Metal(II)-Induced Synthesis of Asymmetric Fluorescence

Article pubs.acs.org/crystal

Metal(II)-Induced Synthesis of Asymmetric Fluorescence Benzimidazoles Complexes and Their Dye-Sensitized Solar Cell Performance as Cosensitizers Xinming Wang,†,‡ Ruiqing Fan,*,† Yuwei Dong,† Ting Su,† Jian Huang,† Xi Du,† Ping Wang,† and Yulin Yang*,† †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, P. R. China ‡ Department of Chemistry, Harbin University of Science And Technology, Harbin 150080, P. R. China S Supporting Information *

ABSTRACT: Four asymmetric complexes with different conjugate structures based on benzimidazole derivatives with formulas of [ZnL1Cl2] (1) (L1: 1((quinoline-2-yl) methylene)-2-quinoline-2-yl benzimidazole), [CdL2Cl2]n (2) (L2: N1,N2-bis((quinoline-2-yl) methylene)o-phenylenediamine), [HgL3Cl2] (3) (L3: 2-(quinoline-2-yl)benzimidazole), and (HgCl4)(L4)2(C6H7)2 (4) (L4: 1,2,3-((quinoline-2-yl) methylene) benzimidazoleion) are obtained. Interestingly, L1−L4 are derived from the half-condensed asymmetrical Schiff base ligand [N1-((quinoline-2-yl) methylene) N2-(H-(quinoline-2-yl) methylenehydroxy)o-phenylenediamine] (L) by means of a metal(II)-induced method. To the best of our knowledge, it is the first example of one Schiff base ligand transforming into three different benzimidazole derivatives from the single quinoline ring (L3), double quinoline rings (L1) to three quinoline rings (L4). Complexes 1, 3, and 4 are new mononuclear complexes, while complex 2 is a one-dimensional (1D) chlorine bridged Cd(II)-bis Schiff base (L2) coordination polymer. Complexes 1−4 are driven mainly by a C−H···Cl hydrogen bond and π−π stacking interactions packing to three-dimensional (3D) supramolecular metal−organic frameworks (SMOFs), respectively. Structural analysis shows that complexes 1−4 possesses a 7-connected, (6,6)-connected, 5-connected, (4,6,6)connected SMOFs topology network with a Schläfli symbol of {34·49·58}, {49·65}2, {46·64}, and {33·42·5}{33·42·53·67}{33·43·53· 66}, respectively. Under the excitation of 330 nm ultraviolet light, 1−4 show stronger blue luminescent emissions in solution comparing to the nearly nonluminous L. The order of the luminous intensity is 2 > 1 > 3 > 4. The strongest luminous intensity from complex 2 is due to its 1D chain conjugated system with the highest extent of electron-donating nature. The absent absorption of ruthenium complex N719 could be overcome by those complexes in the low wavelength region of the visible spectrum. These complexes could reduce the charge recombination of injected electrons and offset the competitive visible light absorption of I3−. Complexes 1−4 could be utilized as cosensitizers in combination with N719 to examine the effect on enhancing the performance of dye-sensitized solar cells (DSSCs). After cosensitization, the incident-photon-to-current conversion efficiency is increased, and the dark current is reduced. Because of utilizing cosensitizers of 1/N719 and 4/N719, the DSSC devices show an overall conversion efficiency of 8.30% and 8.21% with a short circuit current density of 18.69 mA cm−2 and 18.61 mA cm−2, and an open circuit voltage of 0.75 V and 0.74 V, respectively, under AM 1.5 G solar irradiation. The overall conversion efficiency is considerable, 26.72% and 25.34% higher than that by N719. Accordingly, the performance of N719 sensitized solar cells could be enhanced by the prepared fluorescence complexes serving as cosensitizers.

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ation.18−21 Many of these reactions are often associated with a side reaction.22 On account of a greater selectivity in the synthesis of the complex based on benzimidazoles and their derivatives, the search continues. In this work, one “fascinating” novel asymmetrical Schiff base ligand [N1-((quinoline-2-yl) methylene) N2-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L) is synthesized and separated, which could transform into four asymmetrical complexes based on four

INTRODUCTION Benzimidazole derivatives complexes which exhibit excellent light-harvesting, energy transfer properties, and thermostability have attracted much attention and have been used in search of desirable fluorescence signaling,1−8 catalysis,9−11 and dyesensitized solar cells (DSSCs).12−17 The conventional method to form benzimidazole derivatives complexes is a two-step method; namely, the benzimidazole derivatives are synthesized and separated first. The most popular strategy is usually followed by the coupling of o-phenylenediamines (o-PDs) with carboxylic acids or their derivatives or the condensation of oPDs and aldehydes followed by oxidative cyclo-dehydrogen© 2017 American Chemical Society

Received: June 25, 2017 Revised: August 16, 2017 Published: August 23, 2017 5406

DOI: 10.1021/acs.cgd.7b00891 Cryst. Growth Des. 2017, 17, 5406−5421

Crystal Growth & Design

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kinds of benzimidazole derivatives by a metal(II)-induced onestep method. The interest is sequentially increasing for developing fluorescence complexes for photovoltaic performance application as cosensitizers in DSSCs. They show a tailored lightabsorbing property and excellent chemical and photochemical stability.23−28 In our previous work, N-heterocyclic ligands and their transition metal complexes have been prepared and employed in DSSC as cosensitizers to replace precious metal complexes29,30 (compared to IrIII, RuII, OsII, and PtII, which are used as common sensitizers).31,32 Asymmetrical structure cosensitizers have received more attention recently, and it is assumed that the vectorial spatial/energetic orientation between the dye and the TiO2 interface and conduction band potentially hampering charge transfer33 would be disrupted because of the symmetry of the π-electron distribution. A very recent study by our group explores d10 metal complexes based on 4-(1H-imidazol-1-yl)benzoic acid in DSSCs.34 The rational design is mainly focused on the rigid ligand with a single and small conjugated system. However, the scale of the πconjugated system on the ligands plays a role in governing the photoelectric properties of complexes. Therefore, complexes based on benzimidazole derivatives with different conjugation might become very promising materials. Considering the above factors, we synthesize four asymmetric complexes with different conjugated systems by metalinduced methods, namely, [ZnL1Cl2] (1) (L1: 1-((quinoline-2yl) methylene)-2-quinoline-2-yl benzimidazole), [CdL2Cl2]n (2) (L2: N1,N2-bis((quinoline-2-yl) methylene)o-phenylenediamine), [HgL3Cl2] (3) (L3: 2-(quinoline-2-yl)benzimidazole), and [(L4) 2(HgCl4)] (4) (L4: 1, 2, 3-((quinoline-2-yl) methylene) benzimidazoleanion). As far as we know, it is the first example of the half condensed Schiff base ligand transforming into three different benzimidazole derivatives from the single quinoline ring (L3), double quinoline rings (L1) to three quinoline rings (L4). The noncovalent supramolecular interactions, such as hydrogen bonding 35−38 and π−π stacking,39−43 could form supramolecular metal−organic frameworks (SMOFs). Through the main drive force of intermolecular hydrogen bonds C−H···Cl and π−π stacking interaction, complexes 1−4 form the three-dimensional (3D) SMOFs, respectively. Complex 1 displays a 3D 7-connected {34·49·58} supramolecular architecture. 2 has a one-dimensional (1D) chain structure built from chlorine bridge and possesses an interesting 3D (6,6)-connected{49·65}2 supramolecular network. Complexes 3 and 4 exhibits a 5-connected {46·64} bnn and distinct (4,6,6)-connected 3D supramolecular architectures, respectively. Meanwhile, these complexes as potential hydrogen-bonding donors and acceptors enrich the π-electron density and extend the π-conjugated system. They show stronger blue luminescent emissions in solution compared to the nearly nonluminous L. As their strong absorption in the wavelength range 300−450 nm, complexes 1−4 are elected as cosensitizers and are utilized in the DSSC system sensitized by N719. They could cover the shortage of absorption by N719 in the ultraviolet and blue-violet spectrum and offset the competitive visible light absorption of I3−. This results in 26.72% and 25.34% improvement of DSSCs performance than that of a device only sensitized by N719 (6.55%).

Article

EXPERIMENTAL SECTION

Synthesis of Schiff Base Ligand [N1-((quinoline-2-yl)methylene) N2-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L). A dichloromethane (CH2Cl2) solution (20 mL) of 2-quinoline formaldehyde (1.572 g, 10 mmol) was added to a stirred CH2Cl2 solution (10 mL) of o-PD (1.082 g, 10 mmol) under the influence of a catalytic amount of triethylamine. Stirring was continued at 25 °C. After a 0.5 h reaction, a claybank solid was suspended in solution and did not increase. The reaction was stopped and the sample filtered, and a deep yellow solid was obtained. Using acetonitrile (CH3CN) and petroleum ether recrystallization, single crystals of the L suitable for X-ray diffraction study were obtained after 1 day. Yield: 1.250 g, 62.22% (based on o-PD), mp 80−82 °C. Elemental anal. calc. for C26H20N4O (403.45): C, 77.21; H, 4.98; N, 13.85%. Found: C, 77.25; H, 5.00; N, 13.84%. IR (KBr, cm−1): 3431(br), 3060(w), 1630(s), 1597(s), 1500(m), 1443(m), 763(s) (Figure S1). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 8.56 (s, 1H, HCN), 7.81−8.28 (m, 12H, Q-H), 7.29−7.38 (m, 4H, Ph-H), 6.25 (s, 1H, −CH−), 6.60 (s, 1H, −NH−), 5.18 (s, 1H, −OH) ppm. 13C NMR (150 MHz, CD3CN, 298 K): δ 159.3, 151.7, 149.3, 148.5, 147.8, 145.1, 136.5, 135.6, 134.6, 130.9, 129.9, 129.7, 128.9, 128.3, 128.3, 128, 127.8, 127, 126.6, 126.5, 125.9, 122, 121.9, 119.6, 114.8, 90.5 ppm. ESI−MS: m/z = 405.19 [M + H]+. L is soluble in polar organic solvents, for instance, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), methanol (CH3OH), ethanol (CH3CH2OH), which promotes the reactions between it and metal ions in solutions. Synthesis of Complex [ZnL1Cl2] (1) (L1: 1-((quinoline-2-yl) methylene)-2-quinoline-2-ylbenzimidazole). The mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) of ZnCl2 (0.136 g, 1.0 mmol) and L (0.403 g, 1.0 mmol) was refluxed at 80 °C for 5 h and then cooled down to room temperature. Alight yellow solution was filtered to debride any suspended particles and was kept for evaporation at room temperature, and then block colorless crystals were retrieved. Yield: 0.320 g, 70.34% (based on ZnCl2), mp 110−112 °C. Elemental anal. calc. for C26H18N4Cl2Zn (522.71): C, 80.81; H, 4.69; N, 14.50%. Found: C, 80.82; H, 4.70; N, 14.48%. IR (KBr, cm−1): 3063(w), 1624(m), 1600(m), 1519(m), 1480(w), 760(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 7.58−8.68 (m, 12H, Q-H), 7.26−7.40 (m, 4H, Ph-H), 5.30 (s, 2H, −CH2) ppm. 13C NMR (150 MHz, CD3CN, 298 K): δ 167.6, 159.3, 147.8, 146.3, 141.5, 137.9, 137.6, 135.6, 134.2, 129.9, 129.7, 128.3, 127.8, 127, 126.6, 125.9, 125.5, 123, 122, 120.8, 119.3, 115.2, 110, 54.9 ppm. ESI−MS: m/z = 521.31 [M + H]+. It is soluble in polar solvents, for instance, DMSO, DMF, CH3CN, and CH3CH2OH. Synthesis of Complex [CdL2Cl2] (2) (L2: N,N′-bis((quinoline-2yl) methylene)o-phenylenediamine). A mixture of CdCl2 (0.183 g, 1.0 mmol) and L (0.403 g, 1.0 mmol) was dissolved in mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) and stirred for 15 min, then heated to 80 °C for 5 h in sealed vial. After being cooled to room temperature slowly, yellow rectangular block crystals were retrieved. Pure product was obtained in yield 0.228 g, 75.21% (based on CdCl2), mp 121−123 °C. Elemental anal. calc. for C26H18Cl4N4Cd2 (753.04): C, 80.81; H, 4.69; N, 14.50%. Found: C, 80.83; H, 4.66; N, 14.53%. IR (KBr, cm−1): 3063(w), 2931(w), 1644(w), 1586(m), 1504(m), 1438(m), 750(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 8.51 (s, 2H, −CH), 7.66−8.40 (m, 12H, Q-H), 7.33 (d, 2H, Ph-H), 7.19 (d, 2H, Ph-H) ppm. 13C NMR (150 MHz, CD3CN, 298 K): δ 163.7, 147.1, 144.5, 142.5, 136.5, 130.9, 129.9, 128.9, 128.5, 128.3, 127, 123.6, 122.1 ppm. ESI−MS: m/z = 571.13 [L + CdCl2]+. It is soluble in polar solvents, for instance DMSO, DMF, CH3CN, and CH3CH2OH. Synthesis of Complex [HgL3Cl2] (3) (L3: 2-(quinoline-2-yl) benzimidazole). A mixture of HgCl2 (0.271 g, 1. 0 mmol) and L (0.403 g, 1.0 mmol) under a catalytic amount of triethylamine was dissolved in a mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) and stirred at room temperature for 30 min. After that, it was heated to 100 °C for 5 h in a sealed vial. After being cooled to room temperature, colorless needle-like crystals of 3 were obtained by filtration. Pure product was obtained in a yield of 0.283 g, 54.81% (based on HgCl2), mp 110−112 °C. Elemental Anal. Calc. for 5407



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C16H11Cl2N3Hg (516.77): C, 78.35; H, 4.52; N, 17.13%. Found: C, 78.36; H, 4.50; N, 17.11%. IR (KBr, cm−1): 3070(m), 1600(m), 1509(m), 1428(m), 755(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 12.82 (s, 1H, − NH), 7.90−8.67 (m, 6H, Q-H), 7.33 (m, 4H, Ph-H) ppm. 13C NMR (150 MHz, CD3CN, 298 K): δ 167.6, 147.2, 141.5, 138.9, 137.9, 137.6, 129.9, 128.9, 128.3, 127, 125.5, 123, 119.3, 115.2 ppm. ESI−MS: m/z = 518.21 [M + H]+. It is soluble in polar solvents, for instance DMSO, DMF, CH3CN, and CH3CH2OH. Synthesis of Complex (L4)2(HgCl4) (4) (L4: 1, 2, 3-((quinoline2-yl)methylene) benzimidazoleanion). A solution of HgCl2 (0.271 g, 1.0 mmol) in mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) was combined with a mixture of 2-quinoline formaldehyde (0.157 g, 1.0 mmol) and o-PD (0.054 g, 0.5 mmol). The reaction solution was stirred at room temperature for 30 min. After that, it was heated in a sealed vial at 100 °C for 5 h. After slow cooling to room temperature, colorless needle-like crystals of 4 were obtained by filtration. Pure product was obtained in a yield of 0.702 g, 44.35% (based on HgCl2), mp 107−109 °C. Elemental Anal. Calc. for C86H68Cl4N10Hg (1583.89): C, 65.16; H, 4.29; N, 8.84%. Found: C, 65.20; H, 4.30; N, 8.81%. IR (KBr, cm−1): 3060(w), 1598(w), 1500(s), 1428(m), 746(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 7.80−8.66 (m, 18H, Q-H), 7.72−7.75 (m, 4H, Ph-H), 7.35− 7.37 (m, 5H, Ph-H), 2.39 (s, 3H, −CH3) ppm. 13C NMR (150 MHz, CD3CN, 298 K): δ 159.8, 150, 147.8, 146.7, 136.4, 135.6, 129.9, 129.7, 128.9, 128.6, 128.3, 127.8, 127.3, 127, 126.6, 125.9, 122, 121.3, 117.9, 114.3, 50 ppm. ESI−MS: m/z = 528.35 [L]+. It is soluble in polar solvents, for instance, DMSO, DMF, CH3CN, and CH3CH2OH.

interacts with the derivatives of L resulting from the steric hindrance of quinoline and form mononuclear complexes as below. It should be noted that L is virtually insoluble in CH2Cl2 as a result of strong self-dimerization and π-stacking interactions. The molecule of L forms a six-member ring which is coplanar to the adjacent benzene ring via intramolecular hydrogen bond interactions between phenyl H(26A) and the hydroxy oxygen atom O(1) (C26−H26A···O1, H26A··· O1 2.32 Å, ∠CHN 120.70°) (Table 2). This kind of noncovalent bond interaction increases the stability of the single molecular structure of L. The hydroxy O atom is also contained in forming intermolecular hydrogen bonds as an acceptor to give C18−H18A···O1, which connects two adjacent L molecules, yielding 1D chains. Facilitated by the internal πstacking between two quinoline ring of the neighboring compounds with a centroid−centroid distance of 3.58 Å, a two-dimensional (2D) π-stacking network takes shape by these supramolecular chains. In the crystal, molecules are further piled in a 3D network via hydrogen bonds C24−H24A···O1 (H24A···O1 2.59 Å, ∠CHO 141.85°) and strong π−π interactions (centroid−centroid distances 3.61 Å). If the single molecule of L is identified as seven connected nodes and the hydrogen bonds along with π-stacking interaction are simplified to linkers, the 3D structure of Schiff base L can be simplified as a 7-connected {33·411·52·65} net. Synthetic Overview of Complexes 1−4. The quality of α-hydroxy amine in L is such that it is not only easy to further lose a molecule of water to become a polydentate bis-Schiff base ligand, but also it could be hydrolyzed to form a monoamine. In view of the active Schiff base L, our strategy is to prepare a series of M(II) complexes by changing the metal salt ZnCl2, CdCl2, and HgCl2, and four M(II) (M = Zn, Cd, Hg) complexes 1−4 have been successfully prepared. First, the Schiff base L is treated with a methanol and toluene (v:v = 3:1) mixed solvent of ZnCl2 at 80 °C for a reflux reaction. As is well-known, the α-hydroxy amine is not stable, and the condition of heating could facilitate formation of the bis-Schiff base by further dehydration of L. However, the reaction does not proceed to give a conventional bis-Schiff base complex as expected. On the contrary, the 1,2-disubstituted benzimidazole L1 is obtained and coordinated to the Zn(II) ion simultaneously by in an situ metal−ligand reaction, forming Zncomplex (1). Synthetic methods for 1,2-disubstituted benzimidazole often link with several side reactions and byproducts.44−46 It is interesting that ZnCl2 in this reaction serves as more than the metal source; the direct cyclization involving C−N bond-forming from L is mediated by Zn2+ and further obtained 1,2-disubstituted benzimidazole L1 with two quinoline rings. Although the exact mechanism remains a mystery, a Lewis acid ZnCl2-mediated mechanism may well occur (Scheme 2), namely, ZnCl2 as catalyst for the selective preparation of 1,2-disubstituted benzimidazole. L dehydration occurs to form the type of bis-Schiff base L2 first, in the presence of electrophilic catalyst ZnCl2, and the intramolecular 1,3-hydride migration is induced to form the 1,2-disubstituted benzimidazole L1. The existence of ZnCl2 leads to the fourcoordination mononuclear complex 1. Various conditions such as the solvothermal method have been designed to confirm the potency of this model reaction (Table S2). The target product 1 still could be isolated when pressure is applied or the temperature is elevated unless metal salts are replaced. This shows that ZnCl2 is the prime motivator of the reaction.

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RESULTS AND DISCUSSION Analysis of L. Our initial focus was on the monoamine-type Schiff base with 1:1 equiv of 2-quinoline formaldehyde and oPD, to get the asymmetrical complexes. In a surprise twist, halfcondensed Schiff base L which has α-hydroxy amine, was obtained under ambient temperature and short reaction time of 0.5 h (Scheme 1). Scheme 1. Synthetic Route of the Schiff Base L

The single-crystal structure of Schiff base with α-hydroxy amine has not been previously crystallographically analyzed. Crystal data for L are summarized in Table 1, and selected bond angles and bond lengths are given in Table S1. The L crystallizes in the monoclinic P21/c space group, and the molecular structure of L is depicted in Figure 1. L has rigid units including benzene and quinoline rings, which restrict the rotary ability and lead to a good coplanarity. The dihedral angles between the benzene and the two quinoline rings are 12.69° and 17.92°, respectively. The imino group (−CHN−) locates in the relevant quinoline ring plane. Only one metal ion 5408



Article

Table 1. Crystallographic and Structural Determination Data for Schiff Base Ligand L and Complexes 1−4 CCDC no. formula Mr crystal system space group a [Å] b [Å] c [Å] α [°] β [°] γ [°] volume [Å3] Z ρcalcd [Mg m−3] μ, mm−1 F(000) θ limit [°] hkl index ranges

data/restraints/parameters GOF on F2 R1a wR2 [I > 2σ(I)]b R1a wR2 [all data]b max diff. peak/hole [e Å−3] a

L

1

2

3

4

1449510 C26H19N4O 403.45 monoclinic P21/c 12.5599(13) 8.3899(9) 18.5549(18) 90 98.565(3) 90 1933.4(3) 4 1.386 0.087 844 3.05−25.43 −15 ≤ h ≤ 14 −10≤ k ≤ 9 −22 ≤ l ≤ 22 3551/0/281 0.960 0.0632 0.1425 0.1818 0.1999 0.251/−0.442

1449511 C26H18Cl2N4Zn 522.71 monoclinic P21/c 10.0944(12) 19.017(2) 12.0378(14) 90 91.268(2) 90 2310.3(5) 4 1.503 1.317 1060 2.00−25.00 −12 ≤ h ≤ 12 −22≤ k ≤ 18 −14 ≤ l ≤ 14 4072/0/298 1.005 0.0326 0.0689 0.0588 0.0754 0.247/−0.305

1449512 C26H18Cd2Cl4N4 753.04 monoclinic P21/c 9.368(3) 13.506(4) 21.311(6) 90 97.755(3) 90 2671.9(12) 4 1.872 2.015 1464 1.79−25.00 −11 ≤ h ≤ 11 −16 ≤ k ≤ 15 −25 ≤ l ≤ 25 4693/0/325 1.082 0.0536 0.1557 0.0754 0.1686 2.013/−1.594

1449513 C16H11Cl2HgN3 516.77 triclinic P1̅ 7.563(3) 9.992(4) 10.665(4) 100.263(4) 99.789(5) 91.788(5) 779.9(5) 2 2.200 10.207 484 1.97−25.00 −8 ≤ h ≤ 8 −11 ≤ k ≤ 11 −12 ≤ l ≤ 12 2712/0/199 1.064 0.0270 0.0678 0.0311 0.0694 0.841/−1.134

1449514 C86H68Cl4HgN10 1583.89 monoclinic P21/c 14.9695(19) 23.453(3) 24.776(3) 90 115.856(6) 90 7827.6(17) 4 1.344 2.153 3208 2.22−17.34 −16 ≤ h ≤ 17 −18 ≤ k ≤ 27 −28 ≤ l ≤ 28 13251/274/920 0.901 0.0544 0.0691 0.2050 0.0800 0.670/−0.756

R1 = ∑∥F0| − |Fc∥/∑|Fo|. bwR2 = [∑[w(F02 − Fc2)2]/∑[w(F02)2]]1/2.

Figure 1. (a) Crystal structure of L; dotted lines represent the intramolecular hydrogen bond interactions. All hydrogen atoms except hydroxy amine and those participating in hydrogen bonds are omitted for clarity. (b) The 2D layers formed by C18−H18A···O1 hydrogen bonding and π-stacking interactions. (c) 3D supramolecular structure mediated by C24−H24A···O1 and π-stacking interactions. (d) The topological network structure of the 3D supermolecular structure in L (color code: C−H···O hydrogen bonding interactions, green line; π-stacking interactions, blue line).

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Table 2. Important Noncovalent Interactions in Compoundsa compound

D−H···A

H···A/Å

D···A/Å

D−H···A/°

structure

L

C18−H18A···O1 C26−H26A···O1 C24−H24A···O1 πCg1−πCg1 πCg1−πCg1 C9−H9A···Cl1 C15−H15A···Cl1 C2−H2A···Cl1 C13−H13A···Cl2 C20−H20A···Cl2 C7−H7A···Cl2 C6−H6A···Cl2 C24−H24A···Cl2 πCg1−πCg1 πCg1−πCg1 C2−H2A···Cl1 C9−H9A···Cl1 C20−H20A···Cl2 C23−H23A···Cl2 C10−H10A···Cl4 C8−H8A···Cl3 C13−H13A···Cl4 C7−H7A···Cl4 C2−H2A···Cl2 C7−H7A···Cl1 C14−H14A···Cl1 C6−H16A···Cl1 C3−H3A···Cl1 C13−H13A···Cl1 πCg1−πCg2 C3B−H3B···Cl1 C2B−H2B···Cl1 C22B−H22B···Cl1 C33A−H33A···Cl1 C30B−H30B···Cl2 C5A−H5A···Cl2 C27B−H27C···Cl3 C10B−H10B···Cl3 C27A−H27B···Cl3 C29A−H29A···Cl4 C8B−H8B2···Cl4 C5B−H5B···Cl4 C8A−H8A1···Cl4 πCg1−πCg1 πCg2−πCg2

2.722 2.326 2.595 3.58 3.61 3.046 2.997 2.981 2.761 2.759 3.067 2.943 2.930

3.653 2.928 3.393

166.68 120.70 141.85

3D

3.823 3.604 3.593 3.594 3.642 3.636 3.573 3.712 3.75 3.59 3.873 3.783 3.792 3.727 3.645 3.566 3.771 3.695 3.786 3.859 3.787 3.845 3.820 3.778 3.55 3.741 3.781 3.531 3.880 3.898 3.874 3.700 3.775 3.853 3.700 3.842 3.674 3.639 3.59 3.57

142.18 124.42 124.83 149.71 159.05 121.16 126.31 142.67

3D

132.38 130.36 143.50 146.53 160.99 168.81 141.07 142.12 155.52 145.62 129.34 147.71 149.22 128.13

3D

128.31 126.03 140.07 170.67 128.09 129.06 173.53 152.75 164.49 141.40 170.24 166.92 176.84

3D

1

2

3

4

a

3.185 3.114 3.004 2.916 2.753 2.649 3.000 2.917 2.921 3.055 3.128 3.026 2.991 3.132 3.092 3.159 2.768 2.960 3.256 3.018 2.735 2.925 2.912 2.930 2.883 2.764 2.671

3D

Cg1 and Cg2 are the centroids of the quinoline ring and imidazole ring, respectively.

the small units by Cl− anions. This phenomenon obviously demonstrates that the substitution of Cd2+ for Zn2+ could make a difference the in situ metal−ligand reaction of L. We hypothesized that the large ion radius of Hg2+ makes it hard to form a complex in the mild reaction condition, and the experimental results indeed are as expected. The Hg-complex 3 could be produced only under solvothermal conditions with elevating the temperature at 100 °C in the presence of a catalytic amount of triethylamine. The nucleophilic attack of the OH− anion on the imine bond allows for imine hydrolysis to form monamine. The most interesting features for this synthesis are the direct hydride migration coupling cyclization involving C−N bond-forming (selective formation of 2-

With the confirmation of the optimal reaction condition, the application scale of this reaction has been also verified by subjecting diverse IIB metal ions (Cd2+, Hg2+). Owning to the different ionic radii and metallicity of Zn2+, Cd2+, and Hg2+, the reaction of L with CdCl2 and HgCl2 in a similar synthesis with 1 could only produce a large quantity of unidentified white precipitates. Taking account of this, the adopting of the solvothermal method is introduced into the synthetic process. Yellow crystals of Cd-complex (2) are formed from the in situ solvothermal reaction between L and CdCl2. The result shows that dehydration reaction of L under solvothermal conditions at 80 °C leads to form crystallization of 1D chain-typed Cdcomplex containing the bis-Schiff base L2 via self-assembly of 5410



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Scheme 2. Mechanism of the Reaction Furnishing Complexes 1−4

substituted benzimidazole L3 with one quinoline ring) in situ metal−ligand reaction from the Schiff base L. The existence of HgCl2 led to the four-coordination mononuclear complex 3. Experimental results demonstrate that the central metals play the role of induction in the process of the reaction to obtain stable asymmetric complexes 1−3 without isolating ligands L1− L3 which are formed by a C−N/CN bond-forming strategy. Meanwhile, the temperature and autogenous pressure under the conditions of solvothermal reaction have a synergistic effect. In the present case, addition of MCl2 (M = Zn2+, Cd2+, Hg2+) to a suspension of L immediately provoked full dissolution of the solid material. It obviously goes through a process in which the solid quickly dissolves to a precipitation, which means highspeed dehydration (complex 1 and 2) or hydrolysis (complex 3) of L and a rapid coordination process. We also use a one-pot method starting with the reaction raw material of o-PD, 2-quinoline formaldehyde, and MCl2 (M = Zn, Cd, Hg) with a ratio of 1:1:1, under the same reaction conditions of 1−3 respectively. It affords only a large quantity of unidentified precipitates except the single crystals of [(L4)2(HgCl4)], complex 4. Complex 4 is prepared under 100 °C in the sealed vial, which is the same with complex 3. Structural analyses shows that the preparation gives a new ligand L4 containing a benzimidazole unit and three quinoline rings in the presence of HgCl2. This may be caused by an intramolecular cyclization of o-PD through the aldehyde group and amino condensation followed by a covalent cross-linking process. Crystals of L and complexes 1−4 suitable for X-ray diffraction studies are obtained by evaporating the resulting solutions, respectively. The NMR data indicate that L and complexes 1−4 still keep a complex structure in solution and do not decompose, respectively (Figures S2 and S3). Elemental analysis, EIS-MS, and PXRD prove the phase purity and stability of the bulk materials in the solid state upon extended

exposure to air (Figures S4−S9). The variances on intensity of PXRD with the simulated patterns could be caused by the preferred orientation of the crystalline powder samples. Crystal Structure of [ZnL1Cl2] (1), [CdL2Cl2] (2), [HgL3Cl2] (3), and [(L4)2(HgCl4)(C6H7)2] (4). Single crystal X-ray diffraction of all four complexes 1−4 was performed. All the bond lengths and angles of the complexes 1−4 in the crystal structures are shown in Tables S3−S7. The crystal structure analysis and packing diagrams of 1−4 suggest that the basic construction of the title SMOFs are intrinsic C−H···Cl hydrogen bond and π-stacking interactions. [ZnL1Cl2] (1). Complex 1 crystallizes in a monoclinic system with the P21/c space group. The asymmetric unit of 1 consists of a mononuclear core with one crystallographically independent Zn(II) ion (Figure 2). The dihedral angle between the benzimidazolyl and quinoline ring linked by a C−C single bond is 1.42°, which is nearly coplanar. The Zn(II) ion is chelated with the quinolyl and imidazole nitrogen atoms (N1, N2), causing a five-membered ring, which are nearly coplanar with quinoline and imidazole ring forming an extended π-conjugated system. This kind of configuration is beneficial to the electronic transmission. The Zn−N (quinoline) bond length [2.102(3) Å] is longer than the Zn−N (imidazole) bond length [2.017(3) Å]. The average Zn−N distance of 2.059 Å is close to the values found in other Zn(II) complexes.47 The bond angle of N−Zn−N is 79.64°. The two chlorine atoms (Cl1 and Cl2) are located above and below the five-membered chelate ring, saturating the standard tetrahedron geometry (Figure S9). The distances between five-membered chelate ring and the two coordinated chlorine anions are 2.43 and 1.15 Å, respectively. The dihedral angle between the other quinoline ring with the uncoordinated and exposed N atom and the benzimidazolyl unit is 73.17°. This twisted conformation between two quinoline rings reduces the steric congestions and results in 5411



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Figure 2. (a) View of coordination environment of Zn(II) in 1. Dotted lines represent the intramolecular hydrogen bond interaction. All hydrogen atoms except those participating in the hydrogen bonds are omitted for clarity. (b) The 2D layer of 1 generated through C−H···Cl and π-stacking interactions. (c) The 3D supramolecular structure of 1. (d) View of the 3D topological structure generated through noncovalent interaction contacts.

L2. The Cd2 is four-coordinated with chlorine atoms (Cl1, Cl2, Cl3, Cl4), forming a standard tetrahedral geometry {Cd2Cl4}. The 1D polymeric structure is achieved through the (Cd2Cl3)n chain along the crystallographic b axis. This chain is further garnished with two terminal chlorine atoms (Cl3 and Cl4) and the ligand L2. The molecule configuration of L2 is laid out as a plane to encompass the Cd1 atom. Of the Cd−N bonds, the bonds with the quinoline nitrogen atoms are longer (Cd1−N1 of 2.41 Å and Cd1−N3 of 2.42 Å) than those with imine nitrogen atoms (Cd1−N2 of 2.41 Å and Cd1−N4 of 2.32 Å). Cd2 is connected with two terminal chlorine atoms and a chloride bridging in the (Cd2Cl3)n chain constituting a CdCl4 tetrahedron. The structure of 2 is further stabilized by C9− H9A···Cl1 intramolecular interaction. As shown in Figure 3, the combination of adjacent 1D chains with C2−H2A···Cl1, C10− H10A···Cl4, C20−H20A···Cl2, and C23−H23A···Cl2 leads to the 2D layer, and then generates a 3D supramolecular framework by C8−H8A···Cl3, C7−H7A···Cl4, and C13− H13A···Cl4. The hydrogen bonding interactions play important roles in stabilizing the 3D supramolecular structure. If the C− H···Cl interactions are regarded as rods, the Cd(II) ion as a six connected node, the 3D structure of 2 can be classified as a

intramolecular hydrogen bonding interaction. As shown in Figure 2, the combination of adjacent molecule with C2− H2A···Cl1, C15−H15A···Cl1 hydrogen bonds and π-stacking interactions (3.75 Å) leads to the 1D chain. The spreading of 1 building units is oriented in the ac plane through C13−H13A··· Cl2 and C20−H20A···Cl2 interactions forming a 2D layers. The multidirectional self-recognition gives rise to the 3D supramolecular construction of complex 1. The active participation of C7−H7A···Cl2, C6−H6A···Cl2, C24− H24A···Cl2, and π−π stacking interactions (3.59 Å) promote multiway self-recognition. Topologically, if the metal centers and the C−H···Cl hydrogen bond can be seen as nodes and rods respectively, the whole architecture can be simplified as a 7-connected {34·49·58} supramolecular structure. [CdL2Cl2]n (2). Single crystal diffraction has shown that complex 2 is a 1D coordination polymer. The monomeric unit of the coordination polymer comprises two independent Cd(II) ions (Cd1 and Cd2) interconnected by bridging chloride (Figure 3). The coordination environment of the Cd1, which is the central point of the Schiff base L2, can be described as a heavily distorted octahedron [CdN4Cl2] with two chloride atoms and N1, N2, N3, N4 nitrogen atoms from 5412



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Figure 3. (a) Coordination environment of Cd(II) center in 2. Dotted lines represent the intramolecular hydrogen bond interactions. All hydrogen atoms except those participating in the hydrogen bond are omitted for clarity. (b) Coordination polyhedron in 2. (c) The infinite Cd−Cl chain of complex 2. (d) View of the crystal packing of 2 in the ab plane. The polymeric chains are further assembled into a 2D network via intermolecular C− H···Cl hydrogen bonds (represented by dotted lines). (e) The 3D supramolecular structure of complex 2. (f) View of the 3D topology in 2 (coordinate bond, blue line; C−H···Cl hydrogen bond interaction, green line).

(6,6)-connected lattice with a {49·65}2 Schläfli symbol which represents a 3D network topology. [HgL3Cl2] (3). Single crystal X-ray diffraction studies reveal that complex 3 crystallizes in the triclinic space group. Complex 3 has a monomeric structure of [HgL3Cl2] unit (Figure 4), where L3 ligand behaves as a chelated ligand coordinated to the Hg(II) ion by means of the N1 (quinoline) and N2 (imidazole) atoms. Two chlorine atoms Cl1 and Cl2 complete the Hg(II) coordination environment in a tetrahedral geometry (Figure S10). The bond distances of Hg(1)−Cl1, Hg(1)−Cl2, Hg(1)− N1, and Hg(1)−N2 are 2.342(5), 2.488(6), 2.448(4), and 2.228(4) Å, respectively. The tetrahedral angles around Hg(II) range from 72.17(1)° to 137.94(1)°. Each [HgL3Cl2] unit is linked to neighboring molecule via C2−H2A···Cl2 intermolecular bonding and π-stacking interaction (3.55 Å), forming a dimer. The dimers are further bridged by C7−H7A···Cl1 in the ab-plane to generate a 1D chain. The centroid-to-centroid distance of benzene ring planes in adjacent chain is 3.60 Å, indicating there is π−π stacking interaction between the 1D chains, which further expand the structure into a 2D layer. Moreover, C3−H3A···Cl1, C6−H6A···Cl1, C13−H13A···Cl1,

and C14−H14A···Cl1 hydrogen bonds result in a 5-connected {46·64} bnn 3D supermolecular structure. [(L4)2(HgCl4)(C7H8)2](4). Asymmetric unit in 4 comprises a discrete [(L4)2]2+ cation, [HgCl4]2− anion moiety, and two free methylbenzene molecules (Figure 5). Single crystal X-ray crystallographic research illustrates that there is a new rearranged benzimidazole ligand L4 with three quinoline rings. The quinoline rings are all in a near-vertical orientation (73.50−83.78°) to the benzimidazole unit to reduce the steric interactions. The Hg(II) center (Hg1) in the anion part adopts a [HgCl4] tetrahedron geometry bound by four Cl anions (Cl1, Cl2, Cl3, and Cl4) (Figure S8). Hg1−Cl distances lie within the range 2.240(10)−2.507(10) Å as expected.48 The solidstate structure of 4 shows a considerable supramolecular architecture through a combination of hydrogen bonding and π−π interactions. The Cl atoms of the [HgCl4]2− anion are all involved in the formation of C−H···Cl hydrogen bonds (C2B− H2B···Cl1, C3B−H3B···Cl1, C22A−H22A···Cl1, C30B− H30B···Cl2, C27B−H27C···Cl3, C10B−H10B···Cl3, C27A− H27B···Cl3, C10B−H10B···Cl3, C29A−H29A···Cl4, C8B− H8B2···Cl4, C5B−H5B···Cl4, C8A−H8A1···Cl4), which bridge the [HgCl4]2− anion with other four L4 cations. The units are 5413



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Figure 4. (a) Coordination environment of Hg(II) center in 3. All H atoms are omitted for clarity. (b) The infinite 1D chain of complex 3 generated through C−H···Cl and π-stacking interactions. (c) The chains are assembled by strong π-stacking interaction into a 2D network. (d) The 3D supramolecular structure of complex 3. (e) View of the topology structure in 3 (π-stacking interaction, pink line; C−H···Cl hydrogen bond interaction, blue line).

supplemented by π−π stacking interaction (3.57 Å) resulting in a 1D linear polymeric chain. Furthermore, the strong π−π stacking interaction (3.59 Å) between adjacent chains assembles the adjacent 1D chain to form a 2D supramolecular network. The hydrogen bonding interactions (C33A−H33A··· Cl1) generate the supramolecular 3D network arrangement, which can be classified as a (4,6,6)-connected lattice with a {33· 42·5}{33·42·53·67}{33·43·53·66} Schläfli symbol. Photoluminescence Studies. The emission spectra, lifetimes or quantum efficiency of Schiff base ligand L and complexes 1−4 are recorded both in CH3CN and in the solid state at room temperature. The photophysical data are gathered in Table S7. In the CH3CN solution, upon excitation in the UV domain (ca. 330 nm), because of the O−H oscillator, free L exhibits almost negligible emission bands at λmax = 418 nm, which is attributed to the π* → π transition. Meanwhile, the excessive loss of energy leads to the nearly nonemissive nature of L which is due to radiation-less decay in the nonrigid organic molecules. After complexation, which restricts the fluctuations of the organic skeleton, the rigidity given by the metal ion and the crystal structure largely relieves this decay and results in the drastically enlargement of photoluminescence intensity. The complexes display broad and strong emission spectra with maxima at 435, 428, 431, and 444 nm for 1, 2, 3, and 4,

respectively (Figure 6a), assigned as ligand-to-ligand chargetransfer (LLCT).49 Obviously, the luminescent intensity of complexes 1−4 are mainly dominated by the degree of conjugation, supplemented by the coordination environment and the steric proximity of the ligands. The emission intensity of the complexes increases in the order of 4 < 3 < 1 < 2. The strongest luminous intensity from the complex 2 is due to its highest extent of electrondonating nature and conjugated system with chloro-bridge links 1D chain. The introduction of rigid quinoline ring results in stronger emission intensity of 1 than 3. The complex 4 also displays stronger luminescence than that of L with emission maxima λmax = 444 nm, which is attributed to the extent of the conjugated system, although Hg(II) is not involved in coordination. All the studied compounds generate bright blue emission with Commission Internationale de L’Eclairage (CIE) coordinates of 0.15−0.18, 0.05−0.12 (Table S7). The emission decay profiles of L and 1−4 measured in CH3CN at 298 K required biexponential fits. This indicates that the emission spectra may not result from monotransition process,50 and one of the emitting species heavily processes the spectra, which leads to one main emission peak in the fluorescence spectra of L and 1−4. The average lifetime at 298 K is in the range of microseconds (mean life: τ = 6.76 μs for L, 5414



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Figure 5. (a) Molecular structures of 4. All H atoms and two free methylbenzene molecules are omitted for clarity. (b) The infinite chain generated through C−H···Cl and π-stacking interactions. (c) The chains are assembled by strong π-stacking interactions into a 2D network. (d) The 3D supramolecular structure of 4. (e) View of the 3D topology in 4 (color code: [HgCl4]2− anion, green ball; (L4)+ cation, red ball).

Figure 6. (a) Emission spectra of ligand L and complexes 1−4 in CH3CN at 298 K (excitation at 330 nm). Inset: photos of L and complexes 1−4 in CH3CN under 365 nm UV lamp illumination. (b) Emission spectra of ligand L and complexes 1−4 in the solid state at 298 K (excitation at 330 nm). Inset is the graph of the fluorescent intensity.

τ = 19.06, 18.56, 9.70, and 8.85 μs for 1−4, respectively). The lifetime of complexes is much longer than that of ligand L (Figure S11). Especially, the lifetime of 1 is roughly 2.8-fold that of L, which may be explained by the rigidity provided and the improvement of conjugated system, in addition to removing hydroxyl substituent group. For complexes 1−4, the extension of the conjugated π system appears to increase the quantum yield; values of φ = 25%, 23%, 17% have been obtained for, respectively, 1, 4, and 2. A lower quantum yield has been achieved for the complex 3, with φ = 11%. Obviously, an

extensional conjugated system of the construction markedly reinforces the emission efficiency of the complexes. The photophysical properties of L and complexes 1−4 are also examined in solid state at 298 K (Table S8). The ligand L and complexes 1−4 display broad emission spectra with maxima at 470 nm for L, 472 nm for 1, 464 nm for 2, 470 nm for 3, and 472 nm for 4 at 298 K (Figure 6b). The emission intensity of the compoud also increases in the order of L < 4 < 3 < 1 < 2 accompanied by the increased degree of conjugation. The solid state emission spectra emit similar peak profiles 5415



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Figure 7. UV−visible absorption spectra of 1−4 and N719 (a) in CH3CN and (b) on TiO2 films.

semiconductor materials.54 There is a low utilization in the region of short wavelengths that shown by Ruthenium complex N719 as a typical representative organic dye used in DSSCs. Compared with the absorption spectra of N719, complexes 1− 4 could compensate for that of N719 in the low wavelength region of the UV−visible spectrum. The molar extinction coefficients in the LE region are 48 085 M−1 cm−1 for 1, 32 670 M−1 cm−1 for 2, 35 488 M−1 cm−1 for 3, and 45 865 M−1 cm−1 for 4. The higher molar extinction coefficient around 300−400 nm indicates that the complexes 1−4 possess higher light harvesting ability in the wavelength interval compared with N719 and I3− (25 000 M−1 cm−1).55 It can be anticipated that the photon loss result from light absorption by I3− will be possibly suppressed by the use of complexes 1−4 as a cosensitizer used in N719 sensitized DSSCs. To further research if the prepared complexes could be used in DSSCs as cosensitizers, the absorption spectra of 1/N719, 2/ N719, 3/N719, and 4/N719 cosensitized TiO2 films were recorded and are shown in Figure 7b. The absorption of N719 on the TiO2 film in the visible light region is obviously enlarged for the electronic coupling of the dyes on the TiO2 surface. When N719 is unified with complexes 1−4, respectively, its absorption in the interval of 350−700 nm on the TiO2 film is enhanced remarkably. This is in accordance with the result in which prepared complexes could make up for that of N719 in the wavelength region of the visible spectrum. In other words, the adsorption is grown in the region of 350−550 nm, which could give light-harvesting in this region and further improve the performance of cosensitized DSSCs. Application in Dye-Sensitized Solar Cells. Electrochemical Properties of Complexes 1−4. Energy-level matching is important in choosing the sensitizer for DSSCs. The redox properties of the complexes 1−4 are investigated by cyclic voltammetry (CV) values in a three-electrode cell on an electrochemical workstation, in order to appraise whether there are beneficial energy offsets in relation to the TiO2 film and the electrolyte. The experimental data for electrochemical properties of complexes are summarized in Table 3. As predicted from the junction of the absorption and emission spectrum, the excitation transition energy values (E0−0) of 1−4 are 2.89, 2.90, 2.85, and 2.86 eV, respectively. Consequently, the HOMO values of 1−4 are calculated based on their first redox potentials as −5.00, −5.05, −5.08, and −5.06 eV, and the LUMO levels of

compared to those in solutions for all compounds, but the maximums have red-shifted in comparison with those in solutions (mean value: 52, 37, 36, 39, and 28 nm for L and 1− 4). The hydrogen bonds interaction and π-stacking of the aromatic rings may lead to the redshift phenomenon in these molecules in the solid state.51 Aggregation and stacking effects result in the double exponential fitting luminescent lifetimes for L and 1−4 in the solid state at 298 K (Figure S12). Absorption Properties of the Schiff Base L and Complexes in Solution. The UV−vis absorption spectra of L and complexes 1−4 are recorded at room temperature in CH3CN (Figure 7a), and the corresponding data are listed in Table S7. The UV−vis absorption spectrum of the Schiff base L exhibits two obvious absorption bands at 311 nm in the higher energy (HE) band and 356 nm in the lower energy (LE) band, which are attributable to π → π* and n → π* transitions.52,53 We tested the UV−vis absorption spectra of the complexes 1− 4 to obtain insights into the influences of the ZnII/CdII/HgII complexation and the deformations of the ligand L on the absorption properties of the compounds. All spectra illustrate two different absorption bands at 315−323 nm in the HE band and 369−390 nm in the LE band, which are assigned to metal perturbed intraligand π−π* and n−π* transitions. Obviously, the complexations lead to the bathochromic of the LE absorption bands (34 nm for 1, 31 nm for 2, 26 nm for 3, and 13 nm for 4) due to the formation of five-membered chelate rings upon M(II) coordination (complexes 1−3) or cyclization reaction (complex 4). There are also strong hyperchromic effects on the LE absorption bands for complexes 1−4, with the increase of the conjugate system upon the induction of metal ions ZnII/CdII/HgII, which are proved by the growth of the molar absorption coefficient for these transitions (intensity: 1 > 4 > 3 > 2). In addition, the electronic transitions of the HE band center on the quinoline rings. Compared with complex 3, those compounds having two or three rigid quinoline rings in their structures, i.e., L, 1, 2, and 4, present stronger hyperchromic effects on the HE absorption band. All of the complexes exhibit prominent peaks in the blueviolet region with a molar extinction coefficient from 3.3 × 104 to 7.0 × 104 M−1 cm−1. In enhancing the efficiency of photovoltaic energy conversion, one of the main difficulties lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the band gap of 5416



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of prepared complexes and N719. Under illumination (AM 1.5G, 100 mW cm−2), the current−voltage (J−V) characteristics of the DSSC devices based on N719, 1/N719, 2/N719, 3/N719, and 4/N719 photoanodes are shown in Figure 8. The

Table 3. Experimental Data for Spectral and Electrochemical Properties of the Synthesized Complexes λabsa (nm) dye (ε [103 M−1 cm−1]) 1 2 3 4

317(58.1), 390(48.1) 318(59.6), 387(32.7) 323(54.7), 382(35.5) 315(68.1), 369(45.9)

λema,b (nm)

E0−0c (eV)

Eox/V vs SCEd

EHOMOe (eV)

ELUMOe (eV)

435

2.89

0.60

−5.00

−2.11

428

2.90

0.65

−5.05

−2.15

431

2.85

0.68

−5.08

−2.23

444

2.86

0.66

−5.06

−2.20

Absorption and emission spectra were recorded in CH3CN (10−5 M) at room temperature. bComplexes were excited at their absorption maximum value. cOptical band gap calculated from intersection between the absorption and emission spectra. dThe first oxidation potentials of complexes were obtained by CV measurement. eThe values of EHOMO and ELUMO were calculated with the following formula:61 HOMO (eV) = −e(Eox onset V + 4.4 V); LUMO (eV) = EHOMO + E0−0, where E0−0 is the intersection of absorption and emission of the complexes 1−4. a

1−4 calculated from EHOMO + E0−0, are −2.11, −2.15, −2.23, and −2.20 eV, respectively.56 The HOMO and LUMO energy levels of 1−4 are shown in Scheme 3. It illustrates that the

Figure 8. J−V curves for DSSCs based on cosensitized photoelectrodes and single N719 sensitized photoelectrodes under irradiation.

Scheme 3. Electrochemical Potential Diagram of Complexes 1−4, TiO2 CB Edge, and I−/I3− Couple, Showing the Feasibility of Electron Injection and Dye Regeneration Processes

Table 4. J−V Performance of DSSCs Based on Different Photoelectrodes photoelectrode

Jsc(mA/cm2)

Voc/V

FF

η/%

N719/TiO2 1/N719/TiO2 2/N719/TiO2 3/N719/TiO2 4/N719/TiO2

15.43 18.69 18.05 16.76 18.61

0.75 0.75 0.75 0.75 0.74

0.56 0.61 0.59 0.57 0.62

6.55 8.30 8.04 7.85 8.21

corresponding cells performances are summarized in Table 4. The individually N719 sensitized device exhibits an overall conversion efficiency (η) value of 6.55% (with Jsc = 15.43 mA cm−2, Voc = 0.75 V, and FF = 0.56). This low performance could be due a single thin transparent TiO2 film used in these devices. As shown in Table 4, the cosensitized solar cell based on the complex/N719/TiO2 (complex: 1−4) electrode yields are markedly high photocurrent density (Jsc) under standard global AM1.5 solar irradiation conditions without an expense of lower open circuit voltage (Voc), which is around 0.75 V stability. The values of Jsc and η are improved in the order of 1/ N719 > 4/N719 > 2/N719 > 3/N719 > N719. All the values of Jsc and η are significantly higher than that of the device sensitized by N719 only. Especially, the cell cosensitized by 1/ N719 and 4/N719 exhibited effectively elevated η of 8.30% (with Jsc = 18.69 mA cm−2 and FF = 0.61) and 8.21% (with Jsc = 18.61 mA cm−2 and FF = 0.62), which is 26.72% (for 1) and 25.34% (for 4) higher than that of cells sensitized by N719 individually. This result suggests that cosensitization of TiO2 photoelectrode with complexes 1−4 can effectively improve the efficiency of DSSCs. The higher η values of 1/N719, 2/N719, 3/N719, and 4/ N719 cosensitized solar cells are caused by the enhanced photovoltaic parameters of Jsc. Commonly, the Jsc value is

energy levels of 1−4 are suitable for the DSSC system involving TiO2.57 The LUMO levels lay over the conduction band (CB) of the TiO2 semiconductor (−4.40 eV vs vacuum) and that of N719, which lead to an improved injection driving force of electrons compared with N719 alone, indicating efficient electron injection. Therefore, the positive synergistic effect of these complexes and N719 perfects the electron injection efficiency from the LUMO of the dye to the conduction band of TiO2. The HOMO energy levels lay below that of the I−/I3− redox electrolyte (−4.85 eV vs vacuum)58 supplying adequate driving force for dye regeneration.59,60 The thermodynamic force for electron injection and regeneration of the photooxidized dyes is adequate. Photovoltaic Properties of DSSCs. The complexes 1−4 are employed as cosensitizers to fabricate complex/N719 photoanodes of DSSCs, which are fabricated following a stepwise cosensitization procedure by sequentially immersing the TiO2 electrode (with a thickness of ca. 10 μm) in a separate solution 5417



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affected by the incident photon-to-current conversion efficiency (IPCE) response of cells, since the equation: Jsc =

∫ eϕph.AM1.5G(λ) dλ

Where ϕph.AM1.5G is the photon flux at AM 1.5 G solar irradiation and e is the elementary charge.62−64 The IPCE spectra of different devices are collected in order to interpret the enhancement of Jsc (Figure 9). The DSSCs including only

Figure 10. Emission spectra of complexes 1−4 and excitation spectrum of N719 in CH3CN solution.

Dark-current measurement of DSSCs as a qualitative technique is regarded to determine the extent of the back electron transfer. A comparison among the investigated cells in dark current could provide useful information about the back electron transfer process. The results mentioned above are also proven by the dark current voltage (J−V) measurements of the different devices, as is shown in Figure 11. The dark current− Figure 9. Incident photon-to-current conversion efficiency spectra of devices based on single N719 sensitized and cosensitized photoanodes.

N719 dye have a broad IPCE spectrum from 300−750 nm but a drop in 340−450 nm. This is because of the competitive light absorption between I3− and N719. When utilizing the prepared complexes 1−4 as cosensitizer, not only the decrease in the wavelength range of 340−450 nm is restored, but also the IPCE spectrum in the whole visible region is improved. The order of IPCE values of new cosensitized solar cells is as follows: 1/ N719 > 4/N719 > 2/N719 > 3/N719 > N719. This can be ascribed to the fact that the prepared complexes have attached to the TiO2 surface effectively or have alleviated the aggregation of N719. The different IPCE values of four curves from complex 1−4 demonstrate that complexes 1 and 4, which possess uncoordinated nitrogen atoms in pyridine ring, contribute most to the electron injection into the TiO2 conduction band. The uncoordinated nitrogen atoms are immobilized exterior to the molecular surfaces, which provides a more efficient electron collection and injection from the complexes to the semiconductor. On the basis of the absorption spectra and IPCE, it is predictable that the photon lost because of light absorption by I3− will be suppressed by the use of 1−4 as a cosensitizer. The luminescence of complexes 1−4 showing in the wavelength region 460−474 nm in CH3CN solution just overlap with the N719 excitation spectra (Figure 10). It indicates that N719 could accept the energy from incident light and excited compounds 1−4 synchronously. The spectral response of N719 in the region 300−700 nm will be widened. These also are helpful to improve Jsc. This means the cosensitization of new complexes 1−4 and N719 have significant synergistic and compensatory effects on the light harvesting and electron injection. These all contributed to the improvement in Jsc and η.

Figure 11. J−V curves for DSSCs based on co-sensitized photoelectrodes and single N719 sensitized photoelectrode in the dark.

voltage characteristics of the DSSCs based on the 1/N719, 2/ N719, 3/N719, 4/N719, and N719 dyes indicate the recombination of injected electrons with I3−. It illustrates that the dark current for the cosensitized system is lower compared to that of single N719 sensitized DSSCs, which sort by values of 1/N719 < 4/N719 < 2/N719 < 3/N719 < N719. The decrease of the dark current stated that complexes 1−4 effectively restrain the electron back reaction with I3− in the electrolyte by constituting a compact layer with N719, causing Jsc, resulting in a higher overall η value. To further explain the enhancement of photovoltaic performance and IPCE of new cosensitized solar cells, the interfacial charge transportation in the DSSC devices is studied by electrochemical impedance spectroscopy (EIS), which is measured under standard AM1.5 G solar irradiation by utilizing 5418



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Figure 12. (a) Nyquist plots (b) Bode plots of EIS for DSSCs based on different photoelectrodes measured under standard AM 1.5 G solar irradiation.

Figure 13. Possible working mechanism of the DSSC with FTO/TiO2/complex multilayered electrode. (a) Three functions of the complex materials in the working process of a DSSC: (1) converting of UV light to blue light, (2) scattering of light, and (3) recombination centers. (b) Molecular structure of 1. (c) Molecular structure of 4.

a forward bias of −0.75 V. Under light illumination, EIS is applied to analyze the electron transport resistance at the TiO2/ dye/electrolyte interface for its importance on the efficiency of DSSCs.65−67 As shown in Figure 12a, the electron transfer at the TiO2/dye/electrolyte interface and charge transfer in the electrolyte (Nernst diffusion)68,69 lead the three semicircles located in high, middle, and low frequency regions (left to right). After cosensitization with prepared complexes, the radius of the large semicircle located in the middle frequency regions in the Nyquist plots reduce and sort in values of 1/N719 < 4/ N719 < 2/N719 < 3/N719 < N719, which at the interface indexes a rise of electron transfer rate and a drop of the electron transfer impedance (Rct) after cosensitization. In the cosensitization system, better dye coverage promotes the passivation of TiO2 surface or constructs an insulating molecular layer

consisting of complexes 1−4 with the N719 molecule, therefore decreasing the recombination caused by electron back transfer between TiO2 and I3−. The electron transport time (τd) is the average time brought by the injected electron to reach the collecting FTO electrode. A higher photocurrent influences a faster electron transport time. Also, the result could be achieved from the Bode phase plots of the EIS spectra of different solar cells (Figure 12b). On the basis of the relationship: in the intermediate frequency region in Bode phase plot τd = 1/(2πf max), where f max is the frequency at the maximum of the curve. The calculated values of the electron transport time (τd) for different devices are 5.89 ms for 1/N719, 6.66 ms for 2/N719, 7.24 ms for 3/N719, 6.27 ms for 4/N719, and 7.70 ms for individually N719, respectively. 5419



Article

construction of novel asymmetric fluorescent complexes with photoelectricity functions.

Influence of Structures for DSSCs. It is found that the structural features of complexes have an effect on the enhancement of the cosensitized DSSC performance. First, the asymmetry of the π-electron distribution in complex 1−4 is able to support charge transfer without disrupting the vectorial spatial/energetic orientation between the dye and the TiO2 interface; Secondly, the chlorine ion with lone pair electrons in complexes 1−4 could conjugate with the π system to increase electron density and lead to enhance the conjugated delocalized system.34 The performance of cosensitized DSSCs compared with that of single N719 sensitized DSSCs is in the order of 1/ N719 > 4/N719 > 2/N719 > 3/N719 > N719. The structure of complex is closely related to the conversion efficiency of complex 1−4 as cosensitizers in DSSC. In complexes 1 and 4, the uncoordinated nitrogen atom in the pyridine ring (Figure 13) could facilitate the electron transportation from the dye to TiO2. This is owing to its intimate contact with the semiconductor surface via a binding mode by the pyridine nitrogen group, which furnishes more electron pathways. Generally speaking, the molecular structure may show greater tortuosity with the positive rotation of C17−C18 for 1 and C8B−C9B, C27B−C28B single band for 4. Therefore, 1 and 4 provide a more efficient electron injection and collection from the complexes to the semiconductor. The modular units of 2 are assembled together to form 1D ∞-like coordination polymer chains, forming high surface area arrays for photon collection. The additional dyes N719 adsorbed onto the 1D coordination polymer chains could also contribute to the enhancement of Jsc.70 When complex 3 served as the cosensitizer, the performance of cosensitized solar cell is lower than that by N719 and 1, 2, and 4. This difference in performance is attributed to the decrease of absorption of complex 3 caused by a smaller conjugated system.71,72 The strong absorption of complexes 1, 2, and 4 is more suitable for recovering the absorption of N719 and conquering the competitive light absorption of I3−.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00891. Structural Information for L and complexes 1−4, FT− IR, 1H NMR, 13C NMR, PXRD spectra, absorption and luminescent data (PDF) Accession Codes

CCDC 1449510−1449514 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(R.F.) Fax: +86-0451-86413710. E-mail: [email protected]. cn. *(Y.Y.) E-mail: [email protected]. ORCID

Ruiqing Fan: 0000-0002-5461-9672 Yulin Yang: 0000-0002-2108-662X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants 21371040 and 21571042), the National Key Basic Research Program of China (973 Program, No. 2013CB632900).

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CONCLUSION This study provides a new example of efficient synthesis of asymmetric ZnII/CdII/HgII complexes based on benzimidazole with one quinoline ring (3), two quinoline rings (1), three quinoline rings (4), and bis-Schiff base (2) respectively by a metal-induced reaction from asymmetric Schiff base ligand L. All of these compounds have 3D SMOFs held together by hydrogen bonds and π-stacking interactions. The studied complexes generate bright blue emission in solution. These new synthesized complexes 1−4 as cosensitizer could have a significant influence on the performance of DSSCs. The cosensitized device exhibits enhanced performance in the order of 1/N719 > 4/N719 > 2/N719 > 3/N719, which are higher than that of single N719 sensitized solar cells. As the complex 1 is used as a cosensitizer, a short circuit current density of 18.69 mA cm−2, an open-circuit voltage of 0.75 V, and a FF of 0.61 are achieved, amounting to an overall conversion efficiency of 8.30% under AM 1.5 G solar irradiation. The improvement in efficiency is attributed to the fact that these complexes could surmount the shortcomings of N719 absorption under the situation of a short wavelength interval of visible spectrum, keep its aggregation off, counteract competitive visible light absorption of I3−, and suppress the charge recombination as a result of the formation of an effective cover layer of the dye molecules on the TiO2 surface. Obviously, this synthesis strategy may expand interest in the

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Metal(II)-Induced Synthesis of Asymmetric Fluorescence

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