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Photochromic and Non-photochromic Luminescent Supramolecular Isomers Based on Carboxylate-Functionalizedbipyridinium-Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net Jie Wang, Shi-Li Li, and Xian-Ming Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06551 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016
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Photochromic
and
Non-photochromic
Luminescent
Supramolecular Isomers Based on Carboxylate-Functionalizedbipyridinium-Ligand: (4,4)-Net versus Interpenetrated (6,3)-Net Jie Wang,† Shi-Li Li†* and Xian-Ming Zhang†,‡* †
School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004,
P. R. China ‡
Institute of Crystalline Materials, Shanxi University, Taiyuan 030006 P. R. China
ABSTRACT: Photochromic and non-photochromic luminescent bipyridinium-based supramolecular
isomers
[Cd(CPBPY)(BDC)(H2O)]n
(1)
and
{[Cd(CPBPY)-
(BDC)].H2O}n (2) (CPBPY = N-(3-carboxyphenyl)-4,4′-bipyridinium, BDC= terephthalate) have been successfully obtained by solvothermal reactions at 100 ºC via tuning stoichiometric ratios of starting reagents. Isomer 1 features (4,4)-topological layer constructed by edge-shared CdO7 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 has (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are four-fold interpenetrated into 3D network. Although both 1 and 2 contain bipyridinium ligands, only isomer 1 possesses reversible photochromic behavior with quick-switchable luminescence in the solid state. Compound 2 does not show photochromic behavior even after exposure to UV light for more than two hours. Photochromism process of 1 originates from photo-stimulated reduction of CPBPY ligands to generate CPBPY•− radicals after irradiation, confirmed by EPR spectra. Careful check on structure reveals that the offset π-π stacking interaction between the pyridine ring of CPBPY and benzene
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ring of BDC with inter-ring shortest C···C distance of 3.214 Å in 1 is responsible for •
electron transfer to form the CPBPY•− radicals. The speculation is further supported by DFT calculation of frontier orbital matching of electron donor and acceptor. HOMO and HOMO-2 orbitals of BDC involve the carbon atoms of benzene ring while LUMO and LUMO+1 orbitals of CPBPY involve the carbon atoms of pyridinium ring. Importantly, the photoinduced formed CPBPY•− radicals in 1 have a long-lived lifetime (at least six months in air and room temperature condition), which is mainly attributed to the close packing mode. KEYWORDS:
supramolecular isomers, photochromic, luminescent, bipyridinium, air-stable, long-lived lifetime
1. INTRODUCTION Photochromic materials not only exhibit color change but also are often accompanied by the variation of some physicochemical behaviors in response to UV or Vis photoirradiation.1-5 These features make them suitable for many technological applications in fields of smart windows, rewritable copy papers, information storage, sensors, high density optical memories, solar energy conversion, the mimicry of natural photosynthetic reaction processes, display, protection, decoration, photography, photo-mechanics and photo-switches, and so on.6-19 Typical examples of photochromic molecules include azobenzene, spiropyran, Schiff base, furylfulgide and diarylethene.20-21 Viologens, di-substituted bipyridinium derivatives, are usually used as electron acceptors to construct redox photochromic materials and charge transfer molecular systems.22-34 In these systems, the luminescence behavior of
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bipyridinium derivative can be adjusted by molecular modification, but the incorporation of fluorophores on viologens is little reported.35 While due to insufficient stability of purely organic viologens,36-37 increasing attention has been devoted for developing the photochromic inorganic–organic hybrids, metal complexes and metal-organic frameworks based on viologens, which have numerous potential applications in photochemical and optoelectronic material field.15, 27 It is important to note that some metal complexes based on viologens do not show photochromism while their isostructural compounds do. To date, no general photochromic rule has been set up in the complex of viologens, and subtle changes in metal, ligand, local geometry, or superstructure have significant influence on photochromism of metal complexes. To reveal the general photochromic rule, more members in the class of metal complexes of viologens should be synthesized, characterized and carefully analyzed. Some bipyridinium-based metal-organic compounds show unstable coloration due to the fact that the reduced radical is extremely sensitive to oxygen molecules. On the other hand, supramolecular isomerism in coordination compounds plays an important role in crystal engineering of material and pharmaceutical sciences,38-39 which can provide valuable insights in understanding the self-assembly and structure-property
relationships.
Supramolecular
isomers
with
different
superstructures, different conformations of flexible ligands and different catenanes can have an enormous influence on the physical and chemical properties of coordination polymers. A great number of attractive examples of supramolecular
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isomeric architectures are assembled by virtue of various kinds of electrostatic interaction such as H-bond interactions, π-π stacking interactions, covalent bonds and so on,40-46 in which supramolecular isomeric pairs often possess remarkably different physical properties such as different guest inclusion, SHG, electric conductivity, luminescence, magnetism, redox and optical activity, and metal-metal interaction.47-55 However, to our most of knowledge, no or little supramolecular isomer pair showing photochromic and non-photochromic luminescence is documented. In the article, we present two novel supramolecular isomeric compounds constructed
with
cadmium,
bipyridinium
species
N-(3-carboxyphenyl)-4,4′-bipyridinium (CPBPY) and terephthalate (BDC) ligands, namely [Cd(CPBPY)(BDC)(H2O)]n (1) and {[Cd(CPBPY)(BDC)].H2O}n (2). Isomer 1 features (4,4)-topological layer constructed by edge-shared Cd2 SBUs and BDC linkers attached by N-pendent CPBPY groups. Isomer 2 shows (6,3)-topological layers with Cd atoms as nodes and BDC and double CPBPY as linkers, which are four-fold interpenetrated into 3D network. Strikingly, although both 1 and 2 contain viologen ligands, only isomer 1 possesses reversible photochromic behavior in the solid state. Isomer 1 exhibits an obvious colour change from pale-yellow to pale-blue (1a) upon irradiation via Xenon lamp or 365nm UV light, and the photoproduct 1a is stable for months even irradiated by sunlight under oxygen atmosphere. The long-lived charge separation state can be assigned to the close packing pattern. The very long lifetime of stable charge separation state in 1a indicates possible application in the conversion of solar energy to electrical energy.27
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2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization. Compounds 1 and 2 were prepared by a solvothermal
reaction
of
Cd(NO3)2.4H2O,
N-(3-carboxyphenyl)-4,4′-bipyridinium
chloride
terephthalic (HCPBPY·Cl)
acid in
(H2BDC), a
mixture
solvents of dimethylformamide (DMF), ethanol (EtOH), and water. The ligand HCPBPY•Cl was synthesized according to previously reported literature.56 These two compounds were obtained at the same reaction temperature but different stoichiometric ratios of the starting reagents. Synthesis of 1: Pale-yellow block crystals of 1 were synthesized by the following procedure: A solution of Cd(NO3)2·4H2O (0.031 g, 0.1 mmol), H2BDC (0.018 g, 0.1 mmol), HCPBPY·Cl (0.028 g, 0.1 mmol), DMF (2 mL), EtOH (2 mL) and H2O (2 mL) was stirred for 30 min. After being stirred, the solution was placed in a 15 ml Teflon-lined stainless container, which was then heated at 100 ºC for 7 days and then slowly cooled to room temperature at 5 °C/min. After being filtered off and dried at room temperature, 1 was obtained as a pale-yellow block crystal in 65% yield. Anal. Calcd (%) for 1 C25H18CdN2O7: C, 52.63; H, 3.16; N, 4.91. Found: C, 52.72; H, 3.34; N, 4.83. IR (KBr, cm−1): ν 3437(m), 3038(m), 2356(w), 1597(m), 1558(s), 1384(s), 1217(w), 889(w), 824(w), 753(m), 515(m). Synthesis of 2:
This compound was prepared by a similar procedure for 1.
However, the difference is that a mixture of Cd(NO3)2·4H2O (0.155 g, 0.5 mmol), H2BDC (0.086 g, 0.5 mmol), HCPBPY·Cl (0.137 g, 0.5 mmol), DMF (4 mL), EtOH (2 mL) and H2O (1 mL) was stirred for 30 min in air. Yellow block crystals of 2 in 40%
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yield were obtained. Anal. Calcd (%) for 2 C25H18CdN2O7: C, 52.63; H, 3.16; N, 4.91. Found: C, 52.49; H, 3.26; N, 4.82. IR (KBr, cm−1): ν 3445(s), 3116(w), 3039(w), 2929(w), 2858(w), 2350(w), 1609(m), 1558(s), 1384(s), 1217(w), 1095(m), 882(w), 747(m), 619(w), 522(w). The solid-state UV−vis diffuse-reflectance spectra of 1 and 2 were carried out on a TU-1901 spectrophotometer, which is equipped with an IS19-1 integrating sphere in the wavelength range of 200 – 850 nm. Powder X-ray diffraction (PXRD) patterns were obtained using a D8 Bruker ADVANCE powder X-ray diffractometer (Cu Kα, λ=1.5418 Å) at 293 K. Thermogravimetric (TG) analysis data were obtained by heating crystalline products under nitrogen gas flow at a rate of 10 °C min− 1 in the range of 30-1000 °C. The TG curve of 1 indicates that the framework remains stable up to about 240 °C in nitrogen, and then it starts to loss the coordinated water molecules (Fig. S1a). Weight loss for 1 occurs in the temperature range of 240-503 °C, corresponding to the removal of coordinated water molecules and the departure of CPBPY and BDC groups (78.47%, calcd 77.6%), which indicates that the removal process of water molecules is accompanied by the departure of the organic ligands. The complete decomposition of 1 forms final residence CdO (21.53%, calcd 22.4%). For 2, two clear and well-separated weight loss steps are observed. The initial weight loss of about 4.12% in the range of 70-290 °C corresponds to the removal of lattice water molecules. To be noted, the removal of water molecules in 2 take place at lower temperature than that in 1, indicating that removal of the free water molecules is easier than the coordinated water molecules. The second step from 290 to 471 °C with
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74.86% weight loss can be assigned to the departure of carboxy ligands, resulting in the collapse of coordination polymer. The resulting gray residues of 21.0% (calcd 22.4%) are close to the percentage of CdO (Fig. S1b). 2.2. X-ray Crystallography. X-ray single-crystal diffraction data were performed on a Agilent Technologies Gemini EOS diffractometer at 293(2) K with Cu−Kα radiation (λ = 1.5418 Å). Pertinent crystallographic data and structural results for 1 and 2 are summarized in Table 1. Table 1. Crystallographic Data and Structural Refinement for 1 and 2 Compound
1
2
Empirical
C25H18CdN2O7
C25H18CdN2O7
Fw
570.81
570.81
Crystal system
triclinic
monoclinic
Space group
P-1
C2/c
a (Å)
10.7205(4)
25.9358(8)
b (Å)
10.8709(4)
11.97170(10)
c (Å)
11.3879(4)
19.9517(11)
α (deg)
114.594(3)
90
β (deg)
113.628(3)
121.794(5)
γ (deg)
92.020(3)
90
V (Å3)
1072.57(6)
5265.4(3)
Z
2
8
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ρcalc,(g cm-3)
1.767
1.440
µ, (mm-1)
1.071
7.025
F(000)
572
2288
Reflections
17842/4279
15566/5272
Crystal size(mm)
0.48×0.12×0.10
0.40×0.12×0.10
Tmax/Tmin
0.9004/0.6273
0.5401/0.1655
Data/parameters
4279/317
5272/319
S
1.205
1.077
R1 a
0.0225
0.0257
wR2b
0.0600
0.0723
∆ρmax/∆ρmin(eÅ-3)
0.421/-0.525
0.481/-0.492
a
R1 = ∑Fo-Fc/∑Fo. bwR2 = [∑[w(Fo2-Fc2)2]/∑[w(Fo2)2]]1/2.
2.3. Electron Paramagnetic Resonance (EPR). EPR experiments of 1 and 2 at 293K were monitored with a Bruker EMX spectrometer using an X-band microwave bridge (9.45 GHz). The experiments were performed by using a low modulation field (20 G) and 22.5 mW as the microwave power. There is no any distortion about these parameters which are suitable for recording the EPR spectra. 3. RESULTS AND DISCUSSION 3.1. Description of the Crystal Structures. Single-crystal X-Ray diffraction studies show that complex 1 crystallizes in the triclinic space group P-1, and the asymmetric unit consists of one crystallographically independent Cd ion, one CPBPY
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ligand, two half BDC2– ligands and one coordinated water molecule. Cd(1) shows a slightly distorted pentagonal bipyramid, coordinated by seven O atoms from two CPBPY ligands, two BDC2– ligands and one water molecule (Fig. 1a). The Cd-O distances are in the range of 2.2416(17) –2.6339(17) Å. The cis- O-Cd-O angles are within the range 52.01(5)–105.71(7)°, and trans O-Cd-O angles are within the range 125.94(6)–167.99(6)°. The CPBPY ligand with two carboxylate atoms in µ2:η1η2 mode links two Cd(II) ions, forming the Cd2 clusters with a Cd⋅⋅⋅Cd separation of 3.9193(3) Å. Each Cd2 cluster can be seen as the second building unit (SBU), which is surrounded by four BDC2– ligands with mono- and bidentate chelating mode alternately. Topologically, the Cd2 clusters can be considered as nodes linked by the linear BDC2– ligand and the layer network of 1 can be described as a 2D (4,4)-topological net (Fig. 1b，1c and S2). The terminal coordinated water molecules point up and down the Cd2 SBU plane alternately. Finally, it should be noted that the neighboring layers are packed into dense three-dimensional supramolecular array. Calculated with PLATON program, the solvent-accessible volume in 1 is not found, which means that 1 is nonporous.
