as the sacrificial reagent. Prior to irradiation, the photoreactor was ultrasonicated for 15 min and then purged with argon to remove the dissolved air. The H2 evolution volume was analyzed by a gas


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chromatograph (5 Å molecular sieve column with Ar carrier gas, TCD, GC, SP-3420A, BeifenRuili, China). TONs were calculated as (2overall H2 amount)/(dye loading) and apparent quantum yield (AQY) under 420 nm monochromatic light irradiation were calculated as (2 number of evolved H2 molecules)/(number of incident photons)100%.30 3. RESULTS AND DISCUSSION 3.1. Molecular structures and frontier molecular orbital Figure 1 shows the energy-optimized molecular and chemical structures of the three dyes investigated in this work. The Chl dye has been extensively studied in dye-sensitized solar cells. It contains a chlorin macrocycle as the backbone, on which an electron-withdrawing carboxyl group was conjugated at the C32 position. The Ind dye consists of an indoline unit and a rhodanine ring acting as the electron-donating and withdrawing moieties, respectively. Simply esterifying Chl at the C17 substituent of the chlorin with the rhodanine side of Ind gave the ChlInd dyad, Dyad.

Figure 1. Energy-optimized molecular (upper) and chemical structures (lower) of the present dyes, Chl, Ind, and Dyad, bearing a carboxyl group.


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Frontier molecular orbitals of dye molecules play an important role in determining the suitability of dye molecules for charge recombination by evaluating the charge separated states. Figure 2 compares the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) of Chl, Ind and Dyad dyes. A long-range intramolecular charge transfer is observed in the photoexcited state of Ind, which is unfavorable for the charge recombination from the TiO2 to the dye cation. On the other hand, the HOMO and LUMO orbitals of Chl are similar, indicating that the electrons in TiO2 tend to recombine with holes in the chlorin macrocycle. As a result of the combination of Chl and Ind to Dyad, the electrons are partly situated at the carboxyl anchoring group of Dyad at the LUMO orbital to improve the excitonic coupling between Dyad and TiO2, while the electrons are delocalized in the chlorinsystem at the HOMO orbital to avoid consequent rapid charge recombination. In brief, the charge recombination tendency among the three dyes follows the order of Chl > Dyad > Ind.

Figure 2. Frontier orbitals based on DFT/CAM-B3LYP/6-31G(d,p) with PCM(water) for Chl, Ind, and Dyad.


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3.2. Electronic absorption and electrochemical properties Figure 3 shows the UV-vis absorption spectra of Chl, Ind and Dyad dyes in acetonitrile/tertbutyl alcohol (1:1, v/v). Chl exhibits typical Soret, Qx and Qy bands absorption peaks at 350-450, 500-550, and 600-700 nm, respectively. Compared to the major Soret and Qy maxima, the Qx intensities are much weaker. Ind exhibits a strong absorption band at the wavelength region of 450-550 nm peaking at 520 nm, where the absorption of Chl is weak. As a result of the combination of Chl and Ind dyes, the absorption spectrum of Dyad exhibits a very wide spectral feature over the whole visible region (380-750 nm). Since the Chl and Ind moieties are connected without any π-conjugation, the high extinction coefficients of these moieties are maintained in Dyad. The broader bands in Dyad than those of Chl and Ind could be attributed to intramolecular π-π interaction between the Chl and Ind moieties. Besides, a little redshift of the absorption band is observed for the Ind moiety of Dyad compared with the single Ind. This little redshift is mainly because of the real system intramolecular interaction between the Ind part and the Chl part. The panchromatic light absorption capability of the novel Dyad dye allows excellent light harvesting efficiency for the following PHE apparatus.


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Figure 3. UV-vis absorption spectra of Chl, Ind, and Dyad in acetonitrile/tert-butyl alcohol (1:1, v:v).

Figure 4 shows the cyclic voltammetry curves of Chl, Ind, and Dyad. The oxidation potentials of Chl, Ind, and Dyad are observed to be Eox = 1.16, 1.12, and 1.12 V vs. Ag/AgCl, respectively. The half-wave potential of ferrocene/ferrocenium (Fc/Fc+) as an external reference is measured to be 0.69 V vs. Ag/AgCl. The HOMO levels of these molecules could be calculated from the following equation:31 HOMO eV e 4.8 Where is the one electron oxidation onset potential of these dye molecules. The

values of Chl, Ind and Dyad are 0.98, 0.95 and 0.96 V vs. Ag/AgCl, respectively. The LUMO levels of these dyes could be calculated by HOMO + , where

is the optical bandgap

obtained from the absorption spectra. The oxidation potentials and energy levels of Chl, Ind, and Dyad are summarized in Table 1. It shows that the HOMO levels of these dyes are close, which are -5.09, -5.06, and -5.07 eV for Chl, Ind, and Dyad, respectively. The lowest-energy transition


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from HOMO to LUMO of excited-state Dyad is easier upon illumination due to the smaller band gap of Dyad as compared to Chl and Ind. The LUMO energy levels of these dyes are more negative than the conduction band edge (ECB) of TiO2, indicating that they are suitable for efficiently transferring electrons to TiO2. In particular, Ind is theoretically more favorable for electron injection due to its more negative LUMO level than other two dyes. The Dyad is formed by a simple connection of Chl and Ind moieties without π-conjugation. However, as shown in Figure 2, the electron density of the Dyad molecule is clearly different from each Chl and Ind moieties, because these moieties are somehow coupled through intramolecular π-π interaction. The excitonic coupling between the Dyad and TiO2 is obviously stronger than that between Chl and TiO2, owing to the presence of Ind moiety. The stronger excitonic coupling as well as the easier electron transition from HOMO to LUMO of excitedstate Dyad are beneficial for the better electron transfer ability of Dyad to TiO2.

Figure 4. Cyclic voltammograms of Chl, Ind, and Dyad in dry CH2Cl2 with 0.1 M TBAPF6 as a supporting electrolyte at a scan rate of 100 mV/s.


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Table 1. Optical and electronic properties of Chl, Ind, and Dyad. Eox(a)

λonset(b)

(c)

[V]

[nm]

[V]

Chl

1.16

693

Ind

1.12

Dyad

1.12

Dye

(d)

HOMO

LUMO

[eV]

[eV]

[eV]

0.98

1.79

-5.09

-3.30

561

0.95

2.21

-5.06

-2.85

717

0.96

1.73

-5.07

-3.34

a

Oxidation potential vs. Ag/AgCl.

b

Onset wavelength was recorded in acetonitrile/tert-butyl alcohol (1:1, v/v) at 298 K.

c

Onset oxidation potential vs. Ag/AgCl was recorded in dry CH2Cl2 with 0.1 M TBAPF6 as an

electrolyte. d

Optical band gap was estimated from λonset; = 1240/λonset.

3.2. Photocatalytic H2 evolution activity analysis To study the PHE activity of the covalently bound Dyad dye, the single Chl and Ind dyes adsorbed Pt/TiO2 powder (8 µmol g-1 for each dye), and a mixture of Chl and Ind (n:n=1:1) coadsorbed on Pt/TiO2 surface (4 µmol g-1 for each dye) were tested, as shown in Figure 5a. It shows the amount of H2 evolution during 6 h under simulated solar light irradiation (λ > 400 nm) for the dye sensitized Pt/TiO2 photocatalysts. The amount of H2 evolution with light irradiation for 6 h was 1067, 1204, 4176 and 3259 µmol g-1 with corresponding TONs of 267, 301, 1044 and 814, for Chl, Ind, Dyad and co-adsorbed mixture of Chl and Ind, respectively. In addition, the activity of hydrogen production from a physical mixture of Chl adsorbed Pt/TiO2 powder and Ind adsorbed Pt/TiO2 powder (8 µmol g-1 for each dye, and each dye loaded Pt/TiO2 powder


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mass is 2.5 mg) is shown in Figure S4 (Supporting Information). We found that the amount of hydrogen produced is still smaller than that produced from the Dyad-based photocatalyst. Furthermore, under 420 nm light irradiation, the AQY of Dyad-sensitized Pt/TiO2 photocatalyst (1.27%) is much higher than that of Chl- or Ind-sensitized Pt/TiO2 (0.37% or 0.20%, respectively). The above PHE activity of these dye-based systems could be summarized as follows: 1. The amount of H2 production is near a linear relation to the irradiation time for all the samples; 2. The PHE activity of Ind is slightly higher than that of Chl; 3. The PHE activity of Dyad is not only much higher than that of either Chl or Ind, but also higher than those of the cosensitization and physical mix of Chl- and Ind-based systems. We also evaluated the PHE photoactivities at λ > 600 nm as displayed in Figure 5b. Under the same conditions, Dyad still remain a high PHE activity of 1280 µmol g-1, which is 4 times higher than that of Chl with 315 µmol g-1. Obviously, due to the lack of absorption above 600 nm, Ind shows no hydrogen evaluation. Thus, the PHE activities of both Dyad and Chl rely on their Qy absorption bands. Considering that the light harvesting efficiencies of Dyad and Chl dyes are nearly the same at λ > 600 nm, the large difference in their PHE activities should be attributed to their interfacial electron transfer/recombination efficiencies. Moreover, the hydrogen evolution under λ > 600 nm shows the same trend as that under illumination at λ > 400 nm for Dyad and Chl, except that the amount of H2 production dropped to about 1/4, owing to the limited light harvesting capability by Qy band.


