A fluorescent chemosensor for Zn(II). Exciplex formation in solution and the solid state

Article abstract of DOI:10.1039/b403743j

The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9- methylaraino)ethyl]-[9]-1, 10-phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol dm^-3 NMe₄Cl H ₂O-MeCN (1 : 1, v/v) solution at 298.1 ± 0.1 K by means of potentiometric (pH metric) titration. The fluorescence emission properties of these complexes were studied in this solvent. For the Zn(n) complex, steady-state and time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular π-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited state were observed. From the temperature dependence of the photostationary ratio (I_EXC/I_M), the activation energy for the exciplex formation (E_a) and the binding energy of the exciplex (-ΔH) were determined. The crystal structure of the [ZnLBr](ClO₄)·H₂O compound was resolved, showing that in the solid state both intra- and inter-molecular π-stacking interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra, whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no π-stacking (full overlap of the π-electron cloud of the two chromophores - anthracene and phenanthroline) is observed. L is a fluorescent chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to the formation of π-stacking complexes or to exciplex emission in solution.

Full text of DOI:10.1039/b403743j

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F U L L P A P E R
A fluorescent chemosensor for Zn(II). Exciplex formation in
solution and the solid state†
Andrea Bencini,a Emanuela Berni,a Antonio Bianchi,*a Patrizia Fornasari,a Claudia Giorgi,a
João C. Lima,b Carlos Lodeiro,b Maria J. Melo,b J. Seixas de Melo,*c Antonio Jorge Parola,b
Fernando Pina,*b João Pinac and Barbara Valtancolia
a
Department of Chemistry, University of Florence, via della Lastruccia 3,
50019 Sesto Fiorentino, Florence, Italy. E-mail: antonio.bianchi@unifi.it
Departamento de Química, Centro de Química-Fina e Biotecnologia,
b
Faculadade de Ciências e Tecnologia, Universidade Nova de Lisboa, Quinta da Torre 2825
Monte de Caparic, Portugal. E-mail: fjp@dq.fct.unl.pt
Department of Chemistry (CQC), University of Coimbra, 3004-535 Coimbra, Portugal.
c
E-mail: sseixa@ci.uc.pt
Received 11th March 2004, Accepted 20th May 2004
First published as an Advance Article on the web 8th June 2004
The macrocyclic phenanthrolinophane 2,9-[2,5,8-triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-1,10-
phenanthrolinophane (L) bearing a pendant arm containing a coordinating amine and an anthracene group forms stable
complexes with Zn(II), Cd(II) and Hg(II) in solution. Stability constants of these complexes were determined in 0.10 mol
dm−3 NMe4Cl H2O–MeCN (1:1, v/v) solution at 298.1 ± 0.1 K by means of potentiometric (pH metric) titration. The
fluorescence emission properties of these complexes were studied in this solvent. For the Zn(II) complex, steady-state and
time-resolved fluorescence studies were performed in ethanol solution and in the solid state. In solution, intramolecular
-stacking interaction between phenanthroline and anthracene in the ground state and exciplex emission in the excited
state were observed. From the temperature dependence of the photostationary ratio (IExc/IM), the activation energy for
the exciplex formation (Ea) and the binding energy of the exciplex (−H) were determined. The crystal structure of the
[ZnLBr](ClO4)·H2O compound was resolved, showing that in the solid state both intra- and inter-molecular -stacking
interactions are present. Such interactions were also evidenced by UV-vis absorption and emission spectra in the solid
state. The absorption spectrum of a thin film of the solid complex is red-shifted compared with the solution spectra,
whereas its emission spectrum reveals the unique featureless exciplex band, blue shifted compared with the solution. In
conjunction with X-ray data the solid-state data was interpreted as being due to a new exciplex where no -stacking (full
overlap of the -electron cloud of the two chromophores – anthracene and phenanthroline) is observed. L is a fluorescent
chemosensor able to signal Zn(II) in presence of Cd(II) and Hg(II), since the last two metal ions do not give rise either to
the formation of -stacking complexes or to exciplex emission in solution.
The search for new molecules able to perform complex movements
has been an elected field for chemical research in the past two
decades.1 Light has been the principal driving force for the con-
sideration of such supramolecular dynamics.2 However, the idea
of a supramolecular machine as being an entity where individual
molecules are parts of a whole structure, working as cooperating
functional units, involves more steps and procedures than just light
absorption. Donor and acceptor units can be linked and be sensitive
to light, protons or electrons thus performing the role of active/
inactive devices. In recent years, molecules containing polyamine
chains linking aromatic fluorophores have proved useful as fluores-
cent chemosensors for protons, cations and/or anions.3–9
The pursuit for new structures able to respond efficiently to ex-
ternal stimuli has been considered by our groups in recent publica-
tions.10–17 A chemical complex consisting in the binding of Zn(II)
to L and showing to behave as an elementary molecular machine
driven by pH and by light was briefly described in a previous
communication.11 Structurally this new complex results from the
presence of two chromophores (phenanthroline and anthracene)
connected by a polyamine structure (and chain) where Zn(II) is
included. This new complex was shown to display the peculiar
emission from a new species, attributed to exciplex emission result-
ing from the movement of the anthracene moiety to get into contact
with the phenanthroline chromophore.11
order to fully equate the movement leading to exciplex formation.
To this purpose, the photophysical behaviour of the complex was
investigated in the solid state both in a KBr matrix and in a thin
film. The energetics of exciplex formation (activation energy Ea and
binding energy of the exciplex, H) were obtained in ethanol where
a significant range of temperatures is allowed.
In order to investigate the possible use of this ligand as a selective
chemosensor, the solution study was extended to Cd(II) and Hg(II)
metal ions.
Results and discussion
Crystal structure of [ZnLBr](ClO4)·H2O
The aim of the present work was to extend the study to solid
conditions and to obtain thermodynamic parameters in solution in
The crystal structure of [ZnLBr](ClO4)·H2O consists of [ZnLBr]+
complex cations, perchlorate anions and water molecules. Fig. 1
shows an ORTEP drawing of the [ZnLBr]+ cation with atom label-
ling and Table 1 lists bond lengths and angles for the metal coordi-
nation environment.
† Electronic supplementary information (ESI) available: Theoretical basis
for the temperature dependence of fluorescence. See http://www.rsc.org/
suppdata/dt/b4/b403743j/
2 1 8 0
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Table 1 Selected bond lengths (Å) and angles (°) for the Zn(II) coordina-
tion environment in [ZnLBr](ClO4)·H2O
Zn1–N1
Zn1–N2
Zn1–N3
Zn1–N4
2.226(5)
2.200(5)
2.293(6)
2.308(6)
Zn1–N5
Zn1–N6
Zn1–Br1
2.357(6)
2.228(6)
2.7781(15)
N2–Zn1–N1
N2–Zn1–N6
N1–Zn1–N6
N2–Zn1–N3
N1–Zn1–N3
N6–Zn1–N3
N2–Zn1–N4
N1–Zn1–N4
N6–Zn1–N4
N3–Zn1–N4
N2–Zn1–N5
73.2(2)
92.8(2)
88.5(2)
71.2(2)
143.2(2)
102.2(2)
144.8(2)
140.3(2)
80.3(2)
76.5(2)
141.0(2)
N1–Zn1–N5
N6–Zn1–N5
N3–Zn1–N5
N4–Zn1–N5
N2–Zn1–Br1
N1–Zn1–Br1
N6–Zn1–Br1
N3–Zn1–Br1
N4–Zn1–Br1
N5–Zn1–Br1
69.6(2)
97.5(2)
141.2(2)
74.2(2)
90.28(15)
89.92(16)
175.99(14)
81.24(17)
98.62(17)
78.54(18)
Table 2 Protonation and metal ion complexation constants obtained in
0.10 mol dm−3 NMe4Cl H2O–MeCN (1:1, v/v) solution at 298.1 ± 0.1 K
Reaction
logK
L + H+ = LH+
9.0(1)a
8.3(1)
6.1(2)
14.0(1)
5.8(1)
4.6(1)
Fig. 1 ORTEP drawing of the [ZnLBr]+ complex cation.
LH+ + H+ = LH22+
LH22+ + H+ = LH33+
In [ZnLBr]+ the zinc ion is heptacoordinated by all nitrogen
atoms of the ligand and by a bromide anion. The metal coordination
geometry can be best described as a distorted pentagonal bipyramid.
The N1, N2, N3, N4 and N5 nitrogens of the macrocyclic ring de-
fine the basal plane (max deviation from the plane 0.40 Å for N4),
while the N6 amine group of the pendant arm and the Br1 anion
occupy the apical positions. The zinc atom lies 0.092(1) Å above
the basal plane, shifted toward N6. The N6–Zn and Br1–Zn bonds
form angles of 3.46 and 1.66°, respectively, with the normal to the
basal plane.
Zn2+ + L = [ZnL]2+
[ZnL]2+ + H+ = [ZnLH]3+
[ZnLH]3+ + H+ = [ZnLH2]4+
[ZnL]2+ + OH− = [ZnLOH]+
[ZnLOH]+ + OH− = [ZnL(OH)2]
Cd2+ + L = [CdL]2+
6.1(1)
4.3(1)
15.2(1)
5.8(1)
[CdL]2+ + H+ = [CdLH]3+
Hg2+ + L = [HgL]2+
14.4(1)b
4.2(1)b
[HgL]2+ + H+ = [HgLH]3+
a Values in parentheses are standard deviation in the last significant figure.
b Conditional stability constants (see Experimental section).
The Zn–N bonds, ranging between 2.200 and 2.357 Å, are
somewhat longer than those generally found in Zn(II) polyamine
complexes.
It is of interest to compare the present structure with that of the
zinc complex with the ligand 2,5,8-triaza[9]-10,23-phenanthrolino-
phane (L1),18 consisting of the macrocyclic framework of L, the
pendant arm with the anthracene group being replaced by a hydro-
gen atom. In contrast with the present structure in which the macro-
cyclic ring is almost coplanar (Fig. 1), in [ZnL1(H2O)]2+ the ligand
is folded along the axis linking the two benzylic nitrogens, forming
an angle of 93.0° between the plane defined by the phenanthroline
unit and that defined by the amine groups of the aliphatic chain.
Most likely, coordination of the pendant arm prevents the folding
of the macrocyclic ring in the L complex.
However, the most interesting finding is the almost perpendicular
disposition of the two aromatic moieties in [ZnLBr]+. Actually, the
phenanthroline plane and the anthracene unit form a dihedral angle
of 83.6°, with a distance between C25 and C26 anthracene carbons
and the centroid of the central ring of phenanthroline of 3.88 and
3.92 Å, respectively, accounting for edge-to-face stacking interac-
tion between the two aromatic rings.19
Besides this CH stacking interaction, inspection of the crystal
packing (Fig. 2) reveals the presence of intermolecular CH in-
teractions between the centroid of a lateral ring of anthracene (Ct1)
and CH groups (C2, C3, C11) of two phenanthroline units belong-
ing to two different complexes (Ct1C2, 3.63 Å; Ct1C3, 3.48 Å;
Ct1C11, 3.75 Å). Further intermolecular CH interactions are
found between the centroid of the central ring of anthracene (Ct2)
and a CH group (C2) of a phenanthroline (Ct2C2, 3.33 Å). The
shorter intermolecular -stacking distances reveal that the interac-
tion between phenanthroline and anthracene units of different com-
plex cations is stronger than that between the two aromatic rings
within the same complex cation.
Fig. 2 Crystal packing of [ZnLBr](ClO4)·H2O.
protic base. In order to ascertain which amine groups are involved
in the proton transfer processes, protonation of L was followed by
means of 1H NMR measurements in D2O–CD3CN (50:50, v/v) at
different pH values. The spectrum of the ligand at pH 11, where
the free amine is prevalent in solution displays two sets of signals,
six in the aliphatic region and eight in the aromatic one, in accord
with an overall C2v time-averaged symmetry. This symmetry is pre-
served throughout all the pH range investigated. As shown by the
downfield shifts of the resonances of protons H8, H9, H12 and H13,
adjacent to the amine groups N1 and N3 (Fig. 3), taking place upon
ligand protonation, all three protonation stages involve the three
benzylic nitrogens N3, N3 and N1, in agreement with the greater
basicity of secondary amines with respect to tertiary ones in water.
Ligand protonation in H2O–CH3CN (50:50, v/v)
The stepwise protonation constants of L are reported in Table 2. In
the pH range investigated (2.5  pH  10.5) L behaves as a tri-
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7
2 1 8 1

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formed. As can be seen from Table 2, the equilibrium constants
for protonation of [ZnL]2+, to form the mono- and di-protonated
species, are significantly high, if compared with the protonation
constants (Table 2) of the metal-free ligand, suggesting that in
[ZnL]2+ a couple of nitrogen atoms are only weakly coordinated,
or not coordinated at all, to the metal ion. As a matter of fact, in the
crystal structure of the [ZnLBr]+ complex cations some Zn–N bonds
are rather long and complex solvation may cause further weakening,
or breaking, of these bonds. Hence, it is not surprising that facile
protonation of the complex may occur in solution. In addition the
presence of weakly coordinated, or not coordinated, amine groups
rationalises the formation of hydroxylated complexes in alkaline
solution. On the contrary, the larger Cd(II) and Hg(II) cations, which
are not able to form hydroxylate complexes and show a lower ten-
dency to bear protonation, seems to give a better fit of the macrocy-
clic ligand cavity, with stronger metal-to-ligand interactions.
1
Metal coordination was also followed by means of H NMR
measurements carried out in D2O–CD3CN (50:50, v/v) solutions.
By adding 0.1 eq. of M(ClO4)2 (M = Zn(II), Cd(II) or Hg(II)) to a
solution of L, a different set of signals appears in the 1H NMR spec-
tra, together with the resonances of the ligand alone, evidencing the
formation in solution of a metal complex slowly exchanging on the
NMR time scale with the free ligand. On addition of further M(II),
the signals of this species are enhanced, while the resonances of
L show a clear decrease in intensity. When the L:M(II) 1:1 molar
1
ratio is reached, the H NMR L subspectrum disappears and the
1
recorded H NMR spectrum can be reasonably due to a unique
mononuclear M(II) complex. Addition of further M(II) does not
alter the 1H spectrum.As shown in Fig. 4 for Zn(II) and Cd(II), metal
coordination leads to significant changes in both the aromatic and
the aliphatic regions of the spectrum of L. The signals of protons of
the cyclic framework are generally downfield shifted in both Zn(II)
and Cd(II) complexes, as generally found in metal complexes with
polyamine ligands. The most interesting finding is the very large
downfield shift of the signals of aromatic protons upon Zn(II) com-
plexation. Minor shifts, instead, are observed in case of the Cd(II)
complex. The Hg(II) complex shows spectral features similar to the
Cd(II) one.
Fig. 3 (a) pH Dependence of the 1H signals of L. The H10 signal has been
omitted for clarity. It does not significantly shift in the pH range investi-
gated. (b) Distribution diagram of the protonated species of L.
On the other hand, Fig. 3 shows that the formation of the [H3L]3+
species gives rise to marked downfield shifts of the signals of the
aromatic protons of phenanthroline, and to a lesser extent, of those
of anthracene. While the slight shift of the anthracene resonances
can be ascribed to a charge effect due to protonation of the adjacent
N1 nitrogen, the larger downfield displacements of the phenanth-
roline signals suggest a direct participation of phenanthroline nitro-
gens in ligand protonation, most likely via hydrogen bonding to the
aliphatic ammonium groups.
Fig. 4 1H NMR spectra of L (a) and its Zn(II) (b) and Cd(II) (c) com-
plexes.
Although Zn(II) coordination to the nitrogen donors could simply
explain the downfield shifts observed for the phenanthroline pro-
tons, the marked changes in the anthracene subspectrum is difficult
to interpret on the basis of the effects of Zn(II) coordination and
would suggest a -stacking interaction between the two aromatic
units. This prompted us to carry out 1H NMR pH-metric titrations
on solutions containing M(II) and L in 1:1 molar ratio. Fig. 5 re-
Metal complexation in H2O–CH3CN (50:50, v/v)
The binding ability of L towards Zn(II), Cd(II) and Hg(II) were
investigated by performing the speciation of the relevant complex
systems and determining the stability constants of the formed spe-
cies by means of potentiometric titrations in 0.10 mol dm−3 NMe4Cl
H2O–MeCN (1:1, v/v) solution at 298.1 ± 0.1 K. A list of the
complex species with the relevant stability constants are reported
in Table 2. Zn(II) gives rise to the formation of many complex spe-
cies with L. From acidic to alkaline solutions the species [ZnH2L]4+,
[ZnHL]3+, [ZnL]2+, [ZnLOH]+ and [ZnL(OH)2] are consecutively
1
ports the pH dependence of selected H NMR signals of L in the
presence of Zn(II) ([L] = [Zn(II)] = 1 × 10−3 M), compared with
the species distribution diagram determined on the basis of the po-
tentiometric results. The most interesting finding is the significant
downfield shift of the signals of the ethylenic chain H9 and H10 of
the anthracene pendant observed upon deprotonation of [ZnLH2]4+
2 1 8 2
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7

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to give the monoprotonated [ZnLH]3+ species in the pH range 3–6.
This strongly suggests that the formation of [ZnLH]3+ implies metal
coordination by the amine group of the pendant. Simultaneously,
a remarkable upfield shift is observed for the methylenic protons
H8 and for the aromatic ones H1, H2 and H3. Minor shifts are also
observed for the resonances of the remaining anthracene and phen-
anthroline protons. This indicates the formation of intramolecular
-stacking interactions20 between the two aromatic moieties of the
ligand in the [ZnLH]3+ complex, most probably occurring in the
face-to-face mode, in contrast to the edge-to-face mode found in
the solid state. Probably packing effects, including the important
intramolecular -stacking observed in the solid complex, are critical
in determining the intramolecular -stacking interaction mode. As
shown in Fig. 5, the chemical shifts do not show significant varia-
tion in the pH range 6–10.5, where the [ZnL]2+ and [ZnL(OH)]+
species are formed. Very interestingly, the binding of the second
hydroxide anion leads to detachment of the NH group from the
metal, as shown by the upfield shift experienced by the H9 and H10
signals above pH 10.5. The downfield shift displayed by the signals
of the anthracene protons H1, H2 and H3 and by H8 indicates that
the -stacking interaction is lost upon detachment of the N1 amine
group from the Zn(II) ion.
binding of N1 to the metal is accompanied by negligible shifts of the
signals of the protons of the two aromatic units. This suggests that
in the [CdL]2+ complex the two aromatic moieties do not interact, or
they weakly interact via -stacking. The Hg(II) complexes display
an almost equivalent behaviour; coordination of the amine N1 of the
pendant arm in the [HgL]2+ complex does not give any -stacking
interaction between the phenanthroline and the anthracene units.
Fig. 6 pH Dependence of selected 1H NMR signals of L in the presence
of Cd(II) and distribution diagram for the system Cd(II)/L ([L] = [Cd(II)] =
1 × 10−3 M).
Fluorescence emission in H2O–CH3CN (50:50, v/v)
We anticipated in a previous communication11 that the fluorescence
emission from Lsolutions is strictly dependent upon the protonation
state of the compound.Atotal quenching of the emission is observed
for the species L, HL+ and H2L2+, while H3L3+ exhibits an intense
emission. Such quenching effect can be explained by an electron
transfer process from unprotonated amine groups in benzylic posi-
tion to the excited anthracene. Indeed, as shown by the NMR study,
in H3L3+ all three amine groups in benzylic positions are protonated
and the species is emissive.
