Investigating the Structural Features and Spectroscopic Properties of Bis(tetrazolato)-Based Coordination Polymers

Article abstract of DOI:10.1021/acs.cgd.6b01056

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 variable-temperature 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.

Full text of DOI:10.1021/acs.cgd.6b01056

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, Universita
̀
dell’Insubria, Via Valleggio 11, 22100 Como, Italy
di Camerino, Via S. Agostino 1, 62032 Camerino, Italy
‡Scuola del Farmaco e dei Prodotti della Salute, Universita
̀
§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, Universita
̀
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-tetra-
zol-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 variable-
temperature 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.
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 trans-
fer, 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
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
8
2 +
e x a m p l e s , B a M g A l
O
: E u
a n d
1 0
1 7
1
GdMgB5O10:Ce3+,Tb3+,Mn2+ 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
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 con-
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
Received: July 15, 2016
Revised: September 19, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.cgd.6b01056
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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-4-
yl)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 carboxy-
lates,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 nitrogen-
donor ligands, namely, tetrazoles. Tetrazole-containing linkers
have been the target of many research interests, due to their
strong electron-donating ability and rich coordination chem-
istry, 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
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
1
weak; w, weak; m, medium; s, strong; vs, very strong. The H 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
1
following, the H 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,
Aiming at isolating CPs potentially featuring luminescence
properties, we prepared the luminescent ligand xylene-bis(2H-
tetrazol-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 proper-
ties. As far as flexible bis(azolato)-based ligands are concerned,
we have recently shown that 4,4′-bis((3,5-dimethyl-1H-pyrazol-
4-yl)methyl)biphenyl (H2DMPMB)43 (Scheme 1) and 1,4-
bis((3,5-dimethyl-1H-pyrazol-4-yl)methyl)benzene
(H2BDMPX)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
B
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Article
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),
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.
1
1659 (m), 1580 (m), 1520 (m) ν(NN, CC + CN). H 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. Polycrystal-
line 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
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, P1, 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 (VT-
PXRD). 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 delta-
like 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
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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:
Scheme 2. Synthetic Path to Xylene-Bis(2H-tetrazol-5-yl)
(H2BTZPX), as Optimized in the Present Work
N
⎡
⎤
t − t0
y(t) = A
f exp⎢−
⎥ + y
∑
i
0
τ
⎣
⎦
i
(1)
i=1
In eq 1 A represents the zero-time amplitude of the fluorescence decay
pattern, the fi 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 point-
spread function, and compares the obtained numerical string with the
experimental decay histograms by minimizing the χ2 value with respect
to τi, fi, 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 point-
spread 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.
Scheme 3. Synthetic Path to the Three Bis(tetrazolato)-
Based CPs Described in the Present Work
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
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
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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 None-
theless, 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-5-
yl) (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, temper-
atures, 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
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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 c-
axis) 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,N3-
bridging 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.
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
Ag (BTZPX) crystallizes in the triclinic space group P1. The
̅
2
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 coordina-
tion 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%) exo-
tridentate.
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 penta-
atomic 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
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]
f 2
τ2 [ps]
f 3
τ3 [ps]
H2BTZPX
-
-
-
-
0.49 0.02
0.47 0.01
0.23 0.03
0.28 0.03
484
651
509
427
2
0.51 0.02
0.53 0.01
0.07 0.01
0.08 0.01
2585
3144
4
8
Zn(BTZPX)
Hg(BTZPX)
Ag2(BTZPX)
3
7
6
0.70 0.09
0.64 0.08
7
1
1
2503 72
2194 54
12
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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.
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
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)
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-di-
methyl-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.
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.
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,
Ag2(BTZPX) surpasses the thermal stability 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 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
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Figure 13. Representation of the electronic transitions defining the
seventh excited state (λ = 408.8 nm) of the fragment
Ag3(HBTZPX)3(TZ)22−: 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
2−
twelfth excited state (λ = 338.7 nm) of the fragment Zn(HBTZPX)4
:
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 1-
D 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.
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
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.
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
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.
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
3.4. Electronic Transitions Spectroscopy. The absorp-
tion spectral line-shapes of H2BTZPX, 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 red-
shifts in the absorption maxima, particularly when concomitant
with the onset of excited-state proton transfer mecha-
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 metal-
to-metal distance [3.0562(2) Å] emerging from the PXRD
results. However, the complex panorama of the spectral line-
shapes, together with the multiexponential and metal-depend-
ent 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
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when λobs,1 is chosen as the observation wavelength, while it
(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 three-
exponential, 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).
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 interconver-
sion 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
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.
E340
.
Before tackling the bathochromic shift, preliminary calcu-
lations 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 non-
negligible 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 intermo-
lecular 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
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 time-
resolved 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
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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.
(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)+, Zn2(MeTZ)73−, Zn-
3−
(HBTZPX)42−, and Zn2(HBZMeTZ)7
(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
2−
transitions for the Zn(HBTZPX)4 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
4−
Hg4(MeTZ)12 (see Supporting Information, Figure S12).
The calculations on these fragments (see Figure 16 for
Hg4(MeTZ)124−) only yielded high-energy electronic transi-
tions, 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 correspond-
ing 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.
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.
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 fluores-
cence 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 mercu-
rophilic interactions in Hg(BTZPX).
As for the Ag(I) derivative, five models were generated,
2−
namely, Ag(HBTZPX), Ag2(BTZPX), Ag(HBTZPX)3
,
2−
Ag2(HBTZPX)42−, and Ag3(HBTZPX)3(TZ)2 (see Support-
ing 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
r e p r e s e n t a t i v e e l e c t r o n i c t r a n s i t i o n s f o r
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)
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.cgd.6b01056.
■
S
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 measure-
ments carried out in solution; representation of the
model fragments built up for the TD-DFT calculations
(PDF)
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Accession Codes
9232−9242.
CCDC 1493595−1493596 and 1493773 contain the supple-
mentary 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 data_request@ccdc.cam.ac.uk, or by
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: aurel.tabacaru@unicam.it.
*E-mail: simona.galli@uninsubria.it.
Notes
■
Chem. Soc. Rev. 2012, 41, 115−147.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
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S.G. and E.L. acknowledge partial funding by Universita
dell’Insubria. CP acknowledges University of Camerino and
COST Action CM 1302 (SIPs). Angelo Maspero, Universita
dell’Insubria, is acknowledged for experimental help.
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(85) The calculations were carried out on (neutral or negatively
charged) penta-atomic rings only. Indeed, as a first approximation, the
influence of the central benzene ring on the residual charges can be
neglected, due to the absence of conjugation between benzene and
tetrazoles within the H2BTZPX ligand.
(86) In the conceived models, protons have been added to terminal
tetrazole rings in order to simulate the missing metal centres.
(87) No calculated transitions are available to interpret the
absorption maximum experimentally observed at ≈340 nm as, due
to high computational costs, our calculations were stopped at the
twelfth excited state (λ = 396.1 nm), which is composed by the
transitions HOMO-3 → LUMO+1 and HOMO-4 → LUMO+1.
̈
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(69) The torsion angle Ctetrazolate-CH2-CH2-Ctetrazolate is 151.3°, while
the angles between the root-mean-square planes defined by the two
tetrazolate rings and the phenyl ring amount to 66.4 and 89.9°.
(70) The search was performed excluding, from the potential hits,
those having errors and containing ions, and those referring to the
same phase but derived from data acquired in different experimental
conditions.
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(72) The torsion angle Ctetrazolate-CH2-CH2-Ctetrazolate is obviously
180°, while the angle between the root-mean-square planes defined by
the tetrazolate and phenyl rings amounts to 76.9°.
O
DOI: 10.1021/acs.cgd.6b01056
Cryst. Growth Des. XXXX, XXX, XXX−XXX