Exploring the Origin of Aggregation Induced Emission Activity and Crystallization Induced Emission in Organometallic Iridium(III) Cationic Complexes: Influence of Counterions

Article abstract of DOI:10.1021/acs.cgd.6b00810

A new cationic iridium(III) complex exhibiting aggregation induced emission (AIE) activity, [Ir(PPh₃)₂(bipy)(H)₂]A (bipy = 2,2′-bipyridine; A = counterions), has been synthesized in a straightforward synthetic route. Interestingly, the emission color of solid state samples of this complex varied with different counteranions [A = Cl^-, BF₄^-, PF₆^-, N(CN)₂^-] and crystallization induced emission was observed. The emission properties for the compounds with A= Cl^-, PF₆^- are discussed by analyzing the crystal packing, the frontier molecular orbitals, and the calculation of the relevant low-lying excited states using time-dependent density functional theory. The restriction of internal rotation of the phenyl rings in the phosphine ligands due to intermolecular interactions is suggested as the most plausible origin of the observed AIE effect in these crystals.

Full text of DOI:10.1021/acs.cgd.6b00810

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Article
Exploring the Origin of ‘Aggregation-Induced Emission’
Activity and Mechanofluorochromism in Organometallic
Iridium(III) Cationic Complexes: Influence of Counterions
Parvej Alam, Clàudia Climent, Gurpreet Kaur, David Casanova, Angshuman
Roy Choudhury, Ashish Gupta, Pere Alemany, and Inamur Rahaman Laskar
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00810 • Publication Date (Web): 09 Aug 2016
Downloaded from http://pubs.acs.org on August 10, 2016
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Exploring the Origin of ‘Aggregation Induced
Emission’ Activity and ‘Crystallization Induced
Emission’ in Organometallic Iridium(III) Cationic
Complexes: Influence of Counterions
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a‡
b‡,
c
d,e
Parvej Alam , Clàudia Climent Gurpreet Kaur , David Casanova ,
c
f
*b
Angshuman Roy Choudhury , Ashish Gupta , Pere Alemany , Inamur
*a
Rahaman Laskar
a
Department of Chemistry, Birla Institute of Technology and Science, Pilani, Rajasthan, India.
Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i
b
Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1ꢀ11, Barcelona 08028,
Spain.
c
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.
d
Kimika Facultatea, Euskal Herriko Unibertsitatea (UPV/EHU), Donostia International Physics
Center, Paseo Manuel de Lardizabal, 4, Donostia 20018, Spain.
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IKERBASQUE, Basque Foundation for Science, Bilbao 48013, Spain.
Samtel Center for Display Technologies, IIT Kanpur, Kanpur, (U.P), India†
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‡
Both authors have contributed equally to this work developing the experimental (P.Alam)
and computational (C.Climent) parts, respectively.
KEYWORDS. Iridium(III) complex, Aggregation Induced Emission (AIE), Counterions
dependent color, Crystallization Induced Emission, DFT calculations.
ABSTRACT: A new cationic iridium(III) complex exhibiting ‘aggregation induced emission
(
AIE)' activity, [Ir(PPh ) (bipy)(H) ]A (bipy = 2,2′ꢀbipyridine; A = counterions) has been
3 2 2
synthesized in a straightforward synthetic route. Interestingly, the emission colour of solid state
ꢀ
ꢀ
ꢀ
ꢀ
samples of this complex varied with different counteranions [A = Cl , BF , PF , N(CN) ] and
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crystallization induced emission was observed. The emission properties for the compounds with
ꢀ
ꢀ
A= Cl , PF are discussed by analyzing the crystal packing, the frontier molecular orbitals and
6
the calculation of the relevant lowꢀlying excited states using time dependent density functional
theory. The restriction of internal rotation of the phenyl rings in the phosphine ligands due to
intermolecular interactions is suggested as the most plausible origin of the observed AIE effect in
these crystals.
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INTRODUCTION
Smart solid state luminescent materials responsive to external stimuli such as shearing, grinding,
rubbing, solvent exposure, or temperature changes have received considerable attention in recent
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times. The principal interest in these materials stems from their ability to change their emission
colour as a response to an external perturbation that triggers changes in the network of weak nonꢀ
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covalent interactions between individual molecules in molecular aggregates. These materials
have an immense potentiality in various applications such as optical data recorders, mechanical
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sensors, security paper, deformation detectors or storage devices.
In order to enhance the
existing capabilities and guide the design of new improved materials, it is, however, necessary to
develop a deep understanding of the physical nature of the mechanisms involved in their
response to external perturbations.
In the field of luminescent compounds with technological applications, iridium(III) coordination
complexes have drawn enormous attention in the last decade, mainly because of their rich
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excitedꢀstate properties.
The presence of strong spinꢀorbit coupling (SOC) renowned it as
one of the most efficient known triplet emitters, resulting in bright phosphorescence observed
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even at room temperature.
Additionally, the long emissive lifetimes, large Stoke shifts, high
quantum yields, and the existence of straightforward synthetic routes for the preparation of these
compounds has made them specially attractive as novel materials for diverse stateꢀofꢀtheꢀart
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technological applications
such as flatꢀpanel displays, lighting systems or cell imaging.
For the ultimate application of these iridium(III) compounds in optoelectronic devices, it is
evident that an enhanced emission in the solid state would be highly desirable, so that
increasingly improved solid state quantum efficiencies should be obtained.
