Synthesis and spectroscopic studies of diaza-8-crown-4-dinitrophenyl ethers

Article abstract of DOI:10.1080/10610278.2019.1662420

The macrocycles 3,7-bis(3-nitrophenyl)-1,5,3,7-dioxadiazocane 1 and 3,7-bis(4-nitrophenyl)-1,5,3,7-dioxadiazocane 2 were synthesised by a one-pot reaction with substituted nitroanilines and formaldehyde under acidic conditions. The same reaction with 2-ni

Full text of DOI:10.1080/10610278.2019.1662420

Supramolecular Chemistry
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Synthesis and spectroscopic studies of diaza-8-
crown-4-dinitrophenyl ethers
Claudia Galea, Damjan Makuc, Konrad Szaciłowski, Janez Plavec & David C.
Magri
To cite this article: Claudia Galea, Damjan Makuc, Konrad Szaciłowski, Janez Plavec & David
C. Magri (2019): Synthesis and spectroscopic studies of diaza-8-crown-4-dinitrophenyl ethers,
Supramolecular Chemistry, DOI: 10.1080/10610278.2019.1662420
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SUPRAMOLECULAR CHEMISTRY
https://doi.org/10.1080/10610278.2019.1662420
ARTICLE
Synthesis and spectroscopic studies of diaza-8-crown-4-dinitrophenyl ethers
Claudia Galeaa, Damjan Makucb, Konrad Szaciłowskic, Janez Plavecb,d and David C. Magria
b
aDepartment of Chemistry, Faculty of Science, University of Malta, Msida, Malta; Slovenian NMR Centre, National Institute of Chemistry,
c
d
Ljubljana, Slovenia; Academic Centre of Materials and Nanotechnology, AGH University of Science and Technology, Kraków, Poland; EN-
FIST Centre of Excellence, Ljubljana, Slovenia
ABSTRACT
ARTICLE HISTORY
Received 7 May 2019
Accepted 27 August 2019
The macrocycles 3,7-bis(3-nitrophenyl)-1,5,3,7-dioxadiazocane 1 and 3,7-bis(4-nitrophenyl)-1,5,3,7-diox-
adiazocane 2 were synthesised by a one-pot reaction with substituted nitroanilines and formaldehyde
under acidic conditions. The same reaction with 2-nitroaniline yielded N, N’-(oxybis(methylene))bis
(2-nitroaniline) 3 rather than 3,7-bis(2-nitrophenyl)-1,5,3,7-dioxadiazocane 4. The yellow powders 1–3
were characterised by 1H/13C/15N NMR, FTIR and HRMS. The photophysical properties were studied by
UV-visible absorption and steady-state fluorescence spectroscopy. Acid dissociation constants were
determined by UV-visible absorption titrations in 1:1 (v/v) acetonitrile/water to be 0.80 and 3.1 for the
meta- and para-substituted compounds 1 and 2, respectively. Compounds 1 and 2 were discovered to
fluoresce in the solid-state, yellow and green, respectively, but not in solution. Density functional theory
(DFT) calculations for 1–4 provide insight into the frontier molecular orbital energy levels. These
compounds represent a new class of fluorescent heterocyclic building blocks for organic and supra-
molecular applications.
KEYWORDS
Dioxadiazocanes;
macrocycle; internal charge
transfer; nitrogen NMR
spectroscopy; solid-state
fluorescence
1. Introduction
was reported by Gunnlaugsson as part of a fluorescent
chemosensor for Li+ (9, 10).
Crown ethers (1–5) are widely used in the design of photo-
responsive materials (6) biomedical applications (7) and
molecular logic-based computation (8). The standard
crown ethers used in sensing applications typically consist
of 12 or more atoms, although examples with fewer atoms
do exist. For example, a nine-atom diaza-9-crown-3 ether
1,5,3,7-Dioxadiazocanes are a class of eight-atom
heterocycle with two nitrogen and two oxygen atoms
separated by methylene units. A search through the
literature reveals that studies on 1,5,3,7-dioxadiazo-
canes (11–15) and 1,5,3,7-dithiadiazocanes (16) are
rather limited to synthesis and crystallographic analysis.
CONTACT David C. Magri
david.magri@um.edu.mt
Department of Chemistry, Faculty of Science, University of Malta, Msida MSD 2080, Malta
Supplemental data for this article can be accessed here.
© 2019 Informa UK Limited, trading as Taylor & Francis Group

2
C. GALEA ET AL.
They adopt a crown-like conformation with the N-aryl
substituents on the same side of the eight-membered
ring. To the best of our knowledge, there are no
reported studies on the photophysical and physio-
chemical properties of this class of compound.
a Bruker Aspect 3000 computer. Tetramethylsilane (TMS)
was used as the internal reference for H NMR (δ0.00
1
ppm). NMR samples were taken at 298 K using 5–10 mg
of sample dissolved in 0.8 mL of DMSO-d6. Chemical shifts
are reported in parts per million downfield of TMS.
Heteronuclear Multiple Bond Correlation (HMBC) spectra
were recorded on a DD2 Agilent Technology at frequen-
Hence, we were curious to explore whether
1,5,3,7-dioxadiazocanes may be suitable as fluorescent
materials (17, 18), and perhaps, as proton and lithium
chemosensors (19). A promising lead was finding from
the literature that the crown cavity diameter is 3.61 Å
and that the ionic diameter of Li+ is 1.45 Å (3). Because of
the close proximity of the two nitrogen atoms, we also
postulated that perhaps 1,5,3,7-dioxadiazocanes may act
as proton sponges and form an N-H-N bond (20–23).
In this paper, we present our findings including the
synthesis and spectroscopic studies of heterocyclic 3,7-bis
(nitrophenyl)-1,5,3,7-dioxadiazocanes isomers 1 and 2 as
internal charge transfer (ICT) chemosensors (1). The
1,5,3,7-dioxadiazocanes possess two nitrophenyl moieties
according to a chromophore-receptor-spacer-receptor-
chromophore arrangement (24, 25). We studied the
compounds by UV-visible absorption, steady-state
fluorescence and 1H/13C/15N 2D NMR correlation spectro-
scopies and density functional theory (DFT) to gain
insight into their physiochemical properties.
1
cies of 599.66 MHz and 60.78 MHz for H and 15N NMR,
respectively. Chemical shifts are referenced to the residual
solvent signal of DMSO-d6 while 15N chemical shifts were
referenced relative to ammonia (δ0.00 ppm). 13C NMR
spectra were recorded at a frequency of 150.8 MHz. FTIR
spectra were recorded as KBr discs with dried potassium
bromide on a Shimadzu IRAffinity-1 spectrophotometer
calibrated using 1601 cm−1 polystyrene absorption peak.
Low-resolution mass spectra were acquired by a triple
quadrupole mass spectrometer-TQD mass spectrometer
(Waters Ltd. UK) equipped with an Acquity UPLC and an
electrospray ion source. High-resolution mass spectra
were obtained with a quadrupole time-of-flight instru-
ment-Xevo QToF mass spectrometer (Waters Ltd. UK)
equipped with atmospheric pressure gas chromatogra-
phy (APGC) and solids analysis probe (ASAP). Melting
points were obtained using a Stuart melting point
apparatus.
2.3. Synthesis
2. Experimental
2.3.1. Synthesis of 3,7-bis(3-nitrophenyl)-
1,5,3,7-dioxadiazocane 1
2.1. Materials
Formic acid (AnalaR, BDH), formaldehyde (36.5%, Sigma-
Aldrich), 1-amino-4-nitrobenzene (97%, Cambrian
Chemicals), 1-amino-3-nitrobenzene (BDH), 1-amino-
2-nitrobenzene (96%, Cambrian Chemicals), tetrahydro-
furan (99.9%, Sigma-Aldrich), lithium perchlorate (99%,
Hopkin & Williams Ltd), acetonitrile (99.9%, Sigma-
Aldrich), methanol (99.9%, Sigma-Aldrich), chloroform
(99.9%, Fisher Scientific) and dimethyl sulfoxide (99.9
atom D%, Acros Organics) were used as received.
The meta-substituted product was synthesised using
a literature procedure (12). 1.4 g (10 mmol) of 3-nitroani-
line and 20 μL (0.025 g, 0.55 mmol) of formic acid were
dissolved in 50 mL of acetonitrile in a 100 mL round-
bottomed flask. After dissolution, 1.2 mL (1.2 g, 15 mmol)
of formaldehyde was added. The reaction mixture was
stirred at room temperature for 24 h. A yellow powder
was collected by suction filtration and washed with cold
acetonitrile in 80% yield (1.08 g). m.p. 226–227°C; 1H NMR
(250 MHz, DMSO-d6, ppm): 4.90–5.60 (br s, 8H), 7.00–7.60
(m, 8H); 13C NMR (63 MHz, DMSO-d6, ppm): 81.4, 109.0,
112.8, 121.0, 129.9, 145.1, 147.8; IR (KBr, νmax, cm−1): 3110,
2972, 2935, 2885, 1602, 1525, 1350, 1136, 1028, 950, 870,
848, 725; UV-vis (1:1 (v/v) MeCN:H2O, λmax/nm (log ԑ)): 370
(3.50); UV-vis (MeCN, λmax/nm (log ԑ)): 367 (3.38); HRMS
(ESI-TOF): m/z Calc. for C16H17N4O6 [M + H] 361.1156,
found 361.1156.
2.2. Instrumentation
Ultraviolet-visible absorption spectra were measured with
a Jasco V-650 spectrophotometer at a scan speed of 200
nm min–1 between 200 and 500 nm. Steady-state fluores-
cence spectra were recorded on a Jasco FP-8300 fluores-
cence spectrometer as solid samples on glass slides
positioned at 45 degrees to the incident excitation wave-
length. pH measurements were performed with a Hanna
Instruments 210 pH metre calibrated with pH 4.0 and 7.0
2.3.2. Synthesis of 3,7-bis(4-nitrophenyl)-
1,5,3,7-dioxadiazocane 2
The para-substituted product was synthesised
using a similar procedure with 4-nitroaniline (12).
Recrystallisation from THF resulted in a yellow powder
1
buffers. H and 13C NMR spectra were initially measured
with a Bruker AM250 NMR spectrometer fitted with
1H/13C 5 mm dual probe at 250.1 and 62.9 MHz using

SUPRAMOLECULAR CHEMISTRY
3
in 65% yield (0.90 g). m.p. 256–258°C; 1H NMR
(250 MHz, DMSO-d6, ppm): 4.90–5.80 (br s, 8H), 6.95
(d, J = 9.4 Hz, 4H), 7.86 (d, J = 9.4 Hz, 4H); 13C NMR
(63 MHz, DMSO-d6, ppm): 81.6, 114.1, 124.3, 138.5,
150.0; IR (KBr, νmax, cm−1): 3116, 3090, 2950, 1597,
1508, 1323, 1303, 1030, 990, 933, 890, 750; UV-vis (1:1
(v/v) MeCN:H2O, λmax/nm (log ԑ)): 351 (4.53); (MeCN,
λmax/nm (log ԑ)): 346 (4.45); HRMS (ESI-TOF): m/z Calc.
for C16H17N4O6 [M + H] 361.1156, found 361.1133.
pH and pKa is the negative logarithm of acid dissocia-
tion constant.
2.5. DFT computations
The ground and excited states electronic structure was
computed with the GAUSSIAN09 E.01 software pack-
age (27). Hybrid DFT was employed within the
Coulomb-attenuated method, CAM-B3LYP exchange–
correlation functional (28), 6–311 + g(d,p) basis
functions and a def2tzv fitting set. Unconstrained opti-
misation of the molecular geometry was carried out
until the total energy converged to 10 − 8 Hartree in
the self-consistent electronic loop. Electronic transi-
tions were computed using time-dependent DFT
method using the same functional and basis sets as
for geometry optimisation. Electronic absorption spec-
tra were calculated assuming the spectral bandwidth
of 0.333 eV.
2.3.3. Synthesis of N, N’-(oxybis(methylene))bis
(2-nitroaniline) 3
The product was synthesised using a similar procedure
with 2-nitroaniline (12). Recrystallisation from THF
resulted in a yellow powder in 49% (0.67 g). m.p.
1
176–178°C; H NMR (250 MHz, DMSO-d6, ppm): 4.90
(d, J = 6.6 Hz, 4H), 6.80 (t, J = 6.7 Hz, 2H), 7.20 (d, J = 8.7
Hz, 2H), 7.50 (t, J = 7.1 Hz, 2H), 8.10 (d, J = 8.7 Hz, 2H),
8.80 (br t, J = 6.5 Hz, 2H); 13C NMR (63 MHz, DMSO-d6,
ppm): 69.0, 115.6, 116.8, 125.7, 132.3, 135.8, 143.5; IR
(KBr, νmax, cm–1): 3401, 3084, 1614, 1506, 1345, 1271,
1170, 1030, 1006, 890, 746; UV-vis (1:1 (v/v) MeCN:H2O,
λmax/nm (log ԑ)): 406 (4.12); UV-vis (MeCN, λmax/nm
(log ԑ)): 402 (4.09); HRMS (ESI-TOF): m/z Calc. for C14
H14N4O5Na+ 341.0856, found 341.0858.
2.6. Solid-state fluorescence studies
Solid-state fluorescence measurements were done on
solid samples placed between two glass microscope
slides. The slides were set at a 45° angle to the excita-
tion and emission slits. The excitation wavelength was
set at 370 nm, 351 nm and 406 nm for 1–3, respectively,
and scanned from 450 to 600 nm. The excitation and
emission slits were set at 5.0 nm. Measurements were
background corrected.
2.4. pH and metal ion titrations
UV-visible absorption titrations were carried out as
a function of proton concentration with 50 μM solution
of 1–3 in 50 mL of 1:1 (v/v) acetonitrile/water. Stock
solutions were prepared by dissolving ca. 10 mg of
sample in 100 mL of acetonitrile and diluting with 1:1
(v/v) acetonitrile/water to the appropriate absorbance.
The acid dissociation constants (pKa) were determined
from absorbance-pH plots and fitted to a linearised
Henderson-Hasselbalch equation adapted for spectro-
scopic measurements (eq.(1)) (26):
3. Results and discussion
3.1. Synthesis and characterisation
The
3,7-di(nitrophenyl)-1,5-dioxa-3,7-diazacyclooctanes
were synthesised by a one-pot reaction using the proce-
dure by Kakanejadifard (Figure 1) (11). The 2-, 3- and
4-nitroanilines were reacted with formaldehyde in formic
acid (12). 3,7-bis(3-nitrophenyl)-1,5,3,7-dioxadiazocane 1
and 3,7-bis(4-nitrophenyl)-1,5,3,7-dioxadiazocane 2 were
obtained in 80% and 65% yield as yellow powders.
The solids are readily soluble in DMSO, acetonitrile and 1:1
(v/v) acetonitrile/water and insoluble in water, methanol,
ꢀ
ꢁ
Amax ꢀ A
A ꢀ Amin
log
¼ pH ꢀ pKa
(1)
where Amax and Amin are the maximum and minimum
peak absorbances, A is the absorbance at the specific
Figure 1. The one-pot syntheses of 1–3.

4
C. GALEA ET AL.
Table 1. pKa and photophysical parameters of 1–3 in 1:1 (v/v)
water/acetonitrile and acetonitrile determined by UV-visible
absorption spectroscopy.
ethanol, acetone, chloroform and dichloromethane.