Like protonation, also coordination of Zn(II) to the amine groups
precludes the electron transfer quenching effect. Accordingly, the
quenching effect occurs only at pH values that permit the existence
of nitrogen atoms which are neither attached to the metal nor proton-
ated. This occurs for all Zn(II) complexes except [ZnH2L]4+ which
exhibits an intense emission with a maximum at 418 nm (excitation
at 352 nm). The most interesting feature of this system, however,
is an exciplex emission occurring, at about 550 nm (excitation at
352 nm), for all complex species with the exception of [ZnL(OH)2],
revealed by the formation of a non-structured and red-shifted emis-
sion band (Fig. 7) ascribed to an intramolecular -stacking complex
in the excited state, involving phenanthroline and anthracene. As
described above, the -stacking complex is already formed in the
ground state for the species [ZnHL]3+, [ZnL]2+ and [ZnL(OH)]+, but
not in the case of [ZnH2L]4+ and [ZnL(OH)2]. As a consequence,
in the case of [ZnH2L]4+, the -stacking complex is exclusively
formed during the excited state lifetime. The -stacking complex
in the ground state and the exciplex emission from the excited state,
are driven by the coordination of the nitrogen atom of the pendant
arm to the metal, since this coordination forces the phenanthroline
and the anthracene moieties to stay close to each other. Such an
arrangement is maintained upon coordination of a single OH− ion
to the metal, probably because the binding occurs at the opposite
side of the pendant arm, while the exciplex emission, as well as the
-stacking complex, disappear at more basic pH values, in agree-
ment with the detachment of the amine nitrogen of the pendant arm
caused by coordination of the second OH− ion.
Fig. 5 (a) pH Dependence of selected 1H NMR signals of L in the pres-
ence of Zn(II). (b) Distribution diagram for the system Zn(II)/L ([L] =
[Zn(II)] = 1 × 10−3 M).
The pH-controlled coordination of the amine group of the pen-
dant arm is also observed in the Cd(II) and Hg(II) complexes. In
these cases, however, only the monoprotonated [MLH]3+ and the
[ML]2+ complexes are detected in solution. According to Fig. 6, for
the Cd(II) complexes, coordination of the amine group N1 occurs
upon deprotonation of the [CdLH]3+ complex to give the [CdL]2+
one in the pH range 4–7, as supported by the downfield shifts of
the H9 and H10 signals in this pH range. Differently from Zn(II),
In contrast to the Zn(II) behaviour, no exciplex emission is ob-
served with the Cd(II) and Hg(II) complexes of L. As found for
the Zn(II) complex, quenching of the strong emission at 418 nm
is also observed upon formation of [CdL]2+ and [HgL]2+, but the
-stacking complex is not formed by these two compounds, neither
in the ground state, as shown above by the NMR study, nor in the
excited state. Most likely, the larger Cd(II) and Hg(II) cations are
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7
2 1 8 3

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Independently of the temperature in the range 288–353 K, the
metal-free ligand does not show the exciplex emission band, as pre-
viously observed in H2O–CH3CN (50:50, v/v) at room temperature,
revealing that without metal coordination the two aromatic moieties
do not interact with each other.
The steady-state temperature dependence of L, shown in Fig. 9,
reveals a decrease of the fluorescence emission intensity with
increasing temperature. This is a well-known phenomenon oc-
curring with aromatic-like molecules, which translates the gradual
predominance of the radiationless processes relative to the radiative
processes.
Fig. 7 Fluorescence emission spectra (exc = 352 nm) of the Zn(II) com-
plexes with L at different pH values (1.69, 3.73, 4.4, 4.87, 5.1, 5.57, 6.05,
6.55, 10.37). Inset: fluorimetric titration of the same system: (•) emission at
418 nm, (○) exciplex emission at 600 nm. Species distribution curves in the
inset represent mol fractions.
large enough to prevent the two heteroaromatic moieties to come
in contact even when the nitrogen atom of the arm is coordinated to
the metal. Hence, the exciplex emission might be used for selective
signalling of Zn(II) in the presence of Cd(II) and Hg(II).
Unlike the Zn(II) complex, behaving as an elementary molecular
machine driven by pH and by light, the Cd(II) and Hg(II) complexes
can perform only controlled molecular movements driven by pH.
Fluorescence emission and fluorescence temperature
dependence in ethanol
Fig. 9 Fluorescence emission spectra of L in ethanol, as a function of
temperature. Shown, as inset, is the linear dependence of the fluorescence
intensity with temperature.
The fluorescence emission spectra of compounds L and [ZnL]2+
in ethanol are presented in Fig. 8, together with the exciplex band
obtained by subtracting the emission spectra of L from that of the
complex. The emission spectra of [ZnL]2+ and L show the charac-
teristic structured band of anthracene, with maximum intensity at
412 nm. In the case of [ZnL]2+, again an additional structureless
broad band (exciplex band), appears at longer wavelength values
(with maximum at ca. 525 nm). A significant increase in the total
intensity of this band is observed when compared to the behaviour
found in H2O–CH3CN (50:50, v/v).
However, this is not the case occurring with [ZnL]2+, where the
appearance of the new exciplex band gives rise to two different
behaviours, depending on the temperature range studied.
The variation of the ratio of the exciplex to monomer bands
(IExc/IM) with temperature can be described within the framework
of the classic Birks kinetic scheme21 (see ESI†). The Stevens–Ban
plots of ln(IExc/IM) vs. the reciprocal of the absolute temperature for
[ZnL]2+ are presented in Fig. 10. The exciplex-to-monomer inten-
sity ratio (IExc/IM) reaches its maximum at about 268 K. From the
low-temperature regime (LTL) and high temperature regime (HTL)
it is possible to extract respectively the activation energy for the
exciplex formation, Ea = 12.3 kJ mol−1 and the enthalpy of exci-
plex formation (H = −7.9 kJ mol−1).17 As expected, the enthalpy
for exciplex formation is negative thus showing that the process is
thermodynamically favorable.
