New ionic dinuclear Ir(iii) Schiff base complexes with aggregation-induced phosphorescent emission (AIPE)

Article abstract of DOI:10.1039/c4cc01799d

Two new ionic dinuclear Ir(iii) Schiff base complexes which are straightforward to synthesise have luminescence quantum yields as high as 37% in neat films. These are the first examples of dinuclear ionic Ir(iii) complexes that display aggregation-induced

Full text of DOI:10.1039/c4cc01799d

Featuring research of Dr Dongxia Zhu, Guangfu Li et al. from the
Institute of Functional Material Chemistry Group of
Prof. Zhongmin Su, Northeast Normal University,
Faculty of Chemistry, Changchun, China
As featured in:
New ionic dinuclear Ir(III) Schiff base complexes with
aggregation-induced phosphorescent emission (AIPE)
Two new ionic dinuclear Ir(III) Schiff base complexes which are
straightforward to synthesise have luminescence quantum yields
as high as 37% in neat films. These are the first examples of
dinuclear ionic Ir(III) complexes that display aggregation-induced
phosphorescent emission (AIPE).
See Dongxia Zhu, Likai Yan,
Martin R. Bryce et al.,
Chem. Commun., 2014, 50, 6977.
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New ionic dinuclear Ir(III) Schiff base complexes
with aggregation-induced phosphorescent
emission (AIPE)†
Cite this: Chem. Commun., 2014,
50, 6977
Received 10th March 2014,
Accepted 12th May 2014
Guangfu Li,a Yong Wu,a Guogang Shan,a Weilong Che,a Dongxia Zhu,*a
Baiqiao Song,a Likai Yan,*a Zhongmin Sua and Martin R. Bryce*b
DOI: 10.1039/c4cc01799d
www.rsc.org/chemcomm
Two new ionic dinuclear Ir(III) Schiff base complexes which are
However, AIPE materials with high quantum yield are still
straightforward to synthesise have luminescence quantum yields as rare,12 especially ionic transition metal complexes. In this
high as 37% in neat films. These are the first examples of dinuclear ionic regard, dinuclear Ir(III) complexes are promising candidates.
Ir(III) complexes that display aggregation-induced phosphorescent Recently, a series of dinuclear iridium(III) complexes were
emission (AIPE).
exploited in LECs.13 It is clear that the nature of the bridging
atoms or ligand(s) in dinuclear complexes plays a fundamental
Solid-state organic luminescent materials are of interest for both role in adjusting the luminescence properties.
fundamental studies and practical applications due to their impor-
Compared with the typical N^N-chelating ligands, such as
tant roles in such diverse areas as solid-state light-emitting electro- 2,20-bipyridine and 1,10-phenanthroline, Schiff base ligands
chemical cells (LECs),1 chemosensors,2 photocatalysts for hydrogen are structurally versatile and can act as both chelating and
generation,3 and biochemical probes.4 In this regard understanding bridging motifs. The flexible nature of the diimine spacer
and controlling intermolecular interactions is a key topic, as allows the ligand to bend and rotate freely when coordinating
aggregate formation – both in solution and in the solid-state – often to metal centres so as to adopt the optimum coordination
quenches light emission.5 In 2001 Tang et al.6 reported an impor- geometries of the metal ions. The motivation for the present
tant class of compounds which display aggregation-induced emis- work is to study dinuclear iridium complexes with bridging
sion (AIE); they emit weakly when dispersed and show strong diimine ligands that are able to electronically couple the two
emission when aggregated or in the solid state. In 2002, Park metal centres and offer the potential for AIPE.
et al.7 reported aggregation-induced enhanced emission (AIEE) in
The complexes 1 and 2 (Scheme 1 and Scheme S1, ESI†) were
similar materials. Since then, AIE and AIEE have attracted consider- prepared by the standard procedure14 in high yield. The Schiff-
able attention for potential applications in fields such as organic based ligands were synthesised following a previous method
light-emitting devices (OLEDs), bioelectronics and chemosensors.8 (Scheme S1, ESI†).15 The complexes are only very weakly emissive in
It is generally agreed that AIE occurs when molecular packing leads solution, whereas they are brightly luminescent in solid thin films.
to restricted molecular rotation or torsion which blocks non- Thus, to our knowledge, 1 and 2 present the first examples of ionic
radiative channels and effectively suppresses self-quenching. To dinuclear Ir complexes with AIPE. The crystallographic analysis of
date only a few neutral iridium complexes with aggregation induced
phosphorescent emission (AIPE) have been well studied.9 In 2010,
Youngkyu Do et al.10 reported a neutral dinuclear iridium(III)
complex which showed AIE character. In 2012, our group11 reported
the first ionic mono-nuclear Ir(III) complex which displayed AIE.
a Institute of Functional Material Chemistry, Faculty of Chemistry,
Northeast Normal University, Renmin Road 5268, Changchun, 130024,
P.R. China. E-mail: zhudx047@nenu.edu.cn, yanlk924@nenu.edu.cn
b Department of Chemistry, Durham University, Durham, DH1 3LE, UK.
E-mail: m.r.bryce@durham.ac.uk
† Electronic supplementary information (ESI) available: Experimental schemes
for the complexes, cyclic voltammograms and procedures for the DFT calcula-
tions. CCDC 976189 and 970289. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc01799d
Scheme 1 Chemical structures of the complexes, with bridging phenyl (1)
and biphenyl (2) units.
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Fig. 2 (a) UV-Vis absorption and emission spectra of complexes 1 and 2
(1 ꢁ 10ꢀ5 M) in CH3CN solution at room temperature. (b) Emission spectra
of complex 1 in CH3CN–water mixtures with different water fractions
(0–90% v/v) at room temperature.
absorption band in the 200–300 nm range is assigned to spin-
allowed p–p* transitions of the ligands. The weak absorption from
350 nm to the visible region is ascribed to both metal-to-ligand
charge transfer (3MLCT) and ligand-to-ligand charge transfer
(1LLCT) character with reference to reported Ir(III) complexes.17
Upon photoexcitation, complexes 1 and 2 show almost no emis-
Fig. 1 (a) The dication in the X-ray molecular structure of complex 2. The
H atoms, PF6ꢀ anions and solvent molecules are omitted for clarity. (b) Ladder-
like double chain structure formed by p–p stacking in complex 2.
the complexes and density functional theory (DFT) gives insight sion observable to the naked eye in CH3CN solution [Fig. 2(a)]. In
into the origin of their luminescence.
contrast, in neat film the complexes exhibit intense emission with
lmax 644 and 692 nm, respectively, at room temperature (Fig. S10,
Noncovalent intra- and intermolecular effects in single
crystals can be used to interpret the origin of the enhanced ESI†). This indicates that complexes 1 and 2 may be AIE active.
emission.7,9b The X-ray molecular structure of complex 2 is shown The clear red shift of the crystal emission relative to the powder
in Fig. 1(a). (For complex 1, see ESI,† Fig. S5). Complex 2 has (Fig. S10 and Table S4, ESI†) can be ascribed to stronger inter-
distorted octahedral geometry around the two Ir centres, which are molecular interactions in the crystals.18
coordinated by four cyclometalated ligands (C^N, ppy) and one
To investigate the AIE properties of 1 and 2, their PL spectra in
ancillary ligand (N^N), adopting C,C-cis and N,N-trans configura- different ratios of water–acetonitrile mixtures were studied. Both
tions, as reported for mononuclear ionic iridium complexes.1c,16 complexes aggregate at high water ratios because 1 and 2 are
The crystal packing of 2 [Fig. 1(b)] shows 1D ladder-like chains, in insoluble in water. As shown in Fig. 2(b), complex 1 in pure
which the pyridine rings of ppy ligands in adjacent molecules acetonitrile exhibits faint emission, but the intensity is remark-
overlap in a face-to-face stack with a separation of ca. 3.667 Å ably increased when the water fraction exceeds ca. 60%. Complex
(Fig. S6, ESI†). These intermolecular p–p interactions in 2 restrict 2 behaves similarly (Fig. S11(b), ESI†), demonstrating that both 1
the intramolecular motions in the solid state and lead to a more and 2 display AIPE. Previous studies have shown that AIE and/or
planar geometry within the bridging biphenyl unit. J-aggregation AIEE might be related to the restriction of intermolecular rotation,
where the molecules are arranged in head-to-head or head-to-tail formation of specific aggregates, and effects of intramolecular
p–p stacking interactions exists in the crystalline state. DFT calcula- planarization of small organic molecules.12 The Schiff base brid-
tions show that the p–p interaction between the ppy ligands ging ligands with head-to-head or head-to-tail p–p stacking inter-
restricts molecular distortion in the bridging ligand in the solid actions can provide potential supramolecular recognition sites
state (see below). These structural features explain the unusually that can govern the AIPE process.
high solid-state phosphorescence efficiency of 1 and 2.7
The absorption spectra of 1 and 2 in different water–acetonitrile
The UV/vis absorption and emission spectra of the complexes 1 ratios are shown in Fig. S13 (ESI†). Both absorption bands of 1 and
and 2 in acetonitrile solution at room temperature are shown in 2 rise in intensity and move to longer wavelengths when the water
Fig. 2(a) and the data are summarised in Table 1. The dominant fraction increases from 60% to 90%, while the spectra are almost
identical in pure acetonitrile solution and 50% water–acetonitrile
mixtures. Evidence for aggregates of 1 and 2 in the solvent mixture
is seen in the level-off tail in the visible spectral region.6
Table 1 Photophysical data for complexes 1 and 2
tb (ns) krc (ꢁ106 sꢀ1
)
knr (ꢁ107 sꢀ1
)
a
b
b
c
labs (nm) lem (nm) FL
The PL quantum yields (FL) of complexes 1 and 2 in neat film are
37.3% and 26.4%, respectively. Complex 2 is among the most red
emissive ionic Ir(III) complex to date19 with its emission maximum
1
2
252, 369sh 644
254, 376sh 692
0.373 47.66 7.83
0.264 38.68 6.83
1.32
1.90
Measured in CH3CN (1.0 ꢁ 10ꢀ5 M) at room temperature. Measured red-shifted by 48 nm compared to complex 1. This may be due to the
a
b
extension of the conjugation length in the ancillary ligand.13a The
excited state lifetimes of complexes 1 and 2 in neat film at room
temperature (Table 1) are relatively short for phosphorescence
in the film state at room temperature; lexc = 370 nm; error for FL ꢂ 5%.
c
The radiative kr and non-radiative knr values in neat film were calculated
according to the equations: kr = F/t and knr = (1 ꢀ F)/t, from the
quantum yields F and the lifetime t values.
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on the two iridium centres and four cyclometalated ppy ligands,
whereas in solution only one iridium centre and two cyclometalated
ppy ligands participate in the HOMO of 2 (Fig. 4). These results
indicate that much more efficient electronic interaction can be
induced between the Ir-centred moieties and the bridging ligand in
the solid state than in the solution state of 2 due to the planar
geometry of the bridge in the solid.10,22 For complex 1 with a phenyl
bridge, the HOMO is similar in both the solution and solid states
(Fig. 4). The HOMO and LUMO energy levels in solution and solid
states are shown in Fig. 4. The energy levels are lowered in the solid
state, probably due to intramolecular or intermolecular interactions.
The emitting triplet states for 1 and 2 are shown to be a mixture
of metal-to-ligand charge transfer (3MLCT), ligand-to-ligand charge
transfer (3LLCT) and ligand-centered (3LC) transitions (Fig. S15,
ESI†). Nonetheless, another important feature should be consid-
ered. Table S5 (ESI†) lists the selected calculated bond lengths, bond
angles and dihedral angles at both the optimized ground state (S0)
and triplet excited state (T1) for 1 and 2. Structural distortions are
found in the T1 geometry compared to the S0 geometry in both
complexes which induce a larger excited-state relaxation and may
result in an effective pathway for nonradiative decay.
In summary, two new ionic dinuclear Ir(III) Schiff base com-
plexes with unusually high PLQY in neat thin films have been
studied. X-ray crystal structure analysis and TD-DFT calculations
suggest that restricted intramolecular relaxation in the solid state
leads to the observed AIPE. This should be a versatile strategy for
obtaining highly efficient iTMCs for future applications.
The work in China was funded by NSFC (51203017 and
21303012), the Science and Technology Development Planning
of Jilin Province (20100540 and 20130522167JH).
Fig. 3 Optimized geometry of complexes 1 and 2 in solution state (a) and
the solid state (b).
materials. Similar values were reported by Youngkyu Do’s group.10
The high quantum yields of complexes 1 and 2 can be attributed to
their relatively high radiative rates.
To gain further understanding of the unusual solid-state emis-
sion properties, the geometries of 1 and 2 were optimised by
referring to the X-ray diffraction data, and the electronic properties
of the frontier orbitals were studied in solution and the solid state
structures using DFT methods. Fig. 3(b) shows that for complex 1
intermolecular CHꢃꢃꢃp interactions among the adjacent ppy ligands
exist in the solid state, which induce the intermolecular p–p
interactions between the phenyl rings of the cyclometalated ligand
and bridge ligand. However, in the solution state for complex 1
there are no p–p interactions so intramolecular relaxation is not
restricted. As shown in Fig. 3(a) for complex 2, there is a dihedral
angle of 381 between the planes of the two phenyl rings of the
bridging ligand in the solution state, whereas in the solid state a
planar geometry occurs in the same fragment, probably due to the
intermolecular p–p interactions by the adjacent ppy ligands
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