Excimer emission induced by metal ion coordination in 1,8-naphthalimide-tethered iminopyridine ligands

Article abstract of DOI:10.1039/b308439f

Ligands 2-5, containing the light-emitting subunit 1,8-naphthalimide, have been prepared and their photophysical properties studied by absorption and emission spectroscopy. Ligand 2 interacts in solution with several cations according to a 1 : 2 (metal : ligand) stoichiometry, the 1 : 3 species being not favoured probably because of steric hindrance, while ligands 3-5 form 1 : 3 adducts with all the investigated metal ions. The interaction of ligands 2-5 with metal ions induces considerable variations on the photophysical properties of the light-emitting subunit. The coordination of genuine transition metal ions (Fe^II, Co^II, Ni^II, Cu^II) causes the emission intensity to decrease in all the investigated systems, while Zn^II or Cd^II induce a fluorescence enhancement (system 2) or the formation of a new band in the emission spectra (systems 3-5) which can be ascribed to an intramolecular excimeric species. Excimeric emission is not observed in the complexes of 2, possibly because the ethylenic chain bridging the naphthalimide and the iminopyridine units is too short to allow the intramolecular interaction. The excimeric species disappears on increasing the metal ion (Zn^II or Cd^II) concentration, as a result of the disassembling of the 1 : 3 complexes and the consecutive formation of 1 : 2 and 1 : 1 species, in which the intramolecular interaction is less probable or no longer possible. The appearance and disappearance of an excimer band in the emission spectrum can be described as a convenient way to monitor a metal-driven assembling/disassembling process.

Full text of DOI:10.1039/b308439f

View Article Online / Journal Homepage / Table of Contents for this issue
Excimer emission induced by metal ion coordination in
1,8-naphthalimide-tethered iminopyridine ligands
Maurizio Licchelli,*a Alessio Orbelli Biroli,a Antonio Poggi,a Donatella Sacchi,a
Corrado Sangermania and Michele Zemab
a Dipartimento di Chimica Generale, Università di Pavia, via Taramelli 12, I-27100 Pavia, Italy.
E-mail: mali@unipv.it
b Dipartimento di Scienze della Terra, Università di Pavia, via Ferrata 1, I-27100 Pavia, Italy
Received 22nd July 2003, Accepted 10th September 2003
First published as an Advance Article on the web 25th September 2003
Ligands 2–5, containing the light-emitting subunit 1,8-naphthalimide, have been prepared and their photophysical
properties studied by absorption and emission spectroscopy. Ligand 2 interacts in solution with several cations
according to a 1 : 2 (metal : ligand) stoichiometry, the 1 : 3 species being not favoured probably because of steric
hindrance, while ligands 3–5 form 1 : 3 adducts with all the investigated metal ions. The interaction of ligands 2–5
with metal ions induces considerable variations on the photophysical properties of the light-emitting subunit. The
coordination of genuine transition metal ions (FeII, CoII, NiII, CuII) causes the emission intensity to decrease in all
the investigated systems, while ZnII or CdII induce a fluorescence enhancement (system 2) or the formation of a new
band in the emission spectra (systems 3–5) which can be ascribed to an intramolecular excimeric species. Excimeric
emission is not observed in the complexes of 2, possibly because the ethylenic chain bridging the naphthalimide and
the iminopyridine units is too short to allow the intramolecular interaction. The excimeric species disappears on
increasing the metal ion (ZnII or CdII) concentration, as a result of the “disassembling” of the 1 : 3 complexes and the
consecutive formation of 1 : 2 and 1 : 1 species, in which the intramolecular interaction is less probable or no longer
possible. The appearance and disappearance of an excimer band in the emission spectrum can be described as a
convenient way to monitor a metal-driven assembling/disassembling process.
Introduction
Derivatives of 1,8-naphthalimide have been extensively studied
in recent decades owing to their interesting photophysical
properties1 and the wide range of their applications in different
chemical and biochemical areas. Many naphthalimide chromo-
phores, characterised by both strong absorption and good
fluorescence yields were prepared in the last century and used
as dyes2 or brighteners.3 Several 1,8-naphthalimide-containing
compounds have been described as photo-induced DNA-
cleaving agents4 and tumoricidals,5 some of them entering into
clinical trials6 owing to their strong anticancer activity. More-
over, similar derivatives have been proposed as good candidates
for the photochemical inhibition of enveloped viruses in blood
and blood products,7 and for the fluorescent probing of hypoxic
the complexes formed by 2 with zinc() and copper() are also
cells.8
reported.
The 1,8-naphthalimide moiety has also been used to design
fluorescent molecular probes sensitive to variations of pH9 or
metal ion concentration.10
Results and discussion
Although the photophysical properties of 1,8-naphthal-
imides have been widely studied, the formation of excimeric
species has been reported only in a few cases.1d,11,12 In particular,
a broad emission band, ascribed to an intramolecular excimer,
was observed in the fluorescence spectra displayed by com-
pound 1 in which two subunits of naphthalimide are linked
together by an aliphatic chain.11 An excimeric emission was also
observed when naphthalimide was appended by ethylenic
spacers on the aliphatic chains of a poly(allylamine) polymer.12
In this paper we report the synthesis of systems 2–5 in which
the 1,8-naphthalimide moiety is connected by aliphatic spacers
to an iminopyridine bidentate coordinating fragment. The
effects on the absorption and emission properties displayed by
compounds 2–5 induced by their interaction with different
metal ions is discussed; particular attention is focused on the
formation of intramolecular excimers due to the coordination
of metal ions such as Zn2ϩ and Cd2ϩ. The results obtained by
crystallographic studies performed on ligands 2 and 3, and on
Synthesis and crystal structures of ligands and metal complexes
Ligands 2–5 were prepared according to a two-step synthetic
pathway as shown in Scheme 1.
The amino intermediates 6–9 were obtained by reacting an
aqueous suspension of the commercially available 1,8-naph-
thalic anhydride in the presence of excess diamine, according to
a modification of a literature method.13 Although the yields
were not quantitative (ranging between 52.8 and 84.6%),
products were pure enough and did not require any chromato-
graphic purification before the following step. Attempts to
prepare 6 under homogeneous conditions (i.e. with ethylene-
diamine as the solvent) afforded an impure product (column
chromatography was necessary) and lower yields (about 40%)
than the heterogeneous method. Compounds 6–9 were easily
transformed into the corresponding ligands 2–5 by performing
a Schiff condensation in MeOH or CH2Cl2 in the presence
of a slight excess of 2-pyridinecarbaldehyde. No attempts to
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 3
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4537

View Article Online
Several attempts to isolate metal complexes in crystalline
form were performed by allowing very slow evaporation of
MeCN solutions containing the envisaged metal ion and ligand
in 1 : 2 or 1 : 3 molar ratio. The slow diffusion of diethyl ether
(or diisopropyl ether) on similar MeCN solutions was
attempted as well. In most cases the precipitates were not good
enough for X-ray diffraction purposes, and sometimes they
were non-crystalline. Crystals of good quality for the X-ray
crystallographic studies were obtained (by slow diffusion of
diethyl ether in MeCN solutions) only for the trifluoro-
methanesulfonate salt of [Zn(2)2]2ϩ. In the case of [Cu(2)2]2ϩ the
quality of the crystals was not as good as for [Zn(2)2]2ϩ (see
details in the Experimental section), but still sufficient to allow
the determination of the molecular structure of the complex.
