& Photocatalysis

Organic Dyes based on Tetraaryl-1,4-dihydropyrrolo-[3,2-b]-pyrroles for Photovoltaic and Photocatalysis Applications with theSuppressed 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] and Zhen Li*[a, c]

Abstract: A tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles

(TAPP) moiety with the combination of two pyrrole ringsand four phenyl moieties demonstrated strong electron-do-nating ability and nonplanar configuration simultaneously.

Once incorporated into the organic dyes as a novel electrondonor, it was beneficial for the enhancement of light-har-

vesting ability and suppression of electron recombination inthe photovoltaic and photocatalysis systems. With the link-age of tunable conjugated bridges and electron acceptor,

the corresponding organic dyes exhibited improved photo-

voltaic performance in dye-sensitized solar cells and facilitat-ed photocatalytic hydrogen generation with a highest turn-

over number (TON) of 4337. Through the detailed investiga-

tion of relationship between molecular structures and pho-tovoltaic/photocatalysis property, the connection and differ-

ence in molecular design for these two systems are wellexplained, with the aim to promote the application of dye-

sensitized technology in various fields.

Introduction

Photovoltaic and photocatalysis devices, which can generateelectricity and hydrogen directly from sunlight, respectively,

are promising to offer clean solutions to the energy crisis andenvironmental degradation around the globe.[1] In these con-

version processes, dye-sensitized technology plays the essen-tial role in the light-harvesting and electron transitions. Gener-ally, sensitizers were excited when they absorbed sunlight andelectrons were injected into the conduction band of TiO2, then,

the oxidized dye could be regenerated by electrolyte in dyesensitized solar cells (DSCs) or sacrifice regent in water splitting(WS) system.[2] Accompanied with these positive electron pro-cesses, the unfavorable electron recombination always occurson 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 orphotocatalysis performance, the suppression of electron re-combination is essential and becomes the urgent issue, which

is mainly related to the aggregated behavior of organic dyesas the adsorption state on TiO2 surface, also, the dye regenera-

tion process in different environments can affect the electroncollection.[3]

As to organic dyes with typical push-pull structures, theelectron donor (D), which is furthest away from TiO2 surface as

a part of dyes and closest to the electrolyte or sacrifice agenton some level, can be the key correlation among them, andplay the crucial role in the adjustment of electron transitions inthe dye/TiO2/electrolyte or sacrifice regent interface.[4] Untilnow, the most common used one is triarylamine unit, which

contains nitrogen atom as the electron-rich center, demon-strates the strong electron-donating property and unique spa-

tial configurations,[5] and the highest conversion efficiency(14.3 %) of the corresponding DSC is achieved with the opti-mized co-sensitized system.[6] Also, the triphenylamine-based

organic dyes show attractive photocatalytic performance inlight-driven H2 production from water with remarkable H2 turn-

over number (TON) of 10 200 (48 h).[7] However, when the ni-trogen atom is incorporated into cyclics as a part of some elec-tron donors, such as carbazole, indoline, phenothiazine, and so

on, just moderate performance can be obtained. It is mainly re-lated to the planar structure of electron donor with extend p

system, which can induce dye aggregation and aggravate elec-tron recombination to some extent.[8] This phenomenon is

proved by the photovoltaic performance of indoline-based or-ganic dyes with different aromatic rings substitutions.[9] The

[a] J. Wang, Z. Chai, S. Liu, M. Fang, K. Chang, M. Han, Prof. Q. Li, Prof. Z. LiDepartment of ChemistryWuhan UniversityWuhan 430072 (China)E-mail : qianqian-alinda@163.com

lizhen@whu.edu.cn

[b] L. Hong, Prof. H. HanMichael Gr-tzel Centre for Mesoscopic Solar CellsWuhan National Laboratory for OptoelectronicsHuazhong University of Science and TechnologyWuhan, 430074 (China)

[c] Prof. Z. LiInstitute of Molecular Aggregation ScienceTianjin UniversityTianjin, 300072 (China)

Supporting information and the ORCID identification number(s) for the au-thor(s) of this article can be found under :https ://doi.org/10.1002/chem.201803688.

Chem. Eur. J. 2018, 24, 18032 – 18042 T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim18032

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conversion efficiencies of the corresponding DSCs increasedlargely with the enlarged sizes of substituents with twisted

configuration to indoline donor. For instance, when the aro-matics linked to nitrogen atom of indoline ring changed from

phenyl to carbazole moiety, the corresponding conversion effi-ciencies can be enhanced from 5.08 % to 8.49 % (Chart S1).

Thus, the modification of donor moieties by incorporatingtwisted substituents is efficient and necessary, which can sup-

press the dye aggregation in some degree for the decreased

intermolecular interactions and enlarged distance betweenconjugated skeletons.

In our previous study, the twisted configuration with nitro-gen-containing ring has also been exploited. In N-phenylpyr-

role, benzene was almost perpendicular to pyrrole ring withthe carbon-nitrogen linkage, which really demonstrated signifi-cant effect on the suppression of electron recombination as

the conjugated bridge.[10] However, as an electron donor, theelectron-donating ability of 1-phenyl-1H-pyrrole is still not

strong enough for the small conjugated size. Thus, another N-phenylpyrrole was incorporated to construct the fused ring,

and two phenyl unit was linked to a-position of pyrrole to fur-ther expand the conjugated system.[11] In vertical direction, two

phenyl units with the modification of flexible chains were in

the opposite sides of main conjugated skeleton, which can actas the isolation groups to inhibit the possible p-p interactions

of organic dyes. Thus, the formed tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles (TAPP) was considered as the desired electron

donor moiety with strong electron-donating ability and twistedstructure, which can be synthesized by one-pot method

easily.[12]

Accordingly, a series of TAPP-based sensitizers (LI-127–LI-130, Figure 1 A) have been designed and synthesized with the

tunable p-conjugated bridge, also, some other twisted struc-tures, for example, triphenylethylene moiety, have been intro-

duced 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 im-

proved photovoltaic performance, and the similar trend can

also be observed in the photocatalytic hydrogen generationsystem (Figure 1 B). Herein, we would like to report the synthe-

sis of organic dyes, the theoretical calculations, their opticaland electrochemical properties, as well as the photovoltaic and

photocatalytic performance in detail.

Results and Discussion

Molecular design and synthesis

With the incorporation of TAPP moiety bearing the almost or-thorhombic structure as the electron donor (D), four TAPP-

based organic dyes were designed and synthesized with the

linkage of different conjugated bridges and cyanoacetic acidas the electron acceptor. Moreover, the famous AIEgen, tetra-

phenyl ethylene (TPE) with the twisted configuration was con-structed 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]pyrroleswith electron-rich property as the core, together with thetwisted structures surrounding as the isolation groups. Theywere synthesized by one-pot reaction easily, and the modifica-tion by flexible chains and aromatics can be conducted by the

nucleophilic substitution and Suzuki coupling reaction, respec-tively. Through the linkage of conjugated bridge (p) with tuna-

ble electron properties by single carbon-carbon bonds and theelectron acceptor (A) with double bonds, the pull-push conju-gated skeleton with d-p-A type was formed, which was benefi-

cial to the intramolecular charge transfer (ICT) and light har-vesting. 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 solu-

tion and on TiO2 film were shown in Figure 2, and the corre-

Figure 1. A) Chemical structures of dye LI-127–LI-130 ; B) the absorption behavior of organic dyes on TiO2 surface with the suppression effect on electron re-combination process.

