Mechano-fluorochromic PtII Luminogen and Its Cysteine Recognition

Article abstract of DOI:10.1002/chem.201504003

A new triphenylamine-based organometallic Pt^II luminogen (1) and its analogous organic compound (2) are reported. The molecules are decorated with aldehyde functionality to improve their photophysical properties by utilising donor-acceptor inte

Full text of DOI:10.1002/chem.201504003

DOI: 10.1002/chem.201504003
Full Paper
&
Organometallic Chemistry |Hot Paper|
Mechano-fluorochromic PtII Luminogen and Its Cysteine
Recognition
Aniket Chowdhury, Prodip Howlader, and Partha Sarathi Mukherjee*[a]
Abstract: A new triphenylamine-based organometallic PtII
luminogen (1) and its analogous organic compound (2) are
reported. The molecules are decorated with aldehyde
functionality to improve their photophysical properties by
utilising donor–acceptor interactions. The single crystal
X-ray structure analysis of PtII analogue 1 revealed that the
neighbouring molecules were loosely organised by weak
intermolecular CÀH···p interactions. Because of the twisted
nature of the triphenylamine backbone the compounds
showed aggregation-induced emission enhancement in
THF/water mixture. Due to their loose crystal packing, upon
application of external stimuli these luminogens exhibited
mechano-fluorochromic behaviour. The crystalline forms of
the compounds displayed a more superior emission efficien-
cy than the grinded samples. Moreover, the compounds
showed crystallization-induced emission enhancement (CIEE)
and exhibited chemodosimetric response towards cysteine
under physiological condition.
development of new stimuli-responsive smart materials.[7]
Along with several new purely organic molecules, a few lumi-
nogens containing different metal centres like ZnII, IrIII, InIII, AlIII,
AuI, AgI and CuI have shown stimuli-responsive fluorescence
switching along with AIE or AIEE.[8–14] Although several PtII lu-
minogens are reported to exhibit mechanochromic (MC) lumi-
nescence due to their variation in morphological packing as
well as in Pt–Pt interaction in the solid state, none of them has
shown emission enhancement in solution upon aggregate
formation.[15,16] The fact that there are very few reports on or-
ganometallic materials having both mechano-fluorochromic
and AIE behaviour indicates the lack of clear guidelines for
designing such molecules which can exhibit both these
properties together.
Introduction
The recent years have witnessed a steady growth in the explo-
ration of luminogenic materials due to their potential applica-
tions in sensing and optoelectronics.[1] Among them a particular
type of material has attracted considerable attention owing to
its fluorescence switching behaviour between bright and dark
states under different external stimuli like mechanical force,
heat, exposure to chemicals, etc.[2] Stimuli-sensitive compounds
are believed to be potential materials for applications in data
storage, security inks, sensing, optical switches and in forensic
sciences.[1]
Although several luminescent molecules are reported in the
literature which exhibit stimuli-responsive fluorescence switch-
ing in solution, their solid-state fluorescence usually decreases
due to an event known as aggregation-caused quenching
(ACQ).[3] However, new phenomena known as aggregation-
induced emission (AIE) and aggregation-induced emission
enhancement (AIEE) have also been observed in certain class
of materials.[4,5] In these compounds the emission intensity
enhances upon aggregate formation in solution and also in
the solid state.[6] Several mechanisms for AIE/AIEE have been
proposed, including restriction of intramolecular rotation (RIR),
the formation of J-aggregate, intramolecular planarization and
restriction of the transition from locally excited state to the
twisted intramolecular charge-transfer state.[6] Several of the
AIE/AIEE materials are also known to be piezochromic and me-
chanochromic, which makes them suitable candidates for the
Several polyaromatic compounds like pyrene, anthracene,
stilbene, silole, carbazole, triphenylamine, etc., have been used
in developing stimuli-responsive materials.[6,7] The most
common feature of MC-luminogens is their twisted aromatic
backbone which restricts molecular close-packing by decreas-
ing p–p interaction and also enables the molecules to crystal-
lize in a loosely bound manner. Therefore, upon application of
external stimuli, the molecular packing in the crystals can un-
dergo severe deformation which is manifested in their change
in photophysical properties. In this regard, we focused our
attention on the triphenylamine moiety (TPA). To reduce the
steric strain among the neighbouring aromatic hydrogen
atoms, the TPA moiety is inherently twisted. In the solid state
this non-planarity of the TPA core restricts both aggregate and
excimer formation by reducing p–p interaction. Therefore, TPA
is a suitable material, based on which new MC-luminogens can
be developed.
