Tri(N-carbazolyl)triphenylamine-based starburst organic dyes: effects of different acceptors on the optical, electrochemical and photovoltaic properties

Abstract

Two novel tri(N-carbazolyl)triphenylamine-based organic dyes were designed, synthesized and applied as photosensitizers for dye-sensitized solar cells. The superiority of the starburst structure of these dyes is suppressing the intermolecular interactions and guaranteeing a fast electron injection into the TiO₂ semiconductor. Cyanoacrylic acid and rhodanine-3-acetic acid were separately used as electron acceptors/anchoring groups to compose the photosensitizers. Different acceptors led to striking changes in the photophysical, electrochemical and photovoltaic properties. An overall power conversion efficiency of 6.53% with a short-circuit photocurrent density of 13.4 mA cm⁻², an open-circuit photovoltage of 735 mV and a fill factor of 0.66 was achieved by the dye with tri(N-carbazolyl)triphenylamine as the donor and cyanoacrylic acid as the acceptor under AM 1.5 illumination (100 mW cm⁻²).

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Introduction

Efficient utilization of solar energy has attracted increasing interest due to its significant importance with respect to global warming and fossil fuel depletion. As low-cost solar energy-to-electricity conversion devices, dye-sensitized solar cells (DSSCs) have been extensively explored.¹ In DSSCs, the sensitizer is one of the crucial components for efficient light harvesting and high power conversion efficiency (PCE).² Recently, metal-free organic dye sensitizers have been widely investigated for their practical advantages, such as structural diversity, facile synthesis, high molar extinction coefficient, low cost and environment-friendly characteristics.³

Triphenylamine and carbazole are well-known electron-rich aromatic compounds and their derivatives are widely used as the donor units for organic dyes applied in DSSCs⁴ and organic photovoltaic cells (OPVs)⁵ due to their significant electronic and optical properties. 4,4′,4′′-Tri(N-carbazolyl)triphenylamine (TCTA) contains three electron-rich carbazole units linked to one electron-rich triphenylamine core and presents starburst structure. This unique structure endows TCTA with an excellent potential for electron donating and hole-transporting.⁶ Consequently, TCTA derivatives have been considerately used in the field of organic light-emitting diodes (OLEDs).⁷ But up to now, the photovoltaic application of TCTA derivatives in DSSCs has not been reported yet. The starburst structure of organic dyes may be also in favour of suppressing the intermolecular interactions, blocking the approach of triiodide in the electrolyte to the TiO₂ particles for charge recombination, and thus improving photovoltage of DSSCs.⁸ Inspired by this, we report herein the design, synthesis and characterization of novel organic dyes based on TCTA for the first time.

Most of the organic dyes are constructed with an electron donor, a π-conjugated bridge and an electron acceptor to give a typical D–π–A structure.⁹ Generally, organic dyes are required to possess broad and intense spectral absorption in the visible light region.¹⁰ Electron acceptor units also have significant influence on the photo-electronic properties because the excited electrons from the dye molecules are injected to the semiconductor film through the acceptor units.¹¹ To induce intramolecular charge transfer (ICT) from donor to acceptor efficiently, an electron-deficient acceptor is essential.¹² Due to their strong electron-withdrawing ability, cyanoacrylic acid and rhodanine-3-acetic acid have been used as efficient electron acceptors in a series of organic dyes.¹³ Consequently, cyanoacrylic acid and rhodanine-3-acetic acid were selected to act as the electron acceptor/anchoring group to elaborate the TCTA-based organic dyes, resulting in sensitizers JY16 and JY17, respectively. The molecular structures of the two dyes are shown in Fig. 1.

Fig. 1 Chemical structures of the dyes JY16 and JY17.

Results and discussion

Dye synthesis

The synthetic routes of the dyes JY16 and JY17 are shown in Scheme 1. The TCTA core was synthesized according to the previous literatures.¹⁴ Single bromination of the TCTA unit and subsequent Suzuki-coupling reaction produced the π-extended TCTA 2 bearing aldehyde group. Knoevenagel reaction of the resulting aldehyde 2 with cyanoacetic acid and rhodanine-3-acetic acid afforded the target dyes JY16 and JY17, respectively.

Scheme 1 Synthetic route for JY16 and JY17.

Photophysical and electrochemical properties

The UV-vis absorption spectra of the two dyes in THF solutions and on TiO₂ films are depicted in Fig. 2. The data of absorption and electrochemical properties are summarized in Table 1. In THF solutions, each of these TCTA-based dyes exhibited two major absorption bands at 300–370 and 370–550 nm. The high-energy absorption bands ranging from 300 to 370 nm correspond to the π–π* transition of the conjugated aromatic skeleton. The low-energy bands appearing at 370–550 nm can be attributed to the intramolecular charge transfer (ICT) transitions.¹⁵ The intensity of π–π* transitions of the two dyes are obviously higher than that of the ICT transitions. The maximum ICT absorption peak at 472 nm of JY17 is significantly red-shifted in comparison to that of JY16 (400 nm) owing to the relatively stronger electron-withdrawing ability of rhodanine-3-acetic acid relative to cyanoacrylic acid. The molar extinction coefficients at λ_max of JY16 and JY17 are 35 200 and 43 400 M⁻¹ cm⁻¹, respectively, which are more higher than those of conventional polypyridyl Ru(II) complexes dyes and conducive to the light harvesting of the solar cells.¹⁶ As shown in Fig. 2, when anchoring on mesoporous TiO₂ films, both JY16 and JY17 show significantly red-shifted and broadened absorption spectra compared to that in THF. The absorption spectra of JY16 and JY17 with coadsorbent CDCA were also measured, and they show nearly the same maximum absorption as that without CDCA. No obvious absorption shift has been observed, which indicates these TCTA-based dyes have a potential to suppress the J-aggregates of dye molecules onto the TiO₂ surface.^15d

Fig. 2 UV-Vis absorption spectra of JY16 and JY17 (a) in THF solutions and (b) on TiO₂ films.

Table 1 Photophysical, electrochemical data of JY16 and JY17^a

Dye	λmax/nm	ε/10^4 M^−1 cm^−1	Eox/V	E0–0/V	Ered/V
a First oxidation potentials (vs. NHE) in benzonitrile were calibrated with ferrocene (0.63 V vs. NHE). E0–0 values (zeroth–zeroth transition energies) were estimated from the onset wavelength in absorption spectra in THF. Ered = Eox − E0–0.
JY16	400	3.52	1.03	2.38	−1.35
JY17	472	4.34	1.05	2.21	−1.16

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Cyclic voltammetry measurement was performed to study the redox behavior of the dyes in benzonitrile calibrated against Fc/Fc⁺ (0.63 V vs. NHE),¹⁷ using 0.1 M TBAPF₆ as the supporting electrolyte. Cyclic voltammograms of the dyes are depicted in Fig. 3. The dyes JY16 and JY17 both exhibited two reversible oxidation couples. The HOMO levels of the two dyes correspond to the first oxidation potential (E_ox). The LUMO levels could be calculated from E_ox − E_0–0. The zeroth–zeroth transition energies (E_0–0) of the two dyes JY16 and JY17 were 2.38 and 2.21 eV, respectively, which were calculated from their onset wavelength in absorption spectra in THF solutions. The first oxidation potentials of the two dyes could be determined from the cyclic voltammograms. As shown in Fig. 3, the oxidation potential of the two dyes show a first reversible oxidation wave at 1.03 V (JY16) and 1.05 V (JY17), respectively. The HOMO levels of the two dyes are evaluated to be higher than the redox potential of the iodide/triiodide redox couple (0.4 V vs. NHE), which guarantees efficient dye regeneration. On the other hand, the LUMO levels of the two dyes (−1.35 and −1.16 V vs. NHE, respectively) are both more negative than the conduction band (CB) of TiO₂ (−0.5 V vs. NHE), allowing efficient electron injection from the excited dyes into the TiO₂ electrode.¹⁸

Fig. 3 Cyclic voltammograms of JY16 and JY17 recorded in benzonitrile solutions.

Computational analysis

To investigate the geometrical and electronic properties of the two dyes, density functional theory (DFT) calculations were conducted by using the Gaussian 03 program package at the B3LYP/6-31G(d) level. Fig. 4 displays the electron distributions of the HOMO−1, HOMO, LUMO and LUMO+1 of the two dyes. It can be clearly seen that the HOMOs of the two dyes predominantly located on the TCTA core. While the LUMOs delocalized over the cyanoacrylic acid and rhodanine-3-acetic acid acceptor as well as the thiophene unit and the neighboring carbazole unit. For the HOMO−1 and LUMO+1 orbitals, the electron distributions mainly located on the thiophene-linked carbazole unit and the acceptors. Notably, the HOMO, HOMO−1, LUMO and LUMO+1 orbitals are more important than other orbitals because their electron transitions are very favorable for the photon-to-electron conversion. Therefore, this electron distribution will allow significant charge transfer from the donor to the acceptor through the thiophene π-bridge and hence efficient electron injection from the excited dyes into the TiO₂ semiconductor.

