Molecular and Nanoaggregation in Cyclometalated Iridium(III) Complexes through Structural Modification

Article abstract of DOI:10.1002/ejic.201600563

New terpyridyl ligands TP1, TP2 and cyclometalated iridium(III) complexes 1 and 2 based on these ligands have been synthesized. The ligands and complexes have been characterized by elemental analysis and spectroscopic studies (ESI-MS,¹H and¹³C NMR, UV/Vis, fluorescence). The molecular structure of 1 has been verified by X-ray single-crystal analysis. It has been unambiguously established that variation of the substituents on 1 and 2 leads to molecular aggregation in 1, while 2 remains nonaggregated. Furthermore, complexes 1 and 2 have been successfully utilized as capping agents for the stabilization of gold nanoparticles (AuNPs). It is of note that 1 forms discretely, while 2 aggregates AuNPs through the assemblage of ultrasmall nanoparticles. It has been affirmed by¹H NMR titration studies that –NH groups from 1 and 2 are involved in the capping of AuNPs. The role of simple structural variations in directing molecular and nanoaggregation has been clearly established for the first time by spectroscopic (UV/Vis, fluorescence,¹H NMR titration) and morphological studies [SEM, TEM, EDX (energy-dispersive X-ray), DLS (dynamic light scattering)].

Full text of DOI:10.1002/ejic.201600563

DOI: 10.1002/ejic.201600563
Full Paper
Photophysical Properties
Molecular and Nanoaggregation in Cyclometalated Iridium(III)
Complexes through Structural Modification
[
a]
[a]
[a]
[a]
Sujay Mukhopadhyay, Roop Shikha Singh, Arnab Biswas, Biswajit Maiti, and
Daya Shankar Pandey*[
a]
Abstract: New terpyridyl ligands TP1, TP2 and cyclometalated zation of gold nanoparticles (AuNPs). It is of note that 1 forms
iridium(III) complexes 1 and 2 based on these ligands have discretely, while 2 aggregates AuNPs through the assemblage
been synthesized. The ligands and complexes have been char- of ultrasmall nanoparticles. It has been affirmed by 1H NMR
acterized by elemental analysis and spectroscopic studies (ESI- titration studies that –NH groups from 1 and 2 are involved in
1
13
MS, H and C NMR, UV/Vis, fluorescence). The molecular struc- the capping of AuNPs. The role of simple structural variations
ture of 1 has been verified by X-ray single-crystal analysis. It in directing molecular and nanoaggregation has been clearly
has been unambiguously established that variation of the sub- established for the first time by spectroscopic (UV/Vis, fluores-
1
stituents on 1 and 2 leads to molecular aggregation in 1, while cence, H NMR titration) and morphological studies [SEM, TEM,
2
remains nonaggregated. Furthermore, complexes 1 and 2 EDX (energy-dispersive X-ray), DLS (dynamic light scattering)].
have been successfully utilized as capping agents for the stabili-
Introduction
They show a lack of emission or weak emission in solution,
but enhanced emission in the solid state, advocating that self-
Monitoring the photophysical properties of cyclometalated irid-
ium(III) complexes has attracted the attention of many research
groups in contemporary chemical research. Since the last dec-
ade, significant advancements in this area have enabled scien-
tists to develop many efficient emitters with wide applicability
in biological and materials science. Owing to their triplet ex-
cited state, cyclometalated iridium complexes exhibit an intense
long-lived red luminescence with substantial Stokes shift.
Changes in the coordination sphere of iridium by ligand substi-
tution may cause diverse emission behavior. Triplet–triplet an-
nihilation due to strong interactions between closely packed
molecules within iridium(III) complexes limits their scope of ap-
plications in optical devices, which can be circumvented by us-
[6]
quenching is not always momentous. Such an effect has
[7a]
scarcely been explored in cationic iridium(III) complexes.
[
1]
However, the combined effects of rich photophysical properties,
ionic character, and stability in aqueous media can resolve the
problem associated with emission quenching and may aug-
[
2]
[7b]
ment the application of these complexes in optical devices.
Likewise, gold nanoparticles (AuNPs) draw an enormous cur-
rent interest due to their widespread applications in diverse
[
3]
[
8]
areas. The innovative report of Mayer et al. on functionaliza-
tion of AuNPs by iridium(III) polypyridyl complexes has in-
[
4]
[
1]
creased the curiosity on Ir-capped AuNPs. In addition, it is
well known that molecular aggregation and formation of
capped nanoparticles involve various weak interactions. How-
ever, concrete experimental evidences and evaluation of the
structural parameters responsible for weak interactions assist-
ing molecular and nanoaggregation has not yet been ad-
dressed. Motivated by these points we have designed and syn-
thesized new ligands, namely 3-([2,2′:6′,2′′-terpyridin]-4′-yl)-6-
methoxyquinolin-2(1H)-one (TP1) and 3-([2,2′:6′,2′′-terpyridin]-
[
5,4b,4c]
ing bulky ligands leading to steric hindrance.