[a] A. Chowdhury, P. Howlader, Prof. Dr. P. S. Mukherjee
Indian Institute of Science, Department Inorganic and Physical Chemistry
Yesvanthpur, Bangalore, 560012 (India)
It is well known that, crystallization of the luminophores
reduces their emission efficiency in solid state due to different
detrimental effects including p–p interaction, excimer forma-
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.201504003.
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tion, etc.[17] This effect in solid compounds is known as crystalli-
zation-caused quenching (CCQ) which is analogues to ACQ of
luminogens in solution. When crystallized, the common organ-
ic luminogens show CCQ. Therefore, considerable efforts are
being devoted to the fabrication of efficient amorphous thin
films of luminogens for their use in optoelectronics. But it is
well documented in the literature that crystalline materials
have more potential as charge carriers, and therefore, can be
used as semiconductors in organic light-emitting diodes
(OLED’s), organic thin film transistors (OTFTs), etc.[18] Recently,
compounds with higher emission in their crystalline state have
also been reported.[19] In comparison to AIE effect of lumino-
gens in solution, this phenomenon is called crystallization-in-
duced emission enhancement (CIEE). Different factors including
conformational twisting, structural rigidification by weak inter-
molecular interactions along with various packing modes of
the compounds in solid state are known to control the CIEE ef-
ficiency. This efficient emission in crystalline phase makes them
attractive candidates to be used in the field of optoelectronics.
The previously reported CIEE-active compounds were mostly
organic, containing different polyaromatic backbones like
diphenylbenzofulvenes, hexaphenylcyclobutane, diketopyrrolo-
pyrole, phosphorous-containing heterocycles, etc. Unlike
purely organic luminogens, metal-containing CIEE compounds
are rare. Very recently an AuI complex with efficient CIEE effect
was reported.[19e] We decided to use Pt–acetylide with the TPA
backbone to develop new luminogens.
Results and Discussion
Synthesis of compounds 1 and 2
Following Sonogashira coupling, a was treated with trimethyl-
silylacetylene along with [Pd(PPh3)2Cl2] as the catalyst in tri-
ethylamine under reflux condition for two days to obtain b in
approximately 95% yield (Scheme 1). After desilylation of b in
a solvent mixture of methanol/dichloromethane (MeOH/DCM)
at room temperature in the presence of potassium carbonate,
c was obtained. Finally compound 1 was prepared by the reac-
tion of c with trans-[(PEt3)2PtI2] in a mixture of toluene and
diethylamine at room temperature for two days. CuI was used
as catalyst for this reaction and the yield was approximately
80%. Compound 2 was obtained in 75% yield by Sonogashira
coupling of c with a in triethylamine under reflux condition for
two days (Scheme 1).
Characterization
The crude compounds were isolated as yellow solids. After pu-
rification, the final compounds 1 and 2 were obtained as light
and deep yellow solids, respectively. All the compounds were
1
characterized by H, 13C and 31P NMR, IR and finally by mass
spectrometry analysis. Finally, the structure of compound
1 was established from single crystal X-ray diffraction analysis.
The crystallographic data of compound 1 are provided in
Table 1.
Platinum–acetylide derivatives are well known for their
extensive use in nonlinear optics, optoelectronics, cellular
imaging and high-efficiency electroluminescent devices.[20] Our
group has previously reported several interesting luminescent
molecules with the Pt–acetylide backbone for nitroaromatics
detection.[21] Although photophysical properties of the PtII
derivatives in solution phase are well explored, their solid-state
luminescence suffers from ACQ as well as fluorescence
quenching due to heavy metal effect.[22] We decided to synthe-
size TPA-based Pt–acetylide compound 1 and its complemen-
tary organic compound 2 to investigate and compare their
photophysical properties in solution as well as in solid state.
Both compounds have a planar central area which can
function as the stator; and the terminal phenyl rings of the
TPA moiety would behave like rotors.
Single crystals of 1 were grown by slow evaporation of a sa-
turated chloroform solution. A block shaped yellow crystal was
analysed by X-ray crystallography (CCDC 1414114 contains the
supplementary crystallographic data for this paper. These data
are provided free of charge by The Cambridge Crystallographic
Data Centre). The crystal structure of compound 1 is shown in
Figure 1.
The torsion angle of the terminal phenyl rings with the cen-
tral plane is 73.138. Therefore, the TPA rings are nonplanar and
this twisted TPA backbone can have a significant influence on
solid state photophysical properties of the compounds. The
molecular packing of 1 is illustrated in Figure 2. As expected,
the molecules do not show any p–p interaction due to the
inherently twisted TPA backbone, however, there is a CÀH···p
interaction (d=2.820 Š) between the ÀPEt3 group and the ter-
minal aromatic moiety that acts as a rotor. Also, the terminal
aromatic group has a CÀH···p (d=2.885 Š) interaction with the
alkyne moiety. Therefore, the crystal structure is loosely packed
by weak hydrogen bonding. The layers are oriented in fish-
bone pattern. Upon application of mechanical force the weak
interactions can be destroyed and the loosely attached
molecules can slip over one another. This structural change
can directly influence the luminescence of the molecules in
solid state.
Some of the previously reported MC-lumiogens which exhib-
ited AIE/AIEE in solution are also found to be more efficient
emitters in crystalline phase than their amorphous counterpart,
thus, they are CIEE active.[23] In our investigation, we found out
that both molecules 1 and 2 are highly emissive in their
crystalline phase.
In this study, we report the mechano-fluorochromic property
of two AIEE-active compounds (1 and 2). To the best of our
knowledge, the PtII analogue is the first example of mechano-
fluorochromic PtII luminogen exhibiting CIEE property. More-
over, they exhibited selective chemodosimetric response
towards cysteine under physiological condition.
Photophysical studies
The UV/Vis spectra of the compounds were recorded with
varying tetrahydrofuran (THF)/water content as water is a poor
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Scheme 1. Synthesis of compounds 1 and 2.
Table 1. Crystallographic data and refinement parameters for compound
1.
Compound
1
empirical formula
Fw
T [K]
C56H58N2O4P2Pt
1080.1215
298
crystal system
space group
a [Š]
b [Š]
c [Š]
monoclinic
P21/n
10.777 (15)
20.996 (3)
11.227 (17)
90
Figure 1. Ball and stick diagram of the crystal structure of compound 1.
a [8]
b [8]
105.13 (4)
90
2452.6
2
1.462
0.087
g [8]
V [Š3]
Z
solvent for the compounds. Compounds 1 and 2 exhibit broad
absorption spectra with maxima centred around 375 and
380 nm, respectively (Figure 3). For compound 1 the absorb-
ance decreases with increasing water fraction. A 20 nm red
shift (375 to 395 nm) was observed when the water fraction
was 70% (Figure 1). Level-off tails were observed at water frac-
tion higher than 60%. Such tails are proposed to originate
from Mie scattering effect which is generally developed when
spherical nanoaggregates are formed in the medium.[24] For
compound 2 also the emission maxima at 380 nm decreased
1calcd [gcmÀ3
]
m(MoKa) [mmÀ1
l [Š]
]
0.71073
1096
F(000)
no. of collected reflens
no. of unique reflens
goodness of fit (F2)
R1 [I>2s(I)][a]
7250
6895
0.784
0.1628
0.3117
wR2 [I>2s(I)][b]
[a] R1 =SjF0 jÀjFcj/SjF0 j, [b] wR2 =(S[w(F0 ÀFc2]/S[w(F0 )2)1/2
.
2
2
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the Supporting Information). At lower water fraction the peak
maxima at 480 nm decreased, but a new peak at 500 nm ap-
peared at 70% water fraction. Compounds 1 and 2 showed
six- and twelvefold enhancement in emission intensity, respec-
tively. When further water was added to the systems the fluo-
rescence intensity decreased. Thus, these compounds exhibit-
ed aggregation-induced emission enhancement (AIEE) behav-
iour. As observed in the previously reported AIE/AIEE materials,
we speculate that in dilute solutions of the compounds the ex-
cited state energy was dissipated by the rotational motion of
the terminal phenyl rings.[6] Thus, the solution-state lumines-
cence was negligible. When water was added to the solution,
aggregates were formed due to poor solubility of the com-
pounds in water. Upon aggregate formation the molecules
were tightly packed in a very confined space. Therefore, excita-
tion energy release through non-radiative decay processes, like
torsional motion of the phenyl rings were restricted. The
molecules released the excited state energy in a radiative
decay pathway only, and as a result the fluorescence intensity
increased. Therefore, the compounds showed higher
fluorescence in aggregate forms.
Figure 2. Molecular packing of 1 in crystalline state.
and at 70% water fraction a new peak at 400 nm was
observed (see the Supporting Information).
The fluorescence spectra of the compounds were recorded
in THF/water mixtures with varying water fraction. The
compound 1 exhibited a broad spectrum with maximum at
500 nm in 10% water/THF mixture. With increasing water con-
tent the peak intensity gradually decreased, but at 70% water
fraction a new high-intensity peak appeared at lmax =510 nm.
The sixfold enhancement of fluorescence intensity is proposed
to be due to the aggregation of the molecules at higher water
portion. When more water was added the peak intensity slight-
ly decreased (Figure 1). The initial emission intensity of com-
pound 2 also decreased upon increasing water content in the
solvent system, but increased above 70% water fraction (see
We think that the molecules slowly organize in an ordered
crystalline aggregate when the water content is moderate
(70%), but upon addition of excess water the molecules
formed random amorphous aggregates with lower fluores-
cence intensity.[25] Careful inspection of the fluorescence
spectra revealed that the ordered aggregates were more
efficient emitters than the amorphous analogues.
Dynamic light scattering experiments with the samples in
the aggregate state were performed, which revealed the
spherical nature of the aggregates for both compounds
Figure 3. Absorption spectra of 1 in THF/water mixture (top left) and fluorescence spectra (top right) in THF/water mixture. Excitation wavelength=375 nm.
Compound 1 (concentration 10À5 molLÀ1) with varying amount of water fractions under 365 nm UV light in THF/water mixture.
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(Figure 4). For compound 1 the average particle size was
around 152 nm and for compound 2 it was about 120 nm in
diameter. Finally scanning electron microscopy (SEM) of the ag-
gregates coated on the carbon tapes was performed to exam-
ine the spherical shapes of the aggregates. For compound
1 well distinguishable spherical particles of about 150 nm in
diameter were obtained and for compound 2 the size was
approximately 120 nm.
Figure 5. a) Solid-state emission spectra of crystalline, grinded and DCM
fumed samples of 1, and b) their PXRD patterns.
by powder X-ray studies. The crystallinity of the samples was
proven by sharp and intense peaks and the powdered sample
exhibited weak and broad peaks (Figure 5).
Upon photoexcitation at 375 nm the crystals of 1 displayed
yellow fluorescence with a peak centred at 530 nm, but the
powdered sample exhibited faint yellow light at 520 nm
(Table 2). The quantum yield of the crystalline sample (FF =
38.76%) is eightfold higher than that of the powdered sample
(FF =4.52%).
Figure 4. a) Particle size distribution of the aggregate of 1 in 70% water/THF
mixture by DLS experiments, and b) SEM image of 1 drop-casted on carbon
tape.
Table 2. Photophysical data of compounds 1 and 2.
Compound
labs
lem
f (solid)
f (soln)
1
375
510
0.38 (cryst)
0.04 (amor)
0.57 (THF/water)
Crystallization induced emission enhancement of
compounds 1 and 2
2
380
500
0.46 (cryst)
0.05 (amor)
0.65 (THF/water)
Crystallization normally reduces emission intensity of common
fluorophores due to the detrimental effect of p–p interaction
and excimer formation. Therefore, luminogens the crystalline
forms of which are better emitters than amorphous analogues
will have a better prospect in optoelectronics. To verify wheth-
er the crystals of 1 have higher fluorescence than the grinded
powder form, single crystals of 1 were grown by slow evapora-
tion of saturated chloroform solution. The powder sample was
prepared by thorough grinding of the crystals with mortar and
pestle. The crystalline and powder samples were characterized
Therefore, it is evident that compound 1 exhibited crystalli-
zation-induced emission enhancement (CIEE). For compound 2
the grinded sample displayed a faint emission at 525 nm, but
the crystalline phase was highly emissive with maximum inten-
sity at 530 nm (see the Supporting Information). Therefore,
both materials showed variation in solid-state emission when
mechanical force was applied. Hence, both the materials were
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mechano-fluorochromic (MC). Tuning the emission intensity of
mechano-fluorochromic materials due to transition from crys-
talline to amorphous state by different forces like grinding,
pressure, etc., is an active field of research.[12] Solvent fumiga-
tion is another process that induces crystallization from amor-
phous/powder materials. Thus, to explore the reversibility of
the MC-materials from amorphous/powder to crystalline phase,
the grinded powder form of compound 1 was treated with
DCM vapour (Figure 5 and Figure 6). The powder XRD pattern
of the DCM-treated sample was found to be almost identical
with the crystalline material, although the diffraction peaks
slightly decreased in intensity. Thus, it is clear that solvent
treatment and grinding can reversibly alter the states between
highly emissive crystalline phase and poorly emissive pseudo-
amorphous phase. For compound 2 also similar reversible fluo-
rescence switching behaviour was observed when the grinded
sample was exposed to DCM vapour (see the Supporting
Information).
structure, the energy-optimized structure of 1 showed identical
geometry with terminal triphenylamine rings twisted out of
the plane of the central benzene ring. The dihedral angle of
70.588 is very close to the value obtained from single crystal X-
ray analysis which is 73.138 for 1 (Figure 7). Therefore, we pro-
pose that the terminal phenyl rings of 2 will also be twisted
out of the central phenyl ring and the dihedral angle was cal-
culated to be 71.118. As explained earlier this twisted character
of the TPA benzene rings restricted aggregate formation in the
solid state. Also, the weak intermolecular interactions between
the benzene rings helped to prevent the non-radiative decay
process by torsional motion. The energy optimized geometry
of compound 2 also furnished similar twisted geometry. Thus,
we speculate that due to similar geometrical features like 1,
compound 2 also exhibited CIEE behaviour in solid and AIEE
behaviour in solution.
