Aggregation-Induced Emission of Platinum(II) Metallacycles and Their Ability to Detect Nitroaromatics

Article abstract of DOI:10.1002/chem.201600698

Two new acceptors containing platinum-carbazole (1) and platinum-triphenylamine (2) backbones with bite angles of 90° and 120°, respectively, have been synthesised and characterised. Reactions of the rigid acceptor 1 with linear dipyridyl-based donors (3 and 4) generated [4+4] self-assembled molecular squares (5 and 6), and similar treatments with acceptor 2 instead of 1 yielded [6+6] self-assembled molecular hexagons (7 and 8). The metallacycles were characterised by multinuclear NMR spectroscopy (¹H and ³¹P) and ESI-MS. The geometries of the metallacycles were optimised by using the PM6 method. When aggregates of the metallacycles were formed by adding hexane solutions in dichloromethane, aggregation-induced emission was observed for metallacycles 5 and 7, and aggregation-caused quenching was observed for metallacycles 6 and 8. The formation of aggregates was verified by dynamic light scattering and TEM analyses. Macrocycles 5 and 7 are white-light emitters in THF. Moreover, their high luminescence in both solution and the solid state was utilised for the recognition of nitroaromatic explosives.

Full text of DOI:10.1002/chem.201600698

DOI: 10.1002/chem.201600698
Full Paper
&
Macrocycles |Hot Paper|
Aggregation-Induced Emission of Platinum(II) Metallacycles and
Their Ability to Detect Nitroaromatics
Aniket Chowdhury, Prodip Howlader, and Partha Sarathi Mukherjee*[a]
Abstract: Two new acceptors containing platinum–carbazole
(1) and platinum–triphenylamine (2) backbones with bite
angles of 908 and 1208, respectively, have been synthesised
and characterised. Reactions of the rigid acceptor 1 with
linear dipyridyl-based donors (3 and 4) generated [4+4] self-
assembled molecular squares (5 and 6), and similar treat-
ments with acceptor 2 instead of 1 yielded [6+6] self-assem-
bled molecular hexagons (7 and 8). The metallacycles were
characterised by multinuclear NMR spectroscopy (1H and 31P)
and ESI-MS. The geometries of the metallacycles were opti-
mised by using the PM6 method. When aggregates of the
metallacycles were formed by adding hexane solutions in di-
chloromethane, aggregation-induced emission was observed
for metallacycles 5 and 7, and aggregation-caused quench-
ing was observed for metallacycles 6 and 8. The formation
of aggregates was verified by dynamic light scattering and
TEM analyses. Macrocycles 5 and 7 are white-light emitters
in THF. Moreover, their high luminescence in both solution
and the solid state was utilised for the recognition of
nitroaromatic explosives.
Introduction
strategies used to afford molecules with high luminescence in
the solid state and in solution.[5–10]
Over the years, luminescent compounds have advanced from
being simple chemosensors to smart materials for bioimaging,
optoelectronic materials, non-linear optics, and so forth.[1] Sev-
eral new materials with tuneable photophysical properties, in-
cluding high photostability, intense absorption and sharp and
strong emission, have been developed.[2] Common fluoro-
phores are highly luminescent in solution, but, when concen-
trated or solidified, their emission quenches due to excimer/ex-
ciplex formation through intermolecular interactions.[3] This
phenomenon is commonly referred as aggregation-caused
quenching (ACQ). Luminescent films with properly organised
molecules are a basic requirement for materials used as emis-
sive layers in optoelectronic devices. Conventional fluoro-
phores are not preferred as light sources due to ACQ. A new
class of solid emissive materials have emerged that work on
the principle of aggregation-induced emission (AIE).[4]
Coordination-driven self-assembly is a potential approach to
assemble desired building blocks in a specific pattern to
obtain molecular architectures with desirable properties. The
final assemblies inherit the physical and chemical characteris-
tics of both the donors and acceptors.[11–18] Moreover, the self-
assembled discrete molecules have the advantages of solution
processability and stability. Several platinum(II) and palladi-
um(II) fluorescent macrocycles and cages have been reported
that showed solution-phase fluorescence behaviour.[12] Howev-
er, their solid-state emission is less explored.[12d] It is proposed
that incorporating an AIE-active ligand into a self-assembly will
impart the molecule with both solution and solid-state emis-
sion. Recently, AIE-active platinum(II) assemblies were reported
by the groups of Stang, Yang and Huang.[19] The assemblies ex-
hibited solvent-dependent emission, dendrimer-dependent AIE
and heparin-induced AIE. It is known that AIE-active molecules
can display brilliant solid-state emissions, but none of the
aforementioned assemblies were subjected to solid-state lumi-
nescence analysis.
Materials that exhibit intense luminescence in solution and
in the solid state are highly desirable. Several new approaches
involving different methods to afford rigid and twisted mole-
cules with high luminescence in solution and in the solid state
have been reported in the literature.[5–10] The introduction of
bulky substituents on the periphery, conjugation-induced ri-
gidity, locking of the aromatic rings with heteroatoms, hydro-
gen bonding and metal ion complexation are some of the
To afford an AIE-active ligand, we focussed our attention on
the 9,10-divinylanthracene moiety because its derivatives are
highly luminescent in both solution and the solid state.[20] Dif-
ferent divinylanthracene derivatives have been used as mecha-
nochromic materials, acid–base sensors, in spontaneous ampli-
fied emission and as charge-transport materials.[20] We decided
to use 9,10-di(4-pyridylvinyl)anthracene (3) as the pyridyl
donor for self-assembly (Scheme 1) with acceptors with suita-
ble backbones. Carbazole and triphenylamine (TPA) derivatives
have widely been applied as photosensitisers, charge–hole
transport layers and emissive layers in organic light-emitting
diodes (OLEDs), and AIE-active materials.[21–30] Hence, we de-
[a] A. Chowdhury, P. Howlader, Prof. P. S. Mukherjee
Inorganic and Physical Chemistry, Indian Institute of Science
Bangalore 560012 (India)
E-mail: psm@ipc.iisc.ernet.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201600698.
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Scheme 2. Synthesis of the carbazole-based PtII acceptor 1. NBS=N-bromo-
succinimide, TMSA=trimethylsilylacetylene.
Scheme 1. Self-assembly of molecular squares (5, 6) and hexagons (7, 8).
signed and synthesised two new diplatinum(II) organometallic
acceptors 1 and 2 with carbazole and TPA backbones, respec-
tively (see below).
room temperature for 2 days. Treatment of i with AgNO3 in
a mixture of CHCl3 and MeOH (2:1) for 24 h at room tempera-
ture afforded 2. The crude product was washed with diethyl
ether several times to obtain 2 as a yellow precipitate in 60%
yield (Scheme 3).
We report herein the synthesis of a [4+4] and [6+6] self-as-
sembled molecular square (5) and hexagon (7) upon treatment
of linear donor 3 with PtII acceptors 1 and 2, respectively
(Scheme 1). Two more analogous macrocycles (6, 8) were syn-
thesised by using donor 1,4-di(4-pyridylvinyl)benzene (4). The
metallacycles 5 and 7 are fluorescent in solution and in the
solid state and exhibit AIE activity. Their luminescence proper-
ties are influenced by solvent polarity and both 5 and 7 display
white-light emission in THF. The luminescence of complexes 5
and 7 in both solution and the solid state was utilised for the
detection of nitroaromatic compounds (NACs) in solution and
in the vapour phase.
Results and Discussion
Scheme 3. Synthesis of the TPA-based PtII acceptor 2.
Synthesis and characterisation of 1 and 2
Carbazole-based PtII acceptor 1 was prepared by means of the
multi-step synthesis shown in Scheme 2. The ethynyl groups
were introduced by Sonogashira coupling of b with TMSA by
using [Pd(PPh3)2Cl2] and CuI to afford c in 83% yield. Subse-
quent desilylation at room temperature in a mixture of di-
chloromethane and methanol (2:1) in the presence of K2CO3
yielded d. Compound d was treated with trans-[Pt(PEt3)2I2] in
a mixture of diethylamine and toluene at room temperature
with CuI to afford e. Compound e was treated with AgNO3 in
chloroform/MeOH (2:1) to obtain 1, which was triturated with
diethyl ether to afford 1 as a grey solid in 80% yield. Dibromo-
triphenylamine (f) was prepared by following a procedure re-
ported in the literature.[31] Compound f was converted into di-
alkyne (h) through Sonogashira coupling followed by desilyla-
tion of g in the presence of K2CO3 at room temperature. The
platinum–alkyne analogue (i) was synthesised in 57% yield by
treating h with trans-[Pt(PEt3)2I2] in the presence of CuI at
The molecular structures of the iodide analogues e and
i were established by single-crystal XRD analysis (Table 1). Suit-
able crystals of e and i were grown by slow evaporation of sa-
turated solutions of the respective compounds in CHCl3. The
phenyl ring at the carbazole nitrogen in e is twisted out of the
plane of the fused ring system at an angle of 60.288 (Figure 1).
In the case of compound i, the unsubstituted phenyl ring of
the TPA moiety is almost perpendicular to the other two
phenyl rings at an angle of 84.408 (Figure 1).
Synthesis of self-assembled squares
The carbazole acceptor 1 was combined with both donors 3
and 4 separately in equimolar amounts. After heating the mix-
ture of precursors 1 and 3 in acetone for 24 h, the colour of
the solution changed from yellow to red. The solvent was re-
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Table 1. Crystallographic data and refinement parameters for e and i.[43]
e
i
formula
Mw
T [K]
C46H71NP4I2Pt2
1405.88
298
C46H73NP4I2Pt2
1407.89
298
crystal system
space group
a [Š]
b [Š]
c [Š]
triclinic
¯
P1
orthorhombic
Fdd2
32.493(3)
37.744(3)
8.5393(7)
90
15.2106(11)
18.4695(13)
20.6579(15)
100.930(2)
105.096(2)
90.734(2)
5489.7(7)
4
1.701
0.087
0.71073
2704.0
19319
a [8]
b [8]
90
90
g [8]
V [Š3]
Z
10472.7(15)
8
1.786
1calcd [gcmÀ3
]
m(MoKa) [mmÀ1
l [Š]
]
0.087
Figure 3. 31P NMR spectra of 1 and 5 in CDCl3 and a mixture of CDCl3/
[D4]MeOH (3:1), respectively.
0.71073
6800.0
59647
5141
1.055
0.0434
0.0917
F (000)
no. of collected reflns
no. of unique reflns
goodness of fit (F2)
R1 [I>2s(I)][a]
148738
1.073
0.0539
In the 31P NMR spectrum, an upfield shift of about 8 ppm,
with respect to the acceptor, was observed, which was consis-
tent with the increase in electron density on the platinum(II)
centre (Figure 3).
wR2[I>2s(I)][b]
0.1414
[a] R1 =SjF0 jÀjFc j/SjF0 j. [b] wR2 =(S[w(F20ÀFc2]/S[w(F02)2]1/2
.
