A Novel Ratiometric Oxygen Sensor Based On a Sextuple Hydrogen-Bonding Self-Assembly Molecular Heterodimer

Article abstract of DOI:10.1002/cjoc.201600257

A novel sextuple hydrogen-bonding (HB) self-assembly molecular heterodimer bearing an iridium complex as the indicator dye and two carbazoles as the reference dye, namely 6HB-Irbt-Cz, was synthesized, and its molecular structure was confirmed by¹H NMR,¹³CNMR, TOF-MS and 2D NMR. Because of the inefficient energy transfer process between the carbazole and iridium complex units, 6HB-Irbt-Cz exhibits distinct ?uorescence/phosphorescence dual emission in neat film state. More importantly, the neat film sample of 6HB-Irbt-Cz could display linear ratiometric optical response toward oxygen in the full oxygen concentration range from 0 to 100 vol%, together with good stability, reversibility and rapid response-recovery times. Note that this represents the first discovery of neat-film-based oxygen sensor capable of showing strictly linear ratiometric Stern-Volmer behavior in the oxygen concentration of 0–100 vol%.

Full text of DOI:10.1002/cjoc.201600257

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DOI: 10.1002/cjoc.201600257
A Novel Ratiometric Oxygen Sensor Based On a Sextuple
Hydrogen-Bonding Self-Assembly Molecular Heterodimer
Hui Zeng, Cheng Zhang, Yan Huang,* and Zhiyun Lu*
Key Laboratory of Green Chemistry and Technology (Ministry of Education), College of Chemistry,
Sichuan University, Chengdu, Sichuan 610064, China
A novel sextuple hydrogen-bonding (HB) self-assembly molecular heterodimer bearing an iridium complex as
the indicator dye and two carbazoles as the reference dye, namely 6HB-Irbt-Cz, was synthesized, and its molecular
1
structure was confirmed by H NMR, 13CNMR, TOF-MS and 2D NMR. Because of the inefficient energy transfer
process between the carbazole and iridium complex units, 6HB-Irbt-Cz exhibits distinct fluorescence/
phosphorescence dual emission in neat film state. More importantly, the neat film sample of 6HB-Irbt-Cz could
display linear ratiometric optical response toward oxygen in the full oxygen concentration range from 0 to 100 vol%,
together with good stability, reversibility and rapid response-recovery times. Note that this represents the first dis-
covery of neat-film-based oxygen sensor capable of showing strictly linear ratiometric Stern-Volmer behavior in the
oxygen concentration of 0－100 vol%.
Keywords hydrogen-bonding self-assembly molecular heterodimer, oxygen sensor, ratiometric response, 0
－100
vol% oxygen concentration range, linear Stern-Volmer behavior
Introduction
and excitation source fluctuations by taking the ratio of
the indicator peak intensity over the reference peak in-
tensity, leading to better reliability.[8]
Owing to its outstanding binding strength and direc-
tionality, hydrogen bond (HB) has been considered as
an ideal non-covalent interaction to construct supramo-
lecular architectures.[1] In fact, HB-based supramolecu-
lar light-emitting compounds have been successfully
used in a variety of applications like optoelectronic de-
vices[2] and functional materials.[3] However, most of
these light-emitting HB systems are fluorescent materi-
als based on singlet excited state characters. Therefore,
it could be expected that if phosphorescent transi-
tion-metal complexes such as iridium chelates are inte-
grated with the HB systems, it is possible to endow the
objective supramolecular materials with some unique
functional properties, e.g., oxygen sensing capability.
In fact, the detection of molecular oxygen level is of
great importance in chemical analysis,[4] biochemical
detection,[5] and environmental monitoring fields.[6]
Owing to their attractive advantages[7] such as no oxy-
gen consumption in the sensing process and no re-
quirement for a reference electrode, optical oxygen
sensors have received more and more attentions. Com-
pared to the photoluminescence (PL) “turn-off” oxygen
sensors, ratiometric ones capable of showing indica-
tor/reference dual emissive signals are more attractive
because they can minimize the effect of light scattering
As far as ratiometric oxygen probes are concerned,
they can be classified as small organic molecules,[7b,9,10]
polymers,[11] nanoparticles,[12] etc., bearing both fluo-
rescent fluorophors (as the reference dye) and phospho-
rescent luminogens (as the indicator dye). Yet for the
majority of these sensors, ratiometric optical response
toward oxygen could only be realized in dilute solution
state. In neat film state, the decreased intermolecular
spacing between the higher band-gap energetic donor
subunits and the lower band-gap acceptor ones will re-
sult in more efficient Förster resonance energy transfer
(FRET) processes, leading to disappeared donor band
hence vanished ratiometric response. To realize rati-
ometric sensing in condensed state, the reference and
indicator luminogens are generally dispersed into a po-
rous matrix to endow the film sensors with deficient
donor-acceptor FRET processes and effective passage-
way of oxygen molecules.[11b] Based on this strategy,
some thin film ratiometric oxygen sensors[9a,13] have
been developed, yet all of them are not able to show a
linear Stern-Volmer (S-V) behavior in the full oxygen
concentration range of 0－100 vol%, which would limit
their practical applications.[14] Therefore, it is important
to exploit new strategies to develop materials for high-
*
E-mail: huangyan@scu.edu.cn, luzhiyun@scu.edu.cn, Fax: 0086-028-85410059
Received April 28, 2016; accepted June 7, 2016; published online July 12, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600257 or from the author.
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r•min1/40 s) on a 20×9 mm2 glass substrate, and the
thickness of the neat films was measured by a Pro-
filometer (Ambios XP-2). The purity of the oxygen and
nitrogen gas used was 99.99%. In oxygen quenching
experiments, the O2/N2 ratio (vol%) of the mixed gas
was regulated with a mass-flow meter (CS200, seven-
star). The gas with proper O2/N2 ratio was bubbled
through the sample prior to the emissive measurements,
and the PL spectra were measured under different oxy-
gen concentrations. The measurement of the phospho-
rescence lifetimes in the oxygen sensing experiments
was carried out on a Single Photo Counting Controller
FluoroHub-B.
performance ratiometric oxygen sensors in neat film
state.
In this paper, we report a novel supramolecular HB
assembly 6HB-Irbt-Cz (vide Figure 1) capable of
showing distinct fluorescence/phosphorescence dual
emission in matrix-free neat-film states. Consequently,
the neat-film sample of 6HB-Irbt-Cz could show rati-
ometric response toward oxygen with a strictly linear
S-V behavior in the full oxygen concentration range
from 0 to 100 vol%, together with good photostability,
good reversibility and rapid response/recovery times.
Materials and synthesis
The molecular structures of the compounds dis-
cussed here are illustrated in Figure 1, and the synthetic
routes to the target molecules and all the synthetic and
characterization details could be found in Supporting
Information. In 6HB-Irbt-Cz, a rigid and bulky sextuple
HB self-assembly system is employed as the spacer to
isolate the donor and acceptor luminogens; while a
phorsphorescent orange iridium complex Irbt (bis(2-
(biphenyl-3-yl)benzothiazole)iridium(III)(2-(3-phenyl-1,
2,4-triazol-5-yl)pyridine)) and two violet fluorescent Cz
(carbazole) luninogens are employed as the indicator
and reference signals, respectively. In fact, 6HB-Irbt-Cz
is designed rationally to act as a ratiometric oxygen
probe: (1) the emission spectra of Cz and Irbt exhibit
good spectral separation (Figure S1); (2) inefficient en-
ergy transfer (ET) may occur between Cz and Irbt due
to the poor overlap in the metal-to-ligand charge-trans-
fer (MLCT) absorption band of Irbt (440－540 nm) and
the PL emission band of Cz (Figure S2), which is propi-
tious to the acquirement of fluorescence/phospho-
rescence dual emission in the neat-film state; (3) the
presence of the rigid bulky HB spacer between the in-
dicator and reference signalling subunits would further
suppress the intra-assembly and inter-assembly ET pro-
cess;[2f] (4) the presence of the bulky HB spacing moiety
will benefit the enhancement of the morphology of the
films.[2e]
Figure 1 Molecular structures of 6HB-Irbt-Cz, Irbt and Cz.
Experimental
General information and methods
Instruments and characterization All the chem-
icals and reagents were purchased from commercial
sources and used as received without further purifica-
tion. All the solvents were of analytical grade and have
been freshly distilled prior to use. N,N-Dimethylform-
amide and dichloromethane were prepared by freshly
distilling over calcium hydride and diphosphorous
1
pentoxide, respectively. H NMR, 13C NMR and 2D
NMR (NOESY) spectra were recorded on a Bruker
AVANCE-400 spectrometer in CDCl3 or DMSO-d6 us-
ing TMS as the internal standard. High resolution MS
spectra were recorded on a Shimadzu LCMS-IT-TOF
instrument. UV-Vis absorption spectra of the dichloro-
methane (DCM) solution (1×105 mol•L1) and neat
film samples were measured on a Perkin Elmer Lamdba
950 UV-Vis scanning spectrophotometer. PL spectra of
both the dilute solution (1×105 mol•L1 in DCM) and
thin-film samples were recorded on an LS-55 fluores-
cence spectrophotometer at 298 K.
Oxygen sensing experiments Ratiometric oxygen
sensing experiments were carried out in both solution
and neat film states. The solution samples were meas-
ured in a 10 mm×10 mm quartz cuvette with the
1×105 mol•L1 DCM solution; the 11.5 μm thick film
sample was obtained by spin-coating from the 10
mg•mL1 chloroform solution of 6HB-Irbt-Cz (1500
Results and Discussion
Preparation and characterization
The molecular structure of two independent strands
6HB-Irbt and 6HB-Cz was confirmed by 1H NMR, 13
C
NMR and HRMS. The solvent of the chloroform solu-
tion of the 1∶1 ratio mixture of 6HB-Irbt and 6HB-Cz
was removed under vacuum, and then the powder of
objective heterodimer 6HB-Irbt-Cz was obtained.
Moreover, two-dimensional NMR (NOESY) spectra
were recorded on a Bruker-AVANCE-II-400 to provide
detailed evidence supporting the formation of a self-
assembly duplex 6HB-Irbt-Cz in CDCl3 solution. As
shown in Figure S3, many strong or weak NOESY con-
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A Novel Ratiometric Oxygen Sensor Based On a Molecular Heterodimer
tact signals between the two different strands could be
observed, indicating that the two strands have been in-
dubitably assembled into a heterodimer through multi-
ple hydrogen-bonded interactions. The experimental
result is consistent with that reported by Gong’s
group.[15]
Photophysical properties
As shown in Figure S4, in both solution and
thin-film states, the absorption spectra of the objective
assembly 6HB-Irbt-Cz in the region of 250－500 nm
are just the simple overlap of those of the small molec-
Figure
2
Normalized PL spectra of Cz, Irbt and 6HB-Irbt-Cz in
ular luminogens Irbt and Cz, indicating that the elec-
tronic interactions between Irbt and Cz chromophores
in 6HB-Irbt-Cz are negligibly in the ground state.[16] As
expected, under excitation at 290 nm, 6HB-Irbt-Cz
shows obvious dual emission property in dilute di-
chloromethane (DCM) solution (Figure 2), with its two
emission maximum located at 378 nm (λF,PLmax) and 544
nm (λP,PLmax), which could be assigned to the PL peaks
of Cz and Irbt luminogens, respectively. The small
bathochromically shifted (15 nm) λF,PLmax of the carba-
zolyl fluorophors in 6HB-Irbt-Cz relative to that of its
small molecular counterpart Cz could be attributed to
the contribution of the σ-π conjugation of the alkyl
chain connected to the N-atom of Cz in 6HB-Irbt-Cz.
Excitingly, in neat-film state, dual emission could also
be observed in 6HB-Irbt-Cz. It is noteworthy that for
both the two small luminogens Cz and Irbt, their
neat-film samples were observed to show 9－14 nm
red-shifted λPLmax in comparison with their correspond-
ing dilute solution samples, which should be ascribed to
the intensified intermolecular interactions in condensed
states; yet in the case of 6HB-Irbt-Cz, its λPLmax in di-
lute solution and neat-film states is nearly identical (546
nm vs. 549 nm), indicative of the alleviated molecular
packing of Irbt and Cz chromophores in the thin film
sample of 6HB-Irbt-Cz, which should be attributed to
the effective spatial isolation of these luminogens by the
HB moiety. Therefore, the integration of HB units with
fluorophores will benefit the alleviation of their PL
concentration quenching,[2f] leading to higher oxygen
sensing performance.
dichloromethane solution and neat-film states (ex＝290 nm).
Figure
3 PL spectra of 6HB-Irbt-Cz in DCM solution at various
oxygen concentrations from 0 to 100 vol% (ex＝290 nm).
Oxygen sensing properties
Nonetheless, in neat-film state, with increasing O2
concentrations, the intensity of the fluorescence band of
6HB-Irbt-Cz is nearly constant; while that of the or-
ange phosphorescence band decreases gradually (Figure
4a). The quenching ratio at λ＝549 nm was determined
to be 53.7%. Further investigations on the S-V behavior
based on the IF(394 nm)/IP(549 nm) indicated that a strict lin-
earity could be realized with R2 of 0.999 (shown in Fig-
ure 4b). Note that although there were some reports on
neat-film ratiometric oxygen sensors,[15] none of them,
to the best of our knowledge, could display a linear S-V
behaviour in the full oxygen concentration range of
Figure 4 (a) PL spectra of the neat-film sample of 6HB-Irbt-
Cz under various oxygen concentrations; and (b) Stern-Volmer
curve of the neat-film sample of 6HB-Irbt-Cz at 298 K (ex＝
290 nm).
Chin. J. Chem. 2016, 34, 873—877
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0－100%, and 6HB-Irbt-Cz is the first example of ox-
ygen sensor capable of exhibiting linear ratiometric S-V
behavior in neat-film state at oxygen concentrations of 0
－100 vol%.
compounds should be a promising ratiometric oxygen
sensor for practical applications.
Conclusions
Additionally, transient PL measurement results indi-
cated that for the fluorescence band of 6HB-Irbt-Cz, its
lifetime is too short to be measured accurately by our
instrument of FluoroHub-B; yet for the phosphores-
cence band of 6HB-Irbt-Cz, a clear linear (R2＝0.985)
shortening of its lifetime (τ) at λem＝549 nm from 1.24
to 0.74 μs with increasing O2 concentration from 0 to
100% was also observed (vide Figure S5), confirming
the linear S-V behavior of the indicator signals of
6HB-Irbt-Cz.
Figure 5b shows the typical oxygen responses of the
ratiometric sensor based on the neat-film of 6HB-Irbt-
Cz when being switched between fully oxygenated and
fully deoxygenated environments. It can be seen that the
sensor displays stable and reproducible PL quenching
signals toward oxygen, with ＞90% recovery of initial
PL intensity during the ~1 h measurement, indicating
that the sample possesses good photostability. The 95%
response time (t↓95%, by the 95% decrement of PL in-
tensity after switching from pure N2 to pure O2) for
6HB-Irbt-Cz is determined to be 18.3 s; while the 95%
recovery time (t↑95%, by the 95% increase of PL inten-
sity after switching from pure O2 to pure N2) is 21.5 s.
Owing to the relatively rapid response/recovery, excel-
lent photostability, this kind of HB supramolecular
In summary, we have prepared an HB supramo-
lecular material 6HB-Irbt-Cz capable of showing flu-
orescence/phosphorescence dual emission properties in
neat-film state at room temperature, hence showing ra-
tiometric response toward O2. This neat-film-based ra-
tiometric oxygen sensor displays a strictly linear Stern-
Volmer behavior in the full oxygen concentration range
from 0 to 100 vol%, good photostability, good reversi-
bility and rapid response/recovery times. All these pre-
liminary results indicated that HB self-assembly supra-
molecular materials are quite promising materials for
oxygen sensor applications. We believe that these find-
ings are of particular interest for the development of
high-performance ratiomentric oxygen sensor.
Acknowledgements
We acknowledge the financial support for this work
by the National Natural Science Foundation of China
(No. 21372168).
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Figure
5 (a) Phosphorescence lifetime variation of the neat-film
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