Blue fluorescent zinc(II) complexes based on tunable imidazo[1,5-a]pyridines

Article abstract of DOI:10.1016/j.ica.2020.119662

Herein, we investigate the photophysical features of new zinc(II) chlorido complexes containing fluorescent 1,3-substituted imidazo[1,5-a]pyridines as ligands. The derivatives introduce electron-donating or electron-withdrawing moieties in position 3 on the ligand skeleton. The obtained compounds have been characterized with different spectroscopic techniques, their structure has been defined by single-crystal X-ray diffraction and mass spectrometry and their optical properties have been discussed in relation to the chemical structure. The comparison between the emission spectra of the free ligands and of the corresponding zinc(II) chlorido complexes shows an intense hypsochromic shift, due to the modification of the ligands conformation upon metal coordination, and a significant increase of the quantum yield after complexation.

Full text of DOI:10.1016/j.ica.2020.119662

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Research paper
Blue fluorescent zinc(II) complexes based on tunable imidazo[1,5-a]pyridines
Giorgio Volpi, Emanuele Priola, Claudio Garino, Andrea Daolio, Roberto
Rabezzana, Paola Benzi, Alessia Giordana, Eliano Diana, Roberto Gobetto
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DOI:
Reference:
S0020-1693(20)30482-5
https://doi.org/10.1016/j.ica.2020.119662
ICA 119662
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Inorganica Chimica Acta
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6 April 2020
9 April 2020
Please cite this article as: G. Volpi, E. Priola, C. Garino, A. Daolio, R. Rabezzana, P. Benzi, A. Giordana, E.
Diana, R. Gobetto, Blue fluorescent zinc(II) complexes based on tunable imidazo[1,5-a]pyridines, Inorganica
Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119662
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Blue fluorescent zinc(II) complexes based on tunable imidazo[1,5-a]pyridines
Giorgio Volpi,‡* Emanuele Priola,‡ Claudio Garino,* Andrea Daolio, Roberto Rabezzana, Paola
Benzi, Alessia Giordana, Eliano Diana and Roberto Gobetto
Department of Chemistry, NIS Interdepartmental Centre, University of Torino, Via Pietro Giuria 7,
10125, Torino, Italy
[‡] These authors contributed equally to this work
Keywords: imidazo[1,5-a]pyridine, luminescence, zinc complex, quantum yield, Stokes shift,
tunable emission.
Abstract
Herein, we investigate the photophysical features of new zinc(II) chlorido complexes containing
fluorescent 1,3-substituted imidazo[1,5-a]pyridines as ligands. The derivatives introduce electron-
donating or electron-withdrawing moieties in position 3 on the ligand skeleton. The obtained
compounds have been characterized with different spectroscopic techniques, their structure has been
defined by single-crystal X-ray diffraction and mass spectrometry and their optical properties have
been discussed in relation to the chemical structure. The comparison between the emission spectra of
the free ligands and of the corresponding zinc(II) chlorido complexes shows an intense hypsochromic
shift, due to the modification of the ligands conformation upon metal coordination, and a significant
increase of the quantum yield after complexation.
1. Introduction
The demand for optically efficient materials prompts researchers to design new fluorescent organic
compounds and luminescent metal derivatives. Their investigation is driven by the necessity to
control the chemical structure, the excited states and the photophysical behavior of the new
molecules. At this regard, coinage metal derivatives show a great potential, due to their tunable optical
properties and high luminescent performances.[1–3] In particular, low-cost coinage metals (such as
Cu, Zn, Fe and Al) are becoming the focus of an increasingly widespread field of research,[3–12] in
comparison with the most investigated and applied heavy second- or third-row transition metals (such
as Au, Pt, Ir, Ru or Re).[13–20]
Recently, zinc(II) complexes have been reported as rising electroluminescent materials for organic
light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs).[10,11,21] Indeed,
the optical behavior of zinc compounds, as emitters, is comparable with that of tris(8-
hydroxyquinolinato)aluminium (Alq3), a well know component of organic light-emitting diodes
(OLEDs).[11,22] In general, zinc(II) compounds may be important and convenient candidates to
improve the performances of optoelectronic devices as luminescent molecule in doped polymer films
or as fluorescence emitting dopants.
Herein, we report a study on the synthesis and optical characterization of three new blue luminescent
zinc(II) chlorido complexes based on 1,3-substituted-imidazo[1,5-a]pyridine ligands (Figure 1),
which contain both electron-donating and electron-withdrawing chemical groups. Fluorescent
imidazo[1,5-a]pyridine derivatives have been employed for many applications, principally as
bidentate ligands[23–28] and intense emitters.[29–41] In addition, they have been investigated for
biological applications[42–46] and for a wide range of potential pharmaceutical applications.[47–51]
1

In particular, the ligands employed in this work were originally developed to form luminescent Re
and Ir complexes[52–54] and to investigate the relationship between chemical structure and electronic
properties in ligands containing electron-donating or electron-withdrawing moieties in their skeleton.
In the present work, these ligands are combined with zinc(II), to improve their optical behavior
towards promising and useful technological applications, in agreement with the recent interest
addressed to the luminescent and inexpensive coinage metal derivatives. In order to increase the
optical properties, the choice of the ancillary chlorido ligands was based on a previous study
concerning the effect of the halides on similar complexes.[7] The main properties of the obtained
complexes include: high emission quantum yields and a remarkable blue-shift, opposite to what
generally reported for similar compounds, making these compounds unique among the zinc(II)
complexes reported so far.
Figure 1. Imidazo[1,5-a]pyridine ligands and corresponding Zn(II) complexes Zn(L)Cl2.
2. Results and discussion
2.1. Synthesis
The ligands L1, L2 and L3 were obtained as previously reported, by a direct one-pot cyclization
synthetic approach condensing di(2-pyridyl)ketone with benzaldehyde (L1), 4-
(trifluoromethyl)benzaldehyde (L2) or 4-tert-butylbenzaldehyde (L3) in acetic acid and ammonium
acetate.[27,52–54] Their purity was assessed by TLC, 1H and 13C NMR spectroscopy.
The Zn(II) complexes Zn(L1)Cl2, Zn(L2)Cl2 and Zn(L3)Cl2 were prepared by reaction of ZnCl2
with 1 equiv. of the corresponding ligand in methanol at room temperature. All the complexes are
yellow crystalline powders and were characterized by elemental analysis, mass spectrometry,
infrared, UV-visible and emission spectroscopy and single crystal X-ray diffraction. Due to the
extremely low solubility of the complexes in the common deuterated solvents, it was not possible to
perform their NMR characterization.
2.2 Structural characterization and topological-energetic packing analysis
Zn(L1)Cl2, Zn(L2)Cl2 and Zn(L3)Cl2 were successfully crystallized and their crystals investigated
by means of single crystal X-ray diffraction. The crystals were obtained by solvent diffusion of a
solution of the related ligand into a solution of ZnCl2.
2

Figure 2. ORTEP diagrams and numeration of asymmetric units of Zn(L1)Cl2 (a), Zn(L2)Cl2 form-a
(b), Zn(L2)Cl2 form-b (c) and Zn(L3)Cl2 (d), thermal ellipsoids drawn at 50% of probability.
Adding a dichloromethane solution of L1 to a methanolic solution of ZnCl2 the formation of several
colorless platelets of Zn(L1)Cl2 was observed after a day. This zinc complex crystallizes in the
monoclinic centrosymmetric P21/c space group and the asymmetric unit is formed by a single
molecule (Figure 2a).
Considering the para-substituted derivatives, the trifluoromethane derivative Zn(L2)Cl2 crystallizes
in two different solvate forms. Form-a was obtained adding a methanolic solution of ZnCl2 to a
dichloromethane solution of L2 (Figure 2b), while using an acetonitrile solution of the ligand led to
form-b (Figure 2c). The inclusion of the crystallization solvent in these crystals changes noticeably
the packing characteristics, although there are no evident changes in the molecular conformation.
With dichloromethane, Zn(L2)Cl2 crystallizes (form-a) in the monoclinic centrosymmetric P21/n
space group with two units of complex and one disordered solvent molecule included in the
asymmetric unit (Z’ = 3). The two molecules adopt almost the same geometry, and the absence of
crystallographic equivalence can be explained with crystal growth effects and with the difference in
surrounding interactions.[55] On the other hand, form-b shows triclinic P-1 symmetry, a single
molecule of Zn(L2)Cl2 in the asymmetric unit and is dominated by π···π stacking between inverted
molecules. Zn(L3)Cl2 was crystallized by adding a solution of L3 in diethyl ether to an acetonitrile
solution of ZnCl2. The complex adopts the triclinic P-1 space group, with one molecule in the
asymmetric unit (Figure 2d).
In all the four structures obtained, the Zn(II) coordination polyhedron is a distorted tetrahedron with
two chloride and two nitrogen coordination sites (τ4= 0.88 for Zn(L1)Cl2, 0.94 and 0.88 for the two
molecules of Zn(L2)Cl2 form-a, 0.87 for Zn(L2)Cl2 form-b and 0.87 for Zn(L3)Cl2, according to
the definition of Houser and coworkers[56]). In Zn(L1)Cl2, the ligand is almost planar, with the
phenyl substituent turned around the C13–C14 inter-ring bond by 49.16(10)°. This value is similar or
higher to those obtained for the two para-substituted compounds, varying from 41.02(7)° for
Zn(L3)Cl2 to 35.34(8)° in the case of Zn(L2)Cl2 (form-a and form-b). These values are much lower
than those obtained by Ardizzoia and others for Zn(II) complexes containing meta substituted
imidazo[1,5-a]pyridines,[7] thus demonstrating the possibility of unsubstituted and para-substituted
3

complexes to freely rotate around the intra-ring bond and the relation of this dihedral angle only to
steric or packing effects.
Considering the weak interactions that influence the packing environment of Zn(L1)Cl2, directional
C–H···Cl contacts forming a 2D pattern are present (Figure 3a, see Table S3 for distance values).
These interactions are evident in the representation of the shape index projected on the Hirshfeld
surface: for the rapid color change in the chlorine region, in the electron density modification around
some of the aromatic C–H fragments and in the derived fingerprint for the clear stripes (Figure 3b).
Between these planes, the molecules interact each other through π···π interactions, as confirmed by
red and blue triangles in the shape index representation (Figure 3c).
Figure 3. Packing plane formed by weak hydrogen bond interactions in the crystal structure of
Zn(L1)Cl2 (a). (b) C–H···Cl and (c) π···π stacking regions on the Hirshfeld surface and in the
decomposed fingerprint.
In the case of Zn(L2)Cl2 (form-a), the two Hirshfeld surfaces (Figures S7 and S9) highlight a stronger
C–H···Cl interaction for molecule 1 with respect to molecule 2 (this can be appreciated looking the
longer and narrower stripes in the fingerprints, Figure S8c and Figure S10c) and a modification in the
donor-acceptor ratio for the C–H···F interaction component. Considering the other contacts (Figure
S8 and Figure S10), it is worth noting some difference in the π···π stacking: in the packing geometry
of the crystal, in which coplanar inverted dimers are present and, perpendicular to those clusters, the
second molecule is inserted forming layers of organic matter separated by fluorine and chlorine
(Figure 4a). Solvent molecules are inserted in the interlayer without any strong directional interaction.
On the other hand, form-b shows triclinic P-1 symmetry, a single molecule of Zn(L2)Cl2 in the
4

asymmetric unit and is dominated by π···π stacking between inverted molecules. Looking at Figure
4b, it is possible to observe the formation of diagonal columns of parallel molecules (with an average
interplanar distance of 4.1 Å, but with intermolecular C–H···π distance as short as 2.919(10) Å),
shifted by the presence of trifluoromethane substituents and the insertion of acetonitrile molecules.
These columns are laterally connected to form layers, separated by a region of only dispersive
interactions, in the (100) plane. This modification of the packing compared to the previous crystal
(form-a) can be analyzed using the Hirshfeld surface components (Figure S12): in this case there are
stronger stacking interactions and also more directional C–H···Cl and C–H···F interactions, due to
the coupling between inverted molecules.
Complex Zn(L3)Cl2 crystallizes in the triclinic P-1 space group with one molecule in the asymmetric
unit. The geometry is similar to the previous ones, with a comparable dihedral angle between the
central ligand core and the phenyl substituent, suggesting an analogous steric hindrance of the para
substituent (although the tert-butyl substituent does not present the positional disorder observed in
the Zn(L2)Cl2 crystals). The packing is a complex interplay of C–H···Cl between couples of stacked
inverted dimers that form a 3D pattern and are reflected in the fingerprint of the molecule by longer
stripes, if compared to the previously analyzed cases (Figure S14).
Figure 4. (a) Packing projection along a axis in form-a of Zn(L2)Cl2 and (b) (100) plane of
molecules in form-b of Zn(L2)Cl2.
By analyzing the crystal packing of the four crystals from a topological point of view (considering
Van der Waals contacts as defining interactions), the underlying net of Zn(L1)Cl2 and Zn(L3)Cl2
crystals is a simple uninodal net with a molecular coordination number of 14 (calculated from
Voronoj-Dirichlet polyhedron, Figures S15-S18), as usually found for molecular compounds (point
symbol for the two cases: {345.463.512} and {345.468.57} respectively).[57,58] On the other hand, the
peculiarity of form-a and form-b of Zn(L2)Cl2 is the underlying trinodal net with a smaller molecular
5

coordination
number
for
each
node
(point
symbol
for
the
two
cases:
{34.46.55}{37.410.514.65}{37.49.55} and {317.410.5}{32.4}{368.4156.547.65} respectively). These results,
calculated using ToposPro software[59] (complete data can be found in the supplementary material,
Figures S19-S22), make clear the inherent difference between the inequivalent molecules in form-b
and the peculiarities of Z’ > 1 structures.
2.3 Mass Spectrometry
The ESI mass spectrum (positive ion mode) of Zn(L1)Cl2 is reported in Figure 5. The complex does
not display free basic sites where cations usually present in the ESI environment (such as Na+, K+ or
+
NH4 ) may bind and impart a positive charge. As a consequence, neither the protonated molecular
ion, [M+H]+, nor the cationized adducts are present in the spectra. However, the loss of a chloride
ligand yields the [Zn(L1)Cl]+ ion, whose signal is detectable at m/z = 370; this signal provides
indication of a 1:1 ligand-metal complexation ratio, as expected. The other peaks can be ascribed to
equilibria taking place in solution: free ligand protonation (L1H+, m/z = 272) and addition of a ligand
molecule to the [Zn(L1)Cl]+ ion, yielding [Zn(L1)2Cl]+ (m/z = 641) which in turn originates
[Zn(L1)2]2+ (m/z = 303) after the loss of the chloride. The presence of the doubly charged [Zn(L1)2]2+
ion was confirmed by high resolution experiments.
The mass spectra of complexes Zn(L2)Cl2 and Zn(L3)Cl2 (Figures S1 and S2, respectively) are very
similar and do not show remarkable differences with respect to the one of Zn(L1)Cl2. The main peak
is due to the [Zn(L)2Cl]+ ion (m/z = 777, and m/z = 753). Protonated ligands L2H+ (m/z = 340) and
L3H+ (m/z = 328), [Zn(L)Cl]+ (m/z = 440 and m/z = 426), [Zn(L)2]2+ (m/z = 371 and m/z = 359) are
the other main identified species in the spectra, which were already detected in the spectrum of
Zn(L1)Cl2. Moreover, a weak signal due to the [Zn(L)3]2+ ion (m/z = 540 and m/z = 523) is also
present for these two complexes. Finally, the peaks at m/z = 914 (Figure S1) and 891 (Figure S2)
were assigned to the [Zn(L)2Cl + ZnCl2]+ adduct, which is a typical gas phase ion formed under ESI
conditions. The experimental mass distributions for all the observed ions nicely fit with the calculated
isotopic mass distributions, thus representing an additional proof for ion assignment.
Even if the absence of free basic sites in the complexes does not allow formation of cationized
molecular ions, nevertheless the signals of the metal-ligand 1:1 complexes are present in the ESI
spectra of each complex, while the other peaks are attributed to equilibria taking place in the
environment of the ESI source. Moreover, elemental analysis and single-crystal X-ray diffraction
leave no doubts about the 1:1 metal-to-ligand ratio for the synthesized complexes in the solid phase.
6

303.30
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
370.16
640.91
667.91
272.22
605.10
704.20
700
397.08
533.06
549.96
365.14
350
594.84
439.46
450
758.83
750
497.92
500
259.88
250
168.21
150
105.18
100
0
50
200
300
400
550
600
650
800
m/z
Figure 5. ESI mass spectrum of complex Zn(L1)Cl2.
2.4 Vibrational characterization
Raman spectra of Zn(L1)Cl2, Zn(L2)Cl2 and Zn(L3)Cl2 show the pattern of the corresponding
ligand: signals are generally shifted to higher wavenumbers for the rigidity imposed by metal
coordination (see Table 1 and Figure 6 and Figures S3 and S4). In particular the breathing mode of
the pyridyl fragment, around 1000 cm–1, has a shift of around 15 cm–1, confirming the formation of
the chelating ring.[60] Far infrared spectra were recorded to observe signals related to the metal
center: in the spectra of the complexes there are two strong peaks absent in the spectra of the ligands.
According to literature, these signals could be attributed to Zn–Cl stretching modes.[61]
Table 1. Raman and Far IR signals of ligands L1–L3 and of the related Zn(L)Cl2 complexes.
L1
Zn(L1)Cl2
L2
Zn(L2)Cl2
L3
Zn(L3)Cl2 Assigment
1631 m
1602 s
1588 s
1533 vs
1523 s
1507 vs
1638 w
1604 vs
1562 w
1547 s
1533 s
1513 vs
1634 m
1619 s
1588 s
1565 w
1538 s
1512 vs
1642 w
1621 s
1604 m
1567 w
1550 s
1541 s
1519 vs
1632 m
1614 m
1588 s
1563 w
1538 s
1640 w
1609 m
1601 m
1562 w
1548 vs
1517 s
νC―N, νC―C
1512 vs
980 s
997 m
994 s
1012 m
983 m
996 w
997 w
1019 w
983 m
993 w
994 m
1017 m
νC―C
7

1023 m
1030 w
1026 w
331 s
308 s
336 s
304 s
332 s
309 s
νZn―Cl
Figure 6. Raman and FIR (inset) spectra of L1 (black) and Zn(L1)Cl2 (red).
2.5 Optical characterization
The optical data obtained for the free ligands, L1–L3, and for the corresponding complexes,
Zn(L1)Cl2, Zn(L2)Cl2, Zn(L3)Cl2, in dichloromethane solution are collected in Table 2, while their
absorption and emission spectra are presented in Figure 7 and Figure 8.
The free ligands show the main absorption features in the wavelength range between 280 nm and 380
nm, with almost no absorption beyond 400 nm. Each ligand shows one main peak in the 310–330 nm
range and a shoulder (a second peak for L2) centered at 360–380 nm. The various substituents on the
imidazo[1,5-a]pyridine ligands have a definite effect on the absorption bands, in particular L2 shows
an hyperchromic effect in the 360–380 nm region.
The corresponding metal complexes present three main absorptions. One in the 310–330 nm range, a
second one centered at 350–360 nm and a third one at 370–380 nm with a final shoulder at 390–398
nm. Compounds Zn(L1)Cl2, Zn(L2)Cl2, Zn(L3)Cl2 show a smaller red shift if compared to the
corresponding free ligand, as if the various substituents had a reduced effect on the absorption bands
of the complexes. From these data it is evident that both electron-donating and electron-withdrawing
groups, on the phenyl substituent in position 3 of the imidazo[1,5-a]pyridine, have a definite and
opposite effect on the absorption behavior in the 350–390 nm range.
8

Figure 7. Normalized absorption spectra of L1–L3 (left) and of the corresponding Zn(L)Cl2
complexes (right).
The imidazo[1,5-a]pyridine nucleus is known in literature for its emissive properties[31,32,43,62–
64]. The employed derivatives display an intense fluorescence emission centered at about 460 nm,
characterized by quantum yields ranging from 15 to 21% in dichloromethane solutions. All the
ligands show a large Stokes shift (80–140 nm).
The related complexes show similar emission profiles in dichloromethane solution. The luminescence
is positioned in the blue region (430–450 nm) and is due to an intense intra-ligand (π–π*) transitions,
as previously reported by us for similar system.[6,7,65] The emission spectrum consists of an intense
structured band centered at about 440 nm and surrounded by two shoulders at 410–420 and 460–470
nm, more evident than for the free ligands.
Figure 8. Emission spectra of ligand L1–L3 (left) and of the corresponding Zn(L)Cl2 complexes
(right).
According to literature data, the emission maxima for zinc complexes with N,N-bidentate ligands
(such as well known phenanthroline and bipyridine ligands) are generally red shifted of about 30–50
nm with respect to the free ligands.[6,66–68] Conversely, the Zn(L)Cl2 complexes reported in this
study display a remarkable hypsochromic shift (30–40 nm) if compared with the emission of the free
imidazo[1,5-a]pyridine ligands. The strong blue-shift is related to the conformational modification of
9

the ligand (from trans to cis) upon metal coordination. To the best of our knowledge, this interesting
characteristic has never been observed in previous works on Zn(II) with analogous imidazo[1,5-
a]pyridines. The hypsochromic shift of the emission, combined with the red shifted absorption, have
as a consequence the reduction of the Stokes shift in the complexes, if compared with the
corresponding free imidazo[1,5-a]pyridines. Moreover, comparing the Zn(L)Cl2 complexes it is
evident that the different substituents produce an opposite effect on the emission spectra. In particular,
Zn(L2)Cl2 results in the most hypsochromic shifted emission (431 nm), while Zn(L3)Cl2 has the
most bathochromic one (445 nm). Predictable and significant changes in the emission maxima of the
complexes have been obtained introducing electron-donating and electron-withdrawing substituents
in the chemical structure of corresponding ligands.
A significant increase of the quantum yields results comparing the free ligands and the corresponding
complexes. The values change from 19% (L1), 15% (L2) and 21% (L3), for the free ligands to 32%,
33% and 31% for the corresponding complexes, respectively. In particular, Zn(L2)Cl2 exhibits the
maximum quantum yield (33%) which, to the best of our knowledge, is the highest reported in
literature among the Zn(II) imidazo[1.5-a]pyridine complexes in solution.
Table 2. Absorption and emission data for L1–L3 and for the corresponding complexes Zn(L)Cl2.
Compound
Absorption
(nm)
Emission
(nm)
Stokes Shift
(nm)
Quantum Yield
(%)
L1
377sh
323
463
79
19
Zn(L1)Cl2
395sh
376
416sh
440
64
32
362
462sh
325
L2
361
330
458
128
57
15
33
Zn(L2)Cl2
373
358
410sh
431
325
458sh
392sh
L3
381sh
324
466
142
64
21
31
Zn(L3)Cl2
398sh
380
422sh
445
365
470sh
324
This class of complexes is typically non sensitive to molecular oxygen, so that the emission from
aerated solutions is not quenched. Indeed, in our case the emission profile and intensity of Zn(L)Cl2
complexes in aerated and deaerated conditions are perfectly overlapped. The emission at  = 410–
460 nm is mainly LC (π–π*), in agreement with the vibrational profile of the band. Analogies with
the corresponding Cu, Ir and Re derivatives are in agreement with this interpretation.[5–
7,27,53,54,65] Furthermore, as can be seen from the emission spectra, no partial dissociation of the
complexes was observed in dichloromethane solution, contrary to what previously reported in
literature for analogue zinc complexes.[6] Indeed, the typical emission of the free ligands is not
detectable in the solutions of the complexes.
10

3. Experimental
3.1 Material and methods
All solvents and raw materials were used as received from commercial suppliers (Sigma-Aldrich and
Alfa Aesar) without further purification. TLC was performed on Fluka silica gel TLC-PET foils GF
254. particle size 25 nm. medium pore diameter 60 Å. Column chromatography was performed on
Sigma-Aldrich silica gel 60 (70–230 mesh ASTM).
Mass spectra were recorded on a Thermo-Finnigan Advantage Max Ion Trap Spectrometer equipped
with an electrospray ion source (ESI) in positive and negative ion acquiring mode.
UV-Vis absorption spectra were recorded on a Cary60 spectrometer. Photoemission spectra and
luminescence quantum yields were acquired with a HORIBA Jobin Yvon IBH Fluorolog-TCSPC
spectrofluorometer, equipped with a Quanta-φ integrating sphere. The spectral response was
corrected for the spectral sensitivity of the photomultiplier.
Vibrational spectra were recorded for all products on powder samples. FT-Raman spectra were
obtained with a Bruker Vertex 70 spectrometer, equipped with the RAM II accessory, by exciting
with a 1064 nm laser, with a resolution of 4 cm–1. FTIR spectra were obtained with the same
instrument equipped with the Harrick MVP2 ATR cell and using a DTGS detector.
The elemental composition of the solid complexes was determined using a Thermo FlashEA 1112
CHNS–O analyser. Two replicas were performed, and values were presented as % mass mean value.
Zn(L)Cl2 single crystals were analysed with a Gemini R Ultra diffractometer operating at 293(2) K,
using a Mo−Kα source (λ = 0.71073 Å) (Cu-Kα source for the acetonitrile solvate of Zn(L2)Cl2,
form-b, λ = 1.54060 Å, for the small size of the crystal obtained). Data collection and reduction were
performed using the CrysAlisPro software. The crystal structure was solved by direct methods and
refined with the full matrix least-squares technique on F2 using the SHELXS-97[69] and SHELXL-
97[70] programs. All non-hydrogen atoms were refined anisotropically; hydrogen atoms were placed
in geometrical positions and refined using the riding model. Visualization of crystal structures has
been performed using Mercury.[71] All the crystallographic and structural data can be consulted in
the supplementary material. Topological analysis of all crystal structure has been done using
ToposPro[72] and can be found in the supplementary material. The Hirshfeld surface analysis has
been performed using CrystalMaker17[73] and can be found in the supplementary material. In
Zn(L2)Cl2 form-a and Zn(L2)Cl2 form-b the -CF3 disordered groups and solvent disorders have been
modeled in the Hirshfeld surface calculation using the new cycling option of CrystalMaker17. This
option includes all orientations of the disordered molecule with their partial occupancies and results
in a Hirshfeld surface based on the smeared electron distribution based on the average structure with
electron densities weighted by their partial occupancies.
The crystallographic data for Zn(L1)Cl2, Zn(L2)Cl2, and Zn(L3)Cl2 have been deposited within the
Cambridge Crystallographic Data Centre as supplementary publications, under the CCDC numbers
1947720-1947723. This information can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cifcode CCDC.
3.2 Syntheses
General procedure for 1,3-substituted-imidazo[1,5-a]pyridines (L1–L3).
Compounds L1–L3 were prepared as previously reported.[27,53,54,74] Di(2-pyridyl)ketone (800
mg, 4.37 mmol, 1 eq), corresponding aldehyde (6.55 mmol, 1.5 eq.) and ammonium acetate (1704
11

mg, 21.85 mmol, 5 eq) in glacial acetic acid (25 mL). Reaction time: 5 h at 118 °C. The acetic acid
was removed by evaporation under reduced pressure. The obtained solid was dissolved in a saturated
aqueous solution of Na2CO3 and the mixture extracted with CH2Cl2. The organic layer was separated,
dried and the solvent evaporated under vacuum. The obtained crude product was purified via column
chromatography on silica gel (CH2Cl2-CH3OH 98:2) and the product isolated as a yellowish solid.
Synthesis of Zn(L)Cl2 complexes. To a suspension of ZnCl2 (55 mg, 0.400 mmol) in methanol (15
ml) L1–L3 (0.48 mmol, 1.2 eq.) was added, and a yellow suspension immediately formed. The
suspension was stirred at room temperature for 2.5 h and then filtered; the yellow powder obtained
in quantitative yield was washed with diethyl ether three times and dried under vacuum.
Zn(L1)Cl2 elemental analysis (C18H13N3Cl2Zn) – Theoretical: C = 53.04%, H = 3.21%, N = 10.31%;
experimental: C = 53.34%, H = 3.17%, N = 10.17%.
Zn(L2)Cl2 elemental analysis (C19H12N3F3Cl2Zn) – Theoretical: C = 47.98%, H = 2.54%, N = 8.84%;
experimental: C = 47.65%, H = 2.46%, N = 8.51%.
Zn(L3)Cl2 elemental analysis (C22H21N3Cl2Zn) – Theoretical: C = 56.68%, H = 4.56%, N = 9.06%;
experimental: C = 56.35%, H = 4.36%, N = 8.69%.
4. Conclusions
Three new fluorescent 1,3-substituted-imidazo[1,5-a]pyridine zinc(II) chlorido complexes have been
synthesized, structurally and electronically characterized. The luminescent imidazo[1,5-a]pyridine
core, usefully substituted in position 3 with phenyl group (L1) or electron-donating (trifluoromethyl
group L2) and electron-withdrawing pendant group (tert-butyl group L3) on the phenyl, have been
employed to obtain the corresponding Zn(II) complexes in high yields.
The structure of the achieved compounds has been determined by single-crystal X-ray diffraction,
optical and vibrational spectroscopy, electrospray ionization mass spectrometry and elemental
analysis. The comparison of the photophysical properties, induced by the modification of the
substituent on the imidazo[1,5-a]pyridine ligands, led us throughout the synthesis of the
corresponding Zn(II) chlorido complexes. The obtained optical properties of the ZnLCl2 products
demonstrate a suitable optical tunability, related to the chemical structure, and an intense increase of
the quantum yield after complexation. Electronic absorption and emission spectra of the compounds
have been investigated in dichloromethane, showing for the first time a surprising hypsochromic shift
(30-40 nm) in spectra of the Zn(II) complexes if compared with the emission of the free ligands, on
the contrary to what reported in the literature for other N,N-bidentate ligands (such as well know
phenanthroline and bipyridine). In general, the obtained 1,3-substituted-imidazo[1,5-a]pyridine
complexes show absorption maxima below 410 nm, with a suitable transparency in the visible range,
and a powerful emissions in the blue region (430-445 nm).
On the basis of these promising properties further studies are in progress to test these products as
tunable low-cost emitting materials for possible and desirable technological applications.
Funding
This research did not receive any specific grant from funding agencies in the public, commercial, or
not-for-profit sectors.
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CRediT author statement
Giorgio Volpi: Conceptualization, Investigation, Writing - Review & Editing.
Emanuele Priola: Investigation, Formal analysis, Writing - Original Draf.
Claudio Garino: Conceptualization, Writing - Review & Editing, Visualization.
Andrea Daolio: Investigation.
Roberto Rabezzana: Investigation.
Paola Benzi: Investigation.
Alessia Giordana: Investigation.
Eliano Diana: Resources, Supervision, Project administration.
Roberto Gobetto: Resources, Supervision.
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New zinc(II) complexes with fluorescent 1,3-substituted imidazo[1,5-a]pyridines
Zinc(II) complexes showing an intense hypsochromic shift
Significant increase of the quantum yield after complexation to Zn(II)
Fluorescent ligands with electron-donating or electron-withdrawing moieties
Single crystal X-ray diffraction and topological-energetic packing analysis
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