Although multinuclear NMR spectroscopy analysis indicated
ligand to metal coordination and the formation of a single
product in all cases, it did not provide any information about
the compositions of the final assemblies. The compositions of
5 and 6 were determined by ESI-MS. In the ESI-MS spectrum,
signals attributed to the fragments [5À4NO3]4+ (m/z
1598.5459), [5À6NO3]6+ (m/z 1045.0398) and [5À8NO3]8+ (m/z
768.2763) were found. The signals were in good agreement
with theoretical results and their isotopic distributions
matched with the calculated values (Figure 4 and the Support-
ing Information). The formation of [4+4] self-assembled 6 was
also confirmed by ESI-MS analysis (see the Supporting Informa-
tion) in a similar way.
Figure 1. Crystal structures of e (left) and i (right).
moved and the crude product was triturated with diethyl ether
to afford a red solid (5) in high yield. The colour of the reaction
mixture of 1 and 4 changed from light to deep yellow after
heating for 24 h; a similar workup yielded 6 as a yellow solid.
Synthesis of self-assembled hexagons
1
In the H NMR spectrum of assembly 5, an upfield shift of ac-
The hexagons were obtained by equimolar self-assembly treat-
ment of acceptor 2 with the corresponding donors. In the
1H NMR spectrum of 7, the TPA protons of the acceptor exhib-
ited an upfield shift of about 0.1–0.14 ppm (see the Supporting
Information). The 31P NMR spectrum of 7 showed an upfield
shift of around 4 ppm with respect to the free acceptor (2).
The decrease in the coupling constant of the 195Pt satellites (ca.
DJ(Pt,P)=202.80 Hz) is due to back donation of electrons from
the ligand to the metal centre (Figure 5). Similar upfield shifts
ceptor protons was observed, which was expected to be due
to coordination of the pyridyl nitrogen to the platinum(II) ac-
ceptor (Figure 2).
1
in the H and 31P NMR spectra of 8 were also observed (see the
Supporting Information). The formation of [6+6] self-assem-
bled hexagons (7 and 8) was established by the appearance of
signals corresponding to the fragments [7À6NO3]6+ (m/z
1600.5682), [7À8NO3]8+ (m/z 1184.9825) and [8À8NO3]8+ (m/z
1109.8880) in their respective mass spectra (see the Supporting
Information).
1
Figure 2. Partial H NMR spectra of 1 and 5 in CDCl3 and a mixture of CDCl3/
[D4]MeOH (3:1), respectively.
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Photophysical properties
In CH2Cl2, carbazole acceptor 1 exhibited two sets of
absorption bands. Following previous reports in the
literature, both bands in the l=240–260 and 310–
330 nm regions were ascribed to intramolecular p–p*
transitions.[12] Acceptor 2 showed a single absorption
band centred at l=360 nm. Donor 3 exhibited two
sets of absorption bands centred at l=230–260 and
410 nm, which originated from p–p* transitions.[12]
Compound 4 had a single absorption band centred
at l=350 nm. Square 5 showed three absorption
bands at l=260, 320 and 425 nm, whereas 6 exhibit-
ed two absorption bands at l=260 and 325 nm,
along with a shoulder at l=420 nm (see the Sup-
porting Information). The major band at l=360 nm,
with a shoulder at l=435 nm, was obtained for hex-
agon 7. All assemblies exhibited absorption bands of
the acceptors and the donors (see the Supporting In-
formation).
The assemblies have low solubility in hexane. Dif-
ferent hexane/CH2Cl2 compositions were used to ex-
amine aggregate formation in solution. For all com-
Figure 4. Calculated (top) and experimental (bottom) isotopic distribution patterns of
the signals corresponding to [5À6NO3]6+ (m/z 1045.0398) and [5À8NO3]8+
.
Figure 5. 31P NMR spectra of 2 and 7 in CDCl3 and CDCl3/[D4]MeOH (3:1) re-
spectively.
Figure 6. Optimised structures of the macrocycles 5 (a), 6 (b), 7 (c) and 8 (d).
Molecular modelling analysis
Several attempts to obtain suitable single crystals of the mac-
rocycles were unsuccessful. Optimised geometries of the final
assemblies were achieved by using the PM6 method (Figure 6).
The optimised structure of square 5 is almost planar with
phenyl rings of the carbazole moiety twisted out of the plane
at an angle of 55.38. The vinylanthracene moiety is twisted to
reduce internal steric hindrance. The torsional angle between
the anthracene moiety and adjacent pyridyl group is 69.58. The
width of the optimised structure of 5 was calculated to be
4 nm. Similarly, square 6 is also mostly planar with twisted car-
bazole rings and a divinylphenyl moiety with a width of
3.9 nm. The angle between the phenyl rings of the TPA accept-
or in larger hexagon 7 was 63.68. The hexagons 7 and 8 have
large internal dimensions of 6.7”7.5 nm and 6.7”7.4 nm, re-
spectively.
pounds, long-wavelength tails were observed due to scattering
from spherical particles present in the medium (see the Sup-
porting Information).[32] The formation of aggregates was fur-
ther investigated by dynamic light scattering (DLS), SEM and
TEM.
AIE of 5 and 7
In self-assembled macrocycles, the rotational motion of the
ligand is restricted by directional coordinate-bond formation.
Therefore, the assemblies were highly emissive in solution. To
investigate whether the final assemblies were AIE-active, the
fluorescence spectra of the molecules were recorded with
varying concentrations of hexane in CH2Cl2 (Figure 7). Com-
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shows ACQ upon aggregate formation. In discrete molecules
(5 and 7), the motion of the donors was restricted by ligand-
to-metal bond formation, but a considerable amount of excita-
tion energy was released in non-radiative decay pathways.
When aggregated, the rotational motion of the donor mole-
cules was completely arrested by the confined space, leading
to the release of excitation energy only through the radiative
decay pathway.[18,33] By changing the donor from a divinylphen-
yl analogue (4) to a divinylanthracene analogue (3), a change
from ACQ to AIE behaviour was observed for structurally simi-
lar macrocycles.
DLS and microscopy analyses
To investigate the nature of the aggregates, DLS experiments
were performed with all macrocycles in different solvent com-
positions. For a lower hexane fraction (60%), aggregates with
an average diameter of 156 nm were formed in the medium.
The size gradually increased to 190 nm with increasing hexane
fraction (80%) and remained unchanged when more hexane
was added (Figure 9). The spontaneous increase in aggregate
size with variation of the hexane fraction proved that the emis-
sion intensity increased with increasing aggregate size. For
compound 7, the aggregate sizes gradually increased from 90
to 120 nm as the hexane fraction increased from 60 to 80%.
DLS provided evidence for the formation of spherical aggre-
gates in the medium, but TEM analysis was performed to fur-
ther explore the shape and size of the particles (Figure 10). For
TEM analysis, appropriate solutions of the complex with differ-
ent hexane/CH2Cl2 compositions were drop-cast on copper
grids and slowly dried to afford the final sample. For com-
pound 5, the average aggregate size increased from 150 to
190 nm as the hexane fraction changed from 60 to 80%.
Changes in size with variation in solvent composition were
also prominent for sample 7; the sizes gradually changed from
90 to 120 nm with increasing hexane fraction. Two other mac-
Figure 7. Top: Change in fluorescence intensity of 5 with changing hexane/
CH2Cl2 composition; lex =425 nm. Bottom: images of 5 under l=365 nm
light in different solvent compositions.
pound 5 exhibited orange fluorescence with a peak maximum
centred at l=556 nm. When the hexane fraction slowly in-
creased from 10 to 80%, the emission intensity gradually in-
creased and became saturated. The gradual increase in fluores-
cence intensity of 5 with variation of the hexane fraction indi-
cated that the molecule was AIE-active. In pure CH2Cl2, the
quantum yield of 5 was measured to be 8.5%, but after aggre-
gate formation the emission intensity increased and the final
quantum yield was 21%. Analogous macrocycle 6 was excited
at l=325 nm and emission spectra with different hexane frac-
tions were recorded (Figure 8).
On careful inspection, it was found that, with increasing
hexane fraction, the emission intensity started to decrease; this
is a general characteristic of ACQ. Therefore, two different
types of fluorescence responses were observed from structural-
ly similar macrocycles. Macrocycle 5, which contains AIE-active
donor 3, exhibits AIE activity in solution, whereas macrocycle 6
Figure 9. Particle size distribution of the compounds with different hexane/
CH2Cl2 compositions: 60 (a) and 80% hexane in CH2Cl2 (b) for compound 5;
60 (c) and 80% hexane in CH2Cl2 (d) for compound 7.
Figure 8. Fluorescence intensity change with varying hexane/CH2Cl2 fraction
for 6; lex =350 nm.
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polar solvents, including toluene (Tol) and carbon tetrachloride,
as well as polar solvents, such as CH2Cl2, THF, Dioxane, DMSO
and methanol (Figure 12) were chosen to investigate the role
of solvents on the fluorescence of the compounds.
Figure 10. TEM images of macrocycles with various hexane/CH2Cl2 fractions.
Compound 5 in 60 (a) and 80% hexane in CH2Cl2 (b); compound 7 in 60 (c)
and 80% hexane in CH2Cl2 (d).
rocycles (6 and 8) also exhibited aggregate formation in differ-
ent solvent mixtures (see the Supporting Information).
Finally, to explore the morphology of the aggregates, SEM
was performed on aggregated samples deposited on carbon
tape after drying. As observed in Figure 11, compound 5 re-
tained the spherical morphology after drying, and the average
sizes increased with increasing hexane fraction from 60 to
80%.
Figure 12. Top left: Solvent-dependent fluorescence of 5, lex =425 nm. Top
right: CIE diagram of 5. Bottom: images of 5 in different solvents under
l=365 nm UV light. Hex=hexanes, ACN=acetonitrile, Diox=Dioxane.
The emission intensity of 5 in non-polar solvents, such as
CCl4 and Tol, was high, whereas in polar solvents, such as
DMSO, methanol and ethyl acetate (EA), the intensity de-
creased (Figure 12). In non-polar solvents, the emission occurs
from a locally excited (LE) state, but, when polar solvents were
used, the LE state relaxed to a more stable and more polar
state with lower energy. It is well known in the literature that
the emission intensity from the polar state of fluorophores de-
creases due to different factors, including collisions with non-
excited fluorophores and interactions with solvent molecules.
Upon careful inspection, the solution of 5 in THF was found
to emit white light when excited at l=425 nm. The chromatic-
ity diagram (Figure 12, top right) shows that the solution in
THF exhibits stable white-light emission (0.38, 0.43).[34] Com-
pound 7 also exhibited similar white-light emission in THF (see
the Supporting Information).
NAC detection
Figure 11. SEM images of the macrocycles with various hexane/CH2Cl2 frac-
tions. Compound 5: 60 (a) and 80% hexane in CH2Cl2 (b); compound 7: 60
(c) and 80% hexane in CH2Cl2 (d).
In recent years, cost-effective and rapid detection techniques
for NACs have become an interesting field of research. Apart
from being used as explosives, NACs are also valuable in the
dye industry, in the manufacture of rocket fuel and in the phar-
maceutical industry.[35] Due to high water solubility, NACs and
the final explosive residues become sources of environmental
pollution.[36]
Fluorescence studies in combination with DLS experiments
and microscopy images confirmed that the emission enhance-
ment in complexes 5 and 7 was due to aggregate formation.
Trinitrotoluene (TNT) is one of the oldest and most used
NACs due to its abundance and explosive power. Several small
molecules and polymer-based fluorescent sensors have been
reported for the detection of TNT in solution.[37,38] For practical
Solvent-dependent fluorescence
The AIE behaviour of 5 and 7 prompted us to explore the role
of different solvents on the photophysical properties. Non-
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applications, luminescent thin films that can detect explosive
vapour are highly desirable. Several polyaromatic compounds,
including anthracene, pyrene and TPA, have been used as elec-
tron-rich centres and anchoring templates for NACs in various
self-assemblies.[39]
Although we are mainly interested in exploring compounds
5 and 7 for solid-state NACs detection, to get a preliminary
idea on their possibility of sensing NACs, a fluorescence titra-
tion study with NACs was carried out in the solution phase.
Upon TNT addition, the emission intensity of 5 in solution
gradually decreased. The addition of 4 equivalents of TNT
quenched about 80% of the initial emission intensity of 5
(Figure 13).
Figure 14. Titration of 5 with NB (top left) and 2,4-DNT (top right). Fluores-
cence images of thin films of compounds 5 and 7 after exposure to NB.
exhibited reversible sensing of NACs. After exposing the films
to saturated NB vapour for a certain period of time, emission
intensity was regained by washing the films with water fol-
lowed by drying. The recovered film was reused for a certain
number of times (Figure 15) with the retention of significant
efficiency.
Figure 13. Left: Fluorescence spectra of TNT titration with 5 in CH2Cl2 (left),
lex =425 nm. Right: Stern–Volmer (SV) plot.
The SV constants were 6.25”103 and 1.54”104 mÀ1 for 5
and 7, respectively. The change in fluorescence lifetime of 5
and 7 upon the addition of TNT indicated dynamic quenching
as the possible mechanism of quenching (Supporting Informa-
tion).[40]
TNT and several other NACs, including 2,4-dinitrotoluene
(DNT), 3,4-DNT, nitrotoluene (NT), trinitrobenzene (TNB), 1,3-di-
nitrobenzene (DNB), nitrobenzene (NB), nitromethane (NM), 3-
nitrobenzoic acid (NBA) and benzophenone (BPH), were treat-
ed with solutions of 5 and 7 to examine the selectivity. Of all
the NACs, TNT showed maximum quenching efficiency
(>80%), followed by DNT (>30%; see the Supporting Informa-
tion). For compound 7, except TNT and DNTs, other NACs did
not show much change in fluorescence intensity. Therefore,
both compounds were effective for the detection of TNT in so-
lution.
Figure 15. Fluorescence quenching efficiency versus number of cycles for 5.
The reusability of compound 7 was examined, and after five
cycles of use a decrease in emission intensity was observed
(see the Supporting Information). Therefore, both materials
can be reused several times; this makes them potential candi-
dates for in-field applications.
For solid-state sensing, thin films of 5 and 7 were used; the
films of 5 were exposed to saturated NB vapour and the
change in fluorescence with time was monitored (Figure 14).
After, 100 s of NB exposure, the initial emission intensity of 5
was quenched by 50%, and after 400 s of exposure a maximum
of about 80% quenching was observed without any further
quenching, even after longer exposure times. After 300 s of ex-
posure towards saturated DNT vapour, the initial emission in-
tensity of compound 5 was quenched by about 60%. For com-
pound 7, after 200 s, about 64% quenching of fluorescence
was observed (see the Supporting information).
Conclusion
We reported herein the synthesis of a pair of new platinum(II)
ditopic acceptors with bite angles of 908 (1) and 1208 (2), re-
spectively. Self-assembly of these acceptors with linear dipyrid-
yl donors with divinylanthracene (3) and divinylphenyl (4)
backbones yielded molecular squares (5, 6) and hexagons (7,
8), respectively. Complexes 5 and 7 exhibited AIE behaviour
that originated from the highly twisted bis(4-pyridyl)divinylan-
Solid-state quenching was attributed to a combined effect
of the high vapour pressure of the analytes and their binding
affinity towards the sensors. Thin films of compounds 5 and 7
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thracene donor (3). The role of the divinylanthracene backbone
in the luminescence and AIE properties of these macrocycles
(5, 7) was illustrated by comparing the photophysical proper-
ties of a pair of analogous macrocycles (6, 8), which showed
ACQ luminescence. The formation of the aggregates was char-
acterised by DLS, TEM and SEM analyses. The metallacycles (5,
7) with a divinylanthracene backbone exhibited interesting sol-
vent-dependent emissions, especially white-light emissions in
THF; this makes them a rare class of coordination architectures
for stable white-light emitters. Furthermore, thin films of the
highly luminescent molecular square (5) and hexagon (7)
showed efficient sensing of NAC explosives in the vapour
phase through fluorescence quenching.
Synthesis of c
An oven-dried 100 mL two-necked round-bottomed flask was
charged with b (1.00 g, 2.5 mmol), CuI (0.03 g, 8 mol%), triphenyl-
phosphine (0.13 g, 20 mol%) and [Pd(PPh3)2Cl2] (0.08 g, 5 mol%).
Freshly distilled triethylamine (50 mL) was added to the mixture,
which was heated at 508C for 15 min. Trimethylsilylacetylene
(0.65 g, 2.7 mmol) was added to the hot solution and the mixture
was heated at reflux for 2 days. After completion of the reaction
(as monitored by TLC), the solvent was removed and the com-
pound was purified by column chromatography on silica gel
(60:120) with CH2Cl2/hexane (5%) as the eluent to afford c as
1
a yellow product (83%). H NMR (CDCl3, 400 MHz): d=8.25 (s, 2H),
7.62 (m, 2H), 7.51 (m, 5H), 7.28 (m, 2H), 0.30 ppm (s, 18H).
Synthesis of d
A mixture of compound c (0.90 g, 2.06 mmol) and K2CO3 (0.35 g,
3.00 mmol) was dissolved in a mixture of CH2Cl2 and MeOH (20:30)
and stirred for 24 h. The solvent was removed under reduced pres-
sure and the crude product was purified by column chromatogra-
phy with CH2Cl2/hexane (1:4) as the eluent to give d as a yellow
solid (90%). 1H NMR (CDCl3, 400 MHz): d=8.27 (s, 2H), 7.62 (m,
2H), 7.54 (m, 5H), 7.31 (d, 2H), 3.10 ppm (s, 2H); 13C NMR
(100 MHz, CDCl3): d=141.8, 137.2, 131.0, 130.6, 128.7, 127.6, 125.2,
123.3, 121.3, 114.4, 110.6, 85.3, 76.2 ppm.
Experimental Section
General
The starting materials and solvents were purchased from commer-
cially available sources and used without further purification. Com-
pounds 3, f, g, a and b were prepared according to procedures re-
ported in the literature.[19,41] The NMR spectra were recorded on
a 400 MHz Bruker NMR spectrometer. The chemical shifts (d) in the
1H NMR spectra were reported in ppm relative to Me4Si as an inter-
nal standard (d=0.0 ppm) or the proton resonance resulting from
incomplete deuteration of the NMR solvents: CDCl3 (d=7.26 ppm)
and [D6]DMSO (d=2.50 ppm). 13C NMR spectra were recorded at
100 MHz, and the chemical shifts (d) were reported in ppm relative
to external CDCl3 and [D6]DMSO at d=77.8–77.2 and 40.50 ppm,
respectively. The 31P NMR spectra were recorded at 120 MHz and
the chemical shifts (d) were reported in ppm relative to external
83% H3PO4 at d=0.0 ppm. ESI-MS results were recorded by using
an Agilent 6538 ultra-high definition (UHD) accurate mass Q-TOF
spectrometer with standard spectroscopic-grade solvents. Electron-
ic absorption and emission spectra were recorded on a Lambda
750 UV/Vis spectrophotometer and a Horiba Jobin Yvon Fluoro-
max-4 spectrometer. For the solid-state vapour exposure study,
thin films of 5 and 7 were prepared by drop-casting and spin-coat-
ing of concentrated solutions on thin quartz films, which were
vacuum dried for 6 h. Analytes (20 mg) were placed in a glass con-
tainer and a cotton pad was placed on the sample to avoid direct
contact with the sensor films. The containers were sealed with alu-
minium foil and kept for 2 days to generate saturated vapour.
During the experiments, the thin films were placed on the cotton
pad and after a certain period of exposure they were removed and
immediately placed in the solid-state sample holder of the spectro-
fluorimeter. Single-crystal XRD data were collected with a Bruker
D8 Quest diffractometer equipped with a Photon 100 detector
with CMOS technology. The data were integrated by using SAINT
and an empirical absorption correction was applied by SADABS.
The structures were determined by direct methods by using
SHELX-97.[42] Time-resolved fluorescence measurements were per-
formed on an IBH-Data station platform by using a l=390 nm
nano-LED source. DLS experiments were performed on a zeta-sizer
instrument ZEN3600 (Malvern, UK) with a 1738 back scattering
angle and HeÀNe laser (l=633 nm). Mixtures of CH2Cl2/hexane
with various water fractions were prepared by slowly adding ultra-
pure hexane into solutions of the samples in CH2Cl2. SEM images
were obtained by using a Zeiss Ultra-55 SEM instrument with the
sample coated on a carbon tape. ESI mass spectra were recorded
by using an Esquire 3000 plus ESI spectrometer. TEM analysis was
performed on a JEOL 2100F instrument.
Synthesis of e
Compound
d
(0.17 g, 0.60 mmol), trans-[Pt(PEt3)2I2] (1.26 g,
1.81 mmol) and CuI (0.02 g, 0.06 mmol) were added to a freshly
distilled mixture of toluene (30 mL) and diethylamine (15 mL) in
a Schlenk flask. The flask was degassed under vacuum and refilled
with nitrogen three times. The reaction mixture was stirred for
48 h at room temperature. The solvent was removed under
vacuum and the crude product was purified by column chroma-
tography with EA/hexane (1:1) as the eluent to afford e as a light-
1
yellow solid (80%). H NMR (CDCl3, 400 MHz): d=8.02 (s, 2H), 7.61
(m, 5H), 7.53 (m, 2H), 7.34 (d, 2H), 2.29 (m, 24H), 1.22 ppm (m,
36H); 13C NMR (100 MHz, CDCl3): d=139.7, 138.0, 130.3, 129.5,
127.9, 127.3, 123.5, 122.6, 120.7, 110.0, 101.2, 87.0, 17.1, 8.8 ppm;
31P NMR (120 MHz, CDCl3): d=8.63 ppm.