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Figure 5. H2 evolution over Chl, Ind, Dyad and a mixture of Chl and Ind (n:n=1:1) co-sensitized TiO2/Pt photocatalysts in an aqueous 50 mM AA solution at pH = 2.8 with visible light irradiation: (a) λ > 400 nm and (b) λ > 600 nm.

We further investigated the reusability of the dye/TiO2/Pt-based PHE systems by operating it with λ > 400 nm light irradiation for three cycles with every 6 h for each cycle, and the results are shown in Figure 6. After three cycles’ irradiation, these dye-based PHE systems remain 84%, 38%, and 78% of their initial activity for Chl, Ind, and Dyad, respectively. Clearly, Ind and Chl show the least and the highest stability, respectively. The least stability of Ind could be attributed to the desorption of Ind from TiO2 surface because obvious fading color of Ind/TiO2/Pt sample was observed after prolonged illumination. In contrast, no clear discoloration was observed for Chl-based samples. Due to the same reason, Dyad exhibits a similar stability as Chl owing to its connection with TiO2 through a carboxyl group on a chlorin macrocycle.


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Figure 6. Long-term stability for different dye/TiO2/Pt photocatalysts in an aqueous 50 mM ascorbic acid solution at pH = 2.8 with visible light irradiation (λ > 400 nm).

Figure 7a shows the EIS results of these dye-sensitized TiO2 film electrodes. EIS was performed to elucidate the interface charge recombination based on the transmission line model typically proposed by J. Bisquert et al,.32 Rs in the high frequency range represents the sheet resistance of FTO and the contact resistance between the FTO and TiO2.33 A small semicircle in high frequency range corresponds to the charge transfer resistance at the Pt/electrolyte (R1).34 On the other hand, the large semicircle at the intermediate frequency is assigned to charge recombination resistance at the TiO2-dye-electrolyte interface (R2).35 We performed EIS measurement at a fixed applied potential of -0.197 V vs. Ag/AgCl. The semicircular diameter of the Ind sensitized TiO2 sample at intermediate frequency is the greatest among the three, indicating the large recombination resistance at the TiO2-dye-electrolyte interface which can effectively reduce charge recombination. Conversely, the Chl sensitized TiO2 sample exhibits the smallest arc radiuses, which reflects the smallest charge recombination resistance at the Chl-TiO2 interface and the highest recombination probability of the photo-induced electrons and holes.


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Notably, the charge recombination resistance of Dyad is between Ind and Chl. The experimental result that the charge recombination resistance follows the order of Ind > Dyad > Chl is in accordance with the charge recombination tendency (Ind < Dyad < Chl) from the DFT calculations. Figure 7b shows the photocurrent responses of Chl, Ind, and Dyad sensitized TiO2 film electrodes upon visible light irradiation with repeatable on/off cycles. The intensity of response photocurrent is employed to evaluate the electron transfer ability from the dye to TiO2.36-37 The photocurrent is a combination result of a delicate balance between absorbance and injection probability, recombination, a series of transport and resistance. The photocurrent response observed in Dyad sensitized TiO2 sample is obviously higher than those in Chl and Ind, indicating that electrons transferred to TiO2 from Dyad are more effective than those from Chl and Ind. This can be partially attributed to light absorption ability of Dyad.

Figure 7. (a) EIS of dye-sensitized TiO2 electrodes at -0.197 V vs. Ag/AgCl under illumination and (b) Current responses of dye-sensitized TiO2 electrodes under illumination with an applied bias potential of 0.2 V vs. Ag/AgCl.


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As discussed in previous section, the efficiency in the dye-sensitized PHE systems is determined by light-harvesting efficiency and electron transfer/recombination dynamics. In fact, Dyad/TiO2/Pt photocatalyst which exhibits the highest performance of PHE among these three dyes investigated is a result of multiple factors. First, the extremely broad absorption spectra and high molar extinction coefficient of Dyad molecules ensure a high light-harvesting efficiency. Second, the electron transfer ability as determined by the photocurrent responses of Dyad is much better than Chl and Ind. Third, the charge recombination process of the Dyad is moderate among these three samples. Finally, Dyad exhibits a good stability on TiO2 that is similar to Chl but much better than Ind. Chl demonstrates higher light-harvesting and electron transfer abilities compared to Ind, but the amount of H2 evolution is slightly lower than that of Ind. This could be attributed to the relatively faster interfacial charge recombination and weaker electron injection ability of Chl as compared to Ind. Figure 8 shows one possible photocatalytic hydrogen evolution mechanism of the Dyadsensitized Pt/TiO2 system.37 In short, first, the sensitizer Dyad adsorbed on Pt/TiO2 surface by the carboxyl anchoring group is excited from the ground state to the excited state through light absorption, and the photogenerated electrons from the excited-state dye are directly injected into the conduction band of TiO2. Second, the photogenerated electrons are quickly transferred to the TiO2 surface and captured by Pt particles for the hydrogen production reaction from aqueous solution. Third, the oxidized dye can be reduced via obtaining electron from AA to accomplish the cycle, leading to the regeneration of ground-state dye.


