Effective phosphorescence quenching in borylated PtII ppy-type phosphors and their application as I- ion sensors in aqueous medium

Article abstract of DOI:10.1039/c2cc37135a

The first example of phosphorescent I^- ion sensors based on borylated Pt^II ppy-type phosphors (Hppy = 2-phenylpyridine) was reported, which shows good selectivity and sensitivity.

Full text of DOI:10.1039/c2cc37135a

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Effective phosphorescence quenching in borylated PtII
ppy-type phosphors and their application as IÀ ion
sensors in aqueous medium†‡
Xiaolong Yang,a Zuan Huang,a Jingshuang Dang,a Cheuk-Lam Ho,b
Guijiang Zhou*a and Wai-Yeung Wong*b
Cite this: DOI: 10.1039/c2cc37135a
Received 1st October 2012,
Accepted 5th November 2012
DOI: 10.1039/c2cc37135a
www.rsc.org/chemcomm
The first example of phosphorescent IÀ ion sensors based on aqueous or nonaqueous conditions. Inspired by both the nucleo-
borylated PtII ppy-type phosphors (Hppy = 2-phenylpyridine) was philic property of IÀ ions and the inherent character of the Pt center
reported, which shows good selectivity and sensitivity.
in PtII ppy-type phosphors, we conceive the potential of functiona-
lized PtII ppy-type phosphors as novel chemosensors for IÀ ions via
Among the most efficient phosphorescent emitters, PtII ppy-type b- the PtÁÁÁI interactions. More importantly, some merits, such as the
diketonato phosphors (Hppy = 2-phenylpyridine) are almost invari- large Stokes shift, longer excited state lifetime and small spectral
ably employed in fabrication of highly efficient organic light-emitting overlap with the excitation spectra, etc., render phosphorescence a
devices (OLEDs).1 However, exploration of the potential applications promising detecting message in such application.7
of this kind of molecules in other fields remains a great challenge.
In order to fulfil the above proposed idea, highly emissive
PtII ppy-type b-diketonato complexes possess square planar geometry borylated PtII ppy-type phosphors (PtB-1 and PtB-2, see Fig. 1)
with very exposed metal center which will be susceptible to the were prepared and applied successfully to IÀ ion sensing with
attack by external reagents, especially those showing nucleophilic the detection limit (D.L.) as low as 5.6 Â 10À7 M. Besides the
character.2 Owing to the critical role played by the PtII center in the traditional fluorescent chemosensing systems for IÀ ions,8
emissive excited states (triplet metal-to-ligand charge transfer, these complexes, to our knowledge, represent the first example
3MLCT) of the PtII ppy-type phosphors,1a the attack of nucleophilic of phosphorescent IÀ ion sensors in the literature.
species on Pt ions will likely lead to variation in their phosphorescent
properties which can be harnessed as a suitable sensing signal.
Both PtII complexes display two typical UV–vis absorption bands
which can be assigned to the strong absorption p–p* transition
1
Iodide anions are of great importance to some biological band (o350 nm) and the weak 1MLCT and 3MLCT features
processes, such as thyroid hormone biogenesis.3 Mediated by the (>350 nm). These heteroleptic complexes emit intense phosphores-
membrane glycoprotein Na+/IÀ symporter (NIS), the active IÀ ions cence with very high quantum yields (FP, 0.45 for PtB-1 and 0.54 for
can readily transport in the thyroid gland to execute their function.4 PtB-2) under air saturated conditions (Fig. 2 and Table S1, ESI‡),
Hence, the IÀ ions in the thyroid gland should be maintained at a which can overcome the limitation of other traditional phos-
proper level. Otherwise, thyroid diseases could result. According to phorescent systems used as turn-off sensors due to their low FP.
the survey of the World Health Organization (WHO), iodide
Enhanced by its electron-deficient substitution position
deficiency still causes serious public health problems in many shown by the HOMO of [Pt(ppy)acac],1a the electron-accepting
countries.5 As a strong nucleophilic reagent, IÀ ions are also B(Mes)2 group in PtB-1 has switched the MLCT processes by
commonly employed in many important organic reactions.6 accepting the electron density of the MLCT process (as indicated
Therefore, it is desirable to detect IÀ ions and determine their in the LUMO pattern of PtB-1 with a notable contribution from
content rapidly with high sensitivity and selectivity under either the B(Mes)2 unit) (Fig. S1, ESI‡) and hence forms more stable
3MLCT states. So, PtB-1 exhibits lower-energy MLCT absorption
bands and red shifted emission maxima with respect to those of
a Department of Chemistry, Faculty of Science, Xi’an Jiaotong University, Xi’an,
P. R. China. E-mail: zhougj@mail.xjtu.edu.cn; Fax: +86-29-8266 3914;
Tel: +86-29-8266 3914
b Institute of Molecular Functional Materials, Department of Chemistry and Institute
of Advanced Materials, Hong Kong Baptist University, Hong Kong, P. R. China.
E-mail: rwywong@hkbu.edu.hk; Fax: +852-3411 7348; Tel: +852-3411 7074
† This article is part of the ChemComm ‘Emerging Investigators 2013’ themed issue.
‡ Electronic supplementary information (ESI) available: Syntheses and
experimental procedures, photophysical and DFT data, absorption and PL
Fig. 1 The chemical structures of PtB-1 and PtB-2.
response curves and relevant photos. See DOI: 10.1039/c2cc37135a
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Fig. 4 The proposed binding mechanism of IÀ ions with the PtII complex.
Fig. 2 Absorption and photoluminescence (PL) spectra of the PtII complexes in
THF (conc. B10À5 M) at 293 K.
carried out. Based on the observed emission quenching for
[Pt(ppy)acac] by the added IÀ ions and the inert behavior of the
organic ligands towards the external IÀ ions (Fig. S3, ESI‡) together
with the near invariability of the NMR spectra for all the complexes
in the 1H-NMR titration experiments (Fig. S4, ESI‡), it can be safely
concluded that the effective phosphorescence quenching in PtB-1
and PtB-2 is definitely induced by the binding of IÀ ions to the Pt
center. So, the proposed interaction mechanism between IÀ ions
and the Pt center is shown in Fig. 4. When a smaller amount of IÀ
ions is introduced, the nucleophilic IÀ ion will bind with the Pt
center to form a one-to-one nonemissive adduct. With the addition
of IÀ ions to obtain an IÀ : [Pt] ratio of ca. 1.0, nearly all the emissive
PtII molecules are converted into the nonemissive adducts. So, most
of the phosphorescent emission of the complexes is quenched at an
IÀ : [Pt] ratio of ca. 1.0 (Fig. 3). Due to the nonemissive nature of the
one-to-one adduct, adding more IÀ ions will induce a much weaker
quenching effect, as depicted in Fig. 3. After reaching an IÀ : [Pt]
ratio of ca. 2.0, the phosphorescent signal of the complex remains
stable, although more IÀ ions have been added (Fig. 3). Hence, the
nonemissive two-to-one adduct will finally be formed (Fig. 4).
In order to support the above proposed mechanism, electro-
spray ionization time-of-flight mass spectrometry (ESI-TOF-MS)
was employed. In the TOF-MS spectra of the solutions of PtB-1
and PtB-2 at an IÀ : [Pt] ratio of ca. 1.0, an obvious signal at around
m/z 823 (m/z 823.1664 for PtB-1 and m/z 823.1713 for PtB-2) due to
the one-to-one adducts (F.W. = 823.1532) can be clearly observed
(Fig. S5a, ESI‡). However, the TOF-MS signals from the one-to-one
adducts would disappear and the ones caused by the two-to-one
adducts can be detected at ca. m/z 950 (m/z 950.0497 for PtB-1 and
m/z 950.0811 for PtB-2) (Fig. S5b, ESI‡) for the solutions of PtB-1
and PtB-2 at an IÀ : [Pt] ratio of ca. 2.0. Furthermore, PtII ppy-type
complexes can react with I2 to give compounds with similar
structure to those of the proposed two-to-one adducts.9 So, all of
these results have nicely verified the validity of the proposed IÀ ion
sensing mechanism associated with PtB-1 and PtB-2.
[Pt(ppy)acac] (Fig. 2). However, the shifting of the MLCT process
cannot occur in PtB-2, since the B(Mes)2 group in PtB-2 is bonded
to the electron-rich site.1a So, PtB-2 shares similar pyridyl-oriented
MLCT character to that of [Pt(ppy)acac], shown in their allied
LUMO pattern. The weak electron-accepting feature of the B(Mes)2
unit makes PtB-2 to show a slight hypochromic effect in its PL
spectrum relative to that of [Pt(ppy)acac] (Fig. 2).
In order to show whether there is any interaction between IÀ
ions and PtII molecules and an induced variation in the phosphor-
escent emission of the concerned complexes, the phosphorescent
titration experiments for the highly emissive solutions of PtB-1 and
PtB-2 (10À5 M in THF) were carried out by adding NBu4I (10À3 M in
THF) solution as the IÀ source. As expected, the phosphorescence
of each complex is quenched rapidly by the added IÀ ions, showing
the strong interaction between the external IÀ ions and the PtII
complex (Fig. 3 and Fig. S2, ESI‡). When the molar ratio of the IÀ
ions added to the PtII complex equals ca. 1.0 (i.e. IÀ : [Pt] ratio E
1.0), the phosphorescent intensity of the complex is mostly
quenched (ca. 90% for PtB-1 and 80% for PtB-2, respectively)
(Fig. 3 and Fig. S2, ESI‡), which reveals the high sensitivity of
PtB-1 and PtB-2 to the IÀ ions as well as the great potential of both
molecules as novel phosphorescent IÀ ion sensors. When the
IÀ : [Pt] ratio reached 1.0, adding more IÀ ions did not cause
significant quenching of the phosphorescent signal of the PtB-1
and PtB-2 solutions. And the phosphorescent emission of the PtB-1
and PtB-2 solutions is almost totally quenched by the added IÀ ions
at an IÀ : [Pt] ratio of about 2.0.
With the aim to clarify whether the sensing behavior of PtB-1
and PtB-2 is caused by the interaction between the Pt center and IÀ
ions or not, the emission responses of both [Pt(ppy)acac] and the
organic ligands of PtB-1 and PtB-2 to IÀ ions were studied, and the
1H-NMR titration experiments for the concerned complexes were
From the IÀ ion sensing mechanism aforementioned, the
phosphorescence quenching of PtB-1 and PtB-2 by IÀ ions belongs
to the static quenching. The associated Stern–Volmer quenching
constants (Ksv), also known as the binding constants for the
quencher–acceptor system,10 are 4.6 Â 105 MÀ1 for PtB-1 and 3.5
 105 MÀ1 for PtB-2, which are higher than that for [Pt(ppy)acac]
(1.9 Â 105 MÀ1). The higher Ksv for PtB-1 and PtB-2 can be ascribed
to the electron-accepting B(Mes)2 group in their ligands which
would reduce the electron density on the Pt center and facilitate
the nucleophilic interaction with IÀ ions. Our phosphorescent
systems generally exhibit higher Ksv than those of the traditional
fluorescent IÀ ion sensors (Table 1). Furthermore, PtB-1 and PtB-2
Fig. 3 PL spectral changes for PtB-1 and PtB-2 at different IÀ : [Pt] ratios.
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Table 1 Comparison of the IÀ sensing properties of our phosphorescent PtII
complexes and other typical traditional fluorescent counterparts
IÀ sensing systems
Ksv/MÀ1
D.L./M
Ref.
Benzimidazole-based tripod 1 2.2 Â 105 2.10 Â 10À6 8a
Benzimidazole-based tripod 2 1.3 Â 103 7.45 Â 10À6 8b
Ag-DQAg
—
—
7.16 Â 10À6 8c
1.26 Â 10À7 8d
Thymine-HgII-Thymine
Carbazole-based copolymer
PtB-1
Fig. 5 The emission colour response of self-made sensing paper based on PtB-1
and PtB-2 at different concentrations of the IÀ ions in water. (a) Fresh sensing
paper. (b) After dipping into 1.0 Â 10À5 M water solution containing ClÀ, BrÀ,
1.6 Â 105
—
8e
4.6 Â 105 5.6 Â 10À7
3.5 Â 105 5.7 Â 10À7
1.9 Â 105 1.5 Â 10À6
This work
This work
This work
AcÀ, BF4À, PF6À, PO43À, ClOÀ and ClO4À. (c) After dipping into 1.0 Â 10À5 M water
PtB-2
[Pt(ppy)acac]
solution containing IÀ, ClÀ, BrÀ, AcÀ, BF4À, PF6À, PO43À, ClOÀ and ClO4
.
À
show lower D.L. towards IÀ ions than most of the other fluorescent
counterparts as well (Table 1). All of these results establish that our
phosphorescent systems can serve as a new type of IÀ ion sensors.
The IÀ ion sensing selectivity experiments were also carried out. The
In conclusion, the effective phosphorescence quenching in
PtII ppy-type complexes via the binding of the Pt center
with external IÀ ions demonstrates good potential for IÀ ion
sensing with low D.L. in the order of 10À7 M and Ksv as high as
4.6 Â 105 MÀ1. Furthermore, these phosphorescent chemosensors
show good selectivity to IÀ ions. Importantly, the sensing papers
made from the phosphors can be easily applied to detection of IÀ
ions in aqueous solution. These results provide a new outlet to
furnish phosphorescent IÀ ion sensors with very good potential
for practical application.
We thank the Tengfei Project from Xi’an Jiaotong University, the
Program for New Century Excellent Talents in University, the
Ministry of Education of China (NECT-09-0651), the National
Natural Science Foundation of China (No. 20902072), HKBU
(FRG2/10-11/101), Hong Kong Research Grants Council
(HKBU202709 and HKUST2/CRF/10) and Areas of Excellence
Scheme, University Grants Committee of HKSAR (Project No.
[AoE/P-03/08]) for financial support.
phosphorescent signal from PtB-1 and PtB-2 is inert to other anions,
À
such as ClÀ, BrÀ, AcÀ, BF4À, PF6À, PO43À, ClOÀ and ClO4
,
indicating a good selectivity for IÀ ions (Fig. S6, ESI‡). However,
FÀ ions can also quench the phosphorescence of PtB-1 and PtB-2
(Fig. S7, ESI‡). From the reported FÀ sensing systems bearing
B(Mes)2 moieties11 and the lack of phosphorescent response of
[Pt(ppy)acac] to the added FÀ ions (Fig. S8, ESI‡), the quenching
effect induced by external FÀ ions should be their binding to the B
atoms in B(Mes)2 groups, which is different from that of the IÀ ions
(Fig. S9, ESI‡). As aforementioned, the B(Mes)2 moiety in PtB-1
accepts electron density in its MLCT process. The binding of
negatively charged FÀ ions with the B atom in B(Mes)2 will obviously
cause an electrostatic repulsion effect between boron and the FÀ
ion, which would push the electron density on boron in PtB-1 to the
electron-poor pyridyl group in the cyclometallating ligand (Fig. S10,
ESI‡). This implies the switching of the destination for electrons in
the MLCT process from the B atom to the pyridyl group in PtB-1 via
the binding with FÀ ions, which has also been confirmed by our
theoretical computational results. Due to the weaker electron-
accepting capacity of the pyridyl group than the B(Mes)2 unit (vide
infra),1e the shifting of the MLCT process should elevate the energy
level of MLCT states in PtB-1 and may induce a new high-energy
phosphorescent band, as indicated by the emergence of a weak
emission band at around 480 nm (Fig. S7a, ESI‡). The shifting of the
MLCT in PtB-2 cannot happen by binding with FÀ ions, since the
B(Mes)2 group did not accept the electron density in its MLCT
process. Hence, no new emission band at high energy appeared
(Fig. S7, ESI‡). So, both the theoretical and experimental data have
supported the proposed FÀ ion binding mechanism for PtB-1 and
PtB-2. With different sensing mechanisms, the PtII complexes
exhibit different absorption response and solution colour variation
to the external IÀ and FÀ ions (Fig. S11, ESI‡). These differences in
the photophysical properties render our metallophosphors’ strong
ability to differentiate between IÀ and FÀ ions.
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aqueous conditions. So, sensing papers made from PtB-1 and PtB-2
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IÀ ions at a concentration of ca. 1.0 Â 10À5 M, the phosphorescence
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun.