Synthesis and characterization of luminescent zinc(II) complexes with a N,N-bidentate 1-pyridylimidazo[1,5-a]pyridine ligand

Article abstract of DOI:10.1016/j.poly.2015.02.005

Ligand 2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol (L) has been reacted in a 2:1 molar ratio with different Zn(Y)₂nH₂O salts, where Y represents a polyatomic anion (Y = ClO₄^-, NO₃^-, BF₄^-, PF₆^-). The X-ray structural analysis performed on two of the derivatives obtained, namely [Zn(L)₂(ClO₄)](ClO₄)·H₂O (1) and [Zn(L)₂(CH₃CN)₂](BF₄)₂ (6), proves that as expected ligand L binds in a N,N-bidentate mode via the pyridine ring (N_py) and the pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton (N_im). A spectroscopic investigation (including infrared, elemental analysis, ¹H and ³¹P NMR, thermogravimetric analysis) has been performed on these species, allowing complete characterizations. A preliminary survey on their photophysical properties in the solid state was carried out: the complexes showing intense luminescence when excited with UV light (λ_exc = 421-469 nm) with λ_max of emission in the blue-green range (468-500 nm) and moderate absolute photoluminescence quantum yields (PLQYs).

Full text of DOI:10.1016/j.poly.2015.02.005

Accepted Manuscript
Synthesis and characterization of luminescent zinc(II) complexes with a N,N-
bidentate 1-pyridylimidazo[1,5-a]pyridine ligand
G. Attilio Ardizzoia, Stefano Brenna, Sara Durini, Bruno Therrien
PII:
S0277-5387(15)00084-4
DOI:
http://dx.doi.org/10.1016/j.poly.2015.02.005
POLY 11187
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
24 November 2014
7 February 2015
Please cite this article as: G. Attilio Ardizzoia, S. Brenna, S. Durini, B. Therrien, Synthesis and characterization of
luminescent zinc(II) complexes with a N,N-bidentate 1-pyridylimidazo[1,5-a]pyridine ligand, Polyhedron (2015),
doi: http://dx.doi.org/10.1016/j.poly.2015.02.005
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Synthesis and characterization of luminescent zinc(II) complexes with a N,N-
bidentate 1-pyridylimidazo[1,5-a]pyridine ligand
G. Attilio Ardizzoia,[a] Stefano Brenna,[a]* Sara Durini,[a] Bruno Therrien[b]
Keywords: Zinc(II); Crystal structure; N ligands; Pyridylimidazo[1,5-a]pyridine; Photoluminescence
[a] Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria, Via Valleggio, 11 -
22100 Como, Italy
[b] Institute of Chemistry, Université de Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel,
Switzerland
Corresponding author: Stefano Brenna
Tel.: +39-(0)31-2386476; Fax: +39-(0)31-2386119; e-mail address: stefano.brenna@uninsubria.it
POLY-D-14-01040
1

Abstract
Ligand 2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol (L) has been reacted in a 2:1 molar
-
-
ratio with different Zn(Y)2nH2O salts, where Y represents a polyatomic anion (Y = ClO4 , NO3 ,
-
-
BF4 , PF6 ). The X-ray structural analysis performed on two of the derivatives obtained, namely
[Zn( )2(ClO4)](ClO4)·H2O ( ) and [Zn( )2(CH3CN)2[(BF4)2 ( ), proves that as expected ligand
binds in a N,N-bidentate mode via the pyridine ring (Npy) and the pyridine-like nitrogen of the
imidazo[1,5- ]pyridine skeleton (Nim). A spectroscopic investigation (including infrared, elemental
L
1
L
6
L
a
analysis, 1H and 31P NMR, thermogravimetric analysis) has been performed on these species,
allowing complete characterizations. A preliminary survey on their photophysical properties in the
solid state was carried out: the complexes showing intense luminescence when excited with UV
light (λexc = 421-469 nm) with λmax of emission in the blue-green range (468-500 nm) and moderate
absolute photoluminescence quantum yields (PLQYs).
1. Introduction
Transition metals based luminescent compounds are receiving increasing attention due to their
applications for solid state optical devices, such as organic light emitting diodes (OLEDs) [1] and
dye sensitized solar cells (DSSCs) [2] or for fluorescence sensing [3]. Most of the complexes used so
far are based on heavy second- or third-row transition metals, such as Pt(II) [4], Ir(III) [5], Ru(II) [6]
or Re(I) [7]. Unfortunately, these metals are quite rare and expensive, and consequently a crucial aim
in this field is the synthesis of luminescent compounds based on less expensive and more abundant
transition metals. Among other metal centers, those with d10 configuration proved to satisfy this goal
[8] and reports on OLEDs based on M(I) (M = Cu, Ag, Au) [9] or M(II) (M = Zn, Cd) [10] already
appeared in the literature. In this regard, Group 12 metals including both mononuclear [11] and
polynuclear [12] compounds have revealed high luminescent performances and especially
compounds containing neutral N-based ligands have showed great promises. Relevant examples of
M(N∩N)X2 systems (M = Zn, Cd; N∩N = bidentate nitrogen ligand; X = halide) with luminescent
properties have been published. Wang has coordinated substituted 2,2’-bipyrimidines [13] and 2,2’-
POLY-D-14-01040
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dipyridylamino derived ligands [14] to ZnCl2. Other functionalized dipyridylamines have been
recently used by Song [15] and Schindler [16]. Similarly, Ghosh [17] reported on the structure and
photoluminescence behavior of Zn(II) and Cd(II) atropoisomers with unsymmetrical diimines, while
very recently Larionov synthesized luminescent Zn(II) and Cd(II) complexes containing
pyridophenazine derivatives [18]. In addition, the luminescent properties of bi- and tri-substituted
[Zn(N-N)n]2+ compounds (n = 2, 3) have been deeply investigated, with N-N being phenantroline
[19], 2,2'-bipyridyl [20], pyridyl-imidazole [21] or functionalized terpyridine [22].
In the past, we widely explored the coordination chemistry of N-based multidentate ligands [23],
with special attention to pyrazole and its analogues (imidazole, triazole) both in their neutral [24] and
anionic azolate forms [25]. The corresponding transition-metal complexes have found applications in
catalysis, material science or in the synthesis of polynuclear systems. Continuing our investigation
on N-based ligands, we lately conducted a study on the photophysical properties of zinc(II)
compounds with 3-aryl-substituted 1-pyridylimidazo[1,5-
imidazo[1,5- ]pyridine skeleton are known to possess unique photophysical properties [26] with
potential applications in the field of OLEDs [27] and organic thin-layer field effect transistors (FET)
[28]. Moreover, different 1-pyridylimidazo[1,5- ]pyridines have been coordinated to transition
a]pyridine ligands. Heterocycles having the
a
a
metals giving rise to highly luminescent coordination compounds based on Re(I) [29], Zn(II) or
Cd(II) [30] and Ru(II) or Os(II) [31] metal centers. In a recent paper, we reported on the blue
emission of Zn(II) complexes obtained via coordination of two 1-(pyridin-2-yl)imidazo[1,5-
a
]pyridinyl ligands, namely 1-(pyridin-2-yl)-3-
yl)imidazo[1,5- ]pyridin-3-yl)phenol, to ZnX2 fragments (X = Cl, Br, I) [32]. Herein, we reacted
ligand 2-(1-(pyridin-2-yl)imidazo[1,5- ]pyridin-3-yl)phenol ( ) with Zn(Y)2 salts, where Y
o-tolylimidazo[1,5-a]pyridine and 2-(1-(pyridin-2-
a
a
L
-
-
-
-
represents a polyatomic anion (Y = ClO4 , NO3 , BF4 , PF6 ). The synthesis and spectroscopic
characterization of these compounds are reported and show the zinc center surrounded by two
molecules of ligand
L. A preliminary exploration of the photoluminescent properties of these species
in the solid state was also performed: a bathochromic shift with respect to free ligand is observed for
POLY-D-14-01040
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all the derivatives investigated, intra-ligand π-π* transitions being responsible for fluorescence, as
-chelating ligand coordinated to a closed shell Zn2+ ion.
expected for a N,N
2. Results and discussion
2.1 Syntheses and spectroscopic characterization
2-(1-(Pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol (L) was obtained as a light yellow powder according to
1
a published procedure [33] and its purity was assessed by H, 13C{1H} NMR and elemental analysis.
Complexes 1, 2 and 3 have been prepared following similar procedures (Scheme 1), by reacting ligand L
with the corresponding zinc(II) salt, namely Zn(ClO4)2·6H2O, Zn(NO3)2·6H2O and Zn(BF4)2·nH2O, in a 2:1
ligand:metal ratio, in methanol, at room temperature. They all are yellow crystalline powders and have been
characterized by elemental analysis, infrared spectroscopy and thermogravimetric analysis (TGA). They are
formulated respectively as [Zn(L)2(ClO4)](ClO4)·H2O (1), [Zn(L)2(NO3)](NO3)·H2O (2) and
[Zn(L)2(H2O)2](BF4)2·2H2O (3).
Scheme 1. Syntheses of compounds 1-5.
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On the contrary, the direct reaction of Zn(PF6)2·nH2O (prepared as described in the experimental) with two
equivalents of L, following the aforementioned procedure, did not generate complex [Zn(L)2(H2O)](PF6)2
(4). Actually, the product isolated after workup was later formulated as [Zn(L)2(PO2F2)](PF6) (5) in
accordance with the elemental analysis and 31P{1H} NMR evidences (Scheme 1). The water molecules in the
-
-
starting zinc(II) hexafluorophosphate salt caused partial hydrolysis of the PF6 to PO2F2 [34]. Consequently,
a different approach was followed to prepare pure [Zn(L)2(H2O)](PF6)2 (4): first, a methatetic exchange
reaction was performed (Scheme 1, A) by treating a suspension of complex [Zn(L)2(NO3)](NO3)·H2O (2) in
-
acetonitrile with two equivalents of KPF6. Unfortunately, incomplete NO3 /PF6- substitution occurred, and
compound 4 was recovered in very low yield (<10%). Thus, despite silver hexafluorophosphate being known
to undergo partial hydrolysis [30b], a reaction involving AgPF6 was attempted (Scheme 1, B). This salt was
added to a dichloromethane solution of compound [Zn(L)Cl2] (prepared as described in Ref. 32), resulting in
the formation of insoluble AgCl and a solution of 4, which was isolated as a yellow powder after removal of
the solvent and recovery with diethylether.
In their infrared spectrum (nujol mull, Figures S1-S5 in Supporting Information) all zinc derivatives show
the typical pattern of the ligand, with sharp and intense stretching bands in the region 1614-1516 cm-1 and an
additional band of medium intensity around 1640 cm-1 (υC=N). Furthermore, the infrared spectra of
compounds 1-5 show a very broad band at about 3300 cm-1 due to the OH vibration, and a broad stretching
in the region 1300-1000 cm-1 attributable to the counteranions.
The 1H NMR spectra (CD3C(O)CD3, 25°C, Figures S6-S10 Supporting Information) of complexes 1-5 show
the expected pattern of ligand L, with some shifts due to the N,N-coordination to the metal center. Low-
intensity additional peaks associated to free ligand are also detected, thus suggesting a possible lability for
these complexes, with ligand L partially dissociated in solution. The 31P{1H} NMR spectrum of compound
[Zn(L)2(H2O)](PF6)2 (4) (Figure S11, top) shows the expected septet (1JPF = 708 Hz) due to uncoordinated
hexafluorophosphate. As previously mentioned, treatment of Zn(PF6)2 hydrate with two equivalents of ligand
L afforded the complex [Zn(L)2(PO2F2)](PF6)2 (5), which showed a different 31P NMR pattern. Actually, in
-
-
accordance with other PF6 − PO2F2 containing transition metals compounds [30], the spectrum recorded for
5 consists of two sets of distinct sharp signals, respectively centered at -17.9 ppm and -144.4 ppm (Figure
POLY-D-14-01040
5

S11, bottom). The upper field signal appears as a septet (1JPF = 706 Hz) and is associated to uncoordinated
-
-
PF6 , whereas the lower field signal is a triplet (1JPF = 950 Hz) and attributed to µ-PO2F2 .
The presence of water molecules in the complexes have been confirmed by thermogravimetric analysis
(TGA, Figures S12-S15 in Supporting Information). In the TGA plot recorded for [Zn(L)2(ClO4)](ClO4)·H2O
(1) (Figure S12) a weight loss of 1.95% is detected at about 60°C and associated to the presence of one water
molecule (calculated m/m loss = 2.10%) in the crystal packing of 1. This feature was eventually confirmed
by single crystal X-ray analysis performed on 1. For compound [Zn(L)2(H2O)2](BF4)2·2H2O (3), the TGA
trace (Figure S14) shows two consecutive and distinct losses (4.12%), respectively at about 100°C and
260°C, associated to the presence of four water molecules (in accordance to elemental analysis). Most
probably, two different kind of water molecules interact with the complex: the first two molecules, lost at
100°C (calculated m/m loss
= 4.06%) likely belong to the outer coordination sphere of
[Zn(L)2(H2O)2](BF4)2·2H2O. It is then necessary to heat the sample up to 260°C [35] to lose the two
remaining water molecules in the first, inner coordination sphere around the metal center (calculated m/m
loss = 4.23%). Also for compounds [Zn(L)2(NO3)](NO3)·H2O (2) (drop: -2.47% (experimental) vs. -2.30%
(calculated), Figure S13) and [Zn(L)2(H2O)](PF6)2 (4) (drop: -1.99% (experimental) vs. -1.90% (calculated),
Figure S15) the experimental data are in good agreement with the expected values.
2.2 X-ray crystal structures
Complex [Zn(L)2(ClO4)](ClO4)·H2O (1) was characterized via single-crystal X-ray diffraction analysis
(Figure 1). In 1, the zinc center shows a penta-coordination, with a N4O environment deriving from two
molecules of ligand L, each binding in a N,N-bidentate mode via the pyridine moiety (Npy) and the pyridine-
like nitrogen of the imidazol part (Nim) of the molecule. The fifth coordination site is occupied by one
-
oxygen atom of a perchlorato ligand, the other ClO4 anion lying in the outer coordination sphere of the
metal. The zinc atom shows a value for the trigonality index τ = 0.73 [36], thereby suggesting that the
geometry around the metal is mainly described as trigonal bipyramidal. The two Zn-Nim distances (Zn(1)-
N(2) = 2.024(5) Å; Zn(1)-N(5) = 2.049(4) Å) are slightly shorter than the Zn-Npy distances (Zn(1)-N(1) =
-
2.097(5) Å; Zn(1)-N(4) = 2.070(5) Å), while the Zn(1)-O(3) (ClO4 ) bond distance is 2.229(4) Å. All
POLY-D-14-01040
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parametrical data agree with those found in the literature for zinc(II) complexes having an analogous N4O
environment [37]. Dihedral angles within each pyridyl-imidazo[1,5-a]pyridine molecule confirm the nearly
planar configuration expected for this skeleton, N(1)-C(5)-C(6)-N(2) and N(4)-C(23)-C(24)-N(5) measuring
respectively 2.4(7)° and 11.1(7)°. Finally, the plane of the phenol substituent is twisted with respect to the
plane of the imidazo[1,5-a]pyridine moiety by 55.8(2)° and 57.3(2)°, respectively. In Figure 1, a hydrogen
bond (dashed line) between the hydroxyl group of ligand L and a water molecule is highlighted, the O-H···O
distance being 2.658(7) Å with an O-H···O angle of 177.5°.
-
Figure 1. ORTEP representation of [Zn(L)2(ClO4)](ClO4)·H2O (1) at 50% probability level (uncoordinated ClO4 anion
omitted for clarity). For selected bond lengths (Å) and angles (º) see Supporting Information (Table T1).
Also in the case of the tetrafluoborate derivative 3, it was possible to perform an X-ray structural
investigation. Unfortunately, all attempts to grow single crystals of [Zn(L)2(H2O)2](BF4)2·2H2O (3) in non
coordinating solvents failed, and single crystals suitable for X-ray analysis were only obtained by the slow
diffusion of diethylether into an acetonitrile solution of [Zn(L)2(H2O)2](BF4)2·2H2O (3). Therefore solvent
molecules are now replacing the coordinated water molecules from compound 3, thus the species structurally
characterized can be formulated as [Zn(L)2(CH3CN)2](BF4)2 (6).
POLY-D-14-01040
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As expected, in this complex the two molecules of ligand L are N,N-coordinated to the metal center (Figure
2), with Zn-N distances measuring respectively 2.128(3) Å (Zn(1)-N(1)) and 2.169(3) Å (Zn(1)-N(2)). The
octahedral environment around the metal is then filled by the abovementioned acetonitrile molecules (dZn-N
=
2.185(3) Å) (Figure 2). The structure is stabilized by the presence of H-bonding interactions between one
-
fluorine atom of a BF4 and the hydroxo substituent on ligand L (dF···O = 2.687(5) Å).
Figure 2. ORTEP representation of complex [Zn(L)2(CH3CN)2](BF4)2 (6) at 50% probability level (BF4- anions omitted
for clarity). For selected bond lengths (Å) and angles (º) see Supporting Information (Table T1).
2.3
Photoluminescence of complexes 1-5
Due to the partial dissociation of ligand L in solution, as observed in the 1H NMR spectra of the complexes,
their luminescent properties were only investigated in the solid state. Preliminary data obtained for the
complexes and for the free ligand L are collected in Table 1, while their emission spectra are presented in
Figure 3.
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Table 1. Photoluminescent properties of ligand L and compounds 1-5, in the solid state.
λexc (nm)
402
λmax (nm)
452
Y
ΦPL
0.15a
0.14
0.16
0.09
-
L
1
ClO4
NO3
BF4
452
493
421
468
2
3b
451
491
PF6
452
494
0.12
0.17
4
PO2F2
469
500
5
a
b
Taken from ref 32. Values refer to measurements performed on compound
molecules as isolated after synthesis, namely [Zn( )2(H2O)2](BF4)2·2H2O.
3 with water
L
Figure 3. Normalized emission spectra of ligand L and of complexes 1-5, recorded in the solid state.
In the solid state, all complexes show similar emission profiles, positioned in the blue-green region (468-500
nm): such a behavior can be easily rationalized in terms of intra-ligand (π-π*) transitions, as already
documented for analogous systems [32, 19a].
In principle, it is not excluded that compounds 1-5 could exhibit multiple fluorescence emission maxima
when excited at different excitation wavelength, thus studying of compound fluorescence with single
wavelength excitation could lead to omission of potentially important maxima. Therefore three-dimensional

spectra (i.e., the intensity as a function of excitation wavelength and emission wavelength) for ligand L and
zinc derivatives 1-5 were studied, by collecting emission spectra (300-680 nm) registered at different
excitation wavelengths (220-295 nm). The results are reported in the Supporting Information (Figures S16-
S21). In our case, the 3D spectra did not add further information to those previously recorded with single
excitation, being all characterized over the entire range of excitation by one solitary emission maximum,
localized at the same wavelength as obtained before. Nonetheless, this somehow confirms the ligand-
centered (LC) nature of the electronic transition responsible for emission in all complexes investigated.
According to literature data [38] for zinc complexes with N,N-bidentate ligands (phen, bpy), the emission
maxima of compounds 1, 3, 4 and 5 are red shifted by about 40-48 nm with respect to free L, while a
different behavior is noticed for compound [Zn(L)2(NO3)](NO3)·H2O (2), displaying a remarkably minor
bathochromic shift (16 nm). This is somehow also reflected by the excitation spectra of the complexes,
where the trace associated to the nitrato-derivative is clearly characterized by a diverse shape and maximum
peak (λexc = 421 nm) compared to derivatives 1, 3, 4 and 5 (all showing a λexc in the region 451-469 nm).
When measured as powder samples, species 1-5 exhibited moderate absolute photoluminescent quantum
yields (ΦPL), with experimental values comprised between 0.09 and 0.16, slightly lower than those found for
the previously described [Zn(L)X2] derivatives [32]. This is in agreement with the behavior encountered for
[Zn(N-N)n]2+ systems (n = 1, 2 or 3; N-N = phen, bpy) when compared to the corresponding halido species
[Zn(N-N)X2], the latter showing stronger fluorescence which decreases upon increasing the number of aza-
bidentate molecules around the metal [20].