With these
points in mind and with the aim to develop ionic complexes
with extraordinary emissive properties, cyclometalated iridium
precursors containing 2-phenylpyridine (PPy) along with vari-
[
4a,4d]
[4c]
ous luminescent ligands such as bipyridine,
quinolone,
[
4b]
[4e]
1,10-phenanthroline,
terpyridine derivatives
and so on
[
4]
have been largely explored. Consequently, there is eclectic
interest in developing iridium-based light-emitting electro-
chemical cells, aggregation-induced emitters and so on.
Molecular aggregation is an unusual property that prevails in
many organic luminogens and transition-metal complexes.
4
′-yl)quinolin-2(1H)-one (TP2) based on a terpyridyl core and
+
–
+
–
iridium(III) complex [Ir(PPy) TP1] PF (1) or [Ir(PPy) TP2] PF
2
6
2
6
[
5a,5b]
(2), respectively. The quinolone moiety in these systems has
been purposefully incorporated to enhance π–π interactions
between the molecules. In TP1, an –OMe functionality has been
incorporated to enhance the electron cloud at the chelating
site and to provide hydrophilicity to the quinolone moiety with
[
a] Department of Chemistry, Institute of Science, Banaras Hindu University,
Varanasi, 221005 U.P., India
[9]
respect to unsubstituted TP2. It is of note that 1 displayed
E-mail: dspbhu@bhu.ac.in
molecular aggregation, while 2 was inactive in this regard. On
the other hand, for Ir-capped AuNPs (AuNP1 and AuNP2)
nanoaggregation occurred with AuNP2, whereas AuNP1 af-
http://internet.bhu.ac.in/science/chemistry/dspandey.php
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejic.201600563.
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forded discrete nanoparticles. In this contribution we describe 7.23–8.72 ppm and the –NH proton as a singlet at δ =
dissimilarities in the aggregation behavior at molecular and 12.12 ppm. The occurrence and chemical shift of various signals
1
nanolevel between two closely related systems by fine tuning in the H NMR spectra strongly suggests the formation of li-
functional groups for the first time.
gands TP1 and TP2. Upon complexation with cyclometalated
iridium–PPy the singlets of the –OCH and –NH protons in 1
3
exhibit downfield-shifted resonances at
δ = 3.93 and
Results and Discussion
1
2.25 ppm. Furthermore, aromatic protons also display down-
field shift in this complex and appear at about 5.45–9.10 ppm.
Similarly, for complex 2 both aromatic and –NH protons show
downfield-shifted resonances in the range of 6.25–9.31 ppm
Synthesis and Characterization
2
3
-Chloroquinoline-3-carbaldehyde, 2-oxo-1,2-dihydroquinoline-
-carbaldehyde, and its methoxy derivative have been synthe-
1
3
(broad multiplet) and 12.24 ppm (singlet). C NMR spectro-
scopic data of the ligands and complexes 1 and 2 further sup-
sized following standard procedures. Ligands TP1 and TP2
were prepared by the reaction of 2-acetylpyridine in ammonia-
cal MeOH solution with the corresponding aldehydes under re-
port their formation and the proposed structures. These find-
[
11,12]
ings are similar to those in other closely related systems.
[
10b,10c]
flux and continuous stirring for 48 hours.
The chloro-
bridged dimeric precursor [(PPy) Ir(μ-Cl)] was treated with TP1
2
2
Crystal Structure Analysis
or TP2 in CH Cl /MeOH (1:1) under stirring at room temp. for
2
2
1
2 hours. Subsequent reaction with NH PF and stirring for an The molecular structure of 1 has been explicitly validated by X-
4
6
additional hour gave complexes 1 and 2 in good yields (70– ray single-crystal analysis. It crystallizes in a monoclinic system
0 %). The simple strategy adopted for the synthesis of ligands with a P21/c space group. Details about data collection, solu-
8
and complexes is shown below (Scheme 1).
tion, and refinement along with selected geometrical parame-
ters are gathered in Tables S1 and S2 in the Supporting Infor-
mation. A pertinent view of the crystal structure is given in
Figure 1. It reveals that cationic iridium(III) complex 1 shows
N1–N2 in trans position, which is a normal phenomenon result-
[
11]
ing from the conventional synthetic route. The Ir1–C and Ir1-
N bond lengths and C–Ir1–N, C–Ir1–C, N–Ir1–N bond angles
(
Table S2 in the Supporting Information) have a normal size.
