Metal(II)-Induced Synthesis of Asymmetric Fluorescence

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Metal(II)-induced synthesis asymmetric fluorescence benzimidazoles complexes and their dyesensitized solar cells performance as co-sensitizers Ruiqing Fan, Xinming Wang, Yuwei Dong, Ting Su, Jian Huang, Xi Du, Ping Wang, and Yulin Yang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00891 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 30, 2017

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Crystal Growth & Design

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Metal(II)-induced synthesis asymmetric fluorescence benzimidazoles complexes and their dye-sensitized solar cells performance as co-sensitizers

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Xinming Wang,†‡,b Ruiqing Fan,*, † Yuwei Dong, † Ting Su, † Jian Huang, † Xi Du, † Ping Wang, † Yulin Yang,*, †

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†

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. of China

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‡

Department of Chemistry, Harbin University Of Science And Technology, Harbin 150080, P. R. China

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To whom the proofs and correspondence should be sent.

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Professor Ruiqing Fan School of Chemistry and Chemical Engineering

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Harbin Institute of Technology Harbin 150001, P. R. China

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Fax: +86-0451-86413710 E-mail: [email protected] and [email protected]

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ACS Paragon Plus Environment


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ABSTRACT: Four asymmetric complexes with different conjugate structures based on the 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), (HgCl4)(L4)2(C6H7)2 (4)(L4: 1, 2, 3-((quinoline-2-yl) methylene) benzimidazoleion) are obtained. Interestingly, L1–L4 are derived from half-condensed asymmetrical Schiff base ligand [N1-((quinoline-2-yl) methylene) N2-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L) by means of metal(II)-induced. To the best of our knowledge, it is the first examples of one Schiff base ligand transforms 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 is driven mainly by 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 the complex 2 is due to its 1D chain conjugated system with 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−. The complexes 1–4 could be utilized as co-sensitizers in combination with N719 to examine the effect on enhancing the performance of dye-sensitized solar cells (DSSCs). After co-sensitization, the incident-photon-to-current conversion efficiency is increased, also the dark current is reduced. Because of utilizing co-sensitizers 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, 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 co-sensitizers.

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INTRODUCTION

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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 signaling1–8, catalysis9–11 and dye-sensitized solar cells (DSSCs) 12–17. The conventional method to form benzimidazole derivatives complexes, is two-step method, namely, the benzimidazole derivatives are synthesized and separated firstly.. The most popular strategy are usually followed by the coupling of o-phenylenediamines (o-PDs) with carboxylic acids or their derivatives or the condensation of o-PDs and aldehydes followed by oxidative cyclo-dehydrogenation18– 21 . Many of these reactions are often associated with side reaction22. On account of a greater selectivity in the synthesis of the complex based on benzimidazoles and their derivatives, the search remains to be continued. 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 kinds of benzimidazole derivatives by metal(II)-induced one-step method. The interest is sequentially increasing on developing fluorescence complexes for photovoltaic performance application as co-sensitizers in Dye sensitized solar cells (DSSCs). They show a tailored light-absorbing property, excellent chemical and photochemical stability23–28. In our previous work, N-heterocyclic ligands and their transition metal complexes have been prepared and employed in the DSSC as co-sensitizers to replace precious metal complexes29,30 (compared to IrIII, RuII, OsII and PtII, which are used as common sensitizers)31,32. While, asymmetrical structure co-sensitizers received more attention recently, assuming the vectorial spatial/energetic orientation between the dye and the TiO2 interface and conduction band-potentially hampering charge transfer33 would be disrupted since the symmetry of the π-electron distribution. A very recent study by our group explores d10 metal complexes based on the 4-(1H-imidazol-1-yl)benzoic acid in DSSCs detailedly34. The rational design is mainly focused on the rigid ligand with single and small conjugated system. However, the scale of the π-conjugated system on the ligands plays a role on governing the photoelectric properties of complexes. Therefore, complexes based on benzimidazole derivatives with different conjugation might become much promising materials. Considering the above factors, we synthesize four asymmetric complexes with different conjugated systems by metal–induced, namely, [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), [(L4)2(HgCl4)] (4) (L4: 1, 2, 3-((quinoline-2-yl) methylene) benzimidazoleanion). For all we know, it is the first example of the half

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condensed Schiff base ligand transforms 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 1D chain structure built from chlorine bridge and possesses an interesting 3D (6,6)-connected{49·65}2 supramolecular network. Complex 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 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 co-sensitizers 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%).

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EXPERIMENTAL SECTION

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Synthesis

of

Schiff

Base

Ligand

[N1-((quinoline-2-yl)methylene)

23

N2-(H-(quinoline-2-yl) methylene-hydroxy)o-phenylenediamine] (L)

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A dichloromethane (CH2Cl2) solution (20 mL) of 2-quinoline formaldehyde (1.572 g,

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10 mmol) was added to a stirred CH2Cl2 solution (10 mL) of o-PD (1.082 g, 10 mmol)

26

under the influence of a catalytic amount of triethylamine. Stir was continued at 25 °C.

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Reaction after 0.5 h claybank solid suspended in solution and did not increase. Stop

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reaction, filtration, deep yellow solid were obtained. Using acetonitrile (CH3CN) and

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petroleum ether recrystallization, single crystals of the L suitable for X-ray diffraction

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study were obtained after one day. Yield: 1.250 g, 62.22% (based on o-PD), mp 80–

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82 °C. Elemental anal. Calc. for C26H20N4O (403.45): C, 77.21; H, 4.98; N, 13.85%.

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Found: C, 77.25; H, 5.00; N, 13.84%. IR (KBr, cm-1): 3431(br), 3060(w), 1630(s),

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1597(s), 1500(m), 1443(m), 763(s) (Figure S1). 1H NMR (400 MHz, CD3CN, 298 K,

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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

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(s, 1H, –CH–), 6.60 (s, 1H, –NH–), 5.18 (s, 1H, –OH) ppm.

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CD3CN, 298K): δ 159.3, 151.7, 149.3, 148.5, 147.8, 145.1, 136.5, 135.6, 134.6, 130.9,

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129.9, 129.7, 128.9, 128.3, 128.3, 128, 127.8, 127, 126.6, 126.5, 125.9, 122, 121.9,

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119.6, 114.8, 90.5 ppm. ESI–MS: m/z = 405.19 [M + H]+. L is soluble in polar 4


13

C NMR (150 MHz,

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organic solvents for instance dimethylsulfoxide (DMSO), dimethyl formamide (DMF),

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methanol (CH3OH), ethanol (CH3CH2OH), which promotes the reactions between it

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and metal ions in solutions.

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Synthesis of Complex [ZnL1Cl2] (1) (L1: 1-((quinoline-2-yl) methylene)-2-

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quinoline-2-ylbenzimidazole)

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The mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) of ZnCl2 (0.136 g, 1.0

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mmol) and L (0.403 g, 1.0 mmol) was refluxed at 80 °C for 5 hours and then cooled

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down to room temperature. Filtrating light yellow solution to debride any suspended

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particles and kept for evaporation at room temperature, then the block colorless

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crystals were retrieved. Yield: 0.320 g, 70.34% (based on ZnCl2), mp 110–112 °C.

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Elemental anal. Calc. for C26H18N4Cl2Zn (522.71): C, 80.81; H, 4.69; N, 14.50 %.

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Found: C, 80.82; H, 4.70; N, 14.48 %. IR (KBr, cm–1): 3063(w), 1624(m), 1600(m),

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1519(m), 1480(w), 760(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 7.58–8.68 (m,

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12H, Q-H), 7.26–7.40 (m, 4H, Ph-H), 5.30 (s, 2H, –CH2) ppm. 13C NMR (150 MHz,

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CD3CN, 298K): δ 167.6, 159.3, 147.8, 146.3, 141.5, 137.9, 137.6, 135.6, 134.2, 129.9,

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129.7, 128.3, 127.8, 127, 126.6, 125.9, 125.5, 123, 122, 120.8, 119.3, 115.2, 110, 54.9

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ppm. ESI–MS: m/z = 521.31 [M + H]+. It is soluble in polar solvents, for instance

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DMSO, DMF, CH3CN and CH3CH2OH.

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Synthesis

of

Complex

[CdL2Cl2]

(2)

(L2:

N,N′-bis((quinoline-2-yl)

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methylene)o-phenylenediamine)

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A mixture of CdCl2 (0.183 g, 1.0 mmol) and L (0.403 g, 1.0 mmol) was dissolved in

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mixed solvent of methanol and toluene (v:v = 3:1, 8 mL) and stirred for 15 min, then

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heated to 80 °C for 5 hours in sealed vial. After cooling to room temperature slowly,

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the yellow rectangular block crystals were retrieved. Pure product was obtained in

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yield 0.228 g, 75.21% (based on CdCl2), mp 121–123 °C. Elemental anal. Calc. for

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C26H18Cl4N4Cd2 (753.04): C, 80.81; H, 4.69; N, 14.50%. Found: C, 80.83; H, 4.66; N,

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14.53%. IR (KBr, cm–1): 3063(w), 2931(w), 1644(w), 1586(m), 1504(m), 1438(m),

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750(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 8.51 (s, 2H, –CH), 7.66–8.40 (m,

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12H, Q-H), 7.33 (d, 2H, Ph-H), 7.19 (d, 2H, Ph-H) ppm.

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CD3CN, 298K): δ 163.7, 147.1, 144.5, 142.5, 136.5, 130.9, 129.9, 128.9, 128.5, 128.3,

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127, 123.6, 122.1 ppm. ESI–MS: m/z = 571.13 [L + CdCl2]+. It is soluble in polar

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solvents, for instance DMSO, DMF, CH3CN and CH3CH2OH.

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13

C NMR (150 MHz,

Synthesis of Complex [HgL3Cl2] (3) (L3: 2-(quinoline-2-yl) benzimidazole)

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A mixture of HgCl2 (0.271 g, 1. 0 mmol) and L (0.403 g, 1.0 mmol) under

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circumstance of a catalytic amount of triethylamine was dissolved in mixed solvent of 5



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methanol and toluene (v:v = 3:1, 8 mL) and stirred at room temperature for 30 min.

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After that, it was heated to 100 °C for 5 hours in sealed vial. Cooling to room

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temperature then colorless needle-like crystals of 3 was obtained by filtration. Pure

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product was obtained in yield 0.283 g, 54.81% (based on HgCl2), mp 110–112 °C.

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Elemental anal. Calc. for C16H11Cl2N3Hg (516.77): C, 78.35; H, 4.52; N, 17.13%.

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Found: C, 78.36; H, 4.50; N, 17.11 %. IR (KBr, cm–1): 3070(m), 1600(m), 1509(m),

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1428(m), 755(s). 1H NMR (400 MHz, CD3CN, 298 K, TMS): 12.82 (s, 1H, –NH),

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7.90-8.67 (m, 6H, Q-H), 7.33 (m, 4H, Ph-H) ppm.

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298K): δ 167.6, 147.2, 141.5, 138.9, 137.9, 137.6, 129.9, 128.9, 128.3, 127, 125.5,

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123, 119.3, 115.2 ppm. ESI–MS: m/z = 518.21 [M + H]+. It is soluble in polar

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12

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C NMR (150 MHz, CD3CN,

Synthesis of Complex (L4)2(HgCl4) (4) (L4: 1, 2, 3-((quinoline-2-yl)methylene)

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benzimidazoleanion)

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A solution of HgCl2 (0.271 g, 1.0 mmol) in mixed solvent of methanol and toluene

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(v:v = 3:1, 8 mL) was combined with a mixture of 2-quinoline formaldehyde (0.157 g,

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1.0 mmol) and o-PD (0.054 g, 0.5 mmol). The reaction solution was stirred at room

17

temperature for 30 min. After that, it was heated in a sealed vial at 100 °C for 5 h.

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After slow cooling to room temperature, colorless needle-like crystals of 4 were

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obtained by filtration. Pure product was obtained in yield 0.702 g, 44.35% (based on

20

HgCl2), mp 107–109 °C. Elemental anal. Calc. for C86H68Cl4N10Hg (1583.89): C,

21

65.16; H, 4.29; N, 8.84%. Found: C, 65.20; H, 4.30; N, 8.81 %. IR (KBr, cm–1):

22

3060(w), 1598(w), 1500(s), 1428(m), 746(s). 1H NMR (400 MHz, CD3CN, 298 K,

23

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),

24

2.39 (s, 3H, –CH3) ppm.

25

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,

26

122, 121.3, 117.9, 114.3, 50 ppm. ESI–MS: m/z = 528.35 [L]+. It is soluble in polar

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RESULTS AND DISCUSSION

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Analysis of L

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Our initial focus was on the monoamine-type Schiff base with 1:1 equiv of 2-quinoline formaldehyde and o-PD, to get the asymmetrical complexes. In a surprise twist, half-condensed Schiff base L which has α-hydroxy amine, is obtained under ambient temperature and short reaction time for 0.5 h (Scheme 1).

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C NMR (150 MHz, CD3CN, 298K): δ 159.8, 150, 147.8,

6


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1 2

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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 summed up 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 interacts with the derivatives of L result 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 interaction between phenyl H(26A) and the

16

hydroxy oxygen atom O(1) (C26–H26A⋯O1, H26A⋯O1 2.32 Å, ∠CHN 120.70°)

17

(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 one-dimensional (1D) chains. Facilitated by the internal π-stacking between two quinoline ring of the neighboring compouds with a centroid−centroid distance of 3.58 Å, a two-dimensional (2D) π-stacking network is taken shape by these supramolecular chains. In the crystal, molecules are further piled in a three-dimensional (3D) network via hydrogen bond C24–H24A⋯O1

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18 19 20 21 22 23 24

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1

(H24A⋯O1 2.59 Å, ∠CHO 141.85°) and strong π–π interaction (centroid–centroid

2

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 7-connected {33·411·52·65} net.

3 4

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Figure 1. (a) Crystal structure of L; Dotted lines represent the intramolecular hydrogen bond interactions. All hydrogen atoms except hydroxy amine and it participating in hydrogen bond 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) Synthetic Overview of Complexes 1–4. The quality of α-hydroxy amine in L is not only easy to further lose a molecule of water to become polydentate bis-Schiff base ligand, but also could be hydrolyzed to form monoamine. In view of the active Schiff base L, our strategy is to prepare a series of M(II) complexes by changing metal salt ZnCl2, CdCl2 and HgCl2, and four M(II) (M = Zn, Cd, Hg) complexes 1–4 have been successfully prepared. Firstly, the Schiff base L is treated with a methanol and toluene (v:v = 3:1) mixed solvent of ZnCl2 at 80 °C for 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 conventional bis-Schiff base complex as expected. On the contrary, the 1,2-disubstituted benzimidazole L1 is obtained and coordinated to Zn(II) ion 8


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simultaneously by in situ metal-ligand reaction, forming Zn-complex (1). Synthetic methods for 1,2-disubstituted benzimidazole often link with several side reactions and by-products44–46. 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. In spite of the exact mechanism remains a mystery, the Lewis acid ZnCl2-mediated mechanism may well play (Scheme 2), namely, ZnCl2 as catalyst for the selective preparation of 1,2-disubstituted benzimidazole. The L dehydration to form the type of bis-Shiff base L2 firstly, in the presence of electrophilic catalyst ZnCl2, the intramolecular 1,3-hydride migration is induced to form the 1,2-disubstituted benzimidazole L1. Existent of ZnCl2 leads to the four-coordination mononuclear complex 1. Various conditions such as solvothermal method have been designed to confirm the potency of this model reaction (Table S2). The target product 1 is still could be isolated when pressure is applied or elevating temperature unless the replacement of metal salts. This shows that ZnCl2 is the prime motivator of the reaction. 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 radius and metallicity of Zn2+, Cd2+ and Hg2+, reaction of L with CdCl2 and HgCl2 in the similar synthesis with 1 could only get a large quantity of unidentified white precipitates. Taking account of this, the adopting of 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 Cd-complex containing the bis-Schiff base L2 via self-assembly of the small units by Cl– anions. This phenomenon obviously demonstrates that the substitution of Cd2+ for Zn2+could make a difference in situ metal-ligand reaction of L. We hypothesized that the large ion radius of Hg2+ make it hard to form complex in the mild react condition, and the experimental results indeed as expected. The Hg-complex 3 could be only produced under solvothermal condition with elevating 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 is the direct hydride migration coupling cyclization involving C−N bond-forming (selective formation of 2-substituted benzimidazole L3 with one quinoline ring) in situ metal-ligand reaction from the Schiff base L. Existent 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 is formed by C–N/C=N bond-forming strategy. Meanwhile, the temperature 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

and autogenous pressure under the condition of solvothermal reaction play a synergistic effect role. 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 from the solid quickly dissolves to precipitation process, which means high-speed dehydration (complex 1 and 2) or hydrolysis (complex 3) of L and rapid coordination process. We also use one-pot method starting with the reaction raw material of o-PD, 2-quinoline formaldehyde and MCl2 (M = Zn, Cd, Hg) with ratio of 1:1:1, under the same reaction condition of 1–3 respectively. It affords only a large quantity of unidentified precipitates except the single crystals of [(L4)2(HgCl4)], complex 4. The complex 4 is prepared under 100 °C in the seal 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 presence of HgCl2. This maybe caused by an intramolecular cyclization of o-PD through the aldehyde group and amino condensation followed by 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 keeps a complex structure in solution and does not decompose, respectively (Figure S2, 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 (Figure S4–S9). The variances on intensity of PXRD with the simulated patterns could be led by the preferred orientation of the crystalline powder samples.

23 24

10


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Scheme 2. The mechanism of the reaction furnishing complex 1–4

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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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)  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 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

1 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

2 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

R1 = ||Fo| – |Fc||/|Fo|.b wR2 = [[w (Fo2– Fc2)2]/[ w (Fo2)2]]1/2.

12


3 1449513 C16H11Cl2HgN3 516.77 Triclinic

P1

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

4 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

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1

Crystal structure of [ZnL1Cl2] (1), [CdL2Cl2] (2), [HgL3Cl2] (3) and

2

[(L4)2(HgCl4)(C6H7)2] (4).

3

Single crystal X-ray diffraction researches of all four complexes 1–4 are made. All the

4

bond lengths and angles of the complexes 1–4 in the crystal structures are shown in

5

Table S3–S7. The crystal structure analysis and packing diagrams of 1–4 suggest that

6

the basic construction of the title SMOFs are intrinsic C−H···Cl hydrogen bond and

7

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 chelate with the quinolyl and imidazole nitrogen atoms (N1, N2), causing to a five-membered ring, which are nearly coplanar with quinoline and imidazole ring forming 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) complexes47. 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 Å 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 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 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 multi-way 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.

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Figure 2. (a) View of coordination environment of Zn(II) in 1. Dotted lines represent the intramolecular hydrogen bond interaction. All hydrogen atoms except it 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 3D topological structure generated through non-covalent interaction contacts. [CdL2Cl2]n (2) Single crystal diffraction experiment 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 as 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 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 14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14

15 16 17


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, then generates a 3D supramolecular framework by C8–H8A···Cl3, C7–H7A···Cl4, 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, Cd(II) ion as a six connected node, the 3D structure of 2 can be classified as a (6,6)-connected lattice with a {49·65}2 Schläfli symbol which represents a 3D network topology.

Figure 3. (a) Coordination environment of Cd(II) center in 2. Dotted lines represent the intramolecular hydrogen bond interactions. All hydrogen atoms except it 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5

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

6

(coordinate bond, blue line; C–H⋯Cl hydrogen bond interaction, green line).

7

[HgL3Cl2] (3) Single crystal X-ray diffraction studies reveal that complex 3 crystallizes in the triclinic P 1 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 neighbouring molecule via C2−H2A···Cl2 intermolecular bonding and π-stacking interaction (3.55Å), 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.

1 2 3 4

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

16


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1 2

Figure 4. (a) Coordination environment of Hg(II) center in 3. All H atoms are omitted

3

for clarity. (b) The infinite 1D chain of complex 3 generated through C–H⋯Cl and

4 5

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

6

topology structure in 3 (π-stacking interaction, pink line; C–H⋯Cl hydrogen bond

7

interaction, blue line). [(L4)2(HgCl4)(C7H8)2](4) Asymmetric unit in 4 comprises of discrete [(L4)2]2+ cation, [HgCl4]2– anion moiety and two free methylbenzene moleculars (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 toward to reduce the steric interactions. 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 expected48. The solid-state 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 all involve 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–

8 9 10 11 12 13 14 15 16 17 18 19 20

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8

H10B⋯Cl3,

C27A–H27B⋯Cl3,

C10B–H10B⋯Cl3,

Page 18 of 36

C29A–H29A⋯Cl4,

C8B–

2−

H8B2⋯Cl4, C5B–H5B⋯Cl4, C8A–H8A1⋯Cl4), which bridge [HgCl4] anion with other four L4 cations. The units are supplemented by π–π stacking interaction (3.57Å) 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.

9 10 11

Figure 5. (a) Molecular structures of 4. All H atoms and two free methylbenzene

12

moleculars are omitted for clarity. (b) The infinite chain generated through C–H⋯Cl

13 15

and π-stacking interactions. (c) The chains are assembled by strong π-stacking interaction 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).

16

Table 2. Important noncovalent interactions in compounds.

14

Compound

D–H···A

L

C18–H18A···O1 C26–H26A···O1 C24–H24A···O1

πCg1–πCg1 πCg1–πCg1 1

C9–H9A···Cl1 C15–H15A···Cl1 C2–H2A···Cl1 C13–H13A···Cl2 C20–H20A···Cl2

H···A/Å

D···A/Å

D–H···A/°

structure

2.722 2.326 2.595 3.58 3.61 3.046 2.997 2.981 2.761 2.759

3.653 2.928 3.393

166.68 120.70 141.85

3D

3.823 3.604 3.593 3.594 3.642

142.18 124.42 124.83 149.71 159.05

3D

18


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C7–H7A···Cl2 C6–H6A···Cl2 C24–H24A···Cl2

3.067 2.943 2.930

πCg1–πCg1 πCg1–πCg1

3.636 3.573 3.712 3.75

121.16 126.31 142.67

3.59

1

3.873 132.38 3.783 130.36 3.792 143.50 3.727 146.53 3.645 160.99 3.566 168.81 3.771 141.07 3.695 142.12 3.786 155.52 3 3.859 145.62 3.787 129.34 3.845 147.71 3.820 149.22 3.778 128.13 3.55 πCg1–πCg2 C3B–H3B···Cl1 3.092 3.741 128.31 4 C2B–H2B···Cl1 3.159 3.781 126.03 C22B–H22B···Cl1 2.768 3.531 140.07 C33A–H33A···Cl1 2.960 3.880 170.67 C30B–H30B···Cl2 3.256 3.898 128.09 C5A–H5A···Cl2 3.018 3.874 129.06 C27B–H27C···Cl3 2.735 3.700 173.53 C10B–H10B···Cl3 2.925 3.775 152.75 C27A–H27B···Cl3 2.912 3.853 164.49 C29A–H29A···Cl4 2.930 3.700 141.40 C8B–H8B2···Cl4 2.883 3.842 170.24 C5B–H5B···Cl4 2.764 3.674 166.92 C8A–H8A1···Cl4 2.671 3.639 176.84 3.59 πCg1–πCg1 3.57 πCg2–πCg2 Cg1 and Cg2 are the centroids of the quinoline ring and imidazole ring respectively

2

Photoluminescence Studies

3

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

2

4 5 6 7

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

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

19


3D

3D

3D


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

π*→π transition. Meanwhile, the excessive loss of energy leads the nearly nonemissive nature of L which 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 charge-transfer (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 electron-donating 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 mono-transition process50 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, 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 to 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. 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 compared to them in solutions for all compounds, but the maximums have red shifted in 20


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1 2 3 4 5


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 state51. 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).

6 7 8 9 10 11

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.

12

Absorption Properties of the Schiff Base L and Complexes in Solution.

13

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* transitions52,53. 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 (complex 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 compouds 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

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

27 28

prominent peaks in the blue-violet 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 semiconductor materials54. 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 48085 M−1 cm−1 for 1, 32670 M−1 cm−1 for 2, 35488 M−1 cm−1 for 3 and 45865 M−1 cm−1for 4. The higher molar extinction coefficient around 300–400 nm indexes that the complexes 1–4 possess higher light harvesting ability in the wavelength interval compare with N719 and I3− (25 000 M−1 cm−1)55. It can be anticipated that the photon lost result from light absorption by I3− will be possible suppressed by the use of complexes 1–4 as a co-sensitizer used in N719 sensitized DSSCs. To further research if the prepared complexes could apply to use in DSSCs as co-sensitizers, the absorption spectra of 1/N719, 2/N719, 3/N719 and 4/N719 co-sensitized TiO2 films are recorded and 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 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 co-sensitized DSSCs.

29

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

30

Application in Dye-Sensitized Solar Cells

31

Electrochemical Properties of Complexes 1–4 22


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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 1–4 calculated from EHOMO + E0–0, are −2.11, −2.15, −2.23 and −2.20 eV, respectively56. The HOMO and LUMO energy levels of 1–4 are shown in Scheme 3. It illustrates that the energy levels of 1–4 are suitable for the DSSC system involving TiO257. 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 regeneration59,60. The thermodynamic force for electron injection and regeneration of the photo-oxidized dyes is adequate. Table 3. Experimental data for spectral and electrochemical properties of the synthesized complexes

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

λabsa (nm)(ε[103M–1cm–1])

λema,b (nm)

E0-0c(eV)

Eox/V vs SCEd

EHOMOe(eV)

ELUMOe(eV)

1

317(58.1), 390(48.1)

435

2.89

0.60

-5.00

-2.11

2

318(59.6), 387(32.7)

428

2.90

0.65

-5.05

-2.15

3

323(54.7), 382(35.5)

431

2.85

0.68

-5.08

-2.23

4

315(68.1), 369(45.9)

444

2.86

0.66

-5.06

-2.20

Dye

–5

24 25 26 27

a

28 29

with the following formula61: HOMO (eV) = −e( Eonset V + 4.4 V); LUMO (eV) = EHOMO + E0–0, where E0–0 is the intersection of absorption and emission of the complexes 1–4.

Absorption and emission spectra were recorded in CH3CN (10 M) at room temperature. Complexes 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 b

ox

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 4

Scheme 3. Electrochemical potential diagram of complex 1–4, TiO2 CB edge, and I– /I3– couple, showing the feasibility of electron injection and dye regeneration processes.

5

Photovoltaic Properties of DSSCs

6

The complexes 1–4 are employed as co-sensitizers to fabricate complex/N719 photoanodes of DSSCs, which are fabricated following a stepwise co-sensitization procedure by sequentially immersing the TiO2 electrode (with a thickness of ca. 10 μm) in a separate solution 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 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 co-sensitized 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 value of Jsc and η are significantly higher than that of the device sensitized by N719 only. Especially, the cell co-sensitized 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 co-sensitization of

3

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

24


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1 2 3

TiO2 photoelectrode with the complex 1–4 can effectively improve the efficiency of DSSCs. 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

4 20 18 16 2

Current density (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


14 12

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

10 8 6 4 2 0 0.0

5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Voltage (V)

Figure 8. J–V curves for DSSCs based on co-sensitized photoelectrodes and single N719 sensitized photoelectrodes under irradiation The higher η values of 1/N719, 2/N719, 3/N719 and 4/N719 co-sensitized solar cells are caused by the enganced photovoltaic parameters of Jsc. Commonly, the Jsc value is affected by the incident photon-to-current conversion efficiency (IPCE) response of cells, since the equation:

J sc   eph.AM1.5G ( )d  Where

 ph.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 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 co-sensitizer, 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 co-sensitized 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 the complex 1 and 4, which possess uncoordinated nitrogen atom in 25



1 2 3 4 5 6

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. Based on 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 co-sensitizer. N719/TiO2 1/N719/TiO2 2/N719/TiO2 3/N719/TiO2 4/N719/TiO2

80 70

IPCE (%)

60 50 40 30 20 10 0 300

7 8 9 10 11 12 13 14 15 16 17

400

500

600

700

800

Wavelength (nm)

Figure 9. The incident photon-to-current conversion efficiency spectra of devices based on single N719 sensitized and co-sensitized photoanodes. The luminescence of complex 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 co-sensitization 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.

18 1.0

Normalized Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

Ex-N719 1 2 3 4

0.8

0.6

0.4

0.2

0.0 200

19 20

300

400

500

600

700

800

Wavelength (nm)

Figure 10. The emission spectra of complexes 1–4 and excitation spectrum of N719 26


Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14

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 proved by the dark current voltage (J–V) measurements of the different devices, as is shown in Figure 11. The dark current–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 co-sensitized system is lower compared to that of single N719 sensitized DSSCs, which sort by value of 1/N719 N719. The structure of complex is closely related to the conversion efficiency of complex 1–4 as co-sensitizers in DSSC. In complex 1 and 4, the uncoordinated nitrogen atom in 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 28


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1 2 3 4 5 6 7 8 9 10 11 12

13 14


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 provides a more efficient electron injection and collection from the complexes to the semiconductor. The modular units of 2 are assembled together to form a 1D ∞-like coordination polymer chains, forming highly 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 co-sensitizer, the performance of co-sensitized solar cell is lower than that by N719 and 1, 2, 4. This difference in performance is attributed to the decrease of absorption of complex 3 caused by smaller conjugated system71, 72. The strong absorption of complex 1, 2 and 4 is more suitable for recovering the absorption of N719 and conquer the competitive light absorption of I3–.

15

Figure 13. Possible working mechanism of the DSSC with FTO/TiO2/complex multilayered electrode. (a) Three functions of the complex materials in the working

16

process of a DSSC: ① converting of UV light to blue light, ② scattering of light,

17

and ③ recombination centers.(b) Molecular structure of 1. (c) Molecular structure of

18

4.

19

CONCLUSION

20

This study provides a new example of efficient synthesis of asymmetric ZnII/CdII/HgII 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Page 30 of 36

complex based on benzimidazole with one quinoline ring (3), two quinoline rings (1), three quinoline rings (4) and bis-Schiff base (2) respectively by metal induced reaction from asymmetric Schiff base ligand L. All 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 co-sensitizer could make a significant influence on the performance of DSSCs. The co-sensitized 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 co-sensitizer, 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, amount 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 shortcoming of N719 absorption under the situation of short wavelength interval of visible spectrum, keep its aggregation off, counteract competitive visible light absorption of I3–, 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 a interest in the construction of novel asymmetric fluorescent complex with photoelectricity functions.

19 20

ASSOCIATED CONTENT

21

Supporting Information

22

X−ray crystallographic files (CIF), Structural Information for L and complex 1–4,

23

FT−IR, 1H NMR,

24

information is available free of charge via the Internet http://pubs.acs.org.

25

Accession Codes

26

CCDC 1449510, 1449511,

27

supplementary crystallographic data for this paper. These data can be obtained free of

28

charge

29

[email protected], or by contacting The Cambridge Crystallographic

30

Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

31

AUTHOR INFORMATION

32

Corresponding Authors

33

*E−mail: [email protected]

34

*E−mail: [email protected]

35

ORCID

via

13

C NMR, PXRD spectra, absorption and luminescent data. This

1449512,

1449513

and

www.ccdc.cam.ac.uk/data_request/cif,

30


1449514

or

by

contain

the

emailing

Page 31 of 36

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1

Ruiqing Fan: 0000-0002-5461-9672

2

Notes

3

The authors declare no competing financial interest.

4

ACKNOWLEDGEMENTS

5

This work was supported by National Natural Science Foundation of China (Grant

6

21371040 and 21571042), the National Key Basic Research Program of China (973

7

Program, No. 2013CB632900).

8

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R. Dyes and Pigments 2016, 124, 63–71. (72) Huang, Z. S.; Meier, H.; Cao, D. R. J. Mater. Chem. C 2016, 4, 2404−2426.

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For Table of Contents Use Only

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Metal(II)-induced synthesis asymmetric fluorescence benzimidazoles complexes and their dye-sensitized solar cells performance as co-sensitizers

4

Xinming Wang,†‡ Ruiqing Fan,*, † Yuwei Dong, † Ting Su,† Jian Huang,† Xi Du, † Ping

5

Wang,† Yulin Yang,*, †

6 7 8

†

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. of China

9 10 11

‡

Department of Chemistry, Harbin University of Science And Technology, Harbin 150080, P. R. China

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Four blue light Zn(II)/Cd(II)/Hg(II) complexes 1–4 based on the benzimidazole derivatives were obtained. The overall conversion efficiency of DSSC devices using co-sensitizers of 1/N719 and 4/N719 is 26.72% and 25.34%, higher than that of a device solely sensitized by N719 (6.55%).

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Metal(II)-Induced Synthesis of Asymmetric Fluorescence

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