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In recent years, the aggregationꢀinduced emission (AIE)
as well as the enhanced
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phosphorescence emission in the solid state (EPESS)
terms has been coined to describe this
enhancement of the emission in crystalline or aggregated forms as compared to that in dilute
solutions. Although from an experimental point of view it is quite straightforward to identify the
presence of the AIE effect, in most of the cases, its physical origin still remains unclear. Several
alternative mechanisms have been proposed to rationalize its appearance for different families of
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compounds, including the restriction of intramolecular motions (RIM),
E/Z isomerizations,
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Jꢀaggregate formation,
twisted intramolecular charge transfer (TICT),
excitedꢀstate
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intramolecular proton transfer (ESIPT), conformational planarization, or excimer formation.
Although within a short span of time, the number of compounds showing AIE has increased
notably, investigations of the detailed mechanism causing this effect are still scarce and some
controversy on alternative explanations for the origin of AIE in some compounds can be found in
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the literature.
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Recently, some of us reported the synthesis of a new AIE active complex of iridium(III),
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[
Ir(PPh ) (bipyꢀH)(Cl)(H)] or [Ir(bipyꢀH)] (bipyꢀH = κ ꢀN,Cꢀ2,2’ꢀbipyridine) obtained by a
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”
rollover” cyclometalation reaction in which the bipyridine ligand binds to the metal center in a
C^N mode instead of the usual N^N one. For this complex, reversible protonation and
deprotonation of the nonꢀcoordinated nitrogen atom in the bipyridine ligand is possible, both in
solution and in the solid state, leading to a shift in the emission colour that depends on the pK of
a
the employed acid. In addition to this rare rolloverꢀtype complex, a very similar cationic complex
+
+
[
Ir(bipy)H (PPh ) ] or [Ir(bipy)] (1) was also obtained with the expected N^N coordination
2 3 2
mode for the bipyridine ligand. In the present work, we explore the differences in the solid state
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ꢀ
emission of 1 depending on the nature of the counterions (Cl , N(CN) , BF and PF ) used to
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obtain the crystals. Since all the studied solidꢀstate samples for this cationic complex exhibit a
more or less pronounced AIE activity regardless of the counterion that they contain, a careful
comparison of their crystal structures offers an excellent opportunity to try to shed some light in
the effects of the crystal environment and intermolecular interactions on their AIE activity.
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, 63ꢀ67
One of the complexes was also found to exhibit crystallization induced emission (CIE).
This unusual behavior refers to colour changes of the complex in the solid state when it is put
under stress as for instance by mechanical grinding, a phenomenon that can be greatly exploited
for several applications.
Scheme 1. Structure of [Ir(bipy)H (PPh ) ]A
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EXPERIMENTAL SECTION
Materials
Iridium(III) chloride hydrate, 2,2'ꢀbipyridine, triphenylphosphine and 2ꢀethoxyethanol were
purchased from the Sigma Aldrich Chemical Company Ltd., while sodium borohydride,
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potassium hexafluorophosphate, sodium dicyanamide, as well as all spectroscopic grade solvents
(dichloromethane (DCM), methanol, etc.) were procured from the Merck Company.
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Synthesis, characterization, and sample preparation of [Ir(bipy)H (PPh ) ]A (A = Cl ,
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N(CN) , PF , BF )
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To a stirred solution of IrCl ·3H O (0.5025 mmol) in 2ꢀethoxyethanol (6 mL), triphenyl
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phosphine (1.507 mmol) was added and the reaction mixture refluxed at 130°C for 4h.
Afterwards, the reaction mixture was basified with an excess of Na CO (~10 mmol). Then, 2,2'ꢀ
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bipyridine (1.252 mmol) was added to the reaction mixture which was refluxed further for 3h.
The resulting reaction mass was cooled to room temperature. The final solid mass contained two
products, [Ir(bipyꢀH)(PPh ) (H)(Cl)] (minor) and [Ir(bipy)H (PPh ) ]Cl (major), and it was
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purified by column chromatography. Both products were further recrystallized using a mixture of
DCM and hexane (1:1).
The ion exchange reaction was carried out in a microwave reactor (MW) and it was completed
within a very short period of time. The methanolic solution (2 mL) of [Ir(bipy)H (PPh ) ]Cl was
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placed in a microwave tube along with NaBF /KPF /Na[N(CN) ] and 2 mL methanol. The whole
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reaction mixture was heated at 80 C for 5 minutes. After cooling, the solid product was separated
out from the reaction mixture and purified by recrystallization with a mixture of
methanol/hexane (~70ꢀ80%). Single crystals of [Ir(bipy)]Cl and [Ir(bipy)]PF were grown using
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toluene and hexane/DCM mixture, respectively.
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H, C and P NMR spectra were recorded in a 400 MHz Bruker spectrometer using CDCl and
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DMSOꢀd as solvents. Tetramethylsilane (TMS, δ = 0 for H and C NMR) and phosphoric acid
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(H PO , δ = 0 for P NMR) were used as internal standards. UVꢀVIS absorption spectra in
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solution state were recorded in a Shimadzu Spectrophotometer (model UVꢀ1800), while
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absorption spectra for thin films were recorded in a Shimadzu Spectrophotometer (model UV
2550). The steady state photoluminescence spectra were recorded on a FLS920ꢀs Edinburgh
spectrofluorometer, while the solution state and solid state (thin film) photoluminescence (PL)
spectra were recorded on Shimadzu (A40195003382SA) and Fluoromaxꢀ4 (0406Cꢀ0809)
spectrofluorophotometers, respectively. The solid state quantum yield of the powder samples
were measured using a calibrated integrating sphere in a Gemini spectrophotometer (Gemini
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180). Highꢀresolution mass spectrometry (HRMS) experiments were carried out with a (TOF MS
ES+1.38 eV) VG Analytical (70ꢀS) spectrometer and QꢀTof micro mass spectrometer. Infrared
spectra were recorded using FTIR Shimadzu (IR prestigeꢀ21) and Perkin Elmer Spectrum 100
FTIR spectrometers. Microwave assisted reactions were carried out in a CEM Discover (model
l908010) reactor. Powder XRD (PXRD) data were measured by using a Rigaku miniflex II
desktop Xꢀray diffractometer. All the reactions were performed under nitrogen atmosphere and
their progress monitored using thinꢀlayer chromatography (TLC) plates (preꢀcoated with 0.20
mm silica gel). The whole experiment was conducted under inert atmosphere.
