Investigating the Structural Features and Spectroscopic Properties of

Article pubs.acs.org/crystal

Investigating the Structural Features and Spectroscopic Properties of Bis(tetrazolato)-Based Coordination Polymers Elisa Lavigna,† Nertil Xhaferaj,‡ Aurel Tabacaru,*,‡,§ Marco Lamperti,† Luca Nardo,∥ Massimo Mella,† Claudio Pettinari,‡ and Simona Galli*,†,⊥ †

Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy Scuola del Farmaco e dei Prodotti della Salute, Università di Camerino, Via S. Agostino 1, 62032 Camerino, Italy § Department of Chemistry, Physics and Environment, Faculty of Sciences and Environment, “Dunarea de Jos” University of Galati, 111 Domneasca Street, 800201 Galati, Romania ∥ Dipartimento di Medicina e Chirurgia, Università di Milano Bicocca, Via Cadore 48, 20900 Monza, Italy ⊥ Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via Giusti 9, 50121 Firenze, Italy ‡

S Supporting Information *

ABSTRACT: The luminescent compound xylene-bis(2H-tetrazol-5-yl) (H2BTZPX) was prepared and employed in the construction of the novel coordination polymers (CPs) Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX), containing d10 transition metal ions. Powder X-ray diffraction (PXRD) structure determination enabled us to disclose the crystal and molecular structure of the three CPs: while Zn(BTZPX) features a 3-D polymeric network, Ag2(BTZPX) and Hg(BTZPX) show 2-D corrugated layers. Thermogravimetric analysis and variabletemperature PXRD revealed the appreciable thermal robustness of the three CPs, peaking up to 370 °C, in air, in the case of Ag2(BTZPX). Given the sizable fluorescence emission of the ligand in the solid state, both H2BTZPX and the three CPs were characterized as to their electronic-state transition spectroscopic properties. UV−visible absorption unveiled a bathochromic shift for both the ligand and the CPs with respect to the absorption maximum of the main chromophore of the spacer, i.e., the benzene ring. In order to understand this peculiarity, quantum mechanical calculations were performed using the time-dependent density-functional theory approach. Furthermore, absorption spectra of H2BTZPX in different organic solvents were recorded. To investigate how the luminescence properties of H2BTZPX are influenced by complexation, the fluorescence emission spectra of H2BTZPX and the three CPs were recorded, and the fluorescence quantum yields determined by comparison to anthracene. Zn(BTZPX) exhibited enhanced fluorescence with respect to the ligand, while fluorescence was notably reduced for Ag2(BTZPX) and barely detectable for Hg(BTZPX). This occurrence suggests different decay pathways, which were further investigated by reconstructing the time-resolved fluorescence decays by means of time-correlated single-photon counting.

1. INTRODUCTION All-inorganic1−3 and all-organic4−7 luminescent materials have been extensively investigated, in the past, for their potential in functional applications such as sensing, lighting, and optical devices. Among the inorganic materials which underwent commercialization in this field, we recall here, as representative examples, BaMgAl10O17:Eu2+8 and GdMgB5O10:Ce3+,Tb3+,Mn2+1 for blue and green luminescent lamps, respectively. On the other hand, after the introduction of the light-emitting diode (LED) technology in the 1980s, a number of n- and p-type luminescent organic compounds have been isolated and characterized with the aim of producing organic LEDs.4−7 In this context, hybrid inorganic/organic materials may represent an improvement,9−11 as both the organic and inorganic components can be a source of luminescence. To this, coordination polymers (CPs) may add further radiative © XXXX American Chemical Society

decay pathways. Indeed, the way the organic ligands are arranged and oriented, along with the nature of the transition metal centers, can influence luminescence. Linker-based luminescence, ligand-to-metal or metal-to-ligand charge transfer, metal-based emission, antennae effects, or excimer and exciplex emission are among the mechanisms invoked to explain the origin of luminescence in d10 transition metal-based complexes12−14 and CPs.15 Coordination polymers16−18 constitute a huge class of inorganic−organic hybrids which has recorded exponential growth in the past decades, due to the variety of functional properties they might possess, such as heterogeneous catalysis,19−21 luminescence,22,23 magnetism,24−26 and conReceived: July 15, 2016 Revised: September 19, 2016

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DOI: 10.1021/acs.cgd.6b01056 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Scheme 1. Molecular Structure of Xylene-Bis(2H-tetrazol-5-yl) (H2BTZPX) (left), 1,4-Bis((3,5-dimethyl-1H-pyrazol-4yl)methyl)benzene) (H2BDMPX) (center), and 4,4′-Bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)biphenyl (H2DMPMB) (right)

ductivity,27 to quote only a few. Within the crystal structure of these materials, rigid or flexible polytopic ligands act as spacers, and bridge metal ions or oxo-metallic clusters acting as nodes. The organic spacers that have been commonly employed in the construction of first-generation CPs are polytopic carboxylates,28,29 pyrazines, and bipyridines.28−30 To date, a great number of CPs containing other nitrogen-donor ligands, such as poly(azolates) (pyrazolates, imidazolates, triazolates, and tetrazolates) have been reported.31−34 For the preparation of luminescent CPs, π-conjugated ligands are mostly used, due to the possibility of showing, during the radiative process, multiple spin states that correspond to π → π* and n → π* transitions. In the past years, along with acquiring experience on the preparation of poly(pyrazolato)-based CPs and metal−organic frameworks (MOFs) featuring, e.g., remarkable thermal robustness35−38 and chemical resistance to harsh conditions,39 we have focused our attention on another class of nitrogendonor ligands, namely, tetrazoles. Tetrazole-containing linkers have been the target of many research interests, due to their strong electron-donating ability and rich coordination chemistry, thus leading to an impressive library of coordination polymers displaying a variety of physical properties, ranging from fluorescence to second harmonic generation (SHG), ferroelectric, and dielectric behavior.40,41 A handful of tetrazolyl-based ligands have also been considered for the construction of high-energy MOFs with future perspective as high-energy-density materials.42 Aiming at isolating CPs potentially featuring luminescence properties, we prepared the luminescent ligand xylene-bis(2Htetrazol-5-yl) (H2BTZPX) (Scheme 1). Tetrazolate is known to show different coordination modes, its hapticity generally ranging from two to four. Having two tetrazolate rings, each possessing four nitrogen donor atoms, in its dianionic form H2BTZPX may thus adopt different binding modes toward transition metal ions, also as a function of the stereochemical requirements of the latter. Moreover, the flexibility about the torsion angles hinged at the two methylene groups potentially enables H2BTZPX to adopt anti (C2h), syn (C2v), or even lower-symmetry conformations, depending on the relative orientations of the two external arms. This versatility can in principle lead to CPs with diversified structural motifs, this occurrence concurring to modulate their luminescence properties. As far as flexible bis(azolato)-based ligands are concerned, we have recently shown that 4,4′-bis((3,5-dimethyl-1H-pyrazol4-yl)methyl)biphenyl (H2DMPMB)43 (Scheme 1) and 1,4b i s ( ( 3 , 5 -d i m e t h y l -1H - p y r a z o l- 4 - y l ) m e t h y l ) b e nz e n e (H 2 BDMPX) 44 (Scheme 1) can be employed in the construction of isostructural cobalt(II) and zinc(II)-containing CPs. In the present work, we coupled H2BTZPX to the d10 transition metal ions Zn(II), Ag(I), and Hg(II), isolating Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX), respectively. Details about their synthesis, structural features, and thermal behavior are presented hereafter. Furthermore, we report on

the spectroscopic properties of H2BTZPX and the three CPs. Finally, we describe the results of quantum mechanical calculations performed using time-dependent density-functional theory to shed light on the peculiarity emerged from UV−vis absorption spectroscopy.

