luminescent molecules. This may seem something of a
contradiction in that ferrocene has been widely used as an
emission quencher in intramolecular processes taking place
in solution. However, it is also established that incorporating
a ferrocenyl derivative into a luminescent system does not
necessarily extinguish the luminescence.5 Recently, ferrocene
has been advantageously used as a redox center to provide
multiresponsive photo- and electrochemically active com-
pounds.6 On the basis of this body of work, we decided to
combine in an open-shell system the redox activity of the
ferrocene group with the photoactive behavior of the an-
thracene ring and the cation binding ability of the unsaturated
nitrogen atom.7 The combined effects of these structural
features suggest that this kind of compounds might experi-
ence an electron-cloud perturbation upon coordination or/
and protonation, thus functioning as chemosensors for metal
ions or protons.
Compounds 3 were prepared from the readily available
diethyl aminomethylphosphonate 1,8 which was condensed
with the appropriate aromatic or organometallic aldehyde,
to give the corresponding N-substituted diethyl aminometh-
ylphosphonate 2 in excellent yields (94-96%). Generation
of the metalloenamine by reaction with n-BuLi at -78 °C
and subsequent reaction with 1 equiv of the adequate
aldehyde provided the 1,4-disubstituted 2-aza-1,3-butadienes
3 in yields ranging from 90 to 95% (Scheme 1).
nm. In acetonitrile or acetonitrile-water as solvents, the
typical structured pattern of the parent anthracene, charac-
teristic of its lowest energy transition, was absent. However,
upon acidification of the solutions of compound 3a the
anthracene-like spectrum was clearly observable. This was
not the case for compound 3c, under the same acidic
conditions, where the shape and position of the bands were
also strongly modified.
In view of the influence of protonation on the UV-vis
absorption properties, the fluorescence spectra were recorded
for solutions of the different compounds, after addition of
acid or base in variable amounts. The emission spectra
obtained showed a weak intensity and low quantum yield
values (φ ) 0.042-0.008). However, an important observa-
tion was made during our initial experiments: the presence
of water was critical to achieve the desired fluorescence
enhancement. Thus, upon addition of 1 equiv of HBF4 to a
solution of 3c in acetonitrile, a modest enhancement of the
emission intensity was observed (2-fold), whereas in the
presence of water this effect was more remarkable (10-fold).
So, the acetonitrile/water (70:30) mixture was the solvent
of choice. These results are shown in Table 1.
Table 1. Effect of pH on the Relative Fluorescence Intensity
of 3 in the Presence of Li+ Cationa
pH ) 7
pH ) 5
IH/I0
ILi+/IH
pH ) 3
compoundb
IH/I0
ILi+/IH
IH/I0
ILi+/IH
Scheme 1. Preparation of 2-Aza-1,3-butadiene Derivatives
3a
3b
3c
1
1
1
1
1
1
1
1.4
1.5
1
1
7.3
3.5
nd
11
1
1
1
a Added as its perchlorate salt. b Concentration of 3a-c and Li+: 2.5 ×
10-5 M.
Following the general trend, the emission intensities
increased with decreasing pH.6a This effect was already
observable at moderate acidities, reaching its maximum at
pH ca. 3. The shapes of the fluorescence spectra did not
change significantly within the employed pH range except
in the case of 3b, which underwent hydrolysis during the
measurements. This process was easily detectable through
the appearance of a broad and structureless band, shifted to
the red (λmax ca. 520 nm), characteristic of anthraldehyde.
It is worth mentioning that the structurally related com-
pound 1-(9-anthryl)-4-ferrocenyl-2-aza-1-butene 4, prepared
by condensation of 9-anthraldehyde with â-ferrocenylethyl-
amine,9 exhibited the same behavior as 3b when it was
treated under the above-mentioned acidic conditions.
As stated above, the main goal of the present work was
to explore the possibility of developing fluorescent sensors
capable of achieving a selective recognition of Li+ cations
under an aqueous environment. As shown in Table 1, the
presence of this cation produced a remarkable fluorescence
enhancement (ILi+/IH+) for compound 3c at pH ) 5 (Figure
All the synthesized dyads displayed UV-vis absorption
spectra that extended into the UV-vis region, with diag-
nostically important bands at wavelengths longer than 300
(5) (a) Duggen, D. M.; Hendrickson, D. N. Inorg. Chem. 1975, 14, 955-
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Org. Lett., Vol. 6, No. 24, 2004