Indeno[1,2-b]indole-based organic dyes with different acceptor groups for dye-sensitized solar cells

Accepted Manuscript Indeno[1,2-b]indole-based organic dyes with different acceptor groups for dyesensitized solar cells Xing Qian, Rucai Yan, Yongjie Hang, Yuezhen Lv, Lixin Zheng, Chong Xu, Linxi Hou PII:

S0143-7208(16)31055-5

DOI:

10.1016/j.dyepig.2016.12.028

Reference:

DYPI 5653

To appear in:

Dyes and Pigments

Received Date: 26 October 2016 Revised Date:

26 November 2016

Accepted Date: 4 December 2016

Please cite this article as: Qian X, Yan R, Hang Y, Lv Y, Zheng L, Xu C, Hou L, Indeno[1,2-b]indolebased organic dyes with different acceptor groups for dye-sensitized solar cells, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2016.12.028. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract

ACCEPTED MANUSCRIPT

Indeno[1,2-b]indole-based

organic

dyes

with

different

acceptor groups for dye-sensitized solar cells Xing Qian, Rucai Yan, Yongjie Hang, Yuezhen Lv, Lixin Zheng, Chong Xu, Linxi

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Hou* School of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China

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*Corresponding author. E-mail address: [email protected].

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Abstract: Four new metal-free organic dyes QX15–18 based on indeno[1,2-b]indole have been successfully designed and synthesized. These D–π–A type dyes consist of an indeno[1,2-b]indole donor and a benzene or thiophene π-bridge. In an attempt to tune their photoelectric properties, the effects of different acceptor groups including

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cyanoacrylic acid, rhodanine-3-acetic acid, and 2-(1,1-dicyanomethylene)rhodanine (DCRD) have been investigated. The dye QX15 with a cyanoacrylic acid acceptor exhibited a highest Voc of 813 mV and a high Jsc of 11.0 mA cm−2, generating a

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highest power conversion efficiency of 6.29%. Meanwhile, the dye QX18 with a

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DCRD acceptor and a thiophene π-bridge showed a significantly improved Jsc of 11.9 mA cm−2, which could be attributed to its red-shifted absorption spectra and high molar absorption coefficient. Keywords: Indeno[1,2-b]indole; Organic dyes; Acceptor groups; Charge transfer; Photoelectric property; Dye-sensitized solar cells

1. Introduction 1

ACCEPTED MANUSCRIPT Since the pioneering work was first reported by Grätzel et al. in 1991 [1], dye-sensitized solar cells (DSSCs) have attracted great attention for their low-cost, facile manufacture, and environment-friendly characteristics [2]. Up to now, through

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intensive studies, the performances of DSSCs have been steadily improved. Zinc porphyrin sensitizer SM315 with cobalt-based redox couple [Co(bpy)3]2+/3+ was applied in dye-sensitized solar cells and got a landmark power conversion efficiency

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(PCE) of 13% [3]. Meanwhile, the metal-free organic dyes have also been rapidly

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developed for DSSCs due to their low costs, facile synthetic procedures, high extinction coefficients, and relative low toxicity [4]. Currently, a variety of organic dyes containing the conventional electron donors such as triarylamine [5], phenothiazine [6], and carbazole [7], indoline [8], have been synthesized and reported

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in the literatures, which showed promising DSSC performances. Most metal-free organic dyes possess a donor–π-bridge–acceptor (D–π–A) configuration, namely a typical push–pull structure, which can effectively induce

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charge separation and reduce charge recombination [9]. Moreover, it’s easy to tune the

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absorption spectra as well as the HOMO–LUMO energy levels of the dye molecule through changing the donor, π-bridge, and acceptor groups [10]. Recently, indole derivatives as a kind of important heteroaromatic compounds have been extensively investigated as electron donors in organic photoelectric materials due to their electron-rich structure, strong charge transfer, and flexible synthetic characteristics. Various efficient organic dyes employing indole-based fused heterocycle compounds including indolocarbazole [11], indoloquinoxaline [12], triindole [13], and 2

ACCEPTED MANUSCRIPT benzocarbazole [14] as the donor moieties have been researched and reported by our group. Herein, we have also designed and synthesized a new series of D–π–A type organic dyes using indeno[1,2-b]indole as the donor moiety. Indeno[1,2-b]indole can

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be considered as an indole unit fused with an indene unit, and the expanded conjugated system is beneficial for electron delocalization [15]. The alkyl groups introduced on the indene moiety present a certain angle with the indenoindole plane

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which are in favor of suppressing the intermolecular aggregation of dye molecules

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and improving photovoltaic performances.

As another important part of the D–π–A structure, the acceptor groups anchored on TiO2 has an exceeding influence on PCE and DSSC stability. Difference acceptors will affect the adsorption mode and the adsorption status of dye molecules can greatly

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influence the intramolecular charge transfer (ICT) and the electron injection processes [16]. The common acceptor structure is cyanoacrylic acid and rhodanine-3-acetic acid which bear a carboxyl group [17]. 2-(1,1-Dicyanomethylene)rhodanine (DCRD) as a

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new kind of anchoring unit have some advantages compared with the carboxyl group.

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DCRD acceptor group can induce a broad absorption spectrum, and the O and N atoms of rhodanine in DCRD can both chelate to titanium ions on the TiO2 surface and the electron can easily inject from the excited-state energy level to the conduction band of TiO2 [18]. In this study, we use indeno[1,2-b]indole as the electron donor, a benzene ring as the π-bridge, cyanoacetic acid, rhodanine-3-acetic acid, and DCRD as the acceptors, respectively, resulting in three organic dyes QX15, QX16, and QX17. At the same time, to analyze the effects of different π-bridge units for DCRD acceptor, 3

ACCEPTED MANUSCRIPT we have therefore designed and synthesized the dye QX18 by employing a thienylene unit to replace the phenylene unit as the π-bridge group. The molecular structures of the four dyes QX15–18 are shown in Fig. 1.

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2. Experimental 2.1. Materials and methods

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All 1H and 13C NMR spectra were measured with a Bruker 400 MHz spectrometer. HR-MS spectra of all compounds were recorded on a mass spectrometer (Exactive

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plus, Thermo Fisher Corp.). IR spectra were performed on a Spectrum 2000 FT-IR spectrometer (Perkin-Elmer Corp.). UV-Vis spectra of the dyes were recorded on a Genesys 10S UV-visible spectrophotometer (Thermo Fisher Corp.). Cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy (EIS)

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were determined in an electrochemical workstation (CHI660E, CH Instrument). CV experiments were tested by a three-electrode system with a glassy carbon electrode as

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the working electrode, an Ag/AgNO3 electrode as the reference electrode, and a Pt wire as the counter electrode. The redox potentials were measured with 0.1 M

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n-Bu4NPF6 as the supporting electrolyte at a scan rate of 100 mV S–1 in dichloromethane (DCM) solutions. The fluorine-doped SnO2 conducting glasses (FTO) (the sheet resistance, 15 Ω sq–1) were purchased from NSG Corp.. All reagents and solvents used in the experiments were purchased from Sigma-Aldrich, Aladdin, and J&K. 2.2. Fabrication and Characterization of DSSCs

4

ACCEPTED MANUSCRIPT A nanocrystalline TiO2 film (thickness, about 12 µm) was prepared by the screen-printing technique with a commercial TiO2 sol (particle size, about 20 nm, Heptachroma Corp.) on FTO glass. After that, a scattering TiO2 layer (particle size,

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about 200 nm, thickness, about 4 µm) was printed over the transparent layer by the same screen-printing technique. The active area of nanocrystalline film is 0.16 cm2. The prepared electrode was heated to 500 °C in Muffle furnace. After being calcined

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for 1 hour, the TiO2 photoanodes were put into 40 mM TiCl4 aqueous solution at

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70 °C for 1 hour, and then it was sintered again at 500 °C for 1 hour. The Pt counter electrodes obtained by a thermal deposition of 20 mM hexachloroplatinic acid (isopropanol solution) on the surface of FTO glasses at 450 °C for 0.5 hour. The N719-sensitized photoanodes were prepared through immersing the TiO2 electrodes

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into the commercial N719 dye (Solaronix Corp.) solution (0.3 mM in ethanol) for 12 hours. The TiO2 electrodes were soaking in the four prepared dyes’ DCM solutions (0.3 mM) for 12 hours. The iodine-based electrolyte was composed of 0.3 M

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1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M

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4-tert-butylpyridine in CH3CN. The DSSC device was fabricated as a sandwich structure, then it was illuminated under a standard solar simulator (XM-500W, Trusttech Corp.) at 100 mW/cm2 irradiation, which had been calibrated with a standard silicon solar cell (91150V, Newport Corp.). The incident photon-to-current conversion efficiency (IPCE) spectra were obtained by an IPCE system (QTest2000, CrownTech Corp.). The current-voltage (J-V) characteristic curves of the DSSCs

5

ACCEPTED MANUSCRIPT under the simulated irradiation were recorded by a CHI660E electrochemical workstation. 2.3. Synthesis

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2.3.1. Synthesis of compound 2 (5-hexyl-5,10-dihydroindeno[1,2-b]indole) A mixture of compound 1 (5.1 g, 24.9 mmol), potassium tert-butoxide (2.93 g, 26.1 mmol), and THF (15 mL), was stirred at room temperture for 1 hour under N2

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atmosphere. After that, 1-bromohexane (4.48 g, 27.4 mmol) was slowly added to the

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reaction mixture via a syringe, the mixture was refluxed for 5 hours (80 °C) before it was poured into water. The aqueous layer was extracted with DCM and the combined organic layers were washed thoroughly with water. After the solvent was removed under vacuum and dried over anhydrous Na2SO4, the crude product was purified by

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silica-gel column chromatography using petroleum ether (PE)/DCM (4:1) as the eluent to afford compound 2 as a yellow solid (yield: 81%). Mp 50–52 °C. IR (KBr) ν: 2953, 2925, 2846, 1607, 1522, 1497, 1460, 1437, 1402, 1370, 1345, 1172, 1010, 737,

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714 cm–1. 1H NMR (400 MHz, CDCl3) δ 7.64 (d, J = 7.7 Hz, 1H), 7.61–7.53 (m, 2H),

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7.39 (d, J = 8.2 Hz, 1H), 7.36 (t, J = 7.6 Hz, 1H), 7.21 (dd, J = 14.1, 7.2 Hz, 2H), 7.14 (t, J = 7.4 Hz, 1H), 4.42 (t, J = 7.3 Hz, 2H), 3.73 (s, 2H), 1.97–1.88 (m, 2H), 1.43 (t, J = 7.3 Hz, 2H), 1.35–1.27 (m, 4H), 0.87 (t, J = 6.9 Hz, 3H).

13

C NMR (100 MHz,

CDCl3) δ 148.33, 144.25, 141.32, 135.50, 126.60, 125.66, 124.58, 124.13, 121.13, 120.42, 119.48, 119.13, 117.78, 110.04, 44.88, 31.64, 30.89, 30.19, 26.81, 22.59, 14.07. HR-MS (ESI): m/z [M+H]+ calcd for C21H23N, 290.1909; found, 290.1902. 2.3.2.

Synthesis

of 6

compound

3

ACCEPTED MANUSCRIPT (5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indole) A mixture of compound 2 (2.50 g, 8.65 mmol), potassium tert-butoxide(2.91 g, 26.0 mmol), and dimethyl sulfoxide (DMSO) (5 mL) was stirred at 85 °C for 1 hour

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under N2 atmosphere. Then, 1-bromoethane (4.25 g, 26.0 mmol) was slowly added to the reaction mixture via a syringe. The mixture was refluxed at 85 °C for 3 hours, and then it was poured into water. The aqueous layer was extracted with DCM, the

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combined organic layers was washed thoroughly with water and dried over anhydrous

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Na2SO4. After the solvent was removed under reduced pressure, the product was purified by silica-gel column chromatography using PE/DCM (4:1) as the eluent and a white solid product 3 was gained (yield: 70%). Mp 35–36 °C. IR (KBr) ν: 2953, 2923, 2853, 1602, 1524, 1495, 1455, 1430, 1352, 1122, 1010, 729 cm–1. 1H NMR (400 MHz,

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CDCl3) δ 7.62 (d, J = 7.7 Hz, 1H), 7.51 (d, J = 7.4 Hz, 1H), 7.40 (d, J = 8.3 Hz, 1H), 7.38 (d, J = 7.6 Hz, 1H), 7.31 (t, J = 7.4 Hz, 1H), 7.25–7.21 (m, 1H), 7.20 (d, J = 7.8 Hz, 1H), 7.14 (t, J = 7.3 Hz, 1H), 4.43 (t, J = 7.1 Hz, 2H), 2.21–2.10 (m, 2H),

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2.00–1.88 (m, 4H), 1.45–1.22 (m, 8H), 1.14–1.02 (m, 12H), 0.85 (t, J = 6.9 Hz, 5H), 13

C NMR (100 MHz, CDCl3) δ 156.71, 142.71, 141.41,

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0.77 (t, J = 6.8 Hz, 6H).

135.25, 127.24, 126.36, 124.76, 123.96, 122.82, 120.70, 119.22, 119.17, 117.70, 110.03, 51.61, 44.81, 39.09, 31.60, 31.56, 30.59, 29.79, 26.64, 24.43, 22.63, 22.52, 14.00, 13.95. HR-MS (ESI): m/z [M+H]+ calcd for C33H47N, 458.3787; found, 458.3742. 2.3.3.

Synthesis

of

compound

(4-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl)benzaldehyde) 7

4

ACCEPTED MANUSCRIPT The compound 3 (2.43 g, 5.32 mmol) was dissolved in 150 mL of CHCl3 and the mixture was stirred for 10 min. Then, N-bromosuccinimide (0.860 g, 4.83 mmol) dissolved in 15 mL DMF was added dropwise to the mixture. The mixture was stirred

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at 0 °C for 10 min before it was gradually warmed to room temperature and then was stirred for another 5 hours. The resulting mixture was extracted with DCM and the combined organic layers were washed thoroughly with water. The solvent was

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evaporated and then the white thick brominated intermediate product was obtained.

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Under N2 atmosphere, the above product with (4-formylphenyl)boronic acid (1.44 g, 9.57 mmol), Pd(PPh3)4 (217 mg, 0.266 mmol), K2CO3 (3.67 g, 26.6 mmol), toluene (25 mL), and ethanol (5 mL) was heated at 80 °C for 4 hours before the mixture was poured into water. After the mixture was washed thoroughly with water, the organic

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phase was separated and dried over anhydrous Na2SO4. The crude product was purified by silica-gel column chromatography using PE/DCM (2:1) as the eluent to afford the compound 4 as a yellow thick liquid (yield: 94%). IR (KBr) ν: 2955, 2925,

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2854, 1710, 1600, 1497, 1459, 1438, 1377, 1213, 1170, 1018, 833, 806, 747 cm–1. 1H

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NMR (400 MHz, CDCl3) δ 10.07 (s, 1H), 7.98 (d, J = 8.3 Hz, 2H), 7.87 (d, J = 8.2 Hz, 2H), 7.69–7.62 (m, 2H), 7.53 (d, J = 7.4 Hz, 1H), 7.45 (dd, J = 8.3, 1.4 Hz, 1H), 7.39 (d, J = 7.4 Hz, 1H), 7.35–7.30 (m, 1H), 7.26–7.22 (m, 1H), 4.49 (t, J = 7.2 Hz, 2H), 2.22–2.13 (m, 2H), 2.01–1.92 (m, 4H), 1.45–1.38 (m, 2H), 1.35–1.22 (m, 5H), 1.15–1.02 (m, 13H), 0.84 (t, J = 7.1 Hz, 4H), 0.76 (t, J = 6.9 Hz, 7H). 13C NMR (100 MHz, CDCl3) δ 191.97, 156.86, 148.74, 144.43, 141.90, 134.80, 134.48, 132.30, 130.32, 127.61, 127.17, 126.53, 125.31, 124.12, 122.94, 119.57, 119.14, 118.03, 8

ACCEPTED MANUSCRIPT 108.92, 51.64, 44.89, 39.09, 31.61, 31.55, 30.63, 29.77, 26.65, 24.47, 22.62, 22.52, 14.01, 13.94. HR-MS (ESI): m/z [M+H]+ calcd for C40H52NO, 562.4049; found, 562.4056. Synthesis

of

compound

5

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2.3.4.

(5-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl)thiophene-2-carbaldehy de)

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Compound 3 (2.21 g, 4.83 mmol) was used as the precursor to synthesize the

Under

N2

atmosphere,

a

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brominated intermediate product and the synthetic method is similar to the above step. mixture

of

the

brominated

intermediate,

5-formylthiophene-2-boronic acid (1.13 g, 7.25 mmol), Pd(dppf)Cl2 (197 mg, 0.242 mmol), K2CO3 (3.33 g, 24.15 mmol), toluene (25 mL), and ethanol (5 mL) was heated

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at 80 °C for 4.5 hours before it was poured into water. The mixture was washed thoroughly with water, the organic phase was separated and dried over anhydrous Na2SO4. After the solvent was removed under vacuum, the product was obtained by

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silica-gel column chromatography using PE/DCM (2:1) as the eluent and the

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compound 5 as a light yellow thick liquid was gained (yield: 76%). IR (KBr) ν: 2955, 2920, 2848, 1654, 1626, 1457, 1432, 1382 cm–1. 1H NMR (400 MHz, CDCl3) δ 9.88 (s, 1H), 7.75 (d, J = 3.9 Hz, 1H), 7.68 (s, 1H), 7.60 (d, J = 8.3 Hz, 1H), 7.53 (d, J = 7.4 Hz, 1H), 7.47 (d, J = 8.3 Hz, 1H), 7.44 (d, J = 3.9 Hz, 1H), 7.38 (d, J = 7.4 Hz, 1H), 7.32 (t, J = 7.4 Hz, 1H), 7.24 (d, J = 7.5 Hz, 1H), 4.46 (t, J = 7.2 Hz, 2H), 2.19–2.11 (m, 2H), 1.99–1.91 (m, 4H), 1.38–1.22 (m, 8H), 1.12–1.02 (m, 12H), 0.84 (t, J = 7.0 Hz, 4H), 0.75 (t, J = 6.9 Hz, 7H). 9

13

C NMR (100 MHz, CDCl3) δ 182.58,

ACCEPTED MANUSCRIPT 156.95, 156.89, 145.14, 141.54, 141.11, 137.81, 134.54, 127.46, 126.61, 125.65, 125.59, 124.87, 122.96, 122.92, 119.55, 118.36, 118.20, 108.19, 51.64, 44.94, 39.08, 31.58, 31.50, 30.59, 29.72, 26.61, 24.44, 22.59, 22.50, 13.99, 13.93. HR-MS (ESI):

2.3.5.

Synthesis

of

the

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m/z [M+H]+ calcd for C38H50NOS, 568.3613; found, 568.3604. dye

QX15

((E)-2-cyano-3-(4-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl)phenyl)a

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crylic acid)

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A mixture of compound 4 (350 mg, 0.624 mmol), cyanoacetic acid (159 mg, 1.87 mmol), NH4OAc (480 mg, 6.24 mmol), and HOAc (10 mL) was heated at 130 °C for 5 hours under N2 atmosphere. After being cooled to room temperature, it was poured into water and extracted with DCM. The combined organic layer was washed

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thoroughly with water. Then the crude product was purified by silica-gel column chromatography using DCM/MeOH (15:1) as the eluent and the target product QX15 as a crimson solid was gained (yield: 95%). Mp 135–137 °C. IR (KBr) ν: 2954, 2925,

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2854, 1628, 1594, 1458, 1437, 1397, 1190, 837, 808, 746, 704 cm–1. 1H NMR (400

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MHz, DMSO-d6) δ 8.16 (d, J = 10.9 Hz, 1H), 8.06 (d, J = 8.0 Hz, 2H), 7.95 (d, J = 9.5 Hz, 3H), 7.63 (dd, J = 13.0, 7.8 Hz, 2H), 7.45 (dd, J = 13.7, 7.8 Hz, 2H), 7.31 (t, J = 7.2 Hz, 1H), 7.23 (t, J = 7.2 Hz, 1H), 4.58 (s, 2H), 2.13 (d, J = 11.0 Hz, 2H), 2.00–1.91 (m, 2H), 1.81 (s, 2H), 1.31–1.07 (m, 9H), 1.05–0.91 (m, 12H), 0.68 (dd, J = 14.8, 7.4 Hz, 10H). 13C NMR (100 MHz, DMSO-d6/CDCl3 = 1/1) δ 161.30, 150.52, 149.10, 146.70, 139.45, 136.86, 135.82, 135.33, 132.33, 132.10, 131.61, 131.43, 130.16, 130.14, 128.62, 128.15, 127.60, 124.19, 123.68, 122.88, 113.55, 99.99, 56.22, 10

ACCEPTED MANUSCRIPT 49.43, 43.71, 36.25, 36.18, 35.28, 34.35, 31.18, 29.11, 27.23, 27.17, 18.81, 18.73. HR-MS (ESI): m/z [M–H] – calcd for C43H51N2O2, 627.3951; found, 627.3969. 2.3.6.

Synthesis

of

the

dye

QX16

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((Z)-2-(4-oxo-2-thioxo-5-(4-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl) benzylidene)thiazolidin-3-yl)acetic acid)

The synthetic route of the dye QX16 was similar to that of the dye QX15, the

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product was purified by silica-gel column chromatography using DCM/MeOH (15:1)

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as the eluent and the dye QX16 as a crimson solid was gained (yield: 72%). Mp 95–96 °C. IR (KBr) ν: 2954, 2924, 2853, 1706, 1636, 1589, 1458, 1400, 1383, 1326, 1188, 1102, 1048, 1021, 801, 745 cm–1. 1H NMR (400 MHz, DMSO-d6) δ 7.98 (d, J = 6.6 Hz, 3H), 7.87 (s, 1H), 7.75 (d, J = 8.1 Hz, 2H), 7.64 (dd, J = 15.5, 7.9 Hz, 2H),

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7.49–7.43 (m, 2H), 7.32 (t, J = 7.2 Hz, 1H), 7.24 (t, J = 7.5 Hz, 1H), 4.59 (s, 2H), 4.52 (s, 2H), 2.16 (d, J = 7.6 Hz, 2H), 1.97 (d, J = 9.2 Hz, 2H), 1.82 (s, 2H), 1.23 (s, 13H), 1.00 (dd, J = 12.3, 5.9 Hz, 9H), 0.68 (t, J = 7.2 Hz, 9H). 13C NMR (100 MHz,

EP

DMSO-d6) δ 193.51, 167.78, 166.97, 156.45, 144.67, 144.54, 142.32, 134.88, 134.08,

AC C

131.98, 131.71, 131.34, 127.94, 127.20, 126.55, 125.86, 123.83, 123.12, 121.27, 119.50, 119.50, 119.12, 118.80, 109.78, 51.44, 45.98, 38.81, 31.50, 31.40, 30.59, 29.45, 26.14, 24.42, 22.43, 22.37, 14.22, 14.12. HR-MS (ESI): m/z [M–H]– calcd for C45H53N2O3S2, 733.3498; found, 733.3525. 2.3.7.

Synthesis

of

the

dye

QX17

((Z)-2-(4-oxo-5-(4-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl)benzylid ene)thiazolidin-2-ylidene)malononitrile) 11

ACCEPTED MANUSCRIPT A

mixture

of

compound

4

(480

mg,

0.856

mmol),

2-(4-oxothiazolidin-2-ylidene)malononitrile (169 mg, 1.03 mmol), ethanol (50 mL), and 5 drops of NaOH aqueous solution (w/w 20%) was heated at 80°C for 12 hours.

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After being cooled to room temperature, it was poured into water and then extracted with DCM. After being removed the solvent, the crude product was purified by silica-gel column chromatography using DCM/MeOH (15:1) as the eluent and the

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target product QX17 as a brown solid was gained (yield: 79%). Mp > 300 °C. IR

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(KBr) ν: 2954, 2925, 2853, 2205, 1643, 1586, 1465, 1399, 1375, 1280, 1185, 1084, 1015, 801, 739 cm–1. 1H NMR (400 MHz, DMSO-d6) δ 7.93 (d, J = 8.2 Hz, 3H), 7.69 (d, J = 8.3 Hz, 2H), 7.66–7.56 (m, 3H), 7.43 (d, J = 8.5 Hz, 2H), 7.31 (t, J = 7.4 Hz, 1H), 7.22 (t, J = 7.3 Hz, 1H), 4.58 (t, J = 6.1 Hz, 2H), 2.19–2.10 (m, 2H), 1.95 (dd, J

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= 11.3, 8.5 Hz, 2H), 1.81 (d, J = 6.5 Hz, 2H), 1.19 (dd, J = 38.5, 8.3 Hz, 9H), 1.00 (dd, J = 12.2, 6.0 Hz, 12H), 0.69 (dd, J = 15.8, 7.3 Hz, 10H).

13

C NMR (100 MHz,

DMSO-d6/CDCl3 = 1/1) δ 161.23, 148.86, 148.84, 147.63, 146.82, 146.24, 139.58,

EP

137.26, 135.12, 132.27, 131.52, 130.11, 128.32, 127.61, 124.14, 124.13, 123.67,

AC C

122.93, 122.92, 121.33, 116.23, 115.29, 113.53, 113.51, 56.20, 49.41, 43.71, 39.74, 36.28, 36.22, 35.33, 34.33, 31.17, 29.12, 27.22, 18.89, 18.81. HR-MS (ESI): m/z [M–H]– calcd for C46H51N4OS, 707.3784; found, 707.3800. 2.3.8.

Synthesis

of

the

dye

QX18

((Z)-2-(4-oxo-5-((5-(5,10,10-trihexyl-5,10-dihydroindeno[1,2-b]indol-8-yl)thiophe n-2-yl)methylene)thiazolidin-2-ylidene)malononitrile) The synthetic route of the dye QX18 was similar to that of the dye QX17. The 12

ACCEPTED MANUSCRIPT crude product was purified by silica-gel column chromatography using DCM/MeOH (15:1) as the eluent and the dye QX18 as a brown solid was gained (yield: 84%). IR (KBr) ν: 2954, 2923, 2853, 2226, 2208, 1720, 1637, 1578, 1561, 1459, 1421, 1178,

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1069, 1018, 801, 742, 712 cm–1. 1H NMR (400 MHz, DMSO-d6) δ 7.94 (s, 1H), 7.80 (s, 1H), 7.69 (d, J = 3.9 Hz, 1H), 7.65 (d, J = 7.4 Hz, 1H), 7.59 (dd, J = 12.8, 6.1 Hz, 2H), 7.43 (dd, J = 6.9, 4.5 Hz, 2H), 7.31 (t, J = 7.3 Hz, 1H), 7.23 (t, J = 7.4 Hz, 1H),

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4.58 (t, J = 6.3 Hz, 2H), 2.17–2.09 (m, 2H), 1.98–1.91 (m, 2H), 1.81 (s, 2H), 13

C NMR

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1.29–1.14 (m, 10H), 0.99 (dd, J = 12.1, 5.9 Hz, 12H), 0.72–0.66 (m, 9H).

(100 MHz, DMSO-d6/CDCl3 = 1/1) δ 161.26, 157.14, 157.10, 149.64, 146.53, 141.34, 141.31, 139.74, 139.32, 131.86, 131.61, 130.74, 130.42, 128.98, 128.82, 128.82, 127.63, 124.21, 123.75, 123.17, 123.14, 112.63, 112.59, 99.99, 56.20, 49.36, 43.67,

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36.25, 36.15, 35.28, 34.28, 31.04, 29.10, 27.35, 27.19, 18.89, 18.82. HR-MS (ESI): m/z [M–H]– calcd for C44H49N4OS2, 713.3348; found, 713.3367.

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3. Results and discussion

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3.1. Dye synthesis

The syntheses of the dye sensitizers QX15–18 were conducted by five major steps.

The concise synthetic route was displayed in Scheme 1. The compound 1 and 2-(4-oxothiazolidin-2-ylidene)malononitrile were synthesized according to the literature works [19,20]. The alkyl group-substituted compound 3 was obtained through the consecutive two-step alkylation reactions of the compound 1. After the bromination reaction of the compound 3, the subsequent Suzuki cross-coupling 13

ACCEPTED MANUSCRIPT reaction of the bromine-substituted intermediate with (4-formylphenyl)boronic acid or 5-formylthiophene-2-boronic acid produced the two π-extended aldehydes 4 and 5. Knoevenagel reaction of the compound 4 with cyanoacetic acid, rhodanine-3-acetic

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acid, and 2-(4-oxothiazolidin-2-ylidene)malononitrile produced QX15, QX16, and QX17, respectively. And Knoevenagel reaction of the compound 5 with 2-(4-oxothiazolidin-2-ylidene)malononitrile produced the dye QX18.

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3.2. UV-visible absorption and electrochemical characterization

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The UV-vis absorption spectra of the four dyes QX15–18 in DCM solutions and on TiO2 films are shown in Fig. 2, and the data of absorption and electrochemical properties are summarized in Table 1. As can be seen in Fig. 2a, all of these dyes exhibited three distinct absorption bands in DCM solutions. The high-energy

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absorption band (< 400 nm) can be attributed to the localized aromatic π–π* and n–π* transitions of molecular conjugated backbone [21]. The lower energy absorption band (> 400 nm) can be ascribed to the intramolecular charge transfer (ICT) progress from

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the indenoindole-based donor moiety to the acceptor group [22]. The absorption peaks

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of QX16 and QX17 showed a significant red shift compared with that of QX15 due to the introductions of rhodanine-3-acetic acid and DCRD as the acceptor units, which gave more powerful electron-withdrawing capability and promoted the ICT progress. The molar absorption coefficient of QX17 (ε = 17500 M–1 cm–1) are clearly higher than QX15 (ε = 11500 M–1 cm–1) and QX16 (ε = 10300 M–1 cm–1). And compared with the dyes QX15–17 which used benzene ring as the π-bridges, QX18 with thiophene as the π-bridge (498 nm, ε = 24900 M–1 cm–1) showed an obvious red shift 14

ACCEPTED MANUSCRIPT and higher molar absorption coefficient in ICT absorption region. This could be attributed to the more electron enrichment, better planarity, and more effective π-conjugation of the thiophene π-bridge, which strengthened the intramolecular

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interaction and electronic coupling between the donor group and the acceptor group. As shown in Fig. 2b, all the four dyes exhibited significantly more broadened and red-shifted absorption spectra when anchoring on mesoporous TiO2 films compared

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with that in DCM solutions, which may be ascribed to the enhanced intermolecular

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aggregation of the dye molecules on TiO2 surface and/or the strong interaction between dye molecules and TiO2 semiconductor.

To investigate the oxidation-reduction potential information of the dyes, the cyclic voltammetry (CV) measurements were performed in DCM solutions using

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tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Cyclic voltammograms of QX15–18 were shown in Fig. 3, which were calibrated against ferrocene (0.63 V vs. NHE) [23]. The highest occupied molecular orbitals (HOMOs)

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of the dyes QX15–18 corresponding to their first oxidation potentials (Eox = 0.99,

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0.98, 1.02, and 1.03 V, respectively) (vs. NHE). The band gap energies (E0−0 = 2.21, 2.16, 2.15, and 1.93 eV, respectively) were calculated from the onset wavelengths of the absorption spectra in DCM solutions. The values of Ered corresponding to the lowest unoccupied molecular orbitals (LUMOs), which could be calculated from the equation Ered = Eox – E0−0 to obtain corresponding values of –1.22, –1.18, –1.13, and –0.90 V (vs. NHE), respectively. Therefore, the LUMO levels of the dyes are more negative than the conduction band (CB) of TiO2 (−0.5 V vs. NHE), which 15

ACCEPTED MANUSCRIPT demonstrates that electron can be rapidly injected from dye molecules into the CB of TiO2. At the same time, the HOMO levels of the dyes are more positive than the I−/I3− redox couple (+0.4 V vs. NHE), guaranteeing the thermodynamic regeneration

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process of oxidized form of the dyes can quickly happen [24]. 3.3. Theoretical calculations

Density functional theory (DFT) calculations using the Gauss09w software

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(B3LYP/6-31G*) were conducted in order to further research the optimized

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configurations and frontier molecular orbitals of QX15–18. Their optimized configurations and frontier molecular orbital electron distributions are shown in Fig. 4. The optimized configurations of these dyes demonstrate the indeno[1,2-b]indole moiety consists of an indene unit fused with an indole unit and exhibits a rigid planar

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structure which may facilitate the electron delocalization over the molecular skeleton. The two alkyl groups which are attached on the indene unit and present a certain angle with the plane of the indeno[1,2-b]indole moiety are beneficial to suppress the

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intermolecular aggregation of dye molecules and improve photovoltaic performances.

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As shown in Fig. 4, the electron density of the HOMO state of these dyes is mainly distributed on the whole indeno[1,2-b]indole moiety. For the HOMO-1 state, the electron density mainly located on the electron-rich indole unit and nearby thiophene or furan π-bridge. While the LUMO states of the four dyes show localized electron distributions through the cyanoacrylic acid acceptor and its nearby π-bridge. Obviously, the HOMO, HOMO-1, and LUMO are very important for organic dyes because the corresponding electron transition is beneficial to the light-to-electricity 16

ACCEPTED MANUSCRIPT conversion. As a result, the excited electrons can be transferred from HOMO or HOMO-1 to LUMO through photoexcitation and then be efficiently injected into the TiO2 semiconductor.

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The further theory calculations were performed with time-dependent density functional theory (TD-DFT) at B3LYP/6-31G* level. From the results of the TD-DFT calculations in Table 2, the observed ICT absorption band for QX15 can be mainly

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assigned to the electron transfer transition from HOMO−1 to LUMO (99.3%, f =

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0.584). The same situations were also found for QX16 (98.7%, f = 0.919), QX17 (99.2%, f = 0.832), and QX18 (99.0%, f = 0.930). Therefore, these electron transfer transitions in the four dyes are all in favor of electron injections because the HOMO−1 to LUMO transition corresponds to electron transfer from the

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indeno[1,2-b]indole donor group to the acceptor/anchoring group [23]. 3.4. DSSC performance

The DSSC measurements of the dyes QX15–18 have been performed under a

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simulated solar irradiation (AM 1.5G, 100 mW cm–2). The DSSC data of these four

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dyes with the reference N719 dye are summarized in Table 3. The current density-photovoltage (J-V) curves of QX15–18 are shown in Fig. 5. The overall PCEs of the four dyes lay in the range of 2.22–6.29%, with an order of QX16 < QX17 < QX18 < QX15. The dye QX15 with a cyanoacrylic acid acceptor exhibited a highest open-circuit photovoltage (Voc) of 813 mV, a short-circuit photocurrent density (Jsc) of 11.0 mA cm−2, and a fill factor (FF) of 70.1%, generating a highest PCE of 6.29%. When the acceptor group was changed to rhodanine-3-acetic acid, the dye QX16 17

ACCEPTED MANUSCRIPT presented a lowest Jsc of 4.23 mA cm−2, resulting in a lowest PCE of 2.22%. Compared with QX16, the dye QX17 with a DCRD acceptor exhibited a relatively better DSSC performance (a Voc of 695 mV, a Jsc of 7.10 mA cm−2, and a PCE of

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3.60%). Obviously, the π-bridge have a great effect on the DSSC performances of these dyes. Compared with the dyes QX15–17, QX18 with thiophene as the π-bridge showed a significantly higher Jsc of 11.9 mA cm−2, which could be attributed to its

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red-shifted absorption spectra and higher molar absorption coefficient in ICT

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absorption region. Finally, the PCE of QX15-based cell reached about 80% of the commercial Ru(II) dye N719-based cell (Voc: 771 mV; Jsc: 16.0 mA cm−2; PCE: 7.91%) which was fabricated and measured under the same standard conditions. In order to understand the reason of the different Jsc of these dyes, the IPCE

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measurements of QX15–18 based cells have been performed. As shown in their IPCE spectra (Fig. 6), these dyes could make the sunlight-to-photocurrent conversion in the UV-visible range and the spectra onsets of QX15–18 were at 690, 710, 720, and 750

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nm, respectively. Obviously, the IPCE tendency conformed well to the order of their

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Jsc values. The QX18-based DSSC had the highest IPCE response corresponding to the highest Jsc value of 11.9 mA cm−2 and gave over 60% IPCE values from 430 to 630 nm with a maximum of 71% at 530 nm. The highest and broadest IPCE response of QX18 could explain its highest Jsc value among the four dyes from the J-V measurements. 3.5. Electrochemical impedance spectroscopy The electrochemical impedance spectroscopy (EIS) was used to research the 18

ACCEPTED MANUSCRIPT interfacial charge transfer processes in the cells. The Nyquist plots and Bode phase plots were measured from 100 kHz to 0.1 Hz under –0.75 V bias in the dark (Fig. 7). The relatively larger semicircle in the lower frequency region of the Nyquist plots

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corresponds to the charge transfer resistance (Rct) at the dyes/TiO2/electrolyte interface [25]. The bigger radius means the larger Rct value and indicates the increased hindrance of electron recombination at the dyes/TiO2/electrolyte interface and a

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higher photovoltage value [26]. The Rct values of these cells were fitted in the order of

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QX17 (16.2 Ω) < QX16 (42.3 Ω) < QX18 (50.0 Ω) < QX15 (214 Ω). Obviously, the order agreed well with the Voc values of QX17 (695 mV) < QX16 (700 mV) < QX18 (707 mV) < QX15 (813 mV). Additionally, the peak frequency (f) in the lower frequency region of the Bode plots is also closely related to the charge recombination

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rate and the electron lifetime (τ) can be calculated using the equation τ = 1/(2πf) [27]. In general, the bigger τ value also corresponds to the higher photovoltage value [28]. The peaks in the lower frequency region of the Bode plots for the cells based on the

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four dyes decreased in the order of QX17 > QX16 > QX18 > QX15, and the τ values

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increased in the reverse order of QX17 (1.63 ms) < QX16 (10.8 ms) < QX18 (13.1 ms) < QX15 (50.4 ms). Based on the above studies, the best photovoltaic performance of the dye QX15 can be attributed to its good photon-to-current response and obviously lowest electron recombination rate, which results its highest PCE value.

4. Conclusions In summary, four organic dyes QX15–18 based on indeno[1,2-b]indole have been 19

ACCEPTED MANUSCRIPT designed and synthesized for DSSCs. These four D–π–A dyes consist of an indeno[1,2-b]indole donor, benzene ring or thiophene π-bridge, and different acceptor groups including cyanoacrylic acid, rhodanine-3-acetic acid, and DCRD. Among the

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four dyes, QX15 with a cyanoacrylic acid acceptor exhibited a highest Voc of 813 mV and a high Jsc of 11.0 mA cm−2, generating a highest PCE of 6.29%. Meanwhile, the dye QX18 with a thiophene π-bridge and a DCRD acceptor showed a significantly

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improved Jsc of 11.9 mA cm−2, which could be attributed to its red-shifted absorption

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spectra and higher molar absorption coefficient in ICT absorption region. These results demonstrate that indeno[1,2-b]indole is a promising donor group for

Acknowledgments

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constructing efficient organic dyes.

We highly thank the National Natural Science Foundation of China (No: 21676057)

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for its financial support.

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Supplementary material 1

H and

13

C NMR spectra of the new compounds were presented. This material is

available free of charge via the internet.

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coefficient for efficient dye-sensitized solar cells. Dyes Pigm 2016;125:229–40.

27

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Table 1 UV-vis absorption and electrochemical data of QX15–18.a λmax (nm, on

λmax (nm)

cm–1)

TiO2)

Eox (V)

QX15

416

1.15

422

0.99

QX16

460

1.03

468

0.98

QX17

434

1.75

448

1.02

QX18

498

2.49

510

1.03

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a

E0–0 (V)

Ered (V)

2.21

–1.22

2.16

–1.18

2.15

–1.13

1.93

–0.90

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Dye

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ε (104 M–1

First oxidation potentials (vs. NHE) in DCM were calibrated with ferrocene (0.63 V

vs. NHE); E0–0 values were estimated from the onsets of absorption spectra; Ered = Eox

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– E0–0.

28

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Table 2. Excitation wavelengths, orbital energies, oscillator strength (f) and assignment of QX15–18 in vacuum optimized at B3LYP/6-31G* level.

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Calculated λmax (nm)

energy (eV)

f

Transition assignment

QX15

417

2.97

0.584

HOMO–1 → LUMO (99.3%)

QX16

434

2.86

0.919

HOMO–1 → LUMO (98.7%)

QX17

457

2.71

0.832

QX18

489

2.53

SC

Dye

M AN U

HOMO–1 → LUMO (99.2%)

AC C

EP

TE D

0.930

29

HOMO–1 → LUMO (99.0%)

ACCEPTED MANUSCRIPT

Table 3. Photovoltaic data of QX15–18 using the commercial N719 dye as a reference.a Voc (mV)

Jsc (mA cm–2)

FF (%)

QX15

813 ± 5

11.0 ± 0.2

70.1 ± 0.2

6.29 ± 0.16

QX16

700 ± 4

4.23 ± 0.1

75.0 ± 0.1

2.22 ± 0.12

QX17

695 ± 3

7.10 ± 0.1

73.0 ± 0.2

3.60 ± 0.11

QX18

707 ± 3

11.9 ± 0.2

64.4 ± 0.3

5.41 ± 0.15

N719

771 ± 5

16.0 ± 0.2

PCE (%)

RI PT

SC

M AN U

a

Dye

64.1 ± 0.3

7.91 ± 0.18

The active area of all DSSCs was 0.16 cm2; The electrolyte consisted of 0.3 M

AC C

EP

TE D

DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in CH3CN.

30

ACCEPTED MANUSCRIPT Figure captions Fig. 1. Chemical structures of the four dyes QX15–18. Scheme 1. Synthetic routes of the four dyes QX15–18.

RI PT

Fig. 2. UV-vis absorption spectra of QX15–18 (a) in DCM solutions and (b) on TiO2 films.

Fig. 3. (a) Cyclic voltammograms of QX15–18 measured in CH2Cl2 and (b) energy

SC

diagram of HOMO and LUMO levels for QX15–18 (V vs. NHE).

molecular orbitals of QX15–18.

M AN U

Fig. 4. The optimized configurations and electron distributions of the frontier

Fig. 5. The current density-photovoltage (J-V) curves of the cells based on QX15–18. Fig. 6. IPCE spectra of the four dyes QX15–18.

AC C

EP

–0.75 V.

TE D

Fig. 7. (a) The Nyquist plots and (b) Bode phase plots of the solar cells under a bias of

31

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 1. Chemical structures of the four dyes QX15–18.

32

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

Scheme 1. Synthetic routes of the four dyes QX15–18.

33

ACCEPTED MANUSCRIPT (a) QX15 QX16 QX17 QX18

2.5 ε / 104 M-1 cm-1

2.0 1.5

0.5 0.0

300

400 500 600 Wavelength / nm

700

QX15 QX16 QX17 QX18

SC

0.8 0.6 0.4 0.2 0.0 400

M AN U

Normalized absorption

(b)

1.0

RI PT

1.0

500 600 Wavelength / nm

700

AC C

EP

films.

TE D

Fig. 2. UV-vis absorption spectra of QX15–18 (a) in DCM solutions and (b) on TiO2

34

ACCEPTED MANUSCRIPT

QX15 QX16 QX17 QX18

15 10 5 0 -5

0.6 0.8 1.0 1.2 1.4 Pontentials vs. NHE (V)

1.6

M AN U

SC

0.4

RI PT

Current / uA

(a)

Fig. 3. (a) Cyclic voltammograms of QX15–18 measured in CH2Cl2 and (b) energy

AC C

EP

TE D

diagram of HOMO and LUMO levels for QX15–18 (V vs. NHE).

35

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 4. The optimized configurations and electron distributions of the frontier

AC C

EP

TE D

molecular orbitals of QX15–18.

36

14

QX15 QX16 QX17 QX18

12 10 8 6 4 2 0 0.0

0.2

0.4

0.6

Voltage / V

0.8

RI PT

Current density / mA cm-2

ACCEPTED MANUSCRIPT

1.0

AC C

EP

TE D

M AN U

SC

Fig. 5. The current density-photovoltage (J-V) curves of the cells based on QX15–18.

37

ACCEPTED MANUSCRIPT 100 QX15 QX16 QX17 QX18

60 40 20 0

400

500 600 700 Wavelength / nm

AC C

EP

TE D

M AN U

SC

Fig. 6. IPCE spectra of the four dyes QX15–18.

RI PT

IPCE / %

80

38

ACCEPTED MANUSCRIPT (a) 140

QX15 QX16 QX17 QX18

120

Z'' / Ω

100 80 60

20 0

50

100

150

200

250

300

Z' / Ω (b)

QX15 QX16 QX17 QX18

SC

40

20 10

0

10

M AN U

Phase / o

30

0 -1 10

RI PT

40

1

10

2

10

3

10

4

10

Frequence / Hz

AC C

EP

–0.75 V.

TE D

Fig. 7. (a) The Nyquist plots and (b) Bode phase plots of the solar cells under a bias of

39

ACCEPTED MANUSCRIPT Highlights: 1. Four indeno[1,2-b]indole-based new organic dyes have been synthesized. 2. These dyes were designed into a D–π–A structure and applied in DSSCs.

RI PT

3. Different acceptors were used to tune the photoelectric properties of the dyes.

AC C

EP

TE D

M AN U

SC

4. A highest PCE of 6.29% was achieved by the dye with a cyanoacrylic acid acceptor.