Phenothiazine-functionalized push–pull Zn porphyrin photosensitizers for efficient dye-sensitized solar cells

Abstract

A series of zinc porphyrin dyes (JY24–27) featured phenothiazine moieties have been synthesized and applied as photosensitizers in dye-sensitized solar cells. The phenothiazine donors were directly attached to the meso-position of porphyrins, and different π-linkers (benzene, thiophene and ethynyl benzene) and acceptor groups (carboxylic acid and cyanoacrylic acid) were applied to tune the photoelectric properties of these push–pull porphyrin dyes. The photophysical, electrochemical and theoretical studies revealed that the synthesized porphyrin dyes were all capable of being used as photosensitizer. The dye JY27 with an extended conjugation by introducing ethynyl group exhibited a broader absorption region and more significantly improved IPCE values between 550 and 750 nm than the other three dyes, which ensured a good light-harvesting ability and a high short-circuit current density of 15.3 mA cm⁻². Finally, JY27-based cell achieved a high efficiency of 6.25% under standard simulated irradiation.

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Introduction

As low-cost devices for efficient power conversion, dye-sensitized solar cells (DSSCs) have been investigated under intense academic and industrial research during the past two decades.¹ For a DSSC device, the photosensitizer plays a key role in the light-harvesting, charge transfer and cell performance.² Because of their excellent light-harvesting ability and pivotal role in the simulation of photosynthesis, porphyrin-based dyes have drawn great attentions.^3–9 Besides the virtues of broad absorption composed by intense Soret bands (400–450 nm) and moderately intense Q bands (500–650 nm), it is easy to tune the photophysical and electrochemical properties of porphyrin dyes due to the existence of multiple meso- and β-modification sites.⁴ The research works about porphyrin dyes before 2004 was reviewed by Officer et al.⁵ Some more recent works has been reviewed by many groups.^6–9

Most efficient porphyrin dyes adopt a push–pull type structure which has proved to facilitate the electron transfer from donor group to acceptor group, meanwhile the expansion of the π-conjugated linker and taking a unsymmetrical structure has been found to be beneficial for broadening and red-shifting the absorption of porphyrin dyes.¹⁰ Some promising results, much better than the conventional Ru(II) complexes, have recently been reported. Sooner after the landmark power conversion efficiency (PCE) of 12.3% was achieved with the push–pull type zinc porphyrin photosensitizer (YD2-o-C8),¹¹ the PCE of porphyrin-based cell was improved to 12.7% by Yeh and Grätzel et al. via inserting an electron-withdrawing benzothiadiazole (BTD) moiety between the linker and the acceptor group.¹² Imahori and co-workers reported some triarylamine-substituted imidazole- and quinoxaline-fused porphyrins for efficient DSSCs, demonstrating the construction of a push–pull structure in β positions was also effective.¹³ Lately, a record efficiency of 13% has been achieved through a push–pull structured porphyrin dye SM315 with a bulky diphenylamine donor.¹⁴

Phenothiazine is a kind of heterocyclic compounds which possess simultaneously a nitrogen atom with a lone electron pair and an electron-giving sulfur atom in the same six-member ring, and shows a butterfly conformation in the ground state which can impede the intermolecular aggregation. Due to the existence of electron-rich sulfur atom, a better electron-donating ability than commonly used amine-type dyes such as phenylamine and carbazole,¹⁵ has been endowed for phenothiazine. Phenothiazine has therefore been widely used in DSSCs as a donor segment of pure organic dye.¹⁶ In view of the strong electron donating ability and weak molecular aggregating tendency of phenothiazine group, we report herein the design, synthesis and characterization of novel phenothiazine-functionalized push–pull zinc porphyrin dyes JY24–27. Notably, their optical, electrochemical, and photophysical properties can be facilely modulated through different π-linkers (benzene, thiophene, and ethynyl benzene) and electron-withdrawing/anchoring groups (carboxylic acid and cyanoacrylic acid). The chemical structures of the four porphyrin dyes are shown in Fig. 1.

Fig. 1 Structures of the phenothiazine-modified porphyrin dyes.

Results and discussion

Molecular synthesis

The sensitizers JY24–27 were synthesized through the synthetic route depicted in Scheme 1. The precursors 1 and 2 were prepared according to literature procedures.^17,18 Suzuki cross-coupling of porphyrin 1 with borate 2 afforded phenothiazine-substituted porphyrin freebase 3a. Subsequent bromination of 3a with NBS gave brominated porphyrin 3b. Another Suzuki cross-coupling of porphyrin 3b with corresponding boronic acid derivatives were carried out to afford the key intermediates 4a–c. Sonogashira cross-coupling was adopted to produce intermediate 5 by treating porphyrin 3b with 4-(methoxycarbonyl)phenylacetylene. Finally, the porphyrin esters 4a and 5 were hydrolyzed and successively metallized to yield the dyes JY24 and JY27. With respect to the aldehyde-substituted porphyrin 4b and 4c, the Knoevenagel reaction with cyanoacetic acid and subsequent metallization with zinc acetate were performed to give the other dyes JY25 and JY26.

Scheme 1 Synthetic routes to JY24, JY25, JY26 and JY27.

Photophysical and electrochemical measurements

The photophysical properties of JY24, JY25, JY26 and JY27 were investigated by UV-visible absorption spectra. The UV-vis spectra of the four dyes in dichloromethane solutions and on TiO₂ films are displayed in Fig. 2. As can be seen from Fig. 2a, all four porphyrins exhibit two distinct absorption bands at 400–500 and 530–650 nm, corresponding to the Soret and Q bands, respectively. The intensity of Soret bands is clearly higher than that of the Q bands. Absorption peaks (λ_abs) for JY24 in CH₂Cl₂ are at about 422 nm (ε = 4.04 × 10⁵ M⁻¹ cm⁻¹) in the Soret band, 550 nm (ε = 0.25 × 10⁵ M⁻¹ cm⁻¹) and 592 nm (ε = 0.09 × 10⁵ M⁻¹ cm⁻¹) in the Q band. After replacing the electron-withdrawing anchoring group with the cyanoacrylic acid moiety (JY25 and JY26), the absorption peaks are red-shifted and broadened, due to the enhanced electron-withdrawing ability of the cyanoacrylic acid compared to that of carboxylic acid. When introducing ethynyl group between benzoic acid moiety and porphyrin ring, JY27 exhibits the broadest and red-shifted absorption bands with peaks at 442 nm (ε = 3.01 × 10⁵ M⁻¹ cm⁻¹) in the Soret band, 568 nm (ε = 0.12 × 10⁵ M⁻¹ cm⁻¹) and 620 nm (ε = 0.27 × 10⁵ M⁻¹ cm⁻¹) in the Q band, illustrating the addition of the alkyne group is beneficial for the relief of the bulkiness between porphyrin donor and benzoic acid acceptor and facilitating the interactive electronic communication. As shown in Fig. 2b, when adsorbed on TiO₂, all these dyes showed significantly broadened and red-shifted absorption peaks in comparison to the corresponding spectra recorded in dichloromethane solutions. The extended absorption region when anchoring on TiO₂ surface may be caused by the interaction between the dye and titanium dioxide, or the intermolecular interaction, or both of them. The data of photophysical and electrochemical properties of the four dyes are summarized in Table 1.

Fig. 2 UV-vis absorption spectra of the four dyes (a) in DCM solutions and (b) on TiO₂ films.

Table 1 Photophysical, electrochemical data of JY24–27^a

Dye	λabs/nm	ε/10^5 M^−1 cm^−1	λem/nm	Eox/V	E0–0/eV	E^*ox/V
a Absorption and emission spectra were recorded in DCM; first oxidation potentials (vs. NHE) in benzonitrile were calibrated with ferrocene (0.63 V vs. NHE); E0–0 values were estimated from the intersections of normalized absorption and emission spectra; E^*ox = Eox − E0–0.
JY24	422	4.04	597	0.92	2.08	−1.16
	550	0.25
	592	0.09
JY25	424	2.46	602	0.93	2.06	−1.13
	550	0.23
	593	0.13
JY26	425	2.66	612	0.94	2.03	−1.09
	554	0.22
	602	0.13
JY27	442	3.01	624	0.94	1.98	−1.04
	570	0.12
	617	0.27

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To evaluate both the electron injection and the regeneration of the excited porphyrin dyes on TiO₂ electrode, cyclic voltammetry measurements were operated in benzonitrile with a scan rate of 100 mV s⁻¹ and calibrated by ferrocene (0.63 vs. NHE). Cyclic voltammograms of the four dyes are shown in Fig. 3. Obviously, all four dyes exhibited three reversible oxidation couples. The ground-state oxidation potentials (E_ox) of JY24–27 were 0.92, 0.93, 0.94 and 0.94 V (vs. NHE), respectively. The zero–zero excitation energies (E_0–0) of the four dyes JY24–27 were 2.08, 2.06, 2.03 and 1.98 eV, respectively, which were estimated from the intersections (λ_int) of normalized absorption and emission spectra using E_0–0 = 1240/λ_int. The excited-state oxidation potentials (E^*_ox) of JY24–27, calculated from E_ox − E_0–0, were −1.16, −1.13, −1.09 and −1.04 V (vs. NHE), respectively. The energy levels of the four dyes are depicted in Fig. 4. Absolutely, all E^*_ox values of JY24–27 are more negative than the conduction band (CB) edge of the TiO₂ (−0.5 V vs. NHE), suggesting the electron injection from the excited-state dye into the CB of TiO₂ is favorable. On the other hand, the E_ox values of JY24–27 are sufficiently more positive than the I⁻/I₃⁻ redox couple (+0.4 V vs. NHE) indicating the oxidized dyes can be easily reduced through accepting electrons from I⁻ ions.

Fig. 3 Cyclic voltammograms of the four dyes recorded in benzonitrile solutions.

Fig. 4 Schematic energy-level diagrams of the four porphyrin dyes.

Computational analysis

To deep investigate the electron distributions of the frontier molecular orbitals of the four porphyrins, density functional theory (DFT) calculations were performed by the Gaussian03 package at the B3LYP/6-31G* level. All alkyl groups of the dyes were replaced by methyls to shorten the computation time. The frontier molecular orbital distributions of the four porphyrins are shown in Fig. 5. For the HOMO orbitals of all dyes, the electron distributions are mainly delocalized through the phenothiazine donor group and the porphyrin core. The distributions of electronic density in the LUMO states are located primarily at the porphyrin core, the acceptor and π-spacer. Therefore, the excitation of the electrons from HOMO to LUMO can result in efficient transportation of the electrons from the donor groups to the anchoring moieties, indicating the four porphyrin dyes are suitable for application in DSSC devices.

Fig. 5 The HOMO and LUMO distributions of four porphyrin dyes optimized by DFT calculations.