1
The H NMR signal of the metalꢀhydride peak appeared in the range of ꢀ19.55 to
ꢀ19.46 ppm as a triplet with a spinꢀspin coupling constant value, J(PꢀH) ~16.7 Hz, which
was in accordance with each of the hydrides lying cis to two phosphorus atoms. There
were no significant differences found in the m/z values of the HRMS spectra for
complexes [Ir(bipy)H (PPh ) ]A, where A= Cl, BF , PF and N(CN) , showing that in all
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four cases the same cationic species was present (Scheme 1). The presence of counterions
was confirmed by measuring FTIR stretching frequencies. The complex containing the
ꢀ
PF6 counterion was earlier synthesized using [Ir(H) (PPh ) (acetone) ]BF as a
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ꢀ1
70
ꢀ
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precursor
which showed a strong peak at 840 cm (ν ), the BF containing
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ꢀ1
71
ꢀ
complex a stretching frequency at 1058 cm (ν ), while the complex with a N(CN)
BꢀF
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counterion showed peaks at 2128 and 2184 cm (CN), respectively.
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[
Ir(bipy)H (PPh ) ]Cl: H NMR (400 MHz, CDCl ) δ 9.00 (d, J = 8.5 Hz, 2H), 7.92 (t, J = 7.3
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Hz, 4H), 7.30 (d, J = 6.8 Hz, 5H), 7.28 – 7.19 (m, 23H), 6.64 (dd, J = 7.2, 5.1 Hz, 2H), ꢀ19.55 (t,
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J = 16.9 Hz, 2H); P NMR (162 MHz, CDCl ) δ 19.16; C NMR (101 MHz, CDCl ) δ 156.02,
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54.19, 137.92, 133.13, 133.07, 133.01, 131.64, 131.37, 131.10, 130.21, 128.35, 128.30, 128.25,
ꢀ1
+
1
26.24, 125.72.IR (KBr, cm ): 2141, 2168 (m, ν ); ESIꢀHRMS, calculated: ([M] ), m/z
IrꢀH
+
875.2296, found: ([M] ), m/z 875.2355 (Figure S1).
1
[
Ir(bipy)H (PPh ) ]N(CN) : H NMR (400 MHz, CDCl ) δ 8.33 (d, J = 8.2 Hz, 1H), 8.03 (d, J
2
3 2
2
3
=
5.3 Hz, 1H), 7.87 (t, J = 7.8 Hz, 1H), 7.40 – 7.17 (m, 14H), 6.75 – 6.67 (m, 1H), ꢀ19.51 (t, J =
3
1
13
16.7 Hz, 1H); P NMR (162 MHz, CDCl ) δ 20.52; C NMR (101 MHz, CDCl ) δ 155.76,
3
3
1
54.52, 137.57, 133.11, 133.05, 132.99, 131.51, 131.24, 130.97, 130.33, 128.43, 128.38, 128.33,
ꢀ1
1
26.45, 124.28, 120.31. IR (KBr, cm ): 2221, 2190 (m, ν ), 2128 (s, ν ), 2184(m, ν ); ESIꢀ
IrꢀH
CN
CN
+
+
HRMS, calculated: ([M] ), m/z 875.2296, found: ([M] ), m/z 875.2355 (Figure S2).
1
[
Ir(bipy)H (PPh ) ]PF : H NMR (400 MHz, DMSO) δ 8.33 (d, J = 5.3 Hz, 1H), 8.22 (d, J =
2
3 2
6
8.2 Hz, 1H), 7.83 (t, J = 7.4 Hz, 1H), 7.47 – 7.21 (m, 15H), 7.01 – 6.84 (m, 1H), ꢀ19.46 (t, J =
31
ꢀ1
1
6.7 Hz, 1H); P NMR (162 MHz, DMSO) δ 19.17.IR (KBr, cm ): 2136, 2160 (m, νIrꢀH) 840 (s,
+
+
ν ); ESIꢀHRMS, calculated: ([M] ), m/z 875.2296, found: ([M] ), m/z 875.2356 (Figure S3).
PꢀF
1
[Ir(bipy)H (PPh ) ]BF : H NMR (400 MHz, DMSO) δ 8.33 (d, J = 5.3 Hz, 1H), 8.22 (d, J =
2
3 2
4
8
1
1
.2 Hz, 1H), 7.84 (t, J = 7.4 Hz, 1H), 7.42 – 7.20 (m, 14H), 6.98 – 6.89 (m, 1H), ꢀ19.46 (t, J =
3
1
ꢀ1
6.7 Hz, 1H), P NMR (162 MHz, DMSO) δ 19.64.IR (KBr, cm ): 2220, 2150 (m, νIrꢀH),
+
+
058(m, ν ); ESIꢀHRMS, calculated: ([M] ), m/z 875.2296, found: ([M] ), m/z 875.2374
BꢀF
(Figure S4).
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X-ray single crystal diffraction study
Single crystal Xꢀray diffraction data for [Ir(bipy)H (PPh ) ]Cl and [Ir(bipy)H (PPh ) ]PF were
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recorded on a Rigaku XtaLAB mini single crystal Xꢀray diffractometer equipped with a Mercury
75/M CCD detector and graphite monochromated Mo – K radiation. The data set was collected
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α
at 100.0(1) K by using an Oxford cryosystem. The data were processed using the Rigaku Crystal
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SHELXL2013 available within Olex2 .
Computational details
Ground state optimizations and frequency calculations were carried out within the framework of
76ꢀ78
density functional theory (DFT) with the B3LYP
hybrid functional in combination with a
7
9
basis set of doubleꢀζ quality (LANL2DZ) and the effective core potential of Hay and Wadt for
iridium, while the standard 6ꢀ31G(d) basis set was used for the rest of atoms in the molecule.
Vertical transition energies were calculated using time dependent density functional theory (TDꢀ
DFT) with the same exchangeꢀcorrelation functional and basis sets. The geometry for the lowest
excited triplet state was also optimized at the same level of theory.
In order to have a reference structure for the isolated cation, the optimization calculations have
been carried out in the vacuum. Since in this study we are only seeking for a qualitative
explanation of the experimentally observed trends by comparing the calculations for different
geometries of the same cation, we have not considered the necessity of accounting for the effects
of solvation on the photophysical properties in solution by including a solvent model in our
calculations. A guess of the atomic positions of the two hydrogen atoms coordinating the iridium
atom in the [Ir(bipy)H (PPh ) ]Cl and [Ir(bipy)H (PPh ) ]PF crystals has been obtained through
2
3 2
2
3 2
6
restricted geometry optimizations in which all atoms have been frozen in the geometry obtained
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from Xꢀray diffraction and only the two hydrogen atoms were allowed to move in the energy
minimization procedure. The basis set used in this calculation was 6ꢀ31G(d,p). Throughout the
paper, whenever the cation in the crystal’s geometry is mentioned, it will correspond to this latter
geometry. These restricted optimizations were also carried out in the presence of nearby
counterions, but practically the same final positions for the hydrogen atoms were obtained. The
ground state and the lowest triplet state of cation 1 were optimized starting from the monomer's
geometry in the Ir(bipy)H (PPh ) ]Cl and [Ir(bipy)H (PPh ) ]PF crystals. The same ground state
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was obtained starting from either geometry, while for the triplet states some differences in the
relative orientations of the phenyl rings of the phosphine ligands were observed. Nevertheless,
the pseudooctahedral environment of the iridium atom and the geometry for the bipyridine ligand
that determine the energy for the emission band were practically identical in the two geometries
for the T state. For this reason, we will refer only to one optimized S and T geometry for the
1
0
1
cation throughout the paper. All electronic structure calculations reported in this study were
76
performed with the GAUSSIAN 09 suite of programs.
RESULTS AND DISCUSSION
Molecular and crystal structure
The principal building block in the synthesized crystals is a cationic iridium(III) complex 1 with
a distorted octahedral coordination sphere (Figure 1), as confirmed by the SCꢀXRD analysis
obtained for the [Ir(bipy)]Cl and [Ir(bipy)]PF crystals. A bipyridine ligand along with two
6
hydride ligands lies on the equatorial plane while the axial positions are occupied by two
triphenylphosphine ligands. The chiral propellerꢀlike arrangement of the phenyl rings in the
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triphenylphosphine ligand allows the possibility of having enantiomeric molecules. Indeed, the
two possible enantiomers are present in the two studied crystal structures.
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(
a)
(b)
(c)
Figure 1. Structures of 1 in the (a) [Ir(bipy)]Cl and (b) [Ir(bipy)]PF crystals with the optimized
6
hydride ligands and (c) the superposition of their most overlapping enantiomers. Hydrogen
atoms on the ligands are not shown for the sake of clarity.
Table S1 and S2 contains the principal geometrical parameters of the Ir coordination sphere for
the two experimental Xꢀray diffraction structures, revealing that the two geometries are rather
similar. The main departure from a perfect octahedral environment for the iridium atom
corresponds to the offꢀaxis distortion of the two phosphorous atoms, with PꢀIrꢀP angles on the
side of the two hydride ligands about 15º smaller than in a perfect octahedron (180º). Bond
distances between the central metal atom and the coordinating atoms on the ligands are very
similar in the two crystal structures, except for one of the IrꢀN bonds, which is approximately
0
.05 Å longer in the [Ir(bipy)]PF crystal. This is because the bipyridine ligand in both structures
6
has some differences in the adopted geometry, probably due to intermolecular forces induced by
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the bulky phenylphosphine ligands. Nevertheless, these differences do not have a great impact on
the electronic structure of the cation, as suggested by our calculations discussed below. The
major differences between the two geometries essentially correspond to the relative disposition
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of the phosphine ligands (Figure 1c). In the [Ir(bipy)]PF crystal, the two sets of phenyl rings of
6
the two phosphine groups in trans disposition are closer to an eclipsed conformation than in the
[
Ir(bipy)]Cl case, as indicated by their CꢀPꢀPꢀC dihedral angles (Table S1). In close relation to
this distortion, we notice that the molecular geometries for the two crystals exhibit different
degrees of rotation for some of the phenyl rings.
In the unit cell of both crystals, there are four cations and four counterions, but the [Ir(bipy)]PF6
crystal contains six additional DCM molecules. The four cations in the unit cell are symmetry
equivalent, although in the two crystals they appear as two pairs of enantiomers. In both cases,
the cations are arranged in chains along the aꢀdirection. A more detailed discussion of their
disposition along these chains will be given later on, since these are suggested to play a role in
the observed aggregation induced emission effect.
Photophysical properties
+
The qualitative molecular orbital diagram in Figure 2 summarizes the nature of the [Ir(bipy)]
ꢀ
frontier molecular orbitals obtained for the cation’s geometry in the Cl crystal. Due to the loss of
octahedral symmetry, the metal centered t ꢀ and e ꢀtype orbitals, shown in red in the diagram,
2
g
g
mix significantly with themselves as well as with other ligand centered orbitals. Since the PꢀIrꢀP
angle is less than 180º, we expect the interaction with the phosphine lone pairs to cause a
splitting of the t orbitals by destabilizing the d orbital with respect to the d and d orbitals,
2g
xz
xy
yz
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0
which exhibit a small energy splitting between them. As a result, the HOMO in the present
compounds is mainly an iridium d orbital as shown in the diagram, while the other two t2g
xz
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orbitals lie well below in energy, with several πꢀtype orbitals of the phenyl rings of the
phosphines lying in between the highest occupied molecular orbital (HOMO) and the other two
t ꢀtype orbitals. Due to their MꢀL antibonding character, the e ꢀtype orbitals are strongly
2
g
g
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destabilized and appear rather high in energy with a significant mixture with ligand orbitals. A
large number of ligand centered π*ꢀtype MOs either from the phenyl rings of the phosphines or
from the bipyridine appear between the t2g and the e MOs, with the lowest unoccupied
g
molecular orbital (LUMO) being a bipyridine centered π*ꢀtype orbital (Figure 2). The electron
density distribution associated with the HOMO and the LUMO are virtually identical for the
cation in the two crystal structures (See supplementary information, Figure S5).
The absorption and emission spectra at room temperature for all four [Ir(bipy)]A complexes in
DCM solution are shown in Figure 3. The higher energy bands in the UVꢀvis absorption spectra
for all four compounds were found between 250ꢀ300 nm and are assigned to spinꢀallowed
1
ligandꢀcentered (LC) π–π* transitions. The lower energy bands appearing in the 350 ꢀ 425 nm
1
3
range are assigned to metalꢀtoꢀligand charge transfer ( MLCT and MLCT) transitions. As
expected, a similar pattern is observed for the absorbance and emission profiles of all four
compounds in DCM solution (Figure 3a). Emission λmax values are shown in Table 1.
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Figure 2. Molecular orbital energy diagram of 1 in the [Ir(bipy)]Cl crystal’s geometry and
corresponding HOMO and LUMO. Colour code: red for dꢀblock orbitals of the iridium atom;
green for π and π* type orbitals of the phenyl rings of the phosphine ligands; blue for π*ꢀtype
orbitals of the bipyridine ligand and orange for σꢀtype orbitals of the phosphorous’s lone pairs
and the hydrogen atoms bonded directly to iridium.
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(
a)
(b)
ꢀ
ꢀ
ꢀ
Figure 3. Absorption (a) and emission (b, λexc 385 nm) spectra for [Ir(bipy)]A [A = Cl , BF ,
4
ꢀ
ꢀ5
PF , N(CN) ] compounds for 1x10 M solutions in DCM.
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2
Absorption and emission spectra of [Ir(bipy)]A crystals with different counterions are shown in
Figure 4. While the absorption spectra are similar to those in DCM solution, the emission ones
have a much more structured profile. The computed energy for the cation’s LUMO is very
similar in the two compounds for which we were able to solve their crystal structure, i.e.
[Ir(bipy)]Cl and [Ir(bipy)]PF , while the HOMO is 0.15 eV lower in energy in the
6
[
Ir(bipy)H (PPh ) ]PF crystal (see supplementary information, Table S3), leading to a 0.1 eV
2 3 2 6
ꢀ
smaller HOMOꢀLUMO gap for the cation in crystals with PF . This is indeed the difference
6
between the calculated excitation energies to the lowest excited triplet state of the cation, which
are computed at 454 nm and 433 nm for [Ir(bipy)]PF and [Ir(bipy)]Cl, respectively. The lowest
6
singlet and triplet excited states have a predominant HOMO to LUMO contribution (>80%),
except for the lowest triplet state of the cation in the [Ir(bipy)]PF crystal, for which the HOMO
6
to LUMO contribution accounts just for 60% of the transition. The remaining contributions are
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quite small and they basically belong to excitations to the LUMO from a dꢀtype orbital of the
iridium atom (~15%) or a πꢀtype orbital of the bipyridine ligand (10%). Nevertheless, we can
assign a strong metal to bipyridine charge transfer (MLCT) character to the S and T states. The
1
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vertical transition energies to the lowest excited singlet state are computed at 405 nm and 419 nm
for the cation in the [Ir(bipy)]Cl and [Ir(bipy)]PF crystals respectively, suggesting that they are
6
responsible of the observed band just around 400 nm in the absorption spectra.
(a)
(b)
Figure 4. Absorption (a) and emission (b, λexc 385 nm) spectra in the solid state of the
ꢀ
ꢀ
ꢀ
ꢀ
[
Ir(bipy)]A crystals with A = Cl , N(CN) , BF and PF counterions.
2 4 6
Table 1. Wavelengths corresponding to the maximum emission intensity along with the vibronic
progressions of the crystals with different counterions in the solid state and in DCM solution.
The wavelength corresponding to the maximum emission peak is indicated in boldface.
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Counterion
Wavelength / nm
Quantum yield
Life time
Solution
(DCM)
Solid State
Solution
DCM)
%)
Solid
State
Solution
(DCM)
Aggregate
State
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(
(
(f =90%)
H
(%)
(ns)
Cl-
506
508
448 (sh) 512
446 (sh) 479,
0.018
0.019
19.85
33.64
0.95
0.92
1.6
1.8
PF6-
508
N(CN)2-
BF4-
506
505
446 (sh), 513
0.011
0.024
6.75
0.93
0.94
1.6
1.9
448, 479, 514
(sh)
26.76
It is worth to stress that the computed vertical energies to the lowest triplet state (T ) cannot be
1
directly compared to the phosphorescence emission spectra (Figure 4), since the present
calculations neither account for geometry relaxation of the triplet state, nor take into
consideration vibronic coupling effects. The latter will be discussed below. Nevertheless, our
calculations lead us to rule out intramolecular effects as responsible for the different
phosphorescent emission properties observed for the [Ir(bipy)]Cl and [Ir(bipy)]PF crystals.
6
A notable difference in the emission spectra of the two crystals is the much better resolution of
the vibrational progression structure in the [Ir(bipy)]PF case in comparison to the [Ir(bipy)]Cl
6
crystal. Normal mode analysis for the cation at the minimum energy geometry for S reveals that
0
there are two breathing modes of the bipyridine ligand that may be assigned to the vibrational
progression of the emission spectra. They consist of coupled asymmetricꢀsymmetric and
symmetricꢀsymmetric breathing modes of each pyridine unit of the ligand (Figure S6), and their
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ꢀ1
computed wavenumbers are 1030 and 1038 cm respectively, in fairly good agreement with the
,
ꢀ1
gap observed in the vibrational structure of the experimental spectra (1050 cm ). In the
8
1
literature the appearance of a resolved vibronic structure in the emission band of similar
octahedral complexes has been related to a lower MLCT character and more ligandꢀcentered πꢀ
π* type character (LC) of the excited state. As discussed above, our computational study does
not suggest that the nature of the emitting triplet state should differ significantly in the two
crystals, so that we expect a similar MLCT character for emission in the two cases and for this
reason we suggest that the differences observed in the vibronic structure of the cation in the two
studied crystals are caused by different intermolecular interactions. A difference in the colour of
the emitted light was observed for the four compounds under excitation at 365 nm, as shown in
Figure 5. The values of the xy chromaticity coordinates (Table S4) and further details regarding
their calculation may be found as Supplementary Information.
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ꢀ
ꢀ
Figure 5. Left. Pictures of the emitting [Ir(bipy)]A crystals with different counter ions: Cl , BF ,
4
ꢀ
ꢀ
82
PF and N(CN) .Right. Chromaticity diagram (standard CIE (2º) 1931 colour space ) for the
6
2
synthesized complexes in DCM and solid state.
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ꢀ
Amongst the four different crystals, the one with BF shows a more bluish colour than the rest,
4
which follow a trend in their emission colours, going from greenish to more yellow as we move
ꢀ
ꢀ
ꢀ
from PF to N(CN) and Cl . On the other hand, as expected, the colour of the weak emission of
6
2
all the complexes in DCM is very similar, showing that the differences arise from intermolecular
interactions upon aggregation.
Aggregation Induced Emission
All four investigated compounds present quite weak emission intensities when dissolved
in common organic solvents, such as DCM, trichloromethane or tetrahydrofuran, while in
the solid state their phosphorescence emission becomes much more intense. To
investigate and characterize the observed AIE effect, the four complexes were dissolved
in ethyl acetate and then a gradually increasing amount of hexane was added (hexane
ꢀ4
ꢀ1
fraction (fH) 0ꢀ90%, concentration=1x10 molL ). Since the complexes are not soluble in
hexane, the extent of aggregation should increase with higher hexane fractions. As shown
in Figure 6 and 7, the PL intensity of [Ir(bipy)]BF increases steadily with the fraction of
4
hexane (fH = 0ꢀ70%) in the mixture, while a sharp enhancement of the PL intensity was
observed when increasing the hexane fraction from 70% to 90%. The PL intensity at H
f
=
9
0% was found to be about 25.6 times higher than for the solution in pure ethyl acetate.
Similarly, the PL intensity of compounds [Ir(bipy)]PF and [Ir(bipy)]Cl increased at fH
=
6
9
0% to 27.9 and 116 times that in ethyl acetate solutions, respectively. In case of
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[
Ir(bipy)]N(CN) , the PL intensity increased slowly to fH = 70%, but at fH = 90% a
2
decrease of the intensity was observed. The PL intensity of [Ir(bipy)]N(CN) , at fH = 70%
2
and fH = 90% is about 7.8 and 6.9 times larger, respectively as compared to the intensity
for the parent solution in pure ethyl acetate.
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To further investigate the decrease of the PL intensity of complex [Ir(bipy)]N(CN) for f = 70%,
2
H
we took transmission electron microscopy (TEM) images with electron diffraction (ED) for
samples of this complex. The TEM image clearly showed the existence of nanowireꢀlike
aggregates (90ꢀ230 nm range) at f = 90%. The absence of a clear bright ring in electron
H
diffraction supported the formation of amorphous molecular aggregates (Figure 8). The absolute
quantum yields for [Ir(bipy)]A (A = Cl, PF , BF and N(CN) ) were estimated in powder form
6
4
2
and found to be 19.85% (Cl), 33.64% (PF ), 26.76% (BF ) and 6.75% (N(CN) ), respectively,
6
4
2
while the relative quantum efficiencies in solution (in DCM, with respect to coumarin 153
dissolved in degassed ethanol, QE = 0.38) were observed to be very low, i.e., 0.018% (Cl),
0
.019% (PF ), 0.024% (BF ), and 0.011% (N(CN) ). These facts support that all four compounds
6 4 2
have an enhanced emission in the solid state.
(a)
(b)
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(
c)
Figure 6. PL spectra for the [Ir(bipy)]A compounds in ethyl acetate/hexane mixtures with λexc at
85 nm. (a) BF , (b) PF , (c) Cl (d) N(CN) insets: evolution of the PL intensity of complexes at
(d)
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different f (0ꢀ90 %) fractions.
H
(a)
(b)
(c)
(d)
Figure 7. Luminescent images of [Ir(bipy)]A in ethyl acetateꢀhexane at different fractions of
ꢀ4
hexane (0% 50% 70% and 90% , left to right) keeping the 1x10 M concentration of the solution.
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Figure 8. (a) TEM image of nanoaggregates of [Ir(bipy)]A (A= N(CN) ) formed in ethyl
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acetateꢀhexane mixtures with 90% hexane fraction. (b) Electron diffraction (ED) pattern of the
nanoaggregates showing amorphous nature of the aggregate
Let us consider four possible sources for emission enhancement upon aggregation of the
coordination complexes studied in this work:
(
i)
Packing forces in the aggregated state induce a reorganization of the coordination
sphere of the metal atom large enough to change the nature of the emitting state. In this
scenario, the change of the coordination sphere should enhance spinꢀorbit coupling in the
emitting state as compared to that in diluted solutions with a result of an increase in the
efficiency of the radiative deactivation pathway.
(ii)
The aggregation process leads to strong intermolecular delocalization of the ground
state, the emitting state, or both. This change will affect mainly the efficiency of the
radiative pathway, although it is not easy to predict a priory if the emission intensity
should increase or decrease since it will depend largely on the degree of delocalization of
the emitting and the ground states and this degree of localization is difficult to describe
from a computational point of view.
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(
iii)
Packing forces in the aggregate state restrict the internal motion (RIM) in the cations,
most importantly internal rotations and low frequency, large amplitude vibrations in the
emitting molecule. The nonꢀradiative deactivation process is well described by a first
order rate equation whose constant depends on two parameters: the strength of the
coupling between the initial (emitting) state and the final (ground state) as well as the
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3ꢀ86
density of vibronic states of the ground state at the energy of the emitting state.
Considering that the presence of nearby molecules should not induce great changes in the
coupling between the initial triplet and the final singlet state, the leading ingredient
should be a decrease of the ground state's density of vibrational states. Steric congestion
and interlocking of mobile parts would hinder large amplitude movements upon
aggregation with an increase in their vibrational frequencies and the associated decrease
in the density of states leading to a smaller rate constant for the nonꢀradiative process.
Another possibility is that packing forces in the crystal could impose restrictions along
the molecular geometry relaxation on the emitting state potential energy surface.
Thus, the emitting state in the solid would be less distorted than in solution, leading to a
decrease of the FranckꢀCondon factors, which would result in a decrease of the nonꢀ
radiative decay rate constant
(iv)
Of course more than one of these mechanisms may occur simultaneously in a same crystal, and
discerning if there is a clear effect causing the enhancement of emission upon aggregation is
extremely difficult. The present case, where we have the same emitting cation in different
crystalline environments, offers an excellent opportunity to try to shed some light on these
questions, although we will unfortunately have to limit our study to the comparison between
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ꢀ
ꢀ
crystals with the Cl and PF counterions, for which we have been able to obtain Xꢀray resolved
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crystalline structures that will be considered to offer a proper model for the types of interactions
that may be present in amorphous aggregates.
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+
Since the principal features of the common [Ir(bipy)] cation at the molecular level, both
regarding its atomic and electronic structure have already been discussed, and intramolecular
effects don’t seem to be responsible for the enhanced emission in aggregate state, herein we will
focus on the role of the crystal structure. In particular, we will discuss the different molecular
packing modes and their possible implications in the electronic structure of the crystals such as
electronic delocalization and π–stacking.
Intra-chain interactions
+
The [Ir(bipy)] cations are packed into chains along the
a
ꢀdirection of the crystals (Figure
9
). The fundamental difference between the packing modes in the [Ir(bipy)]Cl and
[
Ir(bipy)]PF crystals is the way in which the molecules dispose with respect to the chain
6
direction. In the [Ir(bipy)]PF crystal, the molecular PꢀIrꢀP axis is oriented along the chain
6
axis (Figure 9a), while it lies approximately perpendicular to it in the [Ir(bipy)]Cl crystal
(
Figure 9b). This difference in the orientation of the molecule with respect to the chain
axis in the two cases may be important for the RIM mechanism, since in the [Ir(bipy)]PF6
crystal, for which a relatively stronger AIE effect is observed, interlocking between the
highly movable phenyl rings of neighbouring molecules may effectively restrict their
motion, while in the [Ir(bipy)]Cl crystal, the interaction between neighbours is mainly
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between the rigid bipyridine and mobile triphenylphosphine fragments, which may result
in a less effective RIM.
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(a)
c
a
(b)
a
b
Figure 9. Chains in the (a) [Ir(bipy)]PF and (b) [Ir(bipy)]Cl crystal structures. Colour code: IrꢀP
6
bonds (orange) and IrꢀN bonds (blue). Hydrogen atoms are not shown for the sake of clarity.
Inter-chain interactions
In the [Ir(bipy)]PF crystal structure, shown in Figure 10a in a view along the direction of
6
the chains, and taking the chain with the ligands coloured in red as our reference, there
ꢀ
6
are four chains of alternating PF counterions and DCM molecules forming a more or less
square prism around the reference chain. There are also four additional cation chains that
interact with the reference chain through the faces of this square prism. The neighbouring
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chains at the left and right side of the central one in the figure are shifted along the chain
direction with respect to the central chain so that the iridium atoms do not have the same
coordinate along the axis parallel to the chain direction, in contrast to the neighbouring
chains shown above and below the central one in the figure, which lie on the same plane
as the reference cation’s chain.
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In the [Ir(bipy)]Cl crystal, where anions are much smaller, each chain is surrounded by six
neighboring cationic chains (Figure 10b) in an arrangement reminiscent of a compact hexagonal
packing for perfect cylinders. In this case, the four neighboring chains shown to the left and the
right of the central one are also shifted along the chain direction with respect to the central chain,
while all the iridium atoms from chains represented above and below lie in the same plane
perpendicular to the aꢀdirection (yzꢀplane of Figure 10b). As it can be appreciated in the figure,
since in this case the PꢀIrꢀP axis is approximately perpendicular to the axis of the chains,
molecules from different chains face each other mainly through the triphenylphosphine ligands.
ꢀ
The Cl counterions occupy in this case the voids in the structure left between the rigid bipyridine
ligands of one molecule and the hydride ligands on the neighboring one.
(a)
(b)
c
b
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Figure 10. Interactions between chains that align along the aꢀdirection in the (a) [Ir(bipy)]PF6
and (b) [Ir(bipy)]Cl crystal structures.
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In both crystals, there are relevant interactions between some hydrogen atoms on the bipyridine
ligand or the phenyl rings and the counterions. In the [Ir(bipy)]Cl crystal, there are short
C−H···Cl contacts that range from 2.7 to 3.0 Å (Figure S7 and S8), while in the [Ir(bipy)]PF6
crystal, there are C−H···F interactions that lie in the range of 2.4 to 3.1 Å (Figure S9 and 10).
More important for our purposes, there are also significant interꢀchain interactions in the two
87, 88
cases. In the [Ir(bipy)]PF crystal, C−H···π interactions
are found between the cation chains
6
highlighted in blue and red in Figure 10a. These interactions between the rigid bipyridine
fragment and the mobile phenyl rings (Figure 11) may indeed play an important role in the
restriction of the motion of these rotors. No other relevant interactions between the rigid
fragments of the cations and their phenyl rings were found for the remaining chains of the
[
Ir(bipy)]PF crystal. In contrast, for the [Ir(bipy)]Cl crystal, the CꢀH···π distances are much
6
larger, excluding the possibility of this type of interactions to be relevant in this case. The
principal interactions between the neighboring chains and the reference one highlighted in red in
the [Ir(bipy)]Cl crystal (Figure 10b) are with the triphenylphosphine ligands on the chains
highlighted in blue, in which interlocking of the phenyl rings would restrict motions in the solid
state.
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Figure 11. Interꢀchain CꢀH···π in the [Ir(bipy)]PF crystal structure.
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The only interactions between chains that may have significant electronic implications in their
photophysical properties, that is electronic delocalization and/or πꢀstacking interactions, are
those between the chains highlighted in red and blue in the [Ir(bipy)]PF crystal (Figure 10a),
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where the πꢀsystems of their bipyridine ligands are susceptible to have a non negligible overlap
(Figure 12). The shortest CꢀC distance between the two coplanar bipyridine ligands is
approximately 3.47 Å. Although the distance between the two ligands is short enough to be
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below the upper limit of what may be considered as a πꢀstacking interaction,
the vertical
overlap between them is very poor, since the two ligands are quite displaced with respect of a
complete alignment, limiting the overlap of πꢀtype orbitals to just two carbon atoms on each
bipyridine fragment. In light of this qualitative analysis, we can exclude significant electronic
cooperative effects between neighboring complexes as the source of the different emission
properties in the [Ir(bipy)]Cl and [Ir(bipy)]PF crystals.
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Summarizing the previous discussion, we arrive at the following conclusions: the involvement of
the phenyl rings of the flexible triphenylphosphine ligand in intermolecular interactions is clearly
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recognizable in the two crystal structures. Namely, as interlocking interactions between
neighboring molecules in each chain in the [Ir(bipy)]PF crystal with additional attractive
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C−H···π interactions between chains and interlocking of the phenyl rings of neighboring
molecules on different chains in the [Ir(bipy)]Cl crystal. Since all other sources of possible AIE
can be discarded from the comparative analysis of the crystal structures, it seems plausible to
assume that the establishment of these intermolecular interactions will significantly hinder the
molecular motion of the phenyl rotors in the aggregated state.
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(a)
(b)
Figure 12. Side view (a) and top view (b) of a possible πꢀstacking interaction between two
cations in the [Ir(bipy)]PF crystal.
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Molecular relaxation of the emitting state
The optimized geometry of the lowest excited triplet state of 1 has important differences with
respect to the ground state optimized geometry and the molecular structure in the crystal. In fact,
our calculations point towards a decrease of the PꢀIrꢀP angle of approximately 20º upon
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