Aggregation-Induced Emission Activity in Iridium(III) Diimine Complexes: Investigations of Their Vapochromic Properties

Article abstract of DOI:10.1002/ejic.201402222

Two iridium(III) diimine complexes [mono(1,10-phenanthroline)bis(triphenylphosphine)(dihydrido)iridium(III) hexafluorophosphate (1) and mono(1,10-phenanthroline)bis(triphenylphosphine)(hydrido)(chloro)iridium(III) hexafluorophosphate (2)] have been synthesized from a single two-step reaction. The structures of 1 and 2 both adopt distorted octahedral geometries, as established by single-crystal X-ray diffraction. The complexes, upon irradiation with UV light at 365 nm, emit faint light in solution and bright light in the solid state. The ground- and excited-state properties of these complexes were investigated through density functional theory (DFT) and time-dependent DFT calculations. The calculated energies for the transitions from the ground state to the singlet and triplet excited states were close to those determined from the experimental absorption and emission. Their molecular orbitals were also exploited to compute the ground-state dipole moments and redox potentials. Several experiments were performed to demonstrate the aggregation-induced emission (AIE) activity of these complexes. AIE was triggered by the restricted intramolecular rotation of the rotating units (phenyls in triphenyphosphines) in these molecules in the solid state. The solid thin films of 1 and 2 exhibit solvent-polarity-dependent vapour-responsive emission properties (vapoluminescent). The rationale for the different emission behavior in the solid state has been thoroughly investigated. The packing diagrams of 1 and 2 show that there is enough space available to accommodate small organic solvent molecules inside the crystal lattices.

Full text of DOI:10.1002/ejic.201402222

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DOI:10.1002/ejic.201402222
Aggregation-Induced Emission Activity in Iridium(III)
Diimine Complexes: Investigations of Their Vapochromic
Properties
Parvej Alam,[a] Maheswararao Karanam,[b]
Debashree Bandyopadhyay,*[c] Angshuman Roy Choudhury,*[b] and
Inamur Rahaman Laskar*[a]
Keywords: Luminescence / Iridium / Nitrogen heterocycles / Density functional calculations
Two iridium(III) diimine complexes [mono(1,10-phenanthrol-
ine)bis(triphenylphosphine)(dihydrido)iridium(III) hexa-
fluorophosphate (1) and mono(1,10-phenanthroline)bis(tri-
phenylphosphine)(hydrido)(chloro)iridium(III) hexafluoro-
termined from the experimental absorption and emission.
Their molecular orbitals were also exploited to compute the
ground-state dipole moments and redox potentials. Several
experiments were performed to demonstrate the “aggrega-
tion-induced emission” (AIE) activity of these complexes.
AIE was triggered by the restricted intramolecular rotation
of the rotating units (phenyls in triphenyphosphines) in these
molecules in the solid state. The solid thin films of 1 and 2
exhibit solvent-polarity-dependent vapour-responsive emis-
sion properties (vapoluminescent). The rationale for the dif-
ferent emission behavior in the solid state has been thor-
oughly investigated. The packing diagrams of 1 and 2 show
that there is enough space available to accommodate small
organic solvent molecules inside the crystal lattices.
phosphate (2)] have been synthesized from a single two-step
reaction. The structures of 1 and 2 both adopt distorted octa-
hedral geometries, as established by single-crystal X-ray dif-
fraction. The complexes, upon irradiation with UV light at
365 nm, emit faint light in solution and bright light in the
solid state. The ground- and excited-state properties of these
complexes were investigated through density functional
theory (DFT) and time-dependent DFT calculations. The cal-
culated energies for the transitions from the ground state to
the singlet and triplet excited states were close to those de-
π–π stacking interactions.[17] There have been many reports
of organometallic and coordination complexes of Ru,[18]
Sn,[19] Pt/Pd,[20] Cu,[21] Zn,[22] Au,[23] Ag,[24] and Re/Co[25]
that have been used in vapor-responsive luminescent materi-
als. However, the most extensive studies have been carried
out with platinum(II) and gold(I). Metallophilic interac-
tions between PtII or AuI are either disrupted or enhanced
upon interaction with VOCs, thereby altering the gap be-
tween the highest-occupied molecular orbital (HOMO) and
the lowest-unoccupied molecular orbital (LUMO), thus
leading to distinct changes in the emission or absorption
spectra. In comparison with other luminescent metallic
complexes, several distinct advantages have been observed
with cyclometalated iridium(III) complexes, for example,
superior quantum efficiencies, easy tunability of light emis-
sion wavelength, higher thermal and electrochemical sta-
bility, and straightforward synthetic routes in comparison
with other analogues.[26] Notwithstanding, reports on emit-
ting iridium(III) complexes as vapoluminescent materials
are very limited.[12]
Introduction
Luminescent materials are widely used as vapolumines-
cent materials to detect volatile organic compounds (VOCs)
in the environment or workplace.[1–13] These materials usu-
ally show changes in emission intensity or wavelength when
exposed to specific VOCs. This variation in luminescence
properties triggered by the presence of VOCs results in
changes in, for example, metallophilic interactions,[14]
hydrogen bonding,[15] solvent–metal bonds,[16] and aromatic
[a] Department of Chemistry, Birla Institute of Technology and
Science, Pilani Campus,
Pilani, Rajasthan, India
E-mail: ir_laskar@bits-pilani.ac.in
http://www.bits-pilani.ac.in/Pilani/index.aspx
[b] Department of Chemical Sciences, Indian Institute of Science
Education and Research (IISER),
Mohali, Sector 81, S. A. S. Nagar, Manauli PO, Mohali,
Punjab 140306, India
angshuman@iisermohali.ac.in
http://www.iisermohali.ac.in/
[c] Department of Biological Sciences, Birla Institute of
Technology and Sciences, Hyderabad Campus,
Jawahar Nagar, Shameerpet Mandal, Hyderabad, Andhra
Pradesh 50078, India
Recently we reported[27] a one-pot synthetic route to ag-
gregation-induced-emission (AIE)-active monocyclometal-
ated iridium(III) complexes and their easy tunability
throughout the visible range within the common framework
of an iridium(III) complex.[28] AIE[29–32] is an anomalistic
banerjee@yahoo.com
http://www.bits-pilani.ac.in/hyderabat/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402222.
