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Synthesis of double D–A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs
Accepted Manuscript Synthesis of double D−A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs Yanping Hong, Zafar Iqbal, Xiaoli Yin, Derong Cao PII:
S0040-4020(14)00448-7
DOI:
10.1016/j.tet.2014.03.086
Reference:
TET 25427
To appear in:
Tetrahedron
Received Date: 15 February 2014 Revised Date:
18 March 2014
Accepted Date: 24 March 2014
Please cite this article as: Hong Y, Iqbal Z, Yin X, Cao D, Synthesis of double D−A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs, Tetrahedron (2014), doi: 10.1016/j.tet.2014.03.086. 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
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Comparing to the dye RD with a single D−A unit, the dye SDD4 with phenoxazine as donor shows better photovaltic performance due to its two D−A light-harvesting units and two anchoring groups.
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Synthesis of double D−A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs
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Yanping Hong a, Zafar Iqbal b,c, Xiaoli Yin d, Derong Cao b,* a
Jiangxi Key Laboratory of Natural Product and Functional Food, College of Food Science and Engineering, Jiangxi Agricultural University, Nanchang 330045, PR China b
c
PCSIR Laboratories Complex, Feroze pur Road Lahore 54000, Pakistan
Library of Jiangxi Agricultural University, Nanchang 330045, PR China
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School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, PR China
*Corresponding Author: D. Cao, Tel.: +86-20-87110245. Fax: +86-20-87110245. Email: [email protected] Abstract: Four novel metal-free organic sensitizers bearing double donor-acceptor (D− −A) branches with indole and phenoxazine units as donors (SDD1− −4) and cyanoacrylic acid as electron acceptor were synthesized and characterized for dye-sensitized solar cells (DSSCs). Dyes
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SDD1− −3 were designed with indole as a donor and 1,4-phenylenebis(methylene), 1,4-butylene and 1,6-hexylene as a linker, respectively, while the dye SDD4 was designed with phenoxazine as a core donor. Their photophysical, electrochemical and DSSCs characteristics were investigated. The results show that the architecture structure of the linkage affects the performance of the cells −3, slightly. The DSSCs based on SDD4 shows much higher η than the DSSCs based on SDD1− which indicated that phenoxazine is a better donor than indole. Under standard global AM 1.5
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solar condition (100 mW cm−2), the SDD4 dye sensitized cell gave the highest η of 4.33% with chenodeoxycholic acid as a co-adsorbent, reaching 82% of N719-based DSSCs.
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Keywords: Dye-sensitized solar cell; Organic dye; Double donor-acceptor branches; Photovoltaic; Phenoxazine; Indole 1. Introduction
Since the first successful fabrication of sandwich type solar cell by Grätzel in 1991,1 dye sensitized solar cells (DSSCs) have received significant research interest in both academic and industrial fields, which are owing to their efficient, highly adaptable, economical feasible and relatively less environmental issues than traditional silicon-based solar cells. DSSCs have three main components i.e photoanode, electrolyte and sensitizer. Among these components the sensitizers play an important role of capturing photon and generating electron which are injected into conduction band of semiconductor (e.g TiO2). Tremendous endeavors have been made to search for efficient sensitizers to enhance the efficiency of DSSCs. Power conversion efficiencies over 11% have been achieved with ruthenium (Ru)-polypyridyl sensitizers,2-4 and a record
ACCEPTED MANUSCRIPT efficiency of DSSCs up to 12.3% was achieved with zinc porphyrin by Yella et al in 2011.5 However, the drawbacks concerning Ru-complexes such as rare sources, difficult purification and relatively lower molar extinction coefficients limit its application. Recently, more and more attention has been focused in the application of metal-free organic dyes as auspicious alternatives to the Ru-complexes due to their high molar extinction coefficient, flexible structural tailoring as well as low cost and environmental friendliness. In the design of the organic dyes, the dipolar
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donor−π spacer−acceptor (D−π−A) system is considered as the most widely adopted molecular configuration because the photoinduced intramolecular electron can be transferred into TiO2 via the D−π−A channel6. Up till now, hundreds of metal-free dyes have been employed as sensitizers for DSSCs and have exhibited impressive photovoltaic efficiencies.7-15 To enhance the photovoltaic performance of the DSSCs, considerable research efforts have been
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devoted to optimized the structure of the organic dyes, such as D−D−π−A,16,17 D−π−A−A,18,19 A−π−D−π−A,20,21 D−A−π−A,22-25 (D−π−A)226 and so on. Recently, we designed several novel organic sensitizers featuring double D−π−A branches (DB dyes).27-29 It was found that the conversion efficiency of DSSCs based on the DB dyes was increased as compared to the corresponding single counterparts due to their better light-harvesting ability. In addition, the donors play a critical function in light harvesting and electron injection.30-34 In this study, we designed and synthesized a series of DB dyes SDD1−4 and a reference sensitizer with single D−π−A system (RD). As shown in Scheme 1, SDD1−4 were designed with the following characteristics: (i) two D−π−A branches were linked by non-conjugated alkyl chain, resulting in two separate light harvesting units, which might enhance the absorption intensity due to the
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absorption overlap of both the same donors and π-spacers. (ii) Indole and phenoxazine donors were introduced into the D−π−A branches as the electron donor units. As a consequent, the photovoltaic performance of the cells based on these dyes would be influenced by the structure of the linkages, such as different length of flexible alkyl chains and the rigid alkyl chain. Moreover, the HOMO and LUMO energy levels of the dyes would be tuned by the different donors, as a result, the conversion efficiencies of the DSSCs would be affected due to the changed absorption range and intensity of the dyes.
Fig. 1. The structure of the dyes SDD1− −4 and RD
2. Results and discussion 2.1. Synthesis and structural characterization
ACCEPTED MANUSCRIPT The synthetic route of the dyes is shown in Scheme 1. Intermediate 1 was prepared from arylamine via alkylation reaction with dibromoalkane or 1-bromoalkane in the presence of KOH in DMSO, respectively. The intermediates 1 were then converted to the corresponding dialdehydes 2 by Vilsmeier-Haack reaction in the presence of DMF and POCl3. Finally, the dialdehydes 2 were
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condensed with cyanoacetic acid to give the target dyes (SDD1−4 and RD) by the Knoevenagel condensation reaction in the presence of piperidine. All the intermediates and target dyes were confirmed by standard spectroscopic methods.
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Scheme 1. Synthesis of the dyes SDD1− −4 and RD.
2.2. Photophysical Properties The UV−vis and emission spectra of the dyes in THF solution are displayed in Fig. 2a and Fig. 2b, respectively, and their corresponding photophysical and electrochemical properties are summarized in Table 1. All the dyes exhibit two distinct absorption bands: first absorption band in the region of 270−310 nm is ascribed to the π−π* transition of the conjugated molecules; and the second band in the region of 380−480 nm which is assigned to intramolecular charge transfer (ICT) between the electron donating unit and the electron accepting units. The spectra of SDD1−SDD3 are quite similar, whose maximum absorption are around 380 nm. The maximum absorption peak of SDD4 is red-shifted by near ca. 100 nm compared to the dyes SDD1−3 due to the stronger electron-donating nature of the phenoxazine than indole. It is worth mentioning that the molar absorption coefficient (ε = 39066 M−1 cm−1) of SDD4 is almost twice as high as that of reference dye RD (ε = 19628 M−1 cm−1), which indicates that the absorption of SDD4 is enhanced and
ACCEPTED MANUSCRIPT overlapped by the two D−π−A light-harvesting units in one molecule. The absorption properties of the dyes in the solution are consistent with the energy optical band gap (E0–0) between the HOMO and LUMO energy levels, and can also be proved by the molecular optimization results of the dyes (Fig. 3). 2.3. Electrochemical properties
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To gain deeper insight into the possibility of regeneration of the dyes and electron injection from the excited state of the dyes to the conduction band (CB) of TiO2 photoanode, the electrochemical properties of the dyes were studied by cyclovoltametry (CV) (Table 1). The first half-wave potentials (E1/2 vs. Fc/Fc+) of SDD1-4 and RD adsorbed on TiO2 film was measured at 0.43, 0.40, 0.41, 0.21 and 0.19 V, respectively. Therefore, the first oxidation potentials (Eox) versus normal hydrogen electrode (NHE), corresponding to the highest occupied molecular orbital (HOMO) levels were 1.06, 1.03, 1.04, 0.84 and 0.82 V calibrated by addition of 0.63 V to the potential vs Fc/Fc+ (vs NHE), respectively. On the other hand, the HOMO values versus vacuum
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were estimated via the equation EHOMO= −e [4.88+ E1/2 (vs. Fc/Fc+)]35 and the corresponding EHOMO of SDD1− −4 and RD versus vacuum were −5.31, −5.28, −5.29, −5.09 and −5.07 eV, respectively. The sufficient low oxidation potential of all the dyes were more positive than I−/I3− redox couple potential value (0.4 V vs NHE or −4.60 eV vs vacuum)36 ensuring more effective dye regeneration and restraining the recapture of the injected electrons by the dye cation radical or
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I−/I3− redox couple.
Fig. 2. (a) Absorption spectra and (b) Normalized absorption and emission spectra of the dyes in THF.
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Table 1 Absorption, emission and electrochemical properties of the dyes SDD1− −4 and RD
Absorption
Emission
Oxidation Potential data E0-0
EHOMO/ELUMOc)
εmax
λmax
Eox
Eox − E0−0
Egapb)
THF(nm)
(M−1cm−1)
(nm)
(V, vs NHE)
(V)a)
(V, vs NHE)
(V, vs NHE)
SDD1
378
34686
440(378)
1.06
3.03
−1.97
1.47
−5.31/−2.28
−5.92/−2.52
SDD2
380
43513
466(380)
1.03
3.00
−1.97
1.47
−5.28/−2.28
−5.98/−2.56
SDD3
379
35769
444(379)
1.04
3.02
−1.98
1.48
−5.29/−2.27
−5.94/−2.46
SDD4
476
39066
576(476)
0.84
2.30
−1.46
0.96
−5.09/−2.79
−5.28/−2.42
RD
474
19628
471(474)
0.82
2.32
−1.50
1.00
−5.07/−2.75
−5.17/−2.31
Dye
λmax
a
in
EHOMO/ELUMO
(eV)
(eV)
b
E0-0 were estimated from the intersection of the normalized absorption and emission spectra (see Fig. 2b). Egap is the energy gap
between the Eox − E0-0 of the dye and the conductive band level of TiO2 (−0.5 V vs NHE). c Calculated at the B3LYP-31G(d) level in vacuum.
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Fig. 3. The optimized structure and the frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/6-31+G(d) level of the dyes ( d Energy level of the I3−/I− redox couple (−4.60 eV vs vacuum);36 Energy level of the conduction band edge of TiO2 (−4.00 eV vs vacuum) 37).
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The estimated excited state potentials corresponding to the lowest unoccupied molecular orbital (LUMO) levels, were −1.97, −1.97, −1.98, −1.46 and −1.50 V (vs NHE) calculated from Eox − E0–0, where the E0–0 (E0–0 = 1240/λint) was estimated from the intersection of the normalized absorption and emission spectra (λint) (Fig. 2b). The corresponding ELUMO value versus vacuum which are calculated by [EHOMO + E0–0] are −2.28, −2.28, −2.27, −2.79 and −2.75 eV, respectively.
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The more negative LUMO level of these dyes relative to the conduction band edge of the TiO2 electrode (−0.5 V vs NHE or −4.00 eV vs vacuum37) , providing that an energy gap (Egap) of 0.2 eV is necessary for efficient electron injection.38 Hence, an effective electron transfer from the excited dye molecule to TiO2 should be energetically favorable. As shown in Table 1, the
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HOMO−LUMO band gap (E0-0) of SDD1− −4 and RD dyes were 3.03 eV, 3.00 eV, 3.02 eV, 2.30 eV, 2.32 eV, respectively. Compared to SDD1− −3, SDD4 and RD has a lower HOMO−LUMO energy band gap which is due to stronger electron-donating accepting ability. The stronger donor lifts the HOMO level and decreases the change in the LUMO energy level. Dye SDD4 has the smallest band gap, causing the red-shifted absorption spectra, which is ideally preferred for DSSCs application. On the other hand, the LUMO and HOMO values of SDD1− −3 are similar. The more positive HOMO value is preferable in regenerating the dyes, and the more negative LUMO value results in a large possibility of electronic injection into the CB of TiO2. However, the higher HOMO−LUMO gap of SDD1− −3 leads to absorb shorter wavelength sunlight, which is in accordance with their absorption spectra. The low light-harvesting characteristic of SDD1-3 means inferior performance of their DSSCs. 2.4 Molecular orbital calculations To learn more on the geometry and electronic properties of the dyes, density functional theory (DFT) calculations were performed at a B3LYP/6-31+G (d) level with Gaussian 03. Fig. 3 shows the optimized molecular structures and frontier molecular orbitals of the dyes. The electrons at HOMO level of all the dyes were populated on both the donors (indole and phenoxazine) and π-bridge, and the LUMO levels were sizably delocalized through the cyanoacrylic acid and its
ACCEPTED MANUSCRIPT adjacent π-bridge fragments, indicating that the HOMO−LUMO excitation moves the electron distribution from the donor to the cyanoacrylic acid moiety, thus facilitating an efficient photoinduced/interfacial electron transfer from the excited dye molecule to the TiO2 electrode through the effective intramolecular charge separation under light irradiation. 2.5. Photovoltaic performances of the DSSCs The detailed photovoltaic parameters (short-circuit photocurrent density (Jsc), open-circuit
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photovoltage (Voc), fill factor (ff) and photovoltaic conversion efficiency (η)) of all the dyes are summarized in Table 2 and the current−voltage (J−V) curves of the dyes are demonstrated in Fig. 4. Among these dyes, the cell based on SDD4 gave a maximum η of 3.92% (Jsc = 6.81 mA cm−2, Voc = 743 mV, ff = 0.62) under standard global AM 1.5 solar light irradiation (100 mW cm−2), while
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the cells sensitized with SDD1−3 and RD gave Jsc of 1.63, 1.87, 1.91 and 5.94 mA cm−2, Voc of 632, 640, 692 and 759 mV and ff of 0.70, 0.72, 0.71 and 0.64, corresponding to η of 0.91, 1.07, 1.19 and 3.61%, respectively. Under the same measuring conditions, the cell based on N719
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showed an efficiency of 5.25% with a Jsc of 8.63 mA cm−2, a Voc of 777mV, and a ff of 0.63.
Fig. 4. J−V curves of DSSCs sensitized by SDD1−4, RD and N719.
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Table 2 Photovoltaic performance of DSSCs based on dyes SDD1−4, RD and N719 Dyes
a
Jsc
Voc -2
ff
η
Dye amount
(%)
(10−7mol⋅cm-2)
(mA⋅cm )
(mV)
N719a
8.63
777
0.63
5.25
−
SDD1
1.63
632
0.70
0.91
1.05
SDD2
1.87
640
0.72
1.07
1.15
SDD3
1.91
692
0.71
1.19
1.34
SDD4
6.81
743
0.62
3.92
2.05
RD
5.94
759
0.64
3.61
0.98
The concentration of dye-bath for N719 was 3 × 10-4 M in ethanol.
Table 3 Photovoltaic performance of DSSCs based on SDD4 and RD sensitized in solutions containing different concentration of CDCAa
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2
(mM) 0
0.1
1.0
10.0
Saturated
Voc(mV)
ff
η(%)
Dye amount (10−7mol⋅cm-2)
)
RD
5.94
759
0.64
3.61
0.982
SDD4
6.81
743
0.62
3.92
2.053
RD
4.65
728
0.64
2.71
0.809
SDD4
6.41
749
0.63
3.82
1.981
RD
4.23
732
0.65
2.53
0.712
SDD4
7.11
753
0.65
4.33
RD
3.24
729
0.69
2.04
SDD4
7.04
750
0.64
4.22
RD
2.86
689
0.66
1.64
SDD4
6.64
747
0.65
4.03
−4
1.713
0.335 1.435 0.141 1.205
Dye-bath: SDD4 and RD in THF solution (2 × 10 M) with addition of CDCA solution (10-3 M).
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a
Jsc(mA⋅cm-
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CDCA
Compared to those cells based on the dyes SDD4 and RD, the relatively lower photovoltaic
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performance of double D−A dyes SDD1−3-sensitized cells was due to their relatively lower short-circuit photocurrent density (Jsc), which could be ascribed to the donor of indole with weak and narrow light-harvesting ability in the visible region. On the other hand, the cell based on SDD3 shows higher efficiency compared to the SDD1 and SDD2 based cells due to its higher Jsc and higher Voc. The higher Jsc may be attributed to the relatively bigger dye loading on the TiO2 surface, and the higher Voc may be ascribed to the n-hexyl which hinders the dye SDD3
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aggregation on TiO2 more effectively. Furthermore, the cell based on SDD1 exhibited lower η due to its lower dye loading amount, which resulted from the rigid and large linker of 1,4-phenylenebis(methylene). It has been reported that chenodeoxycholic acid (CDCA) can enhance the performance of the DSSCs because CDCA can reduce the aggregation of the dyes on the TiO2 surface. We studied the photovoltaic performance of the SDD4 and RD-sensitized cells in the presence of CDCA with different concentrations. After the addition of CDCA to the dye bath, the adsorbed amount of the dyes decreased gradually with the further increasing concentration of CDCA (from 0 to a saturated concentration). The drop of the adsorption of SDD4 and RD indicated that CDCA competed for the TiO2 surface sites with the dyes molecules, and coadsorption of CDCA diminished the dye-dye interactions on the TiO2 surface. From the data of Table 3, it can be seen that the efficiencies of the cells based on RD and CDCA as coadsorbent decreased gradually as well as the dye adsorbed amount of RD. However, the efficiencies of the cells with SDD4 and CDCA as coadsorbent increased. A highest η value of 4.33% of the cell sensitized with SDD4 and CDCA as coadsorbent was obtained at 1.0 mM CDCA bath, which increased by 10.5% compared with SDD4 sensitized cell (3.92%), reaching 82% of a standard device fabricated with N719 (η = 5.25%). It could be explained that the suitable coverage density of SDD4 molecules resulted in higher Jsc and higher Voc of cell. A possible explanation is that the dye RD did not aggregate on the TiO2 surface, but the amount of dye adsorbed on the TiO2 surface was reduced by the coadsorption of CDCA, resulting in a loss of active light-harvesting; however, the dye SDD-4 aggregated on the TiO2 surface, and such an aggregation was reduced by CDCA, resulting in an enhancement of the efficiencies of the cells. 3. Conclusion
ACCEPTED MANUSCRIPT In summary, four novel double donor-acceptor (D− −A) branched dyes SDD1−4 with indole and phenoxazine units as donors were designed and synthesized, their photophysical and electrochemical properties were also investigated. The cell sensitized by SDD1−3 with indole as donor exhibited low η because they absorb only short length sunlight. Among SDD1−3, the photophysical and electrochemical properties were influenced slightly by the linker of alkyl chain
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between the D−π−A units. The SDD4-sensitized cell shows much better photovaltic performance, which are attributed to the stronger electron-donating property of the phenoxazine donor. SDD4 shows better photovaltic performance than the reference dye RD, which may be ascribed to its two D−A light-harvesting units and two anchoring groups. An optimal DSSC based on SDD4 with coadsorption of chenodeoxycholic acid exhibited highest η of 4.33%, which reached 82% of N719-based DSSC (5.25%). The results reveal that the organic
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dyes featuring two D−A branches with phenoxazine unit as donor are promising in the development of DSSCs. 4. Experimental section
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4.1. Instrumentation and materials
All reactions were performed under nitrogen atmosphere. The solvents were purified by standard procedures and purged with nitrogen before use. Chromatographic separations were carried out on silica gel (60M, 200-400 mesh). 1H NMR and 13C NMR spectra were recorded on a Bruker 400 MHz spectrometer in CDCl3 or DMSO-d6 with tetramethylsilane as inner reference. MS data were recorded on an Esquire HCT PLUS mass spectrometer. Elementary analyses were performed using a Vario EL Analyzer. The melting point was taken on Tektronix X4 microscopic melting point apparatus and uncorrected. The absorption and emission spectra of the
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dyes in THF solution (2 × 10−5 M) were measured at room temperature by Shimadzu UV-2450, UV−Vis spectrophotometer and Fluorolog III photoluminescence spectrometer, respectively. Electrochemical redox-potentials were obtained by cyclic voltammetry (CV) using a three-electrode cell and an electrochemistry workstation (e-corder (ED 401) potentiostat). The dyes sensitized TiO2 films were used as a working electrode. The Ag/AgCl in saturated KCl solution and Pt wire were utilized as reference and counter electrodes with a scan rate of 50 mV/s. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was used as supporting electrolyte in CH3CN. The measurements were calibrated using ferrocene as standard. The redox potential of ferrocene internal reference is taken as 0.63V versus normal hydrogen electrode (NHE). The nanoporous TiO2/FTO glass films (the thickness of TiO2 photoelectrode and scattering layer were 14 µm and 4 µm, working area: 0.16 cm2) and counter electrode (FTO glass coated Pt) were purchased from Yingkou OPV Tech New Energy Co., Ltd. of Liaoning province. Heated at 450 °C followed by cooling to 80 °C, the TiO2 film was soaked in ethanol solution containing 5×10-4 M N719 or in solution containing 2 × 10-4 M organic sensitizer for 12 h. The film was then rinsed with THF in order to remove physisorbed organic dye molecules. To evaluate their photovoltaic performance, the dye-sensitized TiO2 films were sandwiched together with counter electrode. The counter electrode was placed directly on the top of the dye-sensitized TiO2 film sealed with thermal adhesive film (~25 µm), and the electrolyte (0.6 M 1-methyl-3-propylimidazolium iodide (PMII), 0.10 M guanidinium thiocyanate, 0.03 M I2, 0.5 M tert-butylpyridine in acetonitrile and valeronitrile (85:15)) was injected from a hole made on the counter electrode into the space between the sandwiched cells. The current–voltage measurements were carried out by adopting a
ACCEPTED MANUSCRIPT Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mW cm−2) provided by solar simulator (9600, Oriel). A 1 KW Xenon arc lamp (6271, Oriel) was served as a light source. The incident light intensity was calibrated with a NREL standard Si solar cell. The individual dye amount absorbed on TiO2 film was determined according to the literature procedure.39 4.2 Syntheses
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4.2.1. 1,4-Bis[(indol-1-yl)methylene)]benzene (1a),40 1,4-bis(indol-1-yl)butane (1b),41 1,6-bis(indol-1-yl)hexane (1c),41 1,4-bis[(3-formyl-indol-1-yl)methylene]benzene (2a)42 and 1,4-bis(3-formyl-indol-1-yl)butane (2b)43 were synthesized according to the literature procedure.
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4.2.2. Synthesis of 1,6-Bis(phenoxazin-10-yl)haxane (1d). Phenoxazine (0.732 g, 4.0 mmol) was added to a suspension of potassium hydroxide (0.448 g, 8.0 mmol) in dimethyl sulfoxide (15 mL) under nitrogen. The mixture was stirred for 30min,then 1,6-dibromohexane (0.488 g, 2.0 mmol) was added. The reaction mixture was stirred overnight at room temperature. Water was added and extracted with dichloromethane (3 × 30 mL), the organic phases were then washed with brine, dried with anhydrous sodium sulfate and evaporated. The residue was purified by chromatography
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on silica gel (dichloromethane/petroleum = 1 : 7) to give 1d as a colorless solid (0.84 g, 93.7%). Mp: 163−164°C. 1H NMR (400 MHz, CDCl3): δ 1.48−1.53 (m, 2 × 2H), 1.66−1.74 (m, 2 × 2H), 3.50 (br. s, 2 × 2H), 6.45 (br. s, 2 × 2H), 6.55−6.74 (m, 2 × 4H), 6.76−6.81 (m, 2 × 2H). 13C NMR (100 MHz, CDCl3): δ 25.0, 26.7, 43.8, 111.2, 115.4, 120.8, 123.6, 133.3, 145.0. MS (ESI): m/z [M+Na]+ calcd for C30H28N2O2: 471.2; found: 471.2. Anal. calcd for C30H28N2O2: C, 80.33; H, 6.29; N, 6.25; found: C, 80.07; H, 6.32; N, 6.28.
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4.2.3. Synthesis of 10-Hexyl-phenoxazine (1e). It was prepared as a colorless liquid (0.968 g, 90.8%) from phenoxazine and 1-bromohexane by using the same method established for 1d. 1H NMR (400 MHz, CDCl3): δ 0.93 (t, J = 6.4 Hz, 3H), 1.34−1.44 (m, 6H), 1.63−1.68 (m, 2H), 3.47 (br. s, 2H), 6.46-6.48 (m, 2H), 6.63 (br. s, 4H), 6.78−6.81 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 14.1, 22.7, 24.9, 26.6, 31.6, 44.1, 111.3, 115.3, 120.7, 123.6, 133.5, 145.0.
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4.2.4. Synthesis of 1,6-Bis(3-formyl-indol-1-yl)hexane (2c). To a solution of 1c (0.633 g, 2.0 mmol) and dry N,N-dimethylformamide 1.17 g (16.0 mmol) in 1,2-dichloroethane (15 mL) was slowly
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added phosphorus oxychloride 2.46 g (16.0 mmol) at 0 °C in an ice−water bath. Then the bath was heated to 90 °C and maintained for 5 h. Dilute aqueous solution of sodium acetate was added and stirred for 3 h, and the mixture was extracted three times with dichloromethane(3× 20 mL). The combined organic phases were washed with brine and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by column chromatography with ethyl acetate/dichloromathane (v/v = 1/50) as the eluent to give 2c as a light yellow solid (0.477 g, 64.0%). Mp 211−212 °C. 1H NMR (400 MHz, CDCl3): δ 1.32−1.37 (m, 2 × 2H), 1.83−1.91 (m, 2 × 2H), 4.15 (t, J = 6.8 Hz, 2 × 2H), 7.31−7.35 (m, 2 × 3H), 7.66 (s, 2 × 1H), 8.29−8.31 (m, 2 × 1H), 9.99 (s, 2 × 1H). MS (ESI): m/z [M−H]+ calcd for C24H24N2O2: 371.2; found: 371.2. Anal. calcd for C24H24N2O2: C, 77.39; H, 6.49; N, 7.52; found: C, 77.07; H, 6.52; N, 7.55. 4.2.5. Synthesis of 1,6-Bis(3-formyl-phenoxazin-10-yl)hexane (2d). It was prepared as a greenish-yellow solid (0.786 g, 77.9%) from 1d by using the same method established for 2c. Mp 297−298 °C. 1H NMR (400 MHz, CDCl3): δ 1.50−1.54 (m, 2 × 2H), 1.70−1.77 (m, 2 × 2H), 3.56 (t, J = 8.0Hz, 2 × 2H,), 6.48−6.53 (m, 2 × 2H), 6.65−6.68 (m, 2 × 1H), 6.72−6.76 (m, 2 × 1H),
ACCEPTED MANUSCRIPT 6.79−6.83 (m, 2 × 1H), 7.08−7.09 (m, 2 × 1H), 7.28−7.31 (m, 2 × 1H), 9.67 (s, 2 × 1H). 13C NMR (100 MHz, C5H5N-d5): δ 25.2, 26.7, 44.0, 111.5, 113.0, 114.1, 116.1, 189.8 (the carbon shift values of the aromatic ring can not be observed in the range of 123.0~124.6 beacuse they were shielded by the solvent peaks of C5H5N-d5). MS (ESI): m/z [M]+ calcd for C32H28N2O4: 504.2; found: 503.8. Anal. calcd for C32H28N2O4: C, 76.17; H, 5.59; N, 5.55; found: C, 76.43; H, 5.62; N, 5.53.
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4.2.6. Synthesis of 10-Hexyl-phenoxazine-3-carbaldehyde (2e). It was prepared as a yellow liguid (0.517 g, 87.5%) from 1e by using the same method established for 2c. 1H NMR (400 MHz,
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CDCl3): δ 0.92 (t, J = 7.2Hz, 3H), 1.34−1.42 (m, 6H), 1.60−1.67 (m, 2H), 3.46 (t, J = 8.0 Hz, 2H), 6.44−6.46 (m, 1H), 6.48−6.50 (m, 1H), 6.59−6.62 (m, 1H), 6.67−6.71 (m, 1H), 6.77−6.81 (m, 1H), 7.01−7.02 (m, 1H), 7.24−7.27 (m, 1H), 9.61 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 14.1, 22.7, 25.0, 26.6, 31.6, 44.4, 110.5, 112.1, 114.1, 115.7, 122.5, 124.0, 128.8, 129.8, 131.4, 139.3, 144.7, 145.1, 189.6. MS (ESI): m/z [M+Na]+ calcd for C19H21NO2: 318.2; found: 318.2. Anal. calcd for C19H21NO2: C, 77.26; H, 7.17; N, 4.74; found: C, 77.19; H, 7.20; N, 4.73.
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4.2.7. Synthesis of 1,4-Bis{[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]methylene}benzene (SDD1). A solution of 2a (0.493 g, 1.0 mmol) in chloroform (15 mL) was condensed with 2-cyanoacetic acid (0.850 g, 10.0 mmol) in the presence of piperidine (0.20 mL, 2.0mmol). The mixture was refluxed for 8 h under nitrogen. After cooling to room temperature, the mixture was poured into 2M aqueous of HCl. The crude solid product was filtered and washed with dichloromethane, water and methanol respectively. The final product SDD1 was obtained as a yellow solid (0.448g, 85.2%). Mp 275−276 °C. 1H NMR (400 MHz, DMSO-d6): δ 6.09 (s, 2 × 2H), 7.69−7.73 (m, 2 × 5H), 8.02−8.04 (m, 2 × 1H), 8.39−8.41 (m, 2 × 1H), 8.95 (s, 2 × 1H), 9.13 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 49.6, 99.0, 109.5, 111.5, 118.6, 119.2, 121.9, 123.4, 127.6, 127.6, 133.0,
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136.0, 136.4, 142.8, 164.8. MS (ESI): m/z [M−H]− calcd for C32H22N4O4: 525.2; found: 525.3. Anal. calcd for C18H22BrN: C, 72.99; H, 4.21; N, 10.64; found: C, 73.02; H, 4.23; N, 10.58. 4.2.8. Synthesis of 1,4-Bis[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]butane (SDD2). It was prepared as a yellow solid (0.464 g, 97.2%) from 2b by using the same method established for
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SDD1. Mp 236−237 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.83 (br. s, 2 × 2H), 4.41 (br. s, 2 × 2H), 7.24−7.33 (m, 2 × 2H), 7.65−7.67 (m, 2 × 1H), 7.92−7.94 (m, 2 × 1H), 8.44 (br. s, 2 × 1H), 8.52 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 26.8, 46.1, 95.1, 109.1, 111.4, 118.5, 118.7, 122.2, 123.5, 127.5, 133.8, 136.1, 144.7, 164.6. MS (ESI): m/z [M−H]− calcd for C28H22N4O4: 477.2; found: 477.2. Anal. calcd for C28H22N4O4: C, 70.28; H, 4.63; N, 11.71; found: C, 70.37; H, 4.65; N, 11.66. 4.2.9. Synthesis of 1,6-Bis[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]haxane (SDD3). It was prepared as a yellow solid (0.393 g, 77.7%) from 2c by using the same method established for SDD1. Mp 228−230 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.28−1.33 (m, 2 × 2H), 1.72−1.80 (m, 2 × 2H), 4.33 (t, J = 6.8 Hz, 2 × 2H), 7.25−7.33 (m, 2 × 2H), 7.62−7.64 (m, 2 × 1H), 7.93−7.95 (m, 2 × 1H), 8.46 (s, 2 × 1H), 8.52 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 25.6, 29.2, 46.6, 93.8, 109.0, 111.4, 118.4, 118.8, 122.4, 123.6, 127.5, 134.3, 136.2, 145.4, 164.7. MS (ESI): m/z [M−H]− calcd for C30H26N4O4: 505.2; found: 505.3. Anal. calcd for C30H26N4O4: C, 71.13; H, 5.17; N, 11.06; found: C, 71.29; H, 5.19; N, 11.01. 4.2.10. Synthesis of 1,6-Bis[3-((E)-2-carboxyl-2-cyanovinyl)phenoxazin-10-yl]haxane (SDD4). It
ACCEPTED MANUSCRIPT was prepared as a red−black solid (0.604 g, 94.8%) from 2d by using the same method established for SDD1. Mp 175−177 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.46 (br. s, 2 × 2H), 1.54 (br. s, 2 × 2H), 3.58 (br. s, 2 × 2H), 6.70−6.82 (m, 2 × 5H), 7.35 (br. s, 2 × 1H), 7.46−7.78 (m, 2 × 1H); 7.96 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 24.5, 25.7, 42.5, 97.6, 111.7, 112.9, 114.6, 115.4, 117.1, 122.6, 123.7, 124.3, 130.5, 131.0, 137.8, 143.6, 143.8, 152.4, 164.0. MS (ESI): m/z [M−H]− calcd for C38H30N4O6: 637.2; found: 637.5. Anal. calcd for C38H30N4O6: C, 71.46; H, 4.73; N, 8.77; found: C, 71.40; H, 4.71; N, 8.76.
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4.2.11. Synthesis of (E)-2-Cyano-3-(10-hexyl-10H-phenoxazin-3-yl)acrylic acid (RD). It was prepared as a red solid (0.263 g, 83.1%) from 2e by using the same method established for SDD1.
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Mp 185−186 °C. 1H NMR (400 MHz, DMSO-d6): δ 0.85 (t, 3H), 1.26−1.29 (m, 4H), 1.32−1.39 (m, 2H), 1.47−1.54 (m, 2H), 3.54 (q, 2H), 6.66−6.72 (m, 4H), 6.81−6.84 (m, 1H), 7.33−7.34 (m, 1H), 7.44−7.46 (m, 1H), 7.94 (s, 1H). 13C NMR (100 MHz, DMSO-d6): δ 13.8, 22.1, 24.5, 25.6, 31.0, 43.1, 97.8, 111.6, 112.8, 114.5, 115.3, 117.1, 122.5, 123.7, 124.3, 130.6, 131.0, 137.8, 143.6, 143.8, 152.2, 164.0. MS (ESI): m/z [M−COOH]− calcd for C22H22N2O3: 317.2; found: 316.9. Anal. calcd for C22H22N2O3: C, 72.91; H, 6.12; N, 7.73; found: C, 72.73; H, 6.15; N, 7.76.
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Acknowledgment
The authors thank the National Natural Science Foundation of China (21272079, 61368006) for the financial support. References and notes
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1. O'Regan, B.; Grätzel, M. Nature 1991, 353, 737-40. 2. Nazeeruddin, M. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C.-H.; Graetzel, M. Inorg. Chem. 1999, 38, 6298-6305. 3. Grätzel, M. J. Photochem. Photobiol., A 2004, 164, 3-14. 4. Gao, F.; Wang, Y.; Shi, D.; Zhang, J.; Wang, M.; Jing, X.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. J. Am. Chem. Soc. 2008, 130, 10720-10728. 5. Yella, A.; Lee, H.-W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.; Grätzel, M. Science 2011, 334, 629-634. 6. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595-6663. 7. Yeh-Yung Lin, R.; Lin, H.-W.; Yen, Y.-S.; Chang, C.-H.; Chou, H.-H.; Chen, P.-W.; Hsu, C.-Y.; Chen, Y.-C.; Lin, J. T.; Ho, K.-C. Energy Environ. Sci. 2013, 6, 2477-2486. 8. Liu, J.; Numata, Y.; Qin, C.; Islam, A.; Yang, X.; Han, L. Chem. Commun. 2013, 49, 7587-7589. 9. Iqbal, Z.; Wu, W.-Q.; Zhang, H.; Han, L.; Fang, X.; Wang, L.; Kuang, D.-B.; Meier, H.; Cao, D. Organic Electronics 2013, 14, 2662-2672. 10. Lim, K.; Ju, M. J.; Na, J.; Choi, H.; Song, M. Y.; Kim, B.; Song, K.; Yu, J.-S.; Kim, E.; Ko, J. Chem. Eur. J. 2013, 19, 9442-9446. 11. Zhang, H.; Fan, J.; Iqbal, Z.; Kuang, D.-B.; Wang, L.; Cao, D.; Meier, H. Dyes Pigm. 2013, 99, 74-81. 12. Zhang, H.; Fan, J.; Iqbal, Z.; Kuang, D.-B.; Wang, L.; Meier, H.; Cao, D. Organic Electronics 2013, 14, 2071-2081. 13. Tamilavan, V.; Kim, A. Y.; Kim, H.-B.; Kang, M.; Hyun, M. H. Tetrahedron 2014, 70,
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S0040-4020(14)00448-7
DOI:
10.1016/j.tet.2014.03.086
Reference:
TET 25427
To appear in:
Tetrahedron
Received Date: 15 February 2014 Revised Date:
18 March 2014
Accepted Date: 24 March 2014
Please cite this article as: Hong Y, Iqbal Z, Yin X, Cao D, Synthesis of double D−A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs, Tetrahedron (2014), doi: 10.1016/j.tet.2014.03.086. 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
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Comparing to the dye RD with a single D−A unit, the dye SDD4 with phenoxazine as donor shows better photovaltic performance due to its two D−A light-harvesting units and two anchoring groups.
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Synthesis of double D−A branched organic dyes employing indole and phenoxazine as donors for efficient DSSCs
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Yanping Hong a, Zafar Iqbal b,c, Xiaoli Yin d, Derong Cao b,* a
Jiangxi Key Laboratory of Natural Product and Functional Food, College of Food Science and Engineering, Jiangxi Agricultural University, Nanchang 330045, PR China b
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PCSIR Laboratories Complex, Feroze pur Road Lahore 54000, Pakistan
Library of Jiangxi Agricultural University, Nanchang 330045, PR China
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School of Chemistry and Chemical Engineering, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510641, PR China
*Corresponding Author: D. Cao, Tel.: +86-20-87110245. Fax: +86-20-87110245. Email: [email protected] Abstract: Four novel metal-free organic sensitizers bearing double donor-acceptor (D− −A) branches with indole and phenoxazine units as donors (SDD1− −4) and cyanoacrylic acid as electron acceptor were synthesized and characterized for dye-sensitized solar cells (DSSCs). Dyes
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SDD1− −3 were designed with indole as a donor and 1,4-phenylenebis(methylene), 1,4-butylene and 1,6-hexylene as a linker, respectively, while the dye SDD4 was designed with phenoxazine as a core donor. Their photophysical, electrochemical and DSSCs characteristics were investigated. The results show that the architecture structure of the linkage affects the performance of the cells −3, slightly. The DSSCs based on SDD4 shows much higher η than the DSSCs based on SDD1− which indicated that phenoxazine is a better donor than indole. Under standard global AM 1.5
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solar condition (100 mW cm−2), the SDD4 dye sensitized cell gave the highest η of 4.33% with chenodeoxycholic acid as a co-adsorbent, reaching 82% of N719-based DSSCs.
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Keywords: Dye-sensitized solar cell; Organic dye; Double donor-acceptor branches; Photovoltaic; Phenoxazine; Indole 1. Introduction
Since the first successful fabrication of sandwich type solar cell by Grätzel in 1991,1 dye sensitized solar cells (DSSCs) have received significant research interest in both academic and industrial fields, which are owing to their efficient, highly adaptable, economical feasible and relatively less environmental issues than traditional silicon-based solar cells. DSSCs have three main components i.e photoanode, electrolyte and sensitizer. Among these components the sensitizers play an important role of capturing photon and generating electron which are injected into conduction band of semiconductor (e.g TiO2). Tremendous endeavors have been made to search for efficient sensitizers to enhance the efficiency of DSSCs. Power conversion efficiencies over 11% have been achieved with ruthenium (Ru)-polypyridyl sensitizers,2-4 and a record
ACCEPTED MANUSCRIPT efficiency of DSSCs up to 12.3% was achieved with zinc porphyrin by Yella et al in 2011.5 However, the drawbacks concerning Ru-complexes such as rare sources, difficult purification and relatively lower molar extinction coefficients limit its application. Recently, more and more attention has been focused in the application of metal-free organic dyes as auspicious alternatives to the Ru-complexes due to their high molar extinction coefficient, flexible structural tailoring as well as low cost and environmental friendliness. In the design of the organic dyes, the dipolar
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donor−π spacer−acceptor (D−π−A) system is considered as the most widely adopted molecular configuration because the photoinduced intramolecular electron can be transferred into TiO2 via the D−π−A channel6. Up till now, hundreds of metal-free dyes have been employed as sensitizers for DSSCs and have exhibited impressive photovoltaic efficiencies.7-15 To enhance the photovoltaic performance of the DSSCs, considerable research efforts have been
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devoted to optimized the structure of the organic dyes, such as D−D−π−A,16,17 D−π−A−A,18,19 A−π−D−π−A,20,21 D−A−π−A,22-25 (D−π−A)226 and so on. Recently, we designed several novel organic sensitizers featuring double D−π−A branches (DB dyes).27-29 It was found that the conversion efficiency of DSSCs based on the DB dyes was increased as compared to the corresponding single counterparts due to their better light-harvesting ability. In addition, the donors play a critical function in light harvesting and electron injection.30-34 In this study, we designed and synthesized a series of DB dyes SDD1−4 and a reference sensitizer with single D−π−A system (RD). As shown in Scheme 1, SDD1−4 were designed with the following characteristics: (i) two D−π−A branches were linked by non-conjugated alkyl chain, resulting in two separate light harvesting units, which might enhance the absorption intensity due to the
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absorption overlap of both the same donors and π-spacers. (ii) Indole and phenoxazine donors were introduced into the D−π−A branches as the electron donor units. As a consequent, the photovoltaic performance of the cells based on these dyes would be influenced by the structure of the linkages, such as different length of flexible alkyl chains and the rigid alkyl chain. Moreover, the HOMO and LUMO energy levels of the dyes would be tuned by the different donors, as a result, the conversion efficiencies of the DSSCs would be affected due to the changed absorption range and intensity of the dyes.
Fig. 1. The structure of the dyes SDD1− −4 and RD
2. Results and discussion 2.1. Synthesis and structural characterization
ACCEPTED MANUSCRIPT The synthetic route of the dyes is shown in Scheme 1. Intermediate 1 was prepared from arylamine via alkylation reaction with dibromoalkane or 1-bromoalkane in the presence of KOH in DMSO, respectively. The intermediates 1 were then converted to the corresponding dialdehydes 2 by Vilsmeier-Haack reaction in the presence of DMF and POCl3. Finally, the dialdehydes 2 were
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condensed with cyanoacetic acid to give the target dyes (SDD1−4 and RD) by the Knoevenagel condensation reaction in the presence of piperidine. All the intermediates and target dyes were confirmed by standard spectroscopic methods.
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Scheme 1. Synthesis of the dyes SDD1− −4 and RD.
2.2. Photophysical Properties The UV−vis and emission spectra of the dyes in THF solution are displayed in Fig. 2a and Fig. 2b, respectively, and their corresponding photophysical and electrochemical properties are summarized in Table 1. All the dyes exhibit two distinct absorption bands: first absorption band in the region of 270−310 nm is ascribed to the π−π* transition of the conjugated molecules; and the second band in the region of 380−480 nm which is assigned to intramolecular charge transfer (ICT) between the electron donating unit and the electron accepting units. The spectra of SDD1−SDD3 are quite similar, whose maximum absorption are around 380 nm. The maximum absorption peak of SDD4 is red-shifted by near ca. 100 nm compared to the dyes SDD1−3 due to the stronger electron-donating nature of the phenoxazine than indole. It is worth mentioning that the molar absorption coefficient (ε = 39066 M−1 cm−1) of SDD4 is almost twice as high as that of reference dye RD (ε = 19628 M−1 cm−1), which indicates that the absorption of SDD4 is enhanced and
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To gain deeper insight into the possibility of regeneration of the dyes and electron injection from the excited state of the dyes to the conduction band (CB) of TiO2 photoanode, the electrochemical properties of the dyes were studied by cyclovoltametry (CV) (Table 1). The first half-wave potentials (E1/2 vs. Fc/Fc+) of SDD1-4 and RD adsorbed on TiO2 film was measured at 0.43, 0.40, 0.41, 0.21 and 0.19 V, respectively. Therefore, the first oxidation potentials (Eox) versus normal hydrogen electrode (NHE), corresponding to the highest occupied molecular orbital (HOMO) levels were 1.06, 1.03, 1.04, 0.84 and 0.82 V calibrated by addition of 0.63 V to the potential vs Fc/Fc+ (vs NHE), respectively. On the other hand, the HOMO values versus vacuum
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were estimated via the equation EHOMO= −e [4.88+ E1/2 (vs. Fc/Fc+)]35 and the corresponding EHOMO of SDD1− −4 and RD versus vacuum were −5.31, −5.28, −5.29, −5.09 and −5.07 eV, respectively. The sufficient low oxidation potential of all the dyes were more positive than I−/I3− redox couple potential value (0.4 V vs NHE or −4.60 eV vs vacuum)36 ensuring more effective dye regeneration and restraining the recapture of the injected electrons by the dye cation radical or
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I−/I3− redox couple.
Fig. 2. (a) Absorption spectra and (b) Normalized absorption and emission spectra of the dyes in THF.
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Table 1 Absorption, emission and electrochemical properties of the dyes SDD1− −4 and RD
Absorption
Emission
Oxidation Potential data E0-0
EHOMO/ELUMOc)
εmax
λmax
Eox
Eox − E0−0
Egapb)
THF(nm)
(M−1cm−1)
(nm)
(V, vs NHE)
(V)a)
(V, vs NHE)
(V, vs NHE)
SDD1
378
34686
440(378)
1.06
3.03
−1.97
1.47
−5.31/−2.28
−5.92/−2.52
SDD2
380
43513
466(380)
1.03
3.00
−1.97
1.47
−5.28/−2.28
−5.98/−2.56
SDD3
379
35769
444(379)
1.04
3.02
−1.98
1.48
−5.29/−2.27
−5.94/−2.46
SDD4
476
39066
576(476)
0.84
2.30
−1.46
0.96
−5.09/−2.79
−5.28/−2.42
RD
474
19628
471(474)
0.82
2.32
−1.50
1.00
−5.07/−2.75
−5.17/−2.31
Dye
λmax
a
in
EHOMO/ELUMO
(eV)
(eV)
b
E0-0 were estimated from the intersection of the normalized absorption and emission spectra (see Fig. 2b). Egap is the energy gap
between the Eox − E0-0 of the dye and the conductive band level of TiO2 (−0.5 V vs NHE). c Calculated at the B3LYP-31G(d) level in vacuum.
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Fig. 3. The optimized structure and the frontier molecular orbitals of the HOMO and LUMO calculated with DFT on a B3LYP/6-31+G(d) level of the dyes ( d Energy level of the I3−/I− redox couple (−4.60 eV vs vacuum);36 Energy level of the conduction band edge of TiO2 (−4.00 eV vs vacuum) 37).
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The estimated excited state potentials corresponding to the lowest unoccupied molecular orbital (LUMO) levels, were −1.97, −1.97, −1.98, −1.46 and −1.50 V (vs NHE) calculated from Eox − E0–0, where the E0–0 (E0–0 = 1240/λint) was estimated from the intersection of the normalized absorption and emission spectra (λint) (Fig. 2b). The corresponding ELUMO value versus vacuum which are calculated by [EHOMO + E0–0] are −2.28, −2.28, −2.27, −2.79 and −2.75 eV, respectively.
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The more negative LUMO level of these dyes relative to the conduction band edge of the TiO2 electrode (−0.5 V vs NHE or −4.00 eV vs vacuum37) , providing that an energy gap (Egap) of 0.2 eV is necessary for efficient electron injection.38 Hence, an effective electron transfer from the excited dye molecule to TiO2 should be energetically favorable. As shown in Table 1, the
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HOMO−LUMO band gap (E0-0) of SDD1− −4 and RD dyes were 3.03 eV, 3.00 eV, 3.02 eV, 2.30 eV, 2.32 eV, respectively. Compared to SDD1− −3, SDD4 and RD has a lower HOMO−LUMO energy band gap which is due to stronger electron-donating accepting ability. The stronger donor lifts the HOMO level and decreases the change in the LUMO energy level. Dye SDD4 has the smallest band gap, causing the red-shifted absorption spectra, which is ideally preferred for DSSCs application. On the other hand, the LUMO and HOMO values of SDD1− −3 are similar. The more positive HOMO value is preferable in regenerating the dyes, and the more negative LUMO value results in a large possibility of electronic injection into the CB of TiO2. However, the higher HOMO−LUMO gap of SDD1− −3 leads to absorb shorter wavelength sunlight, which is in accordance with their absorption spectra. The low light-harvesting characteristic of SDD1-3 means inferior performance of their DSSCs. 2.4 Molecular orbital calculations To learn more on the geometry and electronic properties of the dyes, density functional theory (DFT) calculations were performed at a B3LYP/6-31+G (d) level with Gaussian 03. Fig. 3 shows the optimized molecular structures and frontier molecular orbitals of the dyes. The electrons at HOMO level of all the dyes were populated on both the donors (indole and phenoxazine) and π-bridge, and the LUMO levels were sizably delocalized through the cyanoacrylic acid and its
ACCEPTED MANUSCRIPT adjacent π-bridge fragments, indicating that the HOMO−LUMO excitation moves the electron distribution from the donor to the cyanoacrylic acid moiety, thus facilitating an efficient photoinduced/interfacial electron transfer from the excited dye molecule to the TiO2 electrode through the effective intramolecular charge separation under light irradiation. 2.5. Photovoltaic performances of the DSSCs The detailed photovoltaic parameters (short-circuit photocurrent density (Jsc), open-circuit
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photovoltage (Voc), fill factor (ff) and photovoltaic conversion efficiency (η)) of all the dyes are summarized in Table 2 and the current−voltage (J−V) curves of the dyes are demonstrated in Fig. 4. Among these dyes, the cell based on SDD4 gave a maximum η of 3.92% (Jsc = 6.81 mA cm−2, Voc = 743 mV, ff = 0.62) under standard global AM 1.5 solar light irradiation (100 mW cm−2), while
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the cells sensitized with SDD1−3 and RD gave Jsc of 1.63, 1.87, 1.91 and 5.94 mA cm−2, Voc of 632, 640, 692 and 759 mV and ff of 0.70, 0.72, 0.71 and 0.64, corresponding to η of 0.91, 1.07, 1.19 and 3.61%, respectively. Under the same measuring conditions, the cell based on N719
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showed an efficiency of 5.25% with a Jsc of 8.63 mA cm−2, a Voc of 777mV, and a ff of 0.63.
Fig. 4. J−V curves of DSSCs sensitized by SDD1−4, RD and N719.
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Table 2 Photovoltaic performance of DSSCs based on dyes SDD1−4, RD and N719 Dyes
a
Jsc
Voc -2
ff
η
Dye amount
(%)
(10−7mol⋅cm-2)
(mA⋅cm )
(mV)
N719a
8.63
777
0.63
5.25
−
SDD1
1.63
632
0.70
0.91
1.05
SDD2
1.87
640
0.72
1.07
1.15
SDD3
1.91
692
0.71
1.19
1.34
SDD4
6.81
743
0.62
3.92
2.05
RD
5.94
759
0.64
3.61
0.98
The concentration of dye-bath for N719 was 3 × 10-4 M in ethanol.
Table 3 Photovoltaic performance of DSSCs based on SDD4 and RD sensitized in solutions containing different concentration of CDCAa
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2
(mM) 0
0.1
1.0
10.0
Saturated
Voc(mV)
ff
η(%)
Dye amount (10−7mol⋅cm-2)
)
RD
5.94
759
0.64
3.61
0.982
SDD4
6.81
743
0.62
3.92
2.053
RD
4.65
728
0.64
2.71
0.809
SDD4
6.41
749
0.63
3.82
1.981
RD
4.23
732
0.65
2.53
0.712
SDD4
7.11
753
0.65
4.33
RD
3.24
729
0.69
2.04
SDD4
7.04
750
0.64
4.22
RD
2.86
689
0.66
1.64
SDD4
6.64
747
0.65
4.03
−4
1.713
0.335 1.435 0.141 1.205
Dye-bath: SDD4 and RD in THF solution (2 × 10 M) with addition of CDCA solution (10-3 M).
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a
Jsc(mA⋅cm-
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CDCA
Compared to those cells based on the dyes SDD4 and RD, the relatively lower photovoltaic
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performance of double D−A dyes SDD1−3-sensitized cells was due to their relatively lower short-circuit photocurrent density (Jsc), which could be ascribed to the donor of indole with weak and narrow light-harvesting ability in the visible region. On the other hand, the cell based on SDD3 shows higher efficiency compared to the SDD1 and SDD2 based cells due to its higher Jsc and higher Voc. The higher Jsc may be attributed to the relatively bigger dye loading on the TiO2 surface, and the higher Voc may be ascribed to the n-hexyl which hinders the dye SDD3
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aggregation on TiO2 more effectively. Furthermore, the cell based on SDD1 exhibited lower η due to its lower dye loading amount, which resulted from the rigid and large linker of 1,4-phenylenebis(methylene). It has been reported that chenodeoxycholic acid (CDCA) can enhance the performance of the DSSCs because CDCA can reduce the aggregation of the dyes on the TiO2 surface. We studied the photovoltaic performance of the SDD4 and RD-sensitized cells in the presence of CDCA with different concentrations. After the addition of CDCA to the dye bath, the adsorbed amount of the dyes decreased gradually with the further increasing concentration of CDCA (from 0 to a saturated concentration). The drop of the adsorption of SDD4 and RD indicated that CDCA competed for the TiO2 surface sites with the dyes molecules, and coadsorption of CDCA diminished the dye-dye interactions on the TiO2 surface. From the data of Table 3, it can be seen that the efficiencies of the cells based on RD and CDCA as coadsorbent decreased gradually as well as the dye adsorbed amount of RD. However, the efficiencies of the cells with SDD4 and CDCA as coadsorbent increased. A highest η value of 4.33% of the cell sensitized with SDD4 and CDCA as coadsorbent was obtained at 1.0 mM CDCA bath, which increased by 10.5% compared with SDD4 sensitized cell (3.92%), reaching 82% of a standard device fabricated with N719 (η = 5.25%). It could be explained that the suitable coverage density of SDD4 molecules resulted in higher Jsc and higher Voc of cell. A possible explanation is that the dye RD did not aggregate on the TiO2 surface, but the amount of dye adsorbed on the TiO2 surface was reduced by the coadsorption of CDCA, resulting in a loss of active light-harvesting; however, the dye SDD-4 aggregated on the TiO2 surface, and such an aggregation was reduced by CDCA, resulting in an enhancement of the efficiencies of the cells. 3. Conclusion
ACCEPTED MANUSCRIPT In summary, four novel double donor-acceptor (D− −A) branched dyes SDD1−4 with indole and phenoxazine units as donors were designed and synthesized, their photophysical and electrochemical properties were also investigated. The cell sensitized by SDD1−3 with indole as donor exhibited low η because they absorb only short length sunlight. Among SDD1−3, the photophysical and electrochemical properties were influenced slightly by the linker of alkyl chain
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between the D−π−A units. The SDD4-sensitized cell shows much better photovaltic performance, which are attributed to the stronger electron-donating property of the phenoxazine donor. SDD4 shows better photovaltic performance than the reference dye RD, which may be ascribed to its two D−A light-harvesting units and two anchoring groups. An optimal DSSC based on SDD4 with coadsorption of chenodeoxycholic acid exhibited highest η of 4.33%, which reached 82% of N719-based DSSC (5.25%). The results reveal that the organic
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dyes featuring two D−A branches with phenoxazine unit as donor are promising in the development of DSSCs. 4. Experimental section
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4.1. Instrumentation and materials
All reactions were performed under nitrogen atmosphere. The solvents were purified by standard procedures and purged with nitrogen before use. Chromatographic separations were carried out on silica gel (60M, 200-400 mesh). 1H NMR and 13C NMR spectra were recorded on a Bruker 400 MHz spectrometer in CDCl3 or DMSO-d6 with tetramethylsilane as inner reference. MS data were recorded on an Esquire HCT PLUS mass spectrometer. Elementary analyses were performed using a Vario EL Analyzer. The melting point was taken on Tektronix X4 microscopic melting point apparatus and uncorrected. The absorption and emission spectra of the
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dyes in THF solution (2 × 10−5 M) were measured at room temperature by Shimadzu UV-2450, UV−Vis spectrophotometer and Fluorolog III photoluminescence spectrometer, respectively. Electrochemical redox-potentials were obtained by cyclic voltammetry (CV) using a three-electrode cell and an electrochemistry workstation (e-corder (ED 401) potentiostat). The dyes sensitized TiO2 films were used as a working electrode. The Ag/AgCl in saturated KCl solution and Pt wire were utilized as reference and counter electrodes with a scan rate of 50 mV/s. Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was used as supporting electrolyte in CH3CN. The measurements were calibrated using ferrocene as standard. The redox potential of ferrocene internal reference is taken as 0.63V versus normal hydrogen electrode (NHE). The nanoporous TiO2/FTO glass films (the thickness of TiO2 photoelectrode and scattering layer were 14 µm and 4 µm, working area: 0.16 cm2) and counter electrode (FTO glass coated Pt) were purchased from Yingkou OPV Tech New Energy Co., Ltd. of Liaoning province. Heated at 450 °C followed by cooling to 80 °C, the TiO2 film was soaked in ethanol solution containing 5×10-4 M N719 or in solution containing 2 × 10-4 M organic sensitizer for 12 h. The film was then rinsed with THF in order to remove physisorbed organic dye molecules. To evaluate their photovoltaic performance, the dye-sensitized TiO2 films were sandwiched together with counter electrode. The counter electrode was placed directly on the top of the dye-sensitized TiO2 film sealed with thermal adhesive film (~25 µm), and the electrolyte (0.6 M 1-methyl-3-propylimidazolium iodide (PMII), 0.10 M guanidinium thiocyanate, 0.03 M I2, 0.5 M tert-butylpyridine in acetonitrile and valeronitrile (85:15)) was injected from a hole made on the counter electrode into the space between the sandwiched cells. The current–voltage measurements were carried out by adopting a
ACCEPTED MANUSCRIPT Keithley 2400 source meter under simulated AM 1.5 G illumination (100 mW cm−2) provided by solar simulator (9600, Oriel). A 1 KW Xenon arc lamp (6271, Oriel) was served as a light source. The incident light intensity was calibrated with a NREL standard Si solar cell. The individual dye amount absorbed on TiO2 film was determined according to the literature procedure.39 4.2 Syntheses
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4.2.1. 1,4-Bis[(indol-1-yl)methylene)]benzene (1a),40 1,4-bis(indol-1-yl)butane (1b),41 1,6-bis(indol-1-yl)hexane (1c),41 1,4-bis[(3-formyl-indol-1-yl)methylene]benzene (2a)42 and 1,4-bis(3-formyl-indol-1-yl)butane (2b)43 were synthesized according to the literature procedure.
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4.2.2. Synthesis of 1,6-Bis(phenoxazin-10-yl)haxane (1d). Phenoxazine (0.732 g, 4.0 mmol) was added to a suspension of potassium hydroxide (0.448 g, 8.0 mmol) in dimethyl sulfoxide (15 mL) under nitrogen. The mixture was stirred for 30min,then 1,6-dibromohexane (0.488 g, 2.0 mmol) was added. The reaction mixture was stirred overnight at room temperature. Water was added and extracted with dichloromethane (3 × 30 mL), the organic phases were then washed with brine, dried with anhydrous sodium sulfate and evaporated. The residue was purified by chromatography
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on silica gel (dichloromethane/petroleum = 1 : 7) to give 1d as a colorless solid (0.84 g, 93.7%). Mp: 163−164°C. 1H NMR (400 MHz, CDCl3): δ 1.48−1.53 (m, 2 × 2H), 1.66−1.74 (m, 2 × 2H), 3.50 (br. s, 2 × 2H), 6.45 (br. s, 2 × 2H), 6.55−6.74 (m, 2 × 4H), 6.76−6.81 (m, 2 × 2H). 13C NMR (100 MHz, CDCl3): δ 25.0, 26.7, 43.8, 111.2, 115.4, 120.8, 123.6, 133.3, 145.0. MS (ESI): m/z [M+Na]+ calcd for C30H28N2O2: 471.2; found: 471.2. Anal. calcd for C30H28N2O2: C, 80.33; H, 6.29; N, 6.25; found: C, 80.07; H, 6.32; N, 6.28.
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4.2.3. Synthesis of 10-Hexyl-phenoxazine (1e). It was prepared as a colorless liquid (0.968 g, 90.8%) from phenoxazine and 1-bromohexane by using the same method established for 1d. 1H NMR (400 MHz, CDCl3): δ 0.93 (t, J = 6.4 Hz, 3H), 1.34−1.44 (m, 6H), 1.63−1.68 (m, 2H), 3.47 (br. s, 2H), 6.46-6.48 (m, 2H), 6.63 (br. s, 4H), 6.78−6.81 (m, 2H). 13C NMR (100 MHz, CDCl3): δ 14.1, 22.7, 24.9, 26.6, 31.6, 44.1, 111.3, 115.3, 120.7, 123.6, 133.5, 145.0.
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4.2.4. Synthesis of 1,6-Bis(3-formyl-indol-1-yl)hexane (2c). To a solution of 1c (0.633 g, 2.0 mmol) and dry N,N-dimethylformamide 1.17 g (16.0 mmol) in 1,2-dichloroethane (15 mL) was slowly
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added phosphorus oxychloride 2.46 g (16.0 mmol) at 0 °C in an ice−water bath. Then the bath was heated to 90 °C and maintained for 5 h. Dilute aqueous solution of sodium acetate was added and stirred for 3 h, and the mixture was extracted three times with dichloromethane(3× 20 mL). The combined organic phases were washed with brine and dried over anhydrous sodium sulfate. The solvent was removed under reduced pressure, and the residue was purified by column chromatography with ethyl acetate/dichloromathane (v/v = 1/50) as the eluent to give 2c as a light yellow solid (0.477 g, 64.0%). Mp 211−212 °C. 1H NMR (400 MHz, CDCl3): δ 1.32−1.37 (m, 2 × 2H), 1.83−1.91 (m, 2 × 2H), 4.15 (t, J = 6.8 Hz, 2 × 2H), 7.31−7.35 (m, 2 × 3H), 7.66 (s, 2 × 1H), 8.29−8.31 (m, 2 × 1H), 9.99 (s, 2 × 1H). MS (ESI): m/z [M−H]+ calcd for C24H24N2O2: 371.2; found: 371.2. Anal. calcd for C24H24N2O2: C, 77.39; H, 6.49; N, 7.52; found: C, 77.07; H, 6.52; N, 7.55. 4.2.5. Synthesis of 1,6-Bis(3-formyl-phenoxazin-10-yl)hexane (2d). It was prepared as a greenish-yellow solid (0.786 g, 77.9%) from 1d by using the same method established for 2c. Mp 297−298 °C. 1H NMR (400 MHz, CDCl3): δ 1.50−1.54 (m, 2 × 2H), 1.70−1.77 (m, 2 × 2H), 3.56 (t, J = 8.0Hz, 2 × 2H,), 6.48−6.53 (m, 2 × 2H), 6.65−6.68 (m, 2 × 1H), 6.72−6.76 (m, 2 × 1H),
ACCEPTED MANUSCRIPT 6.79−6.83 (m, 2 × 1H), 7.08−7.09 (m, 2 × 1H), 7.28−7.31 (m, 2 × 1H), 9.67 (s, 2 × 1H). 13C NMR (100 MHz, C5H5N-d5): δ 25.2, 26.7, 44.0, 111.5, 113.0, 114.1, 116.1, 189.8 (the carbon shift values of the aromatic ring can not be observed in the range of 123.0~124.6 beacuse they were shielded by the solvent peaks of C5H5N-d5). MS (ESI): m/z [M]+ calcd for C32H28N2O4: 504.2; found: 503.8. Anal. calcd for C32H28N2O4: C, 76.17; H, 5.59; N, 5.55; found: C, 76.43; H, 5.62; N, 5.53.
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4.2.6. Synthesis of 10-Hexyl-phenoxazine-3-carbaldehyde (2e). It was prepared as a yellow liguid (0.517 g, 87.5%) from 1e by using the same method established for 2c. 1H NMR (400 MHz,
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CDCl3): δ 0.92 (t, J = 7.2Hz, 3H), 1.34−1.42 (m, 6H), 1.60−1.67 (m, 2H), 3.46 (t, J = 8.0 Hz, 2H), 6.44−6.46 (m, 1H), 6.48−6.50 (m, 1H), 6.59−6.62 (m, 1H), 6.67−6.71 (m, 1H), 6.77−6.81 (m, 1H), 7.01−7.02 (m, 1H), 7.24−7.27 (m, 1H), 9.61 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 14.1, 22.7, 25.0, 26.6, 31.6, 44.4, 110.5, 112.1, 114.1, 115.7, 122.5, 124.0, 128.8, 129.8, 131.4, 139.3, 144.7, 145.1, 189.6. MS (ESI): m/z [M+Na]+ calcd for C19H21NO2: 318.2; found: 318.2. Anal. calcd for C19H21NO2: C, 77.26; H, 7.17; N, 4.74; found: C, 77.19; H, 7.20; N, 4.73.
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4.2.7. Synthesis of 1,4-Bis{[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]methylene}benzene (SDD1). A solution of 2a (0.493 g, 1.0 mmol) in chloroform (15 mL) was condensed with 2-cyanoacetic acid (0.850 g, 10.0 mmol) in the presence of piperidine (0.20 mL, 2.0mmol). The mixture was refluxed for 8 h under nitrogen. After cooling to room temperature, the mixture was poured into 2M aqueous of HCl. The crude solid product was filtered and washed with dichloromethane, water and methanol respectively. The final product SDD1 was obtained as a yellow solid (0.448g, 85.2%). Mp 275−276 °C. 1H NMR (400 MHz, DMSO-d6): δ 6.09 (s, 2 × 2H), 7.69−7.73 (m, 2 × 5H), 8.02−8.04 (m, 2 × 1H), 8.39−8.41 (m, 2 × 1H), 8.95 (s, 2 × 1H), 9.13 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 49.6, 99.0, 109.5, 111.5, 118.6, 119.2, 121.9, 123.4, 127.6, 127.6, 133.0,
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136.0, 136.4, 142.8, 164.8. MS (ESI): m/z [M−H]− calcd for C32H22N4O4: 525.2; found: 525.3. Anal. calcd for C18H22BrN: C, 72.99; H, 4.21; N, 10.64; found: C, 73.02; H, 4.23; N, 10.58. 4.2.8. Synthesis of 1,4-Bis[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]butane (SDD2). It was prepared as a yellow solid (0.464 g, 97.2%) from 2b by using the same method established for
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SDD1. Mp 236−237 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.83 (br. s, 2 × 2H), 4.41 (br. s, 2 × 2H), 7.24−7.33 (m, 2 × 2H), 7.65−7.67 (m, 2 × 1H), 7.92−7.94 (m, 2 × 1H), 8.44 (br. s, 2 × 1H), 8.52 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 26.8, 46.1, 95.1, 109.1, 111.4, 118.5, 118.7, 122.2, 123.5, 127.5, 133.8, 136.1, 144.7, 164.6. MS (ESI): m/z [M−H]− calcd for C28H22N4O4: 477.2; found: 477.2. Anal. calcd for C28H22N4O4: C, 70.28; H, 4.63; N, 11.71; found: C, 70.37; H, 4.65; N, 11.66. 4.2.9. Synthesis of 1,6-Bis[3-((E)-2-carboxyl-2-cyanovinyl)indol-1-yl]haxane (SDD3). It was prepared as a yellow solid (0.393 g, 77.7%) from 2c by using the same method established for SDD1. Mp 228−230 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.28−1.33 (m, 2 × 2H), 1.72−1.80 (m, 2 × 2H), 4.33 (t, J = 6.8 Hz, 2 × 2H), 7.25−7.33 (m, 2 × 2H), 7.62−7.64 (m, 2 × 1H), 7.93−7.95 (m, 2 × 1H), 8.46 (s, 2 × 1H), 8.52 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 25.6, 29.2, 46.6, 93.8, 109.0, 111.4, 118.4, 118.8, 122.4, 123.6, 127.5, 134.3, 136.2, 145.4, 164.7. MS (ESI): m/z [M−H]− calcd for C30H26N4O4: 505.2; found: 505.3. Anal. calcd for C30H26N4O4: C, 71.13; H, 5.17; N, 11.06; found: C, 71.29; H, 5.19; N, 11.01. 4.2.10. Synthesis of 1,6-Bis[3-((E)-2-carboxyl-2-cyanovinyl)phenoxazin-10-yl]haxane (SDD4). It
ACCEPTED MANUSCRIPT was prepared as a red−black solid (0.604 g, 94.8%) from 2d by using the same method established for SDD1. Mp 175−177 °C. 1H NMR (400 MHz, DMSO-d6): δ 1.46 (br. s, 2 × 2H), 1.54 (br. s, 2 × 2H), 3.58 (br. s, 2 × 2H), 6.70−6.82 (m, 2 × 5H), 7.35 (br. s, 2 × 1H), 7.46−7.78 (m, 2 × 1H); 7.96 (s, 2 × 1H). 13C NMR (100 MHz, DMSO-d6): δ 24.5, 25.7, 42.5, 97.6, 111.7, 112.9, 114.6, 115.4, 117.1, 122.6, 123.7, 124.3, 130.5, 131.0, 137.8, 143.6, 143.8, 152.4, 164.0. MS (ESI): m/z [M−H]− calcd for C38H30N4O6: 637.2; found: 637.5. Anal. calcd for C38H30N4O6: C, 71.46; H, 4.73; N, 8.77; found: C, 71.40; H, 4.71; N, 8.76.
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4.2.11. Synthesis of (E)-2-Cyano-3-(10-hexyl-10H-phenoxazin-3-yl)acrylic acid (RD). It was prepared as a red solid (0.263 g, 83.1%) from 2e by using the same method established for SDD1.
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Mp 185−186 °C. 1H NMR (400 MHz, DMSO-d6): δ 0.85 (t, 3H), 1.26−1.29 (m, 4H), 1.32−1.39 (m, 2H), 1.47−1.54 (m, 2H), 3.54 (q, 2H), 6.66−6.72 (m, 4H), 6.81−6.84 (m, 1H), 7.33−7.34 (m, 1H), 7.44−7.46 (m, 1H), 7.94 (s, 1H). 13C NMR (100 MHz, DMSO-d6): δ 13.8, 22.1, 24.5, 25.6, 31.0, 43.1, 97.8, 111.6, 112.8, 114.5, 115.3, 117.1, 122.5, 123.7, 124.3, 130.6, 131.0, 137.8, 143.6, 143.8, 152.2, 164.0. MS (ESI): m/z [M−COOH]− calcd for C22H22N2O3: 317.2; found: 316.9. Anal. calcd for C22H22N2O3: C, 72.91; H, 6.12; N, 7.73; found: C, 72.73; H, 6.15; N, 7.76.
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Acknowledgment
The authors thank the National Natural Science Foundation of China (21272079, 61368006) for the financial support. References and notes
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