Fig. 4 The frontier molecular orbital distributions of JY16 and JY17 optimized by DFT calculations.

The further quantum chemistry calculation was performed with TDDFT methods. From the analysis of the TDDFT results in Table 2, we can see that the observed ICT absorption band for JY16 stems mainly from the charge-transfer transition from HOMO−1 to LUMO (85%, f = 0.59). The same phenomenon was observed for JY17 (86%, f = 0.81). As a result, the electronic transitions in the two dyes are positively contributed to electron injection since ICT transitions correspond to electron transfer from the donor group to the acceptor group.¹⁹

Table 2 Excitation wavelengths, orbital energies, oscillator strength (f) and assignment of the two dyes in vacuum computed using B3LYP/6-31G(d)

Dye	Excited state	Calculated energy (eV, nm)	f	Transition assignment^a
a H = HOMO, L = LUMO.
JY16	1	2.50, 495	0.19	H → L (96%)
	2	2.78, 446	0.59	H−1 → L (85%)
JY17	1	2.44, 508	0.33	H → L (95%)
	2	2.65, 467	0.81	H−1 → L (86%)

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DSSC performance

The DSSC performance parameters based on JY16 and JY17 are investigated and displayed in Table 3. The photocurrent density–voltage (J–V) curves and the IPCE spectra of the DSSCs based on JY16 and JY17 are plotted in Fig. 5. The DSSC performances of the dyes are evaluated under AM 1.5 G irradiation at 100 mW cm⁻² with 0.196 cm² active surface area, using an iodine electrolyte composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I₂ and 0.5 M 4-tert-butyl pyridine (TBP) in acetonitrile. The JY16-sensitized cell gave a power conversion efficiency (PCE) of 6.53% with a short-circuit photocurrent density (J_sc) of 13.4 mA cm⁻², an open-circuit photovoltage (V_oc) of 735 mV, and a fill factor (FF) of 0.66. At the same conditions, a DSSC based on JY17 showed a significant decreased PCE of 3.47% with a relatively smaller J_sc of 9.2 mA cm⁻² and a lower V_oc of 580 mV. The better efficiency of JY16 reached about 84% of the commercial N719-based cell (J_sc = 16.8 mA cm⁻², V_oc = 740 mV, FF = 0.63, PCE = 7.80%) that fabricated and measured under the same conditions, indicating that the TCTA donor is a promising unit to construct effective organic dyes for DSSCs. Compared to JY17, the significant improvement of photovoltaic performances for JY16 may be attributed to the more effective electron injection. Notably, the PCEs of the rhodanine-3-acetic acid-based JY17 were significant lower than the cyanoacrylic acid-based JY16, which might be ascribed to the broken electron-transfer path from the dyes to the TiO₂ semiconductor caused by the broken (NCH₂COOH) conjugation of rhodanine-3-acetic acid acceptor.²⁰ Such feature was also supported by the results of Gaussian calculations, and the electron distributions were absent in NCH₂COOH group. The influences of CDCA co-adsorption on their photovoltaic performances were also investigated. When co-adsorbed with CDCA, the PCEs of the two dyes were both decreased (JY16: 6.25%; JY17: 3.09%), which was mainly due to the reduction of J_sc (JY16: 12.8 mA cm⁻²; JY17: 8.0 mA cm⁻²). Furthermore, the amounts of dye loading of JY16 and JY17 in the absence of CDCA are measured to be 1.20 × 10⁻⁷ and 3.45 × 10⁻⁷ mol cm⁻², respectively. However, when co-adsorbed with CDCA, the adsorption amounts of the two dyes significantly decreased to 1.05 × 10⁻⁷ and 1.81 × 10⁻⁷ mol cm⁻², respectively, which may be the main reason of the reduction of J_sc.²¹

Table 3 Photovoltaic performance of JY16 and JY17 using N719 as a reference^a

Dye	CDCA	Dye loading/10^−7 mol cm^−2	Voc/mV	Jsc/mA cm^−2	FF	PCE/%
a The active area of the cells was 0.196 cm^2; the electrolyte was composed of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine in acetonitrile.
JY16	0	1.20	735	13.4	0.66	6.53
JY16	10 mM	1.05	739	12.8	0.66	6.25
JY17	0	3.45	580	9.2	0.65	3.47
JY17	10 mM	1.81	583	8.0	0.66	3.09
N719	0	—	740	16.8	0.63	7.80

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Fig. 5 (a) The photocurrent density–voltage (J–V) curves and (b) the IPCE spectra of JY16 and JY17.

Fig. 5 shows the IPCE spectra of DSSCs based on JY16 and JY17. The TCTA-based two dyes could both efficiently convert the light to photocurrents in the UV-Vis region. The IPCE values changing tendency of these dyes were in accordance with their UV-Vis absorption spectra. The significantly broader IPCE response of JY17 results from the relatively stronger electron-withdrawing ability of rhodanine-3-acetic acid relative to cyanoacrylic acid. However, the solar cell based on JY16 showed a significantly higher IPCE value compared to JY17. The higher IPCE value of JY16 explained the higher J_sc value compared with that of JY17 from the J–V measurement and also indicated a more effective electron injection of cyanoacrylic acid acceptor. When co-adsorbed with CDCA, the IPCE values of the cells both decreased due to the reduction of J_sc values.

Electrochemical impedance spectroscopy

To elucidate the correlation between the V_oc and the dyes, electrochemical impedance spectroscopy (EIS) was then performed to investigate the effects on charge recombination, transport, and collection.²² EIS Nyquist plots and Bode phase plots measured in the dark under a forward bias (−0.70 V) with frequency range of 0.1–100 kHz are shown in Fig. 6. The larger semicircle in the lower frequency range in the Nyquist plot represents the interfacial charge transfer resistances (R_ct) at the TiO₂/dye/electrolyte interface.²³ A larger radius of the major semicircle means a larger charge transfer resistance. The fitted R_ct increases in the order of JY17 (14 Ω) < JY16 (141 Ω) implying significantly increasing resistance to charge recombination. This feature coincides with the increase of open-circuit photovoltage, as the suppression of electron recombination between the injected electrons and electrolyte improves open-circuit photovoltage. This trend is in good agreement with V_oc values of the two dyes. Electron lifetime (τ) could be calculated from the peak frequency (f) in the lower frequency region in EIS Bode plots using τ = 1/(2πf).²⁴ Longer electron lifetimes imply increased resistance between the injected electrons and the electrolyte, which consequently improves the V_oc. The fitted electron lifetime (τ) increases from JY17 (6.4 ms) to JY16 (47.0 ms). The increase in electron lifetime for JY16 supports more effective suppression of the back reaction of the injected electrons with the iodide/triiodide redox couple in the electrolyte,²⁵ which leads to the improvement of photocurrent and photovoltage and gives substantial enhancement of the device efficiency. This result is also in agreement with the abovementioned observations.

Fig. 6 (a) Nyquist plots and (b) Bode plots for DSSCs based on JY16 and JY17.

Conclusions

In summary, novel organic dyes based on 4,4′,4′′-tri(N-carbazolyl)triphenylamine have been successfully synthesized and utilized in dye-sensitized solar cells. These type of organic dyes contain tri(N-carbazolyl)triphenylamine acting as the electron donor and a thiophene unit acting as the π-conjugated linker. Different acceptors such as cyanoacrylic acid (JY16) and rhodanine-3-acetic acid (JY17) led to striking changes of photophysical, electrochemical, and photovoltaic properties of TCTA-based dyes. Notably, the dye JY16 exhibited relatively more efficient electron injection ability, lower electron recombination rate and higher PCE than JY17. A solar cell based on JY16 achieved an overall conversion efficiency of 6.53%, with a short-circuit photocurrent density of 13.4 mA cm⁻², an open-circuit photovoltage of 735 mV and a fill factor of 0.66 under AM 1.5 G irradiation at 100 mW cm⁻².

Experimental

Materials and methods

All NMR solvents and other chemicals were used as received without further purification. The solvents were purified according to standard methods. FTO conductive glasses were used (Nippon Sheet Glass, Japan, fluorine-doped SnO₂ over layer, sheet resistance of 15 Ω per sq.). ¹H NMR and ¹³C NMR spectra were recorded on Bruker 400 MHz spectrometer. UV-Vis spectra were obtained on a Varian Cary 300 Conc UV-visible spectrophotometer. HR-MS data was obtained on a Varian 7.0T FTMS. Cyclic voltammetry experiments and electrochemical impedance spectroscopy (EIS) experiments were carried out using an electrochemical workstation (Zennium, Zahner Corporation). Cyclic voltammetry experiments were performed with a conventional three-electrode system employing a glassy carbon electrode as the working electrode, a Ag/Ag⁺ electrode as the reference electrode, and a Pt wire as the counter electrode. The redox potentials were measured in benzonitrile, using 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte with a scan rate of 100 mV S⁻¹. IR spectra were recorded on the Bruker Tensor 27 using KBr disk.

Fabrication and characterization of DSSCs

A 20 nm mesoporous TiO₂ film (∼10 μm) for the transparent nanocrystalline layer was prepared by doctor-blade method using a commercial TiO₂ sol (Heptachroma Corporation) onto the FTO conductive glass. The main function of the mesoporous TiO₂ layer is providing a large surface area to adsorb more dye molecules, and realizing an efficient photoinduced electron transfer process. The 200 nm scattering layer (∼4 μm) was applied over the transparent layer by doctor-blade method, then gradually heated to 500 °C and sintered for 60 min. The resulting TiO₂ electrodes were treated by 40 mM TiCl₄ aqueous solution at 70 °C for 60 min and sintered again at 500 °C for 60 min. The Pt electrode was obtained by thermal deposition a platinum layer (0.02 M hexachloroplatinic acid in isopropanol) on the surface of FTO glass at 450 °C for 30 min. The TiO₂ photoanodes were immersed in the commercial N719 (Solaronix Corporation) dye solution (0.3 mM in ethanol) for 12 h. The adsorption of the TCTA-based dyes on TiO₂ was carried out in 0.3 mM dye solution in THF for 12 h. The electrolyte was composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I₂ and 0.5 M 4-tert-butylpyridine in acetonitrile. The measurement of the incident photon-to-current conversion efficiency (IPCE) was performed using a commercial setup (QE-PV-SI, Newport Corporation). The DSSCs were illuminated by a solar simulator (CHF-XM-500W, Trusttech Co. Ltd.) under 100 mW cm⁻² irradiation, which was calibrated by a standard silicon solar cell (91150V, Newport Corporation). The photocurrent intensity–voltage (J–V) characteristic curves of the DSSC under simulated sunlight were recorded using an electrochemical workstation (Zennium, Zahner Corporation).

Synthetic procedures

Synthesis of 5-(9-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)-9H-carbazol-3-yl)thiophene-2-carbaldehyde (compound 2). A solution of N-bromosuccinimide (NBS) (96 mg, 0.54 mmol) in THF (5 mL) was added dropwise to the chloroform (30 mL) solution containing compound 1 (400 mg, 0.54 mmol) at 0 °C. Then the mixture was slowly warmed to room temperature and stirred for 2 h before it was poured into water. The organic phase was separated and dried over anhydrous Na₂SO₄. After the solvent was evaporated, the crude product was transferred into a two-neck round bottomed flask. 5-Formylthiophene-2-boronic acid (142 mg, 0.91 mmol), Pd(dppf)Cl₂ (44 mg, 0.054 mmol), Cs₂CO₃ (528 mg, 1.62 mmol), and toluene (25 mL) were successively added. The flask was charged with N₂. The mixture was heated under reflux for 5 h before it was poured into water. Then, the organic phase was separated and dried over anhydrous Na₂SO₄. After the solvent was removed under vacuum, the crude product was purified by silica-gel column chromatography using CH₂Cl₂/petroleum ether (1 : 1) as the eluent to afford compound 2 as a yellow solid (yield: 40.4%). IR (KBr) ν: 3349, 3282, 2920, 2849, 1659, 1506, 1450, 1311, 1228, 797, 747 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 9.92 (s, 1H), 8.50 (s, 1H), 8.23 (dd, J = 14.5, 7.7 Hz, 5H), 7.80 (d, J = 5.1 Hz, 2H), 7.65–7.47 (m, 24H), 7.41 (d, J = 7.1 Hz, 1H), 7.35 (t, J = 7.3 Hz, 4H). ¹³C NMR (101 MHz, CDCl₃) δ 182.66, 155.93, 146.89, 146.28, 141.67, 141.56, 141.50, 140.95, 137.85, 133.05, 131.97, 128.33, 128.18, 126.79, 125.98, 125.56, 125.27, 125.11, 124.72, 123.99, 123.38, 123.16, 123.05, 120.67, 120.64, 120.41, 120.01, 118.55, 110.53, 110.27, 109.80. HR-MS (MALDI): m/z [M]⁺ calcd for C₅₉H₃₈N₄OS, 850.2766; found, 850.2760.

Synthesis of (E)-3-(5-(9-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)-9H-carbazol-3-yl)thiophen-2-yl)-2-cyanoacrylic acid (JY16). A mixture of compound 2 (55 mg, 0.065 mmol), ammonium acetate (28 mg, 0.37 mmol), cyanoacrylic acid (23 mg, 0.12 mmol), and acetic acid (10 mL) was heated at reflux for 5 h under a nitrogen atmosphere. After cooling to room temperature, it was precipitated by pouring into water. The resulting solid was filtered, washed thoroughly with water. Then, the crude product was purified by silica-gel column chromatography using CH₂Cl₂/CH₃OH (10 : 1) as the eluent to afford JY16 as a red solid (yield: 85.6%). IR (KBr) ν: 3479, 3050, 2850, 2807, 1663, 1553, 1508, 1451, 1408, 1267, 1231, 1211, 1061, 743 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 13.71 (s, 1H), 8.78 (d, J = 1.9 Hz, 1H), 8.52 (s, 1H), 8.43 (d, J = 7.8 Hz, 1H), 8.27 (d, J = 7.7 Hz, 4H), 8.07 (d, J = 4.1 Hz, 1H), 7.90 (dd, J = 8.6, 1.9 Hz, 1H), 7.85 (d, J = 4.0 Hz, 1H), 7.78–7.64 (m, 6H), 7.62–7.54 (m, 6H), 7.54–7.42 (m, 10H), 7.41–7.32 (m, 1H), 7.34–7.24 (m, 4H), 7.23–7.08 (m, 1H). ¹³C NMR (101 MHz, DMSO-d₆) δ 164.28, 157.85, 155.16, 147.19, 146.85, 146.37, 142.31, 141.56, 141.41, 140.76, 134.09, 132.56, 131.65, 129.37, 128.69, 127.50, 126.70, 125.99, 125.45, 125.36, 125.09, 124.73, 123.94, 123.12, 122.95, 121.71, 121.67, 121.14, 121.01, 120.48, 119.19, 117.15, 111.26, 110.23. HR-MS (MALDI): m/z [M]⁺ calcd for C₆₂H₃₉N₅O₂S, 917.2824; found, 917.2820.

Synthesis of (Z)-2-(5-((5-(9-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)-9H-carbazol-3-yl)thiophen-2-yl)methylene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (JY17). A mixture of compound 2 (60 mg, 0.071 mmol), ammonium acetate (31 mg, 0.41 mmol), rhodanine-3-acetic acid (21 mg, 0.11 mmol), and acetic acid (15 mL) was heated at reflux for 5 h under a nitrogen atmosphere. After cooling to room temperature, it was precipitated by pouring into water. The resulting solid was filtered, washed thoroughly with water. Then, the crude product was purified by silica-gel column chromatography using CH₂Cl₂/CH₃OH (10 : 3) as the eluent to afford JY17 as a red solid (yield: 80.4%). IR (KBr) ν: 3358, 3055, 2919, 2849, 1729, 1704, 1655, 1628, 1577, 1507, 1451, 1266, 744 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 13.50 (s, 1H), 8.79 (s, 1H), 8.44 (d, J = 7.8 Hz, 1H), 8.27 (d, J = 7.8 Hz, 4H), 8.17 (s, 1H), 7.93 (dd, J = 8.6, 1.9 Hz, 1H), 7.87 (d, J = 4.1 Hz, 1H), 7.84 (d, J = 4.0 Hz, 1H), 7.76–7.64 (m, 6H), 7.62–7.46 (m, 16H), 7.40–7.34 (m, 2H), 7.34–7.25 (m, 4H), 4.73 (s, 2H). ¹³C NMR (101 MHz, DMSO-d₆) δ 192.36, 167.78, 166.49, 154.54, 146.77, 146.35, 141.48, 141.18, 140.73, 139.04, 135.74, 132.52, 131.69, 130.77, 128.67, 128.63, 127.59, 127.44, 126.70, 125.95, 125.44, 125.34, 125.24, 124.97, 123.94, 123.11, 123.00, 121.70, 121.06, 121.01, 120.47, 118.86, 118.07, 111.10, 110.61, 110.22. HR-MS (MALDI): m/z [M]⁺ calcd for C₆₄H₄₁N₅O₃S₃, 1023.2372; found, 1023.2368.

Acknowledgements

We thank the 973 Program (2011CB932502), and the National Natural Science Foundation of China (nos 21172126 and 21272123), and NFFTBS (no. J1103306) for their generous financial support.

Notes and references

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