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phenomenon and the most efficient process for tackling the cyclometalated ligand was prevented probably to avoid the
aggregation-caused quenching (ACQ)[30] effect. AIE activity presence of strong trans-influencing substituents (hydride
mainly arises from the restricted rotation of a freely rotating and C of the cyclometalated ligand) trans to each other.
1
part of the molecular system either in concentrated solu-
Complexes 1 and 2 were characterized by H, 13C, and
tion/or in the solid state. This restricted rotation blocks 31P NMR spectroscopy (see Figures S1 and S2 in the Sup-
non-radiative pathways and opens up radiative channels, porting Information). The 31P NMR spectra each show a
thereby facilitating very bright emission in the solid state.
single resonance peak for both complexes 1 and 2 at δ =
1
Herein we report the synthesis of an AIE-active diimine- 20.88 and 29.49 ppm, respectively. H NMR peaks for the
based chromophore ligated to iridium(III) complexes and hydrides can be observed at δ = –19.24 and –17.56 ppm for
their vapor-responsive luminescence properties. The ex- complexes 1 and 2, respectively. The hydride peaks appear
cited-state properties of these complexes have been charac- as triplets due to P–H coupling (JP–H ≈ 15 Hz). The value
terized through DFT chemical quantum calculations.
of JP-H indicates the hydride to be cis to the phosphorus in
both complexes.[33] The 1H NMR spectrum of 1 (Fig-
ure S1a) clearly exhibits seven sets of protons: Four gener-
ated from the resonances of 1,10-phenanthroline, two from
the triphenylphosphines, and one from the hydride. These
observations, along with a single hydride NMR peak, sub-
stantiate the symmetrical geometry of 1 (see Figure S1a). In
the case of 2, more than four sets of protons were observed
from 1,10-phenanthroline, which clearly indicates the un-
symmetrical geometry of 2 (see Figure S2a). The presence
of the Ir–H bond in 1 and 2 was confirmed from the corre-
sponding stretching frequency at 2179 and 2152 cm–1 (see
Figures S1d and S2d), respectively.
Results and Discussion
Syntheses
The strategy for the synthesis of a diimine complex with
iridium(III) originated from our previous report[27] in which
we had synthesized a monocyclometalated iridium(III)
complex in a one-pot reaction. On addition of 1,10-phen-
anthroline to the reaction mixture (in place of 2-phenylpyr-
idine in the same reaction protocol[27]), two spots were ob-
served very close on a TLC plate. These two species were
difficult to separate by using traditional purification tech-
niques, for example, column chromatography and
recrystallization. Therefore these impure products were
mixed with potassium hexafluorophosphate in methanol
and the reaction mixture was treated with microwave irradi-
Structural Characterization Based on X-ray
Crystallography
The structures of complexes 1 and 2 were established by
ation leading to an exchange of counter ions, with Cl– being a single-crystal X-ray diffraction study at 100 K. The
–
replaced by PF6 . A solid residue corresponding to complex ORTEP diagrams (Figure 1 and Figure S3) show the struc-
2 separated from solution. Thus, this process facilitates the tures to be distorted octahedrals, and selected bond lengths
separation and subsequent purification of complexes 1 and and bond angles are presented in Table S1 and Table S2 in
2. In this case, the diimine iridium(III) complex resulted in the Supporting Information. The packing diagrams for the
dihydride (1) and chlorohydride (2) complexes, in contrast two complexes (Figure 2 and Figure S3) show that each of
to the reaction with the cyclometalated ligand in which the these complexes contains eight dichloromethane (dcm) mo-
chlorohydride was the sole product obtained (Scheme 1, D). lecules per unit cell (both complexes were recrystallized
The formation of the dihydride complex in the case of the from dcm). The void space for 1 was calculated to be
Scheme 1. Protocol for the synthesis of complexes 1 and 2.
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980.1 Å3, compared with a cell volume of 4837.5 Å3. For 2, be attributed to ligand-centered transitions. The spectra
the void space and cell volume are 906.2 and 4964.8 Å3, show a long absorption tail starting at 320 nm and ex-
respectively. The ratios of total cell volume to the void vol- tending to 450 nm (λmax = 364 and 410 nm for 1, and 358
ume, occupied by solvent molecules, are 4.9 and 5.3 for and 400 nm for 2; Figure 3). Close scrutiny reveals that this
complexes 1 and 2, respectively. These similar ratios for 1 long tail consists of two partially resolved broad absorption
and 2 suggest similar occupancy values of the dcm mol- bands.
ecules, namely eight dcm molecules per unit cell of 1 and 2
(Figure 2). These dcm molecules in the crystal lattices pre-
sumably substitute other volatile solvent molecules upon
exposure.
Figure 1. ORTEP structures of 1 and 2. Ellipsoids are drawn at the
Figure 3. Solution absorption spectra for 1 and 2 at a concentration
50% probability level. All the H atoms (except hydrides), solvent
of 10–5 m in dcm (inset: enlarged absorption spectra in the region
molecules, and the counter ion have been omitted for clarity.
290–450 nm). The calculated wavelengths are given in spectra (364
and 410 nm for 1, and 358 and 400 nm for 2).
UV/Vis Absorption Spectra
Results from TDDFT Calculations
Complexes 1 and 2 exhibit intense absorption bands in
The molecular orbitals in complexes 1 and 2 that have a
the range 240–320 nm at a concentration of 10–5 m in dcm. significant involvement in low-lying excitation are shown in
The molar extinction coefficients (ε) corresponding to the Figure 4 and Figure S4 in the Supporting Information. All
absorption maxima were found to be higher than the pertinent energy gaps and the assignments of each tran-
4.5ϫ104 m–1 cm–1 (Table 1). Thus, the absorption bands can sition are listed in Table 1.
Figure 2. Unit cell packing diagrams of 1 and 2 showing the locations of the CH2Cl2 solvent molecules.
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Table 1. Calculated excitation wavelengths (λcal), oscillator strengths (f), MLCT, and transition energies (E) determined at the TDDFT/
B3LYP level in dcm for a few transitions. All the excitations reported herein initiate from the singlet ground state (So). The corresponding
experimental wavelengths (λexpt) are shown in separate columns (1 in above and 2 in below).
States
λcal [nm]
λexpt [nm]
E [eV]
f
Assignments
MLCT [%]
S1
S2
S3
T1
379
369
361
524
410
–
364
542
3.27
3.36
3.43
2.36
0.06
0.0003
0.02
0
HOMOǞLUMO (97%)
33.17
90.6
32.3
8.65
5.58
38.5
2.25
HOMO–1ǞLUMO (98.5%)
HOMOǞLUMO+1 (95%)
HOMOǞLUMO+1 (25.3%)
HOMO–3ǞLUMO+1 (25%)
HOMO–1ǞLUMO (81%)
HOMOǞLUMO+1 (15%)
S1
S2
S3
386
374
358
400
3.21
3.31
3.46
0.000023
–
0.00004
HOMOǞLUMO (79%)
HOMO–1ǞLUMO (15%)
11.58
7.12
358
0.036000
HOMO–2ǞLUMO (75.5%)
HOMOǞLUMO+1 (16%)
18.0
2.4
T1
528
531
2.35
0
HOMO–3ǞLUMO+1 (20%)
HOMO–2ǞLUMO+1 (17%)
HOMO–2ǞLUMO (13%)
HOMOǞLUMO (4%)
1.0
4.0
3.1
0.6
1.329 V, respectively. The reduction potentials observed for
1 and 2 at –0.700 and –0.740 V, respectively, correspond to
the LUMO state of the phenanthroline ligand[34] in both
the complexes.
Vapoluminescence
Complex 1 was recrystallized from dcm. It emits greenish
yellow light on exposure to UV radiation (shown as a thin
film in Figure 5, a). The emission color of this complex in
a thin film changes from greenish yellow to yellow upon
exposure to solvents with a higher polarity than dcm [di-
electric constant (ε) = 8.93], for example, acetone (ε = 20.7)
and acetonitrile (ε = 37.5). Upon exposure to solvents with
a lower polarity than dcm, for example, chloroform (ε =
4.81), benzene (ε = 2.27), or 1,4-dioxane (ε = 2.25), the solid
thin films (recrystallized from dcm) emit yellowish green
and bluish green light (Figure 5, a).
Figure 4. Selected frontier molecular orbitals for 2.
Complex 2 recrystallized from dcm emits green light. The
The calculated energy gaps between the S0 and S1 states thin film of 2 analogously emits blue emission upon expo-
are 3.27 eV (379 nm) and 3.36 eV (386 nm) for complexes 1 sure to nonpolar solvents (benzene, 1,4-dioxane) and green
and 2, respectively, and are consistent with the experimen- light in polar solvents (chloroform, acetone, acetonitrile; see
tally observed absorption wavelengths (Table 1). Similarly, Figure S5a in the Supporting Information). A distinct dif-
the S0 ǞT1 transition energies for complexes 1 (524 nm) ference was observed in the emission spectra of thin films
and 2 (528 nm) are consistent with the experimental emis- in polar and nonpolar solvents. The thin films exposed to
sion spectra at the blue edge (Table 1). This consistency be- nonpolar solvents show structured emission, whereas broad
tween the experimental and calculated transition fre- emission was observed from thin films exposed to polar sol-
quencies prove the accuracy of the molecular orbital dia- vents (Figure 5, b). Two peaks separated by approximately
grams. Complexes 1 and 2 differ in their metal-to-ligand 1408 cm–1 are observed in the structured emission spectra
charge-transfer (MLCT) transition character due to dif- recorded in nonpolar solvents (1,4-doxane, benzene), which
ferent extents of orbital contributions from different ligands correspond to the stretching of double bonds in the aro-
(see Table S3).
matic ligands.[35] The maximum emission wavelength grad-
The redox potentials were measured by cyclic voltamme- ually shifts towards a longer wavelength with increasing sol-
try relative to an internal ferrocene reference (Cp2Fe/ vent polarity (Figure 5, b). In the absorption spectra, dif-
Cp2Fe+ = 0.62 V vs. SCE in dcm). Complexes 1 and 2 show ferent band-edge absorptions can be observed for a thin
metal-centered irreversible oxidation potentials at 1.331 and film of 1 exposed to polar and nonpolar solvents. In the
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Figure 6. Solvent-polarity-dependent tuning of metal-to-ligand
charge transfer (MLCT) states lead to different emission colors of
1 and 2 in the solid state (dcm, acetone etc. stabilize the MLCT
states more than 1,4-dioxane, benzene etc.).[20e]
The calculated excited-state dipole moment (μe) is greater
than the ground-state DM (μg; DMs for 1: μg = 45.9 D, μe
= 60.7 D; DM for 2: μg = 31.0 D, μe = 36.4 D; see Tables S4
and S5 and Figures S7 and S8 in the Supporting Infor-
mation). Higher DM values in the excited states support
the lowering in energy of the MLCT state with increasing
solvent polarity. This fact, in combination with the spectro-
scopic observations of the thin films described earlier, sup-
port the switching of the lowest excited state to MLCT from
a ligand-centered (LC) state with increasing polarity of
slovents (Figure 6). In this case, the nature of the excited
state is predominantly of MLCT character. The observed
Figure 5. (a) Thin-film emission color of 1 (bluish-green to yellow)
with systematic increase of solvent polarity (polarity increases in
the
order
1,4-dioxaneϽbenzeneϽchloroformϽdcmϽacet-
oneϽacetonitrile) on exposure to UV radiation (λmax = 365 nm).
(b) Emission spectra of the corresponding thin films moistened
with the solvents.
maximum emission wavelength in chloroform (λmax
=
case of nonpolar solvents, the observed absorption wave-
length is less than 350 nm and for polar solvents is greater
than 400 nm (see Figure S6a). In addition, a long tail fol-
lowed by a band-edge absorption is observed for the latter
case only (see Figure S6b). These spectroscopic observa-
tions suggest that the nature of the lowest excited states of
the thin-film samples exposed to nonpolar solvents pre-
dominantly consist of ligand-centered states. The nature of
the lowest excited states changes to a MLCT state on expo-
sure of the thin films to polar solvents. In other words,
switching of the excited state occurs due to a change in the
dielectric medium around the thin-film samples (Figure 6).
The ground-state dipole moments (DMs) for both 1 and 2
were calculated by DFT calculations. The excited-state di-
pole moments for both complexes were calculated by using
Equation (1)[36,37]
512 nm) shifts to a longer wavelength when the thin film is
exposed to solvents of higher polarity. The thin films of 1
exposed to dcm, acetone, and acetonitrile, respectively, have
λmax values of 518, 524, and 540 nm.
Similarly, thin films of 2 recrystallized from dcm emit
blue light when exposed to relatively low-polar solvents
(1,4-dioxane, benzene), which changes to green in relatively
high-polar solvents [chloroform (λmax = 507 nm), dcm (λmax
= 513 nm), acetone (λmax = 516 nm), acetonitrile (λmax
=
518 nm)]. The vapor-responsive color and data from dipole
moment calculations, and emission and absorption spectra
are shown in Figure S5a,b and Figure 6b in the Supporting
Information.
The original green emission of 2 in dcm changes to blue
on exposure of the film to 1,4-dioxane. This thin film re-
emits green light after heating at 70 °C for 15 min followed
by exposure to dcm vapor (Figure 7). It is observed that the
emission color of the thin film of 2 turns to green on expo-
sure to dcm, and the blue emission reappears when the thin
film is exposed to 1,4-dioxane. These observations support
the switching of the emission color of 2 on reversibly chang-
ing the volatile organic solvent. Similar switching of the
(1)
in which μe is the excited-state dipole moment, μg is the emission color of 1 is also observed (see Figure S9 in the
ground-state dipole moment, a is Onsager’s cavity radii, and Supporting Information). We recrystallized 1·2 dcm from
N
m is the slope of the linear plot of ET versus the Stokes 1,4-dioxane and the resulting 1 showed the same VOC
shift.
property as was observed in the thin film of 1·2 dcm on
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exposure to 1,4-dioxane after heating (as discussed earlier, in other solvents (1@1,4-dioxane, 1@acetonitrile) are sim-
see Figure S10). We recorded the 1H NMR spectrum of the ilar to that of 1·2CH2Cl2 (the structure obtained from sin-
resulting complex. The presence of the proton peak of 1,4- gle-crystal X-ray diffraction; see Figure S12a). The above
dioxane at around 3.7 ppm is clearly observed in the NMR experiment was performed with 2 and gave a similar result
spectrum (see Figure S11), whereas the proton peak of (see Figure S12b). Note that 1 (or 2) exhibits a broad and
dichloromethane (ca. 5.3 ppm) was not observed. The ob- featureless emission spectrum (with faint yellow emission
servation of a proton peak from dioxane supports the re- color) after dissolving it in dcm. This observation remains
placement of dcm molecules with 1,4-dioxane prior to occu- true irrespective of the polarity of the solvent (see Fig-
pying the crystal sites by dcm molecules.
ure S13a,b). The solution absorption spectra of 1 and 2 in
different solvents barely show any difference (see Fig-
ure S13c,d) in contrast to solid thin films (Figure 5, b). Sim-
ilar findings[20e] were found for the (4,4Ј-tert-butyl-
2,2Ј-bipyridine)(p-dimesitylborylphenylacetylene)platinum-
(II) complex, in which the vapoluminescent properties of
the solid thin film depend on the polarity of the solvent,
but the same was not observed in solution. This anomalous
observation can presumably be explained by localized di-
pole–dipole interactions in the thin films with guest solvents
occupying the void space, rather than the interactions be-
tween 1 (or 2) and the continuously changing dipoles in
bulk solution.[20e]
Aggregation-Induced Emission Activity
A number of controlled experiments were performed to
investigate the cause of the strong solid-state emission be-
havior exhibited by 1 and 2 with yellow and green lumines-
cence at λmax = 518 and 513 nm, respectively (Table 2).
Complexes 1 and 2 were both found to be almost nonemis-
sive after dissolution in thf. However, the intensity of the
emission increased significantly in 90:10 water/thf (Figure 8
and Figure S14 in the Supporting Information). As these
complexes are insoluble in water, the observed increase in
PL intensity with increasing concentration of water (visible
as suspended particles) proves that these complexes show
aggregation-induced emission (AIE) activity. The emission
intensities of 1 and 2 in 90:10 water/thf were found to be 30
and 74 times higher, respectively, than their PL intensities in
thf. Note that, unlike the analogous iridium(III) complexes
with cyclometalated ligands published by our group,[27] the
PL intensities of these diimine complexes remain un-
changed up to 60:40 water/thf (Figure 8 and Figure S14).
This disparity is evident from the ionic nature of these spe-
cies (1 and 2), which renders them more soluble than their
neutral cyclometalated analogues (Scheme 1, D). In another
experiment, a slow enhancement of PL emission intensity
Figure 7. (a) Solid-state reversal of the emission color of 2 from
blue to green (on exposure to dcm) and green to blue (on exposure
to 1,4-dioxane). The photograph was taken under excitation at
364 nm. (b1) Reversibility of the solid-state emission spectrum of 2
with repeated VOC exposure: a. 2 Recrystallized from dcm, b. after
exposure to 1,4-dioxane, c. after exposure to dcm after heating the
film at 70 °C for 15 min, and d. after exposure to 1,4-dioxane.
(b2) Switching of the emission wavelength (ca. 515 nm to ca.
465 nm and back) on repeated alternating exposure to dcm and
1,4-dioxane, respectively.