The reaction with 2-nitroaniline yielded N, N’-(oxybis-
(methylene))bis(2-nitroaniline) 3 in 49% yield rather than
3,7-bis(2-nitrophenyl)-1,5,3,7-dioxadiazocane 4. The com-
Parameter
1
2
3.1b
351
4.53
282
3
a
pKa
0.80
370
3.50
<0c
406
4.12
1
pounds were characterised by H/13C/15N NMR, IR, HRMS
λmax (1:1 MeCN/H2O)/nm
Log ε (/cm−1 M−1
λisos/nm
)
(See experimental and Figures S1-S12).
d
275
–
1
The H NMR spectrum of the meta-substituted 1 in
λmax (MeCN)/nm
367
3.38 (3.44)
346
4.45 (4.50)
402
4.08 (3.77)
Log ε (/cm−1 M−1 e
)
DMSO-d6 shows a broad signal at 5.3 ppm correspond-
ing to the eight methylene protons of the macrocycle
(Figure S1). The complex splitting pattern between 7.1
ppm and 7.6 ppm for the aromatic proton resonances is
due to second-order coupling effects. A 1:1 integration
ratio is calculated between the area of the broad singlet
and the total area under the aromatic peaks. The 1H NMR
spectrum of the para-substituted 2 has a broad signal at
5.3 ppm and two doublets at 6.95 ppm and 7.86 ppm,
which is typical of an AB quartet splitting pattern and an
integration ratio of 1:1:2 (Figure S3). The 13C NMR spec-
trum of 1 reveals seven magnetically non-equivalent
carbon atoms (Figure S2), while the spectrum of 2
shows five different carbon atoms (Figure S4). The mass
spectra of 1 and 2 provide a molecular ion peak M+
at m/z 361.1 (Figures S7-S8).
aError ꢁ 0.10. Measurements done in duplicate. bBased on UV-vis data
isosbestic point observed. eIn parentheses from simulated UV-visible
spectra.
c
between pH 1 and 5. Only minor changes on titration to pH 0.0. dNo
367 nm, 346 nm and 402 nm, respectively. The log ε
are 3.38, 4.45 and 4.08 for 1–3, respectively. In 1:1 (v/v)
ACN/water there is a slight bathochromic shift of 3 to
5 nm: the peak maxima appear at 370 nm, 351 nm and
406 nm, respectively (Figure 2). The log ε values in 1:1 (v/v)
aqueous ACN are 3.50, 4.53 and 4.12 for 1–3, respectively.
The peak shift is attributed to an ππ* ICT state, which is
better stabilised in the more polar solvents.
Acid-base titrations were conducted in 1:1 ACN/water
(Figure 3). No significant changes were observed in the
UV-visible absorption spectra above pH 5.0, hence titra-
tions focused on the range between pH 0 and 5. Titration
of 1 with acid resulted in a downward shift at 370 nm,
a dramatic decrease at 230 nm and the appearance of an
isosbestic point at 275 nm. These results suggest proto-
nation occurs at an anilinic nitrogen atom site. Titration of
2 results in an increase in the peak at ca. 350 nm accom-
panied by a pronounced bathochromic shift of 25 nm up
to pH 1 and followed by a decrease on addition of further
acid. An isosbestic point occurs at 282 nm. The pK
As stated previously, our attempts to prepare 3,7-bis
(2-nitrophenyl)-1,5,3,7-dioxadiazocane 4 were unsuccess-
ful, and instead we obtained N, N’-(oxybis(methylene))bis
1
(2-nitroaniline) 3. The H NMR spectrum of 3 is shown in
Figure S5. Four resonances are observed between 6.6 ppm
and 8.2 ppm, while a sharp doublet and a broad distorted
triplet appear at 4.90 ppm and 8.80 ppm, respectively.
A closer analysis of the 1H NMR revealed a signal integra-
tion ratio of 1:4:2. The IR spectrum has a strong sharp peak
at ca. 3400 cm−1 (Figure S12) characteristic of
N-H stretching of a secondary amine, which is absent in
the IR spectra of 1 and 2 (Figures S10-11). The 13C NMR
spectrum of 3 reveals seven magnetically non-equivalent
carbon atoms (Figure S6). The resonance at 69.0 ppm is
attributed to the two methylene carbon atoms, which is
significantly lower than the resonance of 81.4 ppm for the
four methylene carbon atoms of 1 (Figure S2). Correlation
signals in the 2D NMR spectra, which included 1H-13C HSQC
and HMBC (Figures S13-S14) allowed for the unequivocal
structure elucidation of 3. Notably, no correlation was
observed for the NH resonance at 8.80 ppm in the 2D
1H-13C HSQC. The HRMS confirmed an expected molecular
ion peak M+ at m/z 341.1 (Figure S9).
a values
of 1 and 2 were determined from the absorbance-pH
plots fitted to the linearised Henderson-Hasselbalch
(equation 1) to be 0.80 and 3.1, respectively (26). The
pKa values of 4-nitroaniline and 3-nitroaniline of 1.0 and
2.5 are of comparable magnitude to 1 and 2 suggesting
there is little if any eclipsing interaction between the
orbitals of the two anilinic nitrogen atoms. Compound 3
showed no significant change in the UV-visible spectra
even at pH 1 (Figure S15).
The UV-visible absorption titrations of 1–3 with Li+ in
1:1 (v/v) ACN/water showed no significant changes
(Figures S16-18). Therefore, there is no clear evidence
for metal ion-ligand complexation. These results are
contrary to the selective nine-atom diaza-9-crown-3
ether Li+ chemosensor by Gunnlaugsson (9, 10). The
lack of complexation by 1 and 2 is postulated to be
due to two possibilities: 1) the cavity size is perhaps one
atom too small, and 2) one or both of the carbonyl
oxygen atoms adjacent to the diaza-9-crown-3 ether
assist to coordinate Li+.
3.2. UV−vis absorption spectrophotometric studies
The UV-visible absorption spectra of 1–3 were obtained in
acetonitrile (ACN) and 1:1 (v/v) ACN/water. A summary of
the ground state photophysical parameters is tabulated in
Table 1. In ACN peak maxima for 1–3 are observed at

SUPRAMOLECULAR CHEMISTRY
5
Figure 2. UV-visible absorption spectra of 50 μM 1 (black), 2 (red) and 3 (green) in 1:1 (v/v) acetonitrile/water.
Figure 3. UV-visible absorption spectra of 1 and 2 as a function of pH in 1:1 (v/v) acetonitrile/water.

6
C. GALEA ET AL.
a
3.3. 1H NMR titration studies
Table 2. Select N chemical shifts (δ) of 1–3 in DMSO-d6 as
a function of pD.
1H NMR titrations were performed in DMSO-d6 for the
three compounds by adding increasing concentrations
of DCl acid. Figure 4 shows a comparison of the 1H NMR
spectra of meta-substituted 1 at pD 7 and pD 4. Full sets
of NMR titration spectra are shown in the ESI for 1–3
(Figures S19, S22 and S25). At neutral pD, compound 1
has a single set of proton resonances. However, as the
concentration of acid increased, more proton resonances
are observed due to 1H+ from the protonation of an
anilinic nitrogen atom. Notably, the broad peak corre-
sponding to the methylene protons of the macrocycle at
5.3 ppm was replaced by several overlapping resonances
in the region between 4.5 ppm and 6.0 ppm due to
a loss of symmetry. Analogous 1H NMR spectral changes
were observed for 2 (Figure S22). Titration of 3 with DCl
results in the four aromatic resonances between 6.5 ppm
and 8.5 ppm splitting into eight resonances indicating
a loss of symmetry due to protonation (Figure S25). The
broad peak at 8.80 ppm assigned to the NH atom dis-
appears on addition of DCl as a result of deuterium
exchange.
pD 7
pD 3
Anilinic
Compound
Anilinic
NO2
NO2
1
2
3
92
102
86
372
371
373
1
76, 86, 86, 89, 91, 107
76, 90, 101, 118
78, 95
372
371
373
aN NMR δ were determined by H–15N correlations in gHMBC spectra.