Synthesis of 1
Compound e (0.05 g, 0.03 mmol) was dissolved in fresh CHCl3
(7 mL) in a 20 mL vial. When the solution was treated with a solu-
tion of AgNO3 (0.01 g, 0.07 mmol) in methanol (10 mL), a yellow
precipitate started to form. The mixture was covered with alumini-
um foil and stirred in the dark at room temperature for 24 h. The
solvent was completely dried and the crude product was filtered
through Celite by using glass fibre and CHCl3 as the solvent. Dieth-
yl ether was added to induce the formation of an off-white precipi-
tate, which was dried to afford the final product (80%). 1H NMR
(CDCl3, 400 MHz): d=7.95 (s, 2H), 7.62 (m, 2H), 7.51 (m, 3H), 7.28
(m, 4H), 2.03 (m, 24H), 1.28 ppm (m, 36H); 13C NMR (100 MHz,
CDCl3): d=139.9, 130.4, 129.9, 128.0, 127.3, 123.4, 122.9, 120.3,
110.0, 104.3, 15.0, 8.3 ppm; 31P NMR (120 MHz, CDCl3): d= =
20.07 ppm; IR: n˜ =2961(s), 2912 (m), 2105 (w), 1477 (s), 1266 (s),
1074 (s), 1016 (s), 779 cmÀ1 (s).
Synthesis of h
CHCl3 (30 mL) and MeOH (20 mL) were added to a mixture of g
(2.00 g, 4.56 mmol) and K2CO3 (2.5 g, 18.27 mmol) in a 100 mL
Chem. Eur. J. 2016, 22, 7468 – 7478
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round-bottomed flask, and the reaction mixture was stirred at
room temperature for 24 h. The solvent was dried and the crude
product was purified by column chromatography on silica gel
(60:120) with EA/hexane (20%) as the eluent to give h (80%).
1H NMR (CDCl3, 400 MHz): d=7.37 (d, 4H), 7.29 (m, 2H), 7.11 (m,
3H), 7.01 (d, 4H), 3.05 ppm (s, 2H); 13C NMR (100 MHz, CDCl3): d=
148.1, 147.0, 133.7, 130.1, 126.1, 125.6, 124.8, 123.7, 116.4,
84.1 ppm.
Synthesis of 6
A solution of acceptor 1 (12.75 mg, 0.01 mmol) in acetone was
added to a suspension of 4 (2.84 mg, 0.01 mmol) in acetone. After
washing with cold diethyl ether and acetone, the product was ob-
1
tained (86%). H NMR (CDCl3, 400 MHz): d=8.41 (m, 16H), 7.87 (s,
8H), 7.68 (d, 16H), 7.59 (s, 12H), 7.50 (m, 16H), 7.38 (m, 16H), 7.17
(m, 16H), 7.10 (d, 8H) 1.74 (m, 96H), 1.09 ppm (m, 144H); 31P NMR
(120 MHz, CDCl3): d=16.01 ppm; IR: n˜ =3393 (m), 2970 (m), 2111
(w), 1587 9 s), 1458 (m), 1330 (s), 1023 (s), 798 (m), 753 cmÀ1 (s);
À 6+
ESI-MS (m/z): 1186.5584 [6À5NO3
]
À 5+, 978.3634 [6À6NO3
] .
Synthesis of i
Compound h (0.40 g, 1.36 mmol) and trans-[Pt(PEt3)2I2] (2.85 g,
4.09 mmol) were dissolved in a freshly distilled mixture of toluene
(30 mL) and diethylamine (15 mL) in a Schlenk flask. The flask was
degassed under vacuum and refilled with nitrogen three times. CuI
(0.02 g, 0.13 mmol) was added and the reaction mixture was stirred
for 48 h at room temperature. The solvent was removed under
vacuum and the crude product was purified by column chroma-
tography with EA/hexane (1:1) as the eluent to afford i as a light-
Synthesis of 7
A clear solution of 2 (12.78 mg, 0.01 mmol) in acetone was added
to a suspension of 3 (3.84 mg, 0.01 mmol) in acetone (2 mL). After
final purification, a red product was obtained (72%). 1H NMR
(CDCl3, 400 MHz): d=8.46 (s, 24H), 8.39 (d, 12H), 8.14 (m, 24H),
7.69 (s, 24H), 7.39 (m, 24H), 7.08 (m, 18H), 6.95 (m, 24H), 6.90 (m,
24H), 6.80 (m, 24H), 1.70 (m, 144H), 1.05 ppm (m, 216H); 31P NMR
(120 MHz, CDCl3): d=16.06 ppm; IR: n˜ =3393 (w), 2945 (m), 2105
(m), 1599 (s), 1497 (s), 1311 (s), 1247(s), 1093 (s), 1023 (s), 792 (s),
1
yellow solid (57%). H NMR (CDCl3, 400 MHz): d=7.23 (d, 2H), 7.14
(d, 4H), 7.07 (d, 2H), 6.98 (d, 1H), 6.93 (d, 4H), 2.23 (m, 24H),
1.16 ppm (m, 36H); 13C NMR (100 MHz, CDCl3): d=147.9, 145.6,
131.9, 129.6, 124.6, 124.2, 123.2, 123.1, 100.3, 88.8 ppm; 31P NMR
(120 MHz, CDCl3): d=8.55 ppm.
747 cmÀ1 (s); ESI-MS (m/z): 1600.5682 [7À6NO3
]
À 6+, 1184.9825
À 8+
[7À8NO3
] .