Chem. Eur. J. 2018, 24, 18032 – 18042 www.chemeurj.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim18033

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sponding data were collected in Table 1. The organic dyesshow broad absorption ranging from 300 to 650 nm, and ex-

hibited two distinct absorption bands. The absorption regionat 300–420 nm was assigned to the p-p* transition of aromat-

ics, while the absorption in the region of 450–650 nm was as-cribed to the intramolecular charge transfer (ICT) through the

whole push-pull conjugated system. Dye LI-128 and LI-130bearing thiophene as the conjugated bridge demonstrated sig-nificant 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@1) than that of benzene ring (151 kJ mol@1), which

was beneficial to electron transition from the electron donor(TAPP) to acceptor (cyanoacetic acid). Also, the more planarconfiguration of conjugated skeleton in LI-128 and LI-130 af-

forded the positive effect on ICT process, which can be con-firmed by theory calculations. After four organic dyes were ad-

sorbed onto TiO2 surface, the extended absorption spectrawere obtained with the much red-shifted onset wavelengths

(lonset) (Table 1). The changes between absorption spectra in

solution and TiO2 film were related to the arrangement of or-ganic dyes as the aggregated state and the interaction of an-

choring group (-COOH) with TiO2 surface. The conjugatedbridges with different electron properties and the molecular

geometries play the key role to optimize the absorption be-haviors of organic dyes on TiO2 surface. In this case, from ap-

proximate 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 com-

pact alignment with larger dye-loading amounts (Table S6).

Electrochemical characterization

Electrochemical behaviors of these organic dyes were mea-sured by cyclic voltammetry to evaluate their redox potentialsand the corresponding HOMO and LUMO levels (Figure S1 in

Supporting Information). Two reversible oxidation waves wereobserved for these organic dyes. The first oxidation potentials

at around 0.7 V (vs. the normal hydrogen electrode, NHE) wereassigned to the oxidation of the electron donor moiety, whichcorresponded to the HOMO levels. They were almost identical,since their electron-donating abilities were mainly determinedby the same dihydropyrrolo-[3,2-b]pyrrole moiety as the elec-

tron-rich unit. Thus, these oxidized organic dyes can be regen-erated by iodine/triiodide electrolyte theoretically, for theirmore-positive potentials than that of iodine/triiodide electro-lyte (0.4 V vs. NHE) with the energy gaps +0.3 V.[14] The excited

state oxidation potentials (Eox*), which were estimated fromEox@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 theincorporation 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 conductionband of TiO2 is energetically permitted.

Theoretical approach

The optimized molecular structures and intramolecular chargetransfer of these organic dyes could be understood by density

functional theory (DFT) calculations, which were conducted bythe Gaussian 16 software at B3LYP/6-31G* level.[15] As to theelectron donor (TAPP), the four phenyl units around dihydro-pyrrolo-[3,2-b]pyrrole core demonstrated different dihedral

angles with varied linkage positions (Figure S2 in SupportingInformation). The smaller dihedral angles (about 358) along the

conjugated skeleton was favorable to the intramolecularcharge transfer through the whole molecules, and the largerdihedral angles (about 1358) 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 in-termolecular interactions. After the incorporation of different

conjugated bridges (phenyl or thienyl), the more planar geom-

etry with adjacent dihedral angles of about 198 can be formedin LI-128 and LI-130 by thienyl unit as the bridge, which was

beneficial to intramolecular charge transfer through wholemolecules, resulting in the red-shifts of absorption spectra, as

mentioned above. It can be further proved by the larger over-laps of electron distribution in HOMOs and LUMOs.

Figure 2. Absorption spectra of organic dyes LI-127–LI-130 in CH2Cl2

(30 mm) (A) and TiO2 film (6 mm) (B).

Table 1. Optical and Electrochemical Properties of dyes.

Dye labs[a]

[nm]lonset

[a]

[nm]lonset

[b]

[nm]E0@0

[c]

[eV]Eox

[d]

[V]Eox*

[d]

[V]

LI-127 329, 406 525 621 2.53 0.72 @1.81LI-128 372, 506 650 673 2.01 0.71 @1.30LI-129 311, 412 517 614 2.50 0.72 @1.78LI-130 379, 501 650 683 2.01 0.71 @1.30

[a] Absorption spectra of organic dyes were conducted in CH2Cl2 with theconcentration of 3 V 10@5 mol L@1. [b] Absorption spectra of dyes adsorbedon 6 mm TiO2 films. [c] The band gap E0@0 was determined from the ob-served optical edge. [d] cyclic voltammogram were measured in CH2Cl2

with 0.1 m TBAPF6 as electrolyte. The oxidation potential (Eox) referencedto calibrated Ag/AgCl was converted to the NHE reference scale: Eox =

Eoxon + 0.2 V, Eox

* was calculated from Eox@E0@0/e.

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Photovoltaic performance of DSCs

Then, DSCs based on these TAPP-based organic dyes were fab-ricated with sandwich structures, and the electrolyte consisted

of 0.6 m dimethylpropyl imidazolium iodide, 0.1 m lithiumiodide, 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 3 A) were measured under an

irradiance of standard AM 1.5G sunlight (100 mW cm@2), withthe photovoltaic parameters listed in Table 2. The DSCs based

on dye LI-128 and LI-130 with thienyl as the conjugatedbridge exhibited the higher short-circuit photocurrent densities

(Jscs) of 12.46 and 13.78 mA cm@2, respectively, in comparisonwith 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 ab-sorption 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 themonochromatic photoelectric conversion efficiency (IPCE)curves. As shown in Figure 3 B, IPCE curves of LI-128 and LI-130 can extend to 750 nm, while those of LI-127 and LI-129blue-shifted to 700 nm, meaning that the intramolecularcharge transfer was more efficient in organic dyes bearing

thienyl moieties. However, the open-circuit voltages (Vocs) dem-

onstrated different trends, which mainly related to the sub-stituents 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, re-

spectively. However, once the substituents were replaced bytriphenylethylene, Vocs increased obviously in LI-129 (680 mV)

and LI-130 (664 mV)-sensitized solar cells, indicating the impor-tant role of triphenylethylene with large size and twisted con-figurations. It could be helpful to suppressing the electron re-combination with the inhibition of dye aggregation. Therefore,

the highest conversion efficiency (h) of 6.56 % was achieved byDSC based on LI-130 with the balance of Jsc and Voc values.

Furthermore, the co-adsorbent chenodeoxycholic acid(CDCA) was incorporated to further improve the photovoltaicperformance. When the concentration of CDCA was only

5 mm, a tiny improvement was obtained with h of 6.86 %(Table S1). However, it would drop obviously with the in-

creased contents of CDCA, meaning that dye LI-130 did notform 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 or-ganic dyes exhibited increased conversion efficiencies for the

indoor applications (Table S2 and S3), and the highest h valueof 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 intro-

duction of benzothiadiazole unit as the auxiliary electron ac-ceptor 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 strongintramolecular interactions by the aid of benzothiadiazole with

electron-withdrawing property, as well as their relatively planar

structures (Figure S5). Accordingly, the extension of p-bridgeusually 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), respec-tively. The conversion efficiencies of DSCs based on these two

dyes decreased largely in comparison with that of LI-130, withh 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 andintensity-modulated photovoltage spectroscopy (IMVS)(Figure 5). Since the DSCs based on these organic dyes were

fabricated with the same electrolyte, Voc is mainly influencedby the conduction band (ECB) and free charge density of TiO2

film.[16] The shift of ECB was conducted by the CE technique, asshown in Figure 5 A. By observing the shift in the amount ofcharge 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 benzothiadiazolemoiety 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 structureand stronger intramolecular charge transfer demonstrated

lower extracted charge density at the same Voc, suggestingthat ECB of the corresponding DSC was upshifted. However, the

lower electron lifetime in Figure 5 B means the severe electronrecombination at the dye LI-132/electrolyte/TiO2 interface,

Figure 3. J-V characteristic curves (A) and IPCEs (B) for DSCs based on TAPP-based organic dyes LI-127–LI-130.

Table 2. Photovoltaic parameters of DSCs based on TAPP-based organicdyes.

Dye Jsc

[mA cm@2]Voc

[mV]FF h

[%]

LI-127 9.08 625 0.73 4.16LI-128 12.46 639 0.72 5.70LI-129 9.36 680 0.72 4.61LI-130 13.78 664 0.72 6.56

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leading to the much-decreased conversion efficiency. Mean-

while, the electron lifetime of dye LI-131-sensitized solar cell ismuch higher than that of LI-132, but still lower than those ofLI-127–LI-130 without auxiliary electron acceptor. The de-crease of electron lifetime was mainly related to electron trap

effect of benzothiadiazole moiety and the molecular arrange-ments on the TiO2 surface. Compared to dye LI-127–LI-130,

the decreased dye loading amounts of LI-131 and LI-132might mean the tilted adsorption geometries on TiO2 surface,attributing to the increased molecular lengths.[17] As to the

four dyes of LI-127–LI-130, DSCs based on LI-129 and LI-130bearing triphenylethylene substituent displayed longer elec-

tron lifetime than that of LI-127 and LI-128 with alkylbenzenemoiety, further confirmed the key role of triphenylethylene

with twisted configuration and large size. It can decrease the

electron recombination efficiently by suppressing dye aggrega-tion and inhibiting the short contact of electrolyte to TiO2 sur-

face. 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 mea-surement, attributing to the varied Voc values.

Photocatalytic hydrogen generation

The key role of organic dyes in photovoltaics can also be ap-

plied into photocatalytic hydrogen generation with similar

mechanism. As shown in Figure 6, photoredox reaction of or-

ganic dyes can be conducted by the aid of TiO2 and electrolyte

in DSC or sacrifice regent in photocatalytic system with thesuitable energy levels. As to these TAPP-based dyes, theHOMO levels (0.7 V vs. NHE) are much higher than that of tri-

phenylamine ones (0.9 V vs. NHE),[18] mainly due to the strongelectron donating ability of TAPP moiety. Thus, in DSCs, the

gap (DG1) between electrolyte (I@/I3@) and their HOMO levels isrelatively 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 photo-catalytic hydrogen evolution system, this energy barrier can be

overcome by the up-shifted oxidation potentials of sacrifice re-gents. For instance, the oxidation potential of ascorbic acid

(AA) is 0.14 V vs. NHE,[19] leading to the larger gaps (DG2>

0.5 V) for efficient dye regeneration.

Figure 4. The chemical structures of dye LI-131 and LI-132 with red-shifted absorption spectra.

Figure 5. (A) The charges extracted from the dye-grafted TiO2 films at a cer-tain open-circuit photovoltage, (B) the electron lifetimes measured by IMVS.

Figure 6. The similar mechanism for photovoltaics and photocatalytic hydro-gen generation with different driving force for dye regeneration.

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Thus, in comparison with DSCs, the hydrogen evolutionbased on these TAPP-based organic dyes demonstrated differ-

ent trend for the varied energy levels and environment. As

shown in Figure 7, all of them facilitated the photocatalytic hy-drogen generation with the average rate faster than

4000 mL h@1, and the highest hydrogen production of1301 mmol was achieved by LI-131 bearing benzothiadiazole

as the auxiliary electron acceptor, not the best one (LI-130) inDSCs. Also, for the dyes(LI-127–LI-130) with normal d-p-A

structure, the photocatalytic activities were similar with little

difference of hydrogen amount within 5 h. It seemed that therelationship between the molecular structures and photocata-

lytic performance was varied much with that in DSCs, whichmay be related to the different surroundings of dye-loading

TiO2 surface. For DSCs, TiO2 films were immersed in acetoni-trile/3-methoxy propionitrile as the solvent of electrolyte, andthe organic dyes with absorption state were stacked as H-ag-

gregates, the ultrafast electron injection rates can be realizedby the strong electron coupling between organic dyes andTiO2, but the relatively slow rate of dye regeneration wasmainly due to the large reorganization energy of electrolyte

and lower energy gap (DG1). Thus, the electron recombinationwas usually severe in DSCs by the formation of dye aggregates

and electron back transfer, which can be suppressed by thetwisted structures and the tunable electronic properties of or-

ganic dyes. In this case, LI-130, which consist of triphenylethy-lene-substituted TAPP moiety as the electron donor and thio-

phene as conjugated bridge, exhibited the highest conversionefficiency.

However, in hydrogen-evolution experiments, TiO2 were dis-

persed in nanoparticulate form surrounded by ascorbic acid inwater, which could prevent dye aggregation and produce amuch 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 ofelectron back transfer. With the incorporation of electron-with-

drawing unit (benzothiadiazole) into the conjugated bridge,

the absorption regions of organic dyes can be broaden by themultiple charge transfer. Accordingly, dye LI-131 and LI-132exhibited higher light-harvesting abilities, but the linkage be-tween benzothiadiazole and the anchoring group (-COOH) by

thiophene in LI-132 usually led to the severe electron backtransfer, which is mainly due to the electron trap effect of ben-

zothiadiazole and the planar conjugated system, and has been

proved in our previous study.[21] When the electron back trans-fer was inhibited partially through the replacement of thio-

phene by benzene ring, LI-131 exhibited highest photocatalyt-ic activities with turnover number (TON) of 4337. In this

system, dyes were almost completely adsorbed with the sameconcentration, which was confirmed by the adsorption experi-

ment (Figure S7). Thus, TONs of these organic dyes were in ac-

cordance with the amounts of hydrogen production with thevalues of 3663, 4013, 4053, 3966, 4380, 3784 for dye LI-127–LI-132, respectively.

The external or apparent quantum yield (AQY), which can be

viewed as the equivalent of the external quantum efficiency(IPCE) in photovoltaics, could reflect the conversion efficiency

at monochromatic light. According to the absorption spectra

of TAPP-based organic dyes, the monochromatic light irradia-tion was chosen at the wavelengths of 420:10 nm and 500:10 nm, which were close to their absorption peaks to achievethe high AQY values. As shown in Figure 8, dye LI-127 and LI-

Figure 7. Photocatalytic activities over P25@Pt(1 wt %)@dye under visiblelight irradiation. conditions: 30 mg P25@Pt(1 wt %)@dye catalyst, 30 mL ofwater containing AA(50 mm).

Figure 8. The AQY values of sensitizers with the irradiation at 420 and 500 nm.

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129 demonstrated larger AQY values at 420 nm, mainly due totheir maximum absorption wavelengths of about 410 nm.

However, with the extending of light-harvesting region by theplanar structure and/or the incorporation of additional electron

withdrawing moiety, dye LI-128, LI-130, LI-131 and LI-132 ex-hibited the higher photocatalytic activities at 500 nm, the high-

est value of 0.675 % was achieved by LI-131.

Conclusions

In summary, tetraaryl-1,4-dihydropyrrolo-[3,2-b]pyrroles (TAPP)

as a novel electron donor, which possesses strong electron-richproperty and spatial configuration, has been applied into or-

ganic dyes with the incorporation of different conjugated

bridges and substituent groups. The resultant organic dyes ofLI-127–LI-132 exhibited excellent photovoltaic performance

and photocatalytic activities, mainly due to the efficient sup-pression of electron recombination by the special structure of

TAPP unit and the tunable electron property of conjugatedbridge. Finally, the different relation between molecular struc-

tures and their performance in photovoltaic and photocatalysis

devices, were discussed in detail to afford the useful informa-tion for molecular design from the dye sensitized solar cells to

solar hydrogen production.

Experimental Section

Materials and instrumentation:

Tetrahydrofuran (THF) was dried over and distilled from K-Na alloyunder an atmosphere of dry argon. All solvents were analyticalgrade and were used without further purification. All reagentsused 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-bromoben-zo[c] [1,2,5] thiadiazol-4-yl) methyl) phosphonate[22] were preparedaccording to literature procedures.1H and 13C spectra were ob-tained with a Bruker 300 MHz spectrometer using tetramethylsilane(TMS, d= 0 ppm) as internal standard. Elemental analyses were per-formed by a 73 CARLOERBA-1106 micro-elemental analyzer. ESI-MSspectra were recorded with a Finnigan LCQ advantage mass spec-trometer. UV/Visible spectra were conducted on a Shimadzu UV-2550 spectrometer. Cyclic voltammograms were obtained at a CHI660 voltammetric analyzer with a scanning rate of 100 mV s@1 in ni-trogen-purged dichloromethane. In the test system, tetrabutylam-monium hexafluorophosphate (TBAPF6) is used as the supportingelectrolyte, Pt disk, Pt plate, and Ag/AgCl electrode are acted asworking electrode, counter electrode, and reference electrode, re-spectively. The ferrocene/ferrocenium redox couple was used forinternal 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 thick-ness, 7–8 ohms sq@1) were cleaned with detergent, water, ethanoland acetone, respectively, and irradiated at atmosphere of O3 for18 min, then immersed in TiCl4 solution (40 mm) for 30 min at70 8C. After being cooled to room temperature, they were washedwith deionized water and ethanol for three times, and then dried.The TiO2 films (16 mm thickness) were prepared by the screen

printing technique and heated under airflow at 325 8C for 5 min,375 8C for 5 min, 450 8C for 15 min and 500 8C for 1 h gradually,which consist of 12 mm layer of mesoporous TiO2 (18 NR-T, 18–20 nm, Dyesol) and 4 mm scatter layer (18 NR-AO, 20–450 nm,Dyesol). After the films cool to room temperature, they were im-mersed in TiCl4 solution (40 mm) for 30 min at 70 8C once again,then washed clearly and annealed at 500 8C for 30 min. When thetemperature of corresponding TiO2 film cool to 80 8C, they wereimmersed in dye bathes (0.3 mm) in the mixture solvents (CH3OH/CH2Cl2 = 1/1) for 18 h in dark condition. Then, the sensitized elec-trodes were washed with corresponding solvents and dried in air.Counter electrodes were prepared by thermal deposition: FTOglass (2.2 mm thickness, 7–8 ohms sq@1) with two small holes w.scleaned as counter electrode, 10 mL of H2PtCl6 (10 mm) solution inisopropyl alcohol were dispersed on FTO glass and heated at400 8C for 30 min. After then, dye sensitized photoanodes and Pt-counter electrodes were assembled into a sandwich type cell andsealed with a hot-melt gasket (25 mm thickness, from the ionomerSurlyn 1702 (DuPont)). The electrolyte was injected into cells bythe two small holes in after assembled. Lastly, the holes weresealed with a Surlyn sheet (50 mm thickness) and a thin ITO glasscovered by heating. In the process of optimization, co-adsorbent(CDCA) was absorbed at the same time with sensitizers.

Photovoltaic measurements were conducted under AM 1.5 solarsimulator (Model 94023A equipped with a 450 W xenon arc lamp,Newport Co). It was calibrated with a normal silicon solar cellsbefore measurement. The J-V curves is obtained by Keithley model2400 digital source meter when applying an external bias to thecell. Incident photon-current conversion efficiency (IPCE) was re-corded on a DC Power Meter (Model 2931-C equipped with a300 W xenon arc lamp, Newport Co.) under irradiation with a mo-torized monochromator (Oriel). Some electrochemical propertieswere obtained by Modulab XM PhotoEchem system such as IMVS(intensity modulated photovoltage spectroscopy), CE (charge ex-traction), EIS (electrochemical impedance spectroscopy). IMVS andCE were measured under a white light emitting diode (LED) array.CE were conducted in dark with different potential biases with afrequency range from 0.1 Hz to 100 kHz. The performance at dimlight were performed by Modulab XM PhotoEchem systemequipped with warm white light and the light intensity were de-tected by optical power meter.

The preparation of photocatalyst and property tests

The preparation of platinized TiO2 (P25@Pt (1 wt %))

For P25@Pt(1 wt %), TiO2 powder (P25, 2.0 g) and H2PtCl6·6 H2O(38.6 mm, 2.66 mL) were suspended in 50 mL H2O/CH3OH (4/1),then it was subjected to radiation under a 300 W xenon lamp withvigorously stirring for 3 hours. gray powders were obtained by fil-tration through a syringe filter (0.45 mm).

The adsorption of organic dyes onto platinized TiO2

(P25@Pt(1 wt %)@dye)

The prepared P25@Pt(1 wt %) powders (100 mg) were added to10 mL solution of the corresponding organic dyes (20 mm, DCM/CH3OH = 1:1). The mixture was stirred in the dark for 12 h. Then,purple powders were obtained by filtration through a syringe filter(0.45 mm) and then dried, while the filtrate solution was collectedfor the testing of dye-loading amounts.

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The photocatalytic for generation of H2

Typically, the photocatalytic reaction was conducted in ascorbicacid solution (30 mL, 50 mm) containing 30 mg ofP25@Pt(1 wt %)@dye (catalysts) with a double-neck quartz reactorat 5 8C. Prior to irradiation, the suspension of the catalyst was dis-persed in an ultrasonic bath for 10 min then collected to the gas-circulation system, and the gas-circulation system was pumped tothe vacuum for 0.5 h. After that, reactor was irradiated by a 300 WXe lamp (CEL-HXF300, Beijing, China Education Au-light Co., Ltd.)from the top of reactor. H2 evolution amount was analyzed everyone hour with an online gas chromatograph (GC7920, TCD detec-tor, 5 a molecular sieve columns, and N2 carrier and O2 drivevalve). A series of cutoff filters (such as l+420 nm) or band-passfilters (such as l= 420:10 nm, 500:10 nm) were employed togenerate visible or monochromatic light. The parameters such asturnover number (TON) and apparent quantum yield were calculat-ed according to the following equations:

TON ¼ð2 x number of evolved hydrogen moleculesÞnumber of dye molecules adsorbed

AQY ¼ð2 x number of evolved hydrogen moleculesÞnumber of incident photons

Synthetic procedures of sensitizers

The synthesis procedures are summarized in Scheme 1.

Synthesis of compound 1

A mixture of 4-bromobenzaldehyde (11.04 g, 60 mmol), 4-amino-phenol (6.54 g, 60 mmol) and p-toluenesulfonic acid (516 mg,3 mmol) was placed in 30 mL glacial acetic acid and stirred at 90 8Cfor 30 min. Then butane-2,3-dione (5.32 mL, 60 mmol) was addeddropwise and the resultant mixture was refluxed overnight. After

the mixture was cooled to room temperature, the precipitate wasfiltered off and washed with cold glacial acetic acid and used di-rectly for the next step.

Synthesis of compound 2

A mixture of compound 1 (8.09 g, 13.5 mmol), 3-(bromomethyl)heptane (6.26 g, 32.4 mmol), K2CO3 (5.60 g, 40.5 mmol) and KI(0.11 g, 0.68 mmol) was placed in 200 mL dry Schlenk tube withN,N-dimethylformamide (DMF, 60 mL) as the solvent. Then the so-lution was bubbled with N2 for 30 min and stirred at 100 8C for 48hours. After being cooled to room temperature, the inorganic saltswere removed by filtration, and the solvent was evaporated underreduced pressure, then the crude product was washed with waterfor several times and dried. A pale yellow solid was obtained bycolumn chromatography (7.93 g, 71.2 %). mp:191.4–193.2 8C. IR(thin film): n= 2954–2856 cm@1 (-CH3, -CH2-). 1H NMR (300 MHz,CDCl3): d= 7.32 (d, 4 H, J = 8.1 Hz, ArH), 7.17 (d, 4 H, J = 8.7 Hz, ArH),7.07 (d, 4 H, J = 8.1 Hz, ArH), 6.90 (d, 4 H, J = 8.7 Hz, ArH), 6.31 (s,2 H, ArH), 3.85 (d, 4 H, J = 5.4 Hz, OCH2), 1.73 (m, 2 H, CH), 1.51–1.35(m, 16 H, CH2), 0.94 ppm (m, 12 H, CH3). HRMS (ESI, m/z): [M++H]+

calcd for C46H52O2N2Br 823.0731, found 823.0756.

General synthesis of compound 3

Under the atmosphere of nitrogen, a mixture of compound 2(1.0 equiv), arylboronic acid (1.0 equiv), Pd(PPh3)(0.05 equiv) andK2CO3 (5.0 equiv) in the mixture solvent (THF/H2O = 5/1) wasplaced in Schlenk tube and stirred at 80 8C for 12 hours. Afterbeing cooled to room temperature, the mixture was poured intowater and extracted with dichloromethane for three times. The or-ganic layer was combined and dried with anhydrous sodium sul-fate. After the solvent was evaporated, the desired product wasobtained through the purification by column chromatography onsilica gel.

3 a : Compound 2 (2.23 g, 2.70 mmol), (4-formylphenyl)boronic acid(405 mg, 2.70 mmol), yellow powder (700 mg, 30.4 %), mp: 148.1–

Scheme 1. Synthetic Routes for organic dyes LI-127–LI-131. Reagents and conditions: (i) AcOH, 100 8C, 3 h; (ii) RBr, K2CO3, DMF, 100 8C, overnight; (iii)–(vi) boro-nates or boronic acid, K2CO3, Pd(PPh3)4, THF/H2O, reflux, 12 h; (vii) benzothiadiazole phosphate, tBuOK, THF, 0 8C to rt, overnight; (viii) cyanoacetic acid, piperi-dine, CH3CN, reflux.

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150.4 8C. IR (thin film): n= 1702 cm@1 (C=O). 1H NMR (300 MHz,CDCl3): d= 10.04 (s, 1 H, CHO), 7.92 (d, 2 H, J = 6.9 Hz, ArH), 7.73 (d,2 H, J = 6.6 Hz, ArH), 7.51 (d, 2 H, J = 6.6 Hz, ArH), 7.32 (s, br, 4 H,ArH), 7.22–7.18 (m, 4 H, ArH), 7.07 (d, 2 H, J = 6.9 Hz, ArH), 6.92 (d,J = 8.7 Hz, 4 H, ArH), 6.41 (s, 1 H, ArH), 6.34 (s, 1 H, ArH), 3.85 (s, br,4 H, OCH2), 1.74 (m, 2 H, CH), 1.47–1.34 (m, 16 H, CH2), 0.92 ppm (s,br, 12 H, CH3). HRMS: (ESI, m/z) [M++H]+ calcd for C53H57O3N2Br849.3625, found 849.3658.

3 b : Compound 2 (2.23 g, 2.70 mmol), (5-formylthiophen-2-yl)bor-onic acid(421 mg, 2.70 mmol), orange powder (578 mg, 25.1 %).mp:145.3–147.1 8C. IR (thin film): n= 1670 (C=O). 1H NMR (300 MHz,CDCl3): d= 9.86 (s, 1 H, CHO), 7.71 (d, 1 H, J = 3.9 Hz, ArH ), 7.52 (d,2 H, J = 7.8 Hz, ArH), 7.34 (d, 5 H, J = 7.5 Hz, ArH), 7.19 (m, 4 H, ArH),7.08 (d, 2 H, J = 8.1 Hz, ArH), 6.91 (d, 4 H, J = 8.7 Hz, ArH), 6.40 (s,1 H, ArH), 6.32 (s, 1 H, ArH), 3.86 (d, 4 H, J = 5.1 Hz, OCH2), 1.74 (m,2 H, CH), 1.44–1.35 (m, 16 H, CH2), 0.97–0.92 cm@1 (m, 12 H, CH3) ;HRMS (ESI, m/z): [M++H]+ calcd for C51H55O3N2S 855.3190, found855.3172.

General synthesis of compound 4 and 5

Under the atmosphere of nitrogen, a mixture of compound 3(1.0 equiv), arylboronic acid (2.0 equiv), Pd(PPh3)4 (0.05 equiv) andK2CO3 (5.0 equiv) in the mixture solvent (THF/H2O = 5/1) wasplaced in dry Schlenk tube (100 mL) and stirred at 80 8C for12 hours. After being cooled to room temperature reaction, themixture was poured into water and extracted with dichlorome-thane for three times. The organic layer was combined were anddried with anhydrous sodium sulfate. After the solvent was evapo-rated, the desired product was obtained through purification bycolumn chromatography on silica gel.

4 a : Compound 3 a (430 mg, 0.51 mmol), 2-(4-((2-ethylhexyl)oxy)-phenyl)-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane (339 mg,1.02 mmol), yellow powder (170 mg, 35.0 %). mp: 135.3–136.8 8C. IR(thin film): n= 1704 cm@1 (C=O). 1H NMR (300 MHz, CDCl3): d=10.04 (s, 1 H, CHO), 7.93(d, 2 H, J = 7.8 Hz, ArH), 7.75 (d, 2 H, J =7.8 Hz, ArH), 7.50 (s, br, 4 H, ArH), 7.42 (d, 4 H, J = 7.8 Hz, ArH ), 7.33(d, 4 H, J = 7.5 Hz, ArH), 6.92 (s, br, 8 H, ArH), 6.43 (s, 1 H, ArH), 6.38(s, 1 H, ArH), 3.86 (s, br, 6 H, OCH2), 1.75 (m, 3 H, CH), 1.48–1.33 (m,24 H, CH2), 0.94–0.92 ppm (m, 18 H, CH3). HRMS (ESI, m/z): [M++H]+

calcd for C67H78O4N2 975.6034, found 975.5976.

4 b : Compound 3 b (170 mg, 0.20 mmol), 2-(4-((2-ethylhexyl)oxy)-phenyl)-4,4,5,5,-tetramethyl-1,3,2-dioxaborolane (133 mg,0.40 mmol), orange powder (120 mg, 61.2 %). mp: 128.7–131.3 8C.IR (thin film): n= 1671 cm@1 (C=O). 1H NMR (300 MHz, CDCl3): d=9.86 (s, 1 H, CHO), 7.75–7.72 (m, 2 H, ArH), 7.51 (m, 4 H, ArH), 7.42(d, 2 H, J = 8.1 Hz, ArH), 7.26–7.23 (m, 8 H, ArH), 6.92 (d, 6 H, J =8.7 Hz, ArH), 6.43 (s, 1 H, ArH), 6.38 (s, 1 H, ArH), 3.86 (s, br, 6 H,OCH2), 1.74 (m, 3 H, CH), 1.43–1.33 (m, 24 H, CH2), 0.94–0.92 ppm(m, 18 H, CH3). HRMS (ESI, m/z): [M++H]+ calcd for C65H76O4N2S981.5599, found 981.5625.

5 a : Compound 3 a (150 mg, 0.18 mmol), 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-1,3,2-dioxaborolane (135 mg, 0.36 mmol),yellow powder (90 mg, 48.9 %). mp: 87.6–89.7 8C. IR (thin film): n=1708 cm@1 (C=O). 1H NMR (300 MHz, CDCl3): d= 10.03 (s, 1 H, CHO),7.92 (d, 2 H, J = 8.1 Hz, ArH ), 7.74 (d, 2 H, J = 7.5 Hz, ArH), 7.50 (d,2 H, J = 7.8 Hz, ArH), 7.31 (d, 2 H, J = 7.2 Hz, ArH), 7.21–6.93 (m, 27 H,ArH), 6.40 (s, 1 H, ArH), 6.29 (s, 1 H, ArH), 3.86 (s, br, 4 H, OCH2), 1.74(m, 2 H, CH), 1.48–1.25 (m, 16 H, CH2), 0.94–0.92 ppm (m, 12 H, CH3).HRMS (ESI, m/z): [M++H]+ calcd for C73H72O3N2 1025.5622, found1025.5607.

5 b : Compound 3 b (200 mg, 0.23 mmol), 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-1,3,2-dioxaborolane (176 mg, 0.46 mmol),

orange powder (120 mg, 51.3 %). mp: 88.8–90.2 8C. IR (thin film):n= 1666 cm@1 (C=O). 1H NMR (300 MHz, CDCl3): d= 9.86 (s, 1 H,CHO), 7.71 (d, 1 H, J = 4.2 Hz, ArH ), 7.51 (d, 2 H, J = 7.8 Hz, ArH),7.35 (s, br, 1 H, ArH), 7.23–7.20 (d, 4 H, J = 8.1 Hz, ArH), 7.15–7.01(m, 17 H, ArH), 6.96–6.85 (m, 8 H, ArH), 6.40 (s, 1 H, ArH), 6.28 (s, 1 H,ArH), 3.86 (s, br, 6 H, OCH2), 1.74 (m, 3 H, CH), 1.43–1.33 (m, 24 H,CH2), 0.94–0.92 ppm (m, 18 H, CH3). HRMS (ESI, m/z): [M++H]+ calcdfor C71H70O3N2S 1031.5180, found 1031.5219.

Synthesis of compound 6

Under the atmosphere of nitrogen, a mixture of compound 5b(820 mg, 0.78 mmol) and compound diethyl((7-bromobenzo[c][1,2,5]thiadiazol-4-yl)methyl)phosphonate (620 mg, 1.70 mmol) inTHF (10 mL) was placed in Schlenk tube and stirred. After tBuOK(134 mg, 1.19 mmol) in THF (10 mL) was added dropwise at 0 8C, itwas warmed to room temperature slowly and stirred overnight.Then, the mixture was poured into water and extracted with di-chloromethane for three times. The combined organic layer wasdried with anhydrous sodium sulfate. After the solvent was evapo-rated, the crude product was purified by column chromatographyon silica gel to get a red solid (570 mg, 58.2 %). mp: 98.9–100.4 8C.IR (thin film): n= 1662 cm@1 (C=C). 1H NMR (300 MHz, CDCl3): d=8.19 (d, 1 H, J = 16.2 Hz, = CH), 7.81 (d, 1 H, J = 7.8 Hz, ArH ), 7.46(m, 3 H, ArH), 7.29–7.21 (m, 7 H, ArH), 7.16–7.02 (m, 17 H, ArH),6.95–6.86 (m, 8 H, ArH), 6.38 (s, 1 H, ArH), 6.29 (s, 1 H, ArH), 3.86 (s,br, 6 H, OCH2), 1.74 (m, 2 H, CH), 1.51–1.34 (m, 16 H, CH2), 1.00–0.84 ppm (m, 12 H, CH3). HRMS (ESI, m/z): [M++H]+ calcd forC78H73O2N4S2Br 1241.4431, found 1241.4423.

General synthesis of compound 7

Under the atmosphere of nitrogen, a mixture of compound 6(1.0 equiv), arylboronic acid (3.0 equiv), Pd(PPh3)4(0.05 equiv), andK2CO3 (5.0 equiv) in the mixture solvent (THF/H2O = 5/1) wasplaced in Schlenk tube and stirred at 80 8C for 12 hours. Afterbeing cooled to room temperature, the mixture was poured intowater and extracted with dichloromethane for three times. The or-ganic layer was combined and dried with anhydrous sodium sul-fate. After the solvent was evaporated, the desired product wasobtained through the purification by column chromatography onsilica gel.

7 a : Compound 6 (220 mg, 0.18 mmol), (4-formylphenyl)boronicacid (84 mg, 0.54 mmol), red solid (170 mg, 74.6 %). mp: 101.7–104.3 8C. IR (thin film): n= 1660 (C=O), 1600 cm@1 (C=C). 1H NMR(300 MHz, CDCl3): d= 10.11 (s, 1 H, CHO), 8.28 (d, 1 H, J = 16.2 Hz, =CH), 8.16 (d, 2 H, J = 7.8 Hz, ArH ), 8.05 (d, 2 H, J = 6.6 Hz, ArH ), 7.79(d, 1 H, J = 7.8 Hz, ArH), 7.72 (d, 1 H, J = 7.8 Hz, ArH), 7.49 (d, 2 H, J =7.2 Hz, ArH), 7.41 (d, 1 H, J = 14.4 Hz, CH2 =), 7.22–7.16 (m, 8 H,ArH), 7.09–7.02 (m, 16 H, ArH), 6.96–6.87 (m, 7 H, ArH), 6.39 (s, 1 H,ArH), 6.30 (s, 1 H, ArH), 3.87 (s, br, 4 H, OCH2), 1.75 (m, 2 H, CH),1.51–1.35 (m, 16 H, CH2), 1.00–0.95 ppm (m, 12 H, CH3). HRMS (ESI,m/z): [M++H]+ calcd for C85H78O3N4S2 1267.5294, found 1267.5279.