They are comparable to other closely related cyclometalated
[
2d,11c]
iridium(III) polypyridine systems reported in the literature.
Scheme 1. Synthesis of ligands TP1, TP2, and complexes 1 and 2.
The studied complexes are nonhygroscopic air-stable crystal-
line solids, highly soluble in common organic solvents such as
CH Cl , chloroform, MeOH, acetone, DMSO, and acetonitrile.
2
2
They are partially soluble in ethyl acetate, ethanol and insoluble
in diethyl ether, benzene, hexane, and petroleum ether. The
complexes have been meticulously characterized by satisfactory
1
elemental analyses and spectroscopic studies (ESI-MS, H and
1
3
C NMR, UV/Vis). The structure of 1 has been authenticated
by X-ray single-crystal analysis. ESI-MS spectra are gathered in
Figures S5–S6 in the Supporting Information.
Figure 1. ORTEP view of 1 at 30 % thermal ellipsoid probability (H atoms are
omitted for clarity).
NMR Spectroscopic Studies
Photophysical Properties
1H and 13C NMR spectra of TP1, TP2, 1, and 2 have been re- The absorbance behavior of 1 and 2 has been examined in
corded in [D ]DMSO, and the resulting data is gathered in the acetonitrile at room temp. (Figure S9a in the Supporting Infor-
6
experimental section (see also Figures S1–S4 in the Supporting mation). Both complexes display absorptions below 300 nm
Information). The –OCH and –NH protons of TP1 resonate as that can be assigned to ligand-centered (LC) π–π* transitions.
3
singlets at δ = 3.80 and 12.03 ppm, respectively, while aromatic Weak absorptions appearing at 390 (1) and 375 nm (2) can be
protons appear as broad multiplet at δ = 7.22–8.87 ppm. Like- assigned as metal-to-ligand charge-transfer (MLCT) transitions
wise, the aromatic protons of TP2 resonate as multiplet at δ = in analogy with reported systems.[12,4d] Molecular aggregation
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has been perceived by monitoring the photophysical properties in the Supporting Information) in the solid state. This corrobo-
of these compounds in water/acetonitrile mixtures of varying rates well the findings in the emission spectra of 1 at f 100 %.
w
water content. Upon increasing the water fraction (f ), both 1 The PL (photoluminescence) average lifetime experiments have
w
and 2 exhibit a significant decrease in absorbance without any been carried out for 1 and 2 considering their respective emis-
apparent change in the absorption maxima (Figure S10 in the sion band at 600 nm (Figure 3). The average lifetime (τav) for
Supporting Information). However, 1 shows a level-off tail at fw these compounds in acetonitrile is about 1.93 and 1.28 ns, re-
1
00 % indicating a suspension of aggregates. Furthermore, at spectively, whereas in 100 % water τav is about 3.72 ns for 1
fw 100 % compound 1 displays a hump in the red region with and shorter than 0.6 ns for 2 (beyond detection limit). From the
a small increase in absorbance, which again suggests the for- aforementioned results it can be inferred that the PL lifetime
mation of aggregates (Figure S11a in the Supporting Informa- increases only for 1 due to molecular aggregation.
[
13]
tion).
Upon excitation at 390 nm in acetonitrile 1 displays an emis-
sion band at 495 with a hump at about 600 nm. On the other
hand, upon excitation at 375 nm (Figure S9b in the Supporting
Information) compound 2 exhibits dual emission at 520 and
615 nm. To assign these bands, emission spectra of the quinol-
one–terpyridyl ligands have been recorded. They display bands
at 484 (TP1) and 475 nm (TP2), whereas the precursor complex
emits at 515 nm. Thus, high- and low-energy emissions have
been ascribed to bands associated with quinolone–terpyridyl
and precursor complex [(PPy) Ir(μ-Cl)] , respectively (Fig-
Figure 2. Fluorescence spectra of 1 (a) and 2 (b) in acetonitrile/water (c =
10 μ ) with different volume fractions of water (fw).
2
2
9,14]
M
[
ure S11b in the Supporting Information).
Clearly, dual emis-
sion is prominent in 2 relative to 1. In addition, to gain a defini-
tive idea about the aggregation process, fluorescence spectra
of 1 and 2 have been recorded in acetonitrile/water with fw
varying from 10–100 % (Figure 2). Notably, the fluorescence
measurements turned out to be much more informative than
the electronic absorption spectra. Upon increasing fw (0–90 %)
for 1, the intensity of the bands at 495 and 600 nm decreases
along with a blueshift. This is probably due to the addition of
water, which acts as quencher in the medium. When fw is en-
hanced to 95–98 %, the emission bands reduce to humps with
complete quenching. Interestingly, further increase in fw to
Figure 3. Average PL lifetimes of 1 (a) and 2 (b) in acetonitrile (green) and in
water (red).
100 % leads to an insignificant change in intensity of the band
at 495 nm due to the presence of the quinolone–terpyridyl unit.
Contrarily, the band at 600 nm shows an appreciable increase
in intensity; the solution became red fluorescent. This behavior
signifies the occurrence of molecular aggregation in 1. On the
One of the finest tools for the characterization of AuNPs is
UV/Vis absorption spectroscopy. The position and shape of a
surface plasmon band depends on the particle size, shape, and
[15]
dielectric constant of the medium.
The requisite reducing
other hand, the emission of 2 is quenched with increasing fw,
agent NaBH for the creation of nanoparticles has been opti-
4
which suggests a lack of aggregation. Images for 1 and 2 upon
mized by UV/Vis spectral studies. It was observed that 50 μL of
–
2
excitation at 365 nm with varying f are depicted in Figure S18
w
NaBH (c = 5 × 10
M) is sufficient for the complete reduction
4
–2
in the Supporting Information. The fluorescence quantum yield
of gold(III) (HAuCl ; c = 10
M
, 50 μL). Volume and concentra-
4
(
Φ) for 1 and 2 in acetonitrile is 42 and 19 %, respectively. How- tion of both HAuCl and NaBH were kept fixed. Throughout
4
4
ever, at fw 100 % it is reduced to 22 and 5 % (see Supporting the experiments only the volumes of 1 and 2 were varied. Ab-
Information for quantum-yield calculation method). The quan- sorption spectra of the produced AuNPs with variable Au/[1/2]
tum yield reveals that at f 100 % some sort of enhancement in ratio (01, 02, 03, 05) are shown in Figure 4. Clearly, both 1 and
w
the emission intensity is occurring for 1 relative to 2, signifying 2 display a characteristic surface plasmon resonance (SPR) band
molecular aggregation. Excitation spectra of 1 and 2 at their for AuNPs at about 550 nm. The position of the SPR band indi-
emission maxima have also been monitored (Figure S14 in the cates capping of the AuNPs by the nitrogen donor atoms in 1
Supporting Information). The excitation spectra are very similar and 2. Images of the nanoparticle solutions are shown in Fig-
to the absorption spectra, suggesting a lack of contribution ure S19 in the Supporting Information. After addition of the
from a charge-transfer state to the emission of the compounds. abovementioned ratios of NaBH4 and gold solution, almost
This signifies that molecular aggregation hinders the free rota- complete quenching of the emission intensity for both 1 and 2
tion of 1 for energy relaxation through charge transfer.[ Solid- occurred, which may be related to the formation of Ir-capped
state fluorescence experiments on powder samples have also nanoparticles (Figures S12–S13 in the Supporting Information).
been performed, which show significant red fluorescence of 1 It is contextually evident that the ratio of Au/[1/2] was opti-
compared to that of 2 and a band at about 600 nm (Figure S15 mized properly.
13]
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over the whole molecule, while the HOMO–1 can be attributed
to both quinolone–terpyridyl and PPy units (Figure S17 in the
Supporting Information). Weak MLCT absorption bands in the
UV/Vis spectra of 1 and 2 can be assigned to HOMO–1 → LUMO
transitions (3.09 eV in 1; 3.10 eV in 2), while tailing in the lower-
energy region can be correlated with HOMO–LUMO transitions
(2.38 eV in 1; 2.70 eV in 2). Hence, the aforementioned results
are in good accordance with the spectroscopic findings.
Figure 4. UV/Vis spectra of 1 (a), 2 (b), and AuNPs with respect to varying
Au/[1/2] (ratios 1:1, 1:2, 1:3, and 1:5).
Morphological Analysis
To have a clear view of the aggregation and dissimilar optical
behavior, water/acetonitrile mixtures of 1 and 2 (fw = 100 %)
were subjected to scanning electron microscopic (SEM) and
transmission electron microscopic (TEM) studies (Figure 6).