The crystal structure of the [ZnII(2)2(CF3SO3)2] complex, as
determined by X-ray diffraction methods, is shown in Fig. 2.
The ZnII cation is hexa-coordinated, bonding to two nitrogen
atoms from the iminopyridine bidentate fragments of two
different ligands 2, and to one oxygen atom from each triflate
anion. The overall geometry at the metal centre can be
described as a quite regular octahedron. The Zn–N distances
reflect the different hybridization states of the nitrogen atoms
[average Zn–N(py) 2.099 Å; average Zn–N(imine) 2.142 Å]. The
average Zn–O(triflate) distance is 2.203 Å. It is worth noting
that the two ligands adopt a different conformation: in fact,
looking straight down the C–C bonds of the aliphatic spacers,
the 1,8-naphthalimide and the iminopyridine groups are in an
anti conformation in one case (see atoms N2 and N3 in Fig. 2),
and in a gauche conformation in the other case (see atoms N5
and N6 in Fig. 2). The latter conformation allows one of the
oxygen atoms of the carbonyl groups to be closer to the metal
centre: Zn ؒ ؒ ؒ O4 is in fact 3.654 Å, while Zn ؒ ؒ ؒ O2 is
5.903 Å. The proximity of O4 to the coordination sphere of
ZnII forces the N5–Zn1–N2 angle to the value of 111.3(2)Њ,
which represents the largest discrepancy from the octahedral
geometry. The crystal is maintained by van der Waals forces.
Scheme 1 Schematic representation of the synthetic procedure for
systems 2–5.
improve the yields (in the range 49–80%) of this step were
made, since all starting materials are low-cost commercially
available products.
Ligands 2 and 3 were isolated in crystalline form, as pale
yellow crystals, after slow diffusion of diethyl ether into di-
chloromethane solutions. Systems 4 and 5 containing the more
flexible tetra- and pentamethylene spacers were never obtained
as crystals.
The molecular structures of ligands 2 and 3 were determined
by means of X-ray single-crystal diffraction and did not reveal
any unusual features. The corresponding ORTEP diagrams are
reported in Fig. 1. In both cases, the 1,8-naphthalimide moieties
are planar as expected. The mean deviations from the best
plane calculated using all 15 atoms are 0.045 and 0.025 Å for 2
and 3, respectively, while the largest deviations are 0.100(2) and
0.059(3) Å. The 1,8-naphthalimide fragments belonging to dif-
ferent molecules are parallel and face each other, with shortest
distances of 3.584(3) and 3.448(3) Å for 2 and 3, respectively. In
ligand 2, due to the presence of a shorter aliphatic spacer, the
angle formed by the 1,8-naphthalimide with the iminopyridine
mean plane is smaller (114.8Њ) than in 3 (130.9Њ); moreover the
two sub-units are in an anti conformation with respect to the
C7–C8 bond, as illustrated in Fig. 1(a).
Fig. 2 ORTEP view of [ZnII(2)2(CF3SO3)2] with thermal ellipsoids
drawn at 20% probability level. Selected bond lengths (Å) and angles
(Њ): Zn1–N1 2.112(6), Zn1–N2 2.159(6), Zn1–N4 2.086(7), Zn1–N5
2.125(6), Zn1–O5 2.269(5), Zn1–O8 2.137(6); N1–Zn1–N2 78.7(2), N1–
Zn1–N4 175.3(3), N1–Zn1–N5 98.0(3), N1–Zn1–O5 93.0(2), N1–Zn1–
O8 95.0(3), N2–Zn1–N4 99.5(3), N2–Zn1–N5 111.3(2), N2–Zn1–O5
162.2(2), N2–Zn1–O8 87.1(2), N4–Zn1–N5 78.5(3), N4–Zn1–O5
89.9(2), N4–Zn1–O8 89.2(3), N5–Zn1–O5 85.3(2), N5–Zn1–O8
159.2(2), O5–Zn1–O8 77.9(2).
Fig. 1 ORTEP views of (a) ligand 2 and (b) ligand 3 with thermal
ellipsoids drawn at 30% probability level.
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4538

View Article Online
An ORTEP diagram of the [CuII(2)2]2ϩ molecular cation is
reported in Fig. 3. Unfortunately, the poor quality of the
crystallographic data (see Experimental section) does not allow
an accurate analysis of the structural model. At any rate, a
comparison with the analogous ZnII complex can be made on
the basis of these preliminary data. The crystal structure of
[CuII(2)2](CF3SO3)2ؒCH3CNؒH2O consists of separated [CuII-
(2)2]2ϩ molecular cations and triflate anions which do not inter-
act with the metal centre. Moreover, one MeCN molecule and
one water molecule are present in the crystal structure and they
do not interact with CuII. In the [CuII(2)2]2ϩ molecular cation,
the 1,8-naphthalimide and the iminopyridine sub-units of
both the ligands are in a gauche conformation with respect to
the C–C bond of the aliphatic spacer. This implies that two
oxygen atoms, one from each ligand, are quite close to CuII
[Cu–O(carbonyl) 2.69(2) Å], leaving no more room for the tri-
flate anions in the coordination sphere of the metal centre.
Coordination at the metal centre can therefore be described
as 4 ϩ 2, CuII bonding also to two nitrogen atoms from the
iminopyridine groups of two different ligands [Cu–N(py)
1.98(2) Å; Cu–N(imine) 2.01(2) Å].
naphthalimide and other related compounds1d,10d,12 and con-
firm the existence of very short-lived excited species for
all the examined systems. No concentration dependence of
the emission spectrum and, in particular, no bands ascribable
to excimeric species were observed for all ligands, similarly
to N-alkylnaphthalimides,11,12 for which no intermolecular
excimer emission was found up to a concentration value of
ca. 10Ϫ3 M.11
Interaction with metal ions
The behaviour of ligands 2–5 in the presence of metal ions in
solution was studied at first by spectrophotometric titration
experiments in order to assess the stoichiometry of the complex
species. MeCN solutions (5 × 10Ϫ5 M) of the ligand were
titrated with the envisaged metal ions (FeII, CoII, NiII, CuII,
ZnII, CdII) and absorption spectra were taken after each
addition of metal. Similar titration experiments combined with
spectrofluorimetric measurements, allowed to evaluate the
effects induced by the coordination of metal ions on the
emission properties displayed by 2–5. MeCN was chosen as
the solvent for all the measurements since it guarantees a good
solubility of metal salts, ligands and complexes. The results for
system 2 are quite different from those obtained for the other
ligands and therefore are discussed in a separate section.
Behaviour of 2. The addition of metal ions induces in each
case a substantial variation of the electronic spectrum: the
band at 272 nm, due to the absorption of the iminopyridine
moiety, directly involved in the coordination process, shifts to
higher wavelengths (280–290 nm) and undergoes a distinct
intensity increase. An isosbestic point is observed at about
270 nm. The absorption band related to the π,π* transition of
the naphthalimide moiety experiences only very slight vari-
ations, i.e. a small red shift (∆λ ≤ 2 nm) and a slight inten-
sity increase. These changes were observed until 0.5 equivalents
of metal ion were added. As an example, absorption spectra
registered during the titration of 2 with nickel() trifluoro-
methanesulfonate are reported in Fig. 4. Similar families of
spectra were obtained from the titrations with the other
examined metal ions.
Fig. 3 ORTEP view of [CuII(2)2]2ϩ molecular cation with thermal
ellipsoids drawn at 20% probability level. The triflate anions, as well as
MeCN and H2O molecules, have been omitted for clarity. Selected bond
lengths (Å) and angles (Њ): Cu1–N1 1.98(2), Cu1–N2 2.01(2); N1–Cu1–
N1i 104(1), N1–Cu1–N2 86.7(9), N1–Cu1–N2i 152.2(7), N2–Cu1–N2i
95(1), O1–Cu1–O1i 165.7(7). Symmetry code: i Ϫy, Ϫx, Ϫzϩ1/2.
Absorption and emission properties of ligands 2–5
1,8-Naphthalimide and its derivatives are well known chromo-
phores and fluorophores and their photophysical properties
have been the object of detailed studies.1 Ligands 2–5 display
spectroscopic properties in acetonitrile solution which resemble
very closely those reported for similar compounds such as
N-methyl-1d and N-propyl-naphthalimide.12 Absorption and
emission data are summarized in Table 1.
The electronic spectra of all the examined systems show a
band centered at 272 nm ascribed to the iminopyridine moiety
and the typical bands due to the naphthalimide chromophore
with maxima at 234, 332 and 346 nm whose intensities (see ε
values in Table 1) are very close to those reported for N-methyl-
naphthalimide taken as reference. Naphthalimides 2–5 fluor-
esce in the 350–450 nm range with a maximum at about 380 nm,
and their spectra have a mirror image relationship with the
long-wavelength absorption bands. The weak emission (fluor-
escence quantum yields are quite coincident with the values
reported for the reference compound)1d of 1,8-naphthalimides
has been already explained in terms of fast ISC from the excited
π,π* singlet state to a close-lying n,π* triplet state.1d,14
Fig. 4 Absorption spectra recorded during the titration of a MeCN
solution of 2 (5 × 10Ϫ5 M) with nickel(). Titration profiles reported in
the inset show the formation of a 1 : 2 adduct (n=equiv. of metal ion/
equiv. of 2).
The titration profiles obtained by plotting the molar absorb-
ance (ε) values determined at 290 nm vs. n (metal equiv/ligand
equiv.), clearly indicate the formation of adducts with a 1 : 2
stoichiometry (see inset of Fig. 4). Least-squares analysis of the
spectral data gives logβ values of 11.47 0.21 and 11.66 0.11
for [Zn(2)2]2ϩ and [Cd(2)2]2ϩ, respectively. Similar values were
obtained for the other metal complexes. The examination of the
titration profiles does not provide any clear evidence about the
formation of complex species with 1 : 3 stoichiometry, which
The fluorescence lifetimes detected for 2–5, are lower than
1 ns (see τ values in Table 1) as already observed for N-methyl-
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4539

View Article Online
Table 1 Absorption and emission data for 2–5 and the reference compound in acetonitrile
λmax/nm
ε/MϪ1 cmϪ1
λem/nm
Φ
τ/ns
N-Methylnaphthalimidea
234
332
345
234
272
332
346
234
272
332
346
234
272
332
346
234
272
332
346
49700
12500
11600
49800
6800
12500
11400
49900
6950
12100
11200
49780
6700
11700
10800
48600
7050
376
0.027
0.14
2
376
377
378
378
0.026
0.032
0.031
0.030
0.2b
0.8b
0.4b
0.3b
3
4
5
10950
10050
a Data from ref. 1d. b Excitation wavelength was 335 nm; the fluorescence decays were monitored at 380 nm.
could be prevented by the relatively short length of the ethylenic
chain bridging the chelate to the chromophoric subunits. This
may disfavour a comfortable positioning of three bulky naph-
thalimide moieties around the metal center with a resulting
enhanced stability of the 1 : 2 species. Moreover, the six-co-
ordinate arrangement could also be reached by interaction of
the coordinated ion with the carbonyl groups of the naph-
thalimide moieties, forming seven-membered chelate rings, as
indicated in the solid state by the X-ray structure obtained for
[Cu(2)](CF3SO3)2. As an alternative, the coordination spheres
of the metal ions interacting with 2 may be completed by
involving less sterically hindering anionic ligands or solvent
molecules. In the case of zinc() such a possibility is also con-
firmed in the solid state by the structure determined on [Zn(2)-
(CF3SO3)2] in which two trifluoromethanesulfonate anions are
bound to the metal center.
the metal ion (ZnII or CdII) concentration exceeds the 1 : 2 ratio
according to the following equilibrium:
[M(2)2]2ϩ ϩ M2ϩ
2[M(2)]2ϩ (M = Zn, Cd)
(1)
Similar spectroscopic evidence for the formation of the 1 : 1
species after the addition of excess metal ion is not observed in
the case of the other investigated ions.
The changes in the emission properties displayed by the
naphthalimide moiety observed upon metal ion addition
depend on the nature of the added metal ion. FeII, CoII, NiII
and CuII cause the intensity of the emission band, IF, at 380 nm
to decrease to 10–30% of the original value, depending on the
metal ion. The emission spectra registered during the titration
with nickel() trifluoromethanesulfonate are reported in Fig. 6.
A more careful examination of the spectra collected during
the titration experiments with ZnII and CdII reveals that further
small spectral changes take place also after the addition of 0.5
equivalents of the metal ion. In particular, the isosbestic point
at 270 nm is lost and the absorbance in the 300–310 nm range,
which had increased until 0.5 equiv. of metal ion had been
added, decreases again until the 1 : 1 ratio is reached (see Fig. 5).
Such a situation suggests the formation of a new complex spe-
cies (with a 1 : 1 metal : ligand stoichiometry) in solution when
Fig. 6 Emission spectra recorded during the titration of 2 (5 × 10Ϫ5 M
in MeCN) with nickel(). The titration profile in the inset indicates 1 : 2
stoichiometry for the complex species in solution (n=equiv. of metal
ion/equiv. of 2).
Iron(), d6, cobalt(), d7, nickel(), d8, copper(), d9, as
well as other transition metal ions with partially filled d
orbitals, are known to induce fluorescence quenching through
photoinduced electron transfer (eT) or energy transfer (ET)
mechanisms.15 The titration profiles obtained by plotting IF, vs.
n generally confirm the 1 : 2 stoichiometry, already observed in
the spectrophotometric titrations (see inset in Fig. 6).
On the contrary, the emission band displayed by ligand 2 at
380 nm undergoes an intensity enhancement following the addi-
tion of Zn2ϩ or Cd2ϩ by a factor of 6 and 7, respectively, until
Fig. 5 Variation of the molar absorbance at 302 nm during the
titration of a MeCN solution of 2 (5 × 10Ϫ5 M) with zinc(): on
increasing the metal ion concentration the 1 : 2 complex disassembles
yielding a 1 : 1 species. The process is not clearly revealed by variations
in any other part of the spectrum.
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4540

View Article Online
0.5 equiv. of metal ion is added (see Fig. 7). The small variation
of IF in the 0.5 < n < 1 range supports the existence in solution
of the 1 : 1 species, as already observed in the spectrophoto-
metric experiments.
spectral variations are not observed in the case of the other
ions.
As expected, FeII, CoII, NiII and CuII cause the intensity of
the emission band at 380 nm to decrease by a factor of 5–10,
depending on the metal ion and the ligand. However, no clear
correlation between the fluorescence quenching extent and the
length of the polymethylenic spacer has been found.
The results obtained from the spectrofluorimetric titration
experiments with Zn2ϩ or Cd2ϩ are very different from those
described above for the related system 2: the addition of zinc()
or cadmium() trifluoromethanesulfonate to MeCN solutions
of ligands 3–5 induces the appearance in the emission spectra
of an extra band centered at 470 nm whose relative intensity
increases until the metal ion concentration corresponds to a
1 : 3 stoichiometric ratio, then decreases again and almost dis-
appears when the metal ion concentration exceeds the value
corresponding to the 1 : 1 stoichiometry. The emission spectra
registered during the titration of 4 with ZnII triflate are reported
in Fig. 8. Similar families of spectra were obtained from the
titration experiments performed on 3 and 5. The broad red-
shifted band is very similar to the emission band reported for
compound 1 in acetonitrile or methanol (with a maximum at
470 nm)11 and can be ascribed to an excimeric species deriving
from the intramolecular interaction between the naphthalimide
moieties in the complex species.
Fig. 7 Emission spectra recorded during the titration of 2 (5 × 10Ϫ5 M
in MeCN) with cadmium(). The titration profile is reported in the
inset (n=equiv. of metal ion/equiv. of 2).
The fluorescence enhancement can be explained on the basis
of the possible interaction of the metal ion with the C᎐O
᎐
groups belonging to the naphthalimide moieties of the co-
ordinated ligands. The interaction with the n orbitals of the
dicarboxyimide fragment should increase the energy of the
n,π* excited state and prevent the fast ISC process through
which non-radiative deactivation of the excited π,π* singlet
state takes place in the free ligands. This results in a fluor-
escence enhancement in the presence of the d10 cations Zn2ϩ
and Cd2ϩ, which, differently from genuine transition metal ions,
do not have features to induce the fluorescence quenching
through an eT or ET mechanism. A similar hypothesis has
been proposed to explain the fluorescence intensity increase of
N-methylnaphthalimide in the presence of protic solvents able
to interact with the n orbitals on the dicarboxyimide groups via
H-bonding.1d
Behaviour of ligands 3–5 and excimer formation. The changes
induced in the absorption spectra by the addition of any of the
considered metal ions to MeCN solutions containing ligands
3–5 are very similar to those described in the previous section
for ligand 2. Nevertheless, the titration profiles generally indi-
cate the formation of adducts with a 1 : 3 stoichiometry, which
was not observed for the complexes formed by 2. Such a differ-
ence could be mainly ascribed to the different length of the
aliphatic chains in the examined systems: the longer and more
flexible spacers in 3–5 lead to less crowding around the metal
center in the 1 : 3 complexes. Moreover, the interactions of
the metal ion with the carbonyl groups of the naphthalimide
moieties which possibly complete the six-coordinate geometry
in the 1 : 2 complexes formed by ligand 2, in the case of
3–5 cannot help to stabilize the 1 : 2 species, since they would
originate eight- to ten-membered chelate rings which are highly
disfavoured.16
Least-squares analysis of the spectral data obtained in the
titration experiments gives logβ values in the range 15–17
(e.g. logβ = 15.61 0.07 for [Cd(3)3]2ϩ).
Although the spectrophotometric titrations performed with
either zinc() or cadmium() trifluoromethanesulfonate indi-
cate the formation of 1 : 3 complex as the prevailing species, the
small spectral variations in the 300–310 nm range, observed on
increasing the metal ion concentration, suggest the formation
of complex species with different metal : ligand ratios (i.e. 1 : 2
and 1 : 1), in analogy to ligand 2. On the other hand, these
Fig. 8 Emission spectra taken during the titration of 4 (5 × 10Ϫ5 M in
MeCN) with zinc(): (above) the intensity of the broad band at 470 nm
increases as n (equiv. of ZnII/equiv. of 4) is increased from 0 to 0.33;
(below) the band at 470 nm decreases as n becomes greater than 0.33.
The disappearing of the excimeric species in the presence
of excess metal ion can be accounted for by the consecutive
formation of complex species with 1 : 2 and 1 : 1 stoichiometry.
Octahedral tris-chelate complexes of zinc() and cadmium()
are likely to exist in solution as two isomers, the fac and mer
forms, as a rapidly interconverting mixture, owing to their
lability. However, the intramolecular interaction between the
fluorophore subunits which provides the excimer formation
is allowed in both forms since three or two naphthalimide
moieties, respectively, are in a cis arrangement. Similar con-
siderations were reported a few years ago by Fages and co-
workers when discussing the behaviour of a pyrene-labeled
hydroxamate ligand in the presence of GaIII.17 In 1 : 2 com-
plexes formed by 3–5 the probability of an intramolecular
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4541

View Article Online
Table
2 Excimer-to-monomer fluorescence intensity ratios and
favour the formation of excimers even in the case of fluoro-
phores, such as naphthalimide, for which, differently from
pyrene, excimeric emission is not very common.
excimer lifetimes determined for 1 : 3 complexes of zinc() and cad-
mium() with systems 3–5
The formation of intramolecular excimers in the naphthal-
imide derivatives 2–5 is influenced by the nature of the metal
ion involved in the coordination process and by the length of
the aliphatic chain bridging the chelating iminopyridine and
naphthalimide units. In particular, genuine transition metals
induce only a decrease of the monomer fluorescence intensity,
according to their common quenching behaviour, while zinc()
or cadmium(), which are innocent from a photophysical point
of view, induce the appearance of an excimeric band in the
spectrum of ligands 3–5.
The excimer emission is not observed in the complexes of
2 since the coordination of ZnII and CdII induces only an
enhacement of the monomer fluorescence. The different emis-
sion properties displayed by ligand 2 can be ascribed to the
relatively short length of the ethylenic bridge which prevents the
interaction between the aromatic moieties. The behaviour of 2,
whose fluorescence is simply quenched or enhanced depending
on the nature of the metal ion, resembles that commonly
observed in ON/OFF or OFF/ON fluorescent chemosensors,
which have been widely reported in the literature.20
On the contrary, the behaviour of systems 3–5 in the presence
of ZnII or CdII can be described as an example of metal-driven
assembling/disassembling process21 which is monitored by the
appearance and disappearance, respectively, of a new fluor-
escent signal: in fact, the intensity of the excimer emission
increases with the concentration of ZnII or CdII until a 1 : 3
adduct is formed (assembling) and decreases again at higher
metal ion concentration values owing to the consecutive
formation of 1 : 2 and 1 : 1 adducts (disassembling).
In conclusion, the results obtained in the present work can
contribute to both the design of novel molecular sensors for
metal ions based on naphthalimide derivatives, which are
currently investigated in our laboratory, and the study of self-
signalling assembling/disassembling processes sensitive to the
concentration of metal ions, which are potentially applicable
to the development of more sophisticated (supra)molecular
devices.
3
4
5
ZnII
CdII
ZnII
CdII
ZnII
CdII
a
IE/IM
τb/ns
0.719
19.5
0.544
20.9
1.658
23.5
1.465
23.1
1.242
23.9
0.896
23.7
a Emission intensity values measured at 378 and 470 nm for monomer
and excimer, respectively. b Monitored at 520 nm, λexc = 335 nm.
interaction is obviously lower and in 1 : 1 species it is no longer
possible, so that the progressive dissociation of the 1 : 3 com-
plexes (following an increase of the metal ion concentration)
causes the intensity of the excimeric band to decrease.
The efficiency of the excimer formation can be estimated by
the values of excimer-to-monomer fluorescence intensity ratio
(IE/IM), i.e. the ratio of the emission intensities at 470 and at
380 nm, for all 1 : 3 complex species (see Table 2).
The highest IE/IM values are observed for the complexes
formed by ligand 4, suggesting that the tetramethylene chain
displays the best features in terms of length and conformational
flexibility in order to maximize the interaction (in the excited
state) between two aromatic subunits.
Moreover, for each ligand, zinc() induces a more efficient
excimer formation (i.e. higher IE/IM values) than cadmium(),
indicating an influence of the cation size beside that of the
spacer length and flexibility.
The influence of the chain length and flexibility on the form-
ation of intramolecular excimers, has been pointed out in
several systems comprised of two aromatic nuclei (e.g. naph-
thalene, anthracene, pyrene) bridged together in various ways.18
The interaction of the investigated ligands with Zn2ϩ and
Cd2ϩ was also studied by time-resolved fluorescence measure-
ments, observing the effects on the fluorescence decay
behaviour.
The free ligands 3–5 display single exponential fluorescence
decays at 380 nm, with lifetimes shorter than 1 ns, as mentioned
above (see Table 2). When zinc() or cadmium() trifluoro-
methanesulfonate are added to an MeCN solution containing
one of the systems 3–5 (5 × 10Ϫ5 ), the fluorescence decay
profiles at 380 nm can only be fitted by employing two exponen-
tial terms and reveal the presence of a double lifetime. The
relative weight of the long-lived component increases with the
concentration of the metal ion until the 1 : 3 stoichiometry is
reached. As an example, the following values were obtained for
ligand 3 with Zn2ϩ: τ1 = 0.6 ns (88.8%), τ2 = 4.1 ns (11.2%) and
Cd2ϩ τ1 = 0.8 ns (95.4%), τ2 = 4.6 ns (4.6%). On further addition
of Zn2ϩ or Cd2ϩ, the abundance of the long-lived species
decreases again owing to the progressive formation of the 1 : 1
complex species and the consequent disappearing of the
intramolecular excimer.
Experimental
Unless otherwise stated, commercially available reagent grade
chemicals were used as received, except 2-pyridincarbaldehyde
that was freshly distilled prior to use. Cadmium(), cobalt(),
iron() and nickel() trifluoromethanesulfonates were prepared
according to literature methods.22 Spectrophotometric or
fluorimetric grade solvents were used for spectroscopic
measurements.
Column chromatography was carried out on silica gel 60
(Merck 9385).
UV-vis spectra were recorded on a Hewlett-Packard 8453
diode array or on a Varian Cary 100 spectrophotometer;
emission spectra were taken on a Perkin-Elmer LS-50B
luminescence spectrometer.
NMR spectra were recorded on a Bruker AMX400
spectrometer.
Mass spectra were obtained using an LCQ DECA ion trap
mass spectrometer equipped with electrospray ionization (ESI)
ion source and controlled by Xcalibur software 1.1 (Thermo-
Finnigan).
Spectrophotometric and spectrofluorimetric titrations were
performed on 10 ml samples of solutions of the ligand (5 × 10Ϫ5
) in CH3CN, by microadditions of CH3CN stock solutions of
metal triflate. In each experiment the overall addition was
limited to about 200 µl, so that volume increment variation was
not significant.
The fluorescence decays were also monitored at 520 nm
where a 100% contribution of the excimeric emission was
expected. The corresponding lifetimes, which are summarized
in Table 2, are in all cases higher than 20 ns and in good accord-
ance with the value reported for the intramolecular excimer
formed by compound 1 (22–23 ns in methanol).11
Conclusions
The formation of intramolecular excimers resulting from co-
ordination processes has been often exploited in the last years
for sensing purposes: several molecular probes mainly based on
the well known monomer/excimer dual emission properties of
pyrene have been proposed for the sensing of different metal
ions, in particular alkali and alkali-earth cations.19 The study
described in this paper shows that coordinative interactions can
The fluorescence decay curves were recorded on an
Edinburgh Instruments Model FL900 time-correlated single
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4542

View Article Online
photon counting spectrometer, at the Centro Grandi Strumenti
(Università di Pavia). The instrument was operated using a
flash lamp filled with H2 at a pressure of 0.4 atm, with a
frequency of 40 kHz and a pulse-width of 1.0 ns.
The relative quantum yields of fluorescence were obtained
through the optically diluted method,23 using anthracene (Φ =
0.27 in EtOH) as a reference.24
brown–silverish precipitate, which was collected and washed
with water (2.20 g, 32.80%) (Found: C, 71.46; H, 5.95; N, 10.15.
C16H16N2O2 requires C, 71.62; H, 6.01; N, 10.44%). δH (CDCl3):
8.55 (2H, d), 8.17 (2H, d), 7.73 (2H, t), 4.16 (2H, t), 2.74 (2H, t),
1.75 (2H, m), 1.55 (2H, m); m/z 269 (MHϩ, 100%).
Ligand 4. A solution of 2-pyridinecarbaldehyde (0.09 ml,
0.95 mmol) in CH2Cl2 (10 ml) was added dropwise to a stirred
solution of 8 (0.240 g, 0.89 mmol) in CH2Cl2 (50 ml). The
resulting mixture was stirred for 24 h: the solvent was evapor-
ated and the oily product was dried in vacuo. The oil solidified
at room temperature to give a waxy product (0.242 g, 75.7%)
(Found: C, 72.89; H, 5.40; N, 11.60. C22H19N3O2 requires C,
73.93; H, 5.36; N, 11.76%). δH (CDCl3): 8.59 (1H, d), 8.54 (2H,
d), 8.40 (1H, s), 8.34 (2H, d), 7.99 (1H, d), 7.89 (1H, t), 7.81
(2H, t), 7.47 (1H, t), 4.23 (2H, t), 3.77 (2H, t), 1.85 (4H, m); m/z
380 (M ϩ Naϩ, 16%), 358 (MHϩ, 100), 252 (M Ϫ NCHC5H4N,
7).
Complexation constants were determined by using the
HyperQuad software package.25
Syntheses
N-(2-Aminoethyl)-1,8-naphthalimide, 6. Method A. 1,8-naph-
thalic anhydride (1.00 g, 5.05 mmol) was added in small
portions to ethylenediamine (30 ml, 0.4 mol) under magnetic
stirring and the resulting solution was refluxed overnight. The
reaction mixture was concentrated (to about 10 ml) to give a
yellow precipitate, which was filtered off, washed with diethyl
ether and purified by liquid chromatography [SiO2, CH2Cl2–
MeOH–ammonia solution (97 : 2 : 1)] (0.81 g, 40.3%) (Found:
C, 69.53; H, 4.92; N, 11.35. C14H12N2O2 requires C, 69.99; H,
5.03; N, 11.66%). δH (CD3OD): 8.55 (2H, d), 8.29 (2H, d), 7.79
(2H, t), 4.15 (2H, t) 2.85 (2H, t).
Method B. A suspension of 1,8-naphthalic anhydride (5 g,
25 mmol) in water (25 ml) was slowly added to an aqueous
solution of ethylenediamine (11.1 ml, 0.17 mol in 60 ml) which
was stirred and heated at 70 ЊC. The risulting suspension was
heated at 80 ЊC for 10 min, then a light brown precipitate
formed (corresponding to the byproduct containing two naph-
thalimide moieties) which was eliminated by filtration. The
filtered solution was allowed to remain at room temperature
overnight. Pale yellow crystals were obtained (4.65 g, 77.5%)
(Found: C, 69.12; H, 4.95; N, 11.97. C14H12N2O2 requires C,
69.99; H, 5.03; N 11.66%). δH (CDCl3): 8.60 (2H, d), 8.21 (2H,
d), 7.75 (2H, t), 4.28 (2H, t), 3.07 (2H, t); m/z 241 (MHϩ, 100%),
224 (M Ϫ NH2, 11).
N-(5-Aminopentyl)-1,8-naphthalimide, 9. Same experimental
procedure as reported for 8 (2.05 g, 84.64%) (Found: C, 71.96;
H, 5.90; N, 9.60. C17H18N2O2 requires C, 72.32; H, 6.43; N,
9.92%). δH (CDCl3): 8.58 (2H, d), 8.20 (2H, d), 7.74 (2H, t), 4.18
(2H, t), 2.69 (2H, t), 1.75 (2H, m), 1.49 (4H, m); m/z 283 (MHϩ,
100%).
Ligand 5. Same experimental procedure as described for 4,
starting from intermediate 9 (0.273 g, 80.9%) (Found: C, 73.41;
H, 5.69; N, 11.34. C23H21N3O2 requires C, 74.37; H, 5.70; N,
11.31%). δH (CDCl3): 8.62 (1H, d), 8.59 (2H, d), 8.36 (1H, s),
8.20 (2H, d), 7.96 (1H, d), 7.73 (3H, m), 7.29 (1H, t), 4.19 (2H,
t), 3.69 (2H, t), 1.81 (4H, m), 1.52 (2H, m); m/z 382 (MHϩ, 100).
X-Ray structure determinations
Details on the crystallographic study carried out on ligands 2
and 3, and on the complexes formed by ZnII and CuII with
ligand 2, are reported in Table 3. Data on crystal [Cu(2)2]-
(CF3SO3)2ؒ(CH3CN) are not of good quality; the relevant struc-
tural investigation must be considered as preliminary and it will
be revised as soon as better crystals are available. Nevertheless,
it is reported here since the molecular conformation of the
complex was revealed and this allowed to make a comparison
with the analogous ZnII complex.
Ligand 2. A solution of 2-pyridinecarbaldehyde (0.13 ml,
1.37 mmol) in MeOH (10 ml) was added dropwise to a stirred
solution of 6 (0.303 g, 1.26 mmol) in MeOH (30 ml). The result-
ing solution was stirred at room temperature for 3 h. The white
precipitate formed was isolated by in vacuo filtration (0.270 g,
65.11%). (Found: C, 72.60; H, 4.46; N, 12.10. C20H15N3O2
requires C, 72.94; H, 4.59; N, 12.76%). δH (CDCl3): 8.61 (3H,
m), 8.43 (1H, s), 8.23 (2H, d), 8.00 (1H, d), 7.75 (3H, m), 7.30
(1H, t), 4.61 (2H, t), 4.06 (2H, t); m/z 352 (M ϩ Naϩ, 8%), 330
(MHϩ, 100).
Intensity data were obtained for all the crystals on an Enraf-
Nonius CAD4 diffractometer, using graphite-monochromated
Mo-Kα radiation (λ = 0.71073 Å). Unit-cell parameters were
determined by least-squares fitting of 25 centred reflections.
Calculations were performed with the WinGX-97 software.26
Empirical absorption correction27 was applied in all cases
except for 2, for which no suitable reflection for a ψ-scan was
found in the 80 < χ < 90Њ range. All the structures were solved
by direct methods (SIR92)28 and refined on F 2 by full-matrix
least-squares using SHELXL-97.29 In particular, for both lig-
ands, non-H atoms were refined with anisotropic displacement
parameters, and H atoms were located in the difference-Fourier
maps and refined with isotropic displacement parameters. For
[Zn(2)2](CF3SO3)2, non-H atoms were refined with anisotropic
displacement parameters, while H atoms were inserted in calcu-
lated positions with isotropic displacement parameters pro-
portional to those of their neighbouring atoms and not refined.
As stated above, the poor quality of the [Cu(2)2](CF3SO3)2ؒ
CH3CNؒH2O crystal did not allow to achieve a good resolution
for the structural model. In order to reduce the number of
parameters to be refined, only the Cu atom was refined aniso-
tropically. All other non-H atoms were refined isotropically
while the H atoms were inserted in calculated positions and not
refined. Disorder at the triflate anion, which was refined with
alternative positions and with the use of geometrical con-
straints, and the likely presence of one MeCN molecule and one
N-(3-Aminopropyl)-1,8-naphthalimide, 7. Same experimental
procedure as reported for 6, method B (4.41 g, 69.36%) (Found:
C, 70.55; H, 5.32; N, 10.88. C15H14N2O2 requires C, 70.85; H,
5.55; N, 11.02%). δH (CD3OD): 8.55 (2H, d), 8.35 (2H, d), 7.81
(2H, t), 4.23 (2H, t), 2.70 (2H, t), 1.91 (2H, m); m/z 255 (MHϩ,
52%), 238 (M Ϫ NH2, 100).
Ligand 3. Same experimental procedure as described for 2,
starting from intermediate 7 (0.204 g, 49.1%) (Found: C, 73.43;
H, 4.97; N, 12.11. C21H17N3O2 requires C, 73.45; H, 4.99; N,
12.24%). δH (CDCl3): 8.60 (3H, m), 8.40 (1H, s), 8.20 (2H, d),
7.85 (1H, d), 7.75 (2H, t), 7.61 (1H, t), 7.25 (1H, t), 4.37 (2H, t),
3.81 (2H, t), 2.23 (2H, m); m/z 366 (M ϩ Naϩ, 6%), 344 (MHϩ,
100).
N-(4-Aminobutyl)-1,8-naphthalimide, 8. A suspension of
naphthalic anydride (5 g, 25 mmol) in water (25 ml) was added
to an aqueous solution of 1,4-diaminobutane (17 ml, 0.17 mol
in 60 ml) at 70 ЊC over a period of 30 min. Then the resulting
mixture was heated at 80 ЊC for 15 min and the light brown
solid formed was filtered off. The filtered solution was cooled to
room temperature and then at 4 ЊC, giving, after a week, a light
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4543

D
Table 3 Details on the crystallographic studies
Ligand 2
Ligand 3
[ZnII(2)2(CF3SO3)2]
[CuII(2)2](CF3SO3)2ؒCH3CNؒH2O
Formula
Mr
C20H15N3O2
329.35
C21H17N3O2
343.38
C42H30F6N6O10S2Zn
1022.21
C44H30CuF6N6O11S2
1060.43
Crystal colour
Crystal size/mm
Crystal system
Space group
Yellow
Pale yellow
0.14 × 0.34 × 0.63
Triclinic
Colourless
Blue
0.34 × 0.56 × 0.70
Monoclinic
P21/c
0.14 × 0.14 × 0.21
Triclinic
¯
P1
0.10 × 0.14 × 0.14
Tetragonal
P43212
¯
P1
θ range for cell determination/Њ
a/Å
b/Å
c/Å
α/Њ
β/Њ
6.51–10.58
9.756(3)
13.355(6)
12.837(6)
90
109.57(3)
90
1575.9(12)
1.388
0.092
5.43–11.00
8.038(6)
9.247(4)
12.334(3)
81.09(3)
74.06(4)
77.20(5)
855.3(8)
1.333
4.89–9.64
9.357(4)
14.954(5)
16.129(3)
77.93(2)
84.33(3)
79.44(3)
2165.3(12)
1.568
0.755
5.53–9.84
17.810(80)
17.810(80)
14.150(60)
90
90
90
4488(34)
1.569
0.673
γ/Њ
V/Å3
Dc/g cmϪ3
µ/mmϪ1
0.088
Tmin, Tmax
Scan type
θ range/Њ
Reflections measured
–
0.964, 0.988
ω–2θ
2–30
Ϫ11 < h < 11
Ϫ12 < k < 12
0 < l < 17
5170
4956
0.0141
0.0536 (1920)
0.1778
0.881, 0.900
ω–2θ
2–30
Ϫ13 < h < 13
Ϫ21 < k < 21
0 < l < 22
13017
12582
0.1322
0.0882 (3220)
0.3634
0.910, 0.935
ω–2θ
2–25
0 < h < 21
0 < k < 21
0 < l < 16
4393
2296
0.1691
0.1677 (720)
0.4202
ω–2θ
2–30
Ϫ13 < h < 13
0 < k < 18
0 < l < 18
4769
4585
0.0193
0.0469 (2609)
0.1144
Total reflns. measured
Unique reflns.
a
Rint
R1 b (reflns.)
Rall
wR2all
0.1415
1.024
0.1671
1.007
0.2741
0.913
0.5012
1.289
GOFc
Refined parameters
Weighting schemed
(Shift/esd)max
Max., min. ∆ρ/e ÅϪ3
286
303
604
116
1/[σ2Fo2 ϩ (0.055P)2 ϩ 0.242P]
0.000
1/[σ2Fo2 ϩ (0.060P)2 ϩ 0.018P]
0.000
1/[σ2Fo2 ϩ (0.095P)2 ϩ 0.00P]
0.000
1/[σ2Fo2 ϩ (0.200P)2 ϩ 0.00P]
0.005
0.185, Ϫ0.191
0.158, Ϫ0.162
0.700, Ϫ0.905
0.719, Ϫ0.534
a Rint = Σ|Fo2 Ϫ Fo (mean)|/Σ[Fo ]. b R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo| (calculated on reflections with I > 2σ(I )). c GOF = S = [Σ[w(Fo2 Ϫ Fc2)2]/(n Ϫ p)]0.5, where n is the number of reflections and p is the total number of parameters
2
2
refined. d P = [Max(Fo ,0) ϩ 2Fc2)/3]
2

View Article Online
S. Saha and A. Samanta, Tetrahedron, 2000, 56, 7041–7044;
(d ) B. Ramachandram, G. Saroja, N. B. Sankaran and
A. Samanta, J. Phys. Chem. B, 2000, 104, 11824–11832.
water molecule, not precisely located, contribute to the high
final discrepancy factors. Attempts to refine the structure in
lower-symmetry space groups were unsuccessful due to the low
number of observed reflections. The crystal structure was
described in the chiral space group P43212; nevertheless, due to
the low quality of data, Friedel’s opposites were merged and no
attempts to refine the Flack parameter were made.
Atomic scattering factors were taken from ref. 30. Diagrams
of the molecular structures were produced by the ORTEP
program.31
11 T. C. Barros, P. Berci Filho, V. G. Toscano and M. J. Politi,
J. Photochem. Photobiol. A, 1995, 89, 141–146.
12 T. Cao and S. E. Webber, Macromolecules, 1991, 24, 79–86.
13 H. E. Fierz-David and C. Rossi, Helv. Chim. Acta, 1938, 21, 1466–
1489.
14 T. C. Barros, S. Brochsztain, V. G. Toscano, P. Berci Filho and M. J.
Politi, J. Photochem. Photobiol. A, 1997, 111, 97–104.
15 See, for example: (a) L. Fabbrizzi, M. Licchelli, P. Pallavicini,
A. Perotti, A. Taglietti and D. Sacchi, Chem. Eur. J., 1996, 2, 75–82;
(b) A. W. Varnes, R. B. Dodson and E. L. Wehry, J. Am. Chem. Soc.,
1972, 94, 946–950; (c) J. A. Kemlo and T. M. Shepherd, Chem. Phys.
Lett, 1977, 47, 158–162.
CCDC reference numbers 215887–215890.
See http://www.rsc.org/suppdata/dt/b3/b308439f/ for crystal-
lographic data in CIF or other electronic format.
16 K. F. Purcell and J. C. Kotz, Inorganic Chemistry, Holt-Saunders,
Philadelphia, PA, 1977, p. 718.
17 B. Bodenant, F. Fages and M.-H. Delville, J. Am. Chem. Soc., 1998,
120, 7511–7519.
Acknowledgements
The financial support by the Ministero dell’Istruzione, Univer-
sità e Ricerca (MIUR–PRIN “Dispositivi Supramolecolari”) is
gratefully acknowledged. The Authors thank Prof. L. Fabbrizzi
for fruitful discussions.
18 (a) K. A. Zachariasse, A. L. Maçanita and W. Künle, J. Phys. Chem.
B, 1999, 103, 9356–9365; (b) F. M. Winnik, Chem. Rev., 1993, 93,
587–614; (c) F. C. de Schryver, P. Collart, J. Vandendriessche,
R. Goedeweeck, A.-M. Swinnen and M. Van der Auweraer, Acc.
Chem. Res., 1987, 20, 159–166; (d ) W. Rettig, B. Paeplow, H. Herbst,
K. Müllen, J.-P. Desvergne and H. Bouas-Laurent, New J. Chem.,
1999, 23, 453–460; (e) Z. Lin, S. Priyadarshy, A. Bartko and D. H.
Waldeck, J. Photochem. Photobiol., 1997, 110, 131–139.
References
1 (a) A. Pardo, J. M. L. Poyato and E. Martin, J. Photochem., 1987, 36,
323–329; (b) A. Pardo, J. M. L. Poyato, E. Martin, J. J. Camacho,
M. F. Braña and J. M. Castellano, J. Photochem. Photobiol. A,
1987, 41, 69–78; (c) R. W. Middleton, J. Parrick, E. D. Clarke
and P. Wardman, J. Heterocycl. Chem., 1986, 23, 849–855;
(d ) V. Wintgens, P. Valat, J. Kossanyi, L. Biczok, A. Demeter and
T. Berces, J. Chem. Soc., Faraday Trans., 1994, 90, 411–421.
2 (a) A. Pardo, E. Martin, J. M. L. Poyato, J. J. Camacho, J. M.
Guerra, R. Weingand, M. F. Braña and J. M. Castellano,
J. Photochem. Photobiol. A, 1989, 48, 259–263; (b) E. Martin,
R. Weingand and A. Pardo, J. Luminesc., 1996, 68, 157–164.
3 (a) A. Dorlars, C.-W. Schellhammer and J. Schroeder, Angew. Chem.,
Int. Ed. Engl., 1975, 14, 665–679; (b) I. Grabtchev and Tz. Philipova,
Dyes Pigm., 1995, 27, 321–325.
4 (a) I. Saito, Pure Appl. Chem., 1992, 64, 1305–1310; (b) Z.-F. Tao,
X. Quian and D. Wei, Dyes Pigm., 1996, 31, 245–251; (c) W. Yao,
X. Quian and Q. Hu, Tetrahedron Lett., 2000, 41, 7711–7715.
5 (a) M. F. Braña, J. M. Castellano, A. M. Sanz, C. M. Roldan and
C. Roldan, Eur. J. Med Chem., 1981, 16, 207–212; (b) M. F. Braña,
J. M. Castellano, M. Moran, M. J. Perez de Vega, X. D. Qian, C. A.
Romerdahl and G. Keilhauer, Eur. J. Med. Chem., 1995, 30, 325–
329.
6 (a) V. K. Malviya, P. Y. Liu, D. S. Alberts, E. A. Surwit, J. B. Craig
and E. V. Hanningan, Am. J. Clin. Oncol., 1992, 15, 41–44; (b) P. F.
Bousquet, M. F. Braña, D. Conlon, K. M. Fitzgerald, D. Perron,
C. Cocchiaro, R. Miller, M. Moran, J. George, X. D. Qian,
G. Keilhauer and C. A. Romerdahl, Cancer Res., 1995, 55, 1176–
1180.
7 (a) C. T. Chanh, B. J. Archer, R. E. Utecht, d. E. Lewis, M. M. Judy
and J. L. Matthews, Biomed. Chem. Lett., 1993, 3, 555–556; (b) C. T.
Chanh, D. E. Lewis, M. M. Judy, F. Sogandares-Bernal, G. R.
Michalek, R. E. Utecht, H. Skiles, S. C. Chang and J. L. Matthews,
Antiviral Res., 1994, 25, 133–146.
19 See selected example reported in: (a) J.-S. Yang, C.-S. Lin and C.-Y.
Hwang, Org. Lett., 2001, 3, 889–892; (b) Y. Suzuki, T. Morozumi,
H. Nakamura, M. Shimomura, T. Hayashita and R. A. Bartsh,
J. Phys. Chem. B, 1998, 102, 7910–7917; (c) F. Fages, B. Bodenant
and T. Weil, J. Org. Chem., 1996, 61, 3956–3961; (d ) C. Monahan,
J. T. Bien and B. D. Smith, Chem. Commun., 1998, 431–432;
(e) I. Aoki, T. Sakaki and S. Shinkai, J. Chem Soc., Chem. Commun.,
1991, 1771–1773; ( f ) T. Jin, K. Ichikawa and T. Koyama, J. Chem
Soc., Chem. Commun., 1992, 499–501; (g) J. Kawakami, Y. Komai,
T. Sumori, A. Fukushi, K. Shimozaki and S. Ito, J. Photochem.
Photobiol. A, 2001, 139, 71–78; (h) D. Marquis, J.-P. Desvergne and
H. Bouas-Laurent, J. Org. Chem., 1995, 60, 7984–7996.
20 (a) R. A. Bissell, A. P. de Silva, H. Q. N. Gunaratne, P. L. M. Lynch,
G. E. M. Maguire, C. P. McCoy and K. R. A. S. Sandanayake, Top.
Curr. Chem., 1993, 168, 243–264; (b) L. Fabbrizzi and A. Poggi,
Chem. Soc. Rev, 1995, 95, 197–202; (c) L. Fabbrizzi, M. Licchelli,
P. Pallavicini, L. Parodi and A. Taglietti, in Transition Metals in
Supramolecular Chemistry, ed. J.-P. Sauvage, Wiley, Chichester,
1999; (d ) L. Prodi, F. Bolletta, M. Montalti and N. Zaccheroni,
Coord. Chem. Rev., 2000, 205, 59–83; (e) Chemosensors of Ions and
Molecule Recognition, ed. J. P. Desvergne and A. W. Czarnik,
Kluwer, Dordrecht, 1997; ( f ) A. P. de Silva, H. Q. N. Gunaratne,
T. Gunnlaugsson, A. J. M. Huxley, C. P. McCoy, J. T. Rademacher
and T. E. Rice, Chem. Rev., 1997, 97, 1515–1566.
21 M. Licchelli, L. Linati, A. Orbelli Biroli, E. Perani, A. Poggi and
D. Sacchi, Chem. Eur. J., 2002, 8, 5161–5169.
22 (a) T. Fujinaga and I. Sakamoto, Pure Appl. Chem., 1980, 52, 1387–
1396; (b) D. Parker and J. A. G. Williams, J. Chem. Soc., Perkin
Trans. 2, 1995, 1305–1314.
23 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991–1024.
24 S. L. Murov, I. Charmichael and G. L. Hug, Handbook of
Photochemistry, Marcel Dekker, New York, 2nd edn., 1993.
25 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739–1753.
26 L. J. Farrugia, WinGX-97, An integrated system of Publicly
Available Windows Programs for the Solution, Refinement and
Analysis of Single-Crystal X-Ray Diffraction, University of
Glasgow, 1997.
8 R. W. Middleton and J. Parrick, J. Heterocycl. Chem., 1985, 22,
1567–1572.
9 (a) R. A. Bissell, A. P. de Silva, H. Q. N. Gunaratne, P. L. M. Lynch,
G. E. M. Maguire and K. R. A. S. Sandanayake, Chem. Soc. Rev.,
1992, 187–195; (b) L. M. Duffy, A. P. de Silva, H. Q. N. Gunaratne,
C. Huber, P. L. M. Lynch, T. Werner and O. Wolfbeis, Chem. Eur. J.,
1998, 4, 1810–1815; (c) A. P. de Silva, H. Q. N. Gunaratne,
J.-L. Habib-Jiwan, C. P. McCoy, T. E. Rice and J.-P. Soumillion,
Angew. Chem., Int. Ed. Engl., 1995, 34, 1728–1731.
10 (a) F. Cosnard and V. Witgens, Tetrahedron Lett, 1998, 39,
2751–2754; (b) K. A. Mitchell, R. G. Brown, D. Yuan, S.-C. Chang,
R. E. Utecht and D. E. Lewis, J. Photochem. Photobiol. A, 1998, 115,
157–161; (c) B. Ramachandram, N. B. Sankaran, R. Karmakar,
27 A. C. T. North, D. C. Philips and F. S. Mathews, Acta Crystallogr.,
Sect. A, 1968, 24, 351.
28 A. Altomare, G. Cascarano, C. Giacovazzo and A. Gualardi,
J. Appl. Crystallogr., 1993, 26, 343–350.
29 G. M. Sheldrick, SHELXL-97, Programs for Crystal Structure
Analysis, University of Göttingen, Germany, 1998.
30 International Tables for X-ray Crystallography, Kynoch Press,
Birmingham, England, 1974, vol. 4, pp. 99–101 and 149–150.
31 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
D a l t o n T r a n s . , 2 0 0 3 , 4 5 3 7 – 4 5 4 5
4545