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Novel Organic Dyes Featuring with Spiro[dibenzo[3,4:6,7]cyclohepta[1,2b]quinoxaline-10,9'-fluorene] (SDBQX) as Rigid Moiety for Dye-Sensitized Solar Cells Mengchen Xu, Xiangyu Hu, Yingjun Zhang, Xin Bao, Aiying Pang, and Jingkun Fang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00261 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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ACS Applied Energy Materials

Novel Organic Dyes Featuring with Spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline10,9'-fluorene] (SDBQX) as Rigid Moiety for Dye-Sensitized Solar Cells Mengchen Xu,† Xiangyu Hu,† Yingjun Zhang,§ Xin Bao,† Aiying Pang, *,‡ and Jing-Kun Fang*,†

†Department

of Chemistry, School of Chemical Engineering, Nanjing University of Science and

Technology, Xiaolingwei Street No. 200, Nanjing 210094, China ‡State

Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China §State

Key Laboratory of Anti-Infective Drug Development(No. 2015DQ780357), Sunshine

Lake Pharma Co., Ltd, Dongguan 523871, China

1

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ABSTRACT Three

novel

organic

dyes

coded

as

FHD4,

FHD5

and

FHD6

featuring

with

spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fluorene] (SDBQX) moieties were developed for dye-sensitized solar cells (DSSCs). The fluorenyl moiety of SDBQX is perpendicular to the quinoxaline moiety, which is beneficial to inhibit the H-aggregation in DSSCs. The band gap energies according to DFT calculations showed good tendency with the transition energy calculated from the absorption spectra, indicates that the DFT calculations would be an effective method to predict the absorption spectra range for FHD-type dyes. Broad spectral coverage and high molar extinction were observed in absorption spectrum of FHD4, leads to the best power conversion efficiency was obtained for FHD4-based DSSC. Co-adsorption of CDCA improved the power conversion efficiency slightly for FHD4-based (from 4.61% to 4.69%) and FHD6-based (from 3.59% to 3.69%) DSSCs. The co-adsorption of CDCA decreased the dye loading amount of FHD5 significantly, while the power conversion efficiency increased significantly from 3.18% to 3.73%. Finally, we have developed SDBQX as a new architecture for developing efficient organic dyes for DSSC applications. KEYWORDS: Dye-sensitized solar cells, Organic dyes, Spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fluorene], Rigid moiety, D–A–π–A, Chenodeoxycholic acid

2


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1. INTRODUCTION As a good alternative for silicon-based solar cells, dye-sensitized solar cells (DSSCs) have attracted huge interest for the sake of its low cost, well flexibility, easy structural modification and high solar energy conversion efficiency since it was initially reported in 1991.1-3 As a crucial part of DSSCs, dyes exhibit a key role in the function of a DSSC device because dyes exert significant influence on the light harvesting and electron injection process. In recent twenty years, the structural modifications of dyes have attracted great attention from scientists.4-9 Compared with metal-containing dyes especially the Ru-dyes, metal-free organic dyes have already been widely investigated because of their low cost, low toxicity, easy structural modification and high solar energy conversion efficiency and many organic dyes exhibit excellent performance.

10-13

Generally, the common organic dyes feature with a D–π–A system containing an electron donor (D), a π-conjugated spacer (π) between the donor and acceptor, and an electron acceptor (A) for anchoring onto the semiconductor surface.14-16 By adopting an extra electron deficient spacer, many dyes with D–A–π–A system possess impressive performance while used in DSSCs because the system could decrease the band gap resulting in a wide absorption range and strong intramolecular charge transfer.17-19 CR147 is an excellent organic dye containing an electron deficient diphenylquinoxaline (DPQX) moiety.20 A series of organic dyes modified based on CR147 were developed and the result indicated that DPQX moiety is an efficient electron deficient spacer for organic dyes to achieve high performance in DSSCs.21, 22

3



Planar dyes are especially prone to occur H-aggregation, which is typically adverse for DSSC performance.23 The DPQX moiety could diminish the extent of molecular planarity due to its two benzene rings deviate to each other resulting in a nonplanar structure (Figure 1). Obviously, if the two benzene rings were spiro-linked by a biphenyl unit, to give a spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fluorene] (SDBQX), the extent of molecular planarity would reduce significantly because the fluorenyl moiety appears perpendicular to the quinoxaline moiety. This spatial character would hopefully beneficial to inhibit or suppress the H-aggregation in DSSCs.

Figure 1. Optimized structures of DPQX and SDBQX.

Thus, we synthesized three new organic dyes, which are all imparted with an electron deficient SDBQX moiety in the π-conjugated spacer, to study the effect of SDBQX moiety on dyes in the field of DSSCs. The chemical structures of FHD4, FHD5 and FHD6 are shown in Scheme 1. These three dyes were fully characterized by 1H NMR (Nuclear Magnetic Resonance), 13

C NMR and HRMS (High Resolution Mass Spectrometry), etc. The photophysical and 4


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electrochemical properties and photovoltaic parameters of DSSCs sensitized by these three dyes were investigated systematically.

Scheme 1. Molecular structures of dyes FHD4, FHD5 and FHD6.

2. RESULTS AND DISCUSSION 2.1. Design and Synthesis of the Dyes

Scheme 2. Synthetic routes of FHD4, FHD5 and FHD6.

Scheme 2 depicts the synthetic routes for FHD4, FHD5 and FHD6. Suzuki cross-coupling reaction between 1 and (4-(diphenylamino)phenyl)boronic acid would afford the intermediate 2. Suzuki

cross-coupling

reactions

between

2

and

(5-formylfuran-2-yl)boronic

5


acid,


(5-formylthiophen-2-yl)boronic acid or (4-formylphenyl)boronic acid would give the intermediate 3a-3c, respectively. Cyanoacrylic acid groups were introduced by Knoevenagel reactions between 3a-3c and cyanoacetic acid to give FHD4, FHD5 and FHD6, respectively.24 Every step gave the desired product in a moderate to high yields. FHD4, FHD5 and FHD6 were fully characterized with 1H NMR, 13C NMR and HRMS, etc. 2.2. UV–VIS Absorption Spectra The UV-vis absorption spectra of FHD4, FHD5 and FHD6 in CH2Cl2/MeOH (9:1) solutions (5×10-5 M) and adsorbed on 2 µm TiO2 transparent films are depicted in Figure 2. The relevant photophysical data are summarized in Table 1.

Figure 2. UV-Vis absorption spectra of FHD4, FHD5 and FHD6 in solutions (a), adsorbed on TiO2 films (b).

In solution absorption spectra, all of the three dyes exhibited two prominent peaks at around 440-490 nm and 350-400 nm. The intense absorption bands in the near UV region could be assigned to the π-π* transition.25 In the visible region, the absorption peaks corresponding to the intramolecular charge transfer (ICT) from the electron donor to the electron acceptor were 6


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observed at 484, 485 and 443 nm for FHD4, FHD5 and FHD6, respectively. Broader spectral coverages were observed for FHD4 and FHD5 containing electron-rich aromatic moieties, the furan and thiophene rings. In addition, FHD4 with furan moiety exhibited higher molar extinction coefficients (ε = 2.6×104 M-1 cm-1 at 484 nm and 3.7×104 M-1 cm-1 at 403 nm) compared with FHD5 with thiophene moiety (ε = 2.1×104 M-1 cm-1 at 485 nm and 2.3×104 M-1 cm-1 at 396 nm), which suggests that FHD4 possessed stronger light harvesting capability. Table 1 Photophysical and electrochemical data of FHD4, FHD5 and FHD6. Dye FHD4 FHD5 FHD6

λmax/nm a

ε/104 M-1 cm-1 a

484

2.6

403

3.7

485

2.1

396

2.3

443

1.5

λmax/nm b

Eox/V c

E0-0/V d

Ered/V e

Band gap/eV f

553

1.12

2.06

-0.94

2.34

547

1.11

2.03

-0.92

2.32

482

1.11

2.26

-1.15

2.51

a

In CH2Cl2/MeOH (9:1) solution.

b

Adsorbed on 2 µm TiO2 transparent films.

c

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

d

E0-0 transition energy was calculated from optical absorption onset.

e

Ered = Eox - E0-0.

f

DFT/B3LYP calculated values.

Typically, H-aggregation of dye molecules cause a hypsochromic shift and J-aggregation of dye molecules cause a bathochromic shift for the absorption spectra based on the molecular exciton theory.23 The absorption maxima for FHD4, FHD5 and FHD6 on TiO2 transparent films are 553, 547 and 482 nm, which showed an obvious bathochromic shift of 69, 62 and 39 nm compared to their corresponding absorption spectra in solutions, respectively. The broader absorption spectra coverage are beneficial for better light-harvesting capabilities for the DSSCs. 7



The phenomenon implied that strong J-aggregation (edge-to-edge) instead of H-aggregation (face-to-face) is the main state for these dyes with SDBQX moiety on TiO2 film, which would better suited to enhance solar cell performance due to its effective capture of broader spectrum coverage photons. 2.3. Electrochemical Properties The redox potentials of the dyes were measured by cyclic voltammetry to evaluate the feasibilities of the electron injections and dye regenerations.26 The cyclic voltammograms are shown in Figure 3 and the corresponding data are summarized in Table 1.

Figure 3. CV plots of FHD4, FHD5 and FHD6.

The first oxidation potential (Eox) of dyes FHD4 (1.12 V), FHD5 (1.11 V) and FHD6 (1.11 V) were sufficiently more positive than the I-3 /I- redox potential (0.4 V vs. NHE), which would provide efficient dye regeneration of the oxidized dyes by the I-3 /I- electrolyte.24 The energy band diagrams of the dyes were illustrated in Figure 4. The optic band gap energies (E0-0)27 of FHD4 (2.06 V) and FHD5 (2.03 V) were much lower than that of FHD6 (2.26 V), indicated the 8


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broader adsorption spectra for FHD4 and FHD5, which is good for the improvement of the light harvesting capabilities. Ered was calculated from Eox - E0-0. The Ered of dyes FHD4 (-0.94 V), FHD5 (-0.92 V) and FHD6 (-1.15 V) were more negative than the Fermi level of TiO2 (-0.5 V vs. NHE), indicated a sufficient driving force which could guarantee the thermodynamic feasibility of charge injection from the excited dye molecules to the conduction band of TiO2.24

Fig. 4. The energy band diagram of FHD4, FHD5 and FHD6.

2.4. Theoretical Calculation The geometrical structures and electron distributions of FHD4, FHD5 and FHD6 were optimized by density functional theory (DFT) calculations with the B3LYP exchange correlation functional under the 6-31G (d) basis set implemented in the Gaussian 09 program. The simulated electron distributions in HOMO and LUMO levels of the dyes are shown in Figure 5, with the isodensity surface values fixed at 0.02. According to their molecular orbital profiles, the electrons of the HOMOs of FHD4, FHD5 and FHD6 mainly distributed over the triphenylamine moieties with a little contribution over the 9



quinoxaline core and the LUMOs located on the quinoxaline core and electron acceptors (furanylacrylic acid, thiophenylacrylic acid and phenylacrylic acid moieties for FHD4, FHD5 and FHD6, respectively). The HOMOs and LUMOs of these three dyes exhibited a certain degree of overlap, which is beneficial to a fast intramolecular charge transfer.28 Hence, the electrons could easily transfer from the electron donors to the electron acceptors, resulting in efficient electron injection from the excited dyes into the semiconductor TiO2.

Figure 5. HOMO and LUMO electron distributions of FHD4, FHD5 and FHD6.

The band gap energies according to DFT calculations (2.34, 2.32 and 2.51 V for FHD4, FHD5 and FHD6, respectively) showed good tendency with the E0-0 values estimated from the onset of the absorption spectra, featuring approximate deviations (+0.25-0.29 V). Therefore, the DFT calculations would be an effective method to predict the absorption spectra range for analogues of FHD dyes. It is clear that the SDBQX moieties of these dyes exhibited few electron distribution, indicated no conjugation with the other parts of the molecules because of the

10


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perpendicular geometrical structure between SDBQX moiety and the skeleton structure. Therefore, the key effect of SDBQX moieties on dye molecules would be its geometrical effect.

Figure 6. Calculated dihedral angles between quinoxaline moieties and electron acceptors in optimized structures of FHD4, FHD5 and FHD6.

Figure 6 shows the calculated dihedral angles between quinoxaline of SDBQX moieties and the rest parts in the optimized structures of FHD4, FHD5 and FHD6. The dihedral angles between quinoxaline moieties and triphenyl amine groups are almost the same. The dihedral angles between quinoxaline moieties and furan ring, thiophene ring and benzene ring are 6.6°, 25.1° and 40.9° for FHD4, FHD5 and FHD6, respectively. Based on the dihedral angle data, quinoxaline moiety showed an approximate coplanar geometry with the furan ring in FHD4, so higher molar extinction coefficient was observed for FHD4 in the absorption spectrum. For the sake of the obvious twist between quinoxaline moiety and the benzene ring in FHD6, the shortest λmax was observed for FHD6 in the absorption spectrum, which is adverse for the photovoltaic performance. 2.5. Photovoltaic Properties

11



The photovoltaic performances of the DSSCs based on FHD4, FHD5 and FHD6 were evaluated under illumination simulated AM 1.5G irradiation (100 mW cm-2). Figure 7 illustrates the photocurrent density-voltage (J-V) curves and the incident photon-to-current conversion efficiencies (IPCE) characteristic plots and the detailed corresponding photoelectrode chemical properties of short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (η) are summarized in Table 2.