nitro moieties at δN 92 ppm and 372 ppm, respectively
(Figure S20). On addition of acid at pD 3, the same two
15N chemical shifts were observed. Additional correlation
signals were observed for the methylene diaza-crown
ether groups of 1, with intense peaks at δN 76, 86, 86, 89
and 107 ppm (Figure S21). Several nitrogen nuclei
between 76 ppm and 107 ppm could be assigned to the
anilinic moieties, tentatively corresponding to the proto-
nated and perhaps diprotonated forms of the diaza-crown
ethers. Furthermore, multiple signals were observed in the
1H-15N NMR spectra of 1 indicating a loss of the symmetry
due to protonation of an anilinic nitrogen atom.
1H-15N HMBC spectra of the para-nitro analogue 2
showed two nitrogen atoms at pD 7 at δN 102 ppm and
371 ppm (Figure S23). Similar to compound 1, the
increased concentration of DCl to 2 led to the detection
of new resonances at pD 3 (Figure S24). In addition to
unprotonated anilinic and nitro moiety atoms, three corre-
lation signals were observed at δN 76, 90 and 118 ppm.
3.4. 1H-15N NMR titration studies
1H-15N HMBC spectra of 1–3 were obtained in DMSO-d6 at
pD 7 and pD 3 to better understand the protonation
equilibria (29–31). The 15N NMR chemical shifts are pro-
1
Overlapping H NMR resonances assigned to the methy-
1
vided in Table 2 and the accompanying H-15N HMBC
lene protons of the diazacrown ether moieties suggest the
presence of protonated and unprotonated variants of 2 at
acidic conditions. A single set of 1H-15N correlation signals
were also observed for 3 at pD 7. The NH nitrogen atom
resonance appears at δN 86 ppm and the nitro group at δN
spectra are available in the ESI (Figures S20, S21, S23,
S24, S26, S27). Two nitrogen atom resonances are
observed for meta-substituted 1 at pD 7 attributed to
the unprotonated nitrogen atoms of the anilinic and
1
Figure 4. H NMR spectra of compound 1 (meta) at pD 7 (top) and pD 4 (bottom) in DMSO-d6.

SUPRAMOLECULAR CHEMISTRY
7
373 ppm (Figure S26). Upon addition of acid, the
1H-15N HMBC spectra of 3 showed nitrogen atom reso-
nances at δN 78 ppm and 95 ppm (Figure S27). Similar to
1 and 2, additional resonances appear likely due to
a monoprotonated, and perhaps diprotonated species at
lower pD values.
lowest occupied molecular orbitals (LUMOs) reside
extensively on the nitrophenyl moieties in the cases of
1–4. The significant difference in the location of the
HOMOs and LUMOs clearly illustrates the ICT character.
Electron absorption spectra calculated with the TD-
DFT method are in good agreement with the experi-
mental ones (Figure S28). The lowest energy transition
involves various combinations of HOMO→LUMO (1,2,3),
HOMO-1→LUMO (4) and HOMO→LUMO+1 (1, 2) exci-
tations. An exceptionally high absorptivity of the low
energy transition in the case of 2 is attributed to even
distribution of the HOMO-1, HOMO, LUMO and LUMO
+1 over both nitrophenyl rings. The increased molar
absorptivity and higher energy of the lowest excitation
results from the characteristics of the p-nitrophenyl
and m-nitrophenyl chromophore are demonstrated
with the mono-substituted crown ethers (Figure S29).
Simulated UV-visible absorption spectra of the meta 1
and para 2 monosubstituted crown ethers have λmax at
335 nm (log ε 3.20) and 294 nm (log ε 4.29).
3.5. DFT calculations
DFT calculations were performed for compounds 1–4 at
the cam-b3lyp/6-311 + g(d,p)/def2tzv theory level
(Figure 5). Frontier molecular orbital contours were
calculated for the HOMO-1, HOMO, LUMO and LUMO-
1 levels. The heterocyclic cyclooctane ring adopts
a stable crown-like conformation at room temperature
(14) with the two nitrobenzene groups syn to the eight-
membered heterocyclic ring in agreement with the
reported crystal structures of 1 (11, 13) and 2 (14). The
highest occupied molecular orbitals (HOMO, HOMO-1)
describe the charge density of the lone electron pairs.