Within solution spherical particles (average size ca. 40–50 nm,
Figure 6, a) remain aggregated for 1, whereas 2 shows discrete
spherical particles (average size 40 nm, Figure 6, b). The ob-
served disparity reinforced the idea that molecular aggregation
can be controlled by careful selection of the substituents. EDX
Crystal Structure and Theoretical Analysis
Disparate molecular aggregation and distinct photophysical
properties for 1 and 2 unveiled the substantial role of intermo-
lecular interactions and prompted us to examine the crystal
structure of 1. The quinolone moieties of two molecules are
positioned above each other in a slipped manner causing ag-
gregation with an interplanar distance of 3.6 Å, falling in the
range for π–π interactions (Figure S16 in the Supporting Infor-
mation). It is well known that π–π interactions become stronger
with an increase in π-electron density within the moiety. There-
fore, in presence of the electron-releasing –OMe group the π-
electron density in 1 is expected to be much higher compared
to that in 2, which may be the reason for the aggregation in 1.
To further support the spectroscopic results, time-dependent
DFT calculations (TDDFT) have been performed on the energy
levels of 1 and 2 (Figure 5). As 1 and 2 are heteroleptic com-
plexes, the MO (molecular orbital) distribution should be stud-
ied with respect to both ligands (quinolone–terpyridyl and
PPy). The HOMO (highest occupied MO) energy levels are domi-
nated by orbitals from quinolone–terpyridyl units with insignifi-
cant contribution from PPy. However, the LUMO (lowest unoc-
(energy-dispersive X-ray)-TEM analysis also confirmed that the
spherical particles contain 1 and 2 (Figure S22 in the Support-
ing Information).
III
cupied MO) is concerted on the Ir center, which is in accord-
ance with the better chelating ability of quinolone–terpyridyl
over PPy. The broad absorption bands suggest the involvement
of other orbitals in the electronic transitions. A profound evalu-
ation of the MO distribution revealed that the LUMO+1 spreads
Figure 6. SEM images of 1, aggregated (a) and 2, nonaggregated (b). TEM
images from the same solution of 1 (c) and 2 (d). Inset showing a discrete
particle (scale bar 100 nm).
The aggregate formation has been demonstrated by solu-
tion-based DLS (dynamic light scattering) measurements (Fig-
ure S24a in the Supporting Information). From the DLS experi-
ment it can be observed that at fw 100 % 1 shows an average
particle size (Z ) of about 195 nm, whereas 2 shows a Z of
av
av
about 80 nm. The Zav value reveals that 1 remains aggregated
in 100 % water, whereas 2 segregates at this stage. Due to this
solvent-dependent molecular aggregation, the particle sizes
vary for 1 and 2.
Figure 5. HOMO (a), LUMO (b) diagrams of 1 and HOMO (c), LUMO (d) dia-
grams of 2.
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Size and shape of the nanoparticles are usually determined about 20 nm size (Figure 7, a and c, Figure S20 in the Support-
by SEM, TEM/HRTEM (high-resolution TEM), and their composi- ing Information). However, SEM images for AuNP2 show ultra-
tion is established by EDX studies. These studies have small particles, aggregated to form spherical assemblies. The
been performed to validate the formation of the Ir-capped TEM and HRTEM analysis support the same phenomenon (Fig-
–
2
AuNP(1/2) in presence of NaBH (c = 5 × 10
M). SEM images ure 7, b and d, Figure S21 in the Supporting Information). The
4
reveal well-dispersed spherical particles of nanodimension for EDX analyses on AuNPs show the presence of PF6– moieties
AuNP1; TEM and HRTEM analyses affirm discrete particles of along with Au and Ir, suggesting strong binding of 1 and 2
onto the Au surface (Figure S23 in the Supporting Information).
The stability of the nanoparticles has also been assessed by
monitoring their solutions for several days. The colloidal sus-
pensions remained stable without precipitation.
To gain an idea about size of the particles and stability of
the dispersed solutions, DLS and zeta potential measurements
have been performed on AuNP1 and AuNP2 (Figures S24b, S25
in the Supporting Information). From the DLS studies for
capped AuNPs we observed an average particle size of about
6
5 nm for AuNP1 and about 35 nm for AuNP2, which confirms
the presence of tiny particles in AuNP2. The mean zeta poten-
tials of AuNP1 and AuNP2 are –2.0 and 23.1 mV, respectively.
These findings suggest that the solution of AuNP2 has a high
degree of stability in comparison to that of AuNP1, and they
confirm our assumption that 2 is the more efficient capping
agent compared to 1.
Plausible Mechanism
Following overall discussions pertaining to the formation of the
AuNPs, one may ask about the capping mode of 1 and 2 to
form nanoparticles. To ascertain the donor atoms involved in
Figure 7. SEM images of AuNPs of 1 (a), 2 (b). TEM images of AuNPs of 1 (c),
(d).
1
2
binding, H NMR titration studies have been performed with
Scheme 2. Mechanistic model showing molecular and nanoaggregation in 1 and 2, respectively.
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use of [D ]DMSO as the solvent. After addition of 5 μL (0.1
M
)
Ltd., India. Sodium borohydride, 2-acetylpyridine, and sodium acet-
ate were purchased from S. D. Fine-Chem. India, Pvt. Ltd. The chemi-
cals were used as received without further purification. Precursor
complex [(PPy) Ir(μ-Cl)] was synthesized and purified following lit-
6
NaBH to a solution of 1, the signal of the –NH proton was
4
shifted upfield (δ = 12.249–12.157 ppm) along other protons.
2
2
Again, after addition of 0–50 μL (0.01
M) gold solution the –NH
[
17]
erature procedures.
proton showed a greater upfield shift (δ = 12.137–12.108 ppm)
relative to the protons of the aromatic region (Figure S7 in the General Methods: Elemental analyses for C, H, and N were per-
formed with an Elementar Vario EL III Carlo Erba 1108. Electronic
absorption spectra (aqueous acetonitrile) were recorded at room
temp. with a Shimadzu UV-1601 spectrometer. Fluorescence spectra
Supporting Information). The same pattern of upfield shifting
has been observed for 2. In this case the –NH proton shifted
from δ = 12.23 to 12.13 ppm along with moderate upfield shift
in the aromatic protons (Figure S8 in the Supporting Informa-
tion). From these results we conclude that the binding site for
the capping agents 1 and 2 predominantly comprises the –NH
proton.
(aqueous acetonitrile) were recorded at room temp. with a Perki-
nElmer LS 55 Fluorescence Spectrometer. The excitation and emis-
1
sion slit widths were set at 10.0 and 7.5 nm, respectively.
(
H
13
300 MHz) and C (75.45 MHz) NMR spectra at room temp. were
recorded with a JEOL AL300 FT-NMR spectrometer with use of tetra-
From the above discussion, a tentative mechanism for molec- methylsilane [Si(CH ) ] as an internal reference. ESI-MS measure-
3
4
ular aggregation and creation of nonaggregated/aggregated ments were performed with a Bruker Daltonics Amazon SL ion trap
nanoparticles involving 1 and 2 can be proposed (Scheme 2). mass spectrometer. Samples were dissolved in 100 % acetonitrile
with 0.1 % formic acid and introduced into the ESI source through
a syringe pump at a flow rate of 100 μL/h. The capillary voltage
was 4500 V, dry gas flow 8 L/min at 300 °C. The MS scan was re-
corded for 2.0 min, and spectra print outs were averaged of over
Due to the presence of a hydrophilic –OMe group in 1, weak
electronic or H-bonding interaction with water becomes promi-
nent at fw 100 %. The versatility of intermolecular interactions
adjusts the hydrophilic and hydrophobic spheres in such a way
each scan. SEM images were captured on a quanta 200 F micro-
that the polar groups are exposed to the aqueous milieu. Thus,
scope on a silicon wafer. FEI Tecnai 20 U Twin (TEM) and Tecnai
the molecules remain aggregated through the quinolone moi-
ety by π–π stacking, causing aggregation. Owing to the pres-
2
G 20 (HRTEM) were used to analyze the structure and morphology
of samples at 200 kV. Compositional analyses of the samples were
ence of hydrophobic –H instead of –OMe, such hydrophilic in- performed by EDX spectroscopy and TEM-EDX. For TEM analysis,
teractions are not possible with the quinolone moiety in 2. Ac- samples were dissolved in aqueous acetonitrile and water by soni-
cordingly, the absence of π–π stacking may not favor aggrega- cation, and one drop of the sample was poured onto a copper grid
with porous carbon film support and dried in air. DLS and zeta
potential analyses were performed with a Horiba particle size analy-
zer SZ-100 instrument. The lifetime measurements were performed
with use of a TCSPC system from Horiba Yovin (Model: Fluorocube-
tion in 2. For the formation of AuNPs, the aggregated assembly
of 1 is hardly available for capping, contrary to 2 which remains
segregated in solution and is easily available for capping. Thus,
2
can form ultrasmall nanoparticles which in turn ensemble to
01-NL). The samples were excited at 375 nm by using a picosecond
form a spherelike structure.
diode laser (Model: Pico Brite-375L). The data analysis was per-
formed with IBH DAS (version 6, HORIBA Scientific, Edison, NJ) de-
cay analysis software.
Conclusion
Syntheses
In this work two new cyclometalated iridium(III) complexes 1
and 2 derived from new terpyridyl-based ligands have been
synthesized. UV/Vis and fluorescence spectroscopic studies un- literature procedures.
veiled that 1 undergoes molecular aggregation, while 2 is inac-
2
-Chloroquinoline-3-carbaldehyde, 2-oxo-1,2-dihydroquinoline-3-
carbaldehyde, and its methoxy derivatives were prepared following
[
10a]
TP1: 2-Acetylpyridine (250 μL, 2 mmol) was added to a basic meth-
anolic solution (50 mL) of NaOH (40 mg, 1 mmol) and 6-methoxy-2-
oxo-1,2-dihydroquinoline-3-carbaldehyde (203 mg, 1 mmol), made
tive in this regard. These complexes, upon treatment with
gold(III) solution in presence of NaBH , act as good capping
4
agents and afford AuNPs. From the morphological analysis it
has been established that 1 forms discrete nanoparticles, while mixture was stirred for 1 h and stirred under reflux for additional
ammoniacal by adding concentrated NH OH (10 mL). The reaction
4
2
creates nanoaggregates with the help of an assembly of ultra- 48 h. Upon cooling to room temp. an orange precipitate was
small nanoparticles. SEM, TEM, HRTEM, DLS, and zeta potential formed, which was filtered and washed with water and MeOH, yield
7
9.6 % (323 mg). C H N O (390.44): calcd. C 76.91, H 4.65, N 14.35;
analyses confirmed the above findings. Simple structural varia-
tions dictating the selectivity of complexes with regard to mo-
lecular and nanoaggregation have been described for the first
time. Further amplification of this tactics may lead to the de-
sired diversity in aggregates with flexible properties.
25 18 4
1
found C 76.95, H 4.61, N 14.39. H NMR ([D ]DMSO): δ = 3.80 (s, 3
6
H, OCH ), 7.22 (d, 2 H, phenyl), 7.32 (d, 3 H, phenyl), 7.49 (m, 2 H,
3
phenyl), 8.45 (s, 1 H, phenyl), 8.65 (d, 2 H, phenyl), 8.74 (d, 2 H,
1
3
phenyl), 8.87 (s, 2 H, phenyl), 12.03 (s, 1 H, NH) ppm. C NMR
(
[D ]DMSO): δ = 50.14, (OCH ), 117.59, 117.05,119.25, 121.05, 124.33,
6
3
1
27.18, 130.77, 132.55, 135.79, 137.24, 149.24, 155.14 (C H ) ppm.
6 6
+
ESI-MS calcd. for C H N O [M + H] 407.1500; found 407.1505.
25 19 4
Experimental Section
TP2: The product was prepared by following the above procedure
Reagents: All synthetic manipulations were performed under nitro- for TP1 with use of 2-oxo-1,2-dihydroquinoline-3-carbaldehyde
gen atmosphere. Solvents were dried and distilled prior to use fol-
(173 mg, 1 mmol). TP2 was obtained as a pale yellow precipitate,
[16]
lowing literature procedures.
Hydrated iridium(III) trichloride,
yield 78.2 % (295 mg). C H N O (376.42): calcd. C 76.58, H 4.28, N
2
4 16 4
1
PPy, p-anisidine, acetanilide, ammonium hexafluorophosphate, and
14.88; found C 76.51, H 4.33, N 14.92. H NMR ([D ]DMSO): δ = 7.23
6
gold(III) chloride were procured from Sigma Aldrich Chemical Pvt.
(t, 1 H, phenyl), 7.38 (d, 2 H, phenyl), 7.53 (m, 3 H, phenyl), 7.87 (d,
Eur. J. Inorg. Chem. 0000, 0–0
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6 © 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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1
H, phenyl), 7.98 (m, 2 H, phenyl), 8.48 (s, 1 H, phenyl), 8.72 (m, 4
mation).[22] TD-DFT calculations were also performed in B3LYP/
LANL2DZ mode in the excited state.
1
3
H, phenyl), 12.12 (s, 1 H, NH) ppm. C NMR ([D ]DMSO): δ = 114.89,
6
1
1
3
21.46, 124.38, 128.89, 131.17, 137.39, 139.23, 145.99, 149.26,
Supporting Information (see footnote on the first page of this
+
55.01, 160.54 (C H ) ppm. ESI-MS calcd. for C H N O [M + H]
6
6
24 17
4
1
13
article): H, C NMR, FT-IR, ESI-MS, HRMS, UV/Vis titration curves,
fluorescence spectra, SEM, TEM, EDX, images (PDF), Tables and Crys-
tallographic data for 1 (CIF) are provided.
77.1400; found 377.1400.
Compound 1: To a solution of TP1 (100 mg, 0.25 mmol) in CH Cl /
2
2
MeOH (1:1, 50 mL), the precursor complex [(PPy) Ir(μ-Cl)] (120 mg,
2
2
CCDC 1421297 (for 1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre.
0
1
0
.1 mmol) was added, and the resulting solution was stirred for
2 h at room temp. After filtration, a solution of NH PF (40 mg,
4
6
.25 mmol) in MeOH (5 mL) was added, and the reaction mixture
was stirred for 1 h to afford a reddish brown precipitate. It was
filtered and washed with water and diethyl ether. The product was Acknowledgments
obtained as orange crystals after slow evaporation of MeOH over a
The authors gratefully acknowledge the Department of Science
CH Cl2 solution of the complex, yield 71.4 % (75 mg).
2
and Technology (DST), New Delhi, India for providing financial
assistance through Scheme SR/S1/IC-25/2011 and also the Uni-
versity Grants Commission (UGC), New Delhi, India for the
C H F IrN O P (1052.01): calcd. C 53.66, H 3.26, N 7.99; found C
4
7
34
6
6 2
1
5
5
(
3.72, H 3.20, N 8.03. H NMR ([D ]DMSO): δ = 3.93 (s, 3 H, OCH ),
6
3
.45 (d, 2 H, phenyl), 5.87 (d, 2 H, phenyl), 6.28 (d, 2 H, phenyl), 6.31
s, 1 H, phenyl), 6.58 (d, 2 H, phenyl), 6.61 (t, 1 H, phenyl), 6.73 (t, 1 award of a Senior Research Fellowship (19-6/2011(i)EU-IV) to
H, phenyl), 6.76 (s, 1 H, phenyl), 6.89 (d, 2 H, phenyl), 6.93 (t, 1 H, S. M. The authors are thankful to the Head, Department of
phenyl), 7.09 (d, 2 H, phenyl), 7.22 (s, 2 H, phenyl), 7.42 (d, 1 H, Chemistry, Institute of Science, Banaras Hindu University, Vara-
phenyl), 7.58 (d, 2 H, phenyl), 7.70 (d, 2 H, phenyl), 7.80 (d, 2 H,
phenyl), 7.83 (t, 1 H, phenyl), 8.49 (t, 1 H, phenyl), 8.96 (d, 1 H,
phenyl), 9.10 (s, 1 H, phenyl), 12.25 (s, 1 H, NH) ppm. 13C NMR
nasi (U. P.), India, for extending the use of the laboratory facili-
ties.
(
1
1
[D ]DMSO): δ = 55.04 (OCH ), 115.12, 119.55, 122.91, 123.10,
25.14, 128.12, 129.61, 130.41, 131.59, 134.78, 138.05, 142.40,
6
3
Keywords: Iridium · Gold · Terpyridine ligands ·
47.01, 148.69, 151.04, 156.26, 161.11, 166.14, 167.59 (C H ) ppm. Cyclometalation · Aggregation · Nanoparticles
6
6
+
ESI-MS calcd. for C H IrN O [M] 907.2400; found 907.2316.
47
34
6 2
Compound 2: The product was obtained as a red precipitate by
following the above procedure for 1 with use of TP2 (95 mg,
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46 32 6 6
1
C 54.06, H 3.16, N 8.22; found C 54.13, H 3.21, N 8.16. H NMR
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1
0558.
(
6
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6
1
1
1
01.77, 118.86, 119.89, 123.14, 124.33, 125.92, 127.98, 129.97,
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46 32
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Received: May 11, 2016
Published Online: ■
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Photophysical Properties
The role of simple structural variations
in directing molecular and nanoaggre-
gation has been established for the
first time in two new cyclometalated
iridium(III) complexes with the help of
various spectroscopic (UV/Vis, fluores-
S. Mukhopadhyay, R. S. Singh,
A. Biswas, B. Maiti,
D. S. Pandey* ....................................... 1–9
Molecular and Nanoaggregation in
Cyclometalated Iridium(III) Com-
plexes through Structural Modifica-
tion
1
cence, H NMR titration) and morpho-
logical analyses [SEM, TEM, EDX (en-
ergy-dispersive X-ray), DLS (dynamic
light scattering)].
DOI: 10.1002/ejic.201600563
Eur. J. Inorg. Chem. 0000, 0–0
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9
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim