Significantly improved performance of dye-sensitized solar cells by optimizing organic dyes with pyrrole as the isolation spacer and utilizing alkyl chain engineering

Jinfeng Wang a, Siwei Liu a, Zhaofei Chai a, Kai Chang a, Manman Fang a, Mengmeng Han a, Yiyi Wang a, Sheng Li b, Hongwei Han b, Qianqian Li *a and Zhen Li ac
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China. E-mail: qianqian-alinda@163.com
bMichael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan, 430072, China
cInstitute of Molecular Aggregation Science, Tianjin University, Tianjin, 300072, China

Received 29th June 2018 , Accepted 11th September 2018

First published on 11th September 2018


The optimization of interfacial and intramolecular charge transfer of organic dyes is crucial to the photovoltaic performance of dye-sensitized solar cells. In this study, an efficient strategy was afforded by the introduction of alkylated pyrrole as the isolation spacer into organic dyes. The interfacial charge transfer can be organized by the tunable alkyl chains as a barrier layer to inhibit electron recombination, and the pull–push system can be strengthened by the incorporation of electron-withdrawing units (benzothiadiazole) and electron-rich moieties (pyrrole) into the conjugated bridge simultaneously, resulting in excellent light-harvesting capability. Therefore, the high conversion efficiency of 9.70% was achieved by LI-124 with suitable alkyl chains and device optimization.


Introduction

Conversion of sunlight into electrical energy is considered to be the most effective way to solve the energy crisis and environmental problems.1–4 Dye-sensitized solar cells (DSCs) have attracted much attention in both academic and industrial fields because of their low cost and simple construction.5–10 In the development of DSCs, dye sensitizer engineering has proved to be an efficient approach to adjust the light-harvesting ability and regulate the electron processes in devices, resulting in the improved photovoltaic performance.11–14 Until now, the conversion efficiency of 14.3% as a record was achieved using an organic dye with an electron donor–π–bridge–electron acceptor (D–π–A) structure,15 which highlighted the superiority of these pull–push systems with high polarities.

Generally, the optimization of D–π–A structure has focused on the following two aspects: (1) the modulation of intrinsic electron properties to strengthen the intramolecular charge transfer (CT), with the aim of harvesting more sunlight as the input energy; (2) the adjustment of molecular configurations to optimize the interfacial charge transfer, including the suppression of dye aggregation and electron recombination as the energy loss. However, in most cases, the above two points cannot be well combined to generate the synergistic effect. For instance, it is an efficient approach to broaden the absorption spectra of organic dyes by the introduction of an electron-withdrawing unit into the conjugated bridge as the auxiliary acceptor (A′), but the A′ moieties can also act as the “electron trap” to aggravate electron recombination.16–19 Thus, with the aim of achieving high conversion efficiencies, the balance of these two effects should be controlled by well-designed molecular structures. Apart from the modulation of electronic properties of A′ units using different building blocks,20–22 in our previous study, the steric hindrance effect was applied into the isolation spacers between benzothiadiazole (A′) and cyanoacrylic acid (A), which was conducted by the linkage of different substituents to the β-position of thiophene ring (Fig. 1). With the increased sizes of substituents, the involved dihedral angles were enlarged with a poorer conjugation effect, the electron back reaction can be inhibited efficiently, as proven by the increased lifetimes of the electrons in TiO2 film. Accordingly, the conversion efficiencies of the corresponding DSCs increased from 3.44% to 6.48%, affording an efficient way to suppress the electron recombination by the partially twisted strategy.23 However, on the other hand, the absorption spectra became narrower with the onset wavelength which blue-shifted from 693 to 626 nm (Chart S1), leading to lower light harvesting abilities. Thus, the improvement of photovoltaic performance is limited by similar trends of input power and energy loss. Is it possible to obtain the decreased charge recombination and the increased light-harvesting ability simultaneously?


image file: c8ta06258g-f1.tif
Fig. 1 The comparison between organic dyes with different isolation groups and the functionality of alkyl chains in different positions. CT: charge transfer.

Actually, this combined effect is hard to be realized by the modification of thienyl unit as a common building block of organic dyes for the steric hindrance effect. As a similar five-membered ring to thiophene, pyrrole affords an additional functional site at the nitrogen atom, which can incorporate various substituents with nearly no influence on the intramolecular CT effect of the conjugated system, as proved in our previous study.24–28 Also, pyrrole with its electron-rich property can act as the electron donor (D′) to the adjacent A′ unit, leading to the reversed charge transfer. It can relieve the electron-trapping effect of the A′ unit to some extent,29 indicating the key role of pyrrole in the intramolecular CT effect of organic dyes as the isolation spacer.

Besides, the interfacial CT in DSCs can be optimized by the well-organized molecular arrangements of organic dyes on the TiO2 surface, which can be conducted by the alkyl chain engineering. It plays a key role in molecular morphology by adjusting the packing modes and intermolecular interactions, and has been applied in many fields, including organic field-effect transistors (OFETs),30 organic light-emitting diodes (OLEDs)31 and organic photovoltaic devices (OPVs),32,33 and so on. In DSCs, the alkyl chains are mostly linked to the electron donor moieties of organic dyes, which can act as the first barrier layer to inhibit the dye aggregation and block the electron recombination.34–40 Recently, the largely increased conversion efficiency is achieved using cyclopentadithiophene-based organic dyes from 4.74% to 9.07% (Chart S2),41 mainly due to the dual protection of the two barrier layers from alkyl chains in conjugated bridges. These indicated that the alkyl chains as the side chains in organic dyes play an essential role in the interfacial CT, which were varied with the different linkage positions.

Combined with the function of pyrrole and alkyl chains, a series of organic dyes (LI-121–LI-125) with pyrrole as the isolation spacer were designed and synthesized with alkyl chain engineering from methyl to dodecyl (Fig. 1). Through the replacement of thiophene in the dye LI-80 with N-methyl pyrrole moiety, the improved photovoltaic performance was obvious with the increased conversion efficiency from 3.44% to 4.27%. Furthermore, the highest conversion efficiency of 8.75% was achieved by LI-124 with N-decyl pyrrole as the isolation spacer, which can further increase to 9.70% by device optimization, indicating the great functionality of alkyl chain engineering. These alkyl chains play an the essential role in the regulation of dye molecular arrangement and electron process at the dye/TiO2/electrolyte interface, but have nearly no adverse effect on the conjugated effect of the molecular skeleton due to their perpendicular geometry to the pyrrole ring, as proved by the similar absorption spectra of these organic dyes. Thus, an efficient method is afforded to achieve the high conversion efficiency by the suppression of electron recombination with increased light-harvesting abilities, which can be applied to various organic dyes to promote the development of DSCs.

Results and discussion

Synthesis of the sensitizers

The alkyl chain engineering is the common strategy used to tune the molecular morphology in the aggregated state. In this study, various alkyl moieties with different lengths were linked to the nitrogen atom of the pyrrole ring, which was located near the cyanoacrylic acid moiety as the electron acceptor and anchoring group. In this case, these flexible chains close to the TiO2 surface can act as the second barrier layer after the first insulation effect by the alkyl chains linked to the electron donor, with the aim of further inhibiting the electron recombination. Besides, the absorption behaviors of the organic molecules on TiO2 in the aggregated state can be adjusted by different alkyl moieties as the side chains. Moreover, the conjugated effect of the whole molecule is maintained in spite of the varied alkyl moieties, affording an efficient strategy to reduce the energy loss in DSCs without any unfavorable effect on the light-harvesting ability. It is mainly related to the particular structure of pyrrole with the functional site on the nitrogen atom, which can hardly be realized in the similar moieties, including thiophene and furan. Thus, a series of organic dyes (LI-121–LI-125) based on pyrrole with alkyl chain modification have been synthesized (Scheme 1). After the alkylation of pyrrole with different chains, the corresponding heteroarylboronates were obtained followed by the C–H activation, which can be linked to the main light-harvesting moieties of bromine atoms (Ar-Br) by the Suzuki coupling reaction. Then, through the introduction of the aldehyde group by the Vilsmeier–Haack reaction, the organic dyes were obtained by the Knoevenagel condensation of the compound 4a–e and cyanoacrylic acid, which were well characterized by 1H NMR, 13C NMR, ESI and EA.
image file: c8ta06258g-s1.tif
Scheme 1 Synthetic routes to the sensitizers. Reagents and conditions: (i) NaH, RBr, THF; (ii) [Ir(COD)Cl]2,4,4′-di-tert-butyl-2,2′-dipyridyl, (Bpin)2, cyclohexane, 60 °C, 12 h. (iii) K2CO3, Pd(PPh3)4, THF/H2O, reflux, 12 h; (iv) POCl3, DMF, ClCH2CH2Cl, 0 °C to rt, 10 h; (v) cyanoacetic acid, piperidine, CH3CN, reflux, 12 h.

Optical properties

All the organic dyes with the same conjugated skeleton show similar absorption spectra as single molecules in dilute solutions and in the aggregated states (in TiO2 films) (Fig. 2), indicating that the alkyl chain engineering does not affect ICT and the molecular configuration of the conjugated system. There are two main absorption bands in the range of 350–700 nm. The peak located at 380 nm is assigned to the π–π* transitions of aromatics, while the other one located at 517 nm is attributed to the ICT through the whole molecule, providing efficient charge-separation at the excited state. Once these dyes were adsorbed onto the nanocrystalline TiO2 film, the obvious red-shift in the absorption spectra was observed in comparison with those in the solution, mainly attributing to the interactions between TiO2 and organic dyes, together with their intermolecular interactions in the aggregated state. With the increase in thickness of the TiO2 films (from 6 to 9 μm), the absorption spectra can be further extended (Fig. S1). These broader absorption regions are beneficial to the enhancement of the light-harvesting capability, leading to the improved photovoltaic performance.
image file: c8ta06258g-f2.tif
Fig. 2 Absorption spectra of dyes in the CH2Cl2 solution (30 μM) (A) and on the TiO2 films (6 μm) (B).

Electrochemical properties

Regardless of the different lengths of alkyl chains substituted at the nitrogen atom of pyrrole, the electrochemical properties of these dyes are similar to each other (Fig. 3). The reversible redox process was observed for these organic dyes by cyclic voltammetry (Fig. S2 and Table S1). The first oxidation potentials (Eox) (0.8 V vs. NHE), were significantly more positive than that of the liquid electrolyte I/I3 redox potential (0.4 V vs. NHE), indicating that the oxidized dyes could be efficiently regenerated with enough driving force. The excited state oxidation potentials image file: c8ta06258g-t1.tif (−1.2 V vs. NHE), estimated from Eox and zero–zero transition energy (E0–0), were more negative than the conduction band-edge of TiO2 (ECB) for thermodynamically favored electron injection.42 Thus, the dye LI-121–LI-125 can act as efficient organic dyes for the well-matched energy levels and chemical stability.
image file: c8ta06258g-f3.tif
Fig. 3 Energy-level diagram of the sensitizers, electrolyte and TiO2. ΔG1: energy gap for electron injection; ΔG2: energy gap for the regeneration of the oxidized dyes.

Theoretical calculations

Through the incorporation of the pyrrole moiety as the isolation spacer into the organic dyes, the conjugated skeleton of the dye LI-121–LI-125 exhibited an almost planar geometry with a small dihedral angle of 30.5° between benzothiadiazole and pyrrole (Table S2), which is proved using the density functional theory (DFT) calculation at the B3LYP/6-31G* level.43 The efficient charge transfer in the push–pull structure can be observed with the almost separated HOMO and LUMO. Also, the extent of the charge separation upon excitation was estimated by the natural bond orbital analysis (Fig. S3). These dyes were grouped into four segments, including triarylamine-thiophene (TPA-Th), benzothiadiazole (BT), isolation spacer (IS), and cyanoacrylic acid (CAA). The charge differences (Δq, S0 → S1) showed that prominent charge transfers occurred between TPA-Th and BT, acting as the main light-harvesting unit. Moreover, compared to the analogue LI-80 with thiophene as the isolation group, the electron-rich property of pyrrole in LI-121 can promote the ICT effect for larger charge differences, which is beneficial to the light harvesting.

Photovoltaic performance

The current–voltage (JV) curves of the solar cells in conjunction with an iodine electrolyte (0.05 M I2, 0.2 M LiI, 0.6 M DMPII, 0.1 M GuNCS and 0.5 M 4-TBP in acetonitrile) were measured under standard AM 1.5G conditions (100 mW cm−2) (Fig. 4A and Table S3). With the involved alkyl chains lengthened from methyl to dodecyl, the open circuit voltages (Voc) of the corresponding DSCs first increased from 598 to 674 mV, then decreased to 618 mV. Also, their short-circuit current densities (Jsc) exhibited a similar trend, and the highest conversion efficiency of 8.75% was achieved by a DSC based on LI-124 with decyl modification. It is mainly related to the various aggregated states of the organic dyes on the TiO2 film, which can be partially demonstrated by the different dye-loading amounts with alkyl chain engineering.
image file: c8ta06258g-f4.tif
Fig. 4 JV characteristic curves (A) and IPCEs (B) for DSCs based on dye LI-121–LI-125.

The largest dye-loading amount of LI-121 (3.29 × 10−8 mol cm−2 μm−1) with methyl modification meant the densely packed molecules on the TiO2 surface. In this case, the severe dye aggregates are usually formed due to the strong intermolecular interactions with the short contact of the adjacent molecules, resulting in the inhomogeneous distribution of organic dyes on the TiO2 surface (Fig. 5). Thus, severe electron recombination may occur at the bare TiO2 surface, leading to the lower Voc of LI-121. Also, the possible intermolecular energy transfer can be induced with the excited state quenching, resulting in photocurrent loss.44,45 With the increased length of the alkyl moieties as the side chains, the distances among the organic dyes can be enlarged, accompanied with the decreased intermolecular interactions, as demonstrated by the decreased dye-loading amount of LI-124 (2.80 × 10−8 mol cm−2 μm−1). Thus, the arrangement of the organic dyes on the TiO2 surface could be regulated with the almost homogeneous packing modes, which is beneficial to the light-harvesting capability and the inhibition of electron recombination, leading to the improved photovoltaic performance. However, when the length of the involved alkyl chains increases to some extent, the ample sizes of the organic molecules may largely decrease the amount of dyes on the TiO2 surface, leading to the lower light harvesting efficiencies. Also, the barrier effect for the suppression of electron recombination can be weakened with the decreased amount of alkyl chains. Thus, LI-125 bearing the longest alkyl chains (dodecyl) exhibited moderate conversion efficiency. Accordingly, the decyl moiety has proved to be a suitable side chain to realize the increase of Jsc and Voc at the same time, resulting in the best photovoltaic performance of LI-124. The density of the photogenerated electrons could also be analyzed from IPCE, as shown in Fig. 4B. The IPCE curves of these organic dyes exhibited a similar shape with different IPCE values, which was mainly related to the varied electron collection efficiencies with different alkyl chain modification. Dyes LI-123 and LI-124 bearing octyl and decyl moieties exhibited higher values for the well-organized molecular arrangement and dual insulation effect by the alkyl chains in the electron donor and isolation spacer together (Fig. 5).


image file: c8ta06258g-f5.tif
Fig. 5 The molecular arrangements of organic dyes on the TiO2 surface with different alkyl chains.

Besides, the energetic and dynamic origins of the different Voc can be investigated and understood in detail by the charge extraction (CE) method and intensity-modulated photovoltage spectroscopy (IMVS). Since the related DSCs were fabricated with the same electrolyte, Voc is mainly determined by the electron quasi-Fermi-level (EF,n) of the dye-loaded TiO2 film, which is associated with the conduction band (ECB) and the density of free charge.46 As shown in Fig. 6A, the shifts in ECB were much different with the alkyl chain engineering of organic dyes. The up-shift of the conduction band to a greater extent was conducted by the dye LI-121, which was related to their orientation and arrangement on the TiO2 surface as mentioned above. However, it did not result in the enhancement of Voc values, mainly due to the shorter electron lifetime of LI-121-sensitized TiO2 film, as shown in Fig. 6B. While DSCs based on dye LI-123 and LI-124 exhibited the longer electron lifetime, meaning the lower electron recombination rate. Thus, the suitable alkyl chains close to the TiO2 surface indeed can act as the efficient barrier layer to further inhibit the contact of the electrolyte with the electron in TiO2 film, which is beneficial to the suppression of charge recombination. Accordingly, electrochemical impedance spectroscopy (EIS), which was performed in the dark under a forward bias of −0.69 V (Fig. S4), showed a similar trend with IMVS measurement, further confirming the important role of alkyl chains substituted to the pyrrole moiety.


image file: c8ta06258g-f6.tif
Fig. 6 Charge density (A) and electron lifetime (B) at the open circuit as a function of Voc for DSCs based on LI-121–LI-125-sensitized solar cells.

Thus, with different alkyl moieties applied into the organic dyes as the side chains, a well-organized molecular alignment can be achieved by the adjustable intermolecular interactions and dye-loaded amounts, which can be considered as the chemical optimization of organic dyes. On the other hand, physical doping can be conducted by the introduction of chenodeoxycholic acid (CDCA), which is the common coadsorbent used to inhibit the dye aggregation on the TiO2 surface. When it was applied into DSC based on the dye LI-124 with suitable alkyl chains, the high conversion efficiency of 9.70% was achieved by the incorporation of CDCA with low concentration (1 mM) (Fig. 7 and S5–S8), which was mainly attributed to the combined effect of chemical optimization and physical doping. Under the same conditions, the conversion efficiency of N719 was 8.26% with Jsc of 16.76 mA cm−2, Voc of 683 mV and FF of 0.72 (Table S3), meaning that the superiority of the organic dye LI-124 with reasonable molecular structure and the potential higher conversion efficiency by device optimization.


image file: c8ta06258g-f7.tif
Fig. 7 The changes of the photovoltaic parameters of the dyes LI-121–LI-125 by alkyl chain engineering.

The storage stability of DSCs based on these dyes were investigated, and the values for Jsc, Voc, FF and η were recorded over a period of 500 h. Taking the LI-124-sensitized solar cell with the best photovoltaic performance as the example, the overall efficiency remained at 91% after 500 h of aging in the air (Fig. 8). Also, other dyes exhibited excellent storage stabilities (Fig. S9 and S10), indicating that the organic dyes on the TiO2 surface remained intact after long time aging. Also, under continuous full sunlight irradiation at standard AM 1.5G conditions, the dye-sensitized films almost remain intact for 15 h, as proved by the similar absorption spectra and photographs (Fig. S11–S13), further confirming their good photo-stabilities. Besides, they possessed excellent thermal stability with decomposition temperatures (the 5% of weight lost) higher than 220 °C (Fig. S14).


image file: c8ta06258g-f8.tif
Fig. 8 The storage stability of DSC based on LI-124.

Based on these excellent photovoltaic performances, some strategies can be proposed to optimize the molecular design of the organic dyes in DSCs (Fig. 9). As discussed above, once the A′ units were introduced into the conjugated bridges of the organic dyes with the D–π–A structure to broaden the light harvesting regions, it is essential to insert isolation spacers between the A′ and A moieties to suppress the possible “electron trap” effect. In the previous study, some aromatic rings were employed, such as thienyl, phenyl, and so on.23 However, when some alkyl chains were substituted to inhibit the charge recombination, the steric hindrance can result in the partially twisted configuration of the organic dyes, leading to a decreased light-harvesting capability. In this study, alkylated pyrrole was incorporated into the isolation spacers with adjustable lengths of alkyl chains, which can act as the second barrier layer close to the TiO2 surface, after the first insulation by the alkyl chains linked to electron donor part. Meanwhile, the light harvesting region was maintained, since nearly no steric hindrance would be caused by the alkyl chains linked to the nitrogen atom of the pyrrole. Thus, the intramolecular charge transfer of organic dyes can increase by the addition of electron-withdrawing units and electron-rich rings to harvest more sunlight, and the interfacial charge transfer can be optimized by alkyl chain engineering close to the TiO2 surface, which can adjust the molecular arrangement of organic dyes as the aggregated state and act as an efficient contact inhibitor, to suppress the interfacial charge recombination. Therefore, the high conversion efficiency can be gained by the increased input energy and reduced energy loss, which were conducted by the optimization of molecular configuration and alignment, and the pull–push electronic properties of the organic dyes. In future research, more isolation spacers near the electron acceptor will be exploited to extend the conjugated system and organize the interfacial charge transfer together, with the aim to achieve better photovoltaic performance.


image file: c8ta06258g-f9.tif
Fig. 9 The strategy for molecular design of efficient organic dyes in the dye-sensitized solar cells.

Conclusions

A series of organic dyes with alkylated pyrrole moieties as the isolation spacers were designed and synthesized, with enhanced light-harvesting ability and inhibited charge recombination in dye sensitized solar cells. With the adjustment of alkyl chains close to the TiO2 surface, the interfacial charge transfer was optimized by the well-organized molecular arrangement and formation of the contact inhibitor. Accordingly, the highest conversion efficiency that could be achieved was 9.70% for LI-124 with decyl-substituted pyrrole as the isolation spacer. Also, with the investigation of the relationship between molecular structure and photovoltaic performance, some strategies were proposed to promote the development of DSCs using efficient organic dyes.

Experimental

Materials and instrumentation

Tetrahydrofuran (THF) was dried over and distilled from a K–Na alloy under an atmosphere of dry argon. All solvents were of analytical grade and used without further purification. The compound 1a (N-methylpyrrole) was purchased without further purification. The compound Ar-Br was synthesized according to our previous work.231H and 13C spectra were obtained with a Bruker 300 MHz spectrometer or Bruker Avance III HD 400 MHz using tetramethylsilane (TMS; δ = 0 ppm) as the internal standard. Elemental analyses were performed using 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 in dichloromethane. Electrochemical cyclic voltammetry was performed with a CHI 660 voltammetric analyzer with a Pt disk, Pt plate, and Ag/Ag+ electrode as the working electrode, counter electrode, and reference electrode, respectively, in nitrogen-purged anhydrous CH2Cl2 with tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte (scanning rate: 100 mV s−1). The ferrocene/ferrocenium redox couple was used for potential calibration.

DSC device fabrication and measurement

The DSC devices were fabricated according to the literature.45,47 A fluorine doped tin oxide (FTO) conducting glass (3.2 mm thickness, 7–8 ohms sq−1) was cleaned with detergent, water, ethanol and acetone respectively and irradiated in O3 for 18 min, and then immersed in a TiCl4 solution (40 mM) for 30 min at 70 °C. After being cooled to room temperature, it was washed with deionized water and ethanol three times respectively and dried. The photoanodes (16 μm thickness), 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), were prepared using the screen printing technique and gradually heated under air flow at 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min and 500 °C for1 h. After the films were cooled to room temperature, they were immersed in a TiCl4 solution (40 mM) for 30 min at 70 °C once again, then washed and annealed at 500 °C for 30 min. After the TiO2 films were cooled to 80 °C, they were immersed in a dye bath (0.3 mM) of a solution mixture (CH3CN/t-BuOH/CHCl3 = 2/2/1) for 18 h under dark conditions. Then, the sensitized electrodes were washed with the corresponding solvents and dried in air. The counter electrodes were prepared by thermal deposition: FTO glass (2.2 mm thickness, 7–8 ohms sq−1) with two small holes was cleaned and used as photocathodes, 10 μL solution of H2PtCl6 (10 mM) in isopropyl alcohol was dispersed on an FTO glass and heated at 400 °C for 30 min. After that, the dye absorbed photoanodes and Pt-counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket (25 μm thickness) made using an ionomer Surlyn 1702 (DuPont). The electrolyte consisting of 0.05 M I2, 0.2 M LiI, 0.6 M DMPII, 0.1 M GuNCS and 0.5 M 4-TBP in acetonitrile was injected into the cells through the two small holes after being assembled. Lastly, the two holes were sealed with a Surlyn sheet (50 μm thickness) and a thin ITO glass covered by heating.

The photovoltaic measurements were conducted using an 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 cell before measurement. A Keithley model 2400 digital source meter was connected with a light source to get the JV curves while applying an external bias to the cell. The 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 using a Modulab XM PhotoEchem system such as in IMVS (intensity modulated photovoltage spectroscopy), CE (charge extraction), and EIS (electrochemical impedance spectroscopy). IMVS and CE were performed under a white light emitting diode (LED) array. The CE was conducted in the dark with different potential biases with a frequency range from 1 Hz to 100 kHz.

The measurement of the dye loading amounts

The dye loading amounts (DLA) were measured by two steps, firstly, the desorption of organic dyes from the TiO2 films were conducted by the immersion of dye-sensitized TiO2 films into the solution (3.0 mL) of NaOH (0.1 M)/THF (v/v, 1/1). Then, the concentrations (c) of these solutions were determined by the standard curves of organic dyes under the same conditions. Accordingly, the DLA values were calculated using a mathematical equation: DLA = (c × v)/ (0.25 cm2 × 16 μm), where v is the volume of desorption solution (3.0 mL), 0.25 cm2 is the area of TiO2 film, 16 μm is the thickness of the TiO2 film.

General synthesis of the compound 1b–e

Under an atomsphere of nitrogen, a mixture of pyrrole (1.0 equiv.) and RBr (1.5 equiv.) in dry THF (30 mL) was placed in a 200 mL round-bottom and cooled to −20 °C. Then NaH (3.0 equiv.) dissolved in THF (10 mL) was added slowly and the mixture was stirred at −20 °C for 1 h, then 65 °C for 12 h. The solid was filtered out, and the solvent was poured into water and extracted with chloroform three times. The combined organic layers were dried with anhydrous sodium sulfate. After the solvent was evaporated, the crude products were purified by flash column chromatography.
1b . Pyrrole (670 mg, 10 mmol), C6H13Br (2.48 g, 15.0 mmol), and NaH (720 mg, 30.0 mmol). Brown oil (730 mg, 48.0%). 1H NMR (300 MHz, CDCl3) δ (ppm): 6.66 (s, 2H, ArH), 6.15 (s, br, 2H, ArH), 3.87 (t, J = 7.2 Hz, 2H, –NCH2–), 1.74 (m, 2H, –CH2–), 1.29 (m, 6H, –CH2–), and 0.89 (s, br, 3H, –CH3).
1c . Pyrrole (720 mg, 10.7 mmol), C8H17Br (3.10 g, 16.0 mmol), and NaH (770 mg, 32.1 mmol). Brown oil (832 mg, 43.4%). 1H NMR (300 MHz, CDCl3) δ (ppm): 6.63 (s, 2H, ArH), 6.14 (s, 2H, ArH), 3.78 (t, J = 7.5 Hz, 2H, –NCH2–), 1.69 (m, 2H, –CH2–), 1.27 (m, 10H, –CH2–), and 0.89 (s, br, 3H, –CH3).
1d . Pyrrole (700 mg, 10.4 mmol), C10H21Br (3.45 g, 15.6 mmol), and NaH (749 mg, 31.2 mmol). Brown oil (679 mg, 31.5%). 1H NMR (300 MHz, CDCl3) δ (ppm):6.65 (s, 2H, ArH), 6.14 (s, 2H, ArH), 3.77 (t, J = 7.2 Hz, 2H, –NCH2–), 1.70 (m, 2H, –CH2), 1.27 (m, 14H, –CH2), and 0.89 (s, br, 3H, –CH3).
1e . Pyrrole (910 mg, 13.6 mmol), C12H25Br (5.08 g, 20.4 mmol), and NaH (979 mg, 4.8 mmol). Brown oil (972 mg, 30.3%). 1H NMR (300 MHz, CDCl3) δ (ppm): 6.67 (s, 2H, ArH), 6.14 (s, 2H, ArH), 3.77 (s, br, 2H, –NCH2–), 1.71 (m, 2H, –CH2), 1.27 (m, 18H, –CH2), and 0.89 (s, br, 3H, –CH3).

General synthesis of the compound 2a–e

In an atmosphere of nitrogen, a mixture of 1a–e (1.0 equiv.), 4,4,4′,4′,5,5,5′,5′-octamethyl-2,2′-bi(1,3,2-dioxaborolane) (1.2 equiv.), [Ir(COD)Cl]2 (0.05 equiv.) and 4,4′-di-tert-butyl-2,2′-dipyridyl (0.05 equiv.) in degassed cyclohexane (60 mL) was stirred at 65 °C for 14 h. The mixture was poured into water (100 mL) then extracted with dichloromethane. The organic layers were collected and evaporated under reduced pressure, and the crude products were used directly without further purification.

General synthesis of the compound 3a–e

In an atmosphere of nitrogen, a mixture of the compound Ar-Br (1.0 equiv.), compound 2a–e (5.0 equiv.), Pd(PPh3)4 (0.05 equiv.) and K2CO3 (5 equiv.) in 30 mL solvent (THF/H2O = 5/1) was placed in a 100 mL dry Schlenk tube and refluxed for 12 hours. The mixture was poured into water and extracted with chloroform three times. The combined organic layers were dried with anhydrous sodium sulfate. After the solvent was evaporated, the crude products were purified by column chromatography.
3a . Ar-Br (225 mg, 0.30 mmol), compound 2a (310 mg, 1.50 mmol), and red solid (200 mg, 90%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.15 (m, 2H, ArH), 7.70 (d, 1H, J = 6.6 Hz, ArH), 7.59 (d, 2H, J = 8.7 Hz, ArH), 7.49 (s, 1H, ArH), 7.06 (d, 5H, J = 8.4 Hz, ArH), 6.94 (d, 4H, J = 8.4 Hz, ArH), 6.81 (d, 2H, J = 7.8 Hz, ArH), 6.41 (s, 1H, ArH), 6.55 (m, 1H, ArH), 6.20 (s, br, 1H, ArH), 3.95 (m, 4H, –OCH2–), 3.69 (s, 3H, –NCH3), 1.72 (s, br, 4H, –CH2–), 1.43–1.32 (m, 12H, –CH2–), and 0.89 (s, br, 6H, –CH3).
3b . Ar-Br (200 mg, 0.27 mmol), compound 2b (374 mg, 1.35 mmol), and red solid (289 mg, 78%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.14–8.09 (m, 2H, ArH), 7.66 (d, 1H, J = 7.2 Hz, ArH), 7.57 (d, 2H, J = 8.4 Hz, ArH), 7.49 (s, br, 1H, ArH), 7.05 (d, 5H, J = 6.6 Hz, ArH), 6.92 (d, 4H, J = 8.7 Hz, ArH), 6.80 (d, 2H, J = 7.2 Hz, ArH), 6.41 (s, br, 1H, ArH), 6.20 (s, br, 1H, ArH), 3.94 (m, 6H, –OCH2–, –NCH2–), 1.70 (m, 6H, –CH2–), 1.42–1.23 (m, 18H, –CH2–), and 0.88 (s, br, 9H, –CH3).
3c . Ar-Br (224 mg, 0.30 mmol), compound 2c (458 mg, 1.5 mmol), and red solid (200 mg, 79.4%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.13 (d, 1H, J = 4.0 Hz, ArH), 7.89 (d, 1H, J = 8.0 Hz, ArH), 7.55–7.50 (m, 3H, ArH), 7.30 (d, 1H, J = 4.0 Hz, ArH), 7.09 (d, 4H, J = 8.0 Hz, ArH), 7.96–6.91 (m, 3H, ArH), 6.86 (d, 4H, J = 8.0 Hz, ArH), 6.50 (d, 1H, J = 4.0 Hz, ArH), 6.34 (t, 1H, J = 4.0 Hz, ArH), 3.98–3.94 (m, 6H, –OCH2–, –NCH2–), 1.80–1.77 (m, 4H, –CH2–), 1.62 (m, 2H, –CH2–), 1.49–1.33 (m, 12H, –CH2–), 1.21–1.10 (m, 10H, –CH2–), and 0.93–0.80 (m, 9H, CH3).
3d . Ar-Br (265 mg, 0. 35 mmol), compound 2d (583 mg, 1.75 mmol), and red solid (207 mg, 68.1%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.12 (d, 1H, J = 4.0 Hz, ArH), 7.87 (d, 1H, J = 8.0 Hz, ArH), 7.55–7.44 (m, 3H, ArH), 7.28 (d, 1H, J = 4.0 Hz, ArH), 7.08 (d, 4H, J = 8.0 Hz, ArH), 6.98–6.88 (m, 3H, ArH), 6.84 (d, 4H, J = 8.0 Hz, ArH), 6.48 (d, 1H, J = 4.0 Hz, ArH), 6.35–6.29 (m, 1H, ArH), 4.05–3.84 (m, 6H, –OCH2–, –NCH2–), 1.80–1.75 (m, 4H, –CH2–), 1.63–1.58 (m, 2H, –CH2–), 1.49–1.33 (m, 12H, –CH2–), 1.21–1.10 (m, 14H, –CH2–), and 0.93–0.83 (m, 9H, –CH3).
3e . Ar-Br (265 mg, 0. 35 mmol), compound 2e (632 mg, 1.75 mmol), and red solid (178 mg, 56.9%), 1H NMR (400 MHz, CDCl3) δ (ppm): δ 8.04 (d, 1H, J = 4.0 Hz, ArH), 7.82 (d, 1H, J = 8.0 Hz, ArH), 7.48–7.41 (m, 3H, ArH), 7.23 (t, 1H, J = 4.0 Hz, ArH), 6.99 (d, 4H, J = 8.0 Hz ArH), 6.84–6.82 (m, 3H, ArH), 6.77 (d, 4H, J = 8.0 Hz, ArH), 6.33 (m, 1H, ArH), 6.19 (m, 1H, ArH), 3.85 (m, 6H, –OCH2–, –NCH2–), 172–1.65 (m, 4H, –CH2–), 1.52–1.47 (m, 2H, –CH2–), 1.40–1.25 (m, 12H, –CH2–), 1.16–1.00 (m, 18H, –CH2–), and 0.85–0.75 (m, 9H, –CH3).

General synthesis of the compound 4a–e

In an atmosphere of nitrogen, fresh distilled DMF (3.0 equiv.) and POCl3 (2.0 equiv.) were added into a round-bottom flask and stirred at 0 °C. After the resultant solution turns to a glassy solid, compound 3 (1.0 equiv.) dissolved in 1,2-dichloroethane (10 mL) was added dropwise. The solution was warmed to room temperature and stirred for 12 h. Then poured into Na2CO3 solution (1 M, 50 mL), and stirred for another 2 h. The mixture was extracted with chloroform three times. The combined organic layers were dried with anhydrous sodium sulfate. After the solvent was evaporated, the crude product was purified by column chromatography.
4a . 3a (200 mg, 0.27 mmol), DMF (59 mg, 0.81 mmol), POCl3 (83 mg, 0.54 mmol), and red solid (130 mg, 62.6%). 1H NMR (300 MHz, CDCl3) δ (ppm): 9.66 (s, 1H, –CHO), 8.17 (d, 2H, J = 5.1 Hz, ArH), 7.92 (d, 1H, J = 7.2 Hz, ArH), 7.64 (d, 2H, J = 7.2 Hz, ArH), 7.50 (d, 2H, J = 8.8 Hz, ArH), 7.31 (d, 2H, J = 3.9 Hz, ArH), 7.10 (d, 4H, J = 9.0 Hz, ArH), 6.95 (d, 2H, J = 9.0 Hz, ArH), 6.85 (d, 4H, J = 8.7 Hz, ArH), 6.59 (d, 1H, J = 3.9 Hz, ArH), 3.95 (m, 6H, –NCH2–, –OCH2–), 1.80–1.76 (m, 4H, –CH2–), 1.47–1.25 (m, 12H, –CH2–), and 0.92 (s, br, 6H, –CH3).
4b . 3b (270 mg, 0.33 mmol), DMF (73 mg, 1.00 mmol), POCl3 (101 mg, 0.66 mmol), and red oil (180 mg, 65.0%). 1H NMR (300 MHz, CDCl3) δ (ppm): 9.62 (s, 1H, –CHO), 8.21 (m, 2H, ArH), 7.83 (d, 1H, J = 8.0 Hz, ArH), 7.59 (d, 2H, J = 8.7 Hz, ArH), 7.51 (s, br, 1H, ArH), 7.25 (d, 1H, J = 8.4 Hz, ArH), 7.05 (d, 4H, J = 8.4 Hz, ArH), 6.80 (d, 2H, J = 7.8 Hz, ArH), 6.56 (d, 1H, J = 8.4 Hz, ArH), 4.30 (s, br, 2H, –NCH2–), 3.92 (m, 4H, –OCH2–), 1.69 (m, 6H, –CH2–), 1.41–1.23 (m, 18H, –CH2–), and 0.88 (m, 9H, –CH3).
4c . 3c (190 mg, 0.23 mmol), DMF (50 mg, 0.69 mmol), POCl3 (71 mg, 0.46 mmol), and red oil (143 mg, 71.9%). 1H NMR (400 MHz, CDCl3) δ (ppm): 9.63 (s, 1H, CHO), 8.19 (d, 1H, J = 3.9 Hz, ArH), 7.92 (d, 1H, J = 7.4 Hz, ArH), 7.62 (d, 1H, J = 7.5 Hz, ArH), 7.57–7.46 (m, 2H, ArH), 7.31 (d, 1H, J = 4.0 Hz, ArH), 7.15–7.05 (m, 5H, ArH), 6.95 (d, 2H, J = 8.8 Hz, ArH), 6.85 (d, 4H, J = 8.0 Hz, ArH), 6.52 (d, 1H, J = 8.0 Hz, ArH). 4.39–4.29 (m, 2H, –NCH2–), 3.95 (t, 4H, J = 6.5 Hz, –OCH2–), 1.78 (m, 4H, –CH2–), 1.39 (m, 12H, –CH2–), 1.22 (m, 2H, –CH2–), 1.19–0.98 (m, 10H, –CH2–), 0.92 (t, 6H, J = 4.5 Hz, –CH3), and 0.84 (m, 3H, –CH3).
4d . 3d (200 mg, 0.23 mmol), DMF (50 mg, 0.69 mmol), POCl3 (71 mg, 0.46 mmol), and red oil (156 mg, 75.7%). 1H NMR (400 MHz, CDCl3) δ (ppm):9.63 (s, 1H, –CHO), 8.19 (d, 1H, J = 4.0 Hz, ArH), 7.92 (d, 1H, J = 8.0 Hz, ArH), 7.62 (d, 1H, J = 8.0 Hz, ArH), 7.54–7.47 (m, 2H, ArH), 7.31 (d, 1H, J = 4.0 Hz, ArH), 7.14–7.03 (m, 5H, ArH), 6.95 (d, 2H, J = 8.8 Hz, ArH), 6.85 (m, 4H, ArH), 6.52 (d, 1H, J = 8.0 Hz, ArH), 4.41–4.25 (m, 2H, –NCH2–), 3.95 (t, 4H, J = 6.5 Hz, –OCH2–), 1.79 (m, 4H, –CH2–), 1.58–1.33 (m, 12H, –CH2–), 1.20 (m, 2H, –CH2–), 1.20–0.97 (m, 14H, –CH2–), and 0.97–0.78 (m, 9H, –CH3).
4e . 3e (170 mg, 0.19 mmol), DMF (42 mg, 0.57 mmol), POCl3 (58 mg, 0.38 mmol), and red oil (123 mg, 70.3%). 1H NMR (400 MHz, CDCl3) δ (ppm): 9.63 (s, 1H, CHO), 8.19 (d, 1H, J = 4.0 Hz, ArH), 7.92 (d, 1H, J = 8.0 Hz, ArH), 7.62 (d, 1H, J = 8.0 Hz, ArH), 7.50 (d, J = 8.0, 2H, ArH), 7.31 (d, 1H, J = 4.0 Hz, ArH), 7.12–7.07 (m, 5H, ArH), 6.95 (d, 2H, J = 8.8 Hz, ArH), 6.85 (d, 4H, J = 8.0 Hz, ArH), 6.52 (d, 1H, J = 8.0 Hz, ArH), 4.34 (t, 2H, J = 8.0 Hz, –NCH2–), 3.95 (t, 4H, J = 8.0 Hz, –OCH2–), 1.82–1.75 (m, 4H, –CH2–), 1.49–1.34 (m, 12H, –CH2–), 1.17 (m, 2H, –CH2–), 1.15–1.01 (m, 18H, –CH2–), 0.93–0.83 and (m, 9H, –CH3).

General synthesis of the sensitizers

A mixture of the compound 4a–e (1.0 equiv.), cyanoacetic acid (3.0 equiv.) and a catalytic amount of piperidine in 10 mL solvent (MeCN/THF = 4/1) were placed in a 50 mL dry round-bottom with a condensation tube. The mixture was refluxed for 16 h, and then poured into hydrochloric acid solution (50 mL, 2 M). The crude product was extracted with chloroform three times, and the combined organic layer was dried with anhydrous sodium sulfate. After the solvent was evaporated, the crude product was purified by column chromatography.
LI-121 . 4a (165 mg, 0.21 mmol) and cyanoacetic acid (53 mg, 0.63 mmol). Black solid (110 mg, 62.8%). 1H NMR (300 MHz, DMSO-d6) δ (ppm): 8.17 (m, 2H, ArH, –CH[double bond, length as m-dash]), 8.05 (s, 1H, ArH), 7.78 (d, 1H, J = 7.2 Hz, ArH), 7.58 (d, 2H, J = 8.4 Hz, ArH), 7.51 (s, br, 1H, ArH), 7.43 (s, br, 1H, ArH), 7.05 (d, 4H, J = 8.4 Hz, ArH), 6.93 (d, 4H, J = 8.4 Hz, ArH), 6.80 (d, 2H, J = 9.0 Hz, ArH), 6.72 (s, 1H, ArH), 3.94 (m, 4H, –OCH2–), 3.66 (s, 3H, –NCH3), 1.70 (m, 4H, –CH2–), 1.41–1.23 (m, 12H, –CH2–), 0.88 (m, 6H, –CH3). 13C NMR (100 MHz, THF-d8) δ (ppm): 165.21, 157.11, 155.01, 153.14, 149.99, 147.77, 141.26, 140.04, 139.94, 137.65, 131.69, 130.80, 130.72, 128.80, 127.76, 127.19, 126.76, 125.12, 123.60, 123.43, 120.88, 119.12, 117.81, 116.13, 115.86, 95.32, 68.84, 33.21, 32.68, 30.39, 26.84, 23.63, and 14.50. MS (ESI, m/z): calcd for C49H49N5O4S2, 835.3; found, 835.3; anal. calcd for C49H49N5O4S2: C, 70.39; H, 5.91; N, 8.38; S, 7.67; found: C, 69.93; H, 5.78; N, 8.71; S, 7.58.
LI-122 . 4b (160 mg, 0.19 mmol) and cyanoacetic acid (48 mg, 0.57 mmol). Black solid (127 mg, 73.8%). 1H NMR (300 MHz, DMSO-d6) δ (ppm): 8.20 (m, 3H, ArH, –CH[double bond, length as m-dash]), 7.81 (d, 1H, J = 8.1 Hz, ArH), 7.67 (d, 1H, J = 8.7 Hz, ArH), 7.58 (d, 2H, J = 9.0 Hz, ArH), 7.52 (d, 1H, J = 8.4 Hz, ArH), 7.05 (d, 4H, J = 8.1 Hz, ArH), 6.92 (d, 4H, J = 8.4 Hz, ArH), 6.80 (m, 3H, ArH), 4.18 (m, 2H, –NCH2–), 3.94 (m, 4H, –OCH2–), 1.69 (m, 6H, –CH2–), 1.41–1.23 (m, 18H, –CH2–), 0.89 (m, 9H, –CH3). 13C NMR (75 MHz, CDCl3 and DMSO-d6) δ (ppm): 165.57, 155.74, 153.82, 151.98, 148.76, 146.72, 139.94, 139.45, 138.64, 136.29, 130.80, 129.91, 128.21, 127.95, 126.89, 126.34, 125.25, 124.25, 122.67, 122.35, 119.71, 119.10, 117.75, 115.31, 94.36, 83.41, 68.10, 45.05, 31.58, 30.92, 29.65, 29.28, 26.03, 25.74, 25.05, 22.61, 14.16, and 13.93. MS (ESI, m/z): calcd for C54H59N5O4S2, 905.4; found, 905.4. Anal. calcd for C54H59N5O4S2: C, 71.57; H, 6.56; N, 7.73; S, 7.08; found: C, 71.43; H, 6.85; N, 7.65; S, 7.11.
LI-123 . 4c (140 mg, 0.16 mmol) and cyanoacetic acid (40 mg, 0.48 mmol). Black solid (97 mg, 65.1%). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.16 (m, 2H, ArH, –CH[double bond, length as m-dash]), 8.07 (s, 1H, ArH), 7.78 (d, 1H, J = 7.2 Hz, ArH), 7.58 (d, 2H, J = 8.4 Hz, ArH), 7.50 (s, br, 1H, ArH), 7.45 (s, br, 1H, ArH), 7.05 (d, 4H, J = 8.7 Hz, ArH), 6.92 (d, 4H, J = 8.4 Hz, ArH), 6.80 (d, 2H, J = 8.7 Hz, ArH), 6.64 (d, 1H, J = 3.9 Hz, ArH), 4.06 (s, br, 2H, –NCH2–), 3.94 (t, 3H, J = 6.0 Hz, –OCH2–), 1.73–1.68 (m, 5H, –CH2–, –CH–), 1.41–1.30 (m, 16H, –CH2–), 0.88 (m, 16H, –CH2-, –CH3). 13C NMR (100 MHz, THF-d8) δ (ppm): 165.21, 156.72, 154.66, 152.74, 149.61, 147.34, 140.87, 139.43, 138.86, 137.24, 131.43, 130.29, 129.18, 128.42, 127.37, 126.80, 126.39, 124.78, 123.66, 123.21, 120.52, 118.89, 117.72, 117.55, 115.75, 95.84, 68.45, 45.38, 39.85, 32.45, 32.36, 32.30, 30.02, 29.66, 29.51, 26.94, 26.46, 23.26, 23.16, and 14.13. MS (ESI, m/z): calcd for C56H63N5O4S2, 933.4; found, 933.4. Anal. calcd for C56H63N5O4S2: C, 71.99; H, 6.80; N, 7.50; S, 6.86; found: C, 72.16; H, 6.61; N, 7.26; S, 6.61.
LI-124 . 4d (150 mg, 0.17 mmol) and cyanoacetic acid (43 mg, 0.51 mmol). Black solid (102 mg, 62.6%). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.21–8.15 (m, 3H, ArH, –CH[double bond, length as m-dash]), 7.81 (d, 1H, J = 7.5 Hz, ArH), 7.67 (d, 1H, J = 4.3 Hz, ArH), 7.58 (d, 2H, J = 8.7 Hz, ArH), 7.51 (d, 1H, J = 3.9 Hz, ArH), 7.05 (d, 4H, J = 8.9 Hz, ArH), 6.92 (d, 4H, J = 9.0 Hz, ArH), 6.80 (d, 2H, J = 8.8 Hz, ArH), 6.76 (d, 1H, J = 4.3 Hz, ArH), 4.19 (s, br, 2H, –NCH2–), 3.94 (t, 4H, J = 6.4 Hz, –OCH2–), 1.75–1.66 (m, 4H, –CH2–), 1.51–0.95 (m, 28H, –CH2–), 0.90–0.74 (m, 9H, –CH3). 13C NMR (100 MHz, THF-d8) δ (ppm): 164.87, 156.73, 154.65, 152.74, 149.63, 147.42, 140.88, 139.72, 139.23, 137.22, 131.49, 130.31, 129.10, 128.53, 127.36, 126.79, 126.38, 124.74, 123.59, 123.19, 120.52, 119.07, 117.37, 115.73, 115.67, 95.08, 68.43, 45.37, 32.51, 32.45, 32.29, 30.10, 30.01, 29.97, 29.90, 29.52, 26.91, 26.45, 23.24, 23.21, 14.13, and 14.09. MS (ESI, m/z): calcd for C58H67N5O4S2, 961.5; found, 961.5. Anal. calcd for C58H67N5O4S2, C, 72.39; H, 7.02; N, 7.28; S, 6.66; found: C, 72.05; H, 7.15; N, 7.55; S, 6.45.
LI-125 . 4e (120 mg, 0.13 mmol) and cyanoacetic acid (33 mg, 0.39 mmol). Black solid (80 mg, 62.0%). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.23–8.14 (m, 3H, ArH, –CH[double bond, length as m-dash]), 7.79 (d, J = 7.5 Hz, 1H, ArH), 7.68 (d, J = 4.4 Hz, 1H, ArH), 7.55 (d, J = 8.6 Hz, 2H, ArH), 7.48 (d, J = 3.8 Hz, 1H, ArH), 7.03 (d, J = 8.8 Hz, 4H, ArH), 6.91 (d, J = 8.9 Hz, 4H, ArH), 6.79–6.75 (m, 3H, ArH), 4.19 (s, br, 2H, –NCH2–), 3.93 (t, J = 6.3 Hz, 4H, –OCH2–), 1.68 (m, 4H, –CH2–), 1.44–1.08 (m, 32H, –CH2–), 0.89–0.74 (m, 9H, –CH3). 13C NMR (100 MHz, CDCl3 and DMSO-d6) δ (ppm): 167.67, 155.71, 153.88, 151.97, 148.69, 146.48, 139.99, 136.77, 136.71, 136.38, 130.48, 129.59, 128.61, 127.47, 126.81, 126.31, 125.40, 124.23, 123.07, 122.60, 119.79, 116.98, 115.34, 114.53, 68.14, 44.60, 31.81, 31.68, 31.50, 29.53, 29.43, 29.22, 28.77, 26.14, 25.66, 22.57, 22.52, 18.49, 14.13, and 14.07. MS (ESI, m/z): calcd for C60H71N5O4S2, 990.5; found, 990.6. Anal. calcd for C60H71N5O4S2, C, 72.77; H, 7.23; N, 7.07; S, 6.47; found: C, 72.89; H, 7.33; N, 6.90; S, 6.67.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Science Foundation of China (51673151), Hubei Province (2017CFA002), and the Fundamental Research Funds for the Central Universities (2042017kf0247 and 2042018kf0014) for financial support.

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Footnote

Electronic supplementary information (ESI) available: The cyclic voltammogram, results of theoretical calculations, EIS spectra and some photovoltaic parameters. See DOI: 10.1039/c8ta06258g

This journal is © The Royal Society of Chemistry 2018