The electrons are predicted to be evenly delocalised
over the entire molecule in the case of 1, 2 and 4.
The largest electron density associated with HOMOs
resides primarily on the nitrobenzene moieties,
although the heterocyclic ring appears to be involved
in HOMO orbitals as well. In the case of 3, the HOMO is
localised on one aromatic ring and the HOMO-1 on the
other, with a negligible contribution of the linker. The
3.6. Solid-state fluorescence
Compounds 1–3 are yellow powders (Figure 6). We
discovered that the heterocycles 1 and 2 exhibit fluor-
escence in the solid-state (32, 33), while 3 is non-
fluorescent. Irradiated with 365 nm UV light, powder 1
emits yellow light and 2 bluish-green light (Figure 6).
Figure 5. Depiction of the calculated HOMOs and LUMOs for 1–4.

8
C. GALEA ET AL.
Figure 6. The solids 1–3 (left to right) under room light (top) and irradiated with a 365 nm UV lamp in a dark cabinet (bottom).
The solid-state fluorescence spectra confirm these
observations: the emission spectrum of 1 is broadly
centred at 542 nm, while the emission spectrum 2 has
a maximum at 490 nm and a skewed long tail out to
600 nm (Figure 7). The large Stokes shifts of 172 nm
and 139 nm for 1 and 2, respectively, minimise self-
absorption quenching (34). The strong fluorescence in
the solid-state is presumably because of reduced inter-
molecular interactions more typical in solution. The
non-planar crown conformation results in weak inter-
molecular interactions and restricts intramolecular rota-
tion, thus allowing the solids to be emissive (35). In
contrast, all three compounds are non-fluorescent in
acetonitrile or 1:1 (v/v) acetonitrile/water. It is hypothe-
sised that hydrogen bonding between the nitro moi-
eties (and the NH in 3) with acetonitrile or water causes
non-radiative vibrational relaxation of the excited state
energy.
Further insight comes for the quantum-chemical model-
ling, which indicates that the electronic structures of 1 and
2 are significantly different than 3 (Figure 5). The HOMO
(and also the LUMO) orbitals in compounds 1 and 2 are
highly delocalised over the whole molecular frameworks,
whereas in 3 only one ring system is involved in the frontier
orbitals. It can be hypothesised that this particular arrange-
ment is responsible for the photoluminescence of
Figure 7. Fluorescence emission spectra of solid samples of 1 (black), 2 (red) and 3 (green) at ambient temperature excited at
370 nm, 351 nm and 406 nm, respectively.

SUPRAMOLECULAR CHEMISTRY
9
compounds 1 and 2, in which the electronic structure
should efficiently inhibit thermal deactivation of the
HOMO-LUMO emissive excited state. On the other hand,
the electronic structure of 3 should result in significantly
weaker intermolecular interactions in the solid-state, which
in turn will be manifested by efficient energy dissipation
through vibrational degrees of freedom. This phenomenon
is another manifestation of aggregation-induced emission
observed in small molecular compounds (35, 36).
Funding
This research was funded by the University of Malta, the
Slovenian Research Agency [P1-0242] and the Polish
National Science Centre [UMO-2015/17/B/ST8/01783].
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●
●
●
Diaza-8-crown-4-dinitrophenyl ethers were synthesised
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The compounds are fluorescent in the solid-state, but not
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Financial support is acknowledged from the University of
Malta, the Slovenian Research Agency, the Ministry of
Education, Science and Sport of the Republic of Slovenia
(program no. P1-0242), the National Science Centre (Poland)
within the contract number UMO-2015/17/B/ST8/01783, and
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Disclosure statement
Tetranuclear Halocuprate(I) Clusters of
a Clamplike
There are no conflicts of interest to declare by the authors.
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