Figure 7. J-V curves (a) and IPCE plots (b) of DSSCs based on FHD4, FHD5 and FHD6.

The FHD4-based DSSC exhibited a moderate open-circuit photovoltage (Voc = 0.69 V) and the highest short-circuit current density (Jsc = 9.33 mA cm-2), which guaranteed the highest power conversion efficiency (η = 4.61%) among these three dyes. The FHD5-based DSSC exhibited the lowest open-circuit photovoltage (Voc = 0.65 V) and a moderate short-circuit current density (Jsc = 7.46 mA cm-2), resulting in the lowest power conversion efficiency (η = 3.18%). The highest open-circuit photovoltage (Voc = 0.71 V) and the lowest short-circuit current density (Jsc = 6.97 mA cm-2) were observed for FHD6-based DSSC and hence a moderate power 12


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conversion efficiency (η = 3.59%). The incident photon-to-electron conversion efficiency (IPCE) plots were measured to study the Jsc differences of the FHD dyes based DSSCs. As depicted in Figure 7b, the devices based on the three dyes showed broad spectral responses. The spectral responses exhibited well consistency with the absorption spectra on TiO2 films (Figure 2b). The FHD4-based DSSC showed the widest IPCE plateau (over 60% between 400 and 560 nm) among the three dyes, which correlated well with the highest Jsc value. The FHD6-based DSSC shows a high but narrow IPCE plateau (over 60% between 420 and 530 nm), resulting in a low Jsc value. Table 2 a

Photovoltaic performances parameters of DSSCs based on FHD4, FHD5 and FHD6 . Dye

CDCA/mM Jsc/mA cm-2

Voc/V

FF/%

η/%

0

9.33 ± 0.15 0.69 ± 0.01 71.49 ± 0.31 4.61 ± 0.05

2.76×10-7

3

9.74 ± 0.05 0.68 ± 0.00 70.29 ± 0.45 4.69 ± 0.06

1.84×10-7

0

7.46 ± 0.28 0.65 ± 0.01 65.39 ± 2.21 3.18 ± 0.25

3.06×10-7

3

8.16 ± 0.25 0.65 ± 0.01 69.85 ± 0.94 3.73 ± 0.14

1.67×10-7

0

6.97 ± 0.22 0.71 ± 0.00 72.82 ± 0.26 3.59 ± 0.12

2.47×10-7

3

7.47 ± 0.51 0.70 ± 0.00 70.91 ± 0.03 3.69 ± 0.26

1.82×10-7

FHD4

FHD5

FHD6

N719 a

Γ/M cm-2

0

14.06

0.75

67.88

7.15

-

Averaged parameters of 3 cells for FHD4, FHD5 and FHD6.

In many cases, addition of CDCA would improve the photovoltaic performances because CDCA served as an auxiliary role to hamper the intermolecular aggregations of dyes on TiO2 films.23 We next investigated the performances of the DSSCs in the presence of chenodeoxycholic acid (CDCA). The J-V curves and IPCE plots are depicted in Figure 7 and the

13



relevant data are listed in Table 2. After the addition of CDCA, different influences on photovoltaic performances were observed for the DSSCs based on FHD4, FHD5 and FHD6. The power conversion efficiency increased slightly for FHD4-based (from 4.61% to 4.69%) and FHD6-based (from 3.59% to 3.69%) DSSCs. For FHD5-based DSSC, the power conversion efficiency increased significantly from 3.18% to 3.73% after addition of CDCA. The co-adsorption of CDCA exhibited similar tendency for the IPCE study of FHD4, FHD5 and FHD6-based DSSCs (Figure 7b). To scrutinize the influence of co-adsorption of CDCA, dye loading amount (Γ) on TiO2 of FHD4, FHD5 and FHD6 with or without CDCA were measured. As listed in Table 2, the Γ values of FHD4, FHD5 and FHD6 decreased from 2.76×10-7, 3.06×10-7, 2.47×10-7 M cm-2 (no CDCA) to 1.84×10-7, 1.67×10-7, 1.82×10-7 M cm-2 (with 3 mM CDCA), respectively. FHD5 showed the highest Γ value while no CDCA was added. The possible dye aggregation leads to its low power conversion efficiency. The co-adsorption of CDCA decreased the Γ value of FHD5 significantly by hampering the dye aggregation. As a result, an obvious enhancement of power conversion efficiency was observed for FHD5-based DSSCs by co-adsorption of CDCA. 2.6. Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy (EIS) is a useful method for analyzing the interfacial charge transfer dynamics in DSSCs.29 Nyquist and Bode phase plots of DSSCs based on FHD4, FHD5 and FHD6 are depicted in Figure 8, and the relevant data are listed in Table 3. Table 3 14