As described above, the void space available for poten- was observed with a gradual increase in the concentration
tially occupying solvent molecules in 1 and 2 are 980.1 and of viscous poly(ethylene glycol) (PEG) at a fixed concentra-
906.2 Å3, respectively, per unit cell of the crystal lattice tion of 1 (or 2) in thf (see Figure S15). The luminescence
(vide supra). The crystal packing diagrams for 1 and 2 intensity increased by factors of 2.7 and 2.9 for 1 and 2,
(crystals recrystallized from dcm) demonstrate that the in- respectively, in 90:10 PEG/thf, which suggests that the com-
terstitial positions are occupied by dcm molecules (eight plexes possess some rotationally active moiety. Hindrance
dcm molecules per unit cell of the crystals in each case; of the rotationally active group with increasing viscosity of
Figure 2). Powder X-ray diffraction patterns of thin-film the solution may be responsible for the increasing lumines-
samples of 1 exposed to dcm exactly match the structure cence intensity. Based on these experiments, it is proposed
determined by single-crystal X-ray diffraction. On the basis that rotation of the rotationally active group is restricted in
of this similarity, the thin-film structure has been identified the solid state. The particle sizes of the aggregated forms of
as 1·2CH2Cl2 (see Figure S12 in the Supporting Infor- 1 and 2 obtained in 90:10 water/thf determined by dynamic
mation). The powder X-ray patterns for the thin films of 1 light scattering experiments were found to be 146 and
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Table 2. Photophysical properties of 1 and 2.
[b]
Medium
UV/Vis[a] λmax [nm] (ε [104 m–1 cm–1])
PL
λ
max
[nm]
QE[c] [%]
τ [μs]
1
2
solution
solid state
solution
269 (5.0), 364 (0.50), 410 (0.40)
538
518
531
513
0.75
0.00125[d]
2.95[e]
0.93
269 (5.2), 358 (0.40), 400 (0.20)
0. 00630[d]
solid state
1.43[e]
[a] Spectra were recorded in degassed dichloromethane (dcm) at room temperature at a concentration of 10–5 m. [b] Spectra were recorded
in dcm. [c] The quantum yields were measured in degassed dcm using Coumarin 153 (QY = 0.38 in ethanol) as reference. [d] Life-times
were measured in dry thf. [e] Aggregated form of the complexes in 9:1 water/thf (v/v).
186 nm, respectively (see Figure S16). The packing dia- manner, as described below. The ratios of the total inte-
grams of 1 and 2 show that several intermolecular interac- grated area under the solid-state photoluminescence (PL)
tions exist between the phenyls in the triphenylphosphine spectrum (Is) to its absorption (As; absorbance values corre-
units and neighboring molecules, the PF6– counter ions, and spond to their excitation wavelengths), Qs = Is/As, were cal-
the interstitially accommodated dcm solvent molecules (see culated for 1 and 2 and found to be 8.2ϫ109 and 1.4ϫ1010,
Figure S17). Thus, the rotation of the phenyl groups in tri- respectively. Similarly, values of Qsol for 1 and 2 (sol repre-
phenylphosphine is hindered in the solid state due to these sents solution state; Qsol = Isol/Asol, 1 and 2 dissolved in
interactions. This hindered rotation of the phenyl rotors will pure thf) were calculated to be 8.9ϫ108 and 7.2ϫ108,
[39]
block the nonradiative channels, lose energy, and open up respectively. The ratios of Qs/Qsol
for 1 and 2 are thus
new radiative pathways leading to significantly improved 9.2 and 19.7, respectively, which are in support of the strong
solid-state quantum efficiency.[38] Both complexes in di- emission observed for 1 and 2 in the solid state compared
chloromethane exhibit very weak PL intensity in compari- with their solutions. Time-resolved photoluminescence
son with their corresponding solid/aggregated forms (see spectra for 1 and 2 were measured in pure thf and the life-
Figure S18). We have demonstrated this effect in a different times determined were 1.25 and 6.3 ns, respectively. The
life-times of the complexes 1 and 2 in 90:10 (v/v) water/
thf are significantly longer at 2.95 and 1.63 μs, respectively
(Table 2 and Figure S19). These increased life-times in 90:10
water/thf evidence the improved luminescence yield aggre-
gated in comparison with in pure thf.[40] The solution quan-
tum efficiencies (in pure thf) of 1 and 2 were determined to
be 0.75 and 0.93%, respectively (Coumarin 153 with quan-
tum yield = 0.38 in ethanol was used as reference).[41]
Conclusion
Two new mono(diimine)-based complexes of iridium(III)
have been synthesized. In general, diimine complexes of
iridium(III) emit weak light in both solid and solution
states. However, these synthesized diimine complexes emit
strong light in the solid state due to aggregation-induced
emission activity. This approach has led to improved solid-
state light emission properties in the diimine iridium(III)
system. These complexes are useful as vapor-responsive lu-
minescent materials. To the best of our knowledge, these
are the first examples of iridium complexes that are AIE-
active molecules for sensing VOCs.
Experimental Section
Materials: Iridium(III) chloride hydrate, 1,10-phenanthroline, 2-
ethoxyethanol, potassium hexafluorophosphate, and triphen-
ylphosphine were purchased from Sigma–Aldrich Chemical Com-
pany Ltd. Sodium carbonate and the UV/Vis-grade solvents (dcm,
hexane, ethyl acetate) were procured from Merck.
Figure 8. (a) Variation of PL intensity of 2 with increasing concen-
tration of water in a solution of 2 in thf (concd. of 0% water: 1 mg
2 in 10 mL thf; concd. of 30% water: 1 mg 2 in 7 mL thf and 3 mL
water; concd. of 60% water: 1 mg 2 in 4 mL thf and 6 mL water;
concd. of 90% water: 1 mg 2 in 1 mL thf and 9 mL water). (b) Cor-
responding emission spectrum with increasing concentration of
water.
Characterization: 1H, 13C, and 31P NMR spectra were recorded
with a 400 MHz Bruker spectrometer using CDCl3 as solvent and
tetramethylsilane (δ = 0 ppm for 1H and 13C NMR) and phos-
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phoric acid (δ = 0 ppm for 31P NMR) as internal standards. IR
spectra were recorded with FTIR Shimadzu (IR Prestige-21) and
Perkin–Elmer Spectrum 100 FTIR spectrometers. UV/Vis absorp-
tion spectra were recorded with Shimadzu UV-1800 and 2550 spec-
trophotometers. Steady-state photoluminescence spectra were re-
corded with a FLS920-s Edinburgh spectrofluorimeter. Elemental
analyses were carried out by using an Elementar VARIO III instru-
ment. The particle sizes of the nanoaggregates were determined by
using a Malvern Zetasizer (MAL1040152) instrument. Microwave
reactions were carried out in a CEM Discover (mode l908010)
oven. PXRD patterns were recorded with a Rigaku Miniflex II
desktop X-ray diffractometer.
prepared. Then 2–3 drops of the solution were placed on a thin
glass substrate (2ϫ2 cm2) and the solvent allowed to evaporate
slowly.
Single-Crystal X-ray Diffraction Study: Single-crystal X-ray diffrac-
tion data were collected by using a Bruker AXS Kappa Apex II
diffractometer equipped with an Oxford Cryosystem 700Plus liquid
nitrogen based cooling device. The data sets were collected at 100 K
by using φ and ω scans such that the data were completed up to
60° in 2θ. Data reduction, scaling, merging, and space group deter-
minations were performed by using the APEX2[43] suite available
from Bruker AXS. The crystal structures were solved by direct
methods (SHELXS97)[44] available within Olex 2[45] suite and the
structures were refined by full-matrix least-squares refinement
using SHELXL97. All the hydrogen atoms were geometrically fixed
at their calculated positions and refined by using the riding model.
Geometric calculations were carried out by using PARST97.[46]
All the reactions were performed under nitrogen and monitored by
TLC (TLC plates precoated with 0.20 mm silica gel). Cyclic vol-
tammetry measurements were performed with a Potentiostat/
Galvanostate Model 263 A instrument. The platinum, glassy car-
bon, and Ag/AgCl electrodes were used as counter, working, and
reference electrodes, respectively, and the scan rate was maintained
at 50 mVs–1. The complexes and 0.1 m lithium perchlorate (LiClO4,
100 mg, as supporting electrolyte) were dissolved in acetonitrile
(10 mL). The experiments were conducted under an inert atmo-
sphere. The volume occupied by the solvent molecules (void space
in the unit cell of the crystal lattice) was calculated by WinGx.[42]
DFT Quantum Chemical Calculations: The ground-state geometries
of complexes 1 and 2 were optimized by using density functional
theory (DFT) with the B3LYP hybrid functional[47] starting from
their respective crystal structures.
A double-zeta basis set
(LANL2DZ) and the effective core potential was used on the irid-
ium atom.[48a–48c] Diffusion functions were added to the chlorine
and hydrogen atoms coordinated to iridium by using the 6-
311++G(3df,3pd) basis set. The two phosphorus atoms and the
two nitrogen atoms coordinated to the iridium were assigned by
using the 6-31G** basis set. The remaining carbon and hydrogen
atoms were assigned by using the 6-31G* and 3-21G basis sets,
respectively. Time-dependent DFT (TDDFT)[49] calculations were
performed on the lowest-energy singlet ground state to probe the
absorption and emission properties using the same functional and
basis sets. As for the experiment, calculations were performed in
dichloromethane solution (ε = 8.93) using the polarizable con-
tinuum model.[50] The 10 lowest singlet and triplet roots of the non-
Hermitian eigenvalue equations were obtained to determine the
vertical excitation energies. Oscillator strengths were deduced from
the dipole transition matrix elements for singlet states only. All the
calculations were performed by using the GAMESS-US[51] soft-
ware. Canonical molecular orbital analysis (CMO)[52] was per-
formed to analyze the composition and bonding nature of the mo-
lecular orbitals (MOs) by using the NBO 5.0 software.[53] CMO
analysis provided effective information on the bonding nature of
MOs such as HOMO–x and LUMO+y and the energies of individ-
ual MOs. Partial charge transfer (CT) was characterized for the
HOMO–x to LUMO+y transition as shown in Equation (2)
Synthesis of 1 and 2: Triphenylphosphine (622 mg, 2.348 mmol) was
added to iridium(III) chloride (200 mg, 0.6711 mmol) in 2-ethoxy-
ethanol (15 mL) and the reaction mixture was heated at reflux at
135 °C for 4 h. Then 1,10-phenanthroline (241 mg, 1.342 mmol)
and sodium carbonate (211 mg, 2.013 mmol) were added and the
reaction mixture further heated at reflux for 3 h. The resulting reac-
tion mass was then cooled to room temperature and the crude
product was dried under reduced pressure in a rotary evaporator.
Potassium hexafluorophosphate (100 mg) was mixed with the crude
product in methanol (4 mL) and the solution was heated in a mi-
crowave oven (MW) for 10 min at 60 °C (pressure, 100 psi; power,
100 W). The reaction mass was cooled to room temperature and
the solid residue was collected by filtration and washed with cold
methanol several times to obtain pure complex 2. Then the mother
liquor was evaporated under reduced pressure to dryness to collect
complex 1 (mixed with a little impurity of 2). Complex 1 was puri-
fied by column chromatography eluting with methanol/dcm (1:10;
Rf = 0.55; Scheme 1). Single crystals of both complexes suitable
for X-ray diffraction were grown from dcm/hexane (1:1) at room
temperature. Data for 1: Yellow solid; yield: 35.77%. 1H NMR
(400 MHz, CDCl3): δ = 8.46 (d, J = 5.1 Hz, 2 H), 8.30 (d, J =
8.1 Hz, 2 H), 7.91 (s, 2 H), 7.27–7.17 (m, 19 H), 7.13 (m,10 H),
7.05–6.92 (m, 3 H), –19.24 (t, J = 16.4 Hz, 2 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 133.00, 132.14, 132.04, 131.98, 131.96,
128.58, 128.46 ppm. 31P NMR (162 MHz, CDCl3): δ = 20.88 ppm.
CT(M) = [%(M)HOMO–x] – [%(M)LUMO+y]
(2)
in which %(M)HOMO–x and %(M)LUMO+y are the percentages
of metal character obtained by CMO analysis. When the contri-
butions to an excited state come from multiple single-electron exci-
tations, the metal CT character is described[54] by Equation (3)
IR (KBr): ν = 2179 (m, νIr–H) cm–1. C48H40F6IrN2P3 (1043.99):
˜
calcd. C 55.22, H 3.86, N 2.68; found C 55.28, H 3.77, N 2.75.
Data for 2: Pale-green solid; yield: 11.06% 1H NMR (400 MHz,
CDCl3): δ = 9.14 (d, J = 5.0 Hz, 1 H), 8.49 (d, J = 7.7 Hz, 1 H),
8.35 (d, J = 8.2 Hz, 1 H), 8.13 (q, J = 8.9 Hz, 2 H), 7.71–7.53 (m,
2 H), 7.39–7.26 (m, 6 H), 7.27–7.04 (m, 24 H), 6.61 (dd, J = 8.2,
5.5 Hz, 1 H), –17.56 (t, J = 14.3 Hz, 1 H) ppm. 13C NMR
(101 MHz, CDCl3): δ = 133.03, 132.13, 132.04, 131.99, 131.97,
128.57, 128.45 ppm. 31P NMR (162 MHz, CDCl3): δ = 29.49 ppm.
IR (KBr): 2152 (m, νIr–H) cm–1. C48H39ClF6IrN2P3 (1078.43):
calcd. C 53.46, H 3.62, N 2.60; found C 53.43, H 3.60, N 2.65.
(Scheme 1).
CTi(M) = Σ[Ci(i – j)]2[(M)i – (M)j]
(3)
in which Ci(i – j) are coefficients expressed as the excitation ampli-
tudes corresponding to transitions between i and j states. The di-
pole moments of the molecules were computed in the optimized
ground states by using the dipole moment analysis[53] module of
NBO. In the ground state, the total NBO dipole moment reported
includes delocalization correction.
Supporting Information (see footnote on the first page of this arti-
cle): Additional absorption/emission spectra, DFT results, crystal
structural data for 1 and 2.
Fabrication of Thin Films on Thin Glass Substrates for Photolumi-
nescence Measurements: The 10–3 m solutions of 1 and 2 in thf were
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The authors thank the Department of Science and Technology
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Received: March 27, 2014
Published Online: July 15, 2014
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim