Twisting (conformational changes)-based selective 2D chalcogeno podand fluorescent probes for Cr(iii) and Fe(ii)

Article abstract of DOI:10.1039/c1cc10425j

The fluorenoazomethine containing chalcogeno podand fluorescent probes having N, O/S/Se coordinating donor unit show donor specific 'turn-on' recognition property towards metals (Cr(iii), Fe(ii) and Cu(ii)), where the fluorescence signals are controlled b

Full text of DOI:10.1039/c1cc10425j

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COMMUNICATION
Twisting (conformational changes)-based selective 2D chalcogeno podand
fluorescent probes for Cr(III) and Fe(II)w
Snigdha Panda,* Palas Baran Pati and Sanjio S. Zade*
Received 21st January 2011, Accepted 7th February 2011
DOI: 10.1039/c1cc10425j
The fluorenoazomethine containing chalcogeno podand fluorescent
probes having N, O/S/Se coordinating donor unit show donor
specific ‘turn-on’ recognition property towards metals (Cr(III),
Fe(II) and Cu(II)), where the fluorescence signals are controlled
by the conformational change of the ligand framework on
binding with the metal ion.
In this study, we report the fluorenoazomethine containing
podand fluorescent probes having N, O/S/Se coordinating
donor unit and their specific ‘turn-on’ recognition property
towards metals (Cr(III), Fe(II) and Cu(II)), where the fluores-
cence signals are controlled by the conformational change
of the ligand framework on binding with the metal ion.
However, in contrast to the photoinduced electron transfer
(PET)/internal charge transfer (ICT),10a in our approach the
suppression of internal conversion (IC) and inter system
crossing (ISC) produces FE. The podand 1 (N, O donors)
acts as a fluorescent probe to Cu(II), Cr(III) and Fe(II),
whereas, podands 2 (N, S donors), 3 and 4 (N, Se donors)
act as two dimensional (2D) probes for Cr(III) and Fe(II)
(Table 1).
Much attention has been focused on the development of
chemosensors for the selective and efficient detection of
biologically and chemically important cations.1 Fluorescence
quenching is more common when transition metal (TM) ions
are bound to fluorophore–receptor systems. Due to better
selectivity and sensitivity, the fluorescence enhancement (FE,
‘turn-on’) induced by complexation with metal ions is more
desirable than fluorescence quenching. Cr(III), Fe(II) (in the
case of binding with the low-field ligands) and Cu(II) are the
most effective fluorescent quenchers due to their paramagnetic
nature which also makes it difficult to develop the ‘turn-on’
fluorescent probes.2
Iron3 is an important body constituent and chromium4 and
copper5 are essential trace elements for human beings. Reports
on the fluorescent probes that exhibit a ‘turn-on’ response in
the presence of Cr(III),6 Fe(II)7 and Cu(II)8 are scarce. Few
strategies are available to prevent the fluorescence quenching
in the presence of heavy and TMs. FE can be observed
by introducing the communication gap between the metal
ion and the fluorophore via a fluorophore–spacer–receptor
configuration.9 Twisting-based decoupling of a receptor and
fluorophore was exploited for the change in electron/charge
transfer properties which resulted in FE.10 Another strategy is
to reduce the quenching by enhancing the oxidation potential
of the fluorophore which in turn reduces the single electron
transfer from singlet excited state to metal ion.11 The chemo-
dosimetric detection of heavy and TMs by FE has also been
explored.12
The design of fluorescent probes includes initial unbound
fluorenoazomethines podand (1–4) in an ‘off’ state as a result
of the attachment of azomethine substituents at the 2,7-position
of fluorene. In contrast to 2,7-diaminofluorene, the fluores-
cence of fluorenoazomethines is extremely low or nonexistent
due to IC and ISC.13 The idea behind developing such
podands is that the conformational changes (due to the small
rigid cleft of N, E donor sites) upon complex formation may
disturb the conjugation by flipping the azomethine group
nearly orthogonal to the fluorene plane, which would restore
the fluorescence of the 2,7-diaminofluorene unit (Scheme 1).
It is also supported by DFT calculations on the ligand
and a model complex. We note that the conjugated systems
are known to require less energy for twisting, however, the
resulting changes in the HOMO–LUMO gap is expected to be
significant.14 Therefore, the colorimetric changes can also be
expected in this type of model as a result of the change in an
extent of the conjugation on binding with first row TMs. We
believe that this model will be suitable for development of
‘turn-on’ metal probes, in general.
A fluorene unit was introduced into the podands by
condensation of 2,7-diaminofluorene with ortho-chalcogenoether
Department of Chemical Sciences, Indian Institute of Science
Education and Research, Mohanpur Campus, PO: BCKV Campus
Main Office, Mohanpur 741252, India.
Table 1 Two dimensionality of the podand 2–4 as a fluorescent
probes toward Cr(III) and Fe(II)
E-mail: sanjiozade@iiserkol.ac.in, snigdha@iiserkol.ac.in;
Fax: (+)91-33-25873020
Podand 1–4
Fe(II)
Cr(III)
w Electronic supplementary information (ESI) available: Experimental
details and spectral studies for ligands 1–4 and DFT calculations. See
DOI: 10.1039/c1cc10425j
Fluorimetric response
Colorimetric response
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4174 Chem. Commun., 2011, 47, 4174–4176

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Scheme 1 Schematic presentation of the conformational changes in
azomethine-based heterodonor podand upon recognition of specific
metal cations.
substituted phenylaldehyde (see SIw). The condensation
reaction to prepare the Schiff-base podands is straightforward
and clean.
All heterodonor podands 1–4 showed strong absorbance
but weak emission in THF solution. The UV-Vis spectrum
of the podands 1–4 showed three peaks as observed in case of
2,7-diaminofluorene.15 As suggested by Skene and coworker,
PET and ICT are not responsible for fluorescence quenching.13
The evaluation of the calculated (at B3LYP/6-31G*) HOMO
and LUMO of these ligands (Fig. S9w) corroborate the exclusion
of PET and ICT as quenching pathways. Both the HOMO and
LUMO orbitals are evenly distributed throughout the
molecule.
The recognition properties of the the fluorenoazomethine-
based podands 1–4 having different chalcogen donor atoms
have been studied towards the first row transition metals
(Cr(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II)) in THF
solution. The absorption spectrum of 1 showed three peaks at
lmax 382, 285 and 235 nm, respectively. Upon addition of
increasing equivalents of Cr(III), Fe(II) and Cu(II), two peaks
(250 and 305 nm) were observed (Fig. 1a). The significant
change in the highest wavelength lmax values (B80 nm)
corroborated our initial assumption that the binding changes
the conformation by rotating azomethine plane nearly ortho-
gonal to the plane of fluorene moiety.
To model the twisting of the azomethine from the plane of
fluorene, we have optimized podand 2 without any constrains
and further, freezing the dihedral angle between the fluorene
and o-methylthiophenyl substituted azomethine planes at 901
(twisted structure) (Fig. S24w). TD-DFT (B3LYP/6-31G*)
calculations on both the optimized structures showed the
highest excitation wavelength value higher for the former
structure by B90 nm than that of the twisted structure when
the significant oscillator strength values were considered
(Fig. S26w). The highest oscillator strength at 293 nm for
twisted 2 relates to the transition from HOMO to LUMO+2
orbitals (Fig. S25 and S26w). The contours of both the orbitals
are confined to the fluorene moiety with a slight extension over
the CQN. Thus, this transition is similar to that of arising
from 2,7-diaminofluorene.
Fig. 1 Change in absorption spectra upon addition of (a) Cr(III) (b)
Fe(II)-perchlorate salt solution in THF to the solution of podands 1
and 2, respectively. Fluorescence response following excitation at 270
nm from podand 2 solution upon addition of perchlorate salts of (c)
Cr(III) and (d) Fe(II).
The podand 1 showed a blue shift (B50 nm) in emission on
addition of solution of the Cr(III), Fe(II) and Cu(II) with
increasing emission intensity. A similar blue shift was observed
for all the podands studied here. The emission lmax value
is comparable with that of 2,7-diaminofluorene. Cr(III),
Fe(III) and Cu(II) showed 28, 21, and 17-fold enhancement in
fluorescence intensity, respectively, under similar conditions
and concentrations with podand 1. Out of the podands 1–4,
only the podand 1 shows considerable enhancement in fluor-
escence emission for Cu(II).
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Chem. Commun., 2011, 47, 4174–4176 4175

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podand 2 with N, S donors showed the highest FE. Podand 4
with Se, N donors and long alkyl chains showed enhanced
visual selectivity for Fe(II). The fluorenoazomethines have
significant potential in sensing first row TM ions as these are
easy to synthesize in high purity and by changing the donor
atoms, the selectivity of the turn-on fluorescent probe may be
fine tuned towards specific transition metal ions.
Fig. 2 Visual changes in color after addition of aliquots of Fe(II) ion
to the podand (a) 2 and (b) 4.
Upon titration of Cr(III) ion with the podand 2, the intensity
of the peak at 388 nm decreased gradually; however, a new
peak at 274 nm appeared. Upon titration of 2 with Fe(II),
additionally, a new, low intensity, very broad peak at 650 nm
was observed with the peaks at B345 and 483 nm (as a left
shoulder in peak of 388 nm) which are responsible for the
colorimetric detection of Fe(II) (Fig. 1b). A visual change from
yellow to blue to green (Fig. 2a) was gradually observed upon
addition of increasing equivalents of Fe(II) ions. Thus,
compound 2 recognizes the Fe(II) ion colorimetrically, in
addition to the fluorescence response.
SP and SSZ are thankful to DST, India for Fast-Track
Research Grants. PBP is thankful to UGC, India for fellowship.
We thank Dr. Pradipta Purkayastha and Dr. Prasun K. Mandal
(IISER Kolkata) for helpful discissions.
Notes and references
1 (a) B. Valeur, in Molecular Luminescence Spectroscopy, Part 3,
ed. S. G. Schulman, Wiley, New York, 1993; (b) A. W. Czarnik,
Fluorescent Chemosensors for Ion and Molecular Recognition, ACS,
Washington, 1993; (c) V. Balzani, G. Bergamini and P. Ceroni,
Coord. Chem. Rev., 2008, 252, 2456.
The higher wavelength peaks (B345, 483 and broad peak at
650 nm) are responsible for the colorimetric detection of Fe(II).
To explain the nature of the electronic transition in Fe(II)
complexes, we have carried out DFT calculations (at B3LYP/
6-31G*/sdd) on the model complex 5, where only one arm of
o-methylthiophenyl substituted azomethine on the 2-position
of fluorene was considered. Optimization of singlet (low-spin)
and quintet (high-spin) state structures of 5 at B3LYP/
6-31G*/sdd level of theory showed that the quintet state is
more stable by 15.8 kcal molÀ1 than that of the singlet state. In
line with our assumption on the conformational changes, the
azomethine planes are twisted at B781 from the planes of the
fluorene moiety in the optimized structure of high spin complex
5 (Fig. S27w). TD-DFT calculations at the same level of theory
showed that the higher wavelength transitions arise from the
charge transfer from the fluorene moiety to the metal bound
azomethine fragment (b spin) or only azomethine fragment
(a-spin) (Fig. S28 and S29 and Table S2w).
With podand 2, Cr(III) and Fe(II) showed >300 and 270-fold
enhancement in fluorescence intensity, respectively. Absorption
and emission spectra of 3 and 4 on addition of Cr(III) and
Fe(II) showed similar types of changes to that of podand 2.
Upon addition of Cr(III) and Fe(II), a 71- and 51-fold enhance-
ment in fluorescence intensity was observed for podand 3,
respectively, whereas, podand 4 showed a 14- and 5-fold
increase in fluorescence intensity, respectively. The overall
detection limit for metals of these podands is around 10 mM.
In conclusion, we have introduced here a new concept for
the design of fluorescent probes where fluorescence enhance-
ment can be achieved as a result of conformational changes
which suppress the non-radiative modes (IC and ISC) of
fluorescence quenching. The ligand 1 having O, N donors acts
as a ‘turn-on’ probe toward Cu(II) as well as to Cr(III) and
Fe(II). The podands 2–4 having S/Se, N donors behave as
a ‘turn-on’ two dimensional (colorimetric and fluorimetric)
selective fluorescent probe towards Cr(III) and Fe(II) where
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