As a further proof, an additional experiment was conducted: complex [Zn(L)2(ClO4)](ClO4)·H2O (1) was
cautiously oven-dried overnight at 120°C to remove water from the lattice. A yellow crystalline compound
was obtained, later formulated as [Zn(L)2(ClO4)](ClO4) (7) according to elemental analysis and
spectroscopic evidences. The absolute quantum yield of this de-hydrated species 7 was measured, giving an
experimental value of 0.11, very close to that found for its hydrated counterpart [Zn(L)2(ClO4)](ClO4)·H2O
(1) (0.14, entry 2 in Table 1). Then, a possible relation between the presence of water molecules in
complexes 1-5 and their moderate ΦPL was unambiguously excluded, yet again confirming the
abovementioned explanation.
POLY-D-14-01040
10

3. Conclusion
The N,N-bidentate 2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol ligand (L) have been
coordinated to zinc(II) centers in a 2:1 molar ratio, obtaining a series of blue-green emitting complexes
-
-
-
-
containing polyatomic anions (ClO4 , NO3 , BF4 , PF6 ). A full spectroscopic characterization of these species
has determined the water content in each compound, also confirmed by the single crystal X-ray structure
analysis of two derivatives. In the solid state, these species showed blue-green emission with a 40-48 nm red
shift (λmax = 491-500 nm) with respect to the free ligand (λmax = 452 nm). A slightly different behavior was
noticed for the nitrato derivative 5, displaying a remarkably minor bathochromic shift (16 nm) compared to
L. Complexes 1-5 displayed absolute photoluminescent quantum yields (ΦPL) comprised between 0.09 and
0.16, slightly inferior compared to previously reported halido species [Zn(L)X2] (X = Cl, Br, I). This
behavior was attributed to the increasing number of N,N-bidentate ligands L coordinating to zinc.
4. Experimental section
4.1 General remarks
Caution! Although we have experienced no difficulties, perchlorate salts of metal complexes with
organic ligands are potentially explosive and should be handled with care even in small quantities.
All reactions were carried out under inert atmosphere using standard Schlenk techniques. Solvents
were deoxygenated with nitrogen prior to use. NMR spectra were recorded with an AVANCE 400
1
31
Bruker spectrometer at 400 MHz for H NMR and 162 MHz for P{1H} NMR. Chemical shifts were
assigned as δ values in ppm relative to residual solvent peaks as the internal reference. Elemental
analyses were performed with a Perkin-Elmer CHN Analyzer 2400 Series II. Infrared Spectra were
acquired on a Shimadzu Prestige 21 instrument with a 1-cm-1 resolution. Thermogravimetric
analyses (TGA) were performed in a N2 stream on a Netzsch STA 409 PC Luxx (heating rate
10°C/min). For each sample, the end point of analyses was chosen according to its decomposition
temperature. Excitation and emission spectra were recorded on a Photon Technologies International
QuantaMaster QM-40 spectrometer (equipped with Xe arc lamp, 70 W) and were corrected for the
wavelength response of the instrument. Absolute fluorescence quantum yields were determined
POLY-D-14-01040
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using a PhotoMed GmbH K-Sphere Integrating Sphere (3.2 inch. diameter). All reagents used in the
syntheses (SigmaAldrich) were of commercial grade and used as received. Ligand 2-(1-(pyridin-2-
yl)imidazo[1,5-a]pyridin-3-yl)phenol (L) was prepared according to a published procedure [33].
Zn(BF4)2·xH2O and Zn(PF6)2·xH2O [39] were prepared by stirring Zn(NO3)2·6H2O with two
equivalents of NaBF4 and KPF6, respectively, in acetonitrile at room temperature. The white solid
(NaNO3 or KNO3) was filtered off, and the filtrate was evaporated to dryness to give the zinc(II)
starting material.
4.2 Synthesis of [Zn(L)2(ClO4)](ClO4)·H2O (1)
To a suspension of L (200 mg, 0.696 mmol) in methanol (15 mL), Zn(ClO4)2·6H2O (130 mg, 0.349 mmol)
was added and the resulting suspension was stirred at room temperature for 1 h. During this time a clear
yellow solution formed, which was further stirred for other 3 h. Then the solvent was removed under reduced
pressure, the residue was suspended in diethylether (10 mL) and the yellow solid was filtered off, washed
with diethylether (10 mL) and dried under vacuum. Yield: 215 mg (71%). IR (nujol, ν/cm-1): 3354 br (OH),
1
1639m, 1613s, 1567m, 1550m, 1517m, 1132-1099br (ClO4). H NMR (400 MHz, CD3C(O)CD3, 25°C): δ
6.62 (td, JH,H = 7.6 Hz, JH,H = 1.1 Hz, 1H, Ar-H), 6.85 (dd, JH,H = 8.4 Hz, JH,H = 1.1 Hz, 1H, Ar-H), 7.13 (m,
2H, Ar-H), 7.21 (ddd, JH,H = 8.2 Hz, JH,H = 7.3 Hz, JH,H = 1.7 Hz, 1H, Ar-H), 7.34 (ddd, JH,H = 7.6 Hz, JH,H
=
5.3 Hz, JH,H = 1.1 Hz, 1H, Ar-H), 7.49 (ddd, JH,H = 9.6 Hz, JH,H = 6.6 Hz, JH,H = 1.0 Hz, 1H, Ar-H), 8.06 (dt,
JH,H = 7.4, JH,H = 1.1 Hz, 1H, Ar-H), 8.14 (td, JH,H = 7.8 Hz, JH,H = 1.7 Hz, 1H, Ar-H), 8.30 (m, 1H, Ar-H),
8.40 (d, JH,H = 9.5 Hz, 1H, Ar-H), 8.51 (d, JH,H = 5.1 Hz, 1H, Ar-H), 9.15 (s, 1H, OH). Anal. Calc. for
C36H28Cl2N6O11Zn requires: C, 50.46; H, 3.29; N, 9.81. Found: C, 50.51; H, 3.23; N, 9.86%. Single crystals
suitable for X-ray analysis were obtained by slow diffusion of pentane in a saturated solution of 1 in methyl
ethyl ketone.
POLY-D-14-01040
12

4.2
Synthesis of [Zn(L)2(NO3)](NO3)·H2O (2)
To a suspension of L (200 mg, 0.696 mmol) in methanol (15 mL), Zn(NO3)2·6H2O (104 mg, 0.349 mmol)
was added and the resulting suspension was stirred at room temperature for 4 h. Then it was filtered and the
yellow solid was washed with methanol (5 mL), diethylether (10 mL) and dried under vacuum. Yield: 156
mg (57%). IR (nujol, ν/cm-1): 3141br (OH), 1642m, 1613s, 1566m, 1548m, 1516m, 1341m (NO3), 1290s
1
(NO3). H NMR (400 MHz, CD3C(O)CD3, 25°C): δ 6.58 (dd, JH,H = 6.6 Hz, JH,H = 1.6 Hz, 2H, Ar-H), 6.64
(d, JH,H = 8.5 Hz, 2H, Ar-H), 6.97 (m, 2H, Ar-H), 7.15 (dd, JH,H = 7.9 Hz, JH,H = 1.8 Hz, 1H, Ar-H), 7.24 (t,
JH,H = 6.9 Hz, 1H, Ar-H), 7.81 (t, JH,H = 7.7 Hz, 1H, Ar-H), 7.93 (d, JH,H = 9.3 Hz, 1H, Ar-H), 8.06 (t, JH,H
=
8.8 Hz, 1H, Ar-H), 8.32 (d, JH,H = 6.0 Hz, 1H, Ar-H), 9.19 (br, 1H, OH). Anal. Calc. for C36H28N8O9Zn
requires: C, 55.29; H, 3.61; N, 14.33. Found: C, 55.88; H, 3.48; N, 14.25%.
4.3
Synthesis of [Zn(L)2(H2O)2](BF4)2·2H2O (3)
To a suspension of L (200 mg, 0.696 mmol) in methanol (15 mL), Zn(BF4)2·xH2O (83 mg, 0.348 mmol,
anhydrous basis) was added and the resulting suspension was stirred at room temperature for 1 h. During this
time a clear yellow solution formed, which was further stirred for other 3 h. Then the solvent was removed
under reduced pressure, the residue was suspended in diethylether (10 mL) and the yellow solid was filtered
off, washed with diethylether (10 mL) and dried under vacuum. Yield: 248 mg (81%). IR (nujol, ν/cm-1):
1
3481br (OH), 1639m, 1612s, 1568m, 1549m, 1516m, 1053br (BF4). H NMR (400 MHz, CD3C(O)CD3,
25°C): δ 6.66 (t, JH,H = 7.5 Hz, 1H, Ar-H), 6.81 (d, JH,H = 8.3 Hz, 1H, Ar-H), 6.96 (d, JH,H = 7.7 Hz, 1H, Ar-
H), 7.05 (t, JH,H = 6.9 Hz, 1H, Ar-H), 7.25 (t, JH,H = 7.7 Hz, 1H, Ar-H), 7.41 (t, JH,H = 6.3 Hz, 2H, Ar-H),
7.47 (dd, JH,H = 9.6 Hz, JH,H = 6.6 Hz, 1H), 7.86 (d, JH,H = 7.2 Hz, 1H), 8.10 (dt, JH,H = 12.6 Hz, 7.9 Hz, 2H),
8.20 (d, JH,H = 9.5 Hz, 1H), 8.54 (d, JH,H = 5.4 Hz, 1H). Anal. Calc. for C36H34B2F8N6O6Zn requires: C,
48.82; H, 3.87; N, 9.49. Found: C, 48.91; H, 3.68; N, 9.99%.
4.4
Synthesis of [Zn(L)2(H2O)](PF6)2 (4)
Step A. To a solution of AgNO3 (1 g, 5.882 mmol) in CH3CN was added an equimolar amount of KPF6
(1.083 g, 5.883 mmol) and the resulting suspension was stirred at room temperature for 1 h. Then insoluble
POLY-D-14-01040
13

KNO3 was filtered off, and the solvent was removed under reduced pressure from the filtrate. The white
residue was suspended in diethylether and the solid was filtered, dried in vacuo and stored in a dry, dark
atmosphere.
Step B. To a solution of L (400 mg, 1.392 mmol) and [Zn(L)Cl2] [prepared as in Ref. 32] (590 mg, 1.392
mmol) in CH2Cl2 (10 mL), AgPF6 from step A (739 mg, 2.924 mmol) was added and suddenly a yellowish
suspension formed, which was stirred for 1 h. The mixture was separated by filtration affording a white solid
(AgCl) which was discarded, and a yellow solution. The solvent was removed under rotary evaporation, then
the crude product was suspended in diethylether (10 mL) and the yellow solid was filtered off, washed with
diethyl ether, dried and stored in a dry atmosphere. Yield: 1.216 g (94%). IR (nujol, ν/cm-1): 3507br (OH),
1607s, 1533m, 1516s, 1337m, 1295s, 846s (PF6). 1H NMR (400 MHz, CD3C(O)CD3, 25°C): δ 6.58 (t, JH,H
=
7.5 Hz, 1H, Ar-H), 6.83 (d, JH,H = 8.4 Hz, 1H, Ar-H), 7.08-7.06 (m, 2H, Ar-H), 7.19 (t, JH,H = 7.7 Hz, 1H,
Ar-H), 7.31 (t, JH,H = 6.7 Hz, 1H, Ar-H), 7.47 (m, 1H, Ar-H), 7.99 (d, JH,H = 7.2 Hz, 1H, Ar-H), 8.13-8.07
(m, 1H, Ar-H), 8.24 (d, JH,H = 8.2 Hz, 1H, Ar-H), 8.36 (d, JH,H = 9.5 Hz, 1H, Ar-H), 8.47 (d, JH,H = 5.4 Hz,
1H, Ar-H), 9.16 (s, 1H, OH). 31P{1H} NMR (162 MHz, CD3C(O)CD3, 25°C): δ -144.22 (hept, JP,F = 708.0
Hz). Anal. Calc. for C36H28F12N6O3P2Zn requires: C, 45.61; H, 2.98; N, 8.87. Found: C, 45.50; H, 2.82; N,
9.03.
4.5
Synthesis of [Zn(L)2(PO2F2)](PF6) (5)
To a suspension of L (200 mg, 0.696 mmol) in methanol (15 mL), Zn(PF6)2·nH2O (124 mg, 0.348 mmol, on
anhydrous basis) was added and the resulting suspension was stirred at room temperature for 1 h. During this
time a clear yellow solution formed, which was stirred for other 3 h. Then the solvent was removed under
reduced pressure, the residue was suspended in diethylether (10 mL) and the yellow solid was filtered,
washed with diethylether and dried under vacuum. Yield: 214 mg (69%). IR (nujol, ν/cm-1): 3490br (OH),
1607s, 1532m, 1332m, 1288m, 849s (PF6). 1H NMR (400 MHz, CD3C(O)CD3, 25°C): δ 6.61 (m, 1H, Ar-H),
6.83 (t, JH,H = 6.5 Hz, 1H, Ar-H), 6.95 (t, JH,H = 7.9 Hz, 1H, Ar-H), 7.06 (t, JH,H = 6.7 Hz, 1H, Ar-H), 7.12 (t,
JH,H = 6.7 Hz, 1H, Ar-H), 7.15-7.24 (m, 1H, Ar-H), 7.28-7.34 (m, 1H, Ar-H), 7.47 (m, 1H, Ar-H), 8.14 (t,
JH,H = 8.7 Hz, 1H, Ar-H), 8.30 (m, 1H, Ar-H), 8.40 (d, JH,H = 9.4 Hz, 1H, Ar-H), 8.47 (d, JH,H = 6.4 Hz, 1H,
POLY-D-14-01040
14

Ar-H), 9.18 (br, 1H, OH). 31P{1H} NMR (162 MHz, CD3C(O)CD3, 25°C): δ -144.26 (hept, JP,F = 706.4 Hz),
-16.52 (t, JP,F = 949.8 Hz). Anal. Calc. for C36H28F8N6O5P2Zn requires: C, 47.83; H, 3.12; N, 9.30. Found: C,
47.78; H, 3.17; N, 9.12%.
4.6 Synthesis of [Zn(L)2(ClO4)](ClO4) (7)
Complex [Zn(L)2(ClO4)](ClO4)·H2O (1) (100 mg, 0.117 mmol) was heated overnight at 120°C, in a dry
atmosphere. The resulting yellow powder was formulated as [Zn(L)2(ClO4)](ClO4) (7). Yield: 96 mg (98%).
IR (nujol, ν/cm-1): 3351br (OH), 1640m, 1615s, 1568m, 1550m, 1519m, 1130-1095br (ClO4). Anal. Calc. for
C36H26Cl2N6O10Zn requires: C, 51.54; H, 3.12; N, 10.02. Found: C, 51.35; H, 3.08; N, 9.97%.
4.7 Absolute quantum yields measurements
A PTFE-coated integrating sphere (PhotoMed GmbH) with 3.2 inches diameter was mounted into a Photon
Technologies International QuantaMaster QM-40. The top part of the sphere was lifted, and the solid sample
holder was accommodated into the sphere, with the quartz window facing the excitation light beam. The
measured luminescence spectra were background corrected by subtracting the spectrum obtained using a
blank substrate (Al2O3) and subsequently corrected for the wavelength sensitivity of the fluorimeter and the
spectral response of the sphere. This correction was applied to all subsequently recorded spectra in the
measurement of PLQY of samples. The integrated luminescence area were obtained by the software Felix32
Analysis (© 2001) and used to determine the absolute PLQY (ΦPL) (with uncertainties of 5%) according to
the equation below [40]:
Ei (λ) − (1− A) ⋅ E0 (λ)
Φ PL
=
Le (λ) ⋅ A
where
L0 (λ) − Li (λ)
L0 (λ)
A =
Ei(λ) = integrated emission under direct excitation of the sample; E0(λ) = integrated emission under
secondary excitation of the sample; Li(λ) = integrated excitation under direct excitation of the sample; L0(λ)
POLY-D-14-01040
15

= integrated excitation under secondary excitation of the sample; Le(λ) = integrated excitation profile for the
empty sphere.
4.8 Single-crystal X-ray structure analyses
Several attempts to grow crystals for the complexes were performed, with however little success. Only
single-crystals of complex 1 and of the acetonitrile analogue (6) of complex 3 were obtained, and even for
these two complexes the quality of the crystals was somehow limited. In both cases poor quality data were
obtained, despite using different crystals, from different batches, and after collecting data at different
temperature. Nevertheless, the best data sets were resolved and the results are presented here, which overall
confirm the proposed structures.
Crystals of complexes [Zn(L)2(ClO4)](ClO4)·H2O (1) and [Zn(L)2(CH3CN)2](BF4)2 (6) were mounted on a
Stoe Image Plate Diffraction system equipped with a φ circle goniometer, using Mo-Kα graphite
monochromated radiation (λ = 0.71073 Å) with φ range 0-200º. The structures were solved by direct
methods using the program SHELXS-97, while the refinement and all further calculations were carried out
using SHELXL-97 [41]. The H-atoms were found on Fourier difference map or included in calculated
positions and treated as riding atoms using the SHELXL default parameters. The non-H atoms were refined
anisotropically, using weighted full-matrix least-square on F2. Crystallographic details are summarized in
Table 2, while selected bond lengths and angles are listed in Table T1. Figures 1 and 2 were drawn with
ORTEP [42].
Table 2. Crystallographic and structure refinement parameters for complexes [Zn(L)2(ClO4)](ClO4)·H2O (1)
and [Zn(L)2(CH3CN)2](BF4)2 (6).
[Zn(L)2(ClO4)](ClO4)·H2O
C36H28Cl2N6O11Zn
856.91
[Zn(L)2(CH3CN)2](BF4)2
C40H32B2F8N8O2Zn
895.73
Chemical formula
Formula weight
Crystal system
Space group
Monoclinic
Monoclinic
C 2/c (no. 15)
Yellow block
C 2/c (no. 15)
Yellow rod
Crystal colour and shape
Crystal size
0.18 x 0.17 x 0.14
34.213(2)
0.21 x 0.20 x 0.18
17.8778(18)
a (Å)
POLY-D-14-01040
16

b (Å)
10.3413(3)
25.7849(19)
129.302(5)
7059.4(7)
8
20.5170(15)
13.6658(14)
119.331(7)
4370.0(7)
4
c (Å)
β (°)
V (Å3)
Z
T (K)
173(2)
173(2)
Dc (g·cm-3)
µ (mm-1)
Scan range (°)
Unique reflections
1.613
1.361
0.920
0.640
1.60 < θ < 29.29
1.64 < θ < 29.22
9577
5932
Observed refls [I>2σ(I)]
Rint
5326
0.2256
2844
0.1032
Final R indices [I>2σ(I)]*
R indices (all data)
Goodness-of-fit
0.1010, wR2 0.1760
0.1806, wR2 0.2062
1.069
0.0634, wR2 0.1469
0.1332, wR2 0.1690
0.853
Max, Min ∆ρ/e (Å-3)
1.103, -0.981
1.127, -0.665
2
2
2
2
* Structures were refined on F0 : wR2 = [Σ[w (F02 - Fc2)2] / Σw (F0 )2]1/2, where w-1 = [Σ(F0 ) + (aP)2 + bP] and P = [max(F0 , 0) + 2Fc2]/3
Supporting Information material
CCDC-1035129 [Zn(L)2(ClO4)](ClO4)·H2O (1) and 1035130 [Zn(L)2(CH3CN)2](BF4)2 (6) contain the
Supporting Information crystallographic data for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystallographic Data Centre, 12, Union
Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
Acknowledgments
This work was partially supported by University of Insubria (grant CSR-12).
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POLY-D-14-01040
20

Graphical abstract - Synopsis
Zinc(II) complexes with the N,N-bidentate 2-(1-(pyridin-2-yl)imidazo[1,5-a]pyridin-3-yl)phenol ligand (L)
-
-
-
-
and different polyatomic anions (ClO4 , BF4 , PF6 , NO3 ) have been synthesized and spectroscopically
charatcterized. In the solid state, these compounds show intense luminescence with λmax of emission in the
blue-green range and moderate absolute photoluminescence quantum yields.