DSSC performances

The photovoltaic performances of the solar cells based on JY24–27 under AM 1.5G (100 mW cm⁻²) irradiation were investigated using an iodine electrolyte (0.3 M DMPII, 0.1 M LiI, 0.05 M I₂, and 0.5 M 4-TBP in acetonitrile). As is known to all, the dye-bath solvent has an important influence on the photovoltaic performances of DSSCs. The influence of different solvents on the photovoltaic performance of DSSC devices based on dye JY25 were measured (Table 2). The corresponding photocurrent density–voltage (J–V) curves are depicted in Fig. 6. Changes in the dye concentration at the surface of TiO₂ may alter the intermolecular interactions. Apparently, the devices using the photoanodes constructed from toluene performed higher short-circuit photocurrent density (J_sc) and open-circuit photovoltage (V_oc) than those derived from other solutions, resulting in the highest PCE. That indicates toluene is the favorable solvent for these phenothiazine-functionalized porphyrin dyes. Therefore, toluene was used as the dye-bath solvent for the other phenothiazine-functionalized porphyrin dyes JY24, JY26 and JY27.

Table 2 Photovoltaic performances of JY25 sensitized in different solvents^a

Solvents	Voc/mV	Jsc/mA cm^−2	FF	PCE/%
a Active area of the cells was 0.196 cm^2.b The ratio of toluene/EtOH solvent is 1/1 (v/v).
DCM	670	12.0	0.61	4.91
THF	622	12.2	0.60	4.53
DMF	620	10.6	0.63	4.18
Toluene	704	12.4	0.60	5.21
Toluene/EtOH^b	664	11.3	0.62	4.64

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Fig. 6 The J–V curves of JY25-based cells sensitized in different solvents.

The DSSCs performance parameters of JY24–27 and commercial N719 dye are displayed in Table 3. The photocurrent density–voltage (J–V) curves of JY24–27 are plotted in Fig. 7a. The adsorption of the porphyrin dyes on TiO₂ was carried out with 0.2 mM dye solution in toluene. The amounts of dye loading for JY24–27 were determined to be 83.7, 77.0, 87.9 and 91.3 nmol cm⁻², respectively. The overall power conversion efficiencies lie in the range of 4.01–6.25%, with an order of JY24 < JY25 < JY26 < JY27. The dye JY24 exhibited a lowest short-circuit J_sc of 10.5 mA cm⁻², a lowest V_oc of 621 mV and a fill factor (FF) of 0.61, finally generating a lowest PCE of 4.01%. The PCEs of JY25 (5.21%) and JY26 (5.80%) were both much higher than that of JY24. When inserting ethynyl group into the middle of benzoic acid moiety and porphyrin skeleton, the dye JY27 achieved a highest PCE of 6.25%, with a J_sc of 15.3 mA cm⁻², a V_oc of 677 mV and a fill factor (FF) of 0.60. The efficiency of JY27 reached about 88% of the commercial N719-based cell (V_oc = 750 mV, J_sc = 15.4 mA cm⁻², FF = 0.61, PCE = 7.08%) that fabricated and measured under the same conditions, indicating that phenothiazine is a promising donor unit to construct effective push–pull Zn porphyrin dyes for DSSCs.

Table 3 Photovoltaic performances of JY24–27 with N719 as a reference^a

Dye	DL/nmol cm^−2	Voc/mV	Jsc/mA cm^−2	FF	PCE/%
a Active area of the cells was 0.196 cm^2; DL was the amount of dye loading.b The cell immersed in ethanol solution of the commercial N719 dye was used for comparison.
JY24	83.7	621	10.5	0.61	4.01
JY25	77.0	704	12.4	0.60	5.21
JY26	87.9	661	14.1	0.62	5.80
JY27	91.3	677	15.3	0.60	6.25
N719^b	—	750	15.4	0.61	7.08

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Fig. 7 (a) The J–V curves and (b) the IPCE spectra of JY24–27.

The effect of the structural change of dye molecules on the cell photocurrent characteristics was further investigated to analyze the difference between the J_sc values of the four dyes, and the incident photon-to-current conversion efficiency (IPCE) as a function of incident wavelength for DSSCs based on these dyes are plotted in Fig. 7b. As shown in IPCE spectra, all of the dyes JY24–27 can efficiently convert the light to photocurrents in the UV-vis region. In good agreement with the rank of absorption spectra, the onset wavelengths of photocurrent responses were red-shifted from 700 nm for JY24 to 750 nm for JY27. JY27-based cell gave over 50% IPCE values from 390 to 670 nm with two maximum IPCE values at 440 nm (78%) in the Soret band region and at 630 nm in the Q band region (75%), respectively. Benefited from the introduced ethynyl group, JY27 exhibited the significant improvement of IPCE values ranged from 550 to 750 nm, which ensured a better light-harvesting ability and higher photocurrent than the other three dyes. The high IPCE value and broader absorption of JY27 explained the high J_sc value from the J–V measurement. The somewhat higher efficiency observed for JY27 was mainly due to the largest J_sc.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was performed to analyze the complicated interfacial charge transfer processes in DSSCs.¹⁹ The Nyquist plots and equivalent circuit diagram of the solar cells based on JY24, JY25, JY26 and JY27 measured in the dark under forward bias (−0.70 V) are displayed in Fig. 8. The larger semicircle of the Nyquist plot is attributed to the interfacial charge transfer resistances (R_ct) at the TiO₂/dyes/electrolyte interface.²⁰ A larger R_ct indicates the electron recombinations in the cells are reduced and therefore a higher photovoltage. The fitted R_ct increased in the order of JY24 (24.6 Ω) < JY26 (28.5 Ω) < JY27 (41.5 Ω) < JY25 (71.9 Ω). This trend was consistent with the order of V_oc of JY24 (621 mV) < JY26 (661 mV) < JY27 (677 mV) < JY25 (704 mV). Besides, electron lifetime (τ) can be extracted from C_μ (chemical capacitance) and R_ct, using τ = R_ct × C_μ.²¹ Generally, a longer electron lifetime indicates the effective reduction of the reverse reaction of the injected electron with the triiodide in the electrolyte and therefore a higher photovoltage.²² For the four devices, the calculated electron lifetime values increased in the order of JY24 (15.4 ms) < JY26 (30.5 ms) < JY27 (47.8 ms) < JY25 (60.0 ms). So the highest V_oc of JY25-based cell can be explained.

Fig. 8 The Nyquist plots and equivalent circuit diagram (inset) for DSSCs based on the four dyes measured in the dark under −0.70 V bias with frequency range from 100 kHz to 100 mHz.

Electrochemical impedance spectroscopy (EIS) was performed to analyze the complicated interfacial charge transfer processes in DSSCs.¹⁹ The Nyquist plots and equivalent circuit diagram of the solar cells based on JY24, JY25, JY26 and JY27 measured in the dark under forward bias (−0.70 V) are displayed in Fig. 8. The larger semicircle of the Nyquist plot is attributed to the interfacial charge transfer resistances (R_ct) at the TiO₂/dyes/electrolyte interface.²⁰ A larger R_ct indicates the electron recombinations in the cells are reduced and therefore a higher photovoltage. The fitted R_ct increased in the order of JY24 (24.6 Ω) < JY26 (28.5 Ω) < JY27 (41.5 Ω) < JY25 (71.9 Ω). This trend was consistent with the order of V_oc of JY24 (621 mV) < JY26 (661 mV) < JY27 (677 mV) < JY25 (704 mV). Besides, electron lifetime (τ) can be extracted from C_μ (chemical capacitance) and R_ct, using τ = R_ct × C_μ.²¹ Generally, a longer electron lifetime indicates the effective reduction of the reverse reaction of the injected electron with the triiodide in the electrolyte and therefore a higher photovoltage.²² For the four devices, the calculated electron lifetime values increased in the order of JY24 (15.4 ms) < JY26 (30.5 ms) < JY27 (47.8 ms) < JY25 (60.0 ms). So the highest V_oc of JY25-based cell can be explained.

Conclusions

Four phenothiazine-substituted zinc porphyrin dyes have been synthesized and applied as photosensitizers for DSSCs. All the dyes are composed of a porphyrin skeleton with a phenothiazine moiety directly attached to the meso-position of porphyrin. Different π-linkers (benzene, thiophene, and ethynyl benzene) and different acceptor groups (carboxylic acid and cyanoacrylic acid) have been chosen to tune the photoelectric properties of these push–pull porphyrin dyes. The dye JY27 with an acetylenyl benzoic acid unit shows a broader absorption region and more significantly improved IPCE values between 550 and 750 nm than the other three dyes, exhibiting a good light-harvesting ability and a high J_sc of 15.3 mA cm⁻². As a result, JY27-based cell achieved a high PCE of 6.25% under standard conditions, demonstrating the promising application of push–pull type phenothiazine-functionalized zinc porphyrins for efficient DSSCs.

Experimental

Materials and methods

All NMR solvents and other chemicals came from commercial sources and were used without purification. FTO glasses (Nippon Sheet Glass) with sheet resistance of 15 Ω sq⁻¹ were used. ¹H and ¹³C NMR spectra were obtained on Bruker 400 MHz instrument. UV-visible absorption spectra were recorded on Varian Cary 300 Conc. HR-MS spectra were recorded on Varian 7.0T FTMS. IR spectra were recorded on the Bruker Tensor 27 using KBr disk. Electrochemical analyses and the J–V characteristic curves of the solar cells were all recorded on Zennium (Zahner Corporation) electrochemical workstation. All solar cells were illuminated by a sunlight simulator (CHF-XM-500W, Trusttech Co. Ltd.) under AM 1.5 G (100 mW cm⁻²) irradiation calibrated by a standard silicon solar cell (91150V, Newport Corporation). The IPCE spectra were recorded on a IPCE setup (QTest Station 2000 IPCE Measurement System, CROWNTECH, USA).

Fabrication of DSSCs

The detailed procedures for preparing TiO₂ photoelectrodes in photovoltaic measurements were according to our previous work.²³ The Pt counter electrodes were obtained by thermal deposition a platinum layer (0.02 M hexachloroplatinic acid in isopropanol) on the surface of FTO glasses 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 four porphyrin dyes on TiO₂ was carried out with 0.2 mM dye solution in toluene for 6 h. The amounts of dye loading were determined by dye desorption in a basic solution (0.1 M NaOH in THF/H₂O = 1 : 1) followed by spectroscopic measurement. 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 CH₃CN.

Synthetic procedures

Synthesis of compound 3a. Compound 1 (410 mg, 0.535 mmol), compound 2 (475 mg, 1.087 mmol), Cs₂CO₃ (872 mg, 2.761 mmol), Pd(PPh₃)₄ (62 mg, 0.054 mmol) were placed in a two-neck round-bottomed flask, which was flushed with nitrogen and then charged with 55 mL dry toluene/DMF (10 : 1). The mixture was stirred at 80 °C for 4 h, and the solvent was removed under reduced pressure. The residue was dissolved in CH₂Cl₂ and washed with water, dried over anhydrous Na₂SO₄, and evaporated. The crude product was purified on a silica-gel column using CH₂Cl₂/petroleum ether (1 : 3) as the eluent. Recrystallization of the product from CHCl₃/CH₃OH gave compound 3a as a purple solid (yield: 97%). Mp > 300 °C. IR (KBr) ν: 3439, 3311, 2958, 2867, 1592, 1461, 1362, 1246, 796, 715 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 10.24 (s, 1H), 9.37 (d, J = 4.6 Hz, 2H), 9.11 (d, J = 4.6 Hz, 2H), 8.99 (s, 4H), 8.16 (s, 4H), 8.06 (d, J = 1.9 Hz, 1H), 8.02 (dd, J = 8.1, 1.9 Hz, 1H), 7.86 (s, 2H), 7.31 (d, J = 7.6 Hz, 2H), 7.26 (d, J = 8.2 Hz, 1H), 7.13–7.04 (m, 2H), 4.07–3.98 (m, 2H), 2.30–2.21 (m, 1H), 1.72–1.63 (m, 3H), 1.60 (s, 36H), 1.51–1.38 (m, 5H), 1.07 (t, J = 7.4 Hz, 3H), 1.00 (t, J = 7.0 Hz, 3H), −2.88 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 148.97, 148.91, 145.85, 145.47, 140.79, 136.99, 133.53, 133.24, 131.62, 131.40, 131.13, 130.95, 130.14, 130.07, 127.89, 127.36, 125.77, 124.08, 122.66, 121.06, 121.02, 119.40, 116.17, 113.87, 104.59, 51.38, 36.00, 35.10, 31.79, 30.88, 28.73, 24.29, 23.18, 14.17, 10.76. HR-MS (MALDI): m/z [M]⁺ calcd for C₆₈H₇₇N₅S, 995.5900; found, 995.6070. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 414 (2.71 × 10⁵), 511 (1.99 × 10⁴), 548 (1.23 × 10⁴), 585 (9.97 × 10³), 642 (7.92 × 10³).

Synthesis of compound 3b. A solution of N-bromosuccinimide (NBS) (3.56 mg, 0.02 mmol) in CH₂Cl₂ (5 mL) was added dropwise to a solution of compound 3a (19.9 mg, 0.02 mmol) in CH₂Cl₂ (30 mL) at 0 °C. The mixture was then slowly warmed to room temperature and stirred for 4 h before the reaction was quenched with acetone (5 mL). The solvent was removed under reduced pressure. The product was isolated by silica-gel column using CH₂Cl₂ as the eluent and recrystallized from CHCl₃/CH₃OH to give compound 3b as a brown solid (yield: 94%). Mp > 300 °C. IR (KBr) ν: 3319, 2960, 2926, 2868, 1593, 1460, 1362, 1248, 973, 794, 754, 720 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 9.67 (d, J = 4.8 Hz, 2H), 8.94 (d, J = 4.4 Hz, 2H), 8.84 (t, J = 6.6 Hz, 4H), 8.06 (s, 4H), 7.98 (d, J = 1.8 Hz, 1H), 7.94 (dd, J = 8.2, 1.8 Hz, 1H), 7.81 (s, 2H), 7.28 (d, J = 9.0 Hz, 2H), 7.21 (d, J = 8.2 Hz, 1H), 7.10–6.98 (m, 2H), 4.04–3.91 (m, 2H), 2.25–2.15 (m, 1H), 1.67–1.57 (m, 3H), 1.54 (s, 36H), 1.47–1.33 (m, 5H), 1.02 (t, J = 7.4 Hz, 3H), 0.95 (t, J = 7.0 Hz, 3H), −2.71 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 148.90, 148.84, 145.74, 145.60, 140.82, 136.22, 133.51, 133.17, 129.99, 129.95, 127.89, 127.40, 125.67, 124.30, 122.72, 122.21, 121.23, 119.88, 116.20, 114.05, 102.64, 51.36, 35.98, 35.08, 31.76, 30.85, 28.71, 24.25, 23.16, 14.16, 10.74. HR-MS (MALDI): m/z [M]⁺ calcd for C₆₈H₇₆BrN₅S, 1073.5005; found, 1073.5040. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 422 (3.93 × 10⁵), 522 (2.88 × 10⁴), 558 (2.49 × 10⁴), 598 (1.64 × 10⁴), 642 (1.75 × 10⁴).

Synthesis of compound 4a. The synthesis of compound 4a resembles that method of compound 3a and the crude product was purified by silica-gel column using CH₂Cl₂/petroleum ether (2 : 3) as the eluent to give compound 4a as a purple solid (yield: 95%). Mp > 300 °C. IR (KBr) ν: 3434, 3315, 2959, 2929, 2868, 1727, 1593, 1462, 1362, 1274, 1248, 1110, 974, 801, 735, 717 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 8.96–8.85 (m, 6H), 8.78 (d, J = 4.7 Hz, 2H), 8.44 (d, J = 8.3 Hz, 2H), 8.32 (d, J = 8.1 Hz, 2H), 8.09 (s, 4H), 8.03 (d, J = 1.9 Hz, 1H), 7.99 (dd, J = 8.2, 1.9 Hz, 1H), 7.81 (s, 2H), 7.28–7.23 (m, 3H), 7.10–7.00 (m, 2H), 4.11 (s, 3H), 4.05–3.94 (m, 2H), 2.21 (m, 1H), 1.66–1.58 (m, 3H), 1.54 (s, 36H), 1.42 (m, 5H), 1.04 (t, J = 7.4 Hz, 3H), 0.97 (t, J = 7.1 Hz, 3H), −2.72 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 167.43, 148.86, 148.81, 147.40, 145.82, 145.55, 141.09, 136.47, 134.55, 133.60, 133.29, 130.00, 129.96, 129.48, 127.90, 127.39, 125.75, 124.29, 122.71, 121.79, 121.12, 119.46, 118.22, 116.21, 114.04, 52.43, 51.41, 36.03, 35.09, 31.77, 30.89, 28.74, 24.30, 23.18, 14.17, 10.77. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₆H₈₃N₅O₂S, 1129.6267; found, 1129.6258. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 420 (3.49 × 10⁵), 518 (1.80 × 10⁴), 556 (1.16 × 10⁴), 598 (6.36 × 10³), 658 (2.62 × 10³).

Synthesis of compound 4b. The synthesis of compound 4b resembles that method of compound 3a and the crude product was purified by silica-gel column using CH₂Cl₂/petroleum ether (1 : 1) as the eluent to give compound 4b as a purple solid (yield: 95%). Mp > 300 °C. IR (KBr) ν: 3315, 2960, 2928, 2868, 1704, 1600, 1462, 1363, 1248, 1206, 973, 801, 735, 717 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 10.37 (s, 1H), 8.90 (dd, J = 9.1, 4.6 Hz, 6H), 8.77 (d, J = 4.6 Hz, 2H), 8.41 (d, J = 7.9 Hz, 2H), 8.27 (d, J = 8.1 Hz, 2H), 8.08 (s, 4H), 8.02 (d, J = 1.9 Hz, 1H), 7.99 (dd, J = 8.1, 1.9 Hz, 1H), 7.81 (s, 2H), 7.27 (d, J = 7.7 Hz, 2H), 7.22 (s, 1H), 7.10–7.00 (m, 2H), 4.06–3.93 (m, 2H), 2.28–2.16 (m, 1H), 1.67–1.58 (m, 3H), 1.53 (s, 36H), 1.48–1.36 (m, 5H), 1.03 (t, J = 7.4 Hz, 3H), 0.96 (t, J = 7.0 Hz, 3H), −2.73 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 192.41, 149.03, 148.84, 148.79, 145.76, 145.54, 140.99, 136.37, 135.53, 135.13, 133.57, 133.26, 129.98, 129.94, 127.97, 127.87, 127.37, 125.69, 124.25, 122.68, 121.86, 121.11, 119.61, 117.67, 116.18, 114.01, 51.36, 35.98, 35.05, 31.73, 30.85, 29.70, 28.70, 24.25, 23.15, 14.14, 10.73. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₅H₈₁N₅OS, 1099.6162; found, 1099.6171. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 420 (3.85 × 10⁵), 516 (2.39 × 10⁴), 557 (1.58 × 10⁴), 594 (8.15 × 10³), 650 (7.61 × 10³).

Synthesis of compound 4c. The synthesis of compound 4c resembles that method of compound 3a and the crude product was purified by silica-gel column using CH₂Cl₂/petroleum ether (4 : 3) as the eluent to give compound 4c as a purple solid (yield: 94%). Mp > 300 °C. IR (KBr) ν: 3422, 3317, 2959, 2923, 2854, 1675, 1592, 1455, 1362, 1348, 1247, 955, 915, 801, 733 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 10.13 (s, 1H), 8.92 (d, J = 4.5 Hz, 2H), 8.85 (d, J = 4.6 Hz, 4H), 8.80 (d, J = 4.4 Hz, 2H), 8.05 (d, J = 3.6 Hz, 1H), 8.00 (s, 4H), 7.91 (dd, J = 11.2, 6.2 Hz, 3H), 7.73 (s, 2H), 7.19 (t, J = 7.9 Hz, 2H), 7.13 (s, 1H), 7.02–6.91 (m, 2H), 3.90 (d, J = 4.0 Hz, 2H), 2.18–2.07 (m, 1H), 1.61–1.50 (m, 3H), 1.45 (s, 36H), 1.39–1.25 (m, 5H), 0.94 (t, J = 7.4 Hz, 3H), 0.87 (t, J = 7.0 Hz, 3H), −2.76 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 183.52, 154.11, 148.94, 148.88, 145.74, 145.67, 145.45, 140.89, 136.22, 135.16, 134.70, 133.63, 133.30, 130.07, 130.02, 127.92, 127.43, 125.66, 124.30, 122.76, 122.44, 121.24, 120.70, 116.23, 114.07, 108.24, 51.37, 35.97, 35.11, 31.78, 30.85, 28.72, 24.26, 23.19, 14.20, 10.77. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₃H₇₉N₅OS₂, 1105.5726; found, 1105.5723. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 425 (2.98 × 10⁵), 522 (2.03 × 10⁴), 565 (1.98 × 10⁴), 590 (1.27 × 10⁴), 652 (1.16 × 10⁴).

Synthesis of compound 5. The compound 3b (50 mg, 0.047 mmol), methyl 4-ethynylbenzoate (22 mg, 0.1375 mmol), Pd₂(dba)₃ (10 mg, 0.011 mmol) and AsPh₃ (17 mg, 0.056 mmol) were placed in a two-neck round-bottomed flask, which was flushed with nitrogen and then charged with dry THF (20 mL) and Et₃N (4 mL). The mixture was stirred at 60 °C for 3 h. Then, the solvent was removed under reduced pressure, and the residue was dissolved in CH₂Cl₂ and washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was purified on a silica-gel column using CH₂Cl₂/petroleum ether (2 : 5) as the eluent to give compound 5 as a brown solid (yield: 56%). Mp > 300 °C. IR (KBr) ν: 3317, 2959, 2931, 2872, 1723, 1594, 1460, 1278, 1121, 1074, 799, 739 cm⁻¹. ¹H NMR (400 MHz, CDCl₃) δ 9.74 (d, J = 4.6 Hz, 2H), 8.98 (d, J = 4.3 Hz, 2H), 8.83 (d, J = 7.4 Hz, 4H), 8.24 (d, J = 7.8 Hz, 2H), 8.08 (s, 4H), 7.99 (s, 1H), 7.95 (d, J = 8.2 Hz, 1H), 7.83 (s, 2H), 7.65 (d, J = 18.7 Hz, 2H), 7.29 (d, J = 9.1 Hz, 2H), 7.22 (d, J = 8.2 Hz, 1H), 7.11–6.99 (m, 2H), 4.01 (s, 3H), 4.00–3.93 (m, 2H), 2.25–2.15 (m, 1H), 1.73–1.60 (m, 3H), 1.56 (s, 36H), 1.45–1.36 (m, 5H), 1.03 (t, J = 7.3 Hz, 3H), 0.96 (t, J = 6.6 Hz, 3H), −2.24 (s, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 166.70, 148.97, 148.91, 145.70, 145.63, 140.62, 136.22, 133.48, 133.13, 131.39, 129.99, 129.94, 129.88, 129.47, 128.88, 127.87, 127.38, 125.63, 124.25, 122.78, 122.71, 121.31, 121.22, 116.18, 114.01, 97.97, 95.72, 95.59, 52.33, 51.34, 35.96, 35.07, 31.74, 30.83, 28.69, 24.23, 23.14, 14.14, 10.72. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₈H₈₃N₅O₂S, 1153.6267; found, 1153.6272. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 439 (3.78 × 10⁵), 540 (1.41 × 10⁴), 582 (3.91 × 10⁴), 671 (2.25 × 10⁴).

Synthesis of JY24. The compound 4a (40 mg, 0.034 mmol), NaOH (40 mg, 1 mmol) were placed in a two-neck round-bottomed flask, which was charged with MeOH (5 mL), water (5 mL) and THF (10 mL). The mixture was stirred at 90 °C for 4 h, and the reaction was terminated by adding 2 mL concentrated hydrochloric acid. Then, the solvent was evaporated to dryness under reduced pressure and the residue was redissolved in CH₂Cl₂, washed with water, dried over anhydrous Na₂SO₄. After removal of the solvent in vacuo, the residue was purified on a silica-gel column using CH₂Cl₂/CH₃OH (20 : 1) as the eluent, whereafter the chloroform solution of intermediate product was treated with the saturated methanol solution of zinc acetate (4 mL) for 2 h. Then, the mixture was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was recrystallized using CHCl₃/CH₃OH to give a crimson solid (yield: 91%). Mp > 300 °C. IR (KBr) ν: 3439, 2959, 2868, 1689, 1593, 1463, 1363, 1337, 1249, 1000, 797, 718 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 8.78 (d, J = 13.6 Hz, 8H), 8.29 (d, J = 22.7 Hz, 4H), 8.02 (s, 4H), 7.90 (d, J = 13.8 Hz, 2H), 7.81 (s, 2H), 7.37–7.13 (m, 4H), 7.00 (s, 1H), 2.81 (d, J = 6.0 Hz, 2H), 2.04 (s, 1H), 1.50 (s, 36H), 1.37–1.12 (m, 8H), 0.90 (dd, J = 13.2, 7.3 Hz, 6H). ¹³C NMR (101 MHz, DMSO-d₆) δ 149.53, 149.47, 149.31, 148.94, 147.94, 145.54, 145.31, 144.56, 141.78, 137.02, 133.73, 133.31, 132.42, 131.73, 131.65, 131.35, 131.23, 129.29, 127.61, 127.37, 124.64, 123.00, 122.44, 121.39, 120.26, 119.46, 119.07, 116.24, 114.01, 99.49, 50.64, 35.45, 34.59, 31.44, 30.54, 28.03, 23.62, 22.58, 13.86, 10.50. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₅H₇₉N₅O₂SZn, 1177.5246; found, 1177.5238. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 422 (4.04 × 10⁵), 550 (2.52 × 10⁴), 592 (8.94 × 10³).

Synthesis of JY25. A mixture of compound 4b (100 mg, 0.092 mmol), cyanoacetic acid (78 mg, 0.918 mmol), and ammonium acetate (64 mg, 0.919 mmol) in glacial acetic acid (20 mL) and toluene (10 mL) was refluxed for 4 h. The reaction mixture was then cooled to room temperature and diluted with CH₂Cl₂. The organic phase was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The crude product was purified on a silica-gel column using CH₂Cl₂/CH₃OH (20 : 1) as the eluent, whereafter the chloroform solution of intermediate product was treated with the saturated methanol solution of zinc acetate (7 mL) for 3 h. Then, the mixture was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was recrystallized using CHCl₃/CH₃OH to give a deep purple solid (yield: 73%). Mp > 300 °C. IR (KBr) ν: 3609, 2956, 2920, 2850, 1585, 1462, 1363, 1000, 797, 718 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 8.92–8.76 (m, 8H), 8.53 (s, 1H), 8.42 (d, J = 7.3 Hz, 2H), 8.36 (d, J = 7.7 Hz, 2H), 8.03 (s, 4H), 7.99–7.93 (m, 2H), 7.82 (s, 2H), 7.40 (d, J = 8.4 Hz, 1H), 7.33 (t, J = 7.8 Hz, 1H), 7.27 (d, J = 7.2 Hz, 1H), 7.23 (d, J = 8.1 Hz, 1H), 7.04 (t, J = 7.3 Hz, 1H), 4.08–3.95 (m, 2H), 2.15–2.06 (m, 1H), 1.61–1.55 (m, 3H), 1.51 (s, 36H), 1.43–1.28 (m, 5H), 0.98 (t, J = 7.4 Hz, 3H), 0.91 (t, J = 7.0 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆) δ 149.63, 149.45, 149.34, 148.75, 148.03, 146.60, 145.30, 144.63, 141.75, 136.93, 134.85, 133.38, 132.38, 131.96, 131.73, 131.50, 131.16, 129.41, 128.15, 127.43, 121.55, 120.32, 119.38, 48.62, 35.46, 34.63, 31.46, 30.01, 27.98, 23.58, 22.58, 13.89, 10.53. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₈H₈₀N₆O₂SZn, 1228.5355; found, 1228.5354. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 424 (2.46 × 10⁵), 550 (2.26 × 10⁴), 592 (1.27 × 10⁴).

Synthesis of JY26. A mixture of compound 4c (100 mg, 0.090 mmol), cyanoacetic acid (77 mg, 0.905 mmol) and ammonium acetate (64 mg, 0.919 mmol) in glacial acetic acid (20 mL) and toluene (10 mL) was refluxed for 4 h. The reaction mixture was then cooled to room temperature and diluted with CH₂Cl₂. The organic phase was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The crude product was purified on a silica-gel column using CH₂Cl₂/CH₃OH (20 : 1) as the eluent, whereafter the chloroform solution of intermediate product was treated with the saturated methanol solution of zinc acetate (10 mL) for 3 h. Then, the mixture was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was recrystallized using CHCl₃/CH₃OH to give JY26 as a deep purple solid (yield: 95%). Mp > 300 °C. IR (KBr) ν: 3428, 2960, 2928, 2868, 1592, 1462, 1391, 1364, 1248, 1218, 1003, 797, 717 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 9.04 (d, J = 4.6 Hz, 2H), 8.86–8.79 (m, 4H), 8.77 (d, J = 4.4 Hz, 2H), 8.41 (s, 1H), 8.12 (d, J = 3.4 Hz, 1H), 8.02 (s, 5H), 7.94 (d, J = 7.8 Hz, 2H), 7.82 (s, 2H), 7.37 (d, J = 8.8 Hz, 1H), 7.32 (t, J = 7.7 Hz, 1H), 7.26 (d, J = 7.5 Hz, 1H), 7.22 (d, J = 8.1 Hz, 1H), 7.04 (t, J = 7.5 Hz, 1H), 4.10–3.93 (m, 2H), 2.12–2.05 (m, 1H), 1.63–1.54 (m, 3H), 1.51 (s, 36H), 1.40–1.28 (m, 5H), 0.97 (t, J = 7.3 Hz, 3H), 0.90 (t, J = 6.9 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆) δ 150.32, 150.11, 150.04, 150.00, 149.69, 148.71, 148.69, 145.77, 145.24, 142.13, 141.24, 140.37, 139.65, 137.24, 134.48, 134.33, 133.91, 132.84, 132.30, 131.56, 131.54, 129.89, 128.03, 127.87, 124.81, 123.29, 122.51, 121.01, 119.88, 117.10, 114.86, 109.64, 109.59, 50.98, 35.17, 31.96, 30.38, 29.49, 28.39, 24.00, 23.03, 14.39, 11.06. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₆H₇₈N₆O₂S₂Zn, 1234.4919; found, 1234.4917. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 425 (2.66 × 10⁵), 554 (2.16 × 10⁴), 602 (1.22 × 10⁴).

Synthesis of JY27. The compound 5 (70 mg, 0.061 mmol), NaOH (25 mg, 0.625 mmol) were placed in a two-neck round-bottomed flask, which was charged with MeOH (10 mL), water (10 mL), and THF (20 mL). The mixture was stirred at 90 °C for 4 h, and the reaction was terminated by adding 2 mL concentrated hydrochloric acid. Then, the mixture was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was purified on a silica-gel column using CH₂Cl₂/CH₃OH (20 : 1) as the eluent, whereafter the chloroform solution of intermediate product was treated with the saturated methanol solution of zinc acetate (7 mL) for 2 h. Then, the mixture was washed with water, dried over anhydrous Na₂SO₄, and evaporated. The residue was recrystallized using CHCl₃/CH₃OH to give compound 3a as a dark blue solid (yield: 84%). Mp > 300 °C. IR (KBr) ν: 3403, 2954, 2925, 2855, 1688, 1601, 1549, 1461, 1418, 1287, 1245, 1003, 795, 713 cm⁻¹. ¹H NMR (400 MHz, DMSO-d₆) δ 9.79 (d, J = 4.0 Hz, 2H), 8.88 (d, J = 4.0 Hz, 2H), 8.81–8.68 (m, 4H), 8.21 (d, J = 7.7 Hz, 2H), 8.13 (d, J = 7.5 Hz, 2H), 8.03 (s, 4H), 7.96–7.88 (m, 2H), 7.84 (s, 2H), 7.33 (dd, J = 18.3, 8.2 Hz, 2H), 7.24 (dd, J = 17.7, 7.3 Hz, 2H), 7.03 (t, J = 7.4 Hz, 1H), 4.06–3.92 (m, 2H), 2.38–2.26 (m, 1H), 1.72–1.62 (m, 3H), 1.53 (s, 36H), 1.39–1.28 (m, 5H), 0.96 (t, J = 7.3 Hz, 3H), 0.90 (t, J = 6.6 Hz, 3H). ¹³C NMR (101 MHz, DMSO-d₆) δ 151.83, 150.51, 149.71, 149.66, 148.63, 145.74, 145.22, 141.84, 137.13, 133.72, 133.12, 132.79, 132.19, 132.12, 130.81, 130.66, 130.37, 129.87, 128.97, 127.87, 123.12, 121.77, 120.92, 116.75, 114.53, 98.32, 95.97, 65.35, 35.96, 35.11, 31.93, 29.55, 28.51, 23.88, 23.06, 14.34, 10.98. HR-MS (MALDI): m/z [M]⁺ calcd for C₇₇H₇₉N₅O₂SZn, 1201.5246; found, 1201.5244. UV-vis (CH₂Cl₂) λ_abs (nm) (ε/M⁻¹ cm⁻¹): 442 (3.01 × 10⁵), 570 (1.21 × 10⁴), 617 (2.68 × 10⁴).

Acknowledgements

We thanks to the 973 Program (2011CB932502), and the NSFC (No. 21172126, 21272123, and 21572108) for their financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26754d

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