Two new alkaline earth metal organic frameworks with the diamino derivative of biphenyl-4,4′-dicarboxylate as bridging ligand: Structures, fluorescence and quenching by gas phase aldehydes

Article abstract of DOI:10.1016/j.poly.2018.07.010

Alkaline earth metal ion organic frameworks (AEMOFs) represent a relatively underexplored subcategory of MOFs. Two new MOFs [Ca₆(bpdc-(NH₂)₂)₅(μ₃-HCO₂)₂(H₂O)_2.5(DMF)_0.5]·0.5H₂O·2.5DMF (1) and [Sr₄(bpdc-(NH₂)₄)(μ₂-DMF)₂(DMF)_1/3]·2/3(DMF) (2) [H₂bpdc-(NH₂)₂ = 2,2′-diamino-[1,1′-biphenyl]-4,4′-dicarboxylic acid); DMF = N,N-dimethylformamide] are presented here. These MOFs display structural variety with diverse topologies and new structural features. Luminescence studies revealed that both MOFs display ligand based fluorescence with small differences in emission profiles possibly attributable to the difference in charge density of the metal ions combined with the different conformation adopted by the ligand in the crystal structures of 1 and 2. Furthermore, initial sensing studies reveal that both MOFs can potentially function as fluorescent sensors for gas phase aldehydes.

Full text of DOI:10.1016/j.poly.2018.07.010

Polyhedron 153 (2018) 173–180
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Two new alkaline earth metal organic frameworks with the diamino
derivative of biphenyl-4,40-dicarboxylate as bridging ligand: Structures,
fluorescence and quenching by gas phase aldehydes
Stavros A. Diamantis a, Anastasia D. Pournara b, Antonios G. Hatzidimitriou a, Manolis J. Manos b,
Giannis S. Papaefstathiou c, Theodore Lazarides a,
⇑
a Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
b Department of Chemistry, University of Ioannina, Dourouti Campus, 45500 Ioannina, Greece
c Laboratory of Inorganic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou 15771, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 March 2018
Accepted 9 July 2018
Alkaline earth metal ion organic frameworks (AEMOFs) represent a relatively underexplored subcategory
of MOFs. Two new MOFs [Ca6(bpdc-(NH2)2)5(
l
3-HCO2)2(H2O)2.5(DMF)0.5]ꢀ0.5H2Oꢀ2.5DMF (1) and
[Sr4(bpdc-(NH2)4)(
l
2-DMF)2(DMF)1/3]ꢀ2/3(DMF) (2) [H2bpdc-(NH2)2 = 2,20-diamino-[1,10-biphenyl]-4,40-
dicarboxylic acid); DMF = N,N-dimethylformamide] are presented here. These MOFs display structural
variety with diverse topologies and new structural features. Luminescence studies revealed that both
MOFs display ligand based fluorescence with small differences in emission profiles possibly attributable
to the difference in charge density of the metal ions combined with the different conformation adopted
by the ligand in the crystal structures of 1 and 2. Furthermore, initial sensing studies reveal that both
MOFs can potentially function as fluorescent sensors for gas phase aldehydes.
Dedicated to Professor Spyros Perlepes on
the occasion of his 65th birthday.
Keywords:
Metal organic frameworks
Alkaline earths
Fluorescence
Aldehydes
Ó 2018 Elsevier Ltd. All rights reserved.
Sensors
1. Introduction
[29–34], luminescent molecular thermometers [35–37], biomedi-
cal imaging [38–42] and luminescent sensors [14,20,43–49].
Metal organic frameworks (MOFs) or Porous Coordination Poly-
mers (PCPs) are a class of hybrid crystalline materials, assembled
by metal ions or clusters and bridging polydentate organic ligands
to form polymeric network structures containing potential voids
[1–3]. Due to their properties such as structural and functional
diversity, porosity, high surface area, and adsorption affinity
towards various guest species, MOFs have been used for a variety
of applications including gas storage and separation [4–10], catal-
ysis [11–13], ion exchange [14–20], and proton conduction [21].
One more important advantage of MOFs is the facile introduction
of interesting physical properties such as luminescence through
the use of photoactive ligands, metal ions or their combinations.
Thus, luminescent MOFs (LMOFs), represent an important rapidly
growing subcategory of MOFs [22]. These luminescent crystalline
materials show great prospects in a range of technological applica-
tions from phosphors in lighting devices [23–28], photocatalysts
Alkaline earth metal-based MOFs (AEMOFs) are still relatively
rare in comparison to their transition metal and rare earth based
counterparts. Nevertheless, AEMOFs display unique advantages
over transition and lanthanide-based MOFs such as: (i) low cost
because of the high natural abundance of alkaline earths, (ii)
reduced toxicity, (iii) low density of the early members of the alka-
line earth series which is an important property for gas storage
applications and (iv) stability in air and various solvents. Despite
the fact that the research on AEMOFs remains on a rather prelim-
inary level, AEMOFs exhibit interesting properties and they have
been used in several applications [50–62]. Recently, our group
reported a series of AEMOFs based on the bridging ligand 2,5-dihy-
droxy terephthalate, which exhibits unusual luminescence proper-
ties because of excited state intramolecular proton transfer (ESIPT).
The ESIPT process has been shown to be partly hindered due to
electrostatic repulsion between the mobile proton and the coordi-
nated alkaline earth ion with the lighter ions inducing a more pro-
nounced effect as a result of their higher charge density [63,64].
Among the AEMOFs studied by our group, a Mg2+ based MOF can
⇑
Corresponding author.
E-mail address: tlazarides@chem.auth.gr (T. Lazarides).
https://doi.org/10.1016/j.poly.2018.07.010
0277-5387/Ó 2018 Elsevier Ltd. All rights reserved.

174
S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
be used for real time detection of water traces in organic solvents
via an ‘‘turn on” luminescence sensing mechanism [63].
the simulated PXRD patterns of the compounds (Fig. S1 and Fig. S2
respectively) thereby confirming that the analyzed single crystals
are representative of the bulk samples.
In this contribution, we present two new alkaline earth metal
ion MOFs, namely [Ca6(bpdc-(NH2)2)5(
0.5H2Oꢀ2.5DMF (1) and [Sr4(bpdc-(NH2)4)(
l
-HCO2)2(H2O)2.5(DMF)0.5]ꢀ
All structures were solved using the SUPERFLIP [68] package and
were refined by the full-matrix least-squares method on F2 using
the CRYSTALS package version 14.40b [69]. All non-hydrogen atoms
have been refined anisotropically except in the case of disordered
atoms.
All hydrogen atoms were found at their expected positions and
refined using soft constraints. By the end of the refinement, they
were positioned using riding constraints. CCDC 1833862–
1833863 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif. The crystal data, details of data
collection and structure refinement for both MOFs are given in
Table 1. Illustrations were drawn by the Mercury program [70].
Further details on the crystallographic studies as well as atomic
displacement parameters are given as Supporting Information in
the form of cif files.
l
-DMF)2(DMF)1/3]ꢀ
2/3DMF (2). The compounds display structural diversity forming
three dimensional frameworks with different secondary building
units (SBUs) [65] and network topologies. These AEMOFs exhibit
bright blue fluorescence following population of the ligand based
1pp* and 1np* excited states. Furthermore, the presence of free
amino groups which are possibly able to react with carbonyl com-
pounds in the frameworks of 1 and 2 prompted us to study their
fluorescence properties in the presence of gas phase aldehydes.
Indeed, preliminary studies revealed that both MOFs display signif-
icant fluorescence quenching upon their exposure to vapors of
volatile aldehydes thereby showing potential as gas phase alde-
hyde sensors.
2. Experimental part
2.1. Materials and methods
2.4. Compound preparation
All starting materials and solvents were used as received from
the usual commercial sources (Sigma Aldrich, Alfa Aesar and TCI).
2.4.1. Synthesis of H2bpdc-(NH2)2
Dimethyl-2,20-dinitro-[1,10-biphenyl]-4,40-dicarboxylate.
´
Dimethyl-biphenyl-4,4-dicarboxylate (1.00 g, 3.7 mmol) was
added into concentrated H2SO4 (10 mL). The mixture was stirred
2.2. Physical methods
at room temperature for 10 min. Nitric acid (760 lL, 3 eq.) was
added into concentrated H2SO4 (2 mL). This solution was added
dropwise into the first mixture at room temperature over a period
of 20 min. The mixture was stirred at room temperature for 4 h and
then poured into ice (300 mL) to form a beige solid. The resulting
solid was dissolved in dichloromethane and the aqueous phase
was extracted with dichloromethane (3 ꢂ 70 mL). The combined
organic layers were dried over Na2SO4 and evaporated under
reduced pressure to afford a beige solid. The crude mixture was
recrystallized from 2-propanol and washed with diethyl ether to
give the pure product. Yield: 1.2 g (3.33 mmol, 90%). 1H NMR
(500 MHz, CDCl3): d (ppm): 8.90 (s, 2H), 8.37 (d, J = 8.4 Hz, 2H),
7.40 (d, J = 8 Hz, 2H), 4.02 (s, 6H).
Thermogravimetric analysis (TGA) data were recorded on a
Mettler-Toledo TGA/DSC1 instrument. Thermal analysis was con-
ducted from 25 to 800 °C under a N2 atmosphere (50 mL minꢁ1
)
with a heating rate of 5 °C minꢁ1. The 1H NMR spectra of the ligand
were recorded at room temperature and on Agilent 500 MHz
instrument, with the use of the solvent proton as an internal stan-
dard. PXRD patterns were recorded on a Bruker D8 Advance X-ray
diffractometer with Cu-Ka radiation (k = 1.5418 Å). IR spectra were
recorded on KBr pellets in the 4000–400 cmꢁ1 range using a Perki-
nElmer Fourier Transform IR spectrophotometer. The UV–vis spec-
tra
in
solution
were
recorded
on
a
Hitachi-2001
spectrophotometer in the wavelength range of 200–500 nm while
UV–vis diffuse reflectance spectra were obtained at room temper-
ature on a Shimadzu 1200 PC spectrophotometer in the wave-
length range of 200–800 nm. BaSO4 powder was used both as a
reference (100% reflectance) and a base material on which the
powder sample was coated. The reflectance data were converted
to absorption using the Kubelka-Munk function. The fluorescence
spectra were measured on a Hitachi F-7000 spectrofluorometer
equipped with a red sensitive Hamamatsu R928 photomultiplier
tube detector. Appropriate long pass filters were used to remove
scattering from the sample and the monochromators.
Dimethyl-2,20-diamino-[1,10-biphenyl]-4,40-dicarboxylate.
Dimethyl-2,20-dinitro-[1,10-biphenyl]-4,40-dicarboxylate
(1.2 g,
3.33 mmol) was dissolved in 20 mL acetic acid and the solution
was stirred under Ar for 10 min. To this solution iron powder
(3.7 g, 10 eq.) was added and the mixture was stirred at room
Table 1
Selected crystal data for 1 and 2.
Compound
1
2
Chemical formula
Formula mass
Crystal system
a (Å)
b (Å)
c (Å)
C
81H77Ca6N13O30 C75H81Sr4N13O21
1953.05
monoclinic
13.782(3)
30.575 (7)
11.112 (2)
90
105.628 (7)
90
4509.1 (17)
130
1851.02
monoclinic
12.4481 (16)
25.930 (4)
13.2612 (19)
90
109.292 (4)
90
4040.1 (10)
130
P21/c
2
56760
7749
5646
2.3. Single-crystal X-ray crystallography
Single crystals of the MOFs were obtained from reaction mix-
tures according to the described synthetic procedures. For the
structural determination of 1 and 2, single crystals of the respec-
tive MOF were mounted on a Bruker Kappa APEX II diffractometer,
equipped with a triumph monochromator at ambient temperature.
Diffraction measurements were recorded using MoK
The data were collected at 130 K over a full sphere of reciprocal
space. Intensity data were collected using u and scan mode.
a (°)
b (°)
c
(°)
Unit Cell Vol (Å3)
Temperature (K)
Space group
a radiation.
P21/c
2
50527
8284
Z
x
No. of reflections measured
No. of independent reflections
No of observed reflections [I > 2.0
Rint
The frames collected for each crystal were integrated with the Bru-
ker SAINT software package [66] using a narrow-frame algorithm.
Data were corrected for absorption using the numerical method
r
(I)] 5984
0.028
0.038
0.090
0.053
0.047
0.086
R[F2 > 2
wR(F2)
r
(F2)]
(SADABS) based on crystal dimensions [67]. The powder X-ray
diffraction (PXRD) data of 1 and 2 show excellent agreement with

S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
175
temperature for 24 h. The suspension was filtered through Celite,
washed with 40 mL acetic acid and the filtrate was concentrated
under reduced pressure. The solid was dissolved in ethyl acetate
(80 mL) and was extracted with saturated aqueous sodium carbon-
ate solution (2 ꢂ 80 mL) and H2O (3 ꢂ 80 mL). The combined
organic layers were dried over Na2SO4 and the solution was evap-
orated under vacuum to yield the product as a yellow solid. Yield
900 mg (3 mmol, 90%). 1H NMR (500 MHz, DMSO d6): d (ppm):
7.42 (s, 2H), 7.21 (d, J = 7.8 Hz, 2H), 7.06 (d, J = 7.8 Hz, 2H), 4.97
(s, 4H), 3.81 (s, 6H).
1422 w, 1392 s, 1321 w, 1275 w, 1256 w, 1143 w, 1105 w, 1001
w, 909 w, 805 m, 780 m.
3. Results and discussion
3.1. Description of structures
Crystal Structure of 1. Single-crystal X-ray diffraction analysis
reveals that 1 crystallizes in the monoclinic P21/c space group
(Table 1). As seen in Fig. 1, the framework of 1 features of an infi-
2,20-Diamino-[1,10-biphenyl]-4,40-dicarboxylic
acid
H2bpdc-
(NH2)2. Dimethyl 2,20-diamino-[1,10-biphenyl]-4,40-dicarboxylate
(900 mg, 3.0 mmol) was dissolved in THF (20 mL) and 20 mL of
aqueous NaOH (0.6 M) were added dropwise under vigorous stir-
ring. The mixture was stirred overnight at room temperature.
The organic solvent was removed under vacuum and the aqueous
solution was acidified with acetic acid to yield a light brown solid
(735 mg, 2.7 mmol, 90%). 1H NMR (500 MHz, DMSO d6): d (ppm) =
12.64 (br, 2H), 7.40 (s, 2H), 7.20 (d, J = 7.6 Hz, 2H), 7.40 (d, J = 7.8
Hz, 2H), 4.90 (br, 4H).
nite rod SBU which consists of a Ca3(COO)5(l3-HCO2)(H2O)1.5(-
DMF)0.5 repeating unit and comprises three crystallographically
independent coordinated Ca2+ ions (i.e., Ca1, Ca2 and Ca3) and
three crystallographically independent bridging bpdc-(NH2)22ꢁ
dicarboxylate ligands (i.e., L1, L2 and L3). The Ca1 ion is seven-coor-
dinated by seven oxygen atoms to form a distorted face capped
octahedron in which four oxygen atoms are from the carboxylate
groups of L1 and L2 ligands, one formate oxygen (O12) originating
from DMF hydrolysis, one coordinated water molecule (O13), and
another oxygen (O14) which was modeled as originating from a
coordinated water molecule and from the C@O group of a coordi-
nated terminal DMF molecule (50% occupancy). The Ca2 and Ca3
ions are also seven-coordinated by oxygen atoms and each of them
is ligated with six carboxylate groups which belong to three differ-
2.4.2. Synthesis of MOFs 1 and 2
[Ca6(bpdc-(NH2)2)5(l3-HCO2)2(H2O)2.5(DMF)0.5]ꢀ0.5 H2Oꢀ2.5DMF
(1). CaCl2 (6.4 mg, 0.058 mmol) was added as a solid into a solution
of H2bpdc-(NH2)2 (15.0 mg, 0.055 mmol) in 3 mL DMF/H2O (9:1 v/
v), in a Teflon cup. The mixture was sonicated at room temperature
for ca. 3 min and then, the Teflon cup was transferred into a 23 mL
stainless steel autoclave. The autoclave was sealed and placed in an
oven at 120 °C, remained undisturbed at this temperature for 24 h
and was then cooled to room temperature. Plate-like brown crys-
tals of 1 were isolated by filtration, washed with DMF and dried
under vacuum for 20 h. Yield 13.0 mg (ꢃ61%). IR (KBr pellets,
cmꢁ1): 3426 m, 3360 m, 2920 w, 2850 w, 1669 m, 1654 m, 1638
m, 1629 m, 1590 s, 1541 s, 1534 s, 1432 s, 1394 s, 1358 m, 1317
w, 1287 w, 1250 w, 1147 w, 1111 w, 1089 w, 1008 w, 938 w,
899 w, 780 m, 679 m.
ent ligands and one l3- : :
g1 g2 g2 bridging formate oxygen (O11).
The geometries around Ca2 and Ca3 ion are best described as dis-
torted face capped octahedral and distorted pentagonal bipyrami-
dale respectively. Furthermore, the coordinated water molecule
(O13) forms strong hydrogen bonds (1.9–2.5 Å) with neighboring
oxygen and nitrogen moieties (O1, N5, N7). If all DMF (coordinated
and guest molecules) were removed from the structure the sol-
vent-accessible volume calculated by Mercury would be approxi-
mately 7.4% of the unit cell volume. Selected bond lengths and
bond angles for 1 are listed in Table S1.
From the topological view, the Ca2+ ions along with the car-
boxylates and the formate anions form a zig-zag chain (rod) along
the c crystallographic axis (Fig. 2A). By taking under account that
the carboxylate C atoms (C1, C14, C15, C28 and C29) are the con-
nection points along the rod, a ladder is created along c which is
composed by edge and face sharing square pyramids (Fig. 2B–E).
These ladders are connected through ligands L1 and L2 to create a
2D layer parallel to the ac plane (Fig. 2F). Finally, the layers are con-
nected through the third ligand (L3) along b to create a 3D network
Sr4(bpdc-(NH2)4)(l2-DMF)2(DMF)1/3]ꢀ2/3 (DMF) (2). Sr(NO3)2
(11.7 mg, 0.055 mg) was added as solid into a solution of H2-
bpdc-(NH2)2 (15.0 mg, 0.055 mmol) in 3 mL DMF/H2O (9:1 v/v),
in a Teflon cup. The procedure followed was identical to that for
the synthesis of MOF 1. Prism-like brown crystals of 2 were iso-
lated by filtration, washed with DMF and dried under vacuum for
20 h. Yield: 12.0 mg (ꢃ47%). IR (KBr pellets, cmꢁ1): 3436 m, 3336
m, 3208 w, 2923 w, 1654 m, 1648 m, 1583 m, 1540 s, 1487 w,
Fig. 1. Representation of the (a) infinite SBU and (b) 3-D structure along the c axis of 1.

176
S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
Fig. 2. The deconstruction of the framework of 1. Color code: Ca green, C grey, O red, H cyan, L1 and L2 red, L3 blue. (Colour online.)
(Fig. 2G). The resulting unique tri-nodal network comprises one
type of 4-coordinated and two types 7-coordinated nodes with sto-
is ꢃ10.7% of the unit cell volume. Selected bond lengths and bond
angles for 2 are listed in Table S2.
ichiometry (4-c)2(7-c)2(7-c) and has
a
point symbol of
Topologically, the Sr2+ ions along with the carboxylates groups
and the bridging DMF molecules form a chain (rod) along the 1 0
1 vector (Fig. S3A). By taking under account that the carboxylate
C atoms (C1, C14, C15 and C28) are the connection points along
the rod, a ladder is created (Fig. S3B–D). These ladders are con-
nected through bridging ligands to create a uninodal, 4-coordi-
nated sra 3D network (Fig. 3C) with point symbol (42.63.8). The
same sra rod-net has been found in MOF-69 [71], MOF-71 [72]
and MIL47/MIL53/MIL60 [2,73].
(32.42.5.6)2(35.46.54.66)2{37.46.52.86).
Crystal structure of 2. MOF 2 crystallizes in the monoclinic space
group P21/n and displays a three dimensional framework. The
structure of 2 is based on an infinite rod SBU which consists of a
[Sr2(COO)4(
graphically unique Sr2+ ions (Sr1 and Sr2). Sr1 is eight coordinated,
and is ligated to six carboxylate oxygen atoms and two 2-DMF
l-DMF)2(DMF)] unit in which there are two crystallo-
l
ligands (Fig. 3). On the other hand the Sr2 ion is nine coordinated,
and its coordination sphere contains six carboxylate oxygen atoms,
two oxygen atoms originating from two bringing DMF molecules
and an oxygen atom (O11) originating from a disordered terminal
DMF ligand (one third of which was modeled as coordinated and
two thirds as a lattice DMF molecule in a general position). It
should be noted that the amino groups of each dicarboxylic ligand
from intraligand hydrogen bonds (ꢃ3.0 Å). The terminal DMF
ligands are situated inside the cavities of the MOF (Fig. 3B). Ignor-
ing them, the solvent accessible void of 2 calculated by MERCURY
3.2. Thermal stability
The thermal stability of 1 and 2 was studied by thermogravi-
metric analysis (TGA) under N2 atmosphere. The thermograph of
1 is shown in Fig. 4 with red line. The weight loss was observed
in three steps. In the first step (100–170 °C) we observe weight loss
of the coordinated and guest water molecules (experimental loss of
2.85%, theoretically estimated loss of 2.77%). The second stage of
Fig. 3. Representation of the (a) SBU and (b) 3-D structure along the a axis (c) sra network of 2. H atoms and solvent molecules were omitted for clarity. Color code: Sr, green;
C, gray; O, red; N, blue. (Colour online.)

S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
177
gand 1pp* and 1n
p
* transitions. The relatively small variation of ca.
19 nm in the emission maxima of the two MOFs can possibly be
assigned to the different charge density of the coordinated Ca2+
and Sr2+ ions and to the different conformation of the fluorophores
in 1 and 2 [51,64]. As we can see in the crystal structure of 2, only
one type of ligand exists, in contrast to the case 1 where the ligand
exists in three different conformations with torsion angles
between amino groups of 62.7°, 69.4° and 130.5°.
3.4. Fluorescence quenching by volatile aldehydes
Volatile aldehydes such as acetaldehyde are widely used in the
chemical industry as precursors to pyridine derivatives and various
polymers and constitute an important class of organic pollutants
known for their toxicitity and carcinogenicity even at low exposure
levels [74,75]. For example, acetaldehyde is classified as toxic and
possibly carcinogenic upon long term workplace exposure and has
a threshold limit value (TLV) of 25 ppm [76]. Therefore, there is
always a need for the development of new sensors which can
rapidly and reliably detect and quantify aldehyde vapors. Lumines-
cent MOFs are emerging as a new highly promising class of fluores-
cent sensory materials due to their ease of functionalization
combined with their porosity and diverse emission properties
[22,77–80]. However, even though there is a plethora of examples
where MOFs act as sensors in liquid media, sensing of gas phase
analytes by MOFs remains relatively rare [81,82].
The fact that both MOFs 1 and 2 possess three dimensional
structures which contain uncoordinated amino groups which are
highly reactive towards aldehydes, motivated us to investigate if
they can interact with gas phase aldehydes thereby providing a
possible means for their detection. Thus, small quantities of crys-
tals of 1 and 2 were immobilized on a side window of a standard
3.5 mL quartz fluorescence cuvette and their fluorescence spectra
Fig. 4. The TGA curves for MOFs 1 and 2 (red and black line respectively). (Colour
online.)
thermal decomposition begins at 170 °C and ends at 290 °C and
includes the possible loss of the coordinated and guest DMF mole-
cules (experimental loss of 11.35%, theoretically estimated loss of
11.25%), followed by decomposition of the framework up to 800
°C. In the case of 2 shown with black line the weight loss was
observed at two discreet steps. The first weight loss (100–420 °C)
corresponds to the removal of three coordinated and guest DMF
molecules (experimental loss of 11.65%, theoretically estimated
loss of 11.84%) before decomposition of the framework sets in.
3.3. Photoluminescence properties
were recorded. Subsequently, a small amount (50 lL) of the tested
The luminescence properties of MOFs 1 and 2 were studied in
the solid state at room temperature. As shown in Fig. S4 the H2-
bpdc-(NH2)2 ligand displays turquoise fluorescence in methanol
with emission maximum at 475 nm (kexc = 330 nm). The emission
is likely derived from the radiative decay of spin allowed 1pp*
and 1np* excited states (250 and 330 nm respectively) the latter
involving the lone pair of the amino group (Fig. S5). Upon excita-
tion at 350 nm, 1 and 2 exhibit blue-shifted emission (Fig. 5A) in
comparison to the free ligand (1: kem = 445 nm; 2: kem = 464 nm).
Based on the electronic configurations of Ca2+ and Sr2+ ions and
the similarity of the emission spectra to that of the free ligand,
the fluorescence of both MOFs can similarly be assigned to intrali-
aldehyde (acetaldehyde, propionaldehyde and isobutylaldehyde)
was placed with the use of a microsyringe at the bottom of the cuv-
ette taking care that the liquid does not come to direct contract
with the crystals of 1 and 2 and the cuvette was firmly capped
thereby creating a chamber saturated with the vapors of the tested
aldehyde [83]. The emission spectra were recorded over a period of
45 min. Fig. 6 displays the time-dependent emission spectra of 1
and 2 in the presence acetaldehyde vapors. Upon exposure to
acetaldehyde vapors the emission of 1 is instantly quenched by
more than 75% of its original intensity and by 81% after 45 min
(Fig. 6A) while the emission of 2 is quenched by 74% with a con-
comitant modest red shift of ca. 17 nm (Fig. 6B).
Fig. 5. Solid state (A) emission (kexc = 350 nm) and (B) excitation spectra of 1 and 2 at room-temperature.

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S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
Fig. 6. Emission spectra over time of (A) 1 and (B) 2 upon exposure to acetaldehyde vapors (kex = 370 nm).
Fig. 7. Emission spectra over time of 1 upon exposure to (A) propionaldehyde and (B) isobutylaldehyde vapors (kex = 370 nm).
Fig. 8. Emission spectra over time of 2 upon exposure to (A) propionaldehyde and (B) isobutylaldehyde vapors (kex = 370 nm).

S.A. Diamantis et al. / Polyhedron 153 (2018) 173–180
179
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To evaluate the sensing ability of 1 and 2 towards other volatile
aldehydes we performed additional sensing experiments using this
time propionaldehyde and isobutylaldehyde. As seen in Fig. 7A the
presence of propionaldehyde for 6 min leads to a significant loss of
the initial emission intensity of 1 by ca. 59% and a concomitant red-
shift of 16 nm; reaching equilibrium after a period of 12 min, while
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to propionaldehyde (Fig. 8A) and isobutaldehyde (Fig. 8B). It was
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and 39% respectively. It can generally be observed that 1 and 2
show a significantly more pronounced response towards acetalde-
hyde both in terms of kinetics and total quenching percentage in
comparison to propionaldehyde and isobutaldehyde. This differ-
ence may be attributed to the smaller molecular size and the
greater volatility of acetaldehyde in comparison to its bulkier
counterparts.
´
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As shown in the Fig. S6, the aldehydes exhibit a strong absorp-
tion from 240 to 330 nm, with virtually no absorption at the exci-
tation wavelength of 370 nm. Additionally, their absorption
spectra do not show any overlap with the absorption band of nei-
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and 2 have relatively small solvent accessible voids and small
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4. Conclusion
In summary, two new alkaline earth metal ion MOFs with the
H2bpdc-(NH2)2 ligand were isolated. Both MOFs showed three
dimensional structures and displayed interesting structural fea-
tures and diverse structural topologies. 1 and 2 exhibit ligand
based bright blue fluorescence at room temperature in the solid
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Appendix A. Supplementary data
CCDC 1833862–1833863 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at https://doi.org/10.
1016/j.poly.2018.07.010.
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