Of relevance in Fig. 10 is the fact that the difference between
the ln(IExc/IM) at the crossover temperature and that obtained by
extrapolation of the linearizations obtained in the HTL and LTL is
 = 0.35, which is about half ( = ln2 = 0.69) of the expected value
considering the classical Birks’ kinetic scheme, where no ground-
state excimers or exciplex exist. This difference usually implies
that some degree of exciplex is already present in the ground-state
as was previously established to occur in H2O–CD3CN (50:50,
v/v).11
Moreover the fluorescence quantum yield for L and [ZnL]2+
were determined in ethanol solution at room temperature. The fluo-
rescence quantum yield of L (φM = 0.031) was obtained by using
naphthalene (φF = 0.27) as a standard, while the fluorescence quan-
Fig. 8 Fluorescence emission spectra of compound [ZnL]2+ and L in etha-
nol at 293 K. The exciplex band results from the subtraction of the emission
spectra of L from that of [ZnL]2+.
E
tum efficiency for the exciplex (φF ) occurring with [ZnL]2+, was
2 1 8 4
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7

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Conclusions
The ligand L forms very stable metal complexes in solution thanks
to its polyamine macrocyclic moiety behaving as primary coordi-
nation site. Coordination of the additional amine group located in
the pendant arm to the metal ion can be controlled by competitive
reaction with H+ (protonation), hence by modulating the solution
pH. Binding of metal ions such as Zn(II), Cd(II), and Hg(II) de-
termines a total quenching of the intense fluorescence emission
from the anthracene fluorophore, centred at 418 nm, signalling
the binding process. In the case of Zn(II), but not with Cd(II) and
Hg(II), complex formation gives rise, upon excitation, to an exciplex
emission, at about 550 nm, due to the intramolecular association via
-stacking of phenanthroline and anthracene groups in the excited
state. According to such exciplex emission, L behaves as a selective
fluorescent chemosensor for Zn(II) over Cd(II) and Hg(II). A similar
-stacking association occurs also in the ground state, as shown
by means of 1H NMR study, for all Zn(II) complex species except
for [ZnH2L]4+ and [ZnL(OH)2]. The formation of the [ZnL(OH)2]
species occurring at alkaline pH disrupts the -stacking complex,
both in the ground state and in the excited state, switching off the
exciplex emission. On the other hand, [ZnH2L]4+, in contrast with
the ground state behaviour, gives rise to a -stacking complex and
exciplex emission in the excited state. Hence, around pH 4, where
this species is formed, the movement of the pendant arm leading
to the formation of the -stacking complex is controlled by light
irradiation. In terms of intramolecular movement of the pendent
functionality, complexes of L with Zn(II), Cd(II), and Hg(II) behave
as elementary molecular machines driven by pH, and by light in the
case of [ZnH2L]4+.
Fig. 10 Arrhenius plot for [ZnL]2+ in ethanol showing the two temperature
regimes and the crossover temperature.
Exciplex formation, which is a thermodynamically favourable
process in solution, occurs also in the solid state, although different
-stacking modes of interaction, namely face-to-face in solution
and edge-to-face in the solid, are probably operative.
determined by using L as a standard; the total fluorescence quantum
yield was φTF = 0.012. With the use of eqn. (1)
Ap
Exc
φMAp
φM
φ
φFE =
(1)
1−
Experimental
Materials
where φAp and φMAp are the integrated areas under the exciplex
Exc
and monomer bands with φFT = φEAxpc +φMAp , the value for φF was
E
The synthetical procedure to obtain L (2,9-[2,5,8-triaza-5-(N-an-
thracene-9-methyl-amino)ethyl]-[9]-1,10-phenanthrolinophane
trihydrobromide) is depicted in Scheme 1. 2,9-bis(bromomethyl)-
1,10-phenanthroline (1)22 and tris[2-(N-tosylaminoethyl)]amine
(2)23 were prepared as previously described. Anthracene-9-carbal-
dehyde (5) was purchased from Aldrich.
determined to be 0.0064.
Solid state behaviour
Fig. 11 shows the fluorescence and fluorescence excitation spectra
recorded for a thin film of the solid complex and for an ethanol
solution of the complex. Comparison between the two media (solid
and liquid) reveals that the fluorescence excitation spectrum of the
solid (film) is red-shifted (ca. 5 nm) with respect to the absorption
spectra obtained in solution. However, the fluorescence emission
spectrum of the solid is now blue-shifted by about 46 nm. These
observations are compatible with the X-ray data of the complex
(Figs. 1 and 2), i.e., in the solid state a new exciplex-like conforma-
tion, with relative perpendicular orientations of the phenanthroline
and anthracene moieties, is observed.
Scheme 1
2,9-[2,5,8-Triaza-2,8-ditosyl-5-(N-tosylaminoethyl)]-[9]-1,10-
phenanthrolinophane (3)
A solution of 1 (4 g, 3.6 mmol) in dry CH3CN (250 cm3) was added
over a period of 6 h to a refluxing and vigorously stirred suspen-
Fig. 11 Thin film and solution fluorescence and fluorescence excitation
spectra of [ZnL]2+ obtained at 293 K.
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7
2 1 8 5

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sion of 2 (5.3 g, 8.7 mmol) and K2CO3 (25 g, 0.18 mol) in CH3CN
(400 cm3). After the addition was completed, the solution was re-
fluxed for an additional 2 h. The resulting suspension was filtered
out and the solution was vacuum evaporated to give a crude solid.
The product was purified by column chromatography on neutral
alumina using CH2Cl2–acetone (10:1) as eluting solvent, affording
product 3 as a white solid. Yield: 2.7 g (37%). Elemental analysis:
calc. (%) for C41H44N6S3O6: C 60.57, H 5.45, N 10.34; found: C
fined accordingly to the linked atoms. Refinement were performed
using full-matrix least-squares methods with the SHELXL-97
program.25 Final agreement factors were R1 = 0.0487 (I > 2(I)),
wR2 = 0.1498 (all data).
CCDC reference number 220491.
See http://www.rsc.org/suppdata/dt/b4/b403743j/ for crystallo-
graphic data in CIF or other electronic format.
1
60.4, H 5.4, N 10.2. H NMR (300 MHz, CDCl3):  1.39 (s, 6H),
Potentiometric measurements
2.31 (s, 3H), 2.5 (t, 4H), 2.71 (t, 2H), 3.14 (t, 4H), 3.59 (t, 2H),
4.81 (s, 4H), 7.43 (d, 6H), 7.76 (d, 2H), 7.89 (m, 8H), 8.33 (d, 2H);
13C NMR (300 MHz, CDCl3):  21.91, 29.69, 41.35, 429.69, 41.35,
47.87, 53.46, 53.73, 54.96, 123.98, 126.84, 127.46, 128.34, 130.26,
137.05, 137.61, 143.89, 145.45, 156.69.
All pH-metric measurements (pH = −log[H+]) employed for the
determination of ligand protonation and metal complex stability
constants were carried out in 0.10 mol dm−3 NMe4Cl H2O–MeCN
(1:1, v/v) solution at 298.1 ± 0.1 K by means of conventional titra-
tion experiments under inert atmosphere. The choice of this solvent
mixture was dictated by the low solubility of the ligand in pure
water. The combined Ingold 405 S7/120 electrode was calibrated
as a hydrogen concentration probe by titrating known amounts of
HCl with CO2-free NaOH solutions and determining the equivalent
point by Gran’s method26 which allows one to determine the stan-
dard potential Eo and the ionic product of water (pKw = 14.99(1)).
At least three measurements (about 100 data points for each) were
performed for each system in the pH range 2.5–10.5. In all experi-
ments the ligand concentration [L] was about 1 × 10−3 mol dm−3.
In the complexation experiments the metal ion concentration was
[M(II)] = 0.8[L]. The computer program HYPERQUAD27 was used
to calculate the equilibrium constants from emf data. In the case of
Hg(II), under the experimental conditions employed, the formation
of metal–chloride complexes is expected to occur. The formation of
such complexes was not taken into account in calculations; hence,
the stability constants of Hg(II) complexes reported in Table 2 must
be referred to the specific composition of the medium employed
(0.10 mol dm−3 NMe4Cl H2O–MeCN (1:1, v/v)).
2,9-(2,5,8-Triaza-5-aminoethyl)-[9]-1,10-phenanthrolinophane
trihydrobromide (4·3HBr)
Compound 3 (2.7 g, 3.3 mmol) and phenol (25 g, 0.266 mol) were
dissolved in 33% HBr/CH3COOH (200 cm3). The reaction mix-
ture was kept under stirring at 90 °C for 22 h until a precipitate
was formed. The solid was filtered out and washed several times
with CH2Cl2. The trihydrobromide salt was recrystallized from a
EtOH–water 2:1 mixture. Yield 0.56 g (28%). Elemental analysis:
calc. (%) for C20H26N6·3HBr: C 40.49; H, 4.92, N 14.17; found: C
1
40.52, H 5.1, N 14.34. H NMR (300 MHz, D2O, pH = 5):  2.46
(t, 2H), 2.74 (t, 2H), 2.87 (t, 4H), 3.22 (t, 4H), 4.48 (s, 4H), 7.50 (d,
2H), 7.62 (s, 2H), 8.18 (d, 2H); 13C NMR (300 MHz, D2O, pH = 5):
 34.03, 45.95, 46.15, 50.56, 51.18, 122.68, 127.17, 129.08, 139.09,
143.14, 150.36; MS: m/z = 351 [M + H+].
2,9-[2,5,8-Triaza-5-(N-anthracene-9-methylamino)ethyl]-[9]-
1,10-phenanthrolinophane trihydrobromide (L·3HBr)
Compound 4 (0.35 g, 1 mmol) and anthracene-9-carbaldehyde (5)
(0.2 g, 0.97 mmol) were dissolved in ethanol (50 cm3) and allowed
to react for 48 h at room temperature. Then NaBH4 (0.5 g, 13 mmol)
was added and the resulting solution warmed at 50 °C for 4 h. The
solvent was vacuum evaporated; the residue was then dissolved in
water (50 cm3) and extracted with CH2Cl2 for at least four times
(150 cm3). After drying over Na2SO4, the solvent was evaporated
under reduced pressure; the solid residue was washed several times
with cyclohexane and then dissolved in ethanol. Dropwise addition
of concentrated HBr yields L·3HBr as a solid compound, which
was filtered off, washed with ethanol and dried in vacuum. Yield
0.55 g (73%). Elemental analysis: calc. (%) for C35H39N6Br3Br: C
53.66, H 5.02, N 10.73; found: C 53.72, H 5.15, N 10.87. 1H NMR
(300 MHz, CDCl3):  3.16 (m, 12H), 4.19 (s, 4H), 4.49 (s, 2H), 7.30
(m, 6H), 7.72 (s, 2H), 7.79 (d, 2H), 8.00 (s, 1H), 8.05 (d, 2H), 8.14
(d, 2H); 13C NMR (300 MHz, D2O–CD3CN):  46.80, 50.02, 54.38,
58.16, 60.21, 120.54, 120.95, 123.83, 125.24, 126.05, 127.10,
127.60, 128.73, 128.94, 136.98, 148.21; MS: m/z = 541 [M + H+].
NMR measurements
300.07 MHz 1H and 75.46 MHz 13C NMR spectra in D2O–CD3CN
solutions at different pH values were recorded at 298.1 K in a Var-
ian Unity 300 MHz spectrometer. Peak positions are reported rela-
1
1
tive to TMS. H–1H and H–13C 2D correlation experiments were
performed to assign the signals. Small amounts of 0.01 mol dm−3
NaOD or DCl solutions were added to a solution of L in the absence
or in the presence of metal (1:1 molar ratio) to adjust the pH.
Spectroscopic measurements
The solvents used were of spectroscopic or equivalent grade.
Ethanol was previously dried over CaO and then distilled. Water
was twice distilled and passed through a Millipore apparatus. All
aqueous solutions were prepared in 0.1 mol dm−3 NaCl. The mea-
sured pH values were obtained with a Crison micropH 2000 and
adjustments of H+ concentration was made with diluted HCl and
NaOH solutions. [ZnLBr](ClO4)·H2O was used for both solution
and solid-state studies.
Absorption and fluorescence spectra were recorded on Shimadzu
UV-2100 and Jovin-Yvon Spex Fluorog 3–2.2. spectrometers, re-
spectively. The fluorescence quantum yield of compound L was
determined using anthracene (φF = 0.27) as standard. The fluo-
rescence quantum yield of compound [ZnL]2+ was measured using
compound 1 (φF = 0.031) as standard.
Thin films of the Zn(II) complex were obtained with a Desk-
Top Precision Spin Coating System, Model P6700 Series from
Speedline Technologies. The solid-state thin film of the complex
was obtained by deposition of a few drops of an ethanol solution of
[ZnLBr](ClO4)·H2O in a quartz lamella followed by spin coating at
430–635 mmHg and in Ar saturated atmosphere (2 psi).
The IExc/IM ratio was obtained from the decomposed area under
the monomer and exciplex bands. The general procedure to obtain
those values consisted in matching the emission spectra of L in
ethanol with the monomer band of ZnL. The resulting differential
spectrum is the exciplex emission band. In this procedure the x–x-
scale must be in energetic units.
[ZnLBr](ClO4)·H2O
Crystals of [ZnLBr](ClO4)·H2O used for X-ray analysis were ob-
tained by slow evaporation at room temperature of a water–MeCN
(1:1, v/v) solution containing Zn(ClO4)2 and L·3HBr at pH 7.
Crystallographic data collection and refinement of the crystal
structure of [ZnLBr](ClO4)·H2O
Crystal data: C35H38BrClN6O5, M = 803.44, monoclinic, space
group P21, a = 11.540(5), b = 12.480(5), c = 12.160(5) Å,
 = 106.600(5)°, Z = 2, U = 1678.3(12) Å3, T = 298 K,
 = 3.582 mm−1, F(000) = 824. Data collection: P4 X-ray Bruker
X-ray diffractometer,  = 1.5418 Å (Cu-K), graphite monochro-
mated. 3722 Reflections collected, 3211 unique. The structure was
solved by direct methods using the SIR97 program.24 Anisotropic
thermal parameters were used for all the non-hydrogen atoms. Hy-
drogen atoms, except those of the water molecules, were included
in calculated positions and their coordinates and thermal factors re-
2 1 8 6
D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7

View Article Online
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Acknowledgements
This work was supported by the Italian MIUR (PRIN 2002) and
by project POCTI/QUI/97357/2002 FCT-FEDER (Portugal). C. L.
acknowledges the financial support provided through the European
Community’s Human Potential Programme under contract HPRN-
CT-2000-00029 [Molecular Level Devices and Machines].
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D a l t o n T r a n s . , 2 0 0 4 , 2 1 8 0 – 2 1 8 7
2 1 8 7