(a)
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(b)
(c) Figure 1 (a) Coordination environments of the Cd2+ ion, (b) top view of the 2D rhombic grid network, (c) the distance between electron-rich donors and the bipyridinium acceptors in 1. Compound 2 crystallizes in the monoclinic space group C2/c, and the asymmetric unit contains one unique Cd ion, one CPBPY ligand, two half BDC ligands and one free water molecule. Cd(1) center exhibits a coordination geometry between octahedron and trigonal antiprism, coordinated by one nitrogen atom from one CPBPY ligand and five O atoms from one monodentate BDC, one bidentate BDC and one bidentate CYBPY ligand (Fig. 2a). The Cd–O distances are in the range of
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2.2166(17)-2.5172(17) Å, and the L-Cd-L (L = N or O) angles are in the range 80.16(6)–145.85(7)°. Cd(1) can be considered as 3-connected node, which is linked by two BDC2– groups and double-CPBPY group to form a (6,3) honeycomb 2D net with hexagonal rings (Fig. 2b and S3). The hexagonal rings are so large that four identical but independent (6,3) networks form 4-fold interpenetrated 3D framework(Fig. 2c and 2d). Despite of the 4-fold interpenetration, 2 still shows 1D channel along the b-direction and a void space of 20.8% calculated using the PLATON routine.
(a)
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(b)
(c)
(d) Figure 2 (a) The coordination environment of the Cd2+, (b) (6,3)-topological layer with Cd atoms as nodes and BDC and double CPBPY as linkers, (c) space-filling view of the interpenetrated layers, (d) schematic representation of fourfold interpenetration of (6,3) nets in 2. 3.2.
Photochromic
Properties.
Viologen-containing
materials
are
usually
photochromic because of photo-induced electron transfer from electron-rich group to viologen ligand.57 As is expected, compound 1 is very photosensitive and displays a
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rapid visible photochromic transformation in air at room temperature upon exposure to Xenon lamp or ultraviolet light (365 nm). As shown in Figure 3, pale-yellow block crystals of 1 change into pale-blue after irradiation for 1 min and the coloration achieves saturation completely after irradiation for 50 min. Pale-blue crystals (1a) are very stable in air and do not return to original color after being held in the dark or under sunlight for months at room temperature. The diffuse reflectance UV-vis spectrum of 1 exhibits strong absorption bands at 330 nm, corresponding to the n–π* and π–π* charge-transfer transitions of the conjugated CPBPY ligand, which is akin to that of other viologen-based materials.12 After irradiation of 1 upon UV light, new absorption bands centered at 410 nm and 620 nm appear (Fig. 4a), and their intensities increase upon prolonged irradiation, which is believed to be owing to the generation of the viologen radical through photoinduced electron transfer.10,
13
The diffuse
reflectance UV-vis spectra of 1 and 1a are different, but both IR and PXRD spectra reveal that the crystal structure of 1a is identical to that of 1 (Fig. S4a and S5a). These indicate that photo-responsive behavior in 1 may be not caused by a structural transformation or photolysis, but rather by a photo-induced charge-transfer chemical process,58 which can be confirmed by EPR studies. Complex 1 is EPR-silent before irradiation, but presents a strong single-line signal with g = 2.0051 after coloration (Fig. 5a), which is close to that of a free electron (i.e. 2.0023), further confirming that the photochromism process resulted from photostimulated reduction of CPBPY •
ligands to generate CPBPY•− radicals after irradiation.59
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(a)
(b)
Figure 3 Photographs 1 (a) and 2 (b) before and after irradiation or thermal reaction. For some viologen-based compounds, photo-generated radicals are very sensitive to the oxidants and the reversible color processes can occur at room temperature in the dark under air atmosphere.60 However, the photoproduct 1a is stable for months in the dark and sunlight under oxygen atmosphere, which is mainly attributable to the close packing pattern preventing the radical to be attacked by oxygen molecules. The charge-separated state of 1a is so long-lived that it should be a good candidate as solar energy converting material. Its reverse photochromic process takes place when the pale-blue crystals are annealing above 120 °C in air, accompanied by the extinction of the broad UV-vis absorption band and EPR radical signal (Fig. 4b and 5a). It should be noted that its reversible color-changing time varies inversely with the temperature. In details, when 1a is heated at 120, 140, 160 and 200 °C, its color bleaching reaction need to take 90 min, 40 min, 25 min and 10 min, respectively. The reversibility of these processes can be repeated for at least 10 continuous cycles by UV irradiation and heating alternately (Fig. 6a), which implies that these photochromic processes are reversible. The maximum absorption intensity at 620 nm is mostly unchanged (
