Facile tuning of the aggregation-induced emission wavelength in a common framework of a cyclometalated iridium(III) complex: Micellar encapsulated probe in cellular imaging

Article abstract of DOI:10.1039/c4tc00466c

A simple synthetic protocol involving two steps was developed for the syntheses of a series of monocyclometalated iridium(iii) complexes. Initially, an intermediate, [IrHCl[(o-C₆H₃X)P(Ar) _x-(PAr_xR_y)₂] [A (i, j, k, l)], six-coordinated iridium(iii) complex involving a 4-membered chelate was isolated. Then, it was transformed into a monocyclometalated iridium(iii) complex, [(C^N)Ir(PAr_x-1R_y)₂(Cl(H)] (1-12), by the replacement of the 4-membered chelates with 5-membered cyclometalates. The intermediates and the complexes were structurally characterized by FTIR, ¹H, ¹³C and ³¹P NMR spectroscopies. Octahedral coordination for Ir(iii) in 2, 8 and 9 was established by single crystal X-ray diffraction. Photo-physical experiments and quantum chemical calculations revealed a mixed LC/MLCT/LLCT nature for the lowest excited states of all these complexes that emit bright light in the solid state. Fine tuning of the emission wavelength throughout the visible range was achieved by suitable combinations of chromophoric cyclometalates and non-chromophoric aryl phosphine ligands. More interestingly, all the studied complexes were found to be aggregation-induced emission (AIE) active. One of these AIE active materials (6) was encapsulated inside polymeric micelles that inhibit the macroscopic precipitation of the aggregated complex, <200 nm water-soluble particle exhibiting a strong emission. These colloidal luminescent particles were used as a potential non-toxic bio-imaging probe. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4tc00466c

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Materials Chemistry C
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PAPER
Facile tuning of the aggregation-induced emission
wavelength in a common framework of a
cyclometalated iridium(III) complex: micellar
encapsulated probe in cellular imaging†
Cite this: J. Mater. Chem. C, 2014, 2,
5615
a
b
c
d
c
d
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P. Alam, P. Das, C. Climent, M. Karanam, D. Casanova, A. R. Choudhury,
c
b
a
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*
*
P. Alemany, N. R. Jana and I. R. Laskar
A simple synthetic protocol involving two steps was developed for the syntheses of a series of
monocyclometalated iridium(III) complexes. Initially, an intermediate, [IrHCl[(o-C6H3X)P(Ar)xꢀ(PArxRy)2] [A
(i, j, k, l)], six-coordinated iridium(III) complex involving a 4-membered chelate was isolated. Then, it was
transformed into a monocyclometalated iridium(III) complex, [(C^N)Ir(PArxꢀ1Ry)2(Cl(H)] (1–12), by the
replacement of the 4-membered chelates with 5-membered cyclometalates. The intermediates and the
complexes were structurally characterized by FTIR, 1H, 13C and 31P NMR spectroscopies. Octahedral
coordination for Ir(III) in 2, 8 and 9 was established by single crystal X-ray diffraction. Photo-physical
experiments and quantum chemical calculations revealed a mixed LC/MLCT/LLCT nature for the lowest
excited states of all these complexes that emit bright light in the solid state. Fine tuning of the emission
wavelength throughout the visible range was achieved by suitable combinations of chromophoric
cyclometalates and non-chromophoric aryl phosphine ligands. More interestingly, all the studied
complexes were found to be aggregation-induced emission (AIE) active. One of these AIE active
materials (6) was encapsulated inside polymeric micelles that inhibit the macroscopic precipitation of the
aggregated complex, <200 nm water-soluble particle exhibiting a strong emission. These colloidal
luminescent particles were used as a potential non-toxic bio-imaging probe.
Received 8th March 2014
Accepted 19th May 2014
DOI: 10.1039/c4tc00466c
www.rsc.org/MaterialsC
bioimaging applications.1 The study of the fundamental pho-
tophysical properties of the emitting materials, such as exciton
Introduction
lifetimes, luminescence yield, etc., is invariably measured in
dilute solution to evade the formation of aggregated species,
which results in ‘aggregation-caused quenching (ACQ)’ effect.2
The use of dilute solutions of these emitting species raises
several issues, e.g., weak emission leading to poor sensitivity
in sensory systems and a small number of dye molecules in the
solutions, which are quickly photobleached in the presence of
an intense laser beam, etc.3 The ACQ effect is, of course, an
acute problem in solid state applications, such as biosensor
strips or organic light emitting diodes (OLEDs), because the
uorophore reaches its highest level of concentration in the
solid state.2,3 Therefore, several strategies have been developed
over time to circumvent the detrimental effect of ACQ. From a
chemical point of view, branched chains, bulky cyclics, spiro
kinks, or dendritic wedges have been introduced to hinder the
close approach of at conjugated emitting molecules in
concentrated solution.4 As an alternative, in a more physical
approach, the ACQ effect can be eliminated either through
the passivation of luminogens via surfactant encapsulation
or by including them as dopants in the matrices of non-
conjugated transparent polymers.5 However, for both the
During the last decade, there has been a tremendous interest
among the scientic community in designing and synthe-
sizing new luminescent materials for optoelectronic and
aDepartment of Chemistry, Birla Institute of Technology and Science, Pilani Campus,
Pilani, Rajasthan, India. E-mail: ir_laskar@bits-pilani.ac.in
bCentre for Advanced Materials, Indian Association for the Cultivation of Science,
Kolkata-32, India. E-mail: camnrj@iacs.res.in
c
´
´
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Departament de Quımica Fısica and Institut de Quımica Teorica i Computacional
´
`
(IQTCUB), Universitat de Barcelona, Martı i Franques 1-11, 08028 Barcelona, Spain.
E-mail: p.alemany@ub.edu
dDepartment of Chemical Sciences, Indian Institute of Science Education and Research
(IISER), Mohali, Sector 81, S. A. S. Nagar, Manauli PO, Mohali, Punjab, 140306, India.
E-mail: angshurc@iisermohali.ac.in
† Electronic supplementary information (ESI) available: Time period for the
syntheses of the complexes and their yield, crystal data and renement;
selected bond length and bond angles; IR, 1H, 13C and 31P NMR spectra of the
intermediates and complexes; PL emission spectra showing solvatochromic
effect; DLS histogram for PEG–PLA; life-time data; bright eld and
luminescence image; comparison between X-ray and optimized geometries;
computed vertical excitation energies to low-lying states, and computed
emission wavelengths in solution. CCDC 940486–940487. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4tc00466c
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physical and chemical strategies, these attempts have met benzo[h]quinoline, tris(4-(triuoromethyl)phenyl)phosphine,
with limited success.6
methyldiphenylphosphine, dimethylphenylphosphine and 2-
Instead of blocking aggregation, a natural and inherent ethoxyethanol were purchased from Sigma-Aldrich Chemical
tendency of materials, probably the best solution is to utilize Company Ltd 2-(naphthalen-2-yl) pyridine and 2-(naphthalen-5-
this process itself to enhance light emission. A crucial step in yl) pyridine were synthesized according to previously reported
this direction was achieved in 2001 by Tang et al., who obtained methods.10 All the solvents were procured from Merck.
a series of silole molecules that were non-luminescent in solu- PEG(1000)-b-PLA(5000) diblock polymer (MW 6000) was
tion while showing bright emission in the solid state. This purchased from Polysciences, Inc.
unexpected behaviour, which opened a door for the develop-
ment of numerous new applications of luminescent materials,
has been termed ‘aggregation-induced emission’ (AIE).7 The
cause of the AIE effect in these compounds was investigated and
identied as a restricted intramolecular rotation (RIR) in the
aggregates that blocks non-radiative decay, thus favouring the
radiative channels that lead to enhanced emission. Thereaer,
several new AIE-active organic molecules, uorescent in nature,
have been designed and synthesized.4a,6,8 AIE has also been
introduced for cyclometalated complexes of iridium(III), a well-
known and efficient class of triplet emitting materials, to obtain
better quantum efficiency.9 In most of these cases, the AIE
property arises due to large-amplitude conformational changes
of some parts of the molecule that are hindered when other
molecules come closer in the solid or aggregated state. For the
development of new applications, it would be extremely inter-
esting to design a particular AIE-active system where ne tuning
of the emission wavelength could become possible. With this
idea in mind, we have chosen a six-coordinated iridium(III)
system where two different functionalities, chromophoric
cyclometalated ligands and more or less freely rotating non-
planar triaryl phosphines, were connected to a single iridium(III)
centre. The rotating units are expected to show restricted
intramolecular rotation (RIR) in the solid state, triggering AIE
activity in these compounds. By systematic variation of the
cyclometalated ligand, tuning of the emission wavelength
throughout the visible range becomes possible. Here, we report
the synthesis of a series of complexes (1–12) that emit light
throughout the visible range. More interestingly, all these
complexes were found to be AIE-active.
Characterization
1H NMR, 13C NMR and 31P NMR spectra were recorded on a 400
MHz Bruker NMR spectroscope. FTIR Simadzu (IR prestige-21)
and Perkin Elmer Spectrum 100 FTIR were used to record
infrared spectra. UV-vis absorption spectra were recorded on a
Simadzu Spectrophotometer (model UV-1800 and 2550). Steady
state photoluminescence (PL) spectra were recorded on a
Horiba Jobin Yvon spectrouorometer (FluoroMax-4). The solid
state quantum yield of the thin lm sample was measured using
a calibrated integrating sphere in a Gemini spectrophotometer
(Gemini 180). High-resolution MS (HRMS) were carried out
using a (TOF MS ES+ 1.38 eV) VG analytical (70-S) spectrometer
and Q-Tof micro mass spectrometer. The size and shape of the
nanoparticles were measured by scanning electron microscopy
(SEM) using a JEOL JSM-6700F FESEM instrument. The hydro-
dynamic diameter and surface charge of the nanoparticles were
measured using a Nano ZS (Malvern Instruments Ltd) instru-
ment. Time-correlated single photon counting (TCSPC) spectra
of the iridium complex in THF were obtained by exciting the
sample with a picosecond diode laser (IBH Nanoled) using a
Horiba Jobin Yvon IBH Fluorocube instrument. Luminescence
images of HeLa cells and photostability of the aggregated
iridium complex in water and encapsulated in PEG–PLA nano-
particles were acquired by drop casting the sample solution on a
glass slide. Images were captured using an Olympus IX 81
microscope equipped with a digital camera.
