Two efficient near-infrared (NIR) luminescent [Ir(C^N)2(N^O)]-characteristic complexes with 8-hydroxyquinoline (8-Hq) as the ancillary ligand

Article abstract of DOI:10.1016/j.inoche.2019.01.019

Through the utilization of Hiqbt (1-(benzo[b]-thiophen-2-yl)-isoquinoline) or Hdpbq (2,3-diphenyl benzo[g]quinoxaline) as the C^N main ligand and 8-Hq (8-hydroxyquinoline) as the N^O ancillary ligand, two [Ir(C^N)₂(N^O)]-characterized heterolep

Full text of DOI:10.1016/j.inoche.2019.01.019

Inorganic Chemistry Communications 101 (2019) 69–73
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Two efficient near-infrared (NIR) luminescent [Ir(C^N) (N^O)]-characteristic
2
complexes with 8-hydroxyquinoline (8-Hq) as the ancillary ligand
1
1
⁎
Jiahao Guo , Jin Zhou , Guorui Fu, Yani He, Wentao Li, Xingqiang Lü
School of Chemical Engineering, Shaanxi Key Laboratory of Degradable Medical Material, Northwest University, Xi'an, 710069, Shaanxi, China
G R A P H I C A L A B S T R A C T
Replaced by the large π-conjugation-induced C^N ligand Hdpbq, the distinctive bathochromatic shift into a typical NIR region (λem = 786 nm) for [Ir(dpbq) (8-q)]
2
(
2) relative to that (λem = 687 nm with a shoulder at 756 nm) of [Ir(iqbt) (8-q)] (1) while relatively lower quantum efficiency Φem = 0.16 versus Φem = 0.05 are
2
observed.
A R T I C L E I N F O
A B S T R A C T
Keywords:
Through the utilization of Hiqbt (1-(benzo[b]-thiophen-2-yl)-isoquinoline) or Hdpbq (2,3-diphenyl benzo[g]
quinoxaline) as the C^N main ligand and 8-Hq (8-hydroxyquinoline) as the N^O ancillary ligand, two [Ir
[
2
Ir(C^N) (N^O)]-characteristic Ir(III)-complex
NIR luminescence
Phosphorescence
(
C^N)
2
(N^O)]-characterized heteroleptic complexes [Ir(iqbt)
2
(8-q)] (1) and [Ir(dbpq) (8-q)] (2) with desirable
2
soluble and NIR-phosphorescent properties (λem = 687 nm with a shoulder at 756 nm, lifetime τ = 0.73 μs and
quantum efficiency Φem = 0.16 for complex 1 versus λem = 786 nm, lifetime τ = 0.47 μs and quantum efficiency
Ligands-perturbed effect
Φ
em = 0.05 for complex 2) are obtained, respectively. In comparison, the distinctive bathochromatic shift into a
typical NIR region of complex 2, arisen from the large-molecule-conjugation-induced narrow energy gap, gives
rise to its relatively lower quantum efficiency than that of complex 1.
Near-infrared (NIR) emitting materials have aroused particular in-
terest in electroluminescent diodes promising for military optoelec-
tronics [1], telecommunications [2] and wound healing [3]. In this
perspective, although significant efforts have been devoted to inorganic
NIR-emitting materials (nano-crystal [4], halide perovskite [5] or
quantum dot [6], etc) for practical NIR-light-emitting diodes (NIR-
⁎
Corresponding author.
E-mail address: lvxq@nwu.edu.cn (X. Lü).
1
Both authors contributed equally.
https://doi.org/10.1016/j.inoche.2019.01.019
Received 26 December 2018; Accepted 12 January 2019
Available online 15 January 2019
1387-7003/ © 2019 Elsevier B.V. All rights reserved.

