Electronic absorption and emission properties of bishydrazone [2?×?2] metallosupramolecular grid-type architectures

Article abstract of DOI:10.1016/j.ica.2019.05.017

Several ditopic ligands containing two tridentate bishydrazone coordination subunits and their Zn(II) and Cd(II) [2 × 2] grid-type complexes were prepared and their photoluminescent properties studied. A special attention was devoted to the influence of the orientation of the hydrazone group N–N[dbnd]in the core of the ligands and their complexes. Its reversal from [pyridine[dbnd]N–N–pyrimidine] (L1) to [pyridine–N–N[dbnd]pyrimidine] (L2) has a strong impact on the observed absorption and emission behaviour of particular ligands (L1 and L2) as well as of their [2 × 2] grid assemblies. The further lateral functionalization of the ligands led to different emission quantum yields of the resulting grids, while their emission and absorption spectra varied very little. The simplest derivative L1 turned out to have the best performance with, for its Zn(II) complex, relatively high quantum yield 60%.

Full text of DOI:10.1016/j.ica.2019.05.017

Accepted Manuscript
Research paper
Electronic absorption and emission properties of bishydrazone [2x2] metallo-
supramolecular grid-type architectures
Jan Holub, Antonio Santoro, Jean-Marie Lehn
PII:
S0020-1693(19)30078-7
https://doi.org/10.1016/j.ica.2019.05.017
ICA 18918
DOI:
Reference:
Inorganica Chimica Acta
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Accepted Date:
15 February 2019
9 May 2019
9 May 2019
Please cite this article as: J. Holub, A. Santoro, J-M. Lehn, Electronic absorption and emission properties of
bishydrazone [2x2] metallosupramolecular grid-type architectures, Inorganica Chimica Acta (2019), doi: https://
doi.org/10.1016/j.ica.2019.05.017
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Electronic absorption and emission properties of
bishydrazone [2x2] metallosupramolecular grid-
type architectures
Jan Holub*a,b, Antonio Santoroa,c Jean-Marie Lehn*a
a
b
c
Laboratoire de Chimie Supramoléculaire, ISIS, Université de Strasbourg, 8 Allée Gaspard
Monge, 67000 Strasbourg, France
Present address: Institute of Chemical Research of Catalonia (ICIQ) Avinguda Països
Catalans 16, 43007 Tarragona, Spain; e-mail: jholub@iciq.es (Jan Holub)
Present address: Department of Chemical, Biological, Pharmaceutical and Environmental
Sciences, University of Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy
Key words: Hydrazone, Metallosupramolecular grids; Coordination architecture;
Photoluminescence of coordination compounds
Abstract:
Several ditopic ligands containing two tridentate bishydrazone coordination subunits and
their Zn(II) and Cd(II) [2x2] grid-type complexes were prepared and their photoluminescent
properties studied. A special attention was devoted to the influence of the orientation of the
hydrazone group N-N= in the core of the ligands and their complexes. Its reversal from
[pyridine=N-N-pyrimidine] (L1) to [pyridine-N-N=pyrimidine] (L2) has a strong impact on
the observed absorption and emission behaviour of particular ligands (L1 and L2) as well as
of their [2x2] grid assemblies. The further lateral functionalization of the ligands led to
different emission quantum yields of the resulting grids, while their emission and absorption
spectra varied very little. The simplest derivative L1 turned out to have the best performance
with, for its Zn(II) complex, relatively high quantum yield 60%.
1. Introduction
In the last two decades, photoactive organic compounds gained significant attention for
their application in organic light-emitting diodes (OLED) [1], organic solar cells [2] or
fluorescent chemosensors [3]. Nowadays, many of the promising fluorescent and luminescent
1

compounds still rely on the properties of expensive precious metals such as Ir(II) [4][5],
Ru(II) [4][6], Pt(II) [4][7] or expensive lanthanides (Eu, La,) [8]. Therefore, recently much
effort was put into their substitution with less expensive common metals such as Al(III),
Cu(II), Zn(II). [9].
Among these, zinc appears especially appealing for its accessibility and simple chemistry.
Many fluorescent and phosphorescent organo-zinc complexes are showing potential in
biological studies [10] or as emitters for organic light-emitting diodes [11]. Extensive use has
been made of a tridentate NNN terpyridine-type design for the coordination subunits of the
ligands in these complexes, which in some cases present a photoluminescence quantum yield
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ph
N
N
Ph
N
N
N
N
N
N
N
N
N
N
N
N
Zn(OTf)2
Zn(OTf)2
Ph
Ph
Ph
Ph
vs.
Acetonitrile
Acetonitrile
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Ph
Ph
N
N
N
N
LTpy4Zn4
L14Zn4
LTpy
L1
Figure 1: Illustration of the structural and [2x2] grid assembly analogies between
bisterpyridine LTpy ligand (left) and double hydrazone (pyridine=N-N-pyrimidine) L1 ligand
(right).
exceeding 90%. [12]
In addition, bisterpyridine ligands (LTpy, Figure 1 left) are widely recognized
supramolecular building blocks capable of a formation of highly ordered square-shaped
metallo-supramolecular arrays further referred to as [2x2] grids. [13] Initial studies carried out
with the bisterpyridine ligand LTpy (Figure 1 left) showed that an acetonitrile solution of its
Zn(II) grid LTpy4Zn4 displays a bluish fluorescence with an emission maximum at 450 nm
(λexc = 335 nm) and a moderate quantum yield of 11%‡. [14]
Coincidentally, the hydrazone generated by the condensation of pyridine-2-carboxaldehyde
with 2-hydrazinopyridine represents an easily accessible isosteric analogue of the terpyridine
motif with well-described coordination behaviour [15]. By extension, double hydrazone
isosteric analogues of the bisterpyridine ligand LTpy such as L1 (Figure 1 right) and L2
(Figure 2) have been implemented for the assembly of hydrazone-based [2x2] grid complexes
[13,16]. One may also note that the presence of hydrazone functional groups confers dynamic
covalent features to these ligand entities [17].
2

2. Experimental:
2.1. General information
Commercially available chemicals and solvents were purchased from Sigma-Aldrich, Alfa
Aesar, and STREM and were used without any additional purification. The 4,6-dimethyl-2-
phenylpyrimidine [16a, 19], 4,6-bis(1-methylhydrazinyl)-2-phenyl-1-pyrimidine [16a],
methyl-1-(2-pyridyl)hydrazine [20], 2-phenyl-4,6-pyrimidinedicarboxaldehyde [16a, 21] and
ligands L1 [16], L2 [16] and L1H [16] were prepared according the procedures described in
the corresponding literature references.
NMR spectra were recorded on Bruker Avance 400 (400.14 MHz for 1H and 100.62 MHz
for 13C) and Bruker Avance III plus 400 (400.34 MHz for 1H and 100.67 MHz for 13C) NMR
spectrometers. All the collected spectra were referenced on residual solvent signal according
to Nudelman et al. [18]
Mass spectra were obtained on Bruker MicroTOF and HRMS on Bruker MicroTOF-Q,
both with electrospray ionization. Nominal precision of the HRMS analysis is 10 ppm.
UV-VIS spectra were recorded on a JASCO V670 dual beam spectrophotometer. The
fluorescence spectra were recorded on Horiba Jobin-Yvon IBH FL-322 Fluorolog 3
spectrometer equipped with 450 W xenon arc lamp, double grating excitation and emission
monochromators (2.1 nm x mm-1 of dispersion; 1200 grooves mm-1) and a TBX-04 single
photon-counting detection. Emission and excitation spectra were corrected for source
intensity (lamp and grating) and emission spectral response (detector and grating by standard
correction curves. The photoluminescence quantum yield is a Quantaurus C11347
(Hamamatsu, Japan).
Elemental analysis was performed at the Service de Microanalyse, University of
Strasbourg at ThermoFisher Scientific Flash 200 with absolute precision of 0.3 %.
Melting points were measured on a Buchi Melting Point B-540 apparatus and temperature
data are uncorrected.
The X-ray crystallography studies were performed at the Service de Radiocrystallographie,
University of Strasbourg and at the Karlsruhe Institute of Technology. At the University of
Strasbourg crystallographic data were collected on a Bruker Kappa Apex II diffractometer
using graphite-monochromated Mo-Ka radiation (λ = 0.71073 Å). Crystallographic CIF data
can be found deposited in the Cambridge Crystallographic Data Centre database under the
CCDC 1897491 (L15OH4Zn4) and CCDC 1897489 (L1(COOMe)2Zn3) codes.
3

2.2. General procedure for ligand synthesis (Figure 2)
The 4,6-bis(1-methylhydrazinyl)-2-phenylpyrimidine (1 eq., 2.05x10-4 mol) was dissolved
in EtOH (20 ml) followed by the addition of an appropriate aldehyde (2.1 eq, 4.3x10-4 mol).
Reaction mixture was then heated under reflux for 5-12 h. The reaction proceeds well even in
case when the reaction components are not completely soluble in EtOH. For such cases, the
longer reaction time is recommended. In all cases, an appearance of the precipitate was
observed during the reaction period. After the reaction was cooled down to the room
temperature this precipitate was filtered off and washed with ethanol yielding the desired
bishydrazone ligands (L1XX) in high yields (70-95 %). The analytical descriptions and
accompanying spectra of individual ligands are provided in the accompanying electronic
supporting info (ESI).
The same procedure was applied for the preparation of the other constitutional isomer
(L2), only this time using 2-phenyl-4,6-pyrimidinedicarboxaldehyde (1 eq., 2.05x10-4 mol)
and 1-methyl-1-(2-pyridyl)hydrazine. [16] The ligand L2 was isolated as a bright orange solid
in >80 % yield.
2.3. General procedure for complex preparation L4M4
To the preformed ligand (L1XX, L2, 1 eq., 4.73 x 10-4 mol) in acetonitrile (5 ml) was added
M(OTf)2 (Zn(OTf)2 or Cd(OTf)2 x 6 DMSO, 1 eq., 4.73 x 10-4 mol) and the reaction mixture
was heated at 70 ºC overnight. Afterwards, the reaction mixture was cooled down to the room
temperature and filtered through the cotton pad. The assembly is clean and almost quantitative
as checked by NMR of the reaction mixture. The desired grid complex could be
precipitated/crystalised from the acetonitrile solution by the vapour diffusion of diethyl ether
into the reaction solution. The analytic descriptions and accompanying spectra of individual
complexes are provided in the accompanying electronic supporting info (ESI).
2.4. UV-VIS, fluorescence and quantum yield measurements
To obtain UV-Vis absorption spectra the ligands were dissolved in spectrophotometric or
HPLC grade CHCl3 or DMSO regarding their individual solubility. Typically, 1 ml of 1 mM
solution was prepared beforehand from which 50 μl were transferred into the quartz UV-Vis
cuvette and diluted to the 3 ml (1.7 x 10-5 M) with the solvent of choice. This concentration
was chosen so the absorbance of all the measured compounds falls into the range of 0.6 – 1.In
all cases, the background spectrum of the blank solution was collected first and then
subtracted from the sample. All UV-Vis absorption spectra of the complexes were measured
4

in spectroscopic grade acetonitrile by the same procedure using the working concentration of
2.5 x 10-6 M of the L4M4 grid complex.
To obtain the fluorescence spectra of the metal complexes, 1 ml of the 1 mM acetonitrile
solution was prepared from which typically 2-10 μl equivalents were transferred to the quartz
cuvettes, diluted to 3 ml volume and the fluorescence spectra recorded. The reported spectra
were typically measured at the concentration of 6 x 10-7 M of the .L4M4.
The solid-state fluorescence spectra were measured by introducing several crystal grains of
the sample compound between two quartz slides. These slides were fixed together and
introduced at the angle of 45º to the slot in the measuring machine.
The quantum yield was measured by integrating sphere in quartz tubes, using the
previously prepared acetonitrile solution of the ligand or solid powdered sample.
3. Results and Discussion:
3.1. Synthesis and structural features of hydrazone ligands and their[2x2]grid type
complexes
5

EtO
OEt
O
O
HO
OH
Cl
Cl
O
O
N
N
O
O
H
2N
NH2
MeOK,
EtOH
I2, CF3COOH,
DMSO
N
N
N
N
N
N
N
N
N
N
POCl3
6h, 120°C
NH2NHMe
3h, 95°C
K2CO3, H2O
4-5 days
H
2N
NH
H
2N
NH
.HCl
.HCl
6h, 80°C
3h, 160°C
.2H2O
.2H2O
PPHyd
PPAl
N
H
2N
O
N
N
Ethanol,3h,80°C
Ethanol,3h,80°C
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
L1
L2
H
N
H
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
L1H
L16Me
L15Me
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
N
N
N
N
O
O
N
N
N
N
O
HO
OH
O
O
O
O
O
O
L15OH
L1COOMe
L1OAcMe
O
O
N
N
N
N
N
N
N
N
N
N
O
O
O
O
L1Phtim
Figure 2.: Top: Synthesis of the two constitutional isomers of bishydrazone: L1 derived
from a bis-hydrazine core and L2 from a bis-aldehyde core; schematic representation of the
square [2x2] grid-type complexes formed by these ligands. Bottom: Molecular structures of
differently functionalized ligands studied in this work.
The fluorescence of Zn(II)-hydrazone complexes has been known for a long time [22], but
only few studies were performed on oligonuclear hydrazone complexes. [23] Regarding the
bishydrazone-based ligands L1 and L2 investigated herein, some interesting spectroscopic
aspects involving pH dependent colour change were already mentioned in one of the first
reports on this family of grids [16b]. However, despite these ligands being known for a rather
long time, their photoluminescence properties were so far largely overlooked and only briefly
mentioned in the literature [13a, 14‡].
It was thus of interest to perform a spectroscopic study of the differently functionalized
bishydrazone ligands L1 and L2 and of their [2x2] grid-type complexes formed with Zn(II)
and Cd(II) cations. Special attention has been given to the effects of the imine bond position
6

within the hydrazone connection in the two isomeric ligands L1 [16a, c, d, e] and L2 [16a, d,
e] (Figure 2 Top) as well as to the functionalization of the peripheral pyridine moieties
(Figure 2 Bottom).
The synthetic procedures involve hydrazine-aldehyde condensations to generate
coordination subunits of either [pyridine=N-N-pyrimidine] or [pyridine-N-N=pyrimidine]
tridentate NNN sequences in L1 and L2 respectively. (Figure 2 Top; for detailed procedures
see ESI and literature references 16a-d, 19-21).
The complementary aldehydes were either obtained commercially (L1, L15Me, L16Me) or
synthesised according to the available literature (L15OH [24], L1COOMe [25], L1OAcOMe
[adapted from 24], L1PhtIm [adapted from 24], See ESI). Due to the significantly inferior
spectroscopic performance (weak emission and low quantum yield) of the L2 isomer, only the
basic derivative was studied and its spectroscopic behaviour is discussed further below.
Subsequently, the addition of 1 eq. of Zn(OTf)2 (or Cd(OTf)2x6 DMSO) in acetonitrile to the
ligand L1XX or L2 provides the desired [2x2] grid in close to quantitative yields in all cases.
All the complexes have tetrameric [2x2] square geometry of the general formula [L4M4]8+.
This structure is in particular indicated by the presence of corresponding fragments in the HR-
MS spectra as well as by distinctive doublets in the 1H NMR spectrum (5 - 6.5 ppm, Figure 5
top, see also ESI of respective grids), characteristic for this family of grid complexes. [14,
16d] In the case of L15OH4Zn4, its highly ordered grid-like structure was directly ascertained
by the structural X-ray analysis of a suitable single crystal (obtained by vapour diffusion of
the Et2O into an ACN solution, Figure 3, see also ESI L15OH4Zn4). Its crystal structure
illustrates the highly ordered and rigid assembly of the grids based on the bishydrazone
coordination motif. [13, 14, 16] Noteworthy are the parallel layout of the ligands with the
defined distance between them (6.58 Å) and the almost perpendicular angles (91.49º) around
the metal centers. Furthermore, the spatial positioning of the eight hydroxyl groups evenly
distributed around the periphery could make these grids an interesting building block for the
bottom up assembly of the higher 2D structures. [16f]
7

Figure 3: The crystal structure of octahydroxyl grid [L15OH4Zn4](OTf)8.
3.2. Photophysical properties of bishydrazone ligands and grids:
Absorption behaviour of ligands
The absorption spectra of the ligands L1XX
and L2 were measured in chloroform at a
concentration of 1.7x10-5 M. (For L1 and L2
Figure 4, see ESI for the rest). The model
ligands L1 and L2 display three bands with
maxima for L1 at 259 nm (ε = 22 800 M-1
cm-1), 300 nm (44 000 M-1 cm-1) and 347 nm
(22 900-1 cm-1) and for L2 at 259 nm (22 800
M-1 cm-1), 340 nm (40 000 M-1 cm-1) and 378
Figure 4: UV-VIS absorption spectra of the
nm (21 700 M-1 cm-1).
ligands L1 and L2 in CHCl3. (1.7x10-5M in
In generall, the UV-Vis absorption spectra
of L1XX and L2 ligand familly (Figure 2,
acetonitrile)
Table 1) is dominated by π-π* spin allowed transitions. [14, 23] The presence of the different
weakly electron-donating groups in L1XX plays a marginal role on the absorption behaviour,
by varying the maxima peaks of few nm compared to the L1 model (Tab. 1, for absorption
spectra see respective ESI). The only anomaly is presented by ligand L1COOMe (ESI L1COOMe
)
whose absorbance peaks are much broader with the second maximum at 336 nm being
stronger in comparison to the first at 313 nm and also with a visible shoulder around 375 nm.
In a case of L1Phtim, its low solubility in tested solvents prevented a measurement of the
relevant spectroscopical data.
8

Absorption properties of the grids
In the case of the L24Zn4 grid the formation of the complex in acetonitrile is accompanied
by the appearance of a characteristic orange colour while solutions of the L1xx Zn4 are
4
generally colourless, displaying the marked differences of their absorption spectra. (Figure 5
Left, Tab. 2, also see respective ESI) Especially interesting in the case of L24Zn4 absorption,
is the complete loss of the vibrational structure, the consequential broader band and the
pronounced red shift of its visible region maximum (λmax = 464 nm, ε = 152 700 M-1 cm-1,) in
comparison to the L14Zn4 complex. (Figure 5 Left)
Figure 5: Left: Absorption spectra of L14Zn4 (blue), L14Cd4 (green) and L24Zn4 (red).
Right: Absorption spectra of L14Zn4 at various concentrations.
The shape of the double maxima peak (373 nm, ε = 228 300 M-1 cm-1 and 390 nm, ε =
209 400 M-1 cm-1) observed in electronic spectrum of L14Zn4 (Figure 5) is characteristic for
almost all the derivatives of L1 and closely resembles that of related complexes already
described in a literature. [14, 16] The only exception to this is the grid L16Me4Zn4 (ESI Figure
61) which displays a partial loss of structure with a broad peak with only a single maximum at
379 nm and which may be associated with a distortion of the coordination centres caused by
the steric interactions introduced by the methyl groups.
All the L1XX4Zn4 grids from substituted ligands show a bathochromic shift between 6-14
nm (424-969 cm-1) for first maxima and 15-17 nm (949-1071 cm-1) when compared with
L14Zn4 (Table 2, see also ESI of the respective grids). It seems that the steric distortion
influences the shift and shape of an absorption spectrum (L15Me~12 nm, 835 cm-1 vs. L16Me
~7 nm, 424 cm-1, Table 2) more than the electron donating or withdrawing nature of the
substituents (L15Me ~12 nm, 835 cm-1 vs. L1COOMe ~12 nm, 835 cm-1, Table 2).
9

Table 1: UV-Vis absorption maxima and extinction coefficients (ε) of the ligands L1xx and
L2 in chloroform
Name
UV-Vis maxima/nm
(ε/ M-1cm-1)
UV-Vis maxima/nm
(ε/ M-1cm-1)
UV-Vis maxima/nm
(ε/ M-1cm-1)
L1
L2
259 (22 800)
259 (22 800)
249 (24 500)
230 (23 200)
253.5 (26 200)
254 (24 400)
244.5 (27 500)
300 (44 000)
340 (40 000)
302 (54 700)
300.5 (53 900)
313 (34 400)
347 (22 900)
378 (21 700)
344.5 (28 500)
345 (30 300)
336 (36 900)
345 (29 600)
347 (28 400)
L15OH
L1OAcMe
L1COOMe
L15Me
L16Me
30.5 (54 800)
302.5 (49 600)
Table 2: UV-Vis absorption maxima and extinction coefficients (ε) of the [2x2] grids of
L1xx Zn4 and L24Zn4 in acetonitrile
4
Name
UV-Vis
UV-Vis
UV-Vis
maxima/nm
(ε/ M-1cm-1)
373 (228 300)
UV-Vis
maxima/nm
(ε/ M-1cm-1)
390 (209 400)
maxima/nm
maxima/nm
(ε/ M-1cm-1)
(ε/ M-1cm-1)
L14Zn4
-
-
-
-
-
-
L24Zn4
-
464 (152 700)
386 (432 700)
405 (250 300)
407 (317 800)
405 (334 800)
402 (260 600)
394 (250 300)
-
L14Cd4
372 (527 200)
385 (234 800)
387 (297 700)
385 (313 100)
385 (257 700)
376 (234 800)
379 (195 600)
L15OH4Zn4
L1Phtim4Zn4
L1OAcMe4Zn4
L1COOMe4Zn4 247 (97 600)
L15Me4Zn4
L16Me4Zn4
222 (90 200)
219 (390 500)
-
279 (83 500)
286 (103 300)
284 (101 200)
-
222 (9 000)
279 (83 500)
237 (87 000)
269 (74 600)
All the L1XX4Zn4 grids from substituted ligands show a bathochromic shift between 6-14
nm (424-969 cm-1) for first maxima and 15-17 nm (949-1071 cm-1) when compared with
L14Zn4 (Table 2, see also ESI of the respective grids). It seems that the steric distortion
influences the shift and shape of an absorption spectrum (L15Me~12 nm, 835 cm-1 vs. L16Me
~7 nm, 424 cm-1, Table 2) more than the electron donating or withdrawing nature of the
substituents (L15Me ~12 nm, 835 cm-1 vs. L1COOMe ~12 nm, 835 cm-1, Table 2).
Regarding the L4Cd4 grids, in general these complexes display properties similar to those
of their Zn(II) counterparts. With a comparison to L14Zn4, the electronic spectrum of L14Cd4
shows a stronger absorption with the maximum in the same regions of 373 nm (ε = 527 200
M-1 cm-1), however, with the second maximum reduced to a shoulder at 386 nm (ε = 432 700
M-1 cm-1). (Figure 5 Left, ESI Figure 18)
Emission behaviour of ligands and grids
The parent ligands L1 and L2 are insoluble in acetonitrile and show no fluorescence in
chloroform or DMSO, but upon excitation at 360 nm both present emission in the solid state,
10

Figure 6: Left: Powder emission spectra of ligands L1 (Blue) and L2 (orange) under λex =
365 nm. Middle: Colours of individual ligands and grids under white light and UV light.
Right: The powder emission of grids L14Zn4 (blue) and L24Zn4 (orange) and at λex = 365
nm. (ISIS - Institut de Science et d'Ingénierie Supramoléculaires)
pointing to the presence of aggregation enhanced emission [26]. (Figure 6 Left, ESI Figure 4)
The ligand L1, shows weak white-bluish fluorescence with two maxima at 409 nm and 515
nm with low quantum yield ~1 % (λex = 280-380 nm, see ESI Table S.1.). The ligand L2
exhibits orange-yellow fluorescence with one maximum at 503 nm and a quantum yield of ~2
%. (λex = 280-400 nm, see ESI Table S.2.). The emission is slightly enhanced for powdered
crystals of the Zn(II) grids with approximately twice as large quantum yield of ~3 % in case
of L14Zn4 (Figure 6 Right, ESI Figure S12) and ~4 % for L24Zn4. The increase in quantum
yields upon complexation can be explained by a decrease in non-radiative decay rates. This
decrease is caused by straightening of the bent geometry/conformation with inhibited
rotational motion of the central phenyl substituent (Figure 1 Right) [14, 16d] as well as an
inhibition of the photoinduced electron transfer (PET) via binding a lone electron pair of the
nitrogen to the metal cation. [27]
The observed 95 nm red shift of the emission maxima between the L2 and L24Zn4. (Figure
6) may be related to the influence of metal ion binding on the levels involved in the intra-
ligand charge transfer absorption. A contribution can also come from π-π stacking between
ligands and intercalated phenyl rings in the complex. The overall result is a remarkable
difference in the emission wavelengths and intensities between the ligands and grids.
Previous work on terpyridine based compounds [14] indicated that the fluorescence of this
family of grids is closely related to the d10 nature of the Zn(II) cation. In confirmation of these
findings, excitation of the complexes of L1 with different metal cations (Cu(I), Cu(II), Pb(II),
Fe(II), Co(II), Ag(I), Zn (II), Cd(II), Hg(II)) showed that only the Zn(II) and Cd(II)
complexes presented visible fluorescence. In the case of d10 Cu(I) the absence of
11

fluorescence might be attributed to the inadequate double-halicate geometry. As previously
described by our group, in comparison with the grid assembly, the double helicate
arrangement exhibits considerably reduced rigidity and increased rotational modes in the
ligand backbone. [28] However, based on current knowledge it is not possible to completely
exclude other quenching processed such as Cu based electron transfer. In the case of Hg(II)
the heavy metal effect may play a role as well as electrostatic interactions of the valence
electrons with the ligand backbone. [29] The fact that fluorescence was observed only in the
case of the spectroscopically inert d10 Zn(II) and Cd(II) complexes with similar maxima
confirms that the emission comes from the lowest ligand-centered excited states and the metal
cations just serve as connectors assembling the whole structure.
In the present work, all solution absorption and emission spectra of grid complexes were
measured in acetonitrile. The complexes derived from L1xx Zn4 were observed to be
4
somewhat more stable than L24Zn4. This was reflected in the lower emission quantum yield
of L24Zn4 complex in comparison to the L14Zn4. (For L14Zn4 see ESI Figures 6, 8, see ESI
Figure 13). The higher stability of the former can be attributed to the higher affinity of L1
towards the metal ions due to an increase in electron density on the pyrimidine nitrogens, a
consequence of the conjugation of a directly linked electron donating tertiary nitrogen site of
the hydrazine group to the heterocyclic unit.
L14Zn4
L14Cd4
L24Zn4
Figure 7: Left: Emission spectra of L14Zn4 (6x10-7 mol/dm3 acetonitrile) and L14Cd4
,
(6x10-7mol/dm3, acetonitrile) at λex = 365 nm; Middle: Visual comparison of the L14Zn4
(Blue), L24Zn4 (Orange) and L14Cd4 (blue) emissions under λex = 365 nm; Right: The
emission spectra of L24Zn4 (4x10-6 mol/dm3, acetonitrile) at λex = 365 nm;
12

Excitation of the acetonitrile solution of
L14Zn4 and L14Cd4 at 360 nm produced
remarkably intense blue emission (Figure 7
Right), observable by the naked eye even in
daylight. The quantum yield of the L14Zn4
emission was measured to be ~60 %, relatively
high for Zn(II) complex without any auxiliary
sensitizing units [10][11][12], with
a
fluorescence lifetime (τf1/2) of 3.1 ns. (See ESI
Figure 9) In contrast, L14Cd4 has an order of
Figure 8: Emission spectra of selected
L1XX4Zn4 grid complexes in acetonitrile.
(Each c = 6x10-7 mol/dm3 of grid)
magnitude
lower
quantum
yield
of
approximately 8 % (τf1/2 = 1.79 ns, ESI Figure 20), probably due to heavy metal quenching.
The corresponding L24Zn4 complex shows even weaker orange fluorescence (quantum yield
6 %, τf1/2 = 0.48 ns, ESI Figure 15) with a maximum at 585 nm (Figure 7 Left).
In a same way as for the electronic spectra, the peripheral substitution at L1 induces a
small red shift (Figure 8). Almost all emission spectra display two peak maxima except for
L1H Zn4 (ESI Figure 22) and L1COOMe4Zn4 (ESI Figure 48) where there is a partial loss of
4
the vibrational structures and the second maximum is reduced to a shoulder. The
L1COOMe4Zn4 maximum (at 428 nm) also shows the largest red shift of ~20 nm in the
comparison to the maximum displayed by L14Zn4 (at 408 nm). A summary of the emission
maxima and quantum yields measured for the [2x2] grid complexes studied herein is given in
Table 3.
13

Table 3: Quantum yields of chosen grids
Name
First Emission
Maximum (λex =
360 nm, ACN)
409 (s)*
Second Emission
Maximum (λex =
360 nm, ACN)
518 (s)*
Fluorescence
quantum yield Φ(%)
L1 (solid state)
L2 (solid state)º
L14Zn4
L14Zn4 (solid state)
L24Zn4
L24Zn4 (solid state)
L14Cd4
L15OH4Zn4
L1H
<1
2.4
60
~3
6
~4
8
8
503(s)
409
440
585
598
409
419
407
421
420
428
410
415
-
429
-
-
-
429
445
-
447
445
-
2
L1Phtim4Zn4
L1OAcMe4Zn4
L1COOMe4Zn4
L15Me4Zn4
37
36
41
40
5
434
436
L16Me4Zn4
*(s) – solid state
The quantum yields listed in Table 3 show that the strongest emission and highest quantum
yield (φ) correspond to the simplest bishydrazone grid L14Zn4. On the opposite side of the
series is the unsubstituted derivative with ionisable hydrazone N-H L1H Zn4 [17, 18b], which
4
has a low quantum yield of approximately ~2%. The reason for this is possibly the quenching
via hydrogen abstraction in the excited state. [30] A similar effect may be present in the
derivative L15OH4Zn4 whose quantum yield falls to ~8 %. The substitution of the hydroxyl
substantially increases φ as could be observed for L1Phtim4Zn4 (φ = ~37 %) and L1OAcMe4Zn4
(φ = ~36 %). Unfortunately, in the cases of L1OAcMe4Zn4 and L1COOMe4Zn4 the acetonitrile
solution shows dissociation of the grid structure with formation of some amount of a “corner”
complex L1COOMe2Zn3, which was also identified by single-crystal X-ray diffraction (see
ESI). This complex is most likely also fluorescent thus making it difficult to discuss the
quantum yields of these two compounds.
Regarding the position of substituents on the pyridine moieties it can be concluded that the
quantum yields of “5”-substituted pyridines (L15Me4Zn4) are much higher (~40 %) than those
of the “6”-substituted ones (L16Me4Zn4, ~5 %), possibly due to the steric hindrance from
methyl groups when forced into its grid form in the latter case.
4. Conclusion
Herein, we presented a study of spectroscopic features of different Zn(II) and Cd(II) [2x2]
grid-type complexes. Interestingly, the best performer is also the simplest ligand (L14Zn4).
14

The grids which were built from 4,6-bis(1-methylhydrazino)-2-phenylpyrimidine (L1 ligands)
had substantially more intense emission than those built from 2-phenyl-4,6-
pyrimidinedicarboxaldehyde (L2 ligand). It was observed that the electronic absorption,
emission spectra and quantum yields of both ligands and grids were very sensitive to the
sequence direction of the central hydrazone group. The peripheral substitution by (weakly)
electron donating/withdrawing groups was observed to play only a minor influence on the
absorption and emission spectra but the nature of the substituent has a strong impact on the
quantum yields of individual grid complexes. The results described demonstrate the effects of
constitutional isomerism, metal cation and peripheral substitution on spectroscopic features of
bishydrazone ligands and grid-type complexes.
5. Footnotes
‡ I. Manet, V. Balzani, D. Volkmer, J.-M. Lehn, unpublished results. Related species gave
emission quantum yields of < 1x10-3 and 0.11 for the ligand and the Zn(II) grid complex,
respectively. [14]
6. Acknowledgements
We thank Dr. Olaf Fuhr, Prof. Dieter Fenske, Lydia Brelot and Corinne Bailly for the crystal
structure determinations. We thank the ANR (grant DYNAFUN), the ERC (Advanced
Research Grant SUPRADAPT 290585), the University of Strasbourg Institute of Advanced
Study (USIAS), the International Center for Frontier Research in Chemistry (icFRC) and the
CNRS for financial support. J.H. thanks the Université de Strasbourg for a doctoral
fellowship. A.S. thanks Prof. Sebastiano Campagna and the University of Messina for the
Nanosolar project fellowship and the University of Strasbourg for a post-doctoral fellowship.
We also thank Karlsruhe Nano Micro Facility (KNMF) for financial support of the Karlsruhe
X-ray facility.
7. Appendix A. Supplementary data
Electronic supplementary information to this article can be found online…
Crystallographic CIF data can be found deposited in the Cambridge Crystallographic Data
Centre database under the CCDC 1897491 (L15OH4Zn4) and CCDC 1897489 (L1(COOMe)2Zn3)
numbers.
15

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Highlights of the
Electronic absorption and emission properties of bishydrazone [2x2]
metallosupramolecular grid-type architectures
Jan Holub*, Antonio Santoro Jean-Marie Lehn
 Simple synthetic protocols for the synthesis of functionalized bishydrazone grids
 Electronic absorption, emission and quantum yield study
 Influence of the hydrazone bond orientation on spectroscopic characteristics.
 Influence of substitution on the emission quantum yield of the bishydrazone grids.
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