General syntheses of complexes11
These metal complexes are soluble in organic solvents but To a stirred solution of IrCl3$3H2O (0.5025 mmol) in 2-ethoxy-
insoluble in water, and if water is added to their solution, they ethanol (6 mL), substituted phosphines [triphenyl phosphine,
start aggregating along with the appearance of emission.1o This tris(4-(tri uoro methyl) phenyl) phosphine, methyl diphenyl
poor water solubility and macroscopic aggregate formation in phosphine and dimethyl (phenyl) phosphine] (1.507 mmol)
the presence of water limits their practical application. For were added, and the reaction mixture was reuxed at 130 ꢁC
example, water-soluble materials are necessary for biological for 4–7 h. Then, 2-phenyl pyridine derivatives [2,4-diuor-
labeling applications. In addition, in vitro/in vivo imaging ophenylpyridine, 2-phenylpyridine, benzo [h]quinoline,
applications require good colloidal stability of the material dibenzo[f,h]quinoline, 2-(naphthalen-2-yl) pyridine, and 2-
under physiological conditions, along with size preferably (naphthalen-5-yl)pyridine] (1.252 mmol) were added to the
around <200 nm.1m,q Here, a simple technique has been reaction mixture, which was further reuxed for 3–12 h. The
employed for transferring these insoluble materials into water. reaction mass was then brought to room temperature. The
We also show their use as a bio-imaging probe.
resulting solid mass was triturated and washed several times
with hexane followed by ethanol to obtain a solid (31–70%) of 1–
12, which was puried through recrystallization from a mixture
of DCM and hexane (1 : 1). X-ray quality single crystals for
complexes 2, 8 and 9 were collected from the solution.
Experimental section
Materials
Iridium(III) chloride hydrate, 2-phenyl pyridine, 2-bromo pyri-
1HNMR (400 MHz, CDCl3) d 7.57 (dd, J ¼ 14.0, 7.5 Hz, 1H),
dine, 2,4-diuoroboronic acid, palladium (0) tetrakis triphe- 7.45–7.37 (m, 9H), 7.24–7.19 (m, 5H), 7.08 (dt, J ¼ 17.0, 8.1 Hz,
nylphosphine,
triphenylphosphine,
dibenzo[f,h]quinoline 12H), 6.99 (t, J ¼ 7.5 Hz, 12H), 6.82–6.75 (m, 4H), ꢀ19.27 (dt, J ¼
5616 | J. Mater. Chem. C, 2014, 2, 5615–5628
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Journal of Materials Chemistry C
29.2, 8.5 Hz, 1H); 13CNMR (101 MHz, CDCl3) d 207.00, 206.95, 2152 (m, nIr–H). ESI-HRMS calculated: ([M + Na]+): m/z 842.2687
135.30, 135.20, 135.10, 135.05, 135.00, 132.67, 132.40, 132.16, and ([M–HCl + 2Na]+) m/z 828.7975; found: ([M + Na]+), m/z
132.13, 132.06, 131.96, 129.56, 129.19, 128.57, 128.45, 127.16, 842.3762 and ([M–HCl + 2Na]+), m/z 828.9851 for 3 (Fig. S7†).
31
127.11, 127.06, 127.01, 126.90, 30.94. P NMR (162 MHz,
1H NMR (400 MHz, CDCl3) d 8.85 (d, J ¼ 5.4 Hz, 1H), 7.63 (d,
J ¼ 8.4 Hz, 1H), 7.56 (m, 1H), 7.35 (t, J ¼ 7.4 Hz, 1H), 7.26–7.12
(m, 1H), 7.05 (t, J ¼ 7.3 Hz, 1H), 7.00–6.84 (m, 5H), 6.74 (m, 4H),
CDCl3) d 2.41, 7.54, 9.54 for A (i). IR (KBr, cmꢀ1): 2187 (m, nIr–H
for A (i) (Fig. S1†).
)
1H NMR (400 MHz, CDCl3) d 8.11 (dd, J ¼ 13.4, 5.4 Hz, 1H), 6.38–6.24 (m, 1H), 1.74 (t, J ¼ 3.7 Hz, 6H), 1.62 (t, J ¼ 3.7 Hz, 6H),
7.92–7.71 (m, 12H), 7.71–7.37 (m, 19H), 7.26 (d, J ¼ 3.1 Hz, 1H), ꢀ17.87 (t, J ¼ 18.1 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 147.63,
ꢀ21.58 (m, 1H). 13C NMR (101 MHz, CDCl3) d 136.42, 134.74, 135.73, 133.69, 129.47, 128.73, 128.68, 128.64, 128.49, 128.27,
132.55, 132.44, 125.88, 125.55, 125.03, 124.73. 31P NMR (162 127.39, 127.35, 127.30, 121.97, 120.72, 14.79, 14.59, 14.39,
MHz, CDCl3) d 26.39, 4.86, ꢀ24.04 for A(j). IR (KBr, cmꢀ1): 2251 12.68, 12.49, 12.30. 31P NMR (162 MHz, CDCl3) d ꢀ25.62 for 4. IR
(m, nIr–H) for A (j) (Fig. S2†).
(KBr, cmꢀ1): 2106 (m, nIr–H). ESI-HRMS calculated: ([M ꢀ H]+),
1H NMR (400 MHz, CDCl3) d 8.13–8.02 (m, 1H), 7.61 (dt, J ¼ m/z 694.0983 and ([M + K]+): m/z 734.0698; found: ([M ꢀ H]+), m/
8.2, 4.9 Hz, 8H), 7.50 (d, J ¼ 6.7 Hz, 2H), 7.43–7.26 (m, 10H), z 694.0977 ([M + K]+), m/z 734.0695 for 4 (Fig. S8†).
7.26–7.09 (m, 13H), 7.07–6.90 (m, 9H), 6.74 (td, J ¼ 7.9, 2.1 Hz,
1H NMR (400 MHz, CDCl3) d 8.73 (d, J ¼ 5.5 Hz, 1H), 7.49 (m,
1H), 2.34 (t, J ¼ 4.1 Hz, 3H), 2.15 (t, J ¼ 4.1 Hz, 6H), ꢀ19.40 (dt, 26H), 7.24 (d, J ¼ 7.0 Hz, 1H), 6.74–6.59 (m, 2H), 6.27 (d, J ¼ 7.7
J ¼ 17.1, 9.9 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 135.04, Hz, 1H), 6.09–5.98 (m, 1H), ꢀ16.72 (t, J ¼ 16.8 Hz, 1H). 13C NMR
134.68, 134.48, 133.33, 133.29, 133.25, 132.23, 132.15, 131.91, (101 MHz, CDCl3) d 165.21, 149.06, 142.87, 141.95, 136.51, 134.23,
131.48, 131.22, 130.96, 130.22, 129.53, 129.43, 129.41, 128.93, 134.05, 134.00, 133.94, 131.96, 131.63, 124.91, 124.64, 124.60,
128.53, 127.99, 127.86, 127.76, 127.62, 127.48, 127.43, 127.38, 122.20, 121.21, 118.23. 31P NMR (162 MHz, CDCl3) d 10.17 for 5.
15.86, 15.65, 15.44, 11.75, 11.65, 11.55. 31P NMR (162 MHz, IR (KBr, cmꢀ1): 2144 (m, nIr–H)), ESI-HRMS calculated: ([M ꢀ Cl]+):
CDCl3) d ꢀ12.28, ꢀ40.59, ꢀ54.45 for A (k). IR (KBr, cmꢀ1): 2152 m/z 1280.1430, ([M ꢀ H]+): m/z 1314.1040, ([M–H + Li]+), m/z
(m, nIr–H) for A (k) (Fig. S3†).
1321.1200; found: ([M ꢀ Cl]+): m/z 1280.1439, ([M ꢀ H]+): m/z
1H NMR (400 MHz, CDCl3) d 7.55 (dt, J ¼ 7.9, 4.8 Hz, 5H), 1314.1030, ([M–H + Li]+), m/z 1321.1687 for 5 (Fig. S9†).
7.35 (t, J ¼ 7.3 Hz, 3H), 7.31–7.12 (m, 8H), 6.95 (td, J ¼ 7.9, 2.5
1HNMR (400 MHz, CDCl3) 8.9 (d, J ¼ 5.32, 1H), 7.65 (d, J ¼
Hz, 2H), 1.92 (t, J ¼ 4.3 Hz, 15H). 13C NMR (101 MHz, CDCl3) d 8.32, 1H), 7.5 (t, J ¼ 8, 1H), 7.4 (d, J ¼ 7.96, 1H), 7.2–7.1 (m, 30H),
136.23, 135.99, 130.58, 130.54, 130.50, 129.47, 129.39, 128.97, 6.8 (t, J ¼ 7.6, 1H), 6.5 (t, J ¼ 7.16, 1H), 6.2 (d, J ¼ 7.64, 1H), 5.8 (t,
128.54, 128.49, 128.45, 128.31, 128.21, 13.62, 13.20, 11.08, J ¼ 7.2, 1H). 13C NMR (101 MHz, CDCl3) d 166.31, 149.61,
10.88, 10.68. 31P NMR (162 MHz, CDCl3) d ꢀ40.60, ꢀ49.93 for A 143.46, 135.28, 134.10, 134.05, 133.99, 132.03, 131.77, 131.51,
(l). IR (KBr, cmꢀ1): 2139 (m, nIr–H) for A (l) (Fig. S4†).
130.04, 128.88, 127.26, 127.21, 127.16, 122.36, 120.43, 119.21,
1H NMR (400 MHz, CDCl3) d 8.76 (d, J ¼ 5.4 Hz, 1H), 7.90 (d, 117.00. 31PNMR (162 MHz, CDCl3) d 9.25 for 6. IR (KBr, cmꢀ1):
J ¼ 9.6 Hz, 1H), 7.75–7.39 (m, 25H), 6.74 (t, J ¼ 6.5 Hz, 1H), 6.24– 2098 (m, nIr–H). ESI-HRMS calculated: ([M ꢀ H]+): m/z 906.1797,
6.02 (m, 1H), 5.75 (d, J ¼ 8.8 Hz, 1H), ꢀ16.73 (t, J ¼ 16.6 Hz, 1H). ([M ꢀ Cl]+): m/z 872.2187; found: ([M ꢀ H]+): m/z 906.1766, ([M ꢀ
13C NMR (101 MHz, CDCl3) d 149.09, 137.28, 134.03, 133.97, Cl]+): m/z 872.2165 for 6 (Fig. S10†).
133.91, 133.80, 133.53, 132.69, 132.36, 132.04, 131.71, 127.55,
1H NMR (400 MHz, CDCl3) d 8.62 (d, J ¼ 5.4 Hz, 1H), 7.41 (m,
126.36, 124.80, 124.76, 122.48, 122.10, 121.41. 31P NMR (162 5H), 7.35–7.00 (m, 19H), 6.78 (t, J ¼ 7.4 Hz, 1H), 6.64–6.49 (m,
MHz, CDCl3) d 8.85 for 1. IR (KBr, cmꢀ1): 2152 (m, nIr–H). ESI- 2H), 1.65–1.59 (m, 6H), ꢀ16.74 (t, J ¼ 15.9 Hz, 1H). 13C NMR
HRMS calculated: ([M ꢀ Cl]+), m/z 1316.1242 and ([M–H + Li]+): (101 MHz, CDCl3) d 164.64, 148.24, 142.94, 135.07, 132.82,
m/z 1357.1012; found: ([M ꢀ Cl]+), m/z 1316.1243 ([M–H + Li]+), 132.76, 132.71, 132.34, 132.28, 132.23, 129.55, 128.97, 128.90,
m/z 1357.1494 for 1 (Fig. S5†).
127.43, 127.40, 123.36, 120.65, 119.75, 117.37, 14.76. 31P NMR
1H NMR (400 MHz, CDCl3) 8.9 (d, 1H), 7.8 (d, J ¼ 10.8, 1H), (162 MHz, CDCl3) d ꢀ4.57 for 7. IR (KBr, cmꢀ1): 2113 (m, nIr–H).
7.5–7.1 (m, 31H), 6.6 (t, 1H), 6.0 (t, 1H), 5.7 (d, J ¼ 13.6, 1H); 13
C
ESI-HRMS calculated: ([M]+): m/z 783.1562, ([M ꢀ Cl]+): m/z
NMR (101 MHz, CDCl3) d 149.90, 136.06, 134.02, 133.97, 133.92, 748.1874, found: ([M]+): m/z 782.9617, ([M ꢀ Cl]+): m/z 748.4669
131.53, 131.27, 131.01, 129.26, 127.41, 127.36, 127.31, 126.55, for 7 (Fig. S11†).
121.63, 121.46, 120.66. 31P NMR (162 MHz, CDCl3) d 7.78 IR
1H NMR (400 MHz, CDCl3) d 8.76 (d, J ¼ 5.5 Hz, 1H), 7.48 (d, J ¼
(KBr, cmꢀ1): 2152 (m, nIr–H). ESI-HRMS calculated: ([M ꢀ H]+), 7.5 Hz, 1H), 7.32–7.16 (m, 3H), 7.00 (t, J ¼ 7.3 Hz, 2H), 6.90 (t, J ¼
m/z 942.1609 and ([M ꢀ Cl]+): m/z 908.1999; found: ([M ꢀ H]+), 7.6 Hz, 5H), 6.81 (t, J ¼ 7.3 Hz, 1H), 6.76–6.70 (m, 4H), 6.63 (t, J ¼
m/z 942.1600 ([M ꢀ Cl]+), m/z 908.1995 for 2 (Fig. S6†).
6.1 Hz, 1H), 1.79 (t, J ¼ 3.6 Hz, 6H), 1.53 (t, J ¼ 3.6 Hz, 6H), ꢀ17.88
1H NMR (400 MHz, CDCl3) d 8.82 (d, J ¼ 5.3 Hz, 1H), 7.65 (d, (t, J ¼ 18.4 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 164.57, 147.59,
J ¼ 8.4 Hz, 1H), 7.61–7.48 (m, 4H), 7.41 (t, J ¼ 7.6 Hz, 1H), 7.35– 142.78, 134.99, 134.34, 129.70, 128.92, 128.88, 128.83, 127.90,
7.13 (m, 10H), 7.09–7.01 (m, 6H), 6.84 (d, J ¼ 9.1 Hz, 1H), 6.73 (t, 127.21, 127.17, 127.13, 123.25, 120.55, 119.74, 117.37, 14.77, 14.58,
J ¼ 6.4 Hz, 1H), 6.22–6.05 (m, 1H), 1.71 (t, J ¼ 3.5 Hz, 6H), 14.38, 13.34, 13.15, 12.96. 31P NMR (162 MHz, CDCl3) d ꢀ25.45 for
ꢀ16.71 (t, J ¼ 15.5 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 148.31, 8. IR (KBr, cmꢀ1): 2098 (m, nIr–H). ESI-HRMS calculated: ([M ꢀ
135.91, 132.72, 132.66, 132.60, 132.49, 132.39, 132.22, 132.11, H]+): m/z 658.1171, ([M ꢀ Cl]+): m/z 624.1561, found: ([M ꢀ H]+): m/
132.06, 132.00, 129.49, 129.05, 127.77, 127.72, 127.67, 127.47, z 658.0774, ([M ꢀ Cl]+): m/z 624.1204 for 8 (Fig. S12†).
127.42, 127.37, 122.08, 121.87, 120.84, 95.69, 14.13, 13.94,
1H NMR (400 MHz, CDCl3) d 9.13 (d, J ¼ 5.1 Hz, 1H), 7.77 (d,
13.76. 31P NMR (162 MHz, CDCl3) d ꢀ5.05 for 3. IR (KBr, cmꢀ1): J ¼ 7.6 Hz, 1H), 7.54 (d, J ¼ 8.7 Hz, 2H), 7.36–7.20 (m, 15H),
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7.18–7.07 (m, 8H), 7.03–6.92 (m, 9H), 6.68 (d, J ¼ 7.3 Hz, 1H), tetrahydrofuran (THF) and mixed with 500 mL of THF solution
6.46 (t, J ¼ 7.6 Hz, 1H), ꢀ16.80 (t, J ¼ 16.3 Hz, 1H). 13C NMR (101 of the iridium complex (1 mg mLꢀ1). The solution was then
MHz, CDCl3) d 154.91, 148.39, 140.42, 134.13, 133.86, 133.80, stirred for 1 h and the resulting mixture was added drop-wise to
133.75, 131.69, 131.43, 131.17, 129.46, 128.84, 128.51, 127.13, 15 mL water with vigorous stirring for 3 h. The resulting solu-
127.08, 127.04, 125.75, 122.67, 119.86, 117.58. 31P NMR (162 tion was dialyzed to remove the excess THF using a dialysis
MHz, CDCl3) d 10.58 for 9. IR (KBr, cmꢀ1): 2129 (m, nIr–H). ESI- membrane (MWCO ꢃ12 000–14 000 Da). PEG–PLA nano-
HRMS calculated: ([M ꢀ H]+): m/z 930.1797, ([M ꢀ Cl]+): m/z particles were also prepared following the same method without
896.2187; found: ([M ꢀ H]+): m/z 930.1777, ([M ꢀCl]+): m/z adding any iridium complex during their synthesis.
896.2217 for 9 (Fig. S13†).
1H NMR (400 MHz, CDCl3) d 9.20 (d, J ¼ 5.1 Hz, 1H), 8.58 (d,
X-ray single crystal diffraction study
J ¼ 7.6 Hz, 1H), 8.47 (t, J ¼ 7.0 Hz, 2H), 7.78 (d, J ¼ 8.0 Hz, 1H),
Single-crystal X-ray diffraction data were collected on a Bruker
7.67 (m, 2H), 7.36–7.20 (m, 13H), 7.12 (t, J ¼ 7.3 Hz, 6H), 7.07–
AXS Kappa Apex II diffractometer equipped with an Oxford
6.94 (m, 12H), 6.65 (d, J ¼ 7.4 Hz, 1H), 6.44 (t, J ¼ 7.7 Hz, 1H),
Cryosystem 700Plus liquid nitrogen-based cooling device. The
ꢀ16.49 (t, J ¼ 16.4 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 155.93,
data sets were recorded at 100 K using f and u scans to obtain
148.89, 141.45, 133.94, 133.88, 133.83, 131.71, 131.45, 131.28,
complete data up to 60 degrees in 2q. Data reduction and
131.19, 130.30, 129.97, 129.39, 128.86, 128.04, 127.71, 127.17,
standard processing were done using the APEX II (ref. 13) suite
127.13, 127.08, 126.46, 124.15, 123.58, 122.99, 120.17, 112.72 for
available from Bruker AXS. Crystal structures were solved using
10. 31P NMR (162 MHz, CDCl3) d 7.93. IR (KBr, cmꢀ1): 2113 (m,
direct methods (SHELXS97)14 available in the Olex2 (ref. 15)
nIr–H). ESI-HRMS calculated: ([M ꢀ H]+): m/z 956.1954, ([M ꢀ
suite, and the structures were rened by the full matrix least
Cl]+): m/z 922.2342; found: ([M ꢀ H]+): m/z 956.1945, ([M ꢀ Cl]+):
squares renement process using SHELXL97.14 All the hydrogen
m/z 922.2349 for 10 (Fig. S14†).
atoms (except the hydride ion in 9) were geometrically xed at
1H NMR (400 MHz, CDCl3) d 9.17 (d, J ¼ 5.4 Hz, 1H), 8.21 (d,
their calculated positions and rened as a riding model. The
J ¼ 8.6 Hz, 1H), 7.98 (d, J ¼ 8.1 Hz, 1H), 7.74–7.63 (m, 1H), 7.49
hydride ion in 9 was located from the difference Fourier map,
(m, 2H), 7.44–7.30 (m, 11H), 7.28–6.96 (m, 19H), 6.92 (m, 1H),
and was rened isotropically. Geometric calculations were
6.62 (m, 1H), 6.50 (d, J ¼ 8.4 Hz, 1H), 6.24 (d, J ¼ 8.4 Hz, 1H),
ꢀ16.30 (t, J ¼ 16.7 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 165.90,
150.46, 142.03, 135.02, 134.55, 134.55, 134.16, 134.11, 134.11,
134.05, 132.08, 131.77, 131.77, 131.46, 130.89, 130.19, 129.02,
carried out using PARST97.16
Synthesis of aggregated iridium complexes in water
128.97, 128.68, 127.23, 127.23, 127.18, 125.19, 121.86, 121.83, Aggregated iridium complexes in water were synthesized by
121.48, 119.49. 31P NMR (162 MHz, CDCl3) d 6.56 for 11. IR (KBr, dropwise addition of THF solution of iridium complex
cmꢀ1): 2167 (m, nIr–H). ESI-HRMS calculated: ([M ꢀ H]+): m/z (concentration 1 mg mLꢀ1) to water with continuous sonication
980.1954, ([M ꢀ Cl]+): m/z 946.2343; found: ([M ꢀ H]+): m/z for 10 min. Excess THF was removed by dialysis using a dialysis
980.1937, ([M ꢀ Cl]+): m/z 946.2334 for 11 (Fig. S15†).
membrane (MWCO ꢃ 2000 Da).
1H NMR (400 MHz, CDCl3) d 9.08 (d, J ¼ 5.5 Hz, 1H), 7.73 (s,
1H), 7.60 (t, J ¼ 16.4, 7.9 Hz, 2H), 7.45–7.39 (m, 2H), 7.38–7.30
(m, 11H), 7.26–7.18 (m, 1H), 7.17–7.05 (m, 8H), 7.05–6.86 (m,
11H), 6.72 (t, J ¼ 10.3, 4.2 Hz, 2H), 6.64 (s, 1H), ꢀ16.93 (t, J ¼
16.7 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 164.68, 160.17,
149.93, 135.31, 134.07, 134.02, 133.96, 131.70, 131.43, 131.18,
128.93, 127.72, 127.38, 127.15, 127.10, 127.05, 125.98, 124.74,
122.41, 121.33, 121.03, 117.88. 31P NMR (162 MHz, CDCl3) d
10.17 for 12. IR (KBr, cmꢀ1): 2129 (m, nIr–H), ESI-HRMS calcu-
lated: ([M ꢀ Cl]+): m/z 922.2342, found: ([M ꢀ H]+): m/z ([M ꢀ
Cl]+): m/z 922.2385 for 12 (Fig. S16†).
Computational details
Geometric optimizations for complexes 2, 6 and 8–12 in their
ground and lowest triplet states were carried out using quantum
chemical calculations based on density functional theory (DFT)
with a reparametrization of the GGA B97 functional with an
empirical dispersion correction, i.e., the B97-D functional.17
A
basis set of double-z quality (LANL2DZ) and the effective core
potential of Hay and Wadt18c were used for iridium. For all the
other atoms, standard all-electron 6-31+G(d)18d basis sets were
used. Electronic transitions to low-lying singlet and triplet
states were calculated using the B3LYP (ref. 19) hybrid func-
tional and the same basis sets. Solvation effects (dichloro-
Fabrication of thin-lm on substrate for PL measurement
A 10ꢀ3 M solution of each of the complexes (in THF) was methane, 3 ¼ 8.93) were included by the Integral Equation
prepared. 2–3 drops of the solution were then placed on a thin Formalism–Polarizable Continuum Model (IEF–PCM).20 The
glass substrate (2 ꢂ 2 cm2) and the solvent was allowed to Gaussian 09 package21 was used for all these calculations.
evaporate slowly.
Cell imaging
Synthesis of iridium complex encapsulated PEG–PLA
nanoparticles
HeLa cells were cultured in cell culture asks and then sub-
cultured in cell culture plates with 0.5 mL DMEM cultured
PEG–PLA nanoparticles containing the iridium complex were media having 10% fetal bovine serum (FBS) and 1% penicillin/
prepared using the oil-in-water-based emulsion–evaporation streptomycin. Aer maintaining overnight, the cells were
method.12 Typically, 15 mg PEG–PLA were dissolved in 2 mL washed with phosphate buffer solution (PBS), and then 500 mL
5618 | J. Mater. Chem. C, 2014, 2, 5615–5628
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Journal of Materials Chemistry C
of fresh media was added. Next, 100 mL nanoparticle solution
(0.7 mg mLꢀ1) was added and incubated for 1 to 8 h. Finally,
unbound nanoparticles were removed by washing with PBS
buffer solution. The washed cells were then imaged under the
uorescence microscope.
MTT assay
HeLa cells were seeded into 24-well plates in 500 mL DMEM
media. Aer 24 h, cells were incubated with various amounts of
nanoparticles. Aer 24 h incubation, media was removed and
cells were washed with PBS buffer solution. Then, 500 mL of
fresh media and 50 mL of MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyltetrazolium bromide] solution (5 mg mLꢀ1 in PBS
buffer) were added to every well and incubated for an additional
4 h. Next, the supernatant was removed carefully, leaving the
violet formazan in the plate. The formazan was solubilized in
500 mL 4% SDS solution of 1 : 1 water–DMF mixture, and then
absorbance was measured at 570 nm using a microplate reader.
The relative cell viability was calculated assuming 100% cell
viability for a sample without nanoparticles.
Results and discussion
All the monocyclometalated iridium(III) complexes (1–12) were
synthesized by a one-pot synthetic route11 involving two steps.
Initially, an intermediate, [IrHCl[(o-C6H3X)P(Ar)xꢀ1Ry](PArxRy)2]
[A (i, j, k, l)] (Scheme 1) was isolated, in which a six-fold coor-
dination environment on iridium(III) involving a 4-membered
chelate was formed. It was observed that the time required to
obtain the intermediates when using triuoro methyl phos-
phine (j) or dimethyl phenyl phosphine (l) were relatively longer
(ꢃ7 h) than that for the other two intermediates with triphenyl
phosphine or diphenyl methyl phosphine (ꢃ4 h; Table S1†).
The presence of Ir–H in the intermediates A is supported by
the observation of a stretching frequency in the region ꢃ2187–
2252 cmꢀ1 recorded by FTIR and by the appearance of high-eld
lines in the 1H NMR spectra. Although the hydroxylic solvent,
2-ethoxyethanol,22 appears to be the most plausible source of
hydrogen linked to iridium(III) at rst sight, the same inter-
mediate (A) was formed when the reaction was carried out in a
non-hydroxylic solvent (1,4-dioxane). The measured ratios of
the integrated areas of the high-eld lines (for hydride
linked to iridium) vs. aromatic proton resonances were 1 to 50
ꢄ 5 (i–l).
Scheme 1 The synthetic route for the monocyclometalated iridium(III)
complexes B (1–12) via intermediates A (i, j, k, l).
(Table S1†).24 The observed chemical shi (d) in the range of
(ꢀ15)–(ꢀ19) and their low coupling constants (JP-H, 16.0–18.5)
(Table 1), support the existence of a hydride bonded to iridium(III)
and cis conguration with respect to phosphorous coordinated
species (PAr3) in the complex molecule, respectively. The
structure of the complexes 2, 8, and 9 was established by X-ray
single crystal analyses at 100 K.
1
Table 1 The IR, H NMR (for hydride only) and 31P NMR support the
presence of a Ir–H bond and phosphorous coordination, respectively
for 1–12
Complex
IR n(Ir–H)
1HNMR (hydride) ppm
31P NMR ppm
(Calculated for a monohydride, 1 to 44), showing that one
hydrogen atom was abstracted from the ligand.23 The only
reasonable source for the hydrogen atom that migrates to the
iridium(III) centre is the ortho hydrogen adjacent to phospho-
rous, resulting in a 6-fold coordination of iridium(III) with a
strained 4-membered chelate ring.23 In the second step, the 4-
membered chelate ring in the intermediates (i–l) is replaced by
the stable 5-membered cyclometalates to form six-coordinated
monocyclometalated iridium(III) complexes (B, 1–12) (Scheme
1). The yield of the product has been improved by 10–15% on
carrying out the process in the presence of 3 to 4 equivalents
of sodium carbonate, which acts as an acid scavenger
1
2
3
4
5
6
7
8
2152
2152
2152
2106
2144
2090
2113
2098
2129
2167
2113
2129
ꢀ16.73 (J ¼ 16.8 Hz)
ꢀ16.76 (J ¼ 16.8 Hz)
ꢀ16.71 (J ¼ 16.8 Hz)
ꢀ16.87 (J ¼ 16.8 Hz)
ꢀ16.72 (J ¼ 16.8 Hz)
ꢀ16.71 (J ¼ 16.8 Hz)
ꢀ16.74 (J ¼ 16.8 Hz)
ꢀ16.88 (J ¼ 16.8 Hz)
ꢀ16.80 (J ¼ 16.8 Hz)
ꢀ16.30 (J ¼ 16.8 Hz)
ꢀ16.49 (J ¼ 16.8 Hz)
ꢀ16.93 (J ¼ 16.8 Hz)
8.85
7.78
ꢀ5.05
ꢀ25.62
10.17
9.25
ꢀ4.57
ꢀ25.45
10.58
6.56
9
10
11
12
7.79
10.17
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Because of the poor X-ray scattering factor of the hydride ion
in 2, one coordination site around Ir appears to be empty,
although this position is certainly occupied by the hydride ion
as, supported by the IR and 1H NMR spectra. The ORTEP
diagram (Fig. 1) shows an octahedral geometry for the coordi-
nation environment around the iridium(III) centre for 2, 8 and 9.
Crystallographic data and selected geometrical parameters are
given in Tables S2–S3† for all the three structures. In addition,
the most signicant geometrical parameters for the optimized
ground state and lowest triplet structures in DCM solution are
given in Tables S4 and S5,† respectively. All the optimized
structures are very close to X-ray geometries. Remarkably, the
main difference between crystallographic coordinates and
geometries obtained from quantum chemical calculations are
found in the disposition of the phenyl rings in the phosphine
ligands, which can be explained considering the expected Fig. 2 Solution UV-vis absorbance spectra (10ꢀ5 M, DCM) of the
complexes 2–12 [short range spectra are shown in the inset (360–
460 nm)].
greater molecular exibility of these ligands in solution
(Fig. S17†).
The solution UV-vis absorption spectra (DCM, 10ꢀ5 M) show
intense bands below 350 nm for the complexes 1–12 [Fig. 2 and
S17†], which can be assigned to ligand-centered (LC), 1p–p* on the shape, band position and their intensities [Table 2],24
transitions.25 These absorption bands are followed by weaker these bands can be assigned to MLCT transitions. Electronic
bands in the range of ꢃ350–450 nm. In this range, two well- structure calculations of complexes 2, 6 and 8–12 indicate that
resolved broad peaks are observed for 1–9 [inset of (Fig. 2 and the four highest occupied molecular orbitals (HOMOs) corre-
S18†)], whereas a single broad peak is obtained for 10–12. Based spond to different antibonding combinations between the t2g
(dxy, dxz and dyz) orbitals of iridium ‘p’ orbitals of chlorine and p
contributions from the non-pyridine rings of the cyclometalated
ligand, whereas the two lowest unoccupied orbitals (LUMO and
LUMO + 1) correspond to p* orbitals of the cyclometalated
ligand (Fig. 3). There is almost no participation of the phos-
phine ligands in these frontier orbitals. Our TDDFT computa-
tions suggest that the highest wavelength, weak bands
correspond to spin-forbidden transitions to low-lying triplet
states, resulting from HOMO and HOMO-3 to LUMO electronic
excitations (Table 3). In all the studied complexes, the lowest
energy spin-allowed absorption band corresponds to the
promotion of an electron from the HOMO to the LUMO and
presents an important MLCT and LLCT, i.e., p(Cl) / p*,
character (Fig. 3). Complexes 6 and 8 only differ in the phos-
phine ligand and, therefore, their transition energies to S1 are
almost identical. On the other hand, uorine substitution in 2
destabilizes the p*-type LUMO, resulting in a ꢃ0.1 eV increase
in the excitation energy. Alternatively, the modication of the
degree of conjugation in the cyclometalating ligands in 9, 10, 11
and 12 can be used to tune the S0 / S1 transition energy when
required. Details regarding the transition energies, oscillator
strengths and orbital composition of the lowest singlet and
triplet states can be found in the ESI (Table S6†). Overall,
computed transition frequencies to the low-lying states of the
complexes are in very good agreement with experimental
absorption peaks in dichloromethane solution.
The emission spectrum of complexes 1–10 shows a struc-
tured emission (Fig. S19†). Furthermore, these complexes are
insensitive to solvatochromic effects, i.e., the emission spectra
remain practically unchanged irrespective of the polarity of the
solvent (Fig. S20†). This suggests that the lowest excited states
of 1–10 are predominantly of LC character along with a lesser
Fig. 1 ORTEP diagram for complexes, 2,10 8 and 9 showing an octa-
hedral geometry at the Ir site (in 2 and 8, the hydride coordination to
Ir(III) centre could not be detected).
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Table 2 UV-vis absorbance [extinction coefficient (Mꢀ1 cmꢀ1) in parenthesis] and maximum emission wavelengths for the complexes 1–12
PL (nm)
Complex
UV-vis absorbance (nm) (3 ꢂ 104) (Mꢀ1 cmꢀ1
)
Solid
Solution
1
2
3
4
5
6
7
8
268 (9.00), 350 (1.60), 430 (0.15)
441 (sh), 472, 506 (sh)
448 (sh), 476, 507 (sh)
443 (sh), 478, 507 (sh)
481
460 (sh), 493, 529 (sh)
473, 498
473 (sh), 501
475, 507
508, 544
445, 471
451, 476
453, 475
448, 476
463, 494
468, 499
467, 497
468, 497
512, 548
543, 585
524
255 (23.50), 338 (2.30), 369 (1.50), 430 (0.05)
255 (2.80), 339 (1.70), 367 (1.00), 419 (0.02)
262 (4.10), 337 (0.51), 369 (0.33), 419 (0.05)
275 (13.10), 371 (1.70), 430 (0.05)
269 (5.30), 343 (0.65), 381 (0.51), 445 (0.02)
260 (29.45), 347 (2.87), 385 (2.30), 430 (0.02)
259 (35.20), 349 (3.60), 384 (2.70), 430 (0.02)
276 (7.60), 407 (0.71), 451 (0.04)
9
10
11
12
280 (13.00), 319 (5.50), 336 (5.10), 424 (0.59)
254 (12.30), 267 (10.70), 331 (1.86), 400 (0.27)
275 (6.7), 336 (0.96), 423 (0.40)
513, 558
548, 610 (sh)
593
589
Table 3 Comparison between the experimental absorption maxima
lexp and the computed transition energies lcalc (in nm) and orbital
composition (%) of the lowest excited singlet and triplet states for
complexes 2, 6 and 8–12a
Complex
State
lexp
lcalc
H / L
H-n / Lb
2
T1
S1
T1
S1
T1
S1
T1
S1
T1
S1
T1
S1
T1
S1
430
369
445
381
430
384
433
360
451
372
445
368
494
400
553
418
465
393
567
419
56
97
68
97
68
97
47
94
63
94
28
95
84
84
25
19
21
13
16
17
6
8
9
407
424
400
423
10
11
12
a
Complementary for higher singlet and triplet states can be found in
b
the ESI (Table S6†). Contribution from H-n with 1 # n # 3 (see
Table S6†).
The emission color of the complexes has been tuned
throughout the visible range by changes in the cyclometalated
ligands or the combinations of cyclometalated and phosphine
ligands (Fig. 4). The relative differences in the emission wave-
lengths for the studied complexes are well-recovered by
computational results (Table S7†). It is to be noted that a sharp
change in color is observed with the variation of the cyclo-
metalated ligands, whereas a minor effect is reected for
changes in the phosphine ligands (Fig. 4). This is in line with
the low electron population on the phosphine ligands for the
frontier molecular orbitals involved in electron transition
(Fig. 3). The inclusion of a triuoro methyl substituent in tri-
phenyl phosphine (1 vs. 2; 5 vs. 6) has a negligible inuence on
the emission wavelength (Fig. 5 and S20†). Similarly, the
complexes with methyl or dimethyl substituted tri phenyl
Fig. 3 Molecular orbital energy diagram (in eV) with respect to the
HOMO energy of the frontier orbitals of 8. H and L stand for HOMO
and LUMO, respectively.
contribution from MLCT states. This behaviour is also appro-
priately captured by electronic structure calculations. The
comparative analysis between the S1 and T1 wave functions
shows a much lower weight of the HOMO-to-LUMO (MLCT) for
the latter (Table 3). In particular, the lowest triplet wave func-
tion has an important participation from HOMO-3, which is
largely located on the cyclometalated ligand (Fig. 3), and hence
increases the LC nature of the triplet.
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Fig. 6 The relative luminescence intensity of solid state vs. solution for
complexes 9 (left) and 10 (right) under UV lamp (lmax ¼ 365 nm).
(These are chosen as two representatives of all the reported
complexes).
Table 4 Solid state quantum efficiency (fsolid) and solution state
quantum efficiency (fsol) for 1–12
a
b
a
b
Complex
fsol
fsolid
Complex
fsol
fsol
1
2
3
4
5
6
0.011
0.890
0.014
0.127
0.014
0.688
5.41
41.43
10.65
10.95
5.77
7
8
9
10
11
12
0.015
0.048
0.044
0.114
0.012
0.087
9.46
7.28
9.14
26.4
7.99
Fig. 4 Tuning of emission color in solid state throughout the visible
range with variation of the cyclometalated and phosphine ligands.
54.83
11.42
a
Solution QE (fsol) for 1–10 has been measured with respect to quinine
sulfate (in 0.1 M H2SO4, QE ¼ 0.55, excitation, 470–480 nm); and QE
(fsol) for 11–12 has been measured with respect to coumarin 153 (in
degassed ethanol, QE ¼ 0.38, excitation, 400–420 nm). Solid state
phosphorescence QE (fsolid) has been recorded using integrating
b
sphere. Solid state phosphorescence QE (fsolid) has been recorded
using an integrating sphere.
sulphuric acid, quantum yield (QE) ¼ 0.55, Table 4) and 0.012
and 0.087 for complexes 11 and 12, respectively (with respect to
coumarin 153 dissolved in degassed ethanol, QE ¼ 0.38, Table
4). The least solution efficiency was observed in 1 (0.011%),
whereas the maximum efficiency was observed in 2 (0.89%). The
absolute solid state QE was measured26 for all the complexes.
The maximum QE was observed for 2 (54.83%), whereas the
minimum was observed for 1 (5.41%). The ratio of QEsolid/QEsol
(fsolid/fsol) was found to be maximum for 3 (7.34 ꢂ 102) and
minimum for 2 (0.46 ꢂ 102), showing the remarkable AIE
property of these complexes.
Several experiments were carried out to investigate the cause
of the strong solid state emission behaviour exhibited by
complexes 1–12. PL was measured for a series of solutions with
different water–THF ratios, in which the water content was
gradually varied in the range of 0–90% for 1–12. The PL was
found to increase drastically on increasing the proportion of
water in a water–THF solution (Fig. 7 and S22a†). Because water
is a poor solvent for all these complexes, the molecules aggre-
gate with increasing concentration of water. Micrometer-size
non-uniform aggregated particles were observed in the SEM
image for complex 6 in 90% water–THF solution.11 Thus, this
experiment indicates that all these complexes are aggregation
Fig. 5 Solid state photoluminescence emission spectra for complexes
2–12, showing the tuning of emission wavelengths.
phosphine (3 vs. 4; 7 vs. 8) show only minor variations in the
emission wavelength (Fig. 5 and S21†). However, there is a
noticeable difference in color when comparing complexes 1 and
2 with 3 and 4 (a similar change is observed when comparing 5
and 6 with 7 and 8, Fig. 4), where the phenyl groups from
phosphine are replaced by one or two methyl groups. In these
cases, it is evident that the phosphorous atom coordinating to
iridium(III) affects its d-orbitals when the electron-accepting
phenyl substituents are replaced by methyl groups in the tri-
phenylphosphine ligands.
The photoluminescence (PL) intensity of all these complexes
in dichloromethane was found to be very weak when compared
with that of their respective solids or aggregated forms (Fig. 5
and 6).
The quantum efficiencies in solutions of the complexes were
measured, and the observed values are in the range of 0.01–0.89
(for 1–10 measured with respect to quinine sulphate in 0.1 M
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Fig. 8 (a) Luminescent image of complex 9 radiated with UV light at
365 nm in PEG–THF mixed solvents (0, 30, 60 and 90% PEG into THF
solution) with the concentration of 1 ꢂ 10ꢀ4 mol Lꢀ1; (b) PL spectra of
complex 9 in different PEG–THF mixed solvent (0–95% PEG into THF
solution); (c) change in PL intensity of complexes 2, 6 and 9–12 with
change in the PEG fraction.
Fig. 7 (a) Luminescent image of complex 4 radiated with UV light at
365 nm in water–THF mixed solvents (0, 30, 60 and 90% water into
THF solution) with the concentration of 1 ꢂ 10ꢀ4 mol Lꢀ1; (b) PL
spectra of complex 4 in different water–THF mixed solvent (0, 30, 60
and 90% into THF solution); (c) change in PL intensity of complexes 2,
6 and 9–12 with changing the water fraction.
Table 5 Luminescence decay parameters for the complex 6 solution
at 298 Ka
induced emission (AIE) active. In another experiment, the
gradual increase of the emission intensity was observed on
increasing the concentration of polyethylene glycol, a viscous
solvent, in a solution of each of the complexes 1–12 dissolved in
THF (Fig. 8 and S22b†). This observation suggests that the
hindrance of the rotationally active part on increasing the
viscosity of the solution may be responsible for the increase of
luminescence intensity in aggregated samples. Finally, the AIE
effect for complex 6 (as a representative of all other complexes)
was studied by the time-resolved photoluminescence technique
(Table 5, Fig. S23†)4a,27 In THF, the molecules decay
No. Solvent
A1 (%) A2 (%) A3 (%) s1 (ns) s2 (ns) s3 (ns)
1
2
3
4
THF
95
5
10
1
—
77
91
91
2.17
2.00
2.18
2.03
43.39
5.59
—
THF–H2O (30%) 13
THF–H2O (60%)
THF–H2O (90%)
484.14
642.39
92.60 4556.59
8
5
42.69
4
a
These parameters were determined from the equation I ¼ A1 exp(ꢀt/
s1) + A2 exp(t/s2) for solution 1, and for the rest of the solutions (2–3)
I ¼ A1 exp(ꢀt/s1) + A2 exp(t/s2) + A3 exp(ꢀt/s3) was used, where A and s
represent the fractional amount and the lifetime of the shorter,
medium and the longer species, respectively.
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biexponentially (95% of the molecules decay with a lifetime of
2.17 ns and the rest decay with a lifetime of 43.39 ns). With the
addition of water (30%) into THF, excited molecules decay via
three different relaxation pathways (13% and 10% molecules
decay with lifetimes of 2 ns and 5.59 ns, respectively, but 77% of
the molecules decay through a slow channel with a lifetime of
ꢃ484 ns). On further addition of water molecules (60%) in THF,
the decay via the slowest channel is more populated (from 77%
to 91%) and the lifetime increases to ꢃ642 ns. In 90% water, the
lifetime for the slowest component increases to ꢃ4.6 ms without
any noticeable change in population (60%). It has been
observed that the lifetime for the new relaxation pathway
steadily increases (with population as well) when the concen-
tration of water molecules is increased. This observation leads
us to propose that restricted intramolecular rotation (RIR) can
drastically affect the radiative/nonradiative recombination
processes in the excited states.
The packing diagrams for complexes 2, 8 and 9 shows that
there are several intermolecular interactions where phenyl rings
in the triphenyl phosphine units are involved (Fig. 9). Thus, the
internal rotation of these rings is severely restricted in the
crystals. This observation is also consistent with geometry
optimizations using quantum chemical calculations that show
that in solution the minimum energy disposition of these
phenyl rings is markedly different to that adopted in the crystal
structure (Fig. S17†). This hindered rotation of the phenyl rotors
in the crystal state is suggested to block the non-radiative decay
channels, while simultaneously opening the radiative pathways.
Therefore, the solid state emission efficiency for all these
complexes is predicted to improve signicantly, as observed
experimentally.
Application in cell imaging
As an illustration of the possible applications for these new AIE-
active complexes, we employed them in the designing of a non-
toxic bioimaging probe. Because these complexes are soluble in
tetrahydrofuran and dichloromethane but insoluble in water, a
polyethylene glycol–polylactic acid (PEG–PLA) based biode-
gradable polymer was selected, which is known to self-assemble
in water and produce a micellar structure with hydrophobic PLA
inside and hydrophilic PEG outside28 in which the AIE-active
Ir(III) complexes can be encapsulated. In our experiment,
complex 6 was used as a probe molecule for bioimaging.
Micellar encapsulation of AIE-active iridium complexes involves
the solubilization of a mixture of PEG–PLA and the iridium
complex in the organic solvent, followed by the injection of this
mixture in water under vigorous stirring conditions to favour
the self-assembly of PEG–PLA into micelles, which incorporate
the iridium complex in their aggregated form (Scheme 2).
The PL properties of 6 dissolved in THF, in its aggregated
form in water and in a PEG–PLA colloidal solution aer
encapsulation in PEG–PLA micellar nanoparticles, are shown in
Fig. 10 and in the ESI (Fig. S24†). The iridium complex in THF
exhibits weak green emission under 365 nm excitation with the
maximum emission at 470 nm and 495 nm. The aggregated
iridium complex in water, which are encapsulated in PEG–PLA
Fig. 9 Packing diagram for 2 (a), 8 (b) and 9 (c); in 2, the unit cell
contains four molecules, and the green dotted lines denote the
shortest contacts (a ¼ 2.88 A˚; b ¼ 3.42 A˚; c ¼ 2.71 A˚, d ¼ 2.95 A˚ and e ¼
2.70 A˚); in 8, the unit cell contains four molecules (a ¼ 2.73 A˚; b ¼ 2.86 A˚;
c ¼ 2.93 A˚, d ¼ 2.84 A˚, e ¼ 2.84 A˚, f ¼ 2.38 A˚, g ¼ 2.77 A˚, h ¼ 2.88 A˚); in 9,
the green dotted lines denote the shortest contacts (a ¼ b ¼ 2.86 A˚).
micellar nanoparticles, shows strong green emission under
excitation at 365 nm with two emission maxima at 465 nm, 495
nm and 470 nm, 495 nm, respectively (Fig. S24†). The obser-
vation of a similar strong green emission in water and PEG–PLA
micelles indicates that the iridium complex is in its aggregated
form inside the PEG–PLA micelles.
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Journal of Materials Chemistry C
400 nm with a spherical shape (Fig. 11). These results conrm
that the iridium complex aggregates in water to yield micron-
size particles with a wide size distribution (Fig. 11 and S25†). In
contrast, the hydrodynamic size of PEG–PLA nanoparticles and
iridium complex encapsulated PEG–PLA nanoparticles are 35–
150 nm and 65–220 nm, respectively (Fig. 11 and S25†). The
SEM study also conrms that the iridium complex encapsulated
PEG–PLA nanoparticles are spherical in shape with radii in the
range of 40–150 nm (Fig. 11). These results conrm that the
PEG–PLA particles containing the complex in its aggregated
form are smaller than the particles of the pure aggregated
iridium complex in water, with a relatively narrow size distri-
bution for the encapsulated particles. The size of the micelle is
found to increase by about 30 to 70 nm aer the encapsulation
of the iridium complex.
Scheme 2 Schematic representation of the synthesis of luminescent
iridium complex encapsulated PEG–PLA nanoparticles.
The presence of iridium complex inside the PEG–PLA parti-
cles is conrmed by IR spectroscopy. The FTIR spectrum for the
isolated iridium complex shows that the major characteristic
peaks arise at 2092 cmꢀ1 for Ir–H stretching and 3046 cmꢀ1 for
aromatic C–H stretching.11 The spectrum for PEG–PLA nano-
particles shows peaks at 2955–2850, 1752 and 1180 cmꢀ1, which
are characteristic of C–H stretching, C]O stretching and C–O
stretching, respectively.29 In the spectra for PEG–PLA particles,
including the iridium complex vibrational peaks at 2085 cmꢀ1
and 3046 cmꢀ1 corresponding to the iridium complex and those
corresponding to the PEG–PLA are both present (Fig. S26†).
Further, an SEM-based elemental mapping experiment of the
iridium complex encapsulated PEG–PLA particles demon-
strated that the iridium complex lay inside PEG–PLA nano-
particles (Fig. S27†). A PL decay study shows that the lifetime of
iridium complex in THF, water and PEG–PLA is 5.1 ns, 6.3 ms
and 19.8 ms, respectively (Fig. S28†). Lifetimes of the same order
of magnitude of the aggregated iridium complex in water and
inside the PEG–PLA micelles demonstrate that in both the cases
the emitting molecules are found in similar micro environ-
ments, which are notably different from those in THF solu-
tions.30 The stability of the emission of the aggregated iridium
complex under continuous UV irradiation was tested, showing
that the iridium complex is stable under UV irradiation for more
than a minute (Fig. S29†). This result suggests that the emission
of PEG–PLA particles containing the iridium complex is stable
under imaging conditions, and can be thus used as imaging
probes.
Fig. 10 Photoluminescence spectra (left) and digital image (right) of
iridium complex 6 in tetrahydrofuran (THF), water and in aqueous.
The aggregation and particle formation of the iridium
complex in water is also conrmed by dynamic light scattering
(DLS) and scanning electron microscope (SEM) measurements,
as shown in Fig. 11. The hydrodynamic diameter of the particles
of the aggregated complex as determined by DLS ranges from
200 nm to 900 nm. SEM image shows that the particles of the
aggregated complex in water have diameters ranging from 50 to
The micelles of PEG–PLA encapsulated AIE-active iridium
complex were used as in vitro cellular imaging probes for
biomedical applications. For this purpose, HeLa cells were
incubated with the luminescent PEG–PLA particles, and then
labelling was observed under a uorescence microscope. The
results show that the cells show green luminescence under UV
excitation, as shown in Fig. 12, suggesting that the particles can
be used for labelling the cells. As shown in Fig. 12, by varying
the incubation time, it is difficult to understand whether the
particles enter the cytoplasm even aer exposure for 8 h.
Therefore, we obtained a series of uorescence images at
different Z planes with particle-labelled HeLa cells, demon-
strating that the particles are located in the cytoplasm and also
at the cell surface (Fig. S30†). Interestingly, the particles of
Fig. 11 DLS histogram (a and b) and SEM image (c and d) of aggregated
iridium complex (6) in water (a and c) and PEG–PLA particles encap-
sulated with iridium complex (b and d). Note the comparatively smaller
size and narrow size distribution of PEG–PLA particles.
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particles in cell culture medium indicate that there was no
signicant precipitation up to 7 days (Fig. S32†), which ensures
the colloidal stability of iridium complex encapsulated PEG–
PLA particles in cell culture media. The cytotoxicity of the
iridium complex was investigated by the conventional MTT
assay. HeLa cells were incubated with various amounts of
photoluminescent PEG–PLA particles for 24 h and cell viability
was estimated. The result shows that cell survival is >80% in all
the tested concentrations (Fig. 13). This result suggests that the
photoluminescent PEG–PLA particles have low toxicity and can
be used as cell-imaging probes under in vitro conditions.
Conclusion
The one-pot synthetic route for the mono cyclometalated iri-
dium(III) complexes 1–12, which are strongly emissive in the
solid state has been generalized. Tuning of the emission
wavelength was accomplished throughout the visible range by
the introduction of systematic changes in the chromophoric
ligands. Quantum chemistry calculations at the DFT and
TDDFT level were used to explore the intricacies of the elec-
tronic nature of the ground and low-lying excited states involved
in UV-vis absorption and photoemission processes, conrming
the experimental observations. Computed relative transition
energies are in good agreement with the measured absorption
and emission peaks. Investigations were carried out to explore
both the nature of the emitting states and the AIE activity for all
these complexes. The dual functionalities exhibited by a
common iridium(III) framework by proper selection and place-
ment of the chromophoric (cyclometalating) and rotating entity
(triarylphosphine) into the iridium(III) coordination sphere
provide these molecules with interesting photophysical prop-
erties, which can be exploited to obtain new cell-imaging
probes. To illustrate this possibility, the AIE-active iridium(III)
complex 6 was encapsulated inside the hydrophobic core of
PEG–PLA nanoparticles via a simple oil-in-water based emul-
sion–evaporation method. The aggregation of iridium complex
molecules in the hydrophobic core of the PEG–PLA particles
leads to an important increase in their emission intensity. The
colloidal form of these luminescent PEG–PLA particles is shown
to behave as potential cell imaging probe.
Fig. 12 Bright field (BF) and luminescence (L) image of HeLa cells
labelled with iridium complex (6) encapsulated PEG–PLA particles.
Cells are incubated with particles for 1, 4 and 8 hours and then washed
cells are imaged under microscope.
iridium complex aggregated in water do not label the cells even
aer a long incubation time (Fig. S31†). This result suggests that
successful labeling by photoluminescent PEG–PLA particles is
possibly due to their smaller size combined with the lipophilic
property of PEG–PLA, which leads to a strong interaction of the
particles with the cell membrane.31 Encapsulated PEG–PLA
nanoparticles were mixed with the Dulbecco's modied eagle
medium (DMEM) with 10% fetal bovine serum (FBS) to inves-
tigate their stability. The observed digital images of these
Acknowledgements
We gratefully acknowledge the ‘Department of Science and
Technology (DST), Govt. of India’ and ‘Council of Scientic and
Industrial Research (CSIR), Govt. of India’ for nancial support
under project nos. SR/S1/IC-48/2009 (DST) and no. 01/2551/12/
EMR-II (CSIR), respectively. We are also thankful to the ‘UGC-
sponsored Special Assistance Programme (F.540/14/DRS/2007,
SAP-I)’ and DST FIST for instrumental support. Work in Barce-
lona was supported by the MINECO-Spain (Project CTQ2011-
23862-C02-02) and Generalitat de Catalunya (Projects 2009 SGR
1459 and XRQTC). PD acknowledges CSIR India for providing a
research fellowship. MK thanks DST for a fellowship. CC thanks
MINECO-Spain for a predoctoral fellowship. We also thank
Fig. 13 Viability of HeLa cells in the presence of iridium complex (6)
encapsulated PEG–PLA nanoparticles. Cells were incubated in the
presence of different concentrations of iridium complex encapsulated
PEG–PLA nanoparticles for 24 h (the final concentration of iridium
complex encapsulated PEG–PLA nanoparticles used for imaging was
0.117 mg mLꢀ1).
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Journal of Materials Chemistry C
IISER Mohali for the use of their single crystal X-ray diffraction
and NMR research facilities.
Adv. Funct. Mater., 2005, 15, 981; (g) F. He, Y. Tang,
S. Wang, Y. Li and D. Zhu, J. Am. Chem. Soc., 2005, 127,
12343; (h) C. Fan, S. Wang, J. W. Hong, G. C. Bazan,
K. W. Plaxco and A. J. Heeger, Proc. Natl. Acad. Sci. U. S. A.,
2003, 100, 6297; (i) S. Setayesh, A. C. Grimsdale, T. Weil,
References
¨
1 (a) E. M. Linares, A. Formiga, L. T. Kubota, F. Galembeck and
S. Thalhammer, J. Mater. Chem. B, 2013, 1, 2236; (b) K. Yu
and K. S. Schanze, ACS Appl. Mater. Interfaces, 2013, 5,
V. Enkelmann, K. Mullen, F. Meghdadi, E. J. W. List and
G. Leising, J. Am. Chem. Soc., 2001, 123, 946; (j) S. Hecht
and J. M. J. Frechet, Angew. Chem., Int. Ed., 2001, 40, 74; (k)
R. Jakubiak, Z. Bao and L. Rothberg, Synth. Met., 2000, 114,
61; (l) A. Kra, A. C. Grimsdale and A. B. Holmes, Angew.
Chem., Int. Ed., 1998, 37, 402.
¨
Hockner,
¨
2785;
(c)
S.
R.
Sturzenbaum,
M.
A. Panneerselvam, J. Levitt, J.-S. Bouillard, S. Taniguchi,
L.-A. Dailey, R. A. Khanbeigi, E. V. Rosca, M. Thanou,
K. Suhling and A. V. Zayats, Nat. Nanotechnol., 2013, 8, 57;
(d) Y. Zhou, J. Jia, W. Li, H. Feia and M. Zhou, Chem.
Commun., 2013, 49, 3230; (e) X. Zhang, Z. Chi, Y. Zhang,
S. Liu and J. Xu, J. Mater. Chem. C, 2013, 1, 3376; (f)
J. Y. Kim, T. Yasuda, Y. S. Yang and C. Adachi, Adv. Mater.,
2013, 25, 2666; (g) S. C. F. Kui, F.-F. Hung, S.-L. Lai,
M.-Y. Yuen, C.-C. Kkok, K.-H. Low, S. S.-Y. Chui and
C.-M. Chi, Chem.–Eur. J., 2012, 18, 96; (h) Q. Zhao,
C. Huanga and F. Li, Chem. Soc. Rev., 2011, 40, 2508; (i)
Y. Chi and P.-T. Chou, Chem. Soc. Rev., 2010, 39, 638; (j)
S. Coe, W.-K. Woo, M. Bawendi and V. Bulovic, Nature,
5 (a) B. T. Nguyen, J. E. Gautrot, C. Ji, P.-L. Brunner,
M. T. Nguyen and X. X. Zhu, Langmuir, 2006, 22, 4799; (b)
A. P. Kulkarni and S. A. Jenekhe, Macromolecules, 2003, 36,
5285; (c) S. Abe and L. Chen, J. Polym. Sci., Part B: Polym.
Phys., 2003, 41, 1676; (d) B. S. Gaylord, S. Wang,
A. J. Heeger and G. C. Bazan, J. Am. Chem. Soc., 2001, 123,
6417; (e) L. Chen, S. Xu, D. McBranch and D. Whitten, J.
Am. Chem. Soc., 2000, 122, 9302; (f) P. N. Taylor,
M. J. O'Connell, L. A. McNeill, M. J. Hall, R. T. Aplin and
H. L. Anderson, Angew. Chem., Int. Ed., 2000, 39, 3456; (g)
D. Sainova, T. Miteva, G. Nothofer, U. Scherf, I. Glowacki,
J. Ulanski, H. Fujikawa and D. Neher, Appl. Phys. Lett.,
2000, 76, 1810.
´
´
2002, 420, 800; (k) F. Hide, M. A. Dıaz-Garcıa,
B. J. Schwartz, M. R. Andersson, Q. Pei and A. J. Heeger,
Science, 1996, 273, 1833; (l) H. Shi, J. Liu, J. Geng,
B. Z. Tang and B. Liu, J. Am. Chem. Soc., 2012, 134, 9569–
9572; (m) W. Qin, D. Ding, J. Liu, W. Z. Yuan, Y. Hu, B. Liu
and B. Z. Tang, Adv. Funct. Mater., 2012, 22, 771; (n)
I. L. Medintz, H. T. Uyeda, E. R. Goldman and
H. Mattoussi, Nat. Mater., 2005, 4, 435; (o) L. Q. Xiong,
Z. G. Chen, M. X. Yu, F. Y. Li, C. Liu and C. H. Huang,
Biomaterials, 2009, 30, 5592; (p) R. P. Haugland, A Guide to
Fluorescent Probes and Labelling Technologies, Molecular
Probes, Eugene, OR, 10th edn, 2005; (q) D. Wang, J. Qian,
S. He, J. S. Park, K.-S. Lee, S. Han and Yi. Mu, Biomaterials,
2011, 32, 5880.
6 Y. Hong, J. W. Y. Lama and B. Z. Tang, Chem. Soc. Rev., 2011,
40, 5361.
7 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740.
8 (a) Z. Zhang, B. Xu, J. Su, L. Shen, Y. Xie and H. Tian, Angew.
Chem., 2011, 123, 11858; (b) S. Dong, Z. Li and J. Qin, J. Phys.
Chem. B, 2009, 113, 434; (c) Z. Xie, B. Yang, W. Xie, L. Liu,
F. Shen, H. Wang, X. Yang, Z. Wang, Y. Li, M. Hanif,
G. Yang, L. Ye and Y. Ma, J. Phys. Chem. B, 2006, 110,
20993; (d) H. J. Tracy, J. L. Mullin, W. T. Klooster,
J. A. Martin, J. Haug, S. Wallace, W. Rudloe and K. Watts,
Inorg. Chem., 2005, 44, 2003; (e) Y. Ren, J. W. Y. Lam,
Y. Dong, Y. Y. B. Z. Tang and K. S. Wong, J. Phys. Chem. B,
2005, 109, 1135.
9 (a) N. Zhao, Y.-H. Wu, J. Luo, L.-X. Shia and Z.-N. Chen,
Analyst, 2013, 138, 894; (b) H. Wu, T. Yang, Q. Zhao,
J. Zhou, C. Lia and F. Li, Dalton Trans., 2011, 1969; (c)
G.-G. Shan, D.-X. Zhu, H.-B. Li, P. Li, Z.-M. Su and Y. Liao,
Dalton Trans., 2011, 2947; (d) Q. Zhao, L. Li, F. Li, M. 01Yu,
Z. Liu, T. Yi and C. Huang, Chem. Commun., 2008, 685; (e)
Y. You, H. S. Huh, K. S. Kim, S. W. Lee, D. Kim and
S. Y. Park, Chem. Commun., 2008, 3998.
´
2 (a) k. Slavı, J. Fluorescence Microscopy and Fluorescent Probes,
Plenum, New York, 1996; (b) B. Valeur, Molecular
Fluorescence: Principle and Applications, Wiley-VCH,
Weinheim, 2002.
3 (a) Advanced Concepts in Fluorescence Sensing, ed. C. D.
Geddes and J. R. Lakopwicz, Springer, Norwell, 2005; (b)
Fluorescence Sensors and Biosensors, ed. R. B. Thompson,
CRC, Boca Raton, 2006; (c) W. H. Tan, K. M. Wang and
T. J. Drake, Curr. Opin. Chem. Biol., 2004, 8, 547; (d)
K. E. Sapsford, L. Berti and I. L. Medintz, Angew. Chem.,
Int. Ed., 2006, 45, 4562; (e) S. M. Borisov and
O. S. Woleis, Chem. Rev., 2008, 108, 423.
10 C. Xu, Z.-Q. Wang, X.-M. Dong, X.-Q. Hao, X.-M. Zhao,
B.-M. Ji and M.-P. Song, Inorg. Chim. Acta, 2011, 373,
306.
4 (a) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun.,
2009, 4332; (b) J.-S. Yang and J.-L. Yan, Chem. Commun.,
2008, 1501; (c) Y.-T. Lee, C.-L. Chiang and C.-T. Chen, 11 P. Alam, M. Karanam, A. R. Choudhury and I. R. Laskar,
Chem. Commun., 2008, 217; (d) J. Wang, Y. Zhao, C. Dou,
Dalton Trans., 2012, 9276.
H. Sun, P. Xu, K. Ye, J. Zhang, S. Jiang, F. Li and Y. Wang, 12 G. Ruana and S.-S. Feng, Biomaterials, 2003, 24, 5037.
J. Phys. Chem. B, 2007, 111, 5082; (e) C.-W. Wu, C.-M. Tsai 13 APEX2;SADABS; SAINT, Bruker AXS Inc., Madison,
and H.-C. Lin, Macromolecules, 2006, 39, 4298; (f) S.-F. Lim,
Wisconsin, USA, 2008.
R. H. Friend, I. D. Rees, L. Y. J. Robinson, K. Ma, 14 G. M. Sheldrick, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr.,
A. B. Holms, E. Hennebicq, D. Beljonne and F. Cacialli,
Theor. Gen. Crystallogr., 2008, 64, 112.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. C, 2014, 2, 5615–5628 | 5627

View Article Online
Journal of Materials Chemistry C
Paper
15 O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard, 24 (a) C.-H. Chang, C.-L. Ho, Y.-S. Chang, I.-C. Lien, C.-H. Lin,
J. A. K. Puschmann and H. J., J. Appl. Crystallogr., 2009, 42, 339.
16 M. Nardelli, J. Appl. Crystallogr., 1995, 28, 659.
17 S. Grimme, J. Comput. Chem., 2006, 27, 1787.
Y.-W. Yang, J.-L. Liao and Y. Chi, J. Mater. Chem. C, 2013,
1, 2639; (b) C.-H. Yang, S.-W. Li, Y. Chi, Y.-M. Cheng,
Y.-S. Yeh, P.-T. Chou, G.-H. Lee, C.-H. Wang and C.-F. Shu,
Inorg. Chem., 2005, 44, 7770.
18 (a) P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270; (b)
W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284; (c) 25 (a) I. R. Laskar, S.-F. Hsu and T.-M. Chen, Polyhedron, 2005,
P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299; (d)
P. C. Hariharan and J. A. Pople, Mol. Phys., 1974, 27, 209.
19 (a) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
24, 189; (b) S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-
Razzaq, R. Kwong, I. Tsyba, M. Bortz, B. Mui, R. Bau and
M. E. Thompson, Inorg. Chem., 2001, 40, 1704.
Matter Mater. Phys., 1988, 37, 785; (b) A. D. Becke, J. Chem. 26 J. C. de Mello, H. F. Wittmann and R. H. Friend, Adv. Mater.,
Phys., 1993, 98, 5648. 1997, 9, 230.
20 (a) E. Cances, B. Mennucci and J. Tomasi, J. Chem. Phys., 27 Y. Ren, J. W. Y. Lam, Y. dong, B. Z. Tang and K. S. J. Wong, J.
`
`
1997, 1073032; (b) B. Mennucci, E. Cances and J. Tomasi,
Phys. Chem. B, 2005, 109, 1135.
J. Phys. Chem. B, 1997, 101, 10506; (c) E. Cances and 28 R. Z. Xiao, Z. W. Zeng, G. L. Zhou, J. J. Wang, F. Z. Li and
B. Mennucci, J. Math. Chem., 1998, 23, 309. A. M. Wang, Int. J. Nanomed., 2010, 5, 1057.
21 M. J. Frisch, et al., Gaussian 09 Revision B.01, Gaussian, Inc., 29 Q. Wang, Y. Bao, J. Ahire and Y. Chao, Adv. Healthcare Mater.,
`
Wallingford, CT, 2009.
2013, 2, 459.
22 H. D. Kaesz and R. B. Saillant, Chem. Rev., 1972, 72, 231.
23 M. A. Bennett and D. L. Milner, J. Am. Chem. Soc., 1969, 91,
6983.
30 A. M. Talarico, M. Ghedini, C. O. Rossia and E. I. Szerb, So
Matter, 2012, 8, 11661.
31 N. R. Jana, Phys. Chem. Chem. Phys., 2011, 13, 385.
5628 | J. Mater. Chem. C, 2014, 2, 5615–5628
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