Trilateral π-conjugation extensions of phenothiazine-based dyes enhance the photovoltaic performance of the dye-sensitized solar cells

Accepted Manuscript Trilateral π-conjugation extensions of phenothiazine-based dyes enhance the photovoltaic performance of the dye-sensitized solar cells Zafar Iqbal, Wu-Qiang Wu, Zu-Sheng Huang, Lingyun Wang, Dai-Bin Kuang, Herbert Meier, Derong Cao PII:

S0143-7208(15)00347-2

DOI:

10.1016/j.dyepig.2015.09.001

Reference:

DYPI 4911

To appear in:

Dyes and Pigments

Received Date: 5 June 2015 Accepted Date: 2 September 2015

Please cite this article as: Iqbal Z, Wu W-Q, Huang Z-S, Wang L, Kuang D-B, Meier H, Cao D, Trilateral π-conjugation extensions of phenothiazine-based dyes enhance the photovoltaic performance of the dye-sensitized solar cells, Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.001. 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.

ACCEPTED MANUSCRIPT

Graphical Abstract

Trilateral conjugation extensions

R N

N

CN S

CN

S

CO2H

Jsc = 9.01 PCE =4.87

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OR

S Jsc = 14.87 PCE = 7.33

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TE D

M AN U

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RO

CO2H

ACCEPTED MANUSCRIPT

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Trilateral π-conjugation extensions of phenothiazine-based dyes enhance the photovoltaic performance of the dyesensitized solar cells

Zafar Iqbala,c, Wu-Qiang Wub, Zu-Sheng Huanga, Lingyun Wanga, Dai-Bin Kuangb,*, Herbert Meierd, Derong Caoa,* School of Chemistry and Chemical Engineering, State Key Laboratory of

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a

Luminescent Materials and Devices, South China University of Technology,

b

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Guangzhou 510641, China.

MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of

Environment and Energy Chemistry, School of Chemistry and Chemical Engineering,

c

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Sun Yat-sen University, Guangzhou 510275, China.

Applied Chemistry Research Centre PCSIR Laboratories Complex, Lahore 54000,

Institute of Organic Chemistry, University of Mainz, Mainz 55099, Germany.

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d

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Pakistan

* Corresponding Author: Cao, D. Tel.: +86-20-87110245. Fax: +86-20-87110245. Email: [email protected]; Kuang, D. B.: Tel.: +86-20-84113015. E-mail: [email protected].

1

ACCEPTED MANUSCRIPT ABSTRACT Two novel organic dyes TLEP-1 and TLEP-2 based on phenothiazine with trilateral π–conjugation extensions were designed and synthesized for dye-sensitized solar cells, where phenothiazine ring was linked with two phenyl moieties at 7- and

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10-positions as the first and second π–conjugation extensions, and with furan or thiophene ring at 3-position as the third π–conjugation extension for TLEP-1 and TLEP-2, respectively. The influence of the π-conjugation extensions on the photovoltaic performance was evaluated. The cell based on TLEP-2 exhibits an

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impressive short-circuit photocurrent density of 14.87 mA/cm2, which is much higher than the cell based on the reference dye without π-conjugation extension (9.01

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mA/cm2), leading to high power conversion efficiency of 7.33% under simulated AM 1.5 G illumination condition. The results indicate that the trilateral π–conjugation extensions are an effective way to improve the photovoltaic performance of the dyesensitized solar cells.

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Key words: Organic dye; Dye-sensitized solar cells; Trilateral π-conjugation; Phenothiazine; Thiophene; Furan 1. Introduction

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Among various kinds of emerging photovoltaic technologies, dye-sensitized solar cells (DSSCs) have attracted a great deal of academic and industrial interest since the

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pioneer report in 1991 [1-3]. DSSCs are being accepted as a promising substitute to the conventional silicon based solar cells due to its low material cost, facile fabrication process and reasonably high power conversion efficiency [4–6]. Furthermore, DSSCs have been proved to be better in diffused light and possess good tolerance at moderate temperature –up to 50ºC [7,8]. It also has largest potential for mass production.

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ACCEPTED MANUSCRIPT Metal complex dyes based on ruthenium and zinc have been extensively studied for application in dye-sensitized solar cells and efficiencies over 10-11% [9,10] and 12% [6] have been achieved. However, ruthenium is a rare and expensive metal and its dyes have relatively low molar extinction coefficients. Furthermore, its synthesis

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needs tedious purification processes, which somewhat hampers the large-scale production of these complexes [11]. Therefore, more efforts have been devoted to the development of pure organic dyes due to their higher molar extinction coefficients, simple synthesis and purification processes at a considerably lower cost. Tremendous

diphenylamine,

indoline,

carbazole,

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progress has been made in this field and various electron donors like triphenylamine, tetrahydroquinoline,

phenoxazine

and

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phenothiazine [12–21] have been investigated. However, the photovoltaic performance of metal-free organic dyes still lags behind the ruthenium complexes counterpart [11].

Major factors for the low conversion efficiency of DSSCs based on most of organic dyes are the formation of dye aggregates on the semiconductor surface, inefficient

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electron injection and serious recombination reaction with I3– ions in the redox electrolyte [22,23]. Generally, formation of rod-like configuration after being adsorbed on the TiO2 surface provides a path of unexpected recombinations for redox

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couples [24].

Strong electron donor is the basic requirement for effective and efficient dyes,

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which can be increased by incorporating more electron donating groups on the main skeleton of the donor unit. Recently, introduction of two donors in the dyes was reported and D–D–π–A concept was found better in photovoltaic performance [25– 27]. However, their efficiencies still remain low, plausibly due to usage of bulky donor units which leads to molecular aggregation on the surface of photoanode materials. The effectiveness of the dyes can be increased by reduction of π–π stacking on the TiO2 surface by the introduction of alkoxy and aromatic groups [28,29]. Alkoxy and aromatic groups besides electron donating unit also reduce aggregation of 3

ACCEPTED MANUSCRIPT the dyes on TiO2 films [30,31]. Alkyl and alkoxy chains with different donors have been investigated and these modifications can reduce the recombination rate within DSSCs and thus increase open-circuit voltage [32,33]. The π–spacer between the donor and acceptor in the dye influences not only the

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region of light absorbed by the DSSCs but also the degree of electron injection from the excited state of the dye to the conduction band of anode materials. Furan and thiophene have been studied as π–spacer due to their better electron transport properties [34]. Furan has better aromatic ability due to more electronegative oxygen

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and higher oxidation potential which leads to efficient hole location and reinforces the stability of the dye [35]. Furan reveals comparable optical and charge carrier mobility

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to that of thiophene [36]. Furthermore, the sensitizers containing furan have better solubility [37] and efficiency in DSSCs [38]. It was observed that molar extinction coefficient and ICT band of furan and thiophene containing dyes were higher as compared to benzene containing dye [39,40]. Generally, thiophene is preferred due to

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its high polarizability, tunable spectroscopic and electrochemical properties [41]. Phenothiazine (PTZ) is a well-known photo-sensitizer with electron-rich sulfur and a nitrogen heteroatom and its ring is non-planar with a butterfly conformation at the ground state, which can impede the molecular aggregation and the formation of

stronger

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molecular excimers [42]. The additional electron rich sulfur atom renders PTZ a donor

than

other

amines,

even

better

than

triphenylamine,

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tetrahydroquinoline, carbazole and iminodibenzyles [43]. Hence, PTZ based dyes containing various types of modifications have been synthesized for the application in DSSCs [44–49].

Recently we reported two dyes Z2 and Z4 with mono- and bi-lateral π–conjugation extensions of PTZ ring [50]. The results indicate that the bi-lateral conjugate extension of PTZ can increase their light harvesting capacity and electron lifetime, leading to improvement in the efficiency of DSSCs. Keeping in view the advantages 4

ACCEPTED MANUSCRIPT of electron donating groups, alkyl chains and π–spacers, two novel dyes TLEP-1 and TLEP-2 based on phenothiazine with trilateral π–conjugation extensions were designed and synthesized for efficient DSSCs. Octyloxyphenyl moieties were incorporated to phenothiazine skeleton at 7- and 10-positions as the first and second

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π–conjugation extensions, furan or thiophene ring was linked at 3-position as the third π–conjugation extension. Their photophysical, electrochemical and photovoltaic performances were studied in detail. The influence of the monolateral, bilateral and trilateral π-conjugation extensions of phenothiazine ring on the photovoltaic

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performance was evaluated. A detailed discussion on the relationships between the power conversion efficiencies of the DSSCs and the π-conjugation extensions of the

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phenothiazine ring were made in this paper.

Fig. 1. Structures of the organic dyes. Scheme 1

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2. Experimental Section

2.1. Instrumentation and Materials 1

H NMR spectra were conducted on Bruker 400 MHz spectrometer in CDCl3 or

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DMSO-d6 with tetramethylsilane as internal standard. MS data were recorded on a maXis impact Bruker mass spectrometer. Elementary analyses were performed by

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using Vario ELIII Analyzer, while melting points were taken on Tektronix X4 microscopic MP apparatus. Infrared spectra (FT–IR) were recorded with KBr pellets on Bruker Tensor 27 spectrometer. The absorption and emission spectra of the dyes were conducted in THF and CH2Cl2 solutions and measured at room temperature by Shimadzu UV-2450 UV-Vis spectrophotometer and Fluorolog III photoluminescence spectrometer, respectively. The absorption spectra of the dyes on TiO2 film were measured by UV-3010 UV-Vis spectrophotometer.

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ACCEPTED MANUSCRIPT Electrochemical redox potentials were measured by Cyclic Voltammetry (CV), using three electrode cells and an Ingsens 1030 electrochemical work station (Ingsens Instrument Guangzhou, Co. Ltd., China) in one compartment at a scan rate of 50 mV/s. The Ag/AgCl in KCl (3 M) solution and an auxiliary platinum wire were

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utilized as reference and counter electrodes, while dye coated TiO2 films were used as working electrode. Tetrabutylammonium perchlorate (n-Bu4NClO4, 0.1 M) was used as supporting electrolyte in acetonitrile. All electrochemical measurements were calibrated by using ferrocene as standard (0.63V vs. NHE). The photocurrent-

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photovoltage characteristics were recorded by using a Keithley 2400 source meter under simulated AM 1.5 G (100 mW cm2) illumination with a solar light simulator

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(Oriel, Model: 91192). A 1000 W Xenon arc lamp (Oriel, Model: 6271) served as a light source and its incident light intensity was calibrated with a NREL standard Si solar cell. The incident photon-to-current conversion efficiency (IPCE) spectra were measured as a function of wavelength from 380 to 800 nm on the basis of a Spectral Products DK240 monochromator. The electrochemical impedance spectra (EIS)

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measurements were conducted on the electrochemical workstation (Zahner Zennium) in dark conditions, with an applied bias potential of –0.79 V. A 10 mV AC sinusoidal signal was employed over the constant bias with the frequency ranging between 1

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MHz and 10 mHz. The impedance parameters were determined by fitting of the impedance spectra using Z–view software. The amount of dye loading was measured

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by desorbing the dye from the film with 0.1 M NaOH in ethanol/H2O (1:1) and measuring UV-Vis spectrum [51]. The reference dye SP [49] and 10-[4-(octyloxy)phenyl]-10H-phenothiazine (1)

[50] were synthesized according to the previous procedures. The reagents and solvents were purchased as reagent grade and used without purification. All reactions were performed under nitrogen or argon and monitored by TLC (Merck 60 F254) and the products were purified by column chromatography on silica gel (200–300 mesh). 2.2. Synthesis and characterization 6

ACCEPTED MANUSCRIPT 2.2.1. 3,7-Dibromo-10-[4-(octyloxy)phenyl]-10H-phenothiazine (2). 1 (0.81 g, 2 mmol) was dissolved in CHCl3 (150 mL) and maintained at 0 ˚C. Then liquid Br2 (0.22 mL, 4 mmol) dissolved in 20 mL chloroform was dropped into the flask very slowly while keeping the temperature around 0 ˚C. The reaction

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mixture was stirred for 8 h. After a dilute solution of sodium hydroxide was added, the reaction mixture was stirred for additional 1 h. CH2Cl2 (200 mL) and brine solution was added. Organic layer was separated and washed with water (200 mL) by two times, dried over anhydrous Na2SO4. Solvent was removed through rotary

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evaporator and the crude product was purified by column chromatography with CH2Cl2 as mobile phase to give light green product 2 in 92.5% yield, mp 107–108 ºC. H NMR (CDCl3, 400 MHz, ppm): δ 7.25–7.22 (m, 2H), 7.12–7.08 (m, 4H), 6.93–

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1

6.91 (m, 2H), 6.05–6.03 (m, 2H), 4.05 (t, J = 6.4 Hz, 2H), 1.90–1.83 (m, 2H), 1.56– 1.49 (m, 2H), 1.39–1.34 (m, 8H), 0.93 (t, J = 6.4 Hz, 3H). FT-IR (KBr pellet, cm-1): 2921, 2845, 1690, 1567, 1493, 1464.

carbaldehyde (3)

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2.2.2. 5-{7-Bromo-10-[4-(octyloxy)phenyl]-10H-phenothiazin-3-yl}-furan-2-

A mixture of 2 (2.25 g, 4 mmol), potassium carbonate (2.21 g, 16 mmol), Pd(PPh3)4 (10 mole%) in DMF (30 mL) was stirred for half an hour at 40 ˚C under

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argon. Then 5-formylfuran-2yl-boronic acid (0.49 g, 3.5 mmol) dissolved in DMF (5 mL) was injected slowly (drop by drop) with continuous stirring. After that

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temperature was raised to 80 ˚C and maintained for 48 h. After cooling the reaction mixture was dropped into water (200 mL) and the product was extracted with CH2Cl2. Organic layer was washed several times with water for the complete removal of DMF. CH2Cl2 was removed through rotary evaporator and the crude product was purified by column chromatography using petroleum ether (PE, 60–90 ºC) and CH2Cl2 (1:1) as mobile phase to give dark yellow product 3 (0.92 g) in 39.8 % yield, mp 112–114 ºC. 1

H NMR (CDCl3, 400 MHz, ppm): δ 9.59 (s, 1H), 7.42–7.41 (m, 1H), 7.29–7.25 (m,

4H), 7.14–7.10 (m, 3H), 6.93–6.90 (m, 1H), 6.66–6.65 (m, 1H), 6.20–6.18 (m, 1H), 7

ACCEPTED MANUSCRIPT 6.04–6.01 (m, 1H), 4.06 (t, J = 6.4 Hz, 2H), 1.90–1.83 (m, 2H), 1.56–1.49 (m, 2H), 1.41–1.37 (m, 8H), 0.93 (t, J = 6.8 Hz, 3H). FT-IR (KBr pellet, cm-1): 2919, 2853, 1668, 1603, 1585, 1507, 1304, 1240. MS (m/z, APCI) (M+H)+ calcd for C31H31Br NO3S, 576.12; found, 576.10. Anal. calcd for C31H30Br NO3S: C, 64.58; H, 5.24; N,

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2.34; S, 5.56; found: C, 64.72; H, 5.29; N, 2.34; S, 5.75. 2.2.3. 5-{7-[4-Hydroxyphenyl]-10-[4-(octyloxy)phenyl]-10H-phenothiazin-3-yl}furan-2-carbaldehyde (4) A mixture of 3 (0.87 g, 1.5 mmol), potassium carbonate (0.83 g, 6 mmol),

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Pd(PPh3)4 (10 mole%) in DMF (20 mL) was stirred for half an hour at 40 ˚C under argon. Then 4-hydroxyphenyl boronic acid (0.21 g, 1.5 mmol) dissolved in DMF (5

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mL) was injected slowly (drop by drop) with continuous stirring. After that temperature was raised to 80 ˚C and maintained for 48 h. The reaction mixture was cooled and then dropped into water (200 mL) and the product was extracted with CH2Cl2. Organic layer was washed several times with water for the complete removal of DMF. CH2Cl2 was removed through rotary evaporator and the crude product was

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purified by column chromatography using CH2Cl2 as mobile phase. Orange product 4 (0.36 g) was obtained in 40.1 % yield, mp 89–91 ºC. 1H NMR (CDCl3, 400 MHz, ppm): δ 9.56 (s, 1H), 7.72–7.67 (m, 2H), 7.61–7.57 (m, 1H), 7.52–7.47 (m, 2H),

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7.43–7.42 (m, 1H), 7.34–7.28 (m, 3H), 7.25–7.22 (m, 1H), 7.16–7.15 (m, 1H), 7.00– 6.98 (m, 1H), 6.93–6.91 (m, 1H), 6.65–6.64 (m, 1H), 6.20–6.17 (m, 2H), 4.06 (t, J =

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6.4 Hz, 2H), 1.90–1.83 (m, 2H), 1.57–1.50 (m, 2H), 1.40–1.34 (m, 8H), 0.93 (t, J = 6.4 Hz, 3H). FT-IR (KBr pellet, cm-1): 3383, 2920, 2850, 1651, 1608, 1583, 1510, 1309, 1240. MS (m/z, APCI) (M+H)+ calcd for C37H36NO4S, 590.24; found, 590.22. Anal. calcd for C37H35 NO4S: C, 75.35; H, 5.98; N, 2.38; S, 5.44; found: C, 75.51; H, 5.95; N, 2.19; S, 5.50.

2.2.4. 5-[7,10-Bis(4-(octyloxy)phenyl)-10H-phenothiazin-3-yl]-furan-2-carbaldehyde (5)

8

ACCEPTED MANUSCRIPT A mixture of 4 (0.24 g, 0.4 mmol), potassium carbonate (0.28 g, 2 mmol), DMF (15 mL) was stirred for half an hour under nitrogen at ambient temperature. Then 1-bromooctane (0.097 g, 0.5 mmol) in DMF (5 mL) was injected slowly. After that temperature was raised to 130 ˚C and maintained for 24 h. The reaction mixture

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after cooling was dropped into water (200 mL) and the product was extracted with CH2Cl2. Organic layer was washed several times with water to remove DMF. CH2Cl2 was removed through rotary evaporator and the crude product was purified by column chromatography using PE and CH2Cl2 (1:1) as mobile phase. Bright yellow product 5

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(0.25 g) was obtained in 87.3 % yield, mp 136–138 ºC. 1H NMR (CDCl3, 400 MHz, ppm): δ 9.67 (s, 1H), 7.50–7.47 (m, 1H), 7.27–7.25 (m, 2H), 7.14–7.12 (m, 3H),

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7.06–7.04 (m, 2H), 7.00–6.95 (m, 3H), 6.89–6.87 (m, 1H), 6.80–6.78 (m, 2H), 6.09– 6.02 (m, 2H), 3.91–3.82 (m, 4H), 1.71–1.67 (m, 4H), 1.35–1.21 (m, 20H), 0.80–0.77 (m, 6H). FT-IR (KBr pellet, cm-1): 2922, 2849, 1666, 1611, 1590, 1507, 1310, 1243. MS (m/z, APCI) (M+H)+ calcd for C45H52NO4S, 702.36; found, 702.34. Anal. calcd for C45H51NO4S: C, 77.00; H, 7.32; N, 2.00; S, 4.57; found: C, 77.14; H, 7.23; N,

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2.14; S, 4.62.

2.2.5. (E)-2-Cyano-3-{5-[7,10-bis(4-(octyloxy)phenyl)-10H-phenothiazin-3-yl]-furan2-yl}-acrylic acid (TLEP-1)

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A mixture of 5 (0.18 g, 0.25 mmol), cyanoacrylic acid (0.41 g, 5 mmol), piperidine (0.3 mL, 3 mmol) in acetonitrile (30 mL) was refluxed for 8 h under

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nitrogen. After cooling, the mixture was poured into aqueous solution of HCl (2 M, 100 mL). It was stirred for 15 min and CH2Cl2 (200 mL) was added. The organic layer was separated and washed two times with water (200 mL). Then the organic layer was dried over anhydrous Na2SO4. The solvent was removed under reduced pressure and the crude product was purified by column chromatography using silica gel with CH2Cl2 and then CH2Cl2/CH3OH (20:1) as eluents to yield the product TLEP-1 (0.13 g, 64.9%) as black solid, mp 173–175 ºC. 1H NMR (DMSO-d6, 400 MHz, ppm): δ 7.85 (s, br, 1H), 7.57–7.56 (m, 1H), 7.48–7.46 (m, 2H), 7.41–7.38 (m, 9

ACCEPTED MANUSCRIPT 1H), 7.34–7.31 (m, 4H), 7.19–7.17 (m, 2H), 7.15–7.11 (m, 2H), 6.93–6.91 (m, 2H), 6.15–6.11 (m, 2H), 4.03 (t, J = 6.0 Hz, 2H), 3.94 (t, J = 6.4 Hz, 2H), 1.78–1.65 (m, 4H), 1.47–1.35 (m, 4H), 1.29–1.24 (m, 16H), 0.88–0.83 (m, 6H). FT-IR (KBr pellet, cm-1): 3420, 2924, 2853, 2216, 1698, 1608, 1583, 1508, 1310, 1241. MS (m/z, APCI)

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(M+H)+ calcd for C48H53N2O5S, 769.37; found, 769.35. Anal. calcd for C48H52N2O5S: C, 74.97; H, 6.82; N, 3.64; S, 4.17; found: C, 74.82; H, 6.63; N, 3.73; S, 4.11.

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2.2.6. 5-{7-Bromo-10-[4-(octyloxy)phenyl]-10H-phenothiazin-3-yl}-thiophene-2carbaldehyde (6) 6 was synthesized by following the procedure of 3. (5-Formylthiophen-2-

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yl)boronic acid was used instead of (5-formylfuran-2-yl)boronic acid. Orange product 6 (0.87 g) was obtained in 36.9 % yield, mp 104–105 ºC. 1H NMR (CDCl3, 400 MHz, ppm): δ 9.85 (s, 1H), 7.70–7.69 (m, 1H), 7.28–7.24 (m, 4H), 7.14–7.11 (m, 4H), 6.94–6.92 (m, 1H), 6.20–6.18 (m, 1H), 6.06–6.03 (m, 1H), 4.06 (t, J = 6.0 Hz, 2H), 1.90–1.83 (m, 2H), 1.53–1.50 (m, 2H), 1.38–1.34 (m, 8H), 0.93 (t, J = 6.4 Hz, 3H). FT-IR (KBr pellet, cm-1): 2919, 2850, 1670, 1601, 1571, 1504, 1303, 1241. MS (m/z,

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APCI) (M+H)+ calcd for C31H31BrNO2S2, 592.10; found, 592.3. Anal. calcd for C31H30BrNO2S2: C, 62.83; H, 5.10; N, 2.36; S, 10.82; found: C, 62.91; H, 5.09; N,

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2.31; S, 10.97.

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2.2.7. 5-{7-(4-Hydroxyphenyl)-10-[4-(octyloxy)phenyl]-10H-phenothiazin-3-yl}thiophene-2-carbaldehyde (7) 7 was synthesized by following the procedure of 4. Orange product 7 (0.31 g)

was obtained in 34.1 % yield, mp 91–93 ºC. 1H NMR (CDCl3, 400 MHz, ppm): δ 9.84 (s, 1H), 7.70–7.69 (m, 1H), 7.38–7.36 (m, 2H), 7.29–7.28 (m, 3H), 7.25–7.24 (m, 1H), 7.18–7.14 (m, 4H), 7.03–7.00 (m, 1H), 6.90–6.87 (m, 2H), 6.24–6.18 (m, 2H), 4.07 (t, J = 6.4 Hz, 2H), 1.91–1.84 (m, 2H), 1.57–1.50 (m, 2H), 1.40–1.34 (m, 8H), 0.93 (t, J = 6.0 Hz, 3H). FT-IR (KBr pellet, cm-1): 3310, 2923, 2852, 1661, 1607, 1585, 1505, 1309, 1240. MS (m/z, APCI) (M+H)+ calcd for C37H36NO3S2, 606.21; 10

ACCEPTED MANUSCRIPT found: 605.21. Anal. calcd for C37H35NO3S2: C, 73.36; H, 5.82; N, 2.31; S, 10.59; found: C, 73.10; H, 5.70; N, 2.28; S, 10.43.

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2.2.8. 5-{7,10-Bis[4-(octyloxy)phenyl]-10H-phenothiazin-3-yl}-thiophene-2carbaldehyde (8) 8 was synthesized by following the procedure of 5. Dark yellow product 8 (0.23 g) was obtained in 81.9 % yield, mp 161–163 ºC. 1H NMR (CDCl3, 400 MHz, ppm): δ 9.67 (s, 1H), 7.50–7.47 (m, 1H), 7.27–7.25 (m, 2H), 7.14–7.12 (m, 3H),

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7.06–7.04 (m, 2H), 7.00–6.95 (m, 3H), 6.89–6.87 (m, 1H), 6.80–6.78 (m, 2H), 6.09– 6.02 (m, 2H), 3.91–3.84 (m, 4H), 1.73–1.67 (m, 4H), 1.35–1.21 (m, 20H), 0.82–0.79

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(m, 6H). FT-IR (KBr pellet, cm-1): 2920, 2847, 1682, 1609, 1590, 1504, 1305, 1241. MS (m/z, APCI) (M+H)+ calcd for C45H52NO3S2, 718.34; found: 718.32. Anal. calcd for C45H51NO3S2: C, 75.27; H, 7.16; N, 1.95; S, 8.93; found: C, 75.46; H, 7.39; N, 2.08; S, 8.79. 2.2.9.

(E)-2-Cyano-3-{5-[7,10-bis(4-(octyloxy)phenyl)-10H-phenothiazin-3-yl]-

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thiophen-2-yl}-acrylic acid (TLEP-2)

The product TLEP-2 was synthesized by following the procedure of compound TLEP-1. Product TLEP-2 (0.13 g) as black solid was obtained in 67.6 % yield, mp 253–255 ºC. 1H NMR (DMSO-d6, 400 MHz, ppm): δ 8.21 (s, 1H), 7.79–

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7.78 (m, 1H), 7.55–7.54 (m, 1H), 7.49–7.47 (m, 2H), 7.45–7.44 (m, 1H), 7.36–7.34 (m, 2H), 7.29–7.25 (m, 2H), 7.22–7.20 (m, 2H), 7.17–7.14 (m, 1H), 6.95–6.93 (m,

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2H), 6.15–6.11 (m, 2H), 4.05 (t, J = 6.4 Hz, 2H), 3.96 (t, J = 6.4 Hz, 2H), 1.80–1.66 (m, 4H), 1.47–1.38 (m, 4H), 1.34–1.26 (m, 16H), 0.89–0.84 (m, 6H). FT-IR (KBr pellet, cm-1): 3442, 2922, 2851, 2212, 1685, 1608, 1577, 1508, 1310, 1244. MS (m/z, APCI) (M+H)+ calcd for C48H53N2O4S2, 785.34; found, 785.33. Anal. calcd for C48H52N2O4S2: C, 73.43; H, 6.68; N, 3.57; S, 8.17; found: C, 73.32; H, 6.76; N, 3.40; S, 8.23. 2.3. Fabrication of DSSCs 11

ACCEPTED MANUSCRIPT Fluorine-doped tin oxide (FTO) glasses cleaning and anatase TiO2 nanoparticles (20 nm) preparation were conducted according to previous literature. [25]. The TiO2 photoanodes (about 16 µm in thickness) were also prepared according to the literature procedures [25]. The TiCl4-treated films were immersed in a 5.0 × 10-4 M solution of

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TLEP-1 and TLEP-2 for 16 h (0.5 mM dye in a mixture of tetrahydrofuran and CH2Cl2, 1:1 volume ratio). The dye-sensitized TiO2/FTO glass films were assembled into a sandwiched type together with Pt/FTO counter electrode. Pt counter electrodes were fabricated by thermal depositing H2PtCl6 solution (5 mM in isopropanol) onto

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FTO glass at 400 °C for 15 min. The electrolyte (0.6 M 1-methyl-3propylimidazolium iodide (PMII), 0.10 M guanidinium thiocyanate, 0.03 M I2 and 0.5

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M tert-butylpyridine) in acetonitrile/valeronitrile (85:15) was injected from a hole made on the counter electrode into the space between the sandwiched cells. The active area of the dye-sensitized TiO2 film was approximately 0.16 cm2. 3. Results and Discussion

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3.1. Synthesis and Structural Characterization

The synthetic route of TLEP-1 and TLEP-2 is shown in Scheme 1. 10-[4(Octyloxy)phenyl]-10H-phenothiazine (1) was brominated to give 2. 3 and 6 were synthesized from 2 through a Suzuki coupling reaction with (5-formylfuran-2-

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yl)boronic acid and (5-formylthiophen-2-yl)boronic acid, respectively. Then another Suzuki coupling reaction of 3 and 6 with (4-hydroxyphenyl)boronic acid gave 4 and 7,

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respectively. Alkylation of 4 and 7 with 1-bromooctane gave 5 and 8, respectively. Knoevenagel condensation reaction of 5 and 8 with cyanoacetic acid gave TLEP-1 and TLEP-2, respectively. 3.2. Photophysical Properties of TLEP-1 and TLEP-2 UV-Vis absorption and emission spectra of TLEP-1 and TLEP-2 (2 × 10-5 M) in THF/CH2Cl2 (1:1) solution are presented in Table 1 and Fig. 2. The UV spectra of TLEP-1 and TLEP-2 exhibited two distinct bands, one between 358 and 359 nm and 12

ACCEPTED MANUSCRIPT the other between 473 and 476 nm. The first band corresponds to the π–π* transition of the localized conjugated skeleton in the UV region, while the second band was assigned to an intramolecular charge transfer (ICT) in the visible region. The λmax of the dyes were in the order of TLEP-2 > TLEP-1. The HOMO-LUMO energy band

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gap was also reduced by the increase of π-conjugation extensions. Molar extinction coefficients (ε) of TLEP-1 and TLEP-2 were 24195 M-1cm-1 and 40400 M-1cm-1, respectively. Hyperchromic behavior was observed with the increase in π-conjugation extension. The red shift and enhanced molar extinction coefficient may favor light

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harvesting and hence photocurrent generation in DSSCs. Hypsochromic behavior of TLEP-1 as compared to TLEP-2 may arise from the twisted molecular structure

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induced by steric interaction between furan and neighboring units [52]. UV absorption maximum (λmax) of TLEP-1 has a little blue shift compared to λmax of TLEP-2, plausibly caused by the decrease of co-planarity between the electron donor and the electron acceptor in the ground state [27]. The higher value of ε for TLEP-2 was due to better delocalization of electrons over the π–conjugated molecule, which is an

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advantageous spectral property for light-harvesting [26].

After being adsorbed on the surface of TiO2 films, the absorption wavelength region of TLEP-1 and TLEP-2 was increased to ~650 nm (Fig. 3). The λmax of

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TLEP-1 and TLEP-2 were 471 and 478 nm, respectively. Increase of absorption range may be ascribed to the bonding behavior of the anchoring group with TiO2,

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which should favor the light harvesting of the solar cell and thus increase the photocurrent response region, leading to the increase of Jsc. Furthermore,

the

absorption maxima of TLEP-1 and TLEP-2 on the TiO2 films did not show considerable difference in comparison with those in THF/ CH2Cl2 (Table 1), which may be ascribed to that these dyes exhibited less tendency of aggregation, which is favorable for DSSCs applications. Fig. 2. UV-Vis absorption spectra of TLEP-1 and TLEP-2 in THF/CH2Cl2. 13

ACCEPTED MANUSCRIPT Fig. 3. UV-Vis absorption spectra of TLEP-1 and TLEP-2 on TiO2 films.

3.3. Electrochemical properties of TLEP-1 and TLEP-2 In order to investigate the possibilities of electron transfer from the excited

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states of TLEP-1 and TLEP-2 to the conduction band of TiO2 and regeneration of the dyes, the redox behavior was studied by cyclic voltammetry (Fig. 4, Table 2). Onset oxidation potentials of the dyes enabled the calculation of the highest occupied

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molecular orbitals (HOMO) of 1.24 V and 1.23 V (vs. NHE) for TLEP-1 and TLEP2, respectively. The energy gap between HOMO and LUMO (E0-0) were estimated by oxidation potential and the energy at the intersection of absorption and emission

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spectra, which were 2.36 and 2.38 eV for TLEP-1 and TLEP-2, respectively. The LUMO level of TLEP-1 and TLEP-2 were calculated from the Eox – E0-0, which were –1.12 and –1.15 eV, respectively. Overall, both LUMO levels and HOMO levels of TLEP-1 and TLEP-2 are reasonably suitable to provide sufficient thermodynamics driving force for electron injection from the excited state to the conduction band of

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TiO2 (0.5 eV vs. NHE) [53] and regeneration from I–/I3– redox potential (0.4 eV vs. NHE) [54]. The requirement for appropriate electron injection from the excited state to the conduction band of titanium dioxide is 0.2 eV [55] and requirement for efficient

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electron regeneration of oxidized dye is 0.15 eV [56].

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Fig. 4. Cyclic voltammetry of TLEP-1 and TLEP-2.

3.4. Molecular orbital calculations of TLEP-1 and TLEP-2 To gain further insight into the above mentioned results, density function theory

(DFT) calculations were carried out by using Gaussian 09 software at the B3LYP/631G (d, p) levels [57]. Optimized structures and the electronic distribution in HOMO and LUMO levels are presented in Fig. 5, Table 2. The electron distributions of TLEP-1 and TLEP-2 in the HOMO levels were mainly localized on the 14

ACCEPTED MANUSCRIPT phenothiazine moiety, while the LUMOs showed localized electron distribution through the cyanoacrylic acid upon light illumination. Therefore, HOMO–LUMO excitation by sun light induced shift of the electron from the PTZ donor moiety to the acceptor moiety. This separation of electrons ensures the efficient electron injection

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from the dye to the TiO2 film. The octyloxyphenyl moieties at 7- and 10- positions and the π-spacer at 3-position of phenothiazine increase the bulkiness of the molecule, which prevents the formation of excimers and aggregates on the TiO2 surface [58].

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Table 1. Absorption and emission characteristics of TLEP-1 and TLEP-2. Table 2. Electrochemical characteristics of TLEP-1 and TLEP-2.

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Fig. 5. Optimized structures and frontier molecular orbitals HOMO and LUMO calculated by DFT on a B3LYP/6-31+G (d, p) level of TLEP-1 and TLEP-2.

3.5 Photovoltaic performance of the DSSCs based on TLEP-1 and TLEP-2

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In order to investigate the photovoltaic performance of the dyes, a set of DSSCs were fabricated and tested under standard conditions (AM 1.5 G, 100 mW cm-2). The incident photon-to-current conversion efficiency (IPCE) and photocurrent density– photovoltage (J–V) curves of TLEP-1 and TLEP-2 were obtained with a

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electrolyte.

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sandwiched-typed cell comprising of 16 µm TiO2 photoanode and I–/I3– redox

The IPCE spectra of TLEP-1 and TLEP-2 for DSSCs as a function of

wavelength from 380 to 800 nm can be seen in Fig. 7. The IPCE trend of the dyes was in the order of TLEP-2 > TLEP-1, which is well in agreement with the UV-Vis absorption spectra of the dyes and the Jsc variations obtained in J–V measurements. Specifically, the IPCE values of the DSSCs based on TLEP-1 and TLEP-2 were over 80% at 450 and 550 nm while the IPCE value of the dye SP only exceeds 60% [44], respectively. The IPCE values of TLEP-1 and TLEP-2 tail off at 700 and 710 nm, 15

ACCEPTED MANUSCRIPT respectively. It is worth noting that the DSSC based on TLEP-2 has the highest IPCE value, which is due to its broader absorption spectrum and higher molar extinction coefficient as compared to the dye SP [44]. The detailed photovoltaic parameters of Jsc, open-circuit voltage (Voc), fill

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factor (FF) and overall conversion efficiency (η) were obtained from J-V curves (Fig. 6) and the corresponding photovoltaic parameters are summarized in Table 3. The TLEP-1 and TLEP-2 sensitized cells gave a η of 6.44% and 7.33% (Jsc = 12.61 and 14.87 mA cm-2, Voc = 816 and 777 mV, FF = 0.63 and 0.63, respectively) under

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standard global AM 1.5 G one sun illumination (100 mW cm-2). Under similar measuring conditions, SP and Z4 showed Jsc of 9.01, 10.35 mA cm-2; Voc of 758, 802

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mV and FF of 0.71, 0.69, corresponding to η of 4.87% and 5.73%, respectively [44,50]. The DSSC based on TLEP-2 exhibits much higher Jsc than SP, leading to a high power conversion efficiency, which is 50% higher than that of the DSSC based on SP.

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The Jsc values of the DSSCs based on TLEP-1 and TLEP-2 are well in agreement with their UV-Vis absorption spectra (Fig.1 and Table 1). On the other hand, the dye-loading amount of TLEP-2 on the TiO2 film was higher than TLEP-1, which also leads to higher Jsc for the former. Hence, the highest Jsc of the TLEP-2

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sensitized cell can be ascribed to its highest dye loading amount and light adsorption

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capability (Fig. 1 and Table 1), leading to a better light-harvesting efficiency. Fig. 6. J−V curves of the DSSCs sensitized by TLEP-1 and TLEP-2. Fig. 7. IPCE spectra of the DSSCs based on TLEP-1 and TLEP-2.

To illustrate the difference in Voc among the DSSCs based on these dyes and investigate the interfacial charge transfer process within the DSSCs, the electrochemical impedance spectra (EIS) were performed in the dark condition under 16

ACCEPTED MANUSCRIPT a forward bias of –0.79 V. As shown in Fig. 8, two semicircles were observed in each Nyquist plot. The smaller semicircles (higher frequency from 103–106 Hz) and larger semicircles (lower frequency from 1–103 Hz) in the Nyquist plots correspond to the charge

transfer

at

the

counter

electrode/electrolyte

interface

and

the

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TiO2/dye/electrolyte interface, respectively. Specifically, there was a substantial difference in the large semicircles, which indicates that charge transfer behavior between TiO2 and dye or between dye and electrolyte was significantly altered, which was likely due to the different surface modifications of different dyes. The electron

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lifetime (τr) values calculated by fitting the equation τr = Rrec × CPE2 (CPE2, chemical capacitance) [59] were 342 and 217 ms for TLEP-1 and TLEP-2 sensitized solar

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cells, respectively.. Here, the longer electron lifetime of TLEP-1 based cell is associated with the effective suppression of the back reaction of the injected electron with I3– in the electrolyte, leading to higher electron lifetime and thus higher Voc. Table 3. Photovoltaic parameters of the DSSCs.

Conclusion

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Fig. 8 . Electrochemical impedance spectra of the DSSCs in the dark.

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Two novel dyes (TLEP-1 and TLEP-2) based on phenothiazine with a trilateral π–conjugation extensions were synthesized. The power conversion efficiency of the

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DSSCs based on TLEP-1 (6.44%) and TLEP-2 (7.33%) was found superior to the DSSCs based on the reference dye SP (4.87%) and Z4 (5.73%), due to their more active photoresponse in broader wavelength region and more effectiveness in light harvesting, leading to significant improvement of Jsc (9.01, 10.35 and 14.87 mA cm-2 for SP, Z4 and TLEP-2) under standard AM 1.5 G solar illumination conditions. The results indicate that the π–conjugation extension of phenothiazine ring is an effective way to improve the performance of the DSSCs, and the trilateral π–conjugation extensions lead to the most significant improvement. In addition, it should be 17

ACCEPTED MANUSCRIPT mentioned that thiophene ring is a better π-spacer than furan due to its better lightharvesting ability between these two dyes. Acknowledgment

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We are grateful to the National Natural Science Foundation of China (21272079), the Science and Technology Planning Project of Guangdong Province, China (2013B010405003), the Fundamental Research Funds for the Central

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Universities (2014ZP0008) and the Fund from Guangzhou Science and Technology

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Project, China (2012J4100003, 2014J4100016) for the financial support.

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Table 1 Absorption and emission characteristics of TLEP-1 and TLEP-2 Absorption λmax (nm) a

ε (M-1 cm-1)b

Emission λmax (nm) c

TLEP-1

358, 473

24309, 24195

TLEP-2

359, 476

33950, 40400

Absorption λmax (nm)d

Dye loading amount (Mol-1cm-2)

596

471

1.19 × 10-7

571

478

1.38 × 10-7

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dye

Absorption maximum of dyes measured in THF/CH2Cl2 (1:1) with concentration 2 × 10-5 M.

b

The molar extinction coefficient at λmax in solution.

c

Emission maximum of dyes in THF/CH2Cl2 (1:1) with concentration 2 × 10-5 M.

d

Absorption maximum of dyes adsorbed on the surface of TiO2.

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a

Table 2 Electrochemical characteristics of TLEP-1 and TLEP-2.

24

ACCEPTED MANUSCRIPT Dyes

λ(nm) a

TLEP-1 TLEP-2

HOMO (eV) vs. NHEb

E0-0 (eV) vs. NHEc

LUMO (eV) vs. NHEd

HOMO (eV) vs. Vacuume

LUMO (eV) vs. Vacuume

E-gap (eV)

524

1.24

2.36

-1.12

-5.07

-2.57

-2.50

521

1.23

2.38

-1.15

-4.97

-2.52

-2.45

λ intersection obtained from the cross point of absorption and emission spectra in THF/CH2Cl2 (1:1) solution.

b

HOMO of dyes measured by cyclic voltammetery in 0.1 M tetrabutylammonium perchlorate in acetonitrile solution as supporting electrolyte, Ag/AgCl as reference electrode and Pt as counter electrode.

c

E0-0 = 1240/λ intersection.

d

LUMO was estimated by HOMO – E0-0 .

e

HOMO and LUMO were calculated at B3LYP-3 G(d, p) level in vacuum.

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a

Table 3 Photovoltaic parameters of the DSSCs.

25

ACCEPTED MANUSCRIPT η (%)

Electron lifetime (τr, ms)

758

0.71

4.87

-

8.65

785

0.71

4.79

-

Z4**

10.35

802

0.69

5.73

-

TLEP-1

12.61

816

0.63

6.44

342

TLEP-2

14.87

777

0.63

7.33

217

Jsc ( mA/cm2)

SP*

9.01

Z2**

Voc (mV)

* The data of the DSSC based on SP was obtained from our previous report [44].

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** The data of the DSSCs based on Z2 and Z4 were obtained from our previous report [50].

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FF

Dyes

26

ACCEPTED MANUSCRIPT Figure Captions Scheme 1. Syntheses of TLEP-1 and TLEP-2. (i) CHCl3, Br2, 0 oC, 8 h; (ii) DMF, K2CO3, Pd(PPh3)4, (5-formylfuran-2-yl)boronic acid, 80 oC, 48 h; (iii) DMF, K2CO3,

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Pd(PPh3)4, (4-hydroxyphenyl)boronic acid, 80 oC, 48 h; (iv) DMF, K2CO3, 1bromooctane, 130 ˚C, 24 h; (v) Cyanoacetic acid, piperidine, acetonitrile, reflux, 8 h.

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Fig. 1. Structures of the organic dyes.

Fig. 2. UV-Vis absorption spectra of TLEP-1 and TLEP-2 in THF/CH2Cl2.

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Fig. 3. UV-Vis absorption spectra of TLEP-1 and TLEP-2 on TiO2 films. Fig. 4. Cyclic voltammetry of TLEP-1 and TLEP-2.

Fig. 5. Optimized structures and frontier molecular orbitals HOMO and LUMO

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calculated by DFT on a B3LYP/6-31+G (d, p) level of TLEP-1 and TLEP-2. Fig. 6. J−V curves of the DSSCs sensitized by TLEP-1 and TLEP-2. Fig. 7. IPCE spectra of the DSSCs based on TLEP-1 and TLEP-2.

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Fig. 8. Electrochemical impedance spectra of the DSSCs based on TLEP-1 and

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TLEP-2 in the dark.

27

ACCEPTED MANUSCRIPT OC8H17 OC8H17

OC8H17 (i)

(ii)

(iii) N

92.5%

N

N Br

S

39.8%

Br

Br

S

OC8H17 (v)

(iv)

N

N 87.3%

4

HO

O

S CHO

C8H17O

(iii)

Br

S

S

6

CN COOH

OC8H17

34.1% CHO

(iv)

N S

HO

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36.9%

C8H17O

O

S

TLEP-1

N

2

CHO

5

OC8H17

(vi)

64.9%

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O

S

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OC8H17

OC8H17

N

40.1% CHO

3

2

1

O

S

S

81.9%

CHO

7

OC8H17

OC8H17

N S

C8H17O

8

CHO

N

67.6%

S

S

CN COOH

C8H17O

TE D

S

(v)

TLEP-2

AC C

EP

Scheme 1. Synthesis of TLEP-1 and TLEP-2. (i) CHCl3, Br2, 0 oC, 8 h; (ii) DMF, K2CO3, Pd(PPh3)4, (5-formylfuran-2-yl)boronic acid, 80 oC, 48 h; (iii) DMF, K2CO3, Pd(PPh3)4, (4-hydroxyphenyl)boronic acid, 80 oC, 48 h; (iv) DMF, K2CO3, 1bromooctane, 130 ˚C, 24 h; (v) Cyanoacetic acid, piperidine, acetonitrile, reflux, 8 h.

28

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AC C

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Fig. 1

29

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AC C

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Fig. 2

30

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AC C

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Fig. 3

31

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Fig. 4

32

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Fig. 5

33

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AC C

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Fig. 6

34

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AC C

EP

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Fig. 7

35

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AC C

EP

TE D

Fig. 8

36

ACCEPTED MANUSCRIPT

Highlights:

AC C

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1. Two dyes with trilateral π-conjugation extensions of phenothiazine were synthesized. 2. Trilateral π–conjugation extensions enhance short-circuit current density significantly. 3. The DSSC obtains high power conversion efficiency of 7.33%.