Title: Organic Dyes based on Tetraaryl-1,4-dihydropyrrolo-[3,2- b]pyrroles for Photovoltaic and Photocatalysis Applications with the Suppressed Electron Recombination

Authors: Zhen Li

This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article.

To be cited as: Chem. Eur. J. 10.1002/chem.201803688

Link to VoR: http://dx.doi.org/10.1002/chem.201803688

Organic Dyes based on Tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles for Photovoltaic and Photocatalysis Applications with the Suppressed Electron Recombination

Jinfeng Wang [a] , Zhaofei Chai[a] , Siwei Liu[a], Manman Fang[a], Kai Chang[a], Mengmeng Han[a], Li Hong[b], Hongwei Han[b], Qianqian Li [a], Zhen Li* [a][c]

Abstract: Tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles (TAPP) moiety with the combination of two pyrrole rings and four phenyl moieties demonstrated strong electron-donating ability and nonplanar configuration simultaneously. Once incorporated into the organic dyes as a novel electron donor, it was beneficial to the enhancement of light-harvesting ability and suppression of electron recombination in the photovoltaic and photocatalysis systems. With the linkage of tunable conjugated bridges and electron acceptor, the corresponding organic dyes exhibited improved photovoltaic performance in dye-sensitized solar cells and facilitated the photocatalytic hydrogen generation with highest turnover number (TON) of 4337. Through the detailed investigation of relationship between molecular structures and photovoltaic/photocatalysis property, the connection and difference in molecular design for these two systems are well explained, with the aim to promote the application of dye-sensitized technology in various fields.

Introduction

Photovoltaic and photocatalysis devices, which can generate electricity and hydrogen directly from sunlight respectively, are promising to offer clean solutions to the energy crisis and environmental degradation around the globe.[1] In these conversion processes, dye-sensitized technology plays the essential role in the light-harvesting and electron transitions. Generally, sensitizers were excited when they absorbed sunlight and electrons were injected into the conduction band of TiO2, then, the oxidized dye could be regenerated by electrolyte in dye sensitized solar cells (DSCs) or sacrifice regent in water splitting (WS) system.[2] Accompanied with these positive electron processes, the unfavorable electron recombination always occurs on the TiO2 surface among the electrons in TiO2, the oxidized dyes and the regenerating agents (electrolyte or sacrifice agent). Thus, with the aim to achieve excellent photovoltaic or photocatalysis performance, the suppression of electron recombination is essential and becomes the urgent issue, which is mainly related to the aggregated behavior of organic dyes as the adsorption state on TiO2 surface, also, the dye regeneration process in different environments can affect the electron collection.[3]

As to organic dyes with typical push-pull structures, the electron donor (D), which is furthest away from TiO2 surface as a part of dyes and closest to the electrolyte or sacrifice agent on some level, can be the key correlation among them, and play the crucial role in the adjustment of electron transitions in the dye/TiO2/electrolyte or sacrifice regent interface.[4] Until now, the most common used one is triarylamine unit, which contains nitrogen atom as the electron-rich center, demonstrates the strong electron-donating property and unique spatial configurations,[5] and the highest conversion efficiency (14.3%) of the corresponding DSC is achieved with the optimized cosensitized system.[6] Also, the triphenylamine-based organic dyes show attractive photocatalytic performance in light-driven H2 production from water with remarkable H2 turnover number (TON) of 10200 (48 h). [7] However, when the nitrogen atom is incorporated into cyclics as a part of some electron donors, such as carbazole, indoline, phenothiazine, and so on, just moderate performance can be obtained. It is mainly related to the planar structure of electron donor with extend π system, which can induce dye aggregation and aggravate electron recombination to some extent.[8] This phenomenon is proved by the photovoltaic performance of indoline-based organic dyes with different aromatic rings substitutions.[9] The conversion efficiencies of the corresponding DSCs increased largely with the enlarged sizes of substituents with twisted configuration to indoline donor. For instance, when the aromatics linked to nitrogen atom of indoline ring changed from phenyl to carbazole moiety, the corresponding conversion efficiencies can be enhanced from 5.08% to 8.49% (Chart S1). Thus, the modification of donor moieties by incorporating twisted substituents is efficient and necessary, which can suppress the dye aggregation in some degree for the decreased intermolecular interactions and enlarged distance between conjugated skeletons.

Accordingly, a series of TAPP-based sensitizers (LI-127-LI130, Figure 1A) have been designed and synthesized with the tunable π-conjugated bridge, also, some other twisted structures, for example, triphenylethylene moiety, have been introduced to further suppress the possible electron recombination. When they were applied into the DSCs device, the suppression effect on electron recombination was obvious with the improved photovoltaic performance, and the similar trend can also be observed in the photocatalytic hydrogen generation system (Figure 1B). Herein, we would like to report the synthesis of organic dyes, the theoretical calculations, their optical and electrochemical properties, as well as the photovoltaic and photocatalytic performance in detail.

Results and Discussion

With the incorporation of TAPP moiety bearing the almost orthorhombic structure as the electron donor (D), four TAPPbased organic dyes were designed and synthesized with the linkage of different conjugated bridges and cyanoacetic acid as the electron acceptor. Moreover, the famous AIEgen, tetraphenyl ethylene (TPE) with the twisted configuration was constructed in the donor part to further suppress the possible electron recombination at the TiO2/dye/electrolyte interface.[13] Thus, the optimized electron donor, TAPP derivatives can be formed by the combination of dihydropyrrolo-[3,2-b]pyrroles with electron-rich property as the core, together with the twisted structures surrounding as the isolation groups. They were synthesized by one-pot reaction easily, and the modification by flexible chains and aromatics can be conducted by the nucleophilic substitution and Suzuki coupling reaction, respectively. Through the linkage of conjugated bridge (π) with tunable electron properties by single carbon-carbon bonds and the electron acceptor (A) with double bonds, the pull-push conjugated skeleton with D-π-A type was formed, which was beneficial to the intramolecular charge transfer (ICT) and light harvesting. All the organic dyes exhibited good solubility in common solvents and the purity was confirmed by 1H NMR, 13C NMR, mass spectra and elemental analysis.

Optical properties

The UV-Vis absorption spectra of dyes LI-127-LI-130 in solution and on TiO2 film were shown in Figure 2, and the corresponding data were collected in Table 1. The organic dyes show broad absorption ranging from 300 to 650 nm, and exhibited two distinct absorption bands. The absorption region at 300-420 nm was assigned to the p-p* transition of aromatics, while the absorption in the region of 450-650 nm was ascribed to the intramolecular charge transfer (ICT) through the whole push-pull conjugated system. Dye LI-128 and LI-130 bearing thiophene as the conjugated bridge demonstrated significant red-shifted absorption with the peaks at 506 and 505 nm, respectively, in comparison with those of LI-127 (406 nm) and LI-129 (412 nm) at the same conditions. It was mainly due to the lower delocalization energy of thiophene (117 kJ/mol) than that of benzene ring (151 kJ/mol), which was beneficial to electron transition from the electron donor (TAPP) to acceptor (cyanoacetic acid). Also, the more planar configuration of conjugated skeleton in LI-128 and LI-130 afforded the positive effect on ICT process, which can be confirmed by theory calculations. After four organic dyes were adsorbed onto TiO2 surface, the extended absorption spectra were obtained with the much red-shifted onset wavelengths (λonset) (Table 1). The changes between absorption spectra in solution and TiO2 film were related to the arrangement of organic dyes as the aggregated state and the interaction of anchoring group (-COOH) with TiO2 surface. The conjugated bridges with different electron properties and the molecular geometries play the key role to optimize the absorption behaviors of organic dyes on TiO2 surface. In this case, from approximate single molecular (solution) to aggregated state (TiO2 film), the absorption spectra of LI-127 and LI-129 show much larger red-shifts (about 100 nm) than those of LI-128 and LI-130 (about 25 nm), which may be attributed to the more compact alignment with larger dye-loading amounts (Table S6).

Electrochemical characterization

Electrochemical behaviors of these organic dyes were measured by cyclic voltammetry to evaluate their redox potentials and the corresponding HOMO and LUMO levels (Figure S1). Two reversible oxidation waves were observed for these organic dyes. The first oxidation potentials at around 0.7 V (vs NHE) were assigned to the oxidation of the electron donor moiety, which corresponded to the HOMO levels. They were almost identical, since their electron-donating abilities were mainly determined by the same dihydropyrrolo-[3,2-b]pyrrole moiety as the electron-rich unit. Thus, these oxidized organic dyes can be regenerated by iodine/triiodide electrolyte theoretically, for their more-positive potentials than that of iodine/triiodide electrolyte (0.4 V vs NHE) with the energy gaps ³ 0.3 V.[14] The excited state oxidation potentials (Eox*), which were estimated from Eox -E0−0/e and corresponded to the LUMO levels, were -1.81, -1.30, -1.78 and - 1.30 V for LI-127-LI-130, respectively. They can be classified by the different conjugated bridges, while the incorporation of thiophene ring led to the lower LUMO levels. Fortunately, they were more negative than the ECB of the TiO2 electrode (-0.5 V vs NHE), indicating that electron injection from the LUMO orbitals of organic dyes into the conduction band of TiO2 is energetically permitted.

Theoretical approach

The optimized molecular structures and intramolecular charge transfer of these organic dyes could be understood by Density functional theory (DFT) calculations, which were conducted by the Gaussian 16 software at B3LYP/6-31G* level.[15] As to the electron donor (TAPP), the four phenyl units around dihydropyrrolo-[3,2-b]pyrrole core demonstrated different dihedral angles with varied linkage positions (Figure S2). The smaller dihedral angles (about 35o ) along the conjugated skeleton was favorable to the intramolecular charge transfer though the whole molecules, and the larger dihedral angles (about 135o ) in the other orientation results in the twisted configuration between the isolation groups (phenyl units) in the sides and the conjugated skeleton, which was beneficial to suppressing dye aggregates with strong intermolecular interactions. After the incorporation of different conjugated bridges (phenyl or thienyl), the more planar geometry with adjacent dihedral angles of about 19o can be formed in LI128 and LI-130 by thienyl unit as the bridge, which was beneficial to intramolecular charge transfer through whole molecules, resulting in the red-shifts of absorption spectra, as mentioned above. It can be further proved by the larger overlaps of electron distribution in HOMOs and LUMOs.

Photovoltaic performance of DSCs

Then, DSCs based on these TAPP-based organic dyes were fabricated with sandwich structures, and the electrolyte consisted of 0.6 M dimethylpropyl imidazolium iodide, 0.1 M lithium iodide, 0.05 M iodine, 0.5 M tert-butylpyridine in acetonitrile/3- methoxypropionitrile (85:15, v/v). The photocurrent density– photovoltage (J-V) curves (Figure 3A) were measured under an irradiance of standard AM 1.5G sunlight (100 mW cm−2 ), with the photovoltaic parameters listed in Table 2. The DSCs based on dye LI-128 and LI-130 with thienyl as the conjugated bridge exhibited the higher short-circuit photocurrent densities (Jscs) of 12.46 and 13.78 mA cm-2 , respectively, in comparison with that of LI-127 (9.08 mA cm-2 ) and LI-129 (9.36 mA cm-2 ) with similar structures, mainly attributing to their extended absorption spectra with higher light harvesting abilities. It was consistent with the more planar structures of LI-128 and LI-130 by theoretical calculations, and can be also proved by the monochromatic photoelectric conversion efficiency (IPCE) curves. As shown in Figure 3B, IPCE curves of LI-128 and LI-130 can extend to 750 nm, while those of LI-127 and LI-129 blue-shifted to 700 nm, meaning that the intramolecular charge transfer was more efficient in organic dyes bearing thienyl moieties. However, the open-circuit voltages (Vocs) demonstrated different trends, which mainly related to the substituents on electron donor moieties. For dye LI-127 and LI-128 with alkylphenyl moieties linked to TAPP unit, the Voc values of corresponding DSCs were only 625 and 639 mV, respectively. However, once the substituents were replaced by triphenylethylene, Vocs increased obviously in LI-129 (680 mV) and LI-130 (664 mV)-sensitized solar cells, indicating the important role of triphenylethylene with large size and twisted configurations. It could be helpful to suppressing the electron recombination with the inhibition of dye aggregation. Therefore, the highest conversion efficiency (η) of 6.56% was achieved by DSC based on LI-130 with the balance of Jsc and Voc values.

Furthermore, the coadsorbent chenodeoxycholic acid (CDCA) was incorporated to further improve the photovoltaic performance. When the concentration of CDCA was only 5 mM, a tiny improvement was obtained with η of 6.86% (Table S1). However, it would drop obviously with the increased contents of CDCA, meaning that dye LI-130 did not form severe dye aggregates on the TiO2 surface. It was mainly due to the TAPP donor with two isolation groups in the sides, together with twisted triphenylethylene unit at the head, which can decrease the intermolecular interaction in a large degree. Moreover, the DSCs based on these TAPP-based organic dyes exhibited increased conversion efficiencies for the indoor applications (Table S2 and S3), and the highest η value of 13.45% was achieved by LI-130- sensitized solar cells under white LED light (0.56 mW cm-2 ). Based on the improved photovoltaic performance of LI-130 among the four organic dyes, it was further modified by introduction of benzothiadiazole unit as the auxiliary electron acceptor into the conjugated bridge, with the aim to expand the light-harvesting region of DSCs. As shown in Figure 4, dye LI-131 and LI-132 demonstrated red-shift absorption spectra and broad IPCE curves (Figure S3 and S4), mainly due to the strong intramolecular interactions by the aid of benzothiadiazole with electron-withdrawing property, as well as their relatively planar structures (Figure S5). Accordingly, the extension of π-bridge usually resulted in the up-shift of HOMO levels. The Eoxs of dye LI-131 and LI-132 shifted to 0.70 and 0.69 V (Table S4), respectively. The conversion efficiencies of DSCs based on these two dyes decreased largely in comparison with that of LI-130, with η value of 4.48% and 1.98% (Table S5), respectively. These varied conversion efficiencies of DSCs mainly related to the electron recombination at the dye/electrolyte/TiO2 interface, which can be explained by charge extraction (CE) method and intensity-modulated photovoltage spectroscopy (IMVS) (Figure 5). Since the DSCs based on these organic dyes were fabricated with the same electrolyte, Voc is mainly influenced by the conduction band (ECB) and free charge density of TiO2 film.[16] The shift of ECB was conducted by the CE technique, as shown in Figure 5A. By observing the shift in the amount of charge extracted with respect to the open-circuit voltage, the shifts in ECB were not obvious for the four dyes LI-127-LI-130, probably due to their similar dipole moments with the same donor and acceptor. After the introduction of benzothiadiazole moiety as the auxiliary electron acceptor into the conjugated bridge, the resultant dye LI-131 and LI-132 exhibited much different behaviors. Dye LI-132 bearing more planar structure and stronger intramolecular charge transfer demonstrated lower extracted charge density at the same Voc, suggesting that ECB of the corresponding DSC was upshifted. However, the lower electron lifetime in Figure 5B means the severe electron recombination at the dye LI-132/electrolyte/TiO2 interface, leading to the much decreased conversion efficiency. Meanwhile, the electron lifetime of dye LI-131-sensitized solar cell is much higher than that of LI-132, but still lower than those of LI127-LI-130 without auxiliary electron acceptor. The decrease of electron lifetime was mainly related to electron trap effect of benzothiadiazole moiety and the molecular arrangements on the TiO2 surface. Compared to dye LI-127-LI-130, the decreased dye loading amounts of LI-131 and LI-132 might mean the tilted adsorption geometries on TiO2 surface, attributing to the increased molecular lengths. [17] As to the four dyes of LI-127-LI130, DSCs based on LI-129 and LI-130 bearing triphenylethylene substituent displayed longer electron lifetime than that of LI-127 and LI-128 with alkylbenzene moiety, further confirmed the key role of triphenylethylene with twisted configuration and large size. It can decrease the electron recombination efficiently by suppressing dye aggregation and inhibiting the short contact of electrolyte to TiO2 surface. Accordingly, electrochemical impedance spectroscopy (EIS), which was performed in the dark under a forward bias of -0.69 V (Figure S6), showed the similar trend with IMVS measurement, attributing to the varied Voc values.

Photocatalytic hydrogen generation

The key role of organic dyes in photovoltaics can also be applied into photocatalytic hydrogen generation with similar mechanism. As shown in Figure 6, photoredox reaction of organic dyes can be conducted by the aid of TiO2 and electrolyte in DSC or sacrifice regent in photocatalytic system with the suitable energy levels. As to these TAPP-based dyes, the HOMO levels (0.7 V vs NHE) are much higher than that of triphenylamine ones (0.9 V vs NHE), [18] mainly due to the strong electron donating ability of TAPP moiety. Thus, in DSCs, the gap (∆G1) between electrolyte (I- /I3- ) and their HOMO levels is relatively small. For dye LI-131 and LI-132, it decreased to 0.30 and 0.29 V, respectively, which may affect the dye regeneration process with the insufficient driving force. However, in photocatalytic hydrogen evolution system, this energy barrier can be overcome by the up-shifted oxidation potentials of sacrifice regents. For instance, the oxidation potential of ascorbic acid (AA) is 0.14 V vs NHE, [19] leading to the larger gaps (∆G2 > 0.5 V) for efficient dye regeneration.

Thus, in comparison with DSCs, the hydrogen evolution based on these TAPP-based organic dyes demonstrated different trend for the varied energy levels and environment. As shown in Figure 7, all of them facilitated the photocatalytic hydrogen generation 0.0 0.1 0.2 0.3 0.4 0.5 0.291 0.302 0.367 0.476 0.359 420 nm AQY (%) 0.436 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.376 0.675 0.382 0.298 0.394 0.307 LI-128 LI-129 LI-130 LI-131 LI-132 LI-127 LI-128 LI-129 LI-130 LI-131 LI-132 AQY (%) 500 nm LI-127 Figure 8. The AQY values of sensitizers with the irradiation at 420 and 500 nm with the average rate faster than 4000 μL/h, and the highest hydrogen production of 1301 μmol was achieved by LI-131 bearing benzothiadiazole as the auxiliary electron acceptor, not the best one (LI-130) in DSCs. Also, for the dyes(LI-127-LI-130) with normal D-π-A structure, the photocatalytic activities were similar with little difference of hydrogen amount within 5 h. It seemed that the relationship between the molecular structures and photocatalytic performance was varied much with that in DSCs, which may be related to the different surroundings of dyeloading TiO2 surface. For DSCs, TiO2 films were immersed in acetonitrile/3-methoxy propionitrile as the solvent of electrolyte, and the organic dyes with absorption state were stacked as Haggregates, the ultrafast electron injection rates can be realized by the strong electron coupling between organic dyes and TiO2, but the relatively slow rate of dye regeneration was mainly due to the large reorganization energy of electrolyte and lower energy gap (∆G1). Thus, the electron recombination was usually severe in DSCs by the formation of dye aggregates and electron back transfer, which can be suppressed by the twisted structures and the tunable electronic properties of organic dyes. In this case, LI130, which consist of triphenylethylene-substituted TAPP moiety as the electron donor and thiophene as conjugated bridge, exhibited the highest conversion efficiency.

However, in hydrogen-evolution experiments, TiO2 were dispersed in nanoparticulate form surrounded by ascorbic acid in water, which could prevent dye aggregation and produce a much slower rate of electron injection from organic dyes. [20] Thus, the photocatalytic activities of organic dyes were mainly dependent on the light-harvesting ability and the control of electron back transfer. With the incorporation of electron-withdrawing unit (benzothiadiazole) into the conjugated bridge, the absorption regions of organic dyes can be broaden by the multiple charge transfer. Accordingly, dye LI-131 and LI-132 exhibited higher light-harvesting abilities, but the linkage between benzothiadiazole and the anchoring group (-COOH) by thiophene in LI-132 usually led to the severe electron back transfer, which is mainly due to the electron trap effect of benzothiadiazole and the planar conjugated system, and has been proved in our previous study.[21] When the electron back transfer was inhibited partially through the replacement of thiophene by benzene ring, LI-131 exhibited highest photocatalytic activities with turnover number (TON) of 4337. In this system, dyes were almost completely adsorbed with the same concentration, which was confirmed by the adsorption experiment (Figure S7). Thus, TONs of these organic dyes were in accordance with the amounts of hydrogen production with the values of 3663, 4013, 4053, 3966, 4380, 3784 for dye LI-127-LI-132, respectively.

Experimental Section 

Materials and instrumentation:

Tetrahydrofuran (THF) was dried over and distilled from K-Na alloy under an atmosphere of dry argon. All solvents were analytical grade and were used without further purification. All reagents used in this work were purchased. 2-(4-((2- ethylhexyl)oxy)phenyl)-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane1, 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-1,3,2-dioxaborolane2, diethyl ((7-bromobenzo[c] [1,2,5] thiadiazol-4-yl) methyl) phosphonate[22] were prepared according to literature procedures.1H and 13C spectra were obtained with a Bruker 300 MHz spectrometer using tetramethylsilane (TMS, d = 0 ppm) as internal standard. Elemental analyses were performed by a 73 CARLOERBA-1106 micro-elemental analyzer. ESI-MS spectra were recorded with a Finnigan LCQ advantage mass spectrometer. UV-visible spectra were conducted on a Shimadzu UV-2550 spectrometer. Cyclic voltammograms were obtained at a CHI 660 voltammetric analyzer with a scanning rate of 100 mV/s in nitrogen-purged dichloromethane. In the test system, tetrabutylammonium hexafluorophosphate (TBAPF6) is used as the supporting electrolyte, Pt disk, Pt plate, and Ag/AgCl electrode are acted as working electrode, counter electrode, and reference electrode, respectively. The ferrocene/ferrocenium redox couple was used for internal reference. DSC devices fabrication and measurement The devices were fabricated according to the literatures [23] . Firstly, fluorine doped tin oxide (FTO) conducting glasses (3.2 mm thickness, 7-8 ohms/sq) were cleaned with detergent, water, ethanol and acetone respectively, and irradiated at atmosphere of O3 for 18min, then immersed in TiCl4 solution (40 mM) for 30 min at 70 oC. After being cooled to room temperature, they were washed with deionized water and ethanol for three times, and then dried. The TiO2 films (16 μm thickness) were prepared by the screen printing technique and heated under airflow at 325 oC for 5 min, 375 oC for 5 min, 450 oC for 15 min and 500 oC for 1 h gradually, which consist of 12 μm layer of mesoporous TiO2 (18 NR-T, 18-20 nm, Dyesol) and 4 μm scatter layer (18 NR-AO, 20- 450 nm, Dyesol). After the films cool to room temperature, they were immersed in TiCl4 solution (40 mM) for 30 min at 70 oC once again, then washed clearly and annealed at 500 oC for 30 min. When the temperature of corresponding TiO2 film cool to 80 oC, they were immersed in dye bathes (0.3 mM) in the mixture solvents (CH3OH/CH2Cl2 = 1/1) for 18 h in dark condition. Then, the sensitized electrodes were washed with corresponding solvents and dried in air. Counter electrodes were prepared by thermal deposition: FTO glass (2.2 mm thickness, 7-8 ohms/sq) with two small hole were cleaned as counter electrode, 10 mL of H2PtCl6 (10 mM) solution in isopropyl alcohol were dispersed on FTO glass and heated at 400 oC for 30 min. After then, dye sensitized photoanodes and Pt-counter electrodes were assembled into a sandwich type cell and sealed with a hot-melt gasket (25 μm thickness, from the ionomer Surlyn 1702 (DuPont)). The electrolyte was injected into cells by the two small holes in after assembled. Lastly, theholes were sealed with a Surlyn sheet (50 μm thickness) and a thin ITO glass covered by heating. In the process of optimization, coadsorbant (CDCA) was absorbed at the same time with sensitizers. Photovoltaic measurement were conducted under AM 1.5 solar simulator (Model 94023A equipped with a 450 W xenon arc lamp, Newport Co). It was calibrated with a normal silicon solar cells before measurement. The J-V curves is obtained by Keithley model 2400 digital source meter when applying an external bias to the cell. Incident photon-current conversion efficiency (IPCE) was recorded on a DC Power Meter (Model 2931-C equipped with a 300 W xenon arc lamp, Newport Co.) under irradiation with a motorized monochromator (Oriel). Some electrochemical properties were obtained by Modulab XM PhotoEchem system such as IMVS (intensity modulated photovoltage spectroscopy), CE (charge extraction), EIS (electrochemical impedance spectroscopy). IMVS and CE were measured under a white light emitting diode (LED) array. CE were conducted in dark with different potential biases with a frequency range from 0.1 Hz to 100 kHz. The performance at dim light were performed by Modulab XM PhotoEchem system equipped with warm white light and the light intensity were detected by optical power meter.

The photocatalytic for generation of H2

Typically, the photocayalytic reaction was conducted in ascorbic acid solution (30 mL, 50 mM) containing 30 mg of P25@Pt(1wt%)@dye (catalysts) with a double-neck quartz reactor at 5oC. Prior to irradiation, the suspension of the catalyst was dispersed in an ultrasonic bath for 10 min then collected to the gas-circulation system, and the gas-circulation system was pumped to the vacuum for 0.5 h. After that, reactor was irradiated by a 300 W Xe lamp (CEL-HXF300, Beijing, China Education Au-light Co.,Ltd) from the top of reactor. H2 evolution amount was analyzed every one hour with an online gas chromatograph (GC7920, TCD detector, 5 Å molecular sieve columns, and N2 carrier and O2 drive valve). A series of cutoff filters (such as λ ³ 420 nm) or band-pass filters (such as λ = 420±10 nm, 500±10 nm) were employed to generate visible or monochromatic light. The parameters such as turnover number (TON) and apparent quantum yield were calculated according to the following equation: TON = (2 ×number of evolved hydrogen molecules) / (number of dye molecules adsorbed) AQY = (2 ×number of evolved hydrogen molecules) / (number of incident photons)

Synthetic procedures of sensitizers

Synthesis of compound 1

A mixture of 4-bromobenzaldehyde (11.04 g, 60 mmol), 4- aminophenol (6.54 g, 60 mmol) and p-toluenesulionic acid (516 mg, 3 mmol) was placed in 30 mL glacial acetic acid and stirred at 90 oC for 30 min .Then butane-2,3-dione (5.32 mL, 60 mmol) was added dropwise and the resultant mixture was refluxed overnight. After the mixture was cooled to room temperature, the precipitate was filtered off and washed with cold glacial acetic acid and used directly for the next step...