Efficient fluorescent chemosensors for HSO4- based on a strategy of anion-induced rotation-displaced H-aggregates

Article abstract of DOI:10.1039/c3cc42291g

A novel fluorometric sensing strategy based on the anion-induced rotation-displaced H-aggregates of styrylindolium dyes was employed to enhance the selectivity of fluorescent chemosensors for HSO₄^- detection. The marvelous anion-indu

Full text of DOI:10.1039/c3cc42291g

Chemical Communications
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Volume 49 | Number 56 | 18 July 2013 | Pages 6223–6354
ISSN 1359-7345
COMMUNICATION
Xianshun Zeng et al.
Efficient fluorescent chemosensors for HSO4− based on a strategy of
anion-induced rotation-displaced H-aggregates

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À
Efficient fluorescent chemosensors for HSO4 based
on a strategy of anion-induced rotation-displaced
H-aggregates†
Jiajia Chang,a Yan Lu,a Song He,a Chang Liu,a Liancheng Zhaoab and
Xianshun Zeng*a
Cite this: Chem. Commun., 2013,
49, 6259
Received 29th March 2013,
Accepted 8th May 2013
DOI: 10.1039/c3cc42291g
www.rsc.org/chemcomm
A novel fluorometric sensing strategy based on the anion-induced H-aggregates of the dyes. In this paper, 4-hydroxystyrylindolium
rotation-displaced H-aggregates of styrylindolium dyes was employed to derivatives L1 and L2 were chosen as sensitive and selective
enhance the selectivity of fluorescent chemosensors for HSO4À detection. chemosensors for HSO4À anions based on the following considera-
The marvelous anion-induced H-aggregate strategy opens new routes to tions: (i) a hydroxyl on the para-position of the dye offers an anion
simple synthesis of receptors for tetrahedral anionic species.
binding site via hydrogen bonding; (ii) the charged indolium moiety
donates a positive charge for electrostatic interactions with the
Motivated by applications in medical diagnostics, environmental anionic guest; (iii) the steric hindrance arising from the spatial
and industrial monitoring, and nuclear waste cleanup,1 the selec- orientation of the dye’s 3,3-dimethyls prevents them from the
tive sensing of anions by fluorometric methods2 has been the formation of an anion-induced face-to-face H-aggregate. Based on
subject of intensive research effort. Compared with the receptor- this idea, we developed successfully L1 and L2 as selective chemo-
À
based metal ions and small-sized anions sensing,2a the design of sensors for HSO4 anions Scheme 1.
receptors with the complementary conformation for the detection
Firstly, the photophysical properties of the dyes were elucidated
by UV/Vis absorption and fluorescence spectra in the absence and in
À
of tetrahedral anionic species, such as SO42À, HSO4À, H2PO4
,
2À
HPO4 etc., is quite challenging because of their extremely large the presence of various anions in H2O–EtOH (1 : 1, v/v). L1 showed
hydration energy according to the Hofmeister bias.3 Thus, it is the characteristic intense charge-transfer band occurring in the
highly desirable to seek novel, simple and efficient paradigms to long-wavelength region between 400 and 560 nm, which peaked
realize the high selectivity and sensitivity for anion detection.
at 541 nm (e = 1.18 Â 105 MÀ1 cmÀ1) (Fig. S1, ESI†).ÀIt is interesting
It is known that the intriguing optical properties of J- and to note that the addition of 10.0 equiv. of HSO4 into the red
H-aggregates are of significant interest for organic dyes conceived solutions of L1 immediately generated a yellow solution with a new
by supramolecular approaches.4 To date, the photophysical prop- absorption band centred at 434 nm (e = 5.81 Â 104 MÀ1 cmÀ1),
erties of J-aggregates have been widely utilized to develop novel which matched perfectly those of classical H-type dimer aggregates
fluorescent and colorimetric chemosensors for chemical detec- with a pronounced hypsochromic shift.8 However, no obvious
tions, or to develop photosensitizers for photographic applica- absorption and colour changes arose from the addition of other
2À
tions.5 Comparatively, the applications of the optical properties anions, such as PO43À, HPO42À, H2PO4À, NO3À, SO42À, ClÀ, CO3
,
of H-aggregates are rare due to their blue shifted absorption FÀ, and AcÀ. To understand the HSO4À-induced aggregation, the
bands and fluorescence quenching properties.6 In 2006, absorption spectra of the styrylindolium dyes L2–L4 were also
Wu¨rthner et al.7 reported an example of fluorescent H-aggregates investigated under conditions identical to those used for L1. Similar
composed of slight rotation-displaced merocyanine dyes.
Enlightened by Wu¨rthner’s work,7 we reported herein a novel
strategy for anionic receptor design by taking full advantage of the
fluorescent properties of the anion-induced rotation-displaced
to L1, the electronic absorption spectra of L2 in aqueous ethanol
a Key Laboratory of Display Materials and Photoelectric Devices, Ministry of
Education, School of Materials Science and Engineering, Tianjin University of
Technology, Tianjin 300384, China. E-mail: xshzeng@tjut.edu.cn;
Fax: +86-22-60215226
b School of Materials Science and Engineering, Harbin Institute of Technology,
Harbin 150001, China
† Electronic supplementary information (ESI) available: Experimental details;
synthesis of L1–L4 and their spectra. See DOI: 10.1039/c3cc42291g
Scheme 1 Structures of the styrylindolium dyes L1–L4.
Chem. Commun., 2013, 49, 6259--6261 6259
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Scheme 2 The proposed HSO4À-induced rotation-displaced H-aggregate of L
via hydrogen bondings, charge interactions and p–p interactions.
Fig. 1 Photograph of L1 (a and b) and L2 (c and d) in the presence of various
spectroscopy (Fig. S9, ESI†). Upon increasing the temperature,
the transformation from the aggregates to the monomeric species
was observed, thus revealing the reversibility of the anion-induced
aggregation process. On the other hand, although the concen-
tration-dependent absorption spectra6,12 of the free chemosensor
L1 increased nonlinearly at 434 nm provided additional evidence
for the formation of aggregates under higher concentrations,
anions. (a) and (c) Color changes in the presence of different anions; (b) and (d)
fluorescence changes under UV 365 nm excitation in the presence of different
À
anions. (a)–(d) From right to left, (10.0 equiv.) of PO43À, HPO42À, H2PO4À, NO3
,
SO42À, HSO4À, ClÀ, CO32À, FÀ, AcÀ and L1 or L2 alone in H2O–EtOH (1 : 1, v/v).
showed the characteristic charge-transfer band positioned at
580 nm (e = 1.19 Â 105 MÀ1 cmÀ1), which was hypsochromically
shifted to 448 nm upon the addition of HSO4À (Fig. S2, ESI†). Thus,
the HSO4À-induced unique absorption changes of L1 and L2
indicated that they can be used as selective chemosensors for
HSO4À. Meanwhile, the interesting features of HSO4À-induced
colour changes of L1 and L2 revealed that they can serve as selectivÀe
visible chemosensors for HSO4À. As shown in Fig. 1, only HSO4
induced a prominent colour change (Fig. 1a and c). At the same
time, fluorescence of L1 and L2 also showed a characteristic colour
À
prominent changes are not observed, indicating that the HSO4
anion played a key role in inducing the aggregation (Fig. S10,
À
ESI†). Accordingly, we hypothesized that the HSO4 was firstly
bonded with the free chemosensor L1 via hydrogen bonding to
À
form a L1ÁHSO4 complex, then it was further aggregated in a
head-to-tail fashion via intermolecular p–p interactions and electro-
static interactions to form an H-aggregate (Scheme 2). It has been
reported that functionalization of the dye’s electron-donor moiety
with bulky groups induced displacement of the monomer into a
slip-stacked arrangement in the solid state.13
À
change in the presence of HSO4 when excited under a hand-UV
lamp at 365 nm (Fig. 1b and d). While in the case of a styryl moiety
with a hydroxyl at the 3-position, however, L3 and L4 didn’t show a
hypsochromically shifted absorption band, which indicated that the
structure is unfavourable for the formation of HSO4À-induced
H-aggregates (Fig. S3 and S4, ESI†).
To obtain further insight into the bonding properties of
HSO4À with L1 and L2, the absorptionÀand fluorescence spectra
of the dyes upon titration with HSO4 were obtained in H2OÀ–
EtOH (1 : 1, v/v). As shown in Fig. 2a, upon addition of HSO4
(0–25.5 equiv.), a significant decrease in the absorption band of
L1 at 541 nm and an increase in the new band at 434 nm was
observed. Two distinct isosbestic points at 268 nm and 476 nm
were observed, indicating the formation of a new complex
between L1 and HSO4À. The absorption bands at 541 and
434 nm linearly decreased and increased up to a 4 : 1
[HSO4À]/[L1] ratio, respectively (Fig. S11, ESI†). Moreover, the
absorbance at 434 nm remains coÀnstant in the presence of
more than 5.0 equivalents of HSO4 anions (inset of Fig. 2a).
The absorbance ratio R (A434/A541) increased upon increasing
the concentration of HSO4À, which also allowed the use of L1
To gain a general understanding of the fluorescence proper-
ties of the anion-induced aggregates, fluorescence spectra of the
dyes were recorded by excitation at the maximum absorption
band of the H-aggregates. Under these conditions, both L1 and
L2 showed a prominent blue-shifted fluorescence enhancement
À
upon the addition of 10 equiv. of HSO4 anions (Fig. S5 and S6,
ESI†). However, the addition of 10 equivalents of other anions has
no obvious effect on the fluorescence intensity. Upon excitation
at the isosbestic point of L1 at 476 nm, it showed a monomer
emission band centered at 568 nm (Fig. S7, ESI†). Upon the
À
addition of various anions, only HSO4 induced a blue-shifted
emission, whereas other anions afforded no obviously fluores-
cent changes. The inset of Fig. S7 (ESI†) exhibits the dependence
of the intensity ratios of L1 at 528 nm to that at 568 nm (I528/I568
on the anions, which showed specific selectivity toward HSO4
)
À
among all the examined anions. Upon excitation at the isosbestic
point of L2 at 501 nm, however, only a similar blue-shifted
fluorescence enhancement at 562 nm as those excited at the
maximum absorption band of the aggregation was observed
(Fig. S8, ESI†). The HSO4À-induced H-aggregates exhibited signi-
ficant enhanced or ratiometric fluorescence changes, suggesting
that L1 and L2 may be useful as fluorescent chemosensors.
To qualitatively assess the thermodynamic parameters for their
aggregation processes, the HSO4À-induced H-aggregates of L1
À
Fig. 2 (a) Absorption spectra of L1 (10.0 mM) to increasing concentrations of HSO4
in H2O–EtOH (1 : 1, v/v). Inset: the absorbance changes of L1 at 434 nm as a function
À
of the HSO4 concentration. (b) Fluorescent titration spectra of L1 (6.0 mM) in the
À
presence of different concentrations of HSO4 in EtOH–H2O (1 : 1, v/v). Inset: plot of
À
the emission ratio R upon HSO4 titration of L1, wherein R is defined as the ratio of
were studied by temperature-dependent electronic absorption the emission intensities at the two wavelengths [I528nm/I568nm for L1]. lex = 476 nm.
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for the detection of HSO4À by ratiometric absorbance methods dyes as chromophores provided a simple and efficient way for
(Fig. S12, ESI†). From the sigmoidal curves, the apparent the selective recognition of HSO4À, an interesting tetrahedral
dissociation constant (Kd) 7.04 Â 10À5 M (R = 0.997) is obtained. anion owing to its established role in biological and industrial
Stoichiometry for L1 and the HSO4À complex was evaluated on areas and its harmfulness for human health as well.15 The
the basis of the Job’s plot and was found to be 1 : 1 (Fig. S13, chemosensors showed significant fluorescence signal changes
ESI†). Similar experiments were also conducted with L2, and with high sensitivity and selectivity toward HSO4À, allowing its
À
the results imply that L2 is also a good chemosensor for HSO4
selective detection in the presence of a wide range of the
anions (Fig. S14, ESI†). The Kd derived from the titrations was environmentally relevant competing anions. With smart structural
found to be 3.91 Â 10À5 M (R = 0.998) (Fig. S15, ESI†). Job plots design, we anticipate that this paradigm, by taking advantage of
obtained from fluorescence tÀitrations indicated a 1 : 1 stoichio- the anion-induced rotation-displaced H-aggregates, provides a very
metry for the receptor–HSO4 interaction (Fig. S16, ESI†).
By excitation at the maximum absorption band of the aggre-
simple way for designing anion-selective chemosensors.
This work was sponsored by the NNSFC (21272172, 20972111,
gates, both L1 and L2 showed a fluorescence enhancement with 21074093, 21004044), NCET-09-0894, the NSFT (12JCZDJC21000,
the increase of HSO4À concentrations (Fig. S17 and S18, ESI†). The 08JCYBJC26700).
Kd values 5.81 Â 10À5 M (R = 0.993) for L1 and 4.29 Â 10À5 M for
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M
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practical applicability in anion detection, further experiments to
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excitation at 476 nm in the presence of the above-mentioned
anions. Upon addition of HSO4À anions, the solution still displayed
a distinct fluorescence enhancement, indicating that they showed
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