C. Gou et al. / Inorganic Chemistry Communications 14 (2011) 1622–1625
1623
existence of a very effective non-radiative intramolecular rotation
decay of the excited state most likely due to the structure of 1 with
two salicylaldimine moieties connected by a rotatable N\N single
bond [20]. However, the addition of Al3 significantly enhanced the
fluorescence intensity. As shown in Fig. 2a, the emission peak of 1
+
3+
increased with increasing Al concentration. With the concentration
of Al3 up to three equiv of compound 1, an 18-fold increase in
fluorescence intensity was observed. As a bonus, the development of
the strong blue-green emission and the efficient fluorescence turn-on
may be detected by the naked eye, as can be seen in the inset
photograph of Fig. 2a. As shown in the inset pictures of Fig. 2a, a good
linearity of the fluorescence intensity as the function of the
Scheme 1. Chemical structure of sensor 1.
+
bands were observed to be centred at 311 and 363 nm, which could be
attributed to n–π* transition of C_N and π–π* transitions of benzene
rings, respectively [20]. Upon the addition of Cu2 , a new absorption
band centred at 415 nm appeared with increasing intensity. Mean-
while, the original absorption band centred at 363 nm decreased
gradually and the 311 nm absorption band increased, generating two
isosbestic points at 329 and 393 nm, which indicated the formation
+
3+
−8
concentration of Al
between 0 and 1×10
M is established, and
the detective limit of aluminium determination is calculated at around
−
9
1.0×10
sensitive off-on fluorescent chemosensor for Al
The fluorimetric response of 1 to other metal ions was also studied in
M. These results mean that 1 could be used as a highly
3+
.
2+
2+
of a new complex between 1 and Cu . As shown in the inset of
Fig. 1a, a nearly linear dependence of the ratio of absorbance at 415
and 363 nm as a function of Cu2 concentration from 0 to 30 μM was
observed. At the same time, the absorption changes were clearly
visible to the naked eye, showing that the colourless solution of 1
became yellow upon titration with Cu2 even at 1 μM concentration
the same condition. The addition of Zn
induced a 4-fold emission
enhancement which is much smaller than the 15-fold enhancement
+
3+
induced by Al . In addition, a 29 nm blue shift of emission wavelength
from 505 nm to 476 nm was also observed, making it easy to distinguish
from that of Al3 , which is only 7 nm blue shift. However, fluorescence
+
+
2+
quenching was observed upon the addition of Cu . The quenching by
2+
(
inset photograph in Fig. 1a).
The selectivity of 1 toward other metal ions was investigated by
Cu is most likely due to an energy transfer having occurred from 1 to
2+
the open-shell d-orbitals of Cu exhibiting a faster and more efficient
nonradiative decay of the excited states of 1. A similar quenching effect
2
UV–Vis spectroscopy in EtOH/H O buffer solution (30 mM, pH=7.2,
+
2+
v/v=30/70). The competitive metal ion includes heavy, transition
by Cu2 was shown to be the result of the complexation of Cu to a
+
3+
2+
2+
2+
2+
3+
2+
and alkali earth ions such as Ag , Al , Ba , Ca , Cd , Co , Cr
Hg , Mg , Mn , Ni , Pb
,
bisthiazole derivative with phenolic substituents [23]. In contrast, Zn
2
+
2+
2+
2+
2+
2+
3+
and Zn . As shown in Fig. 1b, the
and Al have closed-shell d-orbitals so the energy transfer processes
2+
3+
addition of two equiv of other metal ions resulted in negligible
changes in the absorption spectrum of 1 and the colour of the
solutions containing these ions remained relatively unchanged, only
cannot take place. In addition, coordination of Zn and Al removes
the phenolic proton of 1 and disrupts the ESIPT causing emission with a
normal Stokes' shift (Scheme 2) [21].
2+
Cu gave an obvious absorption change which was clearly visible to
All of these observations showed that common coexisting metal
the naked eye, showing that 1 has a good selectivity for Cu2
+
.
ions did not interfere with the measurement of Al , thus, compound
3+
+
The fluorescent properties of 1 were also studied and a weak
fluorescence emission band centred at around 425 nm and 505 nm
was observed in the spectrum of 1 (15 μM) in ethanol when excited at
1 shows high selectivity toward Al3 over other metal ions tested
(Fig. 2b). Thus the selectivity shown by 1 toward metal cations is
easily selected by an appropriate choice of the output channel, which
2
+
3
60 nm. As has been reported by Yu, [14] compound 1 contains an
is the detection of Cu
via the colour channel (from colourless to
+
intramolecular hydrogen bond between the phenolic O\H and the
nitrogen of the imine group that undergoes excited-state intramo-
lecular proton transfer (ESIPT) and yields a normal emission at
yellow) and signalling of Al3 via a remarkable enhancement of the
emission intensity.
To explore the binding mechanism, the Job's plot of the UV–Vis
+
3+
4
25 nm and a tautomer emission at 505 nm from the proton transfer
titrations of Cu2 and the fluorescence titrations of Al were revealed
tautomer (Scheme 2) [23]. The low fluorescence of 1 is assigned to the
in Fig. S5. A maximum absorption was observed when the molarfraction
2
+
2
Fig. 1. (a) Absorbance spectra of 1 (30 μM) with increasing concentrations of Cu in the form of nitrate salt in EtOH/H O (v/v=30/70). Inset: the ratio of absorbance at 415 nm and
2
+
2+
3
63 nm as a function of Cu concentration. Inset photograph: free 1 solution (left) and solution after addition of 1 μM Cu (right) under natural light. (b) UV–Vis spectral changes
of 1 in EtOH/H O (v/v=30/70) (30 μM) upon addition of two equiv of different nitrate salts (60 μM).
2
Gou, Chao