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Figure 8. Possible mechanism of photocatalytic hydrogen evolution of Dyad/Pt/TiO2 system.

Dyad composed of chlorophyll and bulky indoline moieties linked via a non-π-conjugated bond is very similar to Phe-Car adduct that consists of chlorophyll moiety with a C32-cyano substituent and bulky carotenoid moiety linked by a non-π-conjugated bond.38 Therefore, Dyad is very likely to exhibit similar electron transfer mechanism as Phe-Car adduct. That is, three possible pathways including electron transfer and singlet-energy transfer from indoline moiety to chlorophyll unit as well as the suppression of singlet-triplet annihilation of the whole Dyad molecule contribute to the electron transfer from Dyad to TiO2.38 It is well-known that transition from HOMO to LUMO is the minimum-energy excited state transition. It is not hard to imagine that intramolecular electron transfer from indoline unit to chlorophyll moiety in Dyad can occur in higher-energy excited state transition. Besides, the photocurrent responses of dye-sensitized TiO2 shown in Figure 7b show that the electron transfer ability of Dyad is better than single Chl or Ind dyes, which is partially ascribed to the enhanced light absorption of Dyad. The photocurrent generated by Dyad-sensitized TiO2 is not only more than that of the sum of Chl and Ind dyes but also doubles that of Chl dye, demonstrating both the chlorophyll and indoline


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moieties (i.e., the whole Dyad molecule) contribute to excited electron transfer for the PHE of Dyad/Pt/TiO2 system. The absorption spectra of dye-sensitized TiO2 films provided in Supporting Information (see Figure S5) also proves the excited-state electron transition of Dyad occurs in both the chlorophyll and indoline units. In brief, the overall Dyad molecule acts as effective electron transfer excited state. This is consistent with the proposed electron transfer mechanism.

4. CONCLUSIONS In summary, a novel organic Chl-Ind dye, Dyad, with a panchromatic absorption capability was designed and synthesized. Dyad as a photosensitizer exhibited a superior H2 evolution activity of 4176 µmol g-1 with a TON of 1044 over 6 h, which is higher than the references Chl (1067 µmol g-1) and Ind (1204 µmol g-1) dyes. Notably, the photocatalytic activity of Dyad-based TiO2/Pt is also higher than that based on the co-sensitization of Chl and Ind (3259 µmol g-1), and even that of the physical mixture of Chl and Ind (1792 µmol g-1). Besides, Dyad-based Pt/TiO2 also shows much higher AQY (1.27%) than the references Chl (0.37%) and Ind (0.20%) dyes under 420 nm light irradiation. The superior photocatalytic activity for Dyad is attributed to the following factors: 1) broad absorption response over the visible light region, 2) efficient electron transfer and slow charge recombination with TiO2, and 3) high stability over prolonged light illumination. All these reasons lead to the optimal photocatalytic activity of Dyad among these investigated dyes. This study is expected to open a new insight into traditional dye-sensitized photocatalysis and develop a low cost and highly efficient photosensitizer for the photocatalytic H2 evolution through water splitting.


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ASSOCIATED CONTENT Supporting Information Synthesis of carboxylated chlorin–indoline Dyad and additional data are available free of charge via the internet at DOI: …… AUTHOR INFORMATION Corresponding Author * X.-F. Wang. E-mail: [email protected] Author Contributions X.-F.W. generated the idea and designed the experimental plan. Y.S and Y.S conducted the corresponding photocatalysts preparation and basic characterization. S.S., K.S., and T.I. contributed to the synthesis of Dyad. C.D. participated in the results discussion and manuscript revision. H.T., G.C., X.-F.W. provided related experimental conditions and participated in the technical discussions. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by the Natural Science Foundation of China (No. 11574111 to X.-F.W.), Natural Science Foundation of Jilin Province (No. 20160101303JC to X.-F.W.), JSPS KAKENHI Grant Number JP16K05826 in Scientific Research (C) (to S.S.), and JSPS KAKENHI Grant Numbers JP24107002 and JP17H06436 in Scientific Research on Innovative Areas (to H.T.). The computations were performed using Research Center for Computational Science, Okazaki, Japan.


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Dyad Sensitizer of Chlorophyll with Indoline Dye for Panchromatic

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