J. Guo et al.
Inorganic Chemistry Communications 101 (2019) 69–73
Scheme 1. Reaction scheme for the synthesis of complexes [Ir(iqbt)
2
(8-q)] (1) and [Ir(dpbq) (8-q)] (2).
2
LEDs), organic-counterparts [7] for NIR organic or polymer light-
emitting diodes (NIR-OLEDs or NIR-PLEDs) still dominate in the aca-
demic community, which should be arisen from their more versatile
and advanced properties in terms of broadened photo- and electro-lu-
minescent spectra as well as endless structure modifications. Moreover,
compared with fluorescent small-molecule dyes [8] and conjugated
also makes the molecular design of the fac-[Ir(C^N) ]-complex much
3
challenging in the obtainment of its efficient NIR-OLED or NIR-PLED.
Convincingly, significant electronic perturbation can be achieved by
modification of the L^X ancillary ligands [16], from which, the energy
gap of its typical [Ir(C^N) (L^X)]-complex is actually adjusted, while
2
bathochromic-shift to effectively narrow the energy gap for NIR lumi-
1
polymers [9] with π-π*-transitions for NIR luminescence, transition-
nescence does not have a universal effect. For example, within the ty-
3
+
metal-complex- (Pt(II) or Ir(III)-complex etc) [7] and Ln -complex-
resourced (Ln = Nd, Yb or Er) [10] phosphors are worthy of a parti-
cular interest because of their harvesting of both singlet and triplet
excitons toward a theoretic 100% internal emission efficiency. Notice-
ably, in contrast to the narrow-energy-gap-confined [11] rather low
pical [Ir(C^N) (N^O)]-complexes with pic as the ancillary ligand, con-
2
trast to the rational blue-shift for the well-known Flrpic complex
2
(Flrpic = bis[2-(4,6-difluorophenyl)pyridinato-C ,N](pic)-iridium(III))
[17], a slight red-shifting [18] at 698 nm for [Ir(iqbt) (pic)] is realized
2
in relative to that (690 nm) of the fac-[Ir(iqbt)
consideration of the resolution to [Ir(iqbt) (pic)] insolubility for solu-
tion-processed OLEDs, the success of our reported two [Ir(iqbt) (N^O)]-
3
] [19]. Moreover, in
3
+
NIR quantum efficiency for Ln -complex-based (Ln = Nd, Yb or Er)
with the emissive wavelength above 900 nm and the notorious effi-
ciency-roll-off [12] inherently contributed from facile aggregation of Pt
2
2
complexes [20] with pic-derived hpa or BF -hpa as the N^O ancillary
2
(
II)-centered square-planar system, the compromise of both desirable
ligand, motivates us a particular concern on the evolution of some other
high-efficiency and low-efficiency-roll-off, to the best of our knowledge,
should be expected for iridium(III)-complexes characteristic of typical
octahedral configuration and rather short phosphorescent lifetime.
As a matter of fact, from the viewpoint of lowing the emissive en-
N^O ancillary ligands. Herein, with 8-hydroxyquinoline (8-Hq) as the
N^O ancillary ligand, its two new [Ir(C^N) (N^O)]-heteroleptic com-
2
plexes with different π-conjugation C^N main ligands of Hiqbt or
Hdpbq are rationally designed, from which, the desirable red-shifted
emission within the NIR regime affected by the electronic perturbations
of the C^N main ligand and the N^O ancillary ligand are also explored.
The C^N main ligand Hiqbt was synthesized by an improved Suzuki
coupling reaction [19] between cost-effective 2-Cl-isoquinoline while
not 2-Br-isoquinoline and benzo[b]-thien-2-y boronic acid in 73% yield.
As to the C^N main ligand Hdpbq, it was obtained from the equimolar
condensation of 2,3-naphthalenediamine with benzyl in the presence of
oxalic acid according to the well-established procedure from the lit-
erature [21]. As shown in Scheme 1, each of the μ-chloro-bridged
ergy of iridium(III)-complexes to
a
restrictive NIR region
(
700–2500 nm), several approaches have been reported. Utilizing the
large π-conjugation porphyrin- [13] or corrole-based [14] macrocycles
as the ligands is highly praised at first, since their substantial NIR
phosphorescence is commonly originated from the intraligand charge
3
transfer ( ILCT) in the Ir(III)-complexes. However, despite the desirable
emission wavelength extended to 800 nm or above for these Ir(III)-
complexes, distinctively low NIR quantum efficiencies regulated by
energy-gap law [11] actually limit their use for NIR-OLEDs. In contrast,
one of the most successful approaches relies on the π-conjugated ex-
pansion of the main C^N-cyclometalated ligand especially incorporated
dimmer intermediates [Ir(iqbt)
2
(μ-Cl)]
2
and [Ir(dpbq)
2
(μ-Cl)] were
2
rationally prepared from the reaction of IrCl
3
⋅3H O with the corre-
2
with electron-rich substituents to afford the fac-[Ir(C^N)
3
]-character-
sponding C^N main ligand of Hiqbt or Hdpbq, and used directly for the
next step without further purification. Further through the reaction of
the N^O ancillary ligand 8-Hq with the corresponding μ-chloro-bridged
istic homoleptic complex [15] with the expected NIR luminescence
ranging at 700–800 nm. Nonetheless, severe aggregation-induced
quenching effect from the large π-conjugation of the C^N main ligand,
dimmer intermediate [Ir(iqbt)
2
(μ-Cl)]
2
or [Ir(dpbq)
2
(μ-Cl)] , two
2
70

J. Guo et al.
Inorganic Chemistry Communications 101 (2019) 69–73
Table 1
Photo-physical and electrochemical properties of complexes 1–2 in solution at room temperature.
Complex
Absorption
Emission
Energy level
a
a
τa
a
a
a
HOMOb
LUMOb
(eV)
λ
abs
λ
em
Φ
em
k
r
k
nr
(
nm)
(nm)
(μs)
(×105 s−1
)
(×105 s−1
)
(eV)
[
Ir(iqbt)
2
(8-q)] (1)
229, 284, 367, 443, 500, 654
238, 285, 325, 411, 541, 689
687, 756(sh)
786
0.73
0.47
0.16
0.05
2.19
1.06
1.15
2.02
−5.172
−5.148
−2.662
−3.140
[
Ir(dbpq)
2
(8-q) (2)] (2)
Rate constant kr and knr are calculated using the equations kr = Φem/τ and knr = (1-Φem)/τ on the assumption that ΦISC = 1 (ISC = intersystem crossing).
a
b
In degassed CH
2
Cl solution.
2
HOMO and LUMO levels are obtained from electrochemical determination, respectively.
new Ir(III)-complexes [Ir(iqbt)
isolated, respectively.
2
(8-q)] (1) and [Ir(dpbq)
2
(8-q)] (2) are
complexes 1–2 investigated by thermogravimetric analysis (TGA;
Fig. 2S) shows that their decomposition temperatures (T , corre-
d
The two Ir(III)-complexes 1–2 are much soluble in common organic
sponding to 5% weight loss) can be up to 300 °C.
solvents except water, which significantly different from the relative
The photo-physical properties of the C^N main ligand Hiqbt or
Hdpbq, the N^O ancillary ligand 8-Hq and their two Ir(III)-complexes
1–2 in solution were explored using absorption and photo-lumines-
cence spectrometers, and the results are summarized in Table 1 and
Figs. 1–2 and 3–5S. In contrast to the strong absorption bands limited to
the λabs < 400 nm range for the ligands Hiqbt, Hdpbq, and 8-Hq, as
shown in Fig. 1, both complexes 1–2 exhibit significantly broadened
UV–visible-NIR absorption spectra: the intense absorption bands below
400 nm assigned to the spin-allowed intra-ligand π-π* transitions; the
moderate absorption bands in the 400–600 nm region probably arisen
insolubility [Ir(iqbt) (pic)] [18] while comparable to that of our re-
2
ported two [Ir(iqbt)
BF -hpa as the N^O ancillary ligand, renders the incorporation of the 8-
Hq N^O ancillary ligand for its new [Ir(C^N) (8-q)] complexes
HC^N = Hiqbt or Hdpbq) an opportunity to solution-processed
2
(N^O)]-complexes [20] with pic-derived hpa or
2
2
(
electro-luminescent devices. Moreover, Ir(III)-complexes 1–2 were
1
1
well-characterized by EA, FT-IR, H NMR and ESI-MS. In the H NMR
spectrum (Fig. 1S) of complex 1, the iridium(III)-induced significantly
−
−
n −
spread shifts of the (iqbt) or (dpbq) and (L ) combined proton
resonances (δ = 9.13–6.39 ppm) relative to those (δ = 8.65–7.43 ppm)
of the free Hiqbt ligand are observed. Moreover, the proton signals of
3
1,3
from the mixed LC/ MLCT (LC = ligand-centered; MLCT = metal-to-
ligand charge transfer, d-π*) transitions [20]; and the weak absorption
bands (654 nm for complex 1 or 689 nm for complex 2) extending over
600 nm possibly attributed to the ground-state excitation into the
−
−
the (iqbt) and (8-q) ligands with a stipulated molar ratio of 2:1
could further confirm the [Ir(iqbt) (8-q)] (1) characteristic of the ty-
2
pical [Ir(C^N)
2
(N^O)]-heteroleptic complexes [22]. For comparison,
lowest triplet state (S
0
- T ). Worthy of notice, the low-energy
1
−
−
3
1,3
despite the similar combination of both the (dpbq) and (8-q) proton
resonances (also in Fig. 1S) in a 2:1 M ratio for complex 2, its evident
converge (δ = 8.72–6.25 ppm) of the proton signals relative to those
LC/ MLCT absorption of complex 1 locates at λabs = 500 nm, which
is distinctively blue-shifted by 39 nm relative to that (λabs = 539 nm) of
the parent fac-[Ir(iqbt) ] complex [19]. Similarly, an evident blue-shift
by 25 nm as compared with the low-energy LC/ MLCT absorption
3
3
1,3
(
δ = 9.13–6.39 ppm) of complex 1, should be arisen from the strong
−
current-circular effect with the large π-conjugated (dpbq) C^N ligand
(λabs = 525 nm) of complex [Ir(iqbt) (pic)] [18], should result from the
2
−
for complex 2 as compared to the (iqbt) C^N ligand in complex 1.
change in N^O-chelate ancillary ligand field strength. As to complex 2
based on the larger π-conjugation C^N ligand Hdpbq, besides the de-
creased Ir(III)-centered d-(t2g) orbital energy (541 nm) relative to that
(500 nm) of complex 1, the lowest-energy absorption edge almost ex-
tends to ca. 700 nm, indicating that the molecular conjugation is sig-
Furthermore, the ESI-MS spectra of the two iridium(III)-complexes 1–2
exhibit a similar pattern, where a strongest mass peak at m/z 858.10 (1)
+
or 1000.15 (2) assigned to the major species [M + H] , respectively,
indicates that each of the respective heteroleptic [Ir(C^N) (N^O)] unit
2
retains stable in solution. The thermal stability of the two iridium(III)-
nificantly increased for the formation of [Ir(dbpq) (8-q)] (2) in a de-
2
localized state. Upon photo-excitation (λex = 387 nm, Fig. 3S), as
Fig. 1. Normalized UV–Visible-NIR absorption for the Ir(III)-complexes 1–2 in
contrast to those of the three ligands Hiqbt, Hdpbq, 8-Hq in degassed CH
2
Cl
2
Fig. 2. Normalized NIR photo-luminescence spectra for complexes 1–2 in de-
gassed CH Cl solution at RT.
solution at RT.
2
2
71

J. Guo et al.
Inorganic Chemistry Communications 101 (2019) 69–73
with complex 1, complex 2 exhibits a slightly narrower HOMO-LUMO
gap of 2.008 eV than that (2.662 eV) of complex 1.
In summary, through Hiqbt or Hdpbq as the C^N main ligand and 8-
Hq as the N^O ancillary ligand, two new soluble Ir(III)-complexes [Ir
(
iqbt)
C^N)
2
(8-q)] (1) and [Ir(dbpq) (8-q)] (2) characteristic of a similar [Ir
2
(
2
(N^O)]-heteroleptic configuration are obtained, respectively. In
comparison with the photo-physical property (λem = 687 nm with a
shoulder at 756 nm, lifetime τ = 0.73 μs and quantum efficiency
Φ
em = 0.16) for complex 1, complex 2 with large-molecule-conjugation
exhibits the distinctive bathochromatic shift into a typical NIR region
(λem = 786 nm), endowing a particular opportunity to future NIR-
OLED.
Acknowledgements
This work is funded by the NNSF (21373160 and 21173165) and the
Graduate Innovation and Creativity Fund (YZZ17127) of Northwest
University in P. R. of China.
Fig. 3. Cyclic voltammograms of complexes 1–2 recorded versus Fc+/Fc in
−
1
solution at RT under a N atmosphere (scan rate = 100 mV s ).
2
Appendix A. Supplementary data
shown in Fig. 2, the strong emission peak (687 nm) of complex 1 lies at
the edge of the NIR region besides a shoulder peak at 756 nm. By
contrast, complex 2 exhibits a typical NIR luminescence with the
emissive wavelength up to 786 nm. Due to the non-emissive character
The information of raw materials and methods, and the synthesis
and characterization of the two C^N ligands Hiqbt and Hdpbq, the two
μ-chloro-bridged dimmer intermediates [Ir(iqbt)
2
(μ-Cl)]
2
and [Ir
(
dpbq)
2
(μ-Cl)]
2
and their two Ir(III)-complexes [Ir(iqbt)
2
(8-q)] (1) and
(
Fig. 4S) of both the C^N main ligand Hiqbt or Hdpbq and the N^O
1
[
Ir(dpbq)
2
(8-q)] (2) depicted in the Supporting information. The
H
ancillary ligand 8-Hq in that NIR region, the NIR emissions of com-
3
1,3
NMR spectra, the TG curves, the solution visible emission and/or ex-
plexes 1–2 should originate from the ligands-perturbed LC/ MLCT-
excited state. Moreover, the NIR emissive nature of the two complexes
is characteristic of typical phosphorescence (Fig. 5S), confirming from
the substantial τ = 0.73 μs for complex 1 and τ = 0.47 μs for complex 2,
respectively. Worthy of notice, although the desirable bathochromatic
citation spectra of the ligands Hiqbt, Hdpbq and 8-Hq and the two Ir
(
III)-complexes 1–2 and the time-decayed curves deposited in
Figs. 1–4S, respectively. Supplementary data to this article can be found
online at https://doi.org/10.1016/j.inoche.2019.01.019.
shifts of complex 1 at 687 nm relative to those (682–683 nm) [18] of
+
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