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All the chemicals and reagents were purchased from Sigma-Aldrich Co. and used as received, without further purification. All the solvents were distilled prior to use. Before performing the analytical characterization, all the samples were dried in vacuo (50 °C, ∼10−4 bar) until a constant weight was reached. The IR spectra were recorded from 4000 to 650 cm−1 with a PerkinElmer Spectrum 100 instrument by attenuated total reflectance (ATR) on a CdSe crystal. In the following, the IR bands are classified as vw, very weak; w, weak; m, medium; s, strong; vs, very strong. The 1H NMR spectrum of the ligand was acquired at room temperature in dimethyl sulfoxide (DMSO) with a VXR-300 Varian spectrometer operating at 400 MHz, using tetramethylsilane as internal standard. In the following, the 1H NMR signals are classified as s, singlet; d, doublet. Positive electrospray mass spectrometry was carried out on the ligand with a Series 1100 MSI detector HP spectrometer, using MeOH as the mobile phase. The melting point of the ligand was measured with a Stuart SMP3 instrument equipped with a capillary apparatus. Elemental analyses (C, H, N) were performed with a Fisons Instruments 1108 CHNS-O Elemental Analyzer. Thermogravimetric analyses (TGA) were carried out under a N2 flow and with a heating rate of 7 °C/min with a PerkinElmer STA 6000 Simultaneous Thermal Analyzer. Powder X-ray diffraction (PXRD) qualitative analyses were carried out with a Bruker D8 Advance diffractometer (see Section 2.6 for the instrument specifics), acquiring data at room temperature in the 3−35° 2θ range, with steps of 0.02°, and time per step of 1 s. The nature and purity of all the batches employed for the spectroscopic characterization were assessed by elemental analysis, IR spectroscopy, and PXRD. 2.2. Synthesis of Xylene-bis(2H-tetrazol-5-yl) (H2BTZPX). Xylene-bis(2H-tetrazol-5-yl) was prepared by following a synthetic procedure different from the original one reported as early as 1979.45 2,2′-(1,4-Phenylene)diacetonitrile (3.12 g, 20 mmol), sodium azide (5.2 g, 80 mmol), zinc bromide (18 g, 80 mmol), and water (50 mL) were introduced into a 100 mL round-bottomed flask. The reaction mixture was refluxed under vigorous stirring for 48 h. After cooling down to room temperature, the white suspension that was formed was transferred to a separatory funnel, to which HCl (aq. 3 M, 60 mL) and ethyl acetate (200 mL) were added. Vigorous shaking was then applied until no solid remained and the aqueous layer had a pH = 1. When necessary to complete the dissolution, further ethyl acetate was added. The organic layer was isolated and the aqueous layer extracted with ethyl acetate (2 × 150 mL). After evaporating the organic layer to dryness, NaOH (aq. 0.25 M, 200 mL) was added. The resulting mixture was stirred until the solid was dissolved and a suspension was formed. The suspension was filtered, and the recovered solid was washed with NaOH (aq. 1 M, 50 mL). HCl (aq. 3 M, 40 mL) was then added to the filtered solution under vigorous stirring, promoting the precipitation of H2BTZPX. The latter was filtered off, washed with HCl (aq. 3 M, 2 × 30 mL) and dried in an oven at 50 °C under vacuum to provide the desired compound in the form of a white and fluffy powder. Yield: 65%. Melting point: 260−270 °C (260 °C was originally proposed in ref 45). H2BTZPX is soluble in methanol, B



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dimethylformamide (DMF), and DMSO. IR (ATR) (cm−1) (see also Figure S1 in the Supporting Information): 3200−2500 strong broad absorption band characteristic of hydrogen bonding, 3123 (w) ν(N− H), 3100−3000 (w) ν(C−Haromatic), 3000−2800 (m) ν(C−Haliphatic), 1659 (m), 1580 (m), 1520 (m) ν(NN, CC + CN). 1H NMR (DMSO, 293 K): δ (ppm), 16.11 (s, 2H, N-H), 7.21−7.19 (d, 4H, C6H4), 4.24 (s, 4H, CH2). ESI-MS (+) (MeOH) m/z 241 (M-H, 10%). Elem. Anal. for C10H10N8 (MW = 242.2 g mol−1): calcd. C 49.58%, H 4.16%, N 46.26%; found: C 49.31%, H 4.11%, N 45.95%. 2.3. Synthesis of Zn(BTZPX). H2BTZPX (0.048 g, 0.2 mmol) was dissolved in DMF (5 mL) in a high-pressure glass tube; ZnCl2·2H2O (0.027 g, 0.2 mmol) was then added. The resulting limpid solution was heated, under stirring, up to 150 °C, and maintained at this temperature for 24 h. The white suspension so obtained was filtered off, washed with warm DMF (2 × 5 mL), and then dried in vacuo for 24 h. Yield: 35%. Zn(BTZPX) is insoluble in alcohols, DMF, acetone, acetonitrile, dimethylformamide, and water. Elemental analysis for C10H8N8Zn (FW = 305.6 g mol−1): Calc. C, 39.30%; H, 2.64%; N, 36.67%. Found C, 38.85%; H, 2.58%; N, 36.19%. IR (ATR) (cm−1): 1660 (w), 1517 (w), 1495 (w) ν(NN + CN + CC), 1434 (m), 1247 (w), 1066 (w), 764 (vs), 679 (vs). 2.4. Synthesis of Ag2(BTZPX). H2BTZPX (0.048 g, 0.2 mmol) was dissolved in DMF (5 mL) in a high-pressure glass tube; AgNO3 (0.068 g, 0.4 mmol) was then added. The resulting limpid solution was heated, under stirring, up to 100 °C, and maintained at this temperature for 24 h. The gray46 suspension so obtained was filtered off, washed with warm DMF (2 × 5 mL), and then dried in vacuo for 24 h. Yield: 65%. Ag2(BTZPX) is insoluble in alcohols, DMSO, acetone, acetonitrile, DMF, and water. Elemental analysis for C10H8Ag2N8 (FW = 456.0 g mol−1): Calc. C, 26.34%; H, 1.76%; N, 24.57%. Found C, 25.84%; H, 1.79%; N, 23.96%. IR (ATR) (cm−1): 1653 (m), 1517 (m), 1477 (m) ν(NN + CN + CC), 1434 (m), 1395 (m), 1221 (w), 1163 (m), 1130 (m), 787 (m), 763 (vs), 685 (vs). 2.5. Synthesis of Hg(BTZPX). H2BTZPX (0.048 g, 0.2 mmol) was dissolved in distilled water (10 mL); Hg(CH3COO)2·6H2O (0.064 g, 0.2 mmol) was then added. The resulting white suspension was left at reflux, under stirring, for 24 h. The white precipitate so formed was filtered off, washed with hot water (2 × 10 mL), and dried in vacuo for 24 h. Yield: 60%. Hg(BTZPX) is insoluble in alcohols, DMSO, acetone, acetonitrile, DMF, and water. Elemental analysis for C10H8HgN8 (FW = 440.8 g mol−1): Calc. C, 27.25%; H, 1.83%; N, 25.42%. Found C, 26.92%; H, 1.56%; N, 24.96%. IR (ATR) (cm−1): 1659 (vw), 1511 (m), 1479 (m) ν(NN + CN + CC), 1420 (s), 1205−1072 (m), 769 (vs), 710 (m), 670 (m). 2.6. Powder X-ray Diffraction Structural Analysis. Polycrystalline samples of Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX) were gently ground in an agate mortar. Then, they were deposited in the hollow of a silicon zero-background plate (supplied by Assing Srl, Monterotondo, Italy). After preliminary acquisitions in the 3−35° 2θ range for fingerprinting analysis, diffraction data for structure solution were collected at room temperature by means of overnight scans (∼12 h) in the 2θ range 5.0−105.0°, with steps of 0.02°, on a Bruker AXS D8 Advance diffractometer, equipped with Ni-filtered Cu Kα radiation (λ = 1.5418 Å), a Lynxeye linear position-sensitive detector, and the following optics: primary beam Soller slits (2.5°), fixed divergence slit (0.5°), receiving slit (8 mm). The generator was set at 40 kV and 40 mA. A standard peak search, followed by indexing through the Singular Value Decomposition approach47 implemented in TOPAS-R,48 enabled us to retrieve the approximate unit cell parameters. The space groups were assigned on the basis of the systematic absences. Structure solutions were performed by the simulated annealing method, as implemented in TOPAS-R, employing a rigid body for the crystallographic independent portion of the ligand.49 The bond angles Ctetrazole−CH2−Cphenyl and the torsion angles Ctetrazole−CH2− Cphenyl−Cphenyl were allowed to refine. The final refinements were carried out by the Rietveld method,50 maintaining the rigid body introduced at the solution stage. For all the samples, the peak shapes were described with the fundamental parameters approach,51 while the peak shape anisotropy was modeled with the aid of spherical

harmonics. The background was modeled by means of a polynomial function. An isotropic displacement parameter was assigned to the metal atoms and refined, lighter atoms being given a value 2 Å2 higher. The final Rietveld refinement plots are collectively supplied in Figure S3 of the Supporting Information. Fractional atomic coordinates are provided in the Supporting Information as CIF files. Crystal Data for Zn(BTZPX). C10H8N8Zn, FW = 305.6 g mol−1, monoclinic, P21/c, a = 5.5606(2) Å, b = 20.320(1) Å, c = 10.2300(5) Å, β = 89.886(6)°, V = 1155.87(9) Å3, Z = 4, ρ = 1.76 g cm−3, F(000) = 616, RBragg = 0.033, Rp = 0.039, and Rwp = 0.052, for 4876 data and 50 parameters in the 7.5−105.0° (2θ) range. CCDC No. 1493596. Crystal data for Ag2(BTZPX). C10H8Ag2N8, FW = 455.96 g mol−1, triclinic, P1̅, a = 4.5354(2) Å, b = 5.8373(3) Å, c = 11.9033(4) Å, α = 92.289(3)°, β = 78.727(3)°, γ = 107.039(4)°, V = 295.43(2) Å3, Z = 2, ρ = 5.13 g cm−3, F(000) = 436, RBragg = 0.025, Rp = 0.045, and Rwp = 0.061, for 4951 data and 55 parameters in the 6.0−105.0° (2θ) range. CCDC No. 1493773. Crystal data for Hg(BTZPX). C10H8HgN8, FW = 440.81 g mol−1, monoclinic, P21/a, a = 10.3966(3) Å, b = 17.9002(6) Å, c = 6.1124(1) Å, β = 95.443(2)°, V = 1132.40(6) Å3, Z = 4, ρ = 2.59 g cm−3, F(000) = 816, RBragg = 0.021, Rp = 0.032, and Rwp = 0.042, for 4836 data and 45 parameters in the 8.3−105.0° (2θ) range. CCDC No. 1493595. 2.7. Variable-Temperature Powder X-ray Diffraction (VTPXRD). To complement the thermogravimetric analysis, the thermal behavior of Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX) was investigated by variable-temperature powder X-ray diffraction.52 As a first experiment, 30 mg samples of the three CPs were heated in air from 30 °C up to decomposition, with steps of 20 °C; a PXRD pattern was acquired at each step, covering a sensible low-to-medium-angle 2θ range, using a custom-made sample heater (Officina Elettrotecnica di Tenno, Ponte Arche, Italy). Treating the data acquired before loss of crystallinity by means of a Le Bail parametric refinement enabled us to disclose the variation of the unit cell parameters as a function of the temperature. As a second experiment, in order to evaluate their reusability, 20 mg samples of the CPs were monitored by PXRD during five heating−cooling cycles, in air, in the range 30−200 °C. 2.8. Electronic Transitions Spectroscopy. Electronic transition spectra in the solid state were acquired by means of a PTI Fluorescence Master System spectrofluorimeter. The instrument was driven by dedicated software (Felix 2000), performing automatic corrections for the excitation lamp spectral intensity and the detector spectral quantum efficiency. For each compound, the measurements were repeated on different batches, in order to exclude the presence of contaminants. Reproducibility of the results was also checked by repeating the measurements, on the same batches, at a distance of few months. The powdered samples were introduced in 1-mm-thick quartz cells (Hellma) which were positioned, with the help of a rotator, at 45° with respect to the excitation beam direction. UV−vis absorption spectral line-shapes were reconstructed by operating the instrument in the “parallel scan” configuration, as described in 53. Fluorescence emission spectra were acquired upon excitation at all the detected absorption peak wavelengths and at 355 nm (the emission wavelength of the laser used for time-resolved fluorescence measurements, vide infra). Fluorescence excitation spectra were also acquired with the same setup. Fluorescence quantum yields were determined by comparison with powdered anthracene (ΦFl = 0.9454), upon normalization for the relative absorption (as estimated by means of the above-described reflectance measurements). The extinction spectral line-shapes of H2BTZPX dissolved in dimethyl sulfoxide, acetonitrile, ethanol, and 0.5% v/v dimethyl sulfoxide/acetonitrile mixtures were recorded as a function of time and ligand concentration by means of a PerkinElmer Lambda 2 spectrophotometer. 2.9. Time-Resolved Fluorescence Analysis. Time-resolved fluorescence decays were reconstructed by means of a time-correlated single-photon counting (TCSPC) apparatus endowed with a temporal resolution ≤30 ps (full-width at half-maximum of the detected deltalike laser pulse), which is fully described elsewhere.35,53,55 The fluorescence of the samples was excited at 355 nm by the third harmonic of a SESAM mode locked Nd:VAN laser (GE-100 SHG, Time Bandwidth Products, Zurich, CH), delivering 5 ps pulses at 113 C



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MHz repetition rate. The third harmonic was generated extra-cavity as detailed in ref 55. Fluorescence was selected through a series of cutoff filters (cutoff wavelength spanning from 380 to 650 nm). The fluorescence decay data were fitted to either a double-exponential or a three-exponential decay model above a constant background: N ⎡ t−t ⎤ 0 y(t ) = A ∑ fi exp⎢ − ⎥ + y0 τ ⎣ ⎦ i i=1

Scheme 2. Synthetic Path to Xylene-Bis(2H-tetrazol-5-yl) (H2BTZPX), as Optimized in the Present Work

(1)

In eq 1 A represents the zero-time amplitude of the fluorescence decay pattern, the f i values denote the relative abundance of the species decaying with time constants τi, while y0 is a constant accounting for detector dark counts and non-time-correlated stray light. The fit was performed using a home-written Matlab routine which convolves the above theoretical model function with the system temporal pointspread function, and compares the obtained numerical string with the experimental decay histograms by minimizing the χ2 value with respect to τi, f i, and y0 by means of the built-in interior point convergence algorithm. To mimic the system temporal point-spread function in the measurements with the cutoff filters cutting at 380, 400, and 450 nm, we measured the instrumental response to 4 ps pulses at 420 nm issued by a Ti:sapphire laser (Tiger-ps, Time bandwidth products), while with the cutoff at 500, 550, 600, and 650 nm the instrumental response to the second harmonic pulses of a GE100 laser (wavelength 532 nm, pulse duration 7 ps) was assumed as the temporal pointspread function. Notably, the two pulse responses are minimally different to one another and from the one measured with the fundamental output of the GE100 laser (wavelength 1064 nm, pulse duration 9 ps). Addition of further decay components did not bring to an improvement of the χ2 value and of the residual randomness around zero. Three decay curves were acquired for each sample with each cutoff filter: the fluorescence lifetimes and relative initial amplitudes were calculated as the averages of the values obtained from the fits, with errors given by the standard deviations. In the case of Zn(BTZPX), the TCSPC experiments were repeated upon excitation at 420 nm, by exploiting the Ti:sapphire laser as the source of excitation pulses. 2.10. Quantum-Mechanical Calculations. Quantum-mechanical calculations were performed employing the program Gaussian09;56 the free cross-platform molecule editor Avogadro57 was used to build the model fragments employed to simulate electronic transitions in the CPs. Time-dependent density-functional theory (TD-DFT) was selected to compute excitation energies, thanks to an advantageous balance between accuracy and computational efficiency as compared to traditional ab initio and semiempirical methods. For similar reasons, the hybrid functional B3LYP58 was chosen to carry out the calculations; in spite of its tendency to underestimate charge transfer excitations, it nonetheless provides semiquantitative results.59,60 In the cases we are dealing with, such a level of accuracy suffices, as our interest lies only in determining the origin of the substantial bathochromic shift of the UV−vis absorption maxima of the title compounds compared to that of the isolated main chromophore. For light atoms such as hydrogen, carbon, and nitrogen, we used the split valence basis set 6-31G61,62 augmented with diffuse and polarization functions. While the latter provide enough flexibility to accommodate the polarization of the electron density in these coordination compounds, diffuse functions are needed for the description of orbitals wider than valence ones, which are typically necessary when studying anions or, as in our case, excited states. As for the heavier elements (i.e., mercury, silver, and zinc), to reduce the total amount of electrons, keep calculations manageable, and describe relativistic effects we employed the LANL2DZ basis set and effective core potentials.63−67