7 b : Compound 6 (220 mg, 0.18 mmol), (5-formylthiophen-2-yl)bor-onic acid (81 mg, 0.54 mmol), black red solid (155 mg, 67.7 %). mp:113.5–116.3 8C. IR (thin film): n= 1666 (C=O), 1625 cm@1 (C=C).1H NMR (300 MHz, CDCl3): d= 9.97 (s, 1 H, CHO), 8.29–8.21 (m, 2 H,ArH, = CH), 7.98 (d, 1 H, J = 7.8 Hz, ArH), 7.84 (d, 1 H, J = 3.6 Hz,ArH), 7.64 (d, 1 H, J = 7.8 Hz, ArH), 7.49 (d, 2 H, J = 8.7 Hz, ArH), 7.36(d, 1 H, J = 16.2 Hz, CH =), 7.24–7.21 (m, 8 H, ArH), 7.16–7.02 (m,16 H, ArH), 6.95–6.86 (m, 7 H, ArH), 6.38 (s, 1 H, ArH), 6.29 (s, 1 H,ArH), 3.86 (s, br, 4 H, OCH2), 1.75 (m, 2 H, CH), 1.51–1.35 (m, 16 H,CH2), 1.00–0.92 ppm (m, 12 H, CH3). HRMS (ESI,m/z): [M++H]+ calcdfor C83H76O3N4S3 1273.5152, found 1273.5121.

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General Synthetic procedure for organic dyes

A mixture of aromatic aldehyde (1.0 equiv), cyanoacetic acid(3.0 equiv) and catalytic amount of piperidine in 10 mL solvent(MeCN/THF = 4/1) was placed in dry round-bottom (50 mL). Afterbeing refluxed for 16 h, it was poured into hydrochloric acid aque-ous solution (2 m). The crude product was extracted by chloroformfor three times. The combined organic layer was dried with anhy-drous sodium sulfate. After the solvent was evaporated, the crudeproduct was purified by column chromatography on silica gel.

LI-127: 4 a (170 mg, 0.17 mmol), cyanoacetic acid (44 mg,0.52 mmol), black solid (100 mg, 56.0 %). mp: 184.8–186.4 8C. IR(thin film): n= 3354 (OH), 2221 cm@1 (C/N). 1H NMR (300 MHz,CD3Cl and [D6]DMSO): d= 7.96 (m, 3 H, ArH and CH =), 7.79 (d, 2 H,J = 9.0 Hz, ArH), 7.66 (d, 2 H, J = 8.1 Hz, ArH), 7.57 (d, 2 H, J = 8.7 Hz,ArH), 7.51 (d, 2 H, J = 8.7 Hz, ArH), 7.30 (d, 2 H, J = 8.4 Hz, ArH), 7.24(d, 6 H, J = 8.4 Hz, ArH), 7.02–6.96 (m, 6 H, ArH ), 6.45 (s, 1 H, ArH),6.39 (s, 1 H, ArH), 3.88 (d, 6 H, J = 5.4 Hz, OCH2), 1.70 (m, 3 H, CH),1.41–1.24 (m, 24 H, CH2), 0.91–0.88 ppm (m, 18 H, CH3). 13C NMR(100 MHz, [D8]THF): d= 163.77, 158.89, 157.52, 152.46, 152.31,143.75, 137.86, 136.05, 135.82, 135.31, 134.82, 134.02, 133.17,132.95, 132.85, 132.57, 132.45, 132.03, 131.18, 130.88, 129.44,129.00, 128.41, 127.83, 127.15, 126.53, 126.40, 125.66, 117.34,115.98, 114.59, 114.40, 94.16, 93.50, 70.12, 69.91, 39.54,39.49, 31.80,30.47, 30.32, 29.57, 29.41, 29.0, 26.95, 22.90, 22.50, 13.39,10.46 ppm. MS (ESI, m/z): [M-1]@ calcd for C70H79N3O5 : 1040.6,found 1040.1; Elemental Analysis Calcd for C70H79N3O5 : C, 80.66, H,7.64; N; 4.03; Found C, 80.39; H, 7.43; N; 4.09.

LI-128 : 4 b (120 mg, 0.12 mmol), cyanoacetic acid (31 mg,0.36 mmol), black solid (100 mg, 79.4 %). mp: 198.8–200.2 8C. IR(thin film): n= 3357 (OH), 2221 cm@1 (C/N). 1H NMR (300 MHz,CD3Cl and [D6]DMSO): d= 8.18 (s, 1 H, CH =), 7.76 (s, 1 H, ArH),7.60–7.48 (m, 7 H, J = 8.1 Hz, ArH), 7.28–7.21 (m, 8 H, J = 8.7 Hz,ArH), 6.99–6.95 (m, 6 H, ArH), 6.45 (s, 1 H, ArH), 6.39 (s, 1 H, ArH),3.88 (d, 6 H, J = 5.4 Hz, OCH2), 1.70 (m, 3 H, CH), 1.41–1.24 (m, 24 H,CH2), 0.91–0.88 ppm (m, 18 H, CH3). 13C NMR (100 MHz, [D8]THF):d= 164.36, 160.17, 158.92, 158.79, 154.62, 146.71, 140.36, 139.23,137.53, 136.23, 135.96, 135.78, 134.68, 134.33, 133.78, 133.76,133.17, 132.33, 131.28, 130.95, 129.09, 128.89, 128.42, 127.73,127.51, 127.00, 126.94, 124.91, 116.89, 115.99, 115.87, 115.65, 95.69,94.63, 71.33, 71.14,40.80, 40.74, 31.73, 30.28, 26.04, 25.91, 24.96,24.18, 14.67, 11.73 ppm. MS (ESI, m/z): [M-1]@ calcd for C68H77N3O5S:1046.6; found 1046.0; Elemental Analysis Calcd for C68H77N3O5S, C,77.90, H, 7.40; N; 4.01; Found C, 77.76; H, 7.61; N; 4.16.

LI-129 : 5 a (90 mg, 0.09 mmol), cyanoacetic acid (23 mg,0.27 mmol), black solid (60 mg, 61.8 %). mp: 114.8–116.4 8C. IR (thinfilm): n= 3376 (OH), 2221 cm@1 (C/N). 1H NMR (300 MHz, CD3Cl and[D6]DMSO): d= 8.09 (s, 1 H, CH =), 8.00 (d, 2 H, J = 9.6 Hz, ArH), 7.82(d, 2 H, J = 8.1 Hz, ArH), 7.66 (d, 2 H, J = 8.1 Hz, ArH), 7.29 (d, 2 H, J =8.4 Hz, ArH), 7.20 (d, 2 H, J = 8.7 Hz, ArH), 7.15–7.09 (m, 13 H, ArH ),6.99–6.95 (m, 10 H, ArH), 6.79 (d, 2 H, J = 8.7 Hz, ArH), 6.43 (s, 1 H,ArH), 6.29 (s, 1 H, ArH), 3.87 (d, 4 H, J = 5.4 Hz, OCH2), 1.70 (m, 2 H,CH), 1.41–1.23(m, 16 H, CH2), 0.94–0.87 ppm (m, 12 H, CH3) ;13C NMR (100 MHz, [D8]THF): d= 162.60, 157.13, 156.91, 152.36,144.12, 143.42, 143.33, 143.12, 140.71, 140.43, 140.30, 138.67,135.53, 135.34, 134.39, 133.73, 132.70, 132.61, 132.55, 132.17,131.88, 131.54, 131.18, 130.89, 130.78, 130.73, 130.51, 130.41,130.22, 130.11, 129.06, 127.39, 126.99, 126.95, 126.35, 126.24,126.01, 125.72, 115.13, 114.24, 114.12, 102.75, 93.80, 93.19, 69,66,39.20, 39.13, 31.42, 30.13, 30.07, 29.19, 28.66, 28.61, 23.31, 22.54,22.52, 22.12, 13.04, 10.12, 10.07 ppm. MS (ESI, m/z): [M + 1]+calcdfor C76H73N3O4 : 1092.6; found 1092.5. Elemental Analysis Calcd for

C76H73N3O4 : C, 83.56, H, 6.74; N; 3.85; Found C, 83.47; H, 6.47; N;3.55.

LI-130 : 5 b (120 mg, 0.12 mmol), cyanoacetic acid (31 mg,0.36 mmol), black solid (80 mg, 60.6 %). mp: 117.8–120.4 8C. IR (thinfilm): n= 3394 (OH), 2215 cm@1 (C/N). 1H NMR (300 MHz, CDCl3):d= 8.17 (s, 1 H, CH =), 7.74–7.66 (m, 14 H, ArH), 7.11–6.84 (m, 19 H,ArH), 6.31 (s, 1 H, ArH), 6.16 (s, 1 H, ArH), 3.79 (s, br, 4 H, OCH2), 1.66(m, 2 H, CH), 1.26–1.17 (m, 16 H, CH2), 0.84 ppm (m 12 H, CH3) ;13C NMR (100 MHz, [D8]THF): d= 161.87, 159.33, 158.62, 158.34,154.08, 146.17, 144.77, 144.68, 144.48, 142.16, 142.01, 141.83,141.67, 139.80, 137.11, 136.75, 135.82,135.56, 134.30, 133.99,133.94, 133.59, 133.29, 132.84, 132.13, 132.07, 131.86, 131.50,130.79, 129.48, 128.60, 128.33, 128.30, 127.84, 127.73, 127.38,127.13, 127.14, 126.67, 124.58, 115.68, 115.51, 95.35, 94.44, 71.12,40.57, 40.52, 31.49, 31.44, 30.53, 30.21, 30.02, 29.98, 29.83, 24.74,24.68, 23.88, 14.35, 11.48, 11.41 ppm. MS (ESI, m/z): [M + 1]+ calcdfor C74H71N3O4S:1098.5; found 1098.5. Elemental Analysis Calcd forC74H71N3O4S: C, 80.91, H, 6.52; N; 3.83; Found C, 80.58; H, 6.47; N;3.79.

LI-131: 7 a (170 mg, 0.14 mmol), cyanoacetic acid (36 mg,0.42 mmol), black solid (90 mg, 48.1 %). mp: 162.1–164.4 8C. IR (thinfilm): n= 3359 (OH), 2218 cm@1 (C/N). 1H NMR(300 MHz, CDCl3): d=8.38 (s, 1 H, CH =), 8.28–8.16 (m, 5 H, ArH), 7.99–7.94 (m, 2 H, ArH),7.52 (d, 2 H, J = 7.5 Hz, ArH ), 7.45–7.35 (m, 3 H, ArH), 7.15–7.10 (m,15 H, ArH), 6.95–6.92 (m, 12 H, ArH), 6.76 (m, 2 H, ArH), 6.38 (s, 1 H,ArH), 6.26 (s, 1 H, ArH), 3.85(s, br, 4 H, OCH2), 1.68 (m, 2 H, CH),1.42–1.29 (m, 16 H, CH2), 0.94–0.86 ppm (m, 12 H, CH3) ; 13C NMR(100 MHz, [D8]THF): d= 164.98, 157.41, 157.19, 152.68, 152.53,143.93, 143.69, 143.61, 143.39, 141.93,141.27,140.96, 140.80, 140.62,135.92, 135.02, 134.10, 132.97, 132.77, 132.69, 132.47, 132.00,131.83, 131.47, 131.20, 131.02, 130.82, 130.42, 130.19, 130.04,129.73, 129.49, 129.33, 129.29, 128.97, 128.83, 128.65, 128.19,127.93, 127.65, 127.43, 127.10, 126.79, 126.40, 126.16, 125.46,125.17, 123.58, 123.41, 123.22, 116.88, 115.02, 114.85, 105.49, 94.15,93.69, 70.61, 39.37, 39.31, 31.80, 30.50, 30.43, 29.55, 29.06, 29.02,23.85, 23.78, 22.97, 18.55, 14.18, 11.25, 11.21 ppm. MS (ESI, m/z):[M + 1]+ calcd for C88H79N5O4S2 : 1334.6; found 1334.5; ElementalAnalysis Calcd for C88H79N5O4S2 : C, 79.19, H, 5.97; N; 5.26; Found C,78.93; H, 5.87; N; 5.53.

LI-132 : 7 b (155 mg, 0.12 mmol), cyanoacetic acid (31 mg,0.36 mmol), black solid (60 mg, 38.2 %). mp: 145.1–147.4 8C. IR (thinfilm1): n= 3358 (OH), 2216 cm@ (C/N). 1H NMR(300 MHz, CDCl3): d=

8.50 (s, 1 H, CH =), 8.26 (s, br, 3 H, ArH), 8.06 (s, br, 1 H, ArH), 7.92 (s,br, 1 H, ArH ), 7.55–7.49 (m, 3 H, ArH ), 7.38 (d, 2 H, J = 16.5 Hz, =CH), 7.15–7.10 (m, 15 H, ArH), 6.95–6.92 (m, 12 H, ArH), 6.79 (m, 2 H,ArH), 6.40 (s, 1 H, ArH), 6.27 (s, 1 H, ArH), 3.86 (s, br, 4 H, OCH2), 1.69(m, 2 H, CH), 1.41–1.30 (m, 16 H, CH2), 0.89 ppm (m, 12 H, CH3) ;13C NMR (100 MHz, CDCl3 and [D6]DMSO): d= 162.66, 157.20,156.99, 152.85, 151.95, 147.79, 144.98, 144.06, 143,.82, 143.51,143.42, 143.22, 142.13, 141.64, 140.77, 140.50, 140.40, 137.63,136.32, 135.37, 134.62, 133.29, 133.05, 132.80, 132.73, 132.57,131.98, 131.64, 130.87, 130.81, 130.79, 130.69, 130.59, 130.49,130.45, 130.42, 130.40, 130.27, 129.06, 127.58, 127.36, 127.03,126.83, 126.43, 126.04, 125.82, 124,57, 123.42, 123.14, 122.80,115.32, 114.33, 114.21, 99.07, 93.70, 93.30, 69.80, 69.77, 39.29,39.23, 30.21, 30.16, 29.26, 28.75, 28.70, 24.40, 23.46, 23.40,22.62 ppm. MS (ESI, m/z): [M + 1]+ calcd for C86H77N5O4S3 : 1340.5;found 1340.5; Elemental Analysis Calcd for C86H77N5O4S3 : C, 77.04,H, 5.79; N; 5.22; Found C, 77.20; H, 5.95; N; 5.51.

Chem. Eur. J. 2018, 24, 18032 – 18042 www.chemeurj.org T 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim18041

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Acknowledgements

We are grateful to the National Science Foundation of China

(51673151, 51573140 and 21734007), Hubei Province(2017CFA002), and the Fundamental Research Funds for the

Central Universities (2042017kf0247 and 2042018kf0014) for fi-

nancial support.

Conflict of interest

The authors declare no conflict of interest.

Keywords: dye-sensitized solar cells · electron donatingability · electron recombination · molecular design ·photocatalysis

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Manuscript received: July 18, 2018

Accepted manuscript online: October 11, 2018

Version of record online: November 8, 2018

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