Phosphorescent cyanide sensor based on a 2-phenylpyridine(ppy)-type cyclometalated Ir(III) complex bearing dimesitylboron group with concentration distinguishing ability

Article abstract of DOI:10.1016/j.jorganchem.2020.121274

With a 2-phenylpyridine(ppy)-type cyclometalated Ir(III) complex with –B(Mes)₂ group Ir–B, the CN^ˉ ions can bind effectively with the boron atom in the –B(Mes)₂ group to furnish CN^ˉ sensing ability through induc

Full text of DOI:10.1016/j.jorganchem.2020.121274

Journal of Organometallic Chemistry 917 (2020) 121274
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Phosphorescent cyanide sensor based on a 2-phenylpyridine(ppy)-
type cyclometalated Ir(III) complex bearing dimesitylboron group
with concentration distinguishing ability
,
Yingju Li a b, Chaofan Yao a, Daokun Zhong a, Huiying Li a, Boao Liu a, Zhao Feng a,
,
, **
Yuanhui Sun a, Guijiang Zhou a *, Zhimao Yang b
a MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials,
Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, 710049, PR China
b MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials,
Department of Material Physics, School of Science, Xi’an Jiaotong University, Xi’an, 710049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
With a 2-phenylpyridine(ppy)-type cyclometalated Ir(III) complex with eB(Mes)2 group IreB, the CN
ions can bind effectively with the boron atom in the eB(Mes)2 group to furnish CN sensing ability
through inducing phosphorescence changing of the concerned Ir(III) complex. Uniquely, with increasing
the added amount of CN ions, loose and tight binding between CN and boron atom in the eB(Mes)2
group of IreB can occur in sequence to destabilize metal-to-ligand charge transfer (MLCT) excited states
and then form new MLCT excited states in IreB. Accompanying this process, the red phosphorescence of
IreB can be effectively quenched by adding CN ions, while new yellow and green emission bands can
appear in sequent with increasing the amount of added CN ions, showing CN sensing behavior with
unique concentration distinguishing ability. In addition, the new phosphorescent CN sensor can possess
detection limit (DL) as low as 4.3 ꢀ 10ꢁ6 M and good selectivity to the CN ions. Clearly, these results can
provide important information for developing new CN sensors with high performances.
© 2020 Elsevier B.V. All rights reserved.
Received 26 February 2020
Received in revised form
14 March 2020
Accepted 1 April 2020
Available online 2 April 2020
Keywords:
Phosphorescent sensor
Cyanide ion
Iridium(III) complex
Dimesitylboron group
Concentration distinguishing
1. Introduction
Generally, phosphorescent emission possesses large Stokes shift
which can effectively avoid interference from the background
Recently, 2-phenylpyridine(ppy)-type cyclometalated Ir(III)
complexes have drawn substantial research interests from both
academic and industrial communities due to their strong phos-
phorescent ability and tunable emission color [1e3]. Owing to the
strong spin-orbital coupling (SOC) effect induced by the heavy
Ir(III) center, these complexes possess ability of emitting phos-
phorescent signal even at room temperature [4e6]. Hence, their
phosphorescent ability have been applied to various fields, such as
electroluminescence (EL) [7e9], bioimaging [10e12], optical power
limiting [13e15], organic solar cells [16] and sensing [17] etc.
Among all these applications, ppy-type cyclometalated Ir(III)
complexes as sensors can show two major inherent advantages: 1)
emission; 2) With longer lifetime typically in the order of micro-
second, phosphorescent emission can afford other detecting in-
formation except wavelength. Hence, various ppy-type
cyclometalated Ir(III) complexes have been served as sensors with
high performances [18e20]. Among all the ppy-type cyclo-
metalated Ir(III) complex ion sensors, dimesitylboron (B(Mes)2)
unit is typically introduced into their ppy-type ligands. Owing to
the strong bonding between F ions and the boron atom in the
B(Mes)2 units, ppy-type cyclometalated Ir(III) complexes with
B(Mes)2 group can act as F ion sensors [19,20]. Critically, these
novel sensors can show unique concentration discrimination
behavior in detecting F ions, indicating their great potential [19].
As another kind of important anions, CN ions are well-known for
high toxic due to their inhibiting many heme-containing enzymes
[21]. Hence, the United States Environmental Protection Agency
(EPA) has set the maximum safe concentration for CN in drinking
* Corresponding author.
** Corresponding author.
water at ca. 7.6
mM [22] and even lower safe CN concentration of ca.
E-mail addresses: zhougj@mail.xjtu.edu.cn (G. Zhou), zmyang@mail.xjtu.edu.cn
2.5 M is set by WHO International Standards [23]. As a result,
m
(Z. Yang).
https://doi.org/10.1016/j.jorganchem.2020.121274
0022-328X/© 2020 Elsevier B.V. All rights reserved.

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Y. Li et al. / Journal of Organometallic Chemistry 917 (2020) 121274
sensing CN ions has become a very important research line.
Recently, various CN sensors have been developed with diverse
structures, such as organic molecules [24,25], metal complexes
[26,27], nanoparticles [28] etc. Among them, CN sensors based on
metal complexes can show advantages of easy preparation and
environmental compatibility. Importantly, metal complexes can
possess ability of phosphorescent emission which can guarantee
high sensitivity and avoiding interference from background. For the
time being, handful Zn(II) [29], Cu(II) [30], Co(II) [31], Os(II) [32]
and Ni(II) [33] complexes have been prepared to detect CN ions
with fluorescence or absorbance as reporting signal, which cannot
afford the merits associated with phosphorescent reporting signal.
Recently, phosphorescent Ir(III) [18e20] and Ru(II) [34,35] com-
plexes have been employed to sense CN ions based on either
addition reaction of the organic ligands with CN ions or bonding
functional groups, such as -B(Mes)2 [19,20,35]. Clearly, sensing CN
ions by addition reaction may have chance to induce slow
responding speed and high detection limit (DL). Generally, Ru(II)
phosphorescent complexes possess low phosphorescent quantum
yield (FP), which might disfavor ion sensing with emission as
reporting signal.
2.4. Emission and absorption titration
A THF solution of [nBu4N]CN (ca.1 ꢀ 10ꢁ3 M) was added to a THF
solution of the iridium complex (ca. 1 ꢀ 10ꢁ5 M) in a screw-capped
UV or PL cell according to the target [B]: CN ratio via a micro-
syringe. After gentle mixing, the solution in the UV or PL cell was
allowed to stand in the dark for 5 min before measurement. The
detection limit (DL) was determined from the absorption titration
data based on the reported method [38]. The absorption intensity
data at ca. 341 nm were taken to calculate the DL.
3. Results and discussion
3.1. Chemical structure and electronic spectra
Chemical structure of the employed phosphorescent ppy-type
Ir(III) complex IreB is shown in Fig. 1a. It had been synthesized
by our previous method [36]. In IreB, -B(Mes)2 has been intro-
duced to the pyridyl unit in each of the two ppy-type ligands
(Fig. 1a). With sensing purpose, CN ions are supposed to bind with
boron atom in the -B(Mes)2 unit.
So, in this research, a highly phosphorescent ppy-type Ir(III)
complex has been employed to detecting CN ions. It has been
shown that both absorption and phosphorescent signals can show
fast response to the external CN ions in THF solution with low DL.
Critically, the phosphorescent signal of the this phosphorescent
Ir(III) complex sensor can show multi emission-color response,
indicating concentration distinguishing ability. The concerned re-
sults can provide useful information for design new phosphores-
cent CN ion sensors with high performance.
Fig. 1b shows the UVevis absorption and photoluminescence
(PL) spectra of IreB in THF. In the absorption spectrum of IreB, two
kinds of absorption bands can be seen clearly with the strong p-p*
absorption band from the organic ppy-type ligands and the low-
energy weak absorption bands from metal-to-ligand charge
transfer (MLCT) absorption bands (Fig. 1b). The strong red phos-
phorescent at ca. 600 nm in the PL spectrum of IreB has been
confirmed to be induced by the radiative decay of the triplet MLCT
excited states (3MLCT). Importantly, IreB can possess very high
phosphorescent quantum yield (FP) of ca. 0.95, favoring its appli-
cation in ion sensing by enhancing sensitivity.
2. Experimental
3.2. CN ion sensing behavior
2.1. General information
Based on the reported photophysical property of IreB, its
phosphorescent emission is induced by the 3MLCT excited states
from by transferring electron from d-orbital of Ir(III) center to the
pp* orbital of the boron atom in the eB(Mes)2 group [36]. If CN ions
interact with boron atom, it should have a great chance to change
the character of the 3MLCT excited states of IreB and hence alter
emission behavior of IreB. Obviously, response of the phospho-
rescent emission from IreB to the CN ions can act as the reporting
signal for sensing CN ions. In addition, different from its Pt(II) an-
alogs, IreB can possess two eB(Mes)2 groups, which can facilitate
interaction between eB(Mes)2 groups and cyanide ions at low CN
concentration. Obviously, it can lower the detecting limit in sensing
CN ions. Hence, IreB should possess great potential to serve as
novel CN ion sensor.
All commercially available chemicals used were not subjected to
further purification. All solvents for the reactions were dried and
distilled in the proper way. The reactions were checked with thin-
layer chromatography (TLC) materials purchased from Merck & Co.,
Inc. Flash column chromatography and preparative TLC plates were
made from silica gel from Shenghai Qingdao (300e400 mesh).
2.2. Synthesis
Synthesis for IreB was according to the well-established two-
step strategy (Scheme S1 in Supporting Information, SI) [19,36]. The
synthetic detail and spectral data for IreB was provided in SI.
In order to evaluate ion sensing ability of IreB, with the
controlled molar ratio between boron atom and CN ion (B: CN), THF
solution of IreB had been titrated with the THF solution of [nBu4N]
CN employed to record variation of the phosphorescent spectrum
of IreB at different B: CN. It has been found that the red phos-
phorescent emission at ca. 600 nm is quenched quickly with adding
CN ion, while a new yellow emission band at ca. 550 nm gradually
appears and is enhanced (Fig. 2). When B: CN value at 1:1, nearly
85% phosphorescent intensity at ca. 600 nm is quenched (Fig. 2).
Adding more CN ion to maintain B: CNat about 1:4, the newly
appeared yellow emission band reaches the highest intensity
(Fig. 2). It should be noted that another new green-emission band at
520 nm can be detected obviously with B: CNat about 1:6 (Fig. 2). It
means that low CN concentration can induce yellow emission and
green emission can appear at higher CN concentration. The in-
tensity variation for the three emission bands can be clearly
2.3. Physical measurements
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker
Avance 400 MHz spectrometer. Chemical shifts were referenced to
the solvent residual peak at
NMR spectra, respectively. UVevis absorption spectra were
measured on a PerkinElmer Lambda 950 spectrophotometer.
Emission spectra and lifetimes for the obtained complex were
recorded on an Edinburgh Instruments, Ltd., (FLSP 920) fluores-
d C
7.26 ppm for 1H and 77.0 ppm for 13
cence spectrophotometer. Phosphorescent quantum yields (Fp
)
was measured in CH2Cl2 solution against fac-[Ir(ppy)3] (Fp ¼ 0.97)
standard at 293 K [37]. The data of elemental analyses were ob-
tained on
a Flash EA 1112 elemental analyzer. Fast atom
bombardment (FAB) mass spectra were recorded on a Finnigan
MAT SSQ710 system.

Y. Li et al. / Journal of Organometallic Chemistry 917 (2020) 121274
3
Fig. 1. (a) Chemical structure of IreB. (b) UVevis absorption and PL spectra for IreB in THF at 298 K.
indicated by Fig. 3. Obviously, for ion sensor with emission as
reporting signal, this is very unique sensing behavior with con-
centration distinguishing ability which typically cannot be
observed in traditional sensors. Furthermore, the “off-on” phos-
phorescent response of IreB to the CN ions can benefit its practical
application by avoiding using advanced instrument to detect
emission signal with very low intensity in the “turn-on” ion
sensors.
The unique sensing behavior of IreB can be explained as fol-
lows: When adding CN ions to the solution of IreB, the nucleophilic
ability associated with CN ions make them inline to bind with
empty pp orbital of boron atom in the eB(Mes)2 group of IreB. As
aforementioned, the empty pp orbital of boron atom in the
eB(Mes)2 group has host electron in the MLCT transition process to
form MLCT excited states which induce the red phosphorescence of
IreB (Fig. 4). Obviously, binding negative-charged CN ions will
induce electrostatic repulsion between CN ions and the hosted
electron from the MLCT procedure (Fig. 4). Obviously, the electro-
static repulsion will make the eB(Mes)2 group reluctant to host
electron and hence elevate energy-level of the MLCT excited states
in IreB. As a result, new yellow-emission band with higher energy-
level appears after adding CN ions (Fig. 2). At this stage, there is
loose binding between CN ions and the boron atom in the eB(Mes)2
group due to electrostatic repulsion (Fig. 4).
Fig. 2. Phosphorescence titration of IreB by [nBu4N]CN in THF solution at 293 K.
In addition, the electrostatic repulsion between CN ions and the
hosted electron from the MLCT procedure should make the binding
between CN ions and boron atom as equilibrium process. So, adding
more CN ions should enhance the binding procedure to form “tight”
binding between CN ions and boron atom. Perceptibly, the tight
interaction will make the boron atom lost its ability of hosting
electron in the MLCT excitation process. Alternatively, pyridyl ring
in the ligand of IreB should host the electron in the MLCT process
to form high-energy MLCT excited states due to the much weaker
electron-accepting ability associated with the pyridyl ring. As a
result, green-emission from these high-energy MLCT states can be
observed at high CN concentration (Fig. 4). In addition, from the
absorption titration results in Fig. 5, high-energy absorption bands
at 400 and 470 nm do appear at high CN concentration with B: CN
of 1:5. According to the published results, the complexes
(ppy)2Ir(acac) without eB(Mes)2 can show green-emission (ca.
520 nm) induced by the 3MLCT excited states formed by electron
Fig. 3. Intensity variation for the three emission bands of IreB during the titration
process with [nBu4N]CN in THF solution at 293 K.
transferred from d-orbital of Ir(III) center to
p* orbitals of the

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Y. Li et al. / Journal of Organometallic Chemistry 917 (2020) 121274
Fig. 4. Proposed CNsensing mechanism of IreB.
Fig. 5. Absorption titration of IreB by [nBu4N]CN in THF solution at 293 K.
Fig. 6. Relative phosphorescence intensity at 600 nm for IreB THF solution before and
after adding different anions at the [anion]: [B] value of 5.000 : 1.
pyridyl ring in the ligand. All these results can provide a solid based
for explanation of the sensing mechanism presented in Fig. 4. From
the PL titration result in Fig. 2, the green-emission cannot reach
high intensity even at B: CN of 1:5 (Fig. 2). So, the “tight” binding
between CN ions and boron atom in the eB(Mes)2 group of IreB
should be difficult to occur.
4. Conclusions
Through loose and tight binding with boron atom in the
eB(Mes)2 group of IreB, CN ions can affect the 3MLCT excited states
of IreB in two sequent ways of destabilization MLCT excited states
and then formation new MLCT excited states. By this way, the red
phosphorescence of IreB can be effectively quenched by adding CN
ions, while new yellow and green emission bands can appear in
sequent with increasing the amount of added CN ions, showing CN
sensing behavior with unique concentration distinguishing ability.
Together with the novel sensing behavior to CN ions, the phos-
phorescent sensor IreB can possess DL as low as 4.3 ꢀ 10ꢁ6 M and
good selectivity to the CN ions. The results obtained can provide
important information for developing new CN sensors with high
performances.
Detecting limit and selectivity are two key parameters to char-
acterize the performance of ion sensors. According to the phos-
phorescence titration result, the detecting limit (DL) of IreB to the
CN ions has been obtained as low as 4.3 ꢀ 10ꢁ6 M according to the
reported method, showing potential for application. When five
folds excess different anions were added to the THF solution of
IreB, most of them, such as Cl, Br, I, BF4, Ac, PF6 and ClO4, cannot
quench the red phosphoresce at 600 nm (Fig. 6). However, CN ions
can effectively quench the red phosphorescence of IreB (Fig. 6).
However, as indicated by our previous result, F ions can also quench
the phosphorescence signal at 600 nm [19]. Fortunately, it has been
shown that F ions can show much stronger binding with boron
atom in the eB(Mes)2 group of IreB. Hence, at high concentration,
F ions can induce much stronger and greener emission band than
CN ions (Fig. S1). In addition, wavelength of the green phospho-
rescence induced by CN ions is different from that induced by F ions
as well (Fig. S1). These can be employed to differentiate CN from F
ions in the sensing process. Hence, the sensor of IreB can exhibit
good selectivity to the CN ions. The good selectivity of IreB to the
CN ions can be assigned to the strong nucleophilic interaction be-
tween the CN ions and the empty pp orbital of the boron atom in
the eB(Mes)2 group.
Declaration of competing interest
Manuscript title: Phosphorescent cyanide sensor based on a
2-phenylpyridine(ppy)-type cyclometalated Ir(III) complex
bearing dimesitylboron group with concentration distinguish-
ing ability. The authors whose names are listed immediately below
certify that they have NO affiliations with or involvement in any
organization or entity with any financial interest (such as hono-
raria; educational grants; participation in speakers’ bureaus;
membership, employment, consultancies, stock ownership, or

Y. Li et al. / Journal of Organometallic Chemistry 917 (2020) 121274
5
4775.
other equity interest; and expert testimony or patent-licensing
arrangements), or non-financial interest (such as personal or pro-
fessional relationships, affiliations, knowledge or beliefs) in the
subject matter or materials discussed in this manuscript.
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This research was financially supported by the National Natural
Science Foundation of China (21572176, 21875179, 21602170 and
51803163), the Natural Science Foundation of Shaanxi Province
(2019JZ-29), Fundamental Research Funds for the Central Univer-
sities (cxtd2015003 and 01091191320074), the China Postdoctoral
Science Foundation (2015M580831 and 20130201110034), the Key
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financial supporting from State Key Laboratory for Mechanical
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