Scheme 3. Synthetic Path to the Three Bis(tetrazolato)Based CPs Described in the Present Work

Figure 1. Representation of the crystal structure of Zn(BTZPX): (a) Portion of the 1-D chain of tetrazolate-bridged metal ions running along the [001] direction. The tetrahedral coordination at the metal center can be appreciated. (b) The packing, viewed in perspective along the [001] direction. Horizontal axis, a; vertical axis, b. Carbon, gray; hydrogen, light gray; nitrogen, blue; zinc, yellow. Main bond distances (Å) and angles (deg) at the metal center: Zn−N 1.94(7), 1.96(3), 1.99(2), 2.08(7); N−Zn−N 102(1), 104(1), 107(1), 109(6), 114(1), 121(1).

3. RESULTS AND DISCUSSION 3.1. Synthesis and Preliminary Characterization. Demko and Sharpless successfully showed that, by means of a convenient, safe, and environment friendly synthetic method, a great diversity of 5-substituted-1H-tetrazoles could be

obtained in water by coupling nitrile compounds and sodium azide in the presence of zinc(II) salts, with a molar ratio of D



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Figure 2. Graphical representation of the crystal structure of Ag2(BTZPX): (a) The trigonal coordination at the metal center. (b) Portion of one of the 2-D layers, viewed down the [120] direction. The 1-D ladders of tetrazolate-bridged silver(I) ions can be easily appreciated. (c) Portion of the packing, viewed in perspective along the [110] direction. The 2-D corrugated layers can be appreciated. Carbon, gray; hydrogen, light gray; nitrogen, blue; silver, yellow. Main bond distances (Å) and angles (deg) at the metal center: Ag−N 2.16(5), 2.18(5), 2.33(5); N−Ag−N 106.3(9), 113(1), 141(1).

Figure 3. Graphical representation of the crystal structure of Hg(BTZPX): (a) Tetrahedral coordination at the metal center. (b) Portion of one of the 2-D slabs, viewed in perspective along the [001] direction. Horizontal axis, a; vertical axis, b. The 1-D chains of bridged Hg(II) ions run along the [001] direction. (c) Portion of the packing, viewed along the [001] direction. Horizontal axis, a; vertical axis, b. Carbon, gray; hydrogen, light gray; mercury, yellow; nitrogen, blue. Hydrogen atoms have been omitted for clarity. Main bond distances (Å) and angles (deg) at the metal center: Hg−N 2.19(6), 2.22(9), 2.25(9), 2.36(5); N−Hg−N 88(8), 101(2), 107(1), 111(2), 117(1), 126(4).

1:2:2, respectively.68 In the case of nonsubstituted tetrazoles, this method could be straightforwardly generalized.68 Nonetheless, to prepare poly(tetrazole)-containing ligands bearing different organic “bridges” between the tetrazole rings, synthesis conditions must be optimized case by case. A series of bis(tetrazoles) was already prepared as early as in 1979 by Illy,45 who isolated the ligand xylene-bis(2H-tetrazol-5yl) (H2BTZPX) (Scheme 1) for the first time by coupling 2,2′(1,4-phenylene)diacetonitrile and sodium azide in the presence of ammonium chloride, operating in DMF at 130 °C for 8 h. Conversely, the synthetic path optimized in the present work to isolate H2BTZPX mimics the protocol proposed by Demko and Sharpless, namely, 2,2′-(1,4-phenylene)diacetonitrile, NaN3, and ZnBr2 were reacted with a molar ratio of 1:4:4, respectively, operating in water, under reflux, for 48 h (Scheme 2). This procedure enabled us to isolate H2BTZPX in the form of a white and air stable powdered solid in reasonably good yield (65%). The broad band lying in the range 3200−2500 cm−1 in the infrared spectrum of H2BTZPX (Figure S1 of the Supporting Information), together with the remarkable upfield shift experienced by the N−H proton (16.11 ppm), suggest the presence of NH···N hydrogen bonds through which adjacent H2BTZPX molecules interact in both the solid state and DMSO solution, as already observed for the analogous ligands H2DMPMB43 and H2BDMPX44 (Scheme 1). In the case of H2DMPMB and H2BDMPX, the presence of hydrogen bond interactions in the solid state was confirmed by the determination of their crystal structure.43,44 Unfortunately, no batch of H2BTZPX was crystalline enough to allow a structural analysis. In addition, due to its rather limited solubility (see Section 2.2), all the efforts to recrystallize it were not successful.

Figure 4. TGA traces of Zn(BTZPX) (black), Ag2(BTZPX) (red), and Hg(BTZPX) (blue).

By essaying different reaction solvents, cosolvents, temperatures, times, and concentrations, we were able to optimize the synthesis of the new CPs Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX) (Scheme 3), featuring d10 transition metal ions. Unfortunately, attempts to isolate Cu(I) and Cd(II) derivatives in the form of pure polycrystalline batches invariably failed. The homoleptic coordination polymers Zn(BTZPX) and Ag2(BTZPX) were isolated in 24 h as white and gray46 E



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Figure 5. (a,c,e) Plot of the powder X-ray diffraction patterns measured on Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX), respectively, as a function of temperature heating in air, with steps of 20 °C, from 30 °C. (b,d,f) Percentage variation of the unit cell parameters (pT) of Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX), respectively, as a function of temperature. At each temperature, the actual values (pT) have been normalized with respect to those at 30 °C (p30). a, green squares; b, orange circles; c, red rhombi; α, green open squares; β, orange open circles; γ, red open rhombi; V, blue triangles. The yellow line in (a) and (e) highlights the temperature around which loss of crystallinity starts. The asterisk in (a) indicates the lowest-angle peak of ZnO.

Figure 6. Absorption spectral line-shapes for the ligand H2BTZPX and its CPs Ag2(BTZPX), Hg(BTZPX), and Zn(BTZPX).

Figure 7. Emission spectra of the ligand H2BTZPX and its CPs Ag2(BTZPX), Hg(BTZPX), and Zn(BTZPX).

powders, respectively, by applying solvothermal syntheses mimicking the reaction conditions previously adopted for the F



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mode shown by the tetrazolate anion should not surprise. Zhao and co-workers identified at least nine coordination modes.40 A search70 in the Cambridge Structural Database (v 1.18) for transition metal complexes with ligands containing one or more exo-poly(dentate) tetrazolate moieties, unveiled a diversified landscape, in which the hapticity of the ring may vary from two to four, the exo-bidentate coordination being the most recurring one (see Figure S4 of the Supporting Information for further details). In Zn(BTZPX), 1-D zigzag chains of Zn(II) ions (Figure 1a) run along the [001] direction; along the chains, the metal ions are kept at a distance of 5.1150(5) Å (half of the length of the caxis) by N1,N3-bridging tetrazolate rings. Each chain is connected to neighboring ones by N1,N4-bridging tetrazolate rings, this bringing about the formation of a 3-D (4,6)connected network (Figure 1b). The phenyl rings and the N1,N4-bridging tetrazolate rings stack along the [001] direction with a distance, between their centers of mass, of 5.1150(5) Å, excluding the presence of π−π interactions. The N1,N3bridging tetrazolate rings lay in the crystallographic plane ac, with no stacking among each other or with the other rings. Though two different sets of 1-D channels are apparently present along the c-axis, the software Platon71 calculates a negligible (less than 2%) amount of guest-accessible empty volume. Ag2(BTZPX) crystallizes in the triclinic space group P1̅. The asymmetric unit contains one metal ion, lying on a general position, and one ligand, lying on a crystallographic inversion center. The metal centers possess a distorted trigonal stereochemistry defined by three nitrogen atoms of three BTZPX2− spacers [Ag−N 2.16(5)−2.33(5) Å; N−Ag−N 106.3(9)−140(1)°] (Figure 2a). As in Zn(BTZPX), also in this case the ligands show a chair conformation72 (exactly C2h, in view of the fact that the center of mass of the ligand lie on an inversion center), yet they adopt an exo-hexadentate coordination mode, employing the nitrogen atoms in positions 1, 2, and 3 of each tetrazolate ring. The high hapticity of the ligand toward Ag(I) should not surprise: the search, in the Cambridge Structural Database, quoted above also highlighted that, in silver(I) complexes, tetrazolate is preferably (42%) exotridentate. In Ag2(BTZPX), approximately along the [120] direction the metal ions are alternately kept at distances of 4.026(4) and 3.853(5) Å, this occurrence implying the formation of 1-D ladders (Figure 2b). Incidentally, the Ag···Ag distances are too long for argentophilic interactions to be invoked.73 Each ladder is bridged to two nearby ones by the spacers, this bringing about the formation of 2-D (3,6)-connected corrugated layers (Figure 2b,c) stacking orthogonally to the [120] direction at a distance of 4.5354(3) Å (the length of the a-axis). The pentaatomic rings are orthogonal to the (−213) crystallographic plane, while the hexa-atomic ones lie parallel to the (−435) plane. In both cases, the rings are not eclipsed and the distance

Figure 8. Excitation spectra of the ligand H2BTZPX and its CPs Ag2(BTZPX), Hg(BTZPX), and Zn(BTZPX). The spectrum of the ligand (magenta line) was recorded monitoring the fluorescence intensity at λobs = 450 nm, those of Ag2(BTZPX) (red line) and Hg(BTZPX) (blue line) with λobs = 550 nm. The excitation spectrum of Zn(BTZPX) was acquired at both λobs = 450 nm (black line) and λobs = 550 nm (cyan line).

CPs M(BDMPX)44 (M = Zn, Co, Cd). Zinc chloride dihydrate, silver nitrate, and DMF were used as source of zinc(II) ions, source of silver(I) ions, and solvent, respectively. Conversely, Hg(BTZPX) was isolated in the form of white powders by applying a different route: mercury acetate hexahydrate was allowed to react with H2BTZPX in water under reflux for 24 h. The CPs precipitate in good yields (with the exception of the Zn(II) derivative) as air- and moisture-stable polycrystalline powders, insoluble in water and in several organic solvents, this occurrence suggesting their polymeric nature. Their infrared spectra (Figure S2) present similar absorption bands; in particular, the absence of the N−H stretching band indicates the complete deprotonation of the organic ligand, which is consistent with the formation of metal-bridging bis(tetrazolato) dianions, as eventually confirmed by PXRD structure determination. 3.2. Crystal Structure Analysis. Zn(BTZPX) crystallizes in the monoclinic space group P21/c. The asymmetric unit is composed by one Zn(II) ion and one ligand, both in general positions. The metal centers show a distorted tetrahedral stereochemistry defined by the nitrogen atoms of four BTZPX2− spacers [Zn−N 1.94(8)−2.08(7) Å; N−Zn−N 102(1)−121(1)°] (Figure 1a). The spacers adopt a chair conformation, 69 as already observed in the case of H2DMPMB,43 H2BDMPX,44 and their coordination polymers, and are exo-tetradentate. In more detail, in Zn(BTZPX), the ligands employ, as donor atoms, the nitrogen atoms in positions 1 and 3 of one tetrazolate ring, and the nitrogen atoms in positions 1 and 4 of the other, this occurrence leading to a different structural motif than those of Zn(DMPMB)43 and Zn(BDMPX).44 Incidentally, this versatility in the coordination

Table 1. Fitting Parameters of the Decay Patterns of H2BTZPX and its CPs Ag2(BTZPX), Hg(BTZPX), and Zn(BTZPX) upon Excitation at 355 nm and Observation of the Fluorescence Signal Emitted at λ > 400 nm compound

f1

τ1 [ps]

H2BTZPX Zn(BTZPX) Hg(BTZPX) Ag2(BTZPX)

0.70 ± 0.09 0.64 ± 0.08

7±1 12 ± 1

τ2 [ps]

f2 0.49 0.47 0.23 0.28

± ± ± ±

G

0.02 0.01 0.03 0.03

484 651 509 427

± ± ± ±

2 3 7 6

τ3 [ps]

f3 0.51 0.53 0.07 0.08

± ± ± ±

0.02 0.01 0.01 0.01

2585 3144 2503 2194

± ± ± ±

4 8 72 54



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Figure 9. Relative amplitudes (left) and decay times (right) resulting from the fit to eq 1 of the experimental decay patterns recorded upon excitation at 355 nm as a function of the cutoff filter for H2BTZPX (top), Hg(BTZPX) (center), and Ag2(BTZPX) (bottom).

between their centroids [4.5354(3) Å] is such that no π−π interactions are present. The software Platon71 calculates no guest-accessible empty volume. Hg(BTZPX) crystallizes in the monoclinic space group P21/ a. The asymmetric unit contains one Hg(II) ion and one spacer, both in general positions. The metal centers show a heavily distorted tetrahedral stereochemistry defined by the nitrogen atoms of four different BTZPX2− linkers [Hg−N 2.19(6)−2.36(6) Å; N−Hg−N 88(8)−126(4)°] (Figure 3a). Also in the present case, the ligands adopt a chair conformation.74 As in Zn(BTZPX), in Hg(BTZPX) the spacers are exo-tetradentate and employ, as donor atoms, the nitrogen atoms in positions 1 and 3 of one tetrazolate ring, and the nitrogen atoms in positions 1 and 4 of the other. In Hg(BTZPX), 1-D chains of collinear metal ions (Figure 3b)

are formed along the [001] direction; along the 1-D chains, the Hg(II) ions are maintained at 3.0562(2) Å (half of the length of the c-axis) by N1,N3-bridging tetrazolate rings. Notably, the metal-to-metal distance is such that mercurophilic interactions cannot be excluded.75 Each chain is bridged to two nearby ones by the N1,N4-bridging tetrazolate rings of the spacers. Overall, 2-D corrugated double layers (Figure 2b), stacking along the [010] direction (Figure 3c), are present. The three rings of the ligand stack, staggered, along two different directions. The software Platon71 calculates no guest-accessible empty volume. As for the structural features of the H2BTZPX ligand, we have already anticipated that IR spectroscopy purports the presence of hydrogen-bond interactions of the kind N−H···N in the solid state. By comparing the geometrical features of the ligand in the crystal structure of the three CPs,69,72,74 a certain H



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Figure 10. Relative amplitudes (a) and decay times (b) measured for Zn(BTZPX) upon excitation at 420 nm as a function of the cutoff filter.

Figure 11. Graphical representation of (a) the HOMO and (b) the LUMO of HBTZPX‑. The electron transfer process is evidenced by the different localization of HOMO and LUMO. Carbon, gray; hydrogen, light gray; nitrogen, blue.

similarity of the conformational parameters can be observed. Together with H2BDMPX43 and H2DMPMB,44 also 4,4′((2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene))bis(3,5-dimethyl-1H-pyrazole)76 (Scheme S1 of the Supporting Information) and 4,4′-((2,3,5,6-tetramethyl-1,4-phenylene)bis(methylene))bis(3,5-diphenyl-1H-pyrazole) (Scheme S1) show an anti conformation of the pyrazolyl substituents in position para on the central hexa-atomic ring. Moreover, a search in the Cambridge Structural Database (v 1.18) for organics containing fragments of the kind depicted in Scheme S2 (Supporting Information) revealed that, when the fragment is not involved in extended fused rings or cyclic oligomers, an (ideal or not) anti conformation is adopted in all of the cases. In the light of this, we tentatively propose that also H2BTZPX shows, in the solid state, an anti conformation. 3.3. Thermal Behavior. The thermal behavior of the title CPs was studied by coupling thermogravimetric analysis (TGA), carried out under a N2 flow, to variable-temperature powder X-ray diffraction (VT-PXRD), performed in air. According to the TGA traces, collected in Figure 4, the Zn(II), Ag(I), and Hg(II) derivatives are stable up to 350, 370, and 250 °C, respectively, temperatures at which a slow decomposition begins.77 These decomposition temperatures highlight that the flexibility of the ligand is not necessarily detrimental for the thermal stability of the material. Indeed, Ag 2 (BTZPX) surp asses the t hermal st ability of Ag2(DMPMB)43 (decomposition onset 300 °C). Moreover, Zn(BTZPX) and Ag2(BTZPX) exhibit thermal stabilities comparable to, e.g., the Zn- and Cd-based CPs containing flexible bis(tetrazol-5-yl)alkanes (decomposition onset in the range 350−400 °C).78

Figure 12. Representation of the electronic transitions defining the first excited states of the fragment Ag3(HBTZPX)3(TZ)22−: the first excited state (λ = 496.2 nm) is determined by the transition HOMO → LUMO (a); the second excited state (λ = 469.1 nm) is determined by the transition HOMO−1 → LUMO (b); the third excited state (λ = 452.2 nm) is determined by the transitions HOMO−2 → LUMO (c) and HOMO−3 → LUMO (d). Carbon, gray; hydrogen, light gray; nitrogen, blue; silver, light violet.

Figure 5a shows the PXRD patterns of Zn(BTZPX) acquired as a function of the temperature in the range 30−490 °C. Zn(BTZPX) does not undergo phase transitions up to 390 °C, temperature at which crystallinity loss begins (see the region highlighted with a yellow line in Figure 5a), accompanied by the appearance of ZnO (see the asterisk in Figure 5a). A metastable phase was detected in the temperature range 450− 470 °C, but was not identified. In the range 30−370 °C, the unit cell volume increases by 1.8% as a result of thermal expansion (Figure 5b). The highest variation is observed for the I



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Figure 13. Representation of the electronic transitions defining the seventh excited state (λ = 408.8 nm) of the fragment Ag 3 (HBTZPX) 3 (TZ) 2 2− : HOMO−1 → LUMO+1 (a) and HOMO−2 → LUMO+1 (b). Carbon, gray; hydrogen, light gray; nitrogen, blue; silver, light violet.

Figure 15. Representation of the electronic transitions defining the twelfth excited state (λ = 338.7 nm) of the fragment Zn(HBTZPX)42−: HOMO−7 → LUMO (a), HOMO−8 → LUMO (b), HOMO−9 → LUMO (c), and HOMO−12 → LUMO (d). Carbon, gray; hydrogen, light gray; nitrogen, blue; zinc, violet.

the data in the range 30−390 °C (Figure 5d) highlights an overall unit cell volume increase of 3.0% as a result of thermal expansion. The a-axis increases by 1.1%, occurrence which could be traced back to a slight increase of the distance between the 2-D layers. The c-axis increases by only 0.6%, which may be interpreted as a slight “relaxation” of the chair conformation of the ligand (Scheme S3), possibly as a result of an increase of the angle between the root-mean-square planes defined by the penta- and hexa-atomic rings. Finally, the slight shrink affecting the b-axis (0.3%) could be related to a minor shrinking of the 1D chains. As emerging from Figure 5e, Hg(BTZPX) does not undergo phase transitions until 310 °C, the temperature at which it starts losing crystallinity (see the region highlighted with a yellow line in Figure 5e) and a new phase concomitantly appears. Identification of the latter as a mercury-containing inorganic phase was unsuccessful. The Le Bail parametric treatment of the data acquired in the temperature range 30− 290 °C (Figure 5f) showed that the unit cell volume increases by ca. 1.6%, as a result of thermal expansion. The a-axis increases by 1.0%. Once again, this occurrence may be interpreted as a “relaxation” of the ligand chair conformation, which could bring about a “smoothing” of the 2-D corrugated layers. The slight increase of the b-axis (0.7%) suggests a minor increase of the interlayer distance. Finally, the c-axis undergoes a slight decrease (−0.5%), which might be the consequence of a minor contraction of the metal ion chains. Finally, as evident from Figure S5, the three CPs exhibit no significant alterations along five consecutive heating−cooling cycles in the range 30−200 °C, hence they can be reused as long as the test conditions are fulfilled.

Figure 14. Representation of the electronic transitions defining the first excited states of the fragment Zn(HBTZPX)42−: the first excited state (λ = 456.9 nm) is defined by the transition HOMO → LUMO (a); the second excited state (λ = 430.7 nm) is defined by the transition HOMO−1 → LUMO (b); the third excited state (λ = 404.1 nm) is defined by the transitions HOMO−2 → LUMO (c) and HOMO−3 → LUMO (d). Carbon, gray; hydrogen, light gray; nitrogen, blue; zinc, violet.

b-axis, which increases by 2.3%, whereas both the a- and c-axis shrink (by 0.2% and 0.4%, respectively). Due to the somewhat complicated crystal structure of Zn(BTZPX), rationalizing the variation of each unit cell axis in terms of specific structural modifications is not straightforward. Figure 5c shows the PXRD patterns of Ag2(BTZPX) acquired as a function of the temperature in the range 30− 430 °C. As evident from this figure, Ag2(BTZPX) does not undergo phase transitions up to 410 °C, temperature at which loss of crystallinity begins. The Le Bail parametric treatment of J



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nisms.80−83 To corroborate the hypothesis of ligand−ligand intermolecular hydrogen-bonding patterns as the source of the observed bathochromic shift in the solid state UV−vis absorption spectrum of the ligand, and in spite of its rather low solubility in most organic solvents, a spectroscopic characterization in solution was attempted, on which we report more extensively in Section S.4 of the Supporting Information. Upon shifting from non-H-bonding to highly-H-bonding solvents, and changing the H2BTZPX concentration for a given solvent, the observed features of the extinction spectral line-shapes indicate that oligomerization induces a spectral shift from ∼260 nm (very similar to the absorption peak of isolated benzene and only slightly dependent from the solvent) to ∼330 nm [i.e., in proximity of the main absorption band observed in the solid state for Zn(BTZPX) and Ag2(BTZPX)], thus conferring support to the proposed explanation. Concerning the interpretation of the absorption spectral features of the CPs, quantum-mechanical calculations were undertaken (vide infra). The fluorescence emission spectra are plotted in Figure 7. The ligand H2BTZPX emits sizable fluorescence (ΦFluo = 0.51 ± 0.03) in a band peaked at 430 nm, which has a roughly excitation-wavelength independent spectrum. Interestingly, upon excitation at 340 nm, Zn(BTZPX) preserves the main features of the emission of the ligand: its fluorescence spectrum is only slightly broader and red-shifted than that of the ligand, and the integrated fluorescence intensity is slightly increased (ΦFluo = 0.60 ± 0.04). In contrast, upon excitation at 420 nm, Zn(BTZPX) emits in a band peaked at ∼510 nm, with an almost identical quantum yield (ΦFluo = 0.62 ± 0.03). Ag2(BTZPX) emits fluorescence in a somewhat structured, very broad band with barycenter at ∼500 nm, the line-shape of which is roughly independent from the excitation wavelength and recalls the one observed for Zn(BTZPX) upon excitation at 420 nm. However, in the case of Ag2(BTZPX) the emission intensity is significantly quenched with respect to both the ligand and the Zn(II)-based CP (ΦFluo = 0.23 ± 0.05 for excitation at 410 nm; see the excitation spectra below). Finally, Hg(BTZPX) emits barely detectable fluorescence (ΦFluo = 0.04 ± 0.02 for excitation at 400 nm; see the excitation spectra below) in a very broad band peaked in the green and rather independent from the excitation wavelength. The emerging trend for ΦFluo versus the atomic number Z can be partially explained by considering that an increase in the intersystem crossing rate should be expected upon increasing Z due to the impact of relativistic effects such as spin−orbit coupling. In the specific case of Hg(BTZPX), relativistic effects may be flanked by the mercurophilic interactions73,75 hinted by the short metalto-metal distance [3.0562(2) Å] emerging from the PXRD results. However, the complex panorama of the spectral lineshapes, together with the multiexponential and metal-dependent decay patterns (vide infra) suggests that other excited-state mechanisms and more complex excited-state interconversion dynamics are also taking place. The excitation spectra of the ligand and the CPs were also acquired using λobs,1 = 450 nm and λobs,2 = 550 nm as the observation wavelength. Very interestingly, the excitation spectral line-shape is observation-wavelength independent (i.e.: similar at λobs,1 and λobs,2) for H2BTZPX, Ag2(BTZPX), and Hg(BTZPX). Namely, for the ligand it is peaked at ∼355 nm, while for the two coordination polymers it is peaked slightly above 400 nm. Conversely, for Zn(BTZPX) the excitation spectrum is similar to that measured for the ligand

Figure 16. Representation of the electronic transitions defining the first excited states of the fragment Hg4(MeTZ)124−: the first and second excited states (λ = 321.1 and 307.3 nm, respectively) are defined by the transitions HOMO → LUMO (a) and HOMO → LUMO+1 (b); the third excited state (λ = 302.3 nm) is defined by the transitions HOMO−2 → LUMO (c) and HOMO−2 → LUMO+1 (d). Carbon, gray; hydrogen, light gray; nitrogen, blue; mercury, light violet.

3.4. Electronic Transitions Spectroscopy. The absorption spectral line-shapes of H 2 BTZPX, Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX) are shown in Figure 6. The spectrum of the ligand consists of a broad, almost structure-less band peaked around 390 nm. Coordination to Hg(II) results in a limited perturbation of the absorption features, whereas the absorption spectra of the Ag(I) and Zn(II) CPs appear more structured, with two well separated peaks at ∼340 nm and ∼420 nm in place of the above-mentioned broad band, as well as additional minor bands at ∼530 nm and ∼610 nm, respectively. The two major bands have different relative intensities in Zn(BTZPX) and Ag2(BTZPX), the UV-A band being more prominent and slightly blue-shifted for Ag2(BTZPX). The above complex patterns cannot be trivially explained in terms of the signatures of the chromophores present in the ligand (basically the central aromatic ring, λabs(benzene) ≈ 260 nm,79 which is not conjugated with the tetrazole moieties as it is separated by single C−C bonds). The bathochromic shift experienced by the ligand might be correlated with the pattern of strong N−H···N hydrogen bonds observed in both the IR and NMR spectra. Indeed, the N−H stretching band is shifted to as far as 3500 cm−1, and the hydrogen-bonded proton yields an NMR resonance at ∼16 ppm, which is the indication of a rather strong interaction. Hydrogen bonding has been frequently associated with redshifts in the absorption maxima, particularly when concomitant with the onset of excited-state proton transfer mechaK



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(namely, with λcut = 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm), with the aim of unraveling the putative spectral dependence of the decay photophysics. However, as shown in Figure 9, the lifetimes and relative amplitudes are roughly constant as a function of the portion of the fluorescence emission spectrum selected, suggesting a simple two-state transition. As Zn(BTZPX) exhibited excitation-dependent fluorescence emission, with an emission band centered at ∼430 nm upon excitation in correspondence of the UV-A absorption peak and one centered at ∼500 nm upon excitation in the visible, for this CP we repeated the time-resolved characterization by using the Ti:sapphire second harmonic (420 nm; see Materials and Methods) as the excitation source. Notably, the decay patterns measured in this instance are threeexponential, and the detected decay times resemble those measured for Ag2(BTZPX) and Hg(BTZPX) (see Figure 10 for a summary of the fitting parameters). This supports the hypothesis proposed above that the emission of H2BTZPX can in principle occur from two different energy levels, which interconvert in the case of Ag2(BTZPX) and Hg(BTZPX) and are separated by a high potential barrier in the crystal structure of Zn(BTZPX). 3.5. Quantum-Mechanical Calculations. As discussed in the previous Section, the UV−vis absorption spectra of H2BTZPX and the three CPs appear bathochromically shifted with respect to the expected absorption of the main chromophore, i.e., the benzene ring.79 Quantum-mechanical calculations were carried out on the CPs in order to rationalize the observed phenomenon. Before tackling the bathochromic shift, preliminary calculations were carried out on benzene to evaluate the relative accuracy of the method we chose. Upon evaluating the excitation energy for benzene after geometry optimization, we obtained a HOMO−LUMO transition wavelength λH‑L(benzene; calculated) of 228 nm. By comparison with λabs(benzene; literature),79 it clearly appears that a sizable uncertainty of roughly ±30 nm, intrinsic to the calculation method, must be taken into account. Nevertheless, this extent of error was not critical for our goal, as only excitation energy differences were considered. As a first step toward the understanding of the bathochromic shifts, we studied a single molecule of H2BTZPX with four different geometries, namely, those observed in the crystal structure of the three CPs, and a gas-phase fully optimized one.84 Obviously, the intention behind this approach was to evaluate (i) if the nonconjugated tetrazole rings had a nonnegligible role in defining the absorption wavelength of the ligand due to, e.g., inductive effects, and (ii) if the actual geometry of the ligand had any repercussion on the calculated absorption wavelengths. A red-shift of ∼30 nm is found between the absorption maxima calculated for benzene and H2BTZPX, irrespective of the geometry adopted by the latter; though such an occurrence may be ascribed to the inductive effects of the tetrazole rings, its extent is not sufficiently large to explain the observed absorption patterns. We thus investigated the probability of H2BTZPX to establish N−H···N intermolecular hydrogen bonds, the formation of which was invoked in Section 3.4 to explain the bathochromic shift observed in the absorption spectrum of the ligand. First, we calculated the residual charges on the nitrogen atoms of tetrazole by means of the Natural Bond Order (NBO) analysis method.85 Not unexpectedly, we obtained that, while N2 and N3 are almost neutral [δQ(N2) = −0.08, δQ(N3) = −0.07], both N1(H) and

when λobs,1 is chosen as the observation wavelength, while it resembles those of the other CPs when observed at λobs,2. The above phenomenology denounces the existence of two nearby electronic transitions, coinciding with the two main peaks resolved in the reflectance spectra of Ag2(BTZPX) and Zn(BTZPX), which are probably integrated within the broad band observed for the ligand and Hg(BTZPX). The two corresponding excited states, hereafter referred to as E340 and E420, can undergo radiative decay, but the excited-state deactivation photophysics seems to depend on the structural features of the compound. Namely, in the 3-D network Zn(BTZPX) both states are luminescent, and the interconversion between the two occurs with negligible probability within the excited-state lifetime. Conversely, the green fluorescence observed for Zn(BTZPX) in case of excitation to E420 is quenched in the case of the ligand, irrespective of the excitation wavelength, and the excitation spectrum is peaked in the UV. This implies that, for this compound, E420 is rapidly deactivated by nonradiative pathways. We will further comment on this point while describing the time-correlated single-photon counting (TCSPC) results (vide infra). Finally, although a well-defined UV-A band is not observed in the excitation spectrum of Ag2(BTZPX), nor in that of Hg(BTZPX), excitation at 340 nm of both compounds results in detectable fluorescence, with a spectrum peaked above 500 nm, i.e., in the band characteristic of the emission of Zn(BTZPX) when the compound is excited at E420 (Figure 8). This suggests that, in the 2-D corrugated layers, interconversion dynamics from E340 to E420 occurs at a much faster rate than radiative decay from E340. Besides acquiring the steady-state reflectance, excitation and emission spectra of H2BTZPX and its CPs, we also performed TCSPC measurements with the aim of characterizing the excited-state dynamics of these compounds. First, the timeresolved fluorescence decay distribution was acquired upon excitation in the UV, by exploiting the third harmonic of the Nd:VAN laser (355 nm) as the excitation source (see Materials and Methods and ref 55), and selecting the fluorescence signal by means of a cutoff filter with λcut = 400 nm. The fitting parameters are reported in Table 1. Interestingly, the ligand exhibits a double-exponential decay pattern, with a shorter decay time of ∼0.5 ns (relative amplitude ∼0.5), and a longer one of ∼2.6 ns. The same qualitative features are conserved in Zn(BTZPX), the only CP preserving a high fluorescence quantum yield and an emission spectrum similar to that of the ligand. The slightly higher quantum yield measured for Zn(BTZPX) with respect to H2BTZPX correlates with the somewhat slower decay times measured for both the decay transients, while the relative probability of undergoing the slower with respect to the faster excited-state deactivation pathway remains substantially unaltered. In contrast, for both Ag2(BTZPX) and Hg(BTZPX), which exhibited lower fluorescence quantum yield in steady-state measurements, an additional decay transient is detected, which has a decay time comparable with the TCSPC system temporal resolution and a relative amplitude as high as 0.70 and 0.64 for Hg(BTZPX) and Ag2(BTZPX), respectively. The ∼0.5 ns and ∼2.5 ns transients observed for the ligand are still observed also in the decay patterns of these CPs, which are thus triple-exponential. With the exception of Hg(BTZPX), the fluorescence signal of which was too dim to allow such a characterization, the measurements were repeated by selecting the fluorescence emission through cutoff filters of increasing wavelength L



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(Figure 13) can be associated with the absorption maximum observed at ∼420 nm.87 These findings support the charge transfer origin of the red shift measured in the absorption spectra of this CP, which is connected to the presence of negative partial charges. As for the Zn(II)-containing CP, four fragments were defined, namely, Zn(HBTZPX) + , Zn 2 (MeTZ) 7 3− , Zn(HBTZPX)42−, and Zn2(HBZMeTZ)73− (see Supporting Information, Figure S11). Similarly to the fragments built up for Ag2(BTZPX), the results for the Zn-containing fragments show the presence of a charge transfer, with wavelengths covering most of the UV−vis absorption spectrum, from ∼338 nm onward. Figures 14 and 15 collect representative electronic transitions for the Zn(HBTZPX)42− model: the third excited state (λ = 404.1 nm) (Figure 14b) may be associated with the absorption maximum observed at ∼420 nm, while the twelfth one (λ = 338.7 nm) could be associated with the absorption maximum observed at ∼340 nm. Finally, three models were built up for the Hg(II)-containing CP, namely, Hg(HBTZPX)42−, Hg4(MeTZ)4(MeHTZ)84+, and Hg4(MeTZ)124− (see Supporting Information, Figure S12). The calculations on these fragments (see Figure 16 for Hg4(MeTZ)124−) only yielded high-energy electronic transitions, falling at wavelengths ≤342 nm. However, we remind that any information about the relative probabilities of the computed transitions, i.e., on the intensities of the corresponding bands, is hard to infer. If we assume that the transition computed at 342 nm is the most probable one, and keep in mind that the computational method is affected by an error of ∼30 nm, as shown by the results on benzene, the main absorption peak detected experimentally, falling at ∼380 nm, can be associated with that computed at ∼342 nm, and considered to have, once again, a strong metal-to-ligand charge transfer character, as indicated in Figure 16.

N4 are considerably enriched in electronic density. Namely, δQ(N1(H)) = −0.43, δQ(N4) = −0.31. Although partial charges are not, strictly speaking, quantum-mechanical observables of a system and, as such, their values cannot be univocally and unambiguously determined by DFT analysis, but rather depend on the initialization of the calculation, we note that the previously reported residual charge values are compatible with the formation of N1(H)···N4 intermolecular hydrogen bonds. As a further step, in order to probe the effects of intermolecular H-bonding on the absorption properties of H2BTZPX, calculations on hydrogen-bonded H2BTZPX dimers were attempted. However, the latter returned very small differences of the absorption maximum wavelength with respect to the monomeric species. In this respect, it should be noted that the absorption maxima measured for the ligand in different solvents, and interpreted as fingerprints of the claimed intermolecular H-bonding patterns, might be due to larger oligomers, which could not be modeled in silico. Indeed, in dimers there are no molecules with both tetrazole rings involved in hydrogen bonding. As the crystal structures of the CPs suggest that no π−π stacking interactions are at work (see Section 3.2), clues on the origin of the bathochromic shifts were found when studying the excitations of the monodeprotonated ligand, which presented a red-shift of 280−130 nm with respect to those of the neutral H2BTZPX counterpart. Figure 11a,b shows, respectively, the HOMO and the LUMO of HBTZPX‑ involved in the absorption process. It can be easily observed that, in the fundamental state, the HOMO is localized on the deprotonated tetrazolate ring while, after photoexcitation, the LUMO is localized on the neutral tetrazole ring. In light of this, a charge transfer seems to be at work. Support for the above conclusion was searched for by enlarging our models via the addition of metal centers, in order to verify whether or not the charge transfer would withstand the presence of cations. As calculations on periodic systems involving electronic excitations are not trivial, a number of models were carved out starting from the crystal structures,86 in order to test the robustness of the results. Obviously, any fragment-like model presents shortcomings due to either a nonzero charge, or the coordinatively unsaturated nature of at least one metal center. As for the Ag(I) derivative, five models were generated, namely, Ag(HBTZPX), Ag2 (BTZPX), Ag(HBTZPX) 3 2−, Ag2(HBTZPX)42−, and Ag3(HBTZPX)3(TZ)22− (see Supporting Information, Figure S10). The TD-DFT calculations involving these models predicted absorptions in the range from λabs ∼396 nm up to λabs ∼722 nm. It is worth noting that low-energy electronic transitions are found for the Ag2(BTZPX) and Ag(HBTZPX) models, both presenting coordinatively unsaturated silver ions. As similar transitions are present in the experimental spectra, we interpret them as due to non-negligible surface effects. Notably, the charge transfer unveiled for the monodeprotonated ligand is observed also in the silver-containing models (see Figures 12 and 13 for representative electronic transitions for Ag3(HBTZPX)3(TZ)22−): upon examining Figures 12 and 13, the presence of charge transfers appears clear, as the electronic transitions generating the first excited states take place toward antibonding orbitals. If it had been a dipole-bound state, we would have observed a very diffuse electron density cloud almost out of the tetrazole ring. It is noteworthy that the transition generating the seventh excited state (λ = 408.8 nm)

4. CONCLUSIONS Inspired by the protocol developed by Demko and Sharpless, we optimized an environment friendly synthesis of H2BTZPX, which was exploited to build the coordination polymers Zn(BTZPX), Ag2(BTZPX), and Hg(BTZPX). As inferred by PXRD, Zn(BTZPX) features a 3-D polymeric network, while Ag2(BTZPX) and Hg(BTZPX) show 2-D corrugated layers. The UV−vis absorption spectra of H2BTZPX and the CPs unveiled a bathochromic shift with respect to the absorption maximum of the main chromophore of the spacer − benzene. To explain this shift in the case of the ligand, an active role of the N−H···N hydrogen bond interactions was postulated. On the other hand, quantum mechanical calculations performed with TD-DFT on model fragments suggested the presence of a charge transfer, at least in the case of the Zn(II) and Ag(I) derivatives. Finally, Zn(BTZPX) exhibits enhanced fluorescence with respect to the ligand, while the fluorescence emission intensity is notably reduced for Ag2(BTZPX) and barely detectable for Hg(BTZPX). This occurrence, juxtaposed to the results obtained by reconstructing the time-resolved fluorescence, suggests different decay pathways, possibly related to the dimensionality of the crystal structure, or to the impact of relativistic effects (e.g., spin−orbit coupling) and mercurophilic interactions in Hg(BTZPX). M



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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.6b01056. IR spectra; final Rietveld refinement plots; PXRD spectra acquired during the heating−cooling cycles; details on the search within the Cambridge Structural Database quoted in the text; details on the absorption measurements carried out in solution; representation of the model fragments built up for the TD-DFT calculations (PDF) Accession Codes

CCDC 1493595−1493596 and 1493773 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

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.G. and E.L. acknowledge partial funding by Università dell’Insubria. CP acknowledges University of Camerino and COST Action CM 1302 (SIPs). Angelo Maspero, Università dell’Insubria, is acknowledged for experimental help.

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REFERENCES

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Investigating the Structural Features and Spectroscopic Properties of

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