The dependence of fluorescence intensity IF (ꢀ) (kem = 308 nm) on
the pH was shown in Fig. 10. The plot of fluorescence intensity IF
versus pH displays a typical sigmoidal curve.
Conclusion
Two new fluorenyl-substituted multidentate ligands L1 and L2 were
prepared. The two ligands showed very different coordination
chemistry toward the Cu(II) ion. A dimeric species [Cu2(H−2L1)2]
was isolated when the Schiff base L1 was treat with Cu(II) ions in
the presence of NaOH. The dimeric nature of the complex was
supported by the X-ray crystal structure determined† and ES-MS
data. Structural analysis revealed that the Cu(II) ions coordinated
to the outer N atom and O atom of the two Schiff base and
leaving the inner N4 compartment of L1 unoccupied. The solid-
state variable temperature magnetic susceptibility studies showed
the existence of intra-molecular ferromagnetic exchange coupling
between the two Cu(II) centers.
The interaction of Cu(II) with L2 was examined by pH-
potentiometric titration and ES-MS. Results showed that at
pH 5.8, the main species in solution was a monomeric species
[CuH−1L2]+. Absorption spectroscopy suggested that Cu(II) only
coordinated to the inner N4 compartment of L2. Fluorometric
titration indicated that Cu(II) ions quenched the fluorescence of
the L2 ligand.
Fig. 10 The dependence of fluorescence intensity IF (ꢂ) (kem = 308 nm)
and absorbance A (ꢃ) (k= 601 nm) on the pH for a dioxane–water solution
(3:1 v/v) containing L2 plus 1 equiv. Cu2+ ions.
Acknowledgements
For studying the quenching mechanism, an analogous spec-
trophotometric titration experiment was performed. On addition
of base, the color of the solution becomes purple; the d–d
absorption band of Cu2+ shifting from 830 nm to 601 nm, the
absorption band at 601 nm develops and reaches its maximum
value at about pH 6.00; at this pH the solution is almost complete
fluorescence quenching. The plot of absorbance A versus pH
(ꢁ in Fig. 10) shows a sigmoidal curve shape too, which is
symmetrical to the IF/pH profile and centered at the same pH
(pH ≈ 4.30). The dependence of the absorbance A on the pH
is contrary to that of the fluorescence intensity. The curve of
A vs.pH can be conveniently superimposed on the distribution
diagram of [CuH−1L2]+ (Fig. 7. curve 3). This indicates that the
absorption band at 601 nm is caused by the Cu2+ d–d jump of
species [CuH−1L2]+, and that the formation of [CuH−1L2]+ leads to
quenching the fluorescence.
Making a comparison between the ultraviolet spectrum of L2 +
Cu2+ (1:1) (pH = 3.35) and that of Cu(NO3)2 aqueous solution, we
find the absorption band at 830 nm of L2 + Cu2+ (1:1) (pH = 3.35)
is very similar to that of Cu(NO3)2 aqueous solution. Moreover,
the mass spectrum (ES-MS) of an acidic solution of L2 + Cu2+ (1:1)
(pH = 3.72) demonstrates that [L2 + H+]+ is a main species [m/z
(%) = 565.1 (100)], and the peak of complex [m/z (%) = 625.9 (<
5)] is very weak. But when the pH is adjusted to 5.80, [CuH−1L2]+
become a main species [m/z (%) = 625.9 (55)]. It is therefore
suggested that in an acid solution the absorption band at 830 nm
is caused by the d–d jump of hydrated Cu2+. Namely, in a lower pH
the complexes were almost not formed, they were formed with the
pH increasing. Contrasting the ultraviolet spectrum of complexes
with that of the reported ditopic dioxotetraamine ligand L3
(Scheme 1),17 we find the ultraviolet spectrum of complexes (601
nm) are close to that of [CuH−2L3]2+ (590 nm), in which the Cu2+
is coordinated in compartment B. It is therefore suggested that
in L2 the Cu2+ is only coordinated in compartment B and leads
to quenching of the fluorescence of the fluorenyl group; it cannot
move to compartment A at a lower pH value.
This project was supported by the National Natural Science
Foundation of China (20571058).
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