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Data based on the Nyquist and Bode phase plots. Dye

Rct/Ω

f /Hz

τe/ms

FHD4

41.7

18.5

8.60

FHD5

26.1

2562.3

0.062

FHD6

53.2

12.9

12.34

The big semicircles of Nyquist plots are attributed to the charge transfer at the interface of TiO2/dye/electrolyte (Rct). A large Rct indicates a small dark current and a low charge recombination rate.30 The Rct values increased in the order of FHD5 (26.1 Ω) < FHD4 (41.7 Ω) < FHD6 (53.2 Ω), which are in well agreement with the Voc values.

Figure 8. Nyquist plots (a) and Bode plots (b) of the DSSCs.

Electron lifetime (τe) can be evaluated from the peak frequency (f) at lower frequency region of Bode phase plots (Figure 8b) from equation τe = 1/2πf.31 The lower f value represents longer electron lifetime as well as slower electron recombination. The electron lifetime values increased in the order of FHD5 (0.062 ms) < FHD4 (8.60 ms) < FHD6 (12.34 ms), which are also correlated well with the changing tendency of Voc values.

15



3. EXPERIMENTAL SECTION 3.1. Materials and Instruments All reagents obtained from commercial sources were used as received, unless otherwise noted. All solvents were purified according to the standard methods. Column chromatography were carried out on chromatography silica gel (300-400 mesh). The air-sensitive reactions were carried out under nitrogen atmosphere and monitored by thin layer chromatography (TLC). Melting points were measured on an X-4A apparatus. 1H NMR and 13C NMR spectra were recorded at room temperature on Bruker DRX300 or Bruker AVANCE III 500 instruments and calibrated with tetramethylsilane (TMS) as an internal reference. HRMS were recorded by Waters Q-TOF MicroTM. UV-Vis absorption spectra were recorded by PerkinElmer Lambda 950 at room temperature. Cyclic voltammetry measurements of dyes were carried out with Zahner IM6 electrochemical workstation in tetrahydrofuran (THF) (1.0×10−3 M) containing 0.1 M Bu4NPF6 as the supporting electrolyte, and a three-electrode system (glassy carbon as the working electrode, platinum wire as the counter electrode, and Hg/Hg2Cl2 as reference electrode). Ferrocene was used as the external standard. The incident photon-to-current efficiencies (IPCE) were measured with mono-chromatic incident light under 100 mW cm-2 with bias light in DC mode (PEC-S20, using a 1KW Xe Arc lamp). The current density-voltage (J-V) characteristics of the DSSCs were measured by recording J-V curves using a Keithley 2400 source meter using simulated 1.5 AM sunlight with an output power of 100 mW cm-2 with a solar light simulator (PEC-L11, using a 1KW Xe Arc lamp) without a mask. Electrochemical 16


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impedance spectroscopy (EIS) were measured on Zahner IM6 electrochemical workstation under dark station under a forward bias of -0.70 V with a frequency range of 0.1 Hz-0.1 MHz. 3.2. Synthesis of Dyes 1,4-dibromospiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fluorene]

(1)

was

synthesized according to the literature.32 4-(1-Bromospiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fluoren]-4-yl)-N,N-dip henylaniline (2) To a flask were added 1 (903 mg, 1.50 mmol), (4-(diphenylamino)phenyl)boronic acid (289 mg, 1.00 mmol), Pd(PPh3)4 (69 mg, 0.06 mmol), Na2CO3 (318 mg, 3.00 mmol), THF (60 mL) and H2O (15 mL), and the mixture was stirred at 60°C overnight. Then the reaction mixture was poured into aqueous NH4Cl and extracted with CH2Cl2. The organic layer was washed with saturated brine and dried over Na2SO4. After filtration, solvents were removed by rotary evaporation. The crude product was subjected to column chromatography (SiO2; eluent, PE/CH2Cl2, 3:1) to give 2 as orange powder (445 mg, 58% yield). M.p.: 191-193°C. 1H NMR (300 MHz, CDCl3) δ (ppm): 8.53 (d, J=7.5 Hz, 1H), 8.33 (d, J=7.8 Hz, 1H), 8.21 (d, J=7.8 Hz, 1H), 7.78 (d, J=7.8 Hz, 1H), 7.73 (d, J=8.4 Hz, 4H), 7.47 (dt, J=7.2 Hz, 1.8 Hz, 1H), 7.36-7.02 (m, 21H), 6.76 (d, J=7.5 Hz, 2H). 13C NMR (126 MHz, CDCl3) δ (ppm): 152.99, 152.63, 149.50, 147.66, 147.54, 145.68, 145.49, 140.20, 140.07, 139.99, 139.57, 137.63, 137.58, 134.21, 133.76, 133.34, 131.57, 131.20, 129.82, 129.59, 129.26, 128.40, 127.97, 127.91, 127.65, 127.15, 124.75, 123.13, 122.85, 122.48, 120.38, 66.53. 17



General procedures for the preparation of intermediates 3a-3c. To a flask were added 2 (307 mg, 0.40 mmol), the corresponding arylboronic acid (0.48 mmol), Pd(PPh3)4 (28 mg, 0.024 mmol), Na2CO3 (127 mg, 1.20 mmol), THF (18 mL) and H2O (12 mL), and the mixture was stirred at 60°C overnight. Then the reaction mixture was poured into aqueous NH4Cl and extracted with CH2Cl2. The organic layer was washed with saturated brine and dried over Na2SO4. After filtration, solvents were removed by rotary evaporation. The crude product was subjected to column chromatography (SiO2; eluent, PE/CH2Cl2, 1:1) to give 3a-3c. 5-(1-(4-(Diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fl uoren]-4-yl)furan-2-carbaldehyde (3a). Dark red powder, 77% yield. M.p.: 312-314°C. 1H NMR (500 MHz, CDCl3) δ (ppm): 9.74 (s, 1H), 8.68 (d, J=8.0 Hz, 1H), 8.44 (d, J=7.5 Hz, 1H), 8.29 (d, J=7.5 Hz, 1H), 8.03 (d, J=8.0 Hz, 1H), 7.99 (d, J=3.5 Hz, 1H), 7.81 (d, J=8.5 Hz, 2H), 7.75 (d, J=8.0 Hz, 2H), 7.49 (t, J=7.0 Hz, 1H),7.40 (d, J=3.5 Hz, 1H), 7.38-7.15 (m, 19H), 7.05 (t, J=7.0 Hz, 4H), 6.77 (br. s, 2H). 13C NMR (126 MHz, CDCl3) δ (ppm): 177.29, 156.21, 152.33, 151.92, 151.30, 149.47, 147.88, 147.50, 145.76, 145.61, 141.46, 140.06, 139.47, 138.65, 138.15, 137.73, 133.95, 133.40, 131.78, 131.29, 129.56, 129.28, 128.52, 128.49, 128.10, 127.98, 127.82, 127.76, 127.16, 125.53, 124.82, 124.34, 123.22, 122.36, 120.43, 115.68, 66.52. 5-(1-(4-(Diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fl uoren]-4-yl)thiophene-2-carbaldehyde (3b). 18


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Dark red powder, 83% yield. M.p.: 303-305°C. 1H NMR (500 MHz, CDCl3) δ (ppm):9.95 (s, 1H), 8.49 (dd, J=8.0 Hz, 1.0 Hz, 1H), 8.39 (d, J=7.5 Hz, 1H), 8.37 (d, J=8.0 Hz, 1H), 8.01 (s, 1H), 7.99 (d, J=7.5 Hz, 1H), 7.79-7.82 (m, 3H), 7.74 (d, J=7.5 Hz, 2H), 7.54 (dt, J=7.5 Hz, 1.0 Hz, 1H), 7.15-7.37 (m, 17H), 7.05 (t, J=7.5 Hz, 4H), 6.77 (br. s, 2H).