Figure 7. Geometry-optimized structures of the compounds with side view
(top) and top view (bottom), compound 1 (left) and compound 2 (right).
Figure 6. Images of the crystalline, grinded and DCM-fumed samples of
1 under UV light (256 nm).
Vapochromism
From the fluorescence experiments and quantum yield mea-
surement, it became evident that the crystalline forms of the
compounds are better emitters than their grinded forms. The
effect of DCM vapour on the grinded powder forms of the ma-
terials encouraged us to explore the possibility of using them
as potential substrates to sense volatile organic chemicals
(VOC).
The crystal structure of 1 was examined to explore different
weak interactions which might help molecular packing. In
dilute solution and in the grinded state the twisted terminal
phenyl rings acted as rotors. After photoexcitation, energy was
dissipated through a non-radiative pathway by the torsional
motion of the phenyl rings. In crystalline form (before grind-
ing), the intermolecular weak interactions restricted the tor-
sional motion of the rings by different weak CÀH···p interaction
as discussed before. Due to such restricted motion of the
rotors, the excited state energy was released by radiative
decay process only. Therefore, the crystalline forms are more
emissive than the grinded forms. These weak intermolecular
interactions not only restrict movement of the terminal phenyl
ring but also facilitate the loose packing of 1 in the solid state
(Figure 2). Upon application of mechanical force (grinding), the
molecules can slip along their long axis to form a metastable
poorly crystalline phase, where the weak interactions are
mostly absent; and thus the energy decay by the torsional
motion of the phenyl rings become prominent.
Recognition of volatile solvents like DCM or chloroform is
important for environmental and social safety. When the
powder sample of 1 was exposed to different solvents like
DCM, chloroform, tetrahydrofuran (THF), hexane and methanol
(MeOH), emissions of different intensity with different peak
maxima were obtained (Figure 8). Fumigation with hexane and
MeOH showed emission maxima centred at 505 nm with differ-
ent intensity, for chlorinated solvents CH2Cl2 and CHCl3 the
maxima were around 520 nm and in THF it was at 532 nm.
Therefore, vapour infusion induced emission intensity showed
a broad range of 30 nm. Compound 2 also exhibited similar
characteristics upon exposure to different solvent vapours (see
the Supporting Information).
Several attempts to obtain suitable single crystals of com-
pound 2 were unsuccessful. Therefore, we calculated the
energy optimized structures of both the compounds (1 and 2)
for a comparative study to further elucidate the CIEE mecha-
nism. When compared to already obtained single crystal X-ray
The fluorescence switching by grinding and vapour
exposure was repeated several times for these compounds.
Such change in photophysical properties is presumably due to
the change in the crystalline forms of the compounds.
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Figure 8. Vapochromism of 1 after exposure to different solvent vapours.
Cysteine sensing
The interesting effect of structural rigidity on the photo-
physical properties of compounds 1 and 2 prompted us to
explore the materials as sensors. In this context, one of the
essential amino acid, cysteine caught our attention. Cysteine is
known to have a vital role in protein synthesis and cellular
detoxification.[26] Low cysteine levels in blood plasma is linked
with different conditions, such as liver damage, hair depigmen-
tation, skin lesions, lethargy.[27] Excess cysteine in body is re-
ported to be associated with neurodegenerative diseases such
as Alzheimer’s and Parkinson’s disease and also different
cardiovascular diseases.[28] Therefore, considerable effort has
been given for the development of sensitive and selective sen-
sors for cysteine. Fast response time and emission enhance-
ment upon analyte binding are some of the most crucial char-
acteristics of an efficient sensor. Also, designing a sensor for se-
lective detection of a single amino acid is a significant chal-
lenge. Among several reported procedures, including thiol-in-
duced cleavage, cyclization of amino-thiol with aldehydes,
Michael-type addition reaction and ligand exchange, we chose
to explore the chemodosimetric approach by thiazolidine ring
formation for cysteine detection.[28] The first chemodosimetric
approach for cysteine detection was reported by Rusin, Strong-
in et al.[30] Subsequently, several different chemodosimeters
with different fluorophore backbones have been reported.[31]
Our molecules contain an aldehyde group on the terminal
phenyl rings. We speculated that the thiazolidine ring forma-
tion would restrict the torsional movement of the terminal
phenyl rings leading to enhanced luminescence.
Figure 9. Absorption spectral changes of 1 upon gradual addition of
cysteine (top) and absorption spectra of 1 with various other amino acids
(bottom) in DMF/HEPES buffer (pH 7.4, 4:1, v/v).
in cysteine concentration. Compound 2 also exhibited high
selectivity towards cysteine in the presence of other amino
acids (see the Supporting Information). Therefore, the com-
pounds are quite selective towards cysteine.