Synthesis of 8
Synthesis of 2
Acceptor 2 (12.78 mg, 0.01 mmol) was treated with donor 4
(2.84 mg, 0.01 mmol) in acetone (5 mL) to afford the desired final
product 5 (68%). H NMR (CDCl3, 400 MHz): d=8.34 (m, 24H), 7.61
(m, 24H), 7.54 (s, 24H), 7.47 (d, 12H), 7.09 (m, 18H), 6.96 (m, 24H),
6.92 (d, 24H), 6.80 (m, 24H), 1.69 (m, 144H), 1.02 ppm (m, 216H);
31P NMR (120 MHz, CDCl3): d= =15.88 ppm; IR: n˜ =3406 (w), 2964
Compound i (0.10 g, 0.07 mmol) was dissolved in a mixture of
CHCl3 (15 mL) and MeOH (10 mL). A yellow precipitate was formed
upon the addition of AgNO3 (0.03 g, 0.17 mmol) to the above mix-
ture. The mixture was stirred for 24 h at room temperature in the
dark. Upon completion, the solvents were removed and the prod-
uct was extracted with chloroform. The product was isolated as
a grey solid (60% yield) upon treating a concentrated solution in
chloroform with diethyl ether. 1H NMR (CDCl3, 400 MHz): d=7.22
(m, 2H), 7.07 (m, 6H), 7.00 (m, 1H), 6.92 (d, 4H), 1.95 (m, 24H),
1.23 ppm (m, 36H); 13C NMR (100 MHz, CDCl3): d=147.8, 145.9,
132.2, 129.7, 124.6, 124.1, 123.4, 122.7, 103.5, 65.8, 30.2, 14.9,
8.4 ppm; 31P NMR (120 MHz, CDCl3): d=20.03 ppm; IR: n˜ =2964
(m), 2932 (m), 2880 (m), 2105 (w), 1580 (m), 1471 (s), 1273 (s), 1023
(m), 984 (s), 824 (s), 734 cmÀ1 (s).
1
(m), 2111 (w), 1605 (m), 1497 (m), 1323 (m), 1266 (s), 1074 (s), 1016
À 8+
(s), 792 cmÀ1 (s); ESI-MS (m/z): 1109.8880 [8À8NO3
] .
Synthesis of nanoaggregates
Stock solutions (10À3 m) of 5, 6, 7 and 8 were prepared by dissolv-
ing an appropriate amount of the complexes in spectroscopy-
grade CH2Cl2. Calculated amounts of aliquots from the stock solu-
tions were transferred to a 4 mL glass vial and diluted with an ap-
propriate amount of CH2Cl2. To generate aggregates, different
amounts of hexane were added to the vials under vigorous stirring
at room temperature to obtain solutions (10À5 m) with hexane frac-
tions from 10 to 90%. The photophysical studies were carried out
immediately because precipitates started to form if the solutions
were kept for longer times.
General procedure for the synthesis of the macrocycles
To a stirred solution of the appropriate donor in acetone, a clear
solution of the respective acceptor in acetone was added drop-
wise. The obtained reaction mixture was heated at reflux for 24 h.
The volatile solvent was evaporated and the mixture was washed
with cold diethyl ether and acetone to afford the desired final
product.
DLS measurements
For DLS analysis, solutions (2 mL) with varying hexane fractions
were placed in a quartz cuvette with all transparent sides; the cu-
vette was placed in the instrument chamber for data collection.
Each set of data was collected 10 times to obtain reproducible re-
sults.
Synthesis of 5
Acceptor 1 (12.75 mg, 0.01 mmol) was treated with 3 (3.84 mg,
0.01 mmol) in acetone (5 mL) to afford the desired final product 5
(82%). 1H NMR (CDCl3, 400 MHz): d=8.60 (m, 16H), 8.49 (d, 8H),
8.25 (m, 16H), 7.93 (s, 8H), 8.87 (m, 16H), 7.48 (m, 24H), 7.43 (m,
16H), 7.21 (m, 12H), 6.94 (d, 8H), 1.83 (m, 96H), 1.18 ppm (m,
144H); 31P NMR (120 MHz, CDCl3): d=16.02 ppm; IR: n˜ =3450 (w),
2951 (m), 2111 (w), 1599 (s), 1471 (m), 1323 (s), 1260 9 s), 1086À(s4)+,
SEM analysis
For SEM analysis, carbon tapes of 1 mm2 in size were placed in the
SEM holder. Different solutions (10 mL) of 5, 6, 7 and 8 in various
solvent mixtures were drop-cast on the carbon tapes and the sam-
ples were air-dried overnight and then in vacuum for 6 h before
data collection.
1009(s), 818 (s), 734 cmÀ1 (s); ESI-MS (m/z): 1598.5459 [5À4NO3
]
À 8+
, 1045.0398 [5À6NO3
]
À 6+, 768.2763 [5À8NO3
] .
Chem. Eur. J. 2016, 22, 7468 – 7478
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2
ꢀc ¼ fꢀr½ð1À10ÀA Þ Â Nc  DcŠg=½ð1À10ÀA Þ Â Nr  DrŠ
c
r
ð1Þ
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For fluorescence titration of the samples with NACs, a solution of
5/7 (10À5 m, 2 mL) was placed in a quartz cuvette. Fresh stock solu-
tions of NACs in MeOH (10À3 m, 10 mL) were prepared. The NAC
solutions were gradually added to the sensor solution.
Acknowledgements
A.C. gratefully acknowledges the CSIR (New Delhi) for a Re-
search Fellowship. P.S.M is thankful to the CSIR for financial
support.
Keywords: fluorescence
platinum · sensors
· luminescence · macrocycles ·
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Received: February 15, 2016
Published online on April 23, 2016
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