13

C NMR (126 MHz,

CDCl3) δ (ppm): 183.27, 152.35, 152.26, 149.51, 148.44, 147.85, 147.48, 145.70, 145.59, 144.79, 140.99, 140.04, 139.51, 138.24, 137.72, 137.38, 135.50, 134.72, 133.50, 131.70, 131.20, 129.99, 129.82, 129.56, 129.28, 128.47, 127.92, 127.67, 127.12, 124.85, 123.24, 122.30, 120.41, 66.52. 4-(1-(4-(Diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxaline-10,9'-fl uoren]-4-yl)benzaldehyde (3c). Orange powder, 83% yield. M.p.: 319-320°C. 1H NMR (500 MHz, CDCl3) δ (ppm): 10.10 (s, 1H), 8.37 (d, J=8.0 Hz, 1H), 8.21 (d, J=7.5 Hz, 1H), 8.08 (d, J=8.0 Hz, 2H), 8.02 (s, 2H), 8.01 (d, J=8.5 Hz, 2H), 7.81 (d, J=8.5 Hz, 2H), 7.73 (d, J=7.5 Hz, 2H), 7.26-7.37 (m, 8H), 7.14-7.22 (m, 10H), 7.05 (t, J=7.0 Hz, 4H), 6.79 (br. s, 2H). 13C NMR (126 MHz, CDCl3) δ (ppm): 192.17, 152.07, 151.90, 149.54, 147.58, 145.45, 144.76, 140.75, 140.04, 139.58, 139.43, 138.02, 137.97, 137.80, 135.24, 133.57, 133.53, 131.68, 131.52, 130.61, 129.47, 129.27, 128.36, 127.92, 127.69, 127.10, 124.78, 123.13, 122.47, 120.39, 66.54. General procedures for the preparation of dyes FHD4, FHD5 and FHD6. To a flask were added 3a-3c (0.30 mmol), 2-cyanoacetic acid (77 mg, 0.90 mmol), ammonium acetate (9 mg, 0.12 mmol), acetic acid (5 mL) and toluene (15 mL), and the mixture 19



was heated to reflux overnight. Then the reaction mixture was poured into water and extracted with CH2Cl2. The organic layer was washed with saturated brine and dried over Na2SO4. After filtration, solvents were removed by rotary evaporation. The crude product was subjected to column chromatography (SiO2; eluent, CH2Cl2/MeOH, 30:1 to 10:1) to give FHD4, FHD5 and FHD6, respectively. 2-Cyano-3-(5-(1-(4-(diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxal ine-10,9'-fluoren]-4-yl)furan-2-yl)acrylic acid (FHD4). Dark red powder, 95% yield. M.p.: 241-243°C. 1H NMR (500 MHz, DMSO-d6) δ (ppm): 13.77 (br. s, 1H), 8.62 (d, J=7.5 Hz, 1H), 8.33 (t, J=8.0 Hz, 2H), 8.14-8.16 (m, 2H), 8.00 (s, 1H), 7.96(d, J=6.5 Hz, 2H), 7.88 (d, J=8.0 Hz, 2H), 7.60-7.63 (m, 2H), 7.27-7.42 (m, 9H), 7.08-7.16 (m, 12H), 6.63 (br. s, 2H).

13

C NMR (126 MHz, CDCl3) δ (ppm): 157.91, 152.36, 151.98,

151.39, 149.49, 147.99, 147.54, 147.28, 145.84, 145.67, 141.80, 140.10, 139.46, 138.75, 138.20, 137.75, 133.91, 133.40, 131.85, 131.26, 129.59, 129.31, 129.06, 128.57, 128.51, 128.02, 127.84, 127.78, 127.20, 124.93, 123.28, 122.29, 120.46, 117.39, 116.08, 96.04, 66.56. HRMS (ESI, m/z): Calcd for [M-CO2H]- C58H35N4O-: 803.2816, found: 803.2822. 2-Cyano-3-(5-(1-(4-(diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxal ine-10,9'-fluoren]-4-yl)thiophen-2-yl)acrylic acid (FHD5). Dark red powder, 87% yield. M.p.: 258-259°C. 1H NMR (500 MHz, DMSO-d6) δ (ppm): 8.65 (d, J=7.5 Hz, 1H), 8.58 (d, J=8.0 Hz, 1H), 8.29 (d, J=8.0 Hz, 1H), 8.20 (s, 1H), 8.12 (d, J=4.0 Hz, 1H), 8.09 (d, J=8.0 Hz, 1H), 7.95 (d, J=7.0 Hz, 2H), 7.85-7.87 (m, 3H), 7.68 (t, J=7.5 20


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Hz, 1H), 7.26-7.43 (m, 9H), 7.07-7.15 (m, 12H), 6.68 (br. s, 2H).

13

C NMR (126 MHz,

DMSO-d6) δ (ppm): 151.83, 148.57, 147.38, 146.92, 145.07, 144.81, 140.06, 139.59, 138.98, 138.44, 137.48, 137.26, 136.70, 134.56, 133.73, 131.88, 130.83, 130.12, 129.84, 129.66, 128.92, 128.44, 128.20, 127.89, 127.76, 127.54, 127.33, 124.92, 124.58, 123.58, 121.69, 121.10, 65.93. HRMS (ESI, m/z): Calcd for [M-CO2H]- C58H35N4S-: 819.2588, found: 819.2594. 2-Cyano-3-(4-(1-(4-(diphenylamino)phenyl)spiro[dibenzo[3,4:6,7]cyclohepta[1,2-b]quinoxal ine-10,9'-fluoren]-4-yl)phenyl)acrylic acid (FHD6). Orange powder, 80% yield. M.p.: 268-269°C. 1H NMR (500 MHz, DMSO-d6) δ (ppm): 13.97 (br. s, 1H), 8.42 (s, 1H), 8.34 (d, J=7.5 Hz, 1H), 8.28 (d, J=8.0 Hz, 1H), 8.21 (d, J=8.5 Hz, 2H), 8.19(d, J=8.0 Hz, 1H), 8.13-8.15 (m, 3H), 7.95 (d, J=7.5 Hz, 2H), 7.89 (d, J=8.5 Hz, 2H), 7.44 (t, J=7.5 Hz, 2H), 7.37-7.39 (m, 2H), 7.34 (t, J=8.0 Hz, 4H), 7.26-7.30 (m, 2H), 7.07-7.14 (m,12H), 6.63 (br. s, 2H). 13C NMR (126 MHz, CDCl3) δ (ppm): 156.00, 152.18, 152.00, 149.60, 147.72, 147.63, 145.53, 144.26, 140.99, 140.09, 139.64, 139.47, 138.03, 137.97, 137.46, 133.60, 131.73, 131.06, 130.60, 130.32, 129.54, 129.29, 128.40, 127.95, 127.84, 127.72, 127.14, 124.83, 123.17, 122.50, 120.42, 115.50, 101.69, 66.58. HRMS (ESI, m/z): Calcd for [M-CO2H]C60H37N4-: 813.3024, found: 813.3028. 3.3. Fabrication of DSSCs The active area of the TiO2 film was 0.25 cm2. The TiO2 photoanodes were sintered at 475 °C for 2 hours. After being cooled to 80 °C, the electrodes were immersed in the dye solutions (0.3 mM in CH2Cl2/MeOH (9:1)) unless otherwise noted for 12 hours. For the co-adsorbed 21



DSSCs, CDCA (3 mM) was added into the dye solutions. The electrodes and counter electrodes were sandwiched by using heat-sealing film. The internal space was filled with liquid electrolytes, which consists of LiI (0.1 M), I2 (0.05 M), 4-tert-butylpyridine (0.4 M), 1,2-dimethyl-3-propylimidazolium iodide (0.68 M) and guanidine thiocyanate (0.05 M) in acetonitrile/pentanenitrile (85:15).

4. CONCLUSIONS In summary, we have designed and synthesized three novel organic dyes featuring with SDBQX moieties (FHD4, FHD5 and FHD6). These dyes were fully characterized with 1H NMR,

13

C NMR and HRMS, etc. The fluorenyl moiety of SDBQX is perpendicular to the

quinoxaline moiety, which is beneficial to inhibit the H-aggregation in DSSCs. The band gap energies according to DFT calculations showed good tendency with the transition energy calculated from the absorption spectra, indicates that the DFT calculations would be an effective method to predict the absorption spectra range for FHD-type dyes. Broad spectral coverage and high molar extinction were observed in absorption spectrum of FHD4, which implied well light harvesting capability. As a result, the best power conversion efficiency was obtained for FHD4-based DSSC. Co-adsorption of CDCA improved the power conversion efficiency slightly for FHD4-based (from 4.61% to 4.69%) and FHD6-based (from 3.59% to 3.69%) DSSCs. The co-adsorption of CDCA decreased the dye loading amount of FHD5 significantly by hampering the dye aggregation, while the power conversion efficiency increased significantly from 3.18% to 3.73%. 22


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXXXXXX. 1

H and 13C NMR spectra of 2, 3a, 3b, 3c, FHD4, FHD5 and FHD6; HRMS spectra of FHD4,

FHD5 and FHD6 (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (J.-K.F.). *E-mail: [email protected] (A.P.). ORCID Jing-Kun Fang: 0000-0003-2606-2640 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions; Supported by the National Natural Science Foundation of China (21603230); Supported by Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06Y616); Supported by DongGuan Innovative Research Team Program (No. 201460720200028). 23



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