In the emission spectra, a new peak with maxima at 410 nm
appeared upon incremental addition of cysteine solution to
1 (Figure 10). The emission enhancement can be attributed to
the thiazolidine ring formation upon reaction of cysteine with
the terminal aldehyde groups along with ICT turn-off process
of the aldehyde. When compared with other amino acids, no
new peaks were observed (Figure 10). When the fluorescence
response of 2 was examined in the presence of cysteine,
a new emission peak appeared at 420 nm. Compound 2 was
also found to be highly selective for cysteine.
For practical application water compatibility as well as sensi-
tivity at physiological pH is essential for a fluorescence sensor.
Therefore, for all the photophysical experiments DMF/HEPES
buffer was chosen as the solvent. Upon gradual addition of
cysteine, a new band was observed centred at 300 nm at the
expense of the 375 nm band for compound 1. The data were
recorded after 30 min of cysteine addition to the sensor at
room temperature. Compared with other amino acids no new
absorption band was observed (Figure 9). Similarly for
compound 2 a new peak at around 315 nm appeared and the
original peak intensity at 380 nm decreased with increase
When the interference of other amino acids on the sensitivi-
ty of 1 towards cysteine was monitored, only glutathione
(GSH) exhibited minor influence (Figure 11). A similar response
was also observed when compound 2 was treated with cys-
teine in the presence of other amino acids (see the Supporting
Information).
For practical application paper-strip-based sensors are fav-
oured due to their low production cost and fast response time.
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Test strips of compound 1 were prepared by immersing filter
paper (ꢀ1 cm2) in a saturated solution of 1 in DCM followed
by vacuum drying. Saturated solutions of different amino acids
were drop casted by glass capillary and were air dried. Only for
cysteine a blue fluorescent spot was obtained which indicated
the usefulness of paper strips for fast detection of cysteine
(Figure 12).
Figure 12. Paper strips of compound 1: a) before, and b) after addition of
amino acids, illuminated under 365 nm UV/Vis lamp.
The mechanism of cyclized ring formation was also
1
monitored by H NMR spectroscopy. When the partial NMR of
compound 2 and its cysteine adduct were compared, the
aldehyde peak at 9.91 ppm vanished and two new peaks
appeared at 5.48 and 5.65 ppm, which were assigned to the
protons of thiazolidine diastereomer (Figure 13). From control
experiments with other amino acids it was observed that
compounds 1 and 2 were quite selective for cysteine.
Figure 10. Fluorescence spectral changes upon addition of cysteine to
compound 1 (top) and fluorescence intensity change in the presence of
GSH (bottom) in DMF/HEPES buffer upon excitation at 310 nm.
Figure 13. 1H NMR spectra of 2 (above) and its cysteine adduct (below) in
[D6]DMSO.
Conclusions
In summary, we report here the mechano-fluorochromic be-
haviour of two AIEE-active compounds having a triphenylamine
backbone. One of them contains PtII in the backbone while the
other is an organic analogue. The compounds also turned out
to be crystallization-induced emission active. The presence of
weak intermolecular hydrogen bonding and CÀH···p interac-
tions in the crystalline forms were responsible for the above-
mentioned fascinating optical behaviour. Both compounds also
displayed variation in fluorescence intensity upon exposure to
different solvent vapours. Therefore, they are potential
materials for the development fast responsive sensors for the
detection of trace amounts of toxic solvents. The compounds
Figure 11. Fluorescence responses of 1 towards cysteine in the presence of
various amino acids in DMF/HEPES buffer after excitation at 310 nm. Black
bars represent the intensity after addition of various amino acids to the
stock solution of 1. Red bars represent the intensity upon further addition of
cysteine to that solution.
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were also found to be selective for the detection of cysteine in
the presence of several other amino acids under physiological
conditions.
Synthesis of c
A mixture of compound b (2.00 g, 5.31 mmol) and K2CO3 (2.78 g,
20.12 mmol) was dissolved in a solvent mixture of dichlorome-
thane (DCM) and methanol (MeOH) (20:30; v/v) and stirred for
24 h. The solvents were removed under reduced pressure and the
crude was purified by column chromatography using DCM/hexane
(1:4) as eluent to obtain a yellow solid in 70% yield. 1H NMR
(CDCl3, 400 MHz): d=9.89 (s, 2H), 7.78 (d, J=8 Hz, 4H), 7.48 (d, J=
8 Hz, 2H), 7.18 (d, J=8 Hz, 4H), 7.10 (d, J=8 Hz, 2H), 3.11 ppm (s,
1H); 13C NMR (100 MHz, CDCl3): d=190.97, 152.03, 146.39, 134.29,
132.23, 131.86, 126.47, 123.84, 119.77, 83.33, 78.51 ppm.
Experimental Section
Materials and methods
All the reactions were carried out under N2 atmosphere by using
standard Schlenk technique unless stated otherwise. All the start-
ing materials and reagents were purchased from commercially
available sources and were used without further purification. Com-
pound a was prepared following the procedure described in the
literature.[32] The NMR spectra were recorded using a 400 MHz NMR
spectrometer. The chemical shifts (d) in 1H NMR are reported in
ppm relative to Me4Si as internal standard (0.0 ppm) or proton res-
onance resulting from incomplete deuteration of NMR solvents:
CDCl3 (7.26) and [D6]DMSO (2.50). 13C NMR spectra were recorded
at 100 MHz, and the chemical shifts (d) are reported in ppm rela-
tive to external CDCl3 and [D6]DMSO at 77.8–77.2 ppm and
40.50 ppm, respectively. The 31P NMR spectra were recorded at
120 MHz and the chemical shifts (d) are reported in ppm relative
to external 83% H3PO4 at 0.0 ppm. Electrospray ionization mass
spectra were recorded using Esquire 3000 plus ESI mass spectrom-
eters. Electronic absorption spectra and emission spectra were re-
corded on a LAMBDA 750 UV/Vis spectrophotometer and Horiba
Jobin Yvon made Fluoromax-4 spectrometer. For absorption and
emission studies, solutions were prepared using a microbalance
and volumetric glassware and charged into quartz cuvettes. For
solid state vapour exposure studies similar amounts of the com-
pounds (in grinded form) were exposed to different solvent va-
pours under a closed system and then the spectra were recorded
using quartz plate and solid state fluorescence holder. Single crys-
tal X-ray diffraction data were collected on a Bruker D8 Quest dif-
fractometer with CMOS technology. The structure was solved by
direct methods using SHELX-97.[33] The absolute fluorescence quan-
tum yields were measured by Quanta-f Horiba instrument. Time-
resolved fluorescence measurements were carried out on an IBH-
Data station platform using 370 nm nano-LED source. Dynamic
light scattering (DLS) measurements were performed on the zeta-
seizer instrument ZEN3600 (Malvern, UK) with a 1738 back scatter-
ing angle and He-Ne laser (l=633 nm). The THF/water mixtures
with various water fractions were prepared by slowly adding ultra-
pure water into the THF solution of samples. The SEM image was
obtained using Zeiss Ultra-55 SEM instrument with the sample
coated on a carbon tape.
Synthesis of 1
To a freshly distilled mixture of toluene (30 mL) and diethylamine
(15 mL) in a Schlenk flask were added c (0.80 g, 2.46 mmol), trans-
[Pt(PEt3)2I2] (0.68 g, 0.98 mmol), CuI (0.03 g, 0.19 mmol) and the
flask was degassed under vacuum and refilled with nitrogen three
times. The reaction mixture was stirred for 48 h at room tempera-
ture. The solvent was removed under vacuum and the crude was
purified by column chromatography using ethyl acetate/hexane
(1:1) as eluent to afford 1 as light yellow solid in 80% yield.
1H NMR (CDCl3, 400 MHz): d=9.88 (s, 4H), 7.76 (d, J=8 Hz, 8H),
7.27 (d, J=8 Hz, 4H), 7.19 (d, J=8 Hz, 8H), 6.70 (d, J=8 Hz, 4H),
2.19 (m, 12H), 1.25 ppm (m, 18H);13C NMR (100 MHz, CDCl3): d=
190.94, 152.35, 142.66, 132.92, 131.73, 131.66, 127.39, 127.15,
123.12, 109.18, 60.84, 16.87, 8.85 ppm; 31P NMR (120 MHz, CDCl3):
d=11.31 ppm; IR: n˜ =2931 (w), 2104 (s), 1690 (s), 1575 (s), 1494 (s),
1275 (m), 1210 (m), 1156 (s), 821 cmÀ1 (s); HRMS (ESI):
C56H58N2O4P2Pt, [M+H]+ =1081.1295 (calcd) found: 1081.3704
(100%); elemental analysis calcd [%] (vacuum dried sample) for
C56H58N2O4P2Pt: C 62.27, H 5.41, N 2.59; found: C 62.87, H 5.34, N
2.93; melting point range: 210–2158C.
Synthesis of 2
A flame-dried 100 mL two-neck round bottom flask was charged
with c (0.32 g, 1.00 mmol), CuI (0.005 g, 0.03 mmol), PPh3 (0.02 g,
0.10 mol) and [Pd(PPh3)2Cl2] (0.03 g, 0.05 mmol) in freshly distilled
triethylamine (40 mL) under nitrogen atmosphere and heated for
15 min at 508C. Compound b (0.57 g, 1.5 mmol) was added to the
mixture under high nitrogen flow and the reaction mixture was re-
fluxed for 36 h. The solvent was removed under vacuum and the
crude was purified by column chromatography using 40% (v/v)
ethyl acetate (EA) and hexane mixture to afford deep yellow solid
product in 75% yield. 1H NMR (CDCl3, 400 MHz): d=9.91 (s, 4H),
7.80 (d, J=8 Hz, 8H), 7.52 (d, J=8 Hz, 4H), 7.22 (d, J=8 Hz, 8H),
7.14 ppm (d, J=8 Hz, 4H); 13C NMR (100 MHz, CDCl3): d=190.59,
151.75, 145.73, 133.39, 131.97, 131.53, 126.25, 123.53, 120.43,
89.69 ppm; IR: n˜ =2726 (w), 1684 (s), 1585 (s), 1503 (s), 1316 (m),
Synthesis of b
A flame-dried 100 mL two-neck round bottom flask was charged
with a (2.00 g, 5.26 mmol), CuI (0.03 g, 0.15 mmol), PPh3 (0.13 g,
0.52 mol) and [Pd(PPh3)2Cl2] (0.18 g, 0.26 mmol) in freshly distilled
triethylamine (40 mL) under nitrogen atmosphere and heated for
15 min at 508C. Trimethylsilylacetylene (2.5 mL, 15.77 mmol) was
added dropwise to the mixture under high nitrogen flow and the
reaction mixture was refluxed for 36 h. The solvent was removed
under vacuum and the crude was purified by column chromatog-
raphy using 20% ethyl acetate (EA)/hexane (20%) mixture to
afford the product as yellow solid in 95% yield. 1H NMR (CDCl3,
400 MHz): d=9.90 (s, 2H), 7.78 (d, J=8 Hz, 4H), 7.46 (d, J=8 Hz,
2H), 7.18 (d, J=8 Hz, 4H), 7.08 (d, J=8 Hz, 2H), 0.25 ppm (s, 9H);
13C NMR (100 MHz, CDCl3): d=190.97, 152.06, 146.04, 134.14,
132.18, 131.84, 126.47, 123.75, 120.92, 104.60, 95.77, 0.39 ppm.
1269 (m), 1211 (s), 1161 (w), 823 cmÀ1 (s); HRMS (ESI): [M+H]+
625.7040 (calcd); found: 625.6950 (100%); elemental analysis calcd
[%] (vacuum dried sample) for C42H28N2O4: C 80.75, H 4.52, N 4.48;
found: C 80.43, H 4.43, N 4.21; melting point range: 220–2258C.
=
Synthesis of nanoaggregates
Initially, stock solutions (10À3 m) of both 1 and 2 were prepared
using spectroscopy grade THF. The required amounts of the ali-
quots from the stock solutions were transferred to 4 mL glass vials.
After addition of an appropriate amount of THF for dilution, dis-
tilled water was added to the solutions under vigorous stirring to
afford (10À5 m) solutions with varying water/THF ratio (10–90%).
Chem. Eur. J. 2016, 22, 1424 – 1434
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1432
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
The photophysical studies were carried out immediately. For SEM
analysis 10 mL of the aggregate solutions were drop-casted on
1 cm”1 cm carbon tape and dried under vacuum for 6 h.
Acknowledgements
A.C. gratefully acknowledges the CSIR (New Delhi) for a research
fellowship. We are thankful to Mr. Rudra Narayan Samajdar for
his help in recording SEM images, Mr. Sourav Bhattacharya for
helpful suggestion on structure solution, Mr. Sourav Ghosh for
PXRD measurements. Mr. Samir Kumar Sarkar and Dr. P.
Thilagar are acknowledged for generous help in solid-state
quantum yield measurements.
CIEE study
For CIEE study 20 mg of crystals of sample 1 was taken and its
solid state fluorescence was measured. The crystalline sample was
recovered and ground thoroughly using mortar and pestle.
Powder X-ray diffraction pattern of the grinded sample was ana-
lysed before solid-state fluorescence measurement. To verify the re-
versibility of the sample, the amorphous sample was exposed to
DCM vapour for 30 min in a sealed container. The solid-state fluo-
rescence of the solvent treated sample was almost similar to the
crystalline phase. This process of grinding and solvent treatment
was repeated several times to obtained consistent values.
Compound 2 also showed similar reversible mechanochromic
behaviour (see the Supporting Information).
Keywords: coordination compounds
organometallic chemistry · platinum · sensing
·
fluorescence
·
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Received: October 7, 2015
Published online on January 8, 2016
Chem. Eur. J. 2016, 22, 1424 – 1434
www.chemeurj.org
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