Unexpected Emission Properties of a 1,8-Naphthalimide Unit Covalently
Appended to a Zn-Salophen
Martina Piccinno,[a][b] Gemma Aragay,[b] Francesco Yafteh Mihan,[a] Pablo Ballester,*[b][c]
and Antonella Dalla Cort*[a]

Abstract: We report the synthesis, characterization and binding properties of a Zn-salophen
complex, 1a, functionalized with a 1,8-naphthalimide unit. Unexpectedly, the emission
spectrum of 1a shows a remarkable quenching of the band assigned to the naphthalimide unit.
To better understand this phenomenon, a supramolecular model system constituted by a
symmetric Zn-salophen and a pyridyl derivative of 1,8-naphthalimide, 1b•2a, is investigated.
We propose the existence of a photoinduced energy transfer process between the
naphthalimide (donor) and the salophen (acceptor) units in 1b•2a. A similar process must be
operative in the covalent receptor 1a. Nevertheless, the results deriving from steady-state
fluorescence experiments do not rule out the occurrence of a photoinduced electron transfer
process as alternative pathway for the quenching. We also describe the chemosensing
properties of the receptor 1a and the supramolecular system 1b•2a towards acetate. The nonsymmetrically substituted salophen receptor 1a only transduces the binding of the anion to
the Zn-metal centre in significant spectroscopic changes in its absorption spectrum. On the
other hand, we exploit the strong emission quenching experienced by the naphthalimide
component in the supramolecular complex 1b•2a to detect anions (e.g. acetate) by means of a
typical “turn-on” fluorescent indicator-displacement assay.

Introduction
The importance and ubiquity of anions in many chemical and biological processes have
boosted the field of anion coordination chemistry research. 1,2,3 Anions are not only important
in many sustaining life processes (i.e. enzymatic processes) but also represent a serious
environmental problem (e.g. eutrophication).
Many elegant studies have been reported dealing with the development of optical sensors for
anions and the understanding of their binding modes and sensing mechanisms. 1,4,5 Of
particular interest are optical molecular probes based on a recognition site covalently linked to
a chromophore/fluorophore unit. 6,7,8 In these systems, the recognition unit binds selectively
the target species and the molecular recognition event is transduced through the modulation
of the optical properties of the signaling unit (chromophore). For decades,
chromogenic/absorbing organic dyes have been widely used as signaling units. Nowadays,
however, fluorescent dyes are preferred because they provide increased sensitivity to the
sensing methodology. 5,9
In the last years, zinc-salophen complexes have attracted increasing attention owing to the
Lewis acidity of the Zn(II) center that makes them excellent receptors for electron-rich
substrates including neutral molecules and anions10 . Such interesting properties come along

with an easy synthetic access and reasonable chemical stability. The recognition event is
generally monitored by absorption spectroscopy because the quantum yield of the Znsalophen fluorescence is usually very low. 11,12 We envisaged that the decoration of the Znsalophen scaffold with a highly fluorescent unit must produce a chemosensor able to signal the
binding of target molecules to the Zn-center through changes in the emission properties of the
attached chromophore. Our goal was to increase the sensibility of the sensing system by
changing the transduction of the binding event from absorption to fluorescence signaling. We
focused our attention on the 1,8-naphthalimide fluorophore as reporter unit. The 1,8naphthalimide unit is characterized by unique photophysical properties (i.e. high quantum
yield), and straightforward tunability by simple modifications of the naphthalene skeleton
and/or at the imide unit. 13 1,8-naphthalimide based receptors covalently linked to Zn2+
complexes have already been reported in the literature. 14, 15, 16 Recognition of an anion or a
cation by the binding center is able to modulate the emission properties of the naphthalimide
fluorophore, giving rise to a signaling optical response.
Herein, we report the synthesis and characterization of a non-symmetrically substituted Znsalophen having one 1,8-naphthalimide unit tethered by a triazole ring (1a, Figure 1).
Figure 1.
The presence of two tert-butyl groups in one half of the salophen unit of 1a was aimed to
prevent its dimerization through intermolecular Zn-O interactions 17 . On the other hand, the
N-octyl chain introduced in the naphthalimide component served to increase the solubility of
1a in organic solvents. We investigated the absorption and emission properties of 1a, as well
as its binding properties towards acetate anion that was used to test the behavior of the
system in the presence of coordinating anions. Our studies revealed unexpected features in
the emission properties of 1a which stopped us to further evaluate the effect that the
coordination of other anions produced on the system. To rationalize the obtained results, we
designed and studied a supramolecular model system based on the pyridine naphthalimide
derivative 2a and the symmetrically substituted Zn-salophen 1b. In solution, these components
assembled into a 1:1 complex through axial coordination of the pyridine group of 2a with the
Zn metal center of 1b. We describe the spectroscopic and thermodynamic characterization of
the 1b•2a complex. The obtained results with the supramolecular model 1b•2a indicated that
the quenching of the naphthalimide emission was caused by a photoinduced process that
involved the Zn-salophen component. This result also explains the unexpected emission
properties found for the naphthalimide unit in the covalently connected receptor 1a.

Results and Discussion
Synthesis and characterization of Zn-salophen 1a
5-(azidomethyl)-2-hydroxybenzaldehyde 3 was obtained by reacting sodium azide with 5(chloromethyl)-2-hydroxybenzaldehyde (See SI). On the other hand, commercially available 4Bromo-1,8-naphthalic anhydride was first converted to the corresponding imide derivative.
Then, an ethynyl sustituent was introduced by Sonogashira coupling followed by deprotection
of the trimethylsilyl group to finally afford the ethynyl-naphthalimide derivative 4 (See SI).
Cu(I) catalyzed reaction of the azide salycilaldehyde derivative 3 with alkyne 4 afforded the

1,8-naphthalimide triazole 5 in 69% yield after column chromatography purification. The
condensation reaction of monoimine 6 with the functionalized salycilaldehyde 5 produced Znsalophen 1a as an orange solid in 75% yield (Scheme 1). Salophen 1a was characterized by a
full set of high resolution spectra (see SI).
Scheme 1.

The 1H NMR spectrum of salophen 1a in CDCl3 solution displayed broad signals for the
aromatic protons of the salophen core. Most likely, the broadening of the signals is caused by a
chemical exchange process between monomer and dimer forms of 1a that occurred at an
intermediate rate on the chemical shift timescale. The tendency of Zn-salophen derivates to
dimerize at millimolar concentration in weakly or non-coordinating solvents is well-known. 17
Conversely, the 1H NMR spectrum of 1a in DMSO-d6 solution showed sharp and well-defined
proton signals, which were easily assigned using 2D NMR experiments (Figure S5 and S6 SI).
The imine protons H3 and H8 resonated separately at  = 8.99 and 8.94 ppm respectively,
confirming the non-symmetrical nature of the Zn-salophen core. Moreover, the triazole proton
H13 appeared as a sharp singlet at  = 8.89 ppm. NOE cross-peaks were observed between the
triazole proton H13 and aromatic protons H14 and H18 present in the naphthalimide unit. This
is an expected result if we consider the free rotation of the C-C bond connecting the triazole
ring and the 1,8-naphthalimide unit.
A DOSY experiment performed on a DMSO-d6 solution of 1a at 298 K (Figure S7 SI) assigned a
diffusion coefficient of
1.41×10-10 m2/s to the molecule. This diffusion coefficient value corresponds to a
hydrodynamic radii (rexp) of 7.66 Å, which is in good agreement with the dimensions of the
molecule estimated using molecular modeling (Figure 2). 18
Figure 2.

Solution binding studies
Thermodynamic characterization of the complex of 1a with acetate anion.
Zinc-salophens are known to be good receptors for anions. 19,20 Preliminary studies to examine
the behavior of 1a in the presence of an anion were performed with acetate.
The interaction of 1a with acetate was probed using 1H NMR spectroscopy (Figure 3). The
addition of incremental amounts of tetrabutylammonium acetate, TBA(AcO), to a 1mM
solution of 1a in CDCl3 resulted in chemical shift changes of its proton signals and the
sharpening of some of them that initially were broad.
Figure 3.

This observation confirmed that the acetate anion coordinated to the zinc metal center of the
salophen core of 1a and disrupted its dimerization. The addition of more than 1 equivalent of
acetate did not induce any additional change in the NMR spectra. We concluded that a 1:1
complex 1a•AcO- is formed with an association constant higher than 104 M-1. In order to
assess an accurate stability constant value for the 1a•AcO- complex we next performed a UVVis titration experiment.
The UV-Vis spectrum of the zinc-salophen 1a in chloroform solution displayed two broad
absorption bands with maxima centered at 300 nm and 365 nm.
This spectrum closely resembles the sum of the absorption profiles of the two separated
chromophores in 1b and 2b. This result suggested us that the electronic interaction of the two
chromophores in the ground state of 1a is weak (Figure 4).

Figure 4.

A dilution experiment of 1a in a range of concentrations of 10-7 ÷ 10-5 M showed that the
absorption spectra in epsilon scale remained constant (Figure S15 SI). Hence, in complete
agreement with a previous report for a related system we concluded that in this range of
concentrations 1a should exist in solution as a monomeric species.
The addition of increasing amounts of TBA(AcO) salt (0-6 eq) to a 10-5 M chloroform solution of
1a caused the decrease of the absorption bands centered at 300 and 365 nm and the
simultaneous appearance of a new band with a maximum at 405 nm (Figure 5).

Figure 5.

The titration spectra provided a clear isosbestic point centered at 380 nm suggesting the
existence in solution of a single equilibrium involving the two species. Accordingly, the UV-Vis
titration data were analyzed using a theoretical 1:1 binding model that considered two colored
species (free 1a and 1a•AcO- complex). The fit was good and returned an association constant
value of Ka = (1.73 ± 0.10)×105 M-1 for the 1a•AcO- complex. The determined association
constant value is in agreement with those reported for analogous zinc-salophen receptors
binding acetate anion19

Emission spectroscopy studies of salophen 1a.

The decoration of the salophen complex with the 1,8-naphthalimide unit was expected to
improve the spectroscopic emission properties of the system and allow its use at low
concentrations (10-6 – 10-7 M) to detect the presence of anions. To test this hypothesis we
decided to study the complexation of acetate ion by 1a at micromolar concentration using
emission spectroscopy.
Figure 6 shows the emission spectrum in chloroform of 1a (λexc = 360 nm). Two broad
emission bands centered at 420 and 620 nm, respectively, are visible. By comparison with the
emission spectra of 1b and 2b we attributed them to separate emission of the two
chromophores present in the structure of 1a: the 1,8-naphthalimide (420 nm) and Zn-salophen
(620 nm).
However, the 1,8-naphthalimide chromophore in 2b is a significantly better emitter than zincsalophen chromophore of 1b, see Figure 6. In fact, the emission quantum yield (Φem) of the
1,8-naphthalimide chromophore 2b was measured to be 0.8, much higher than the reported
value for the zinc-salophen complex 1b that was described to be Φem = 0.3.22 Notably, the
high emission exhibited by the 1,8-naphthalimide in 2b is strongly quenched when the
chromophore is covalently incorporated in the structure of 1a. This result indicates the
existence of a quenching mechanism for the emission of the naphthalimide unit in 1a, i.e.
photoinduced electron transfer (PET) or energy transfer (ET).
Figure 6.

The emission spectrum of 1a showed quite similar intensities for the bands assigned to the 1,8naphthalimide and zinc-salophen units which did not match with the different quantum yield
observed for the two separated units. The observed unexpected quenching in the emission of
the 1,8-naphthalimide covalently attached to the Zn-salophen in 1a prompted us to further
investigate the system by using a model in which the two chromophores were kept close in
space through a coordination bond, yielding a supramolecular complex.
Synthesis of the components of the supramolecular complex. Thermodynamic
characterization of the complex.
The designed supramolecular system is based on the symmetrically substituted Zn-salophen 1b
and pyridine derivative 2a having a 1,8-naphthalimide unit (Figure 1). The Zn-salophen 1b was
synthesized following a procedure reported in literature23 . Naphthalimides 2a and 2b were
prepared by Cu(I) catalyzed reactions of N-ethyl-6-ethynil-1,8-naphthalimide S1 with 4(azidomethyl)pyridine S2 or benzyl azide S3, respectively (See SI).
Compounds 1b and 2a are expected to self-assemble in solution through axial coordination of
the nitrogen atom of the pyridine-ligand to the Zn metal center. Based on literature
precedents the association constant for the assembly 1b•2a was estimated as Ka ~ 105 M1 24
. Derivative 2b was used as a control compound because clearly it could not form a
coordination complex with 1b.

The 1H NMR spectrum of 1b in CDCl3 showed broad proton signals (similar to 1a) due to the
dimerization process experienced by Zn-salophen at millimolar concentration in non-polar
solvents (vide supra). When increasing amounts of 2a were added to the solution of 1b, the
proton signals of the latter became sharper. No further changes were observed in the chemical
shift values of 1b when more than 1 equivalent of 2a was added. These results indicated the
disruption of the dimeric aggregates of 1b and the formation a 1:1 coordination complex
between 1b and 2a for which we assessed a binding constant higher than 104 M-1 in complete
agreement with the previous estimate (Figure S14 SI). In the early stages of the titration the
proton signals corresponding to the pyridyl group of the naphthalimide 2a appeared upfield
shifted compared to those of free 2a. Most likely, the magnetic anisotropy caused by the
nearby Zn-salophen unit in the coordination complex 1b•2a are responsible for the observed
upfield shift.24 A 2D-NOESY spectrum showed an intense cross peak between the protons α to
the nitrogen atom of the pyridine ring in 2a and those of the tert-butyl group in the salophen
1b (Figure 7) providing support to the formation of the 1b•2a supramolecular complex with
axial coordination geometry.
Figure 7.

The interaction of 1b with 2a was also studied in chloroform using UV-Vis absorption
spectroscopy. The coordination of the pyridine of 2a to the Zn-salophen 1b resulted in a slight
increase and bathochromic shift of the absorption band at 420 nm (∆λ6 nm, Figure S16 SI),
which is the expected behavior for the formation of an axial pyridine:Zn-salophen coordination
complex.23 The fit of the titration data to a 1:1 binding model was good and the calculated
stability constant for the 1b•2a complex was Ka = (4.66± 0.41)×105 M-1.
An analogous titration experiment performed with the reference compound 2b did not
produce noticeable changes in the absorption spectra of the Zn-salophen 1b (Figure S17 SI).
Emission properties of the supramolecular complex 1b•2a
The emission spectra of the symmetrically substituted Zn-salophen 1b (λexc = 360 nm) in
chloroform solution displayed a weak band centered at 550 nm (Figure 6). In contrast, the
separate emission spectra of compounds 2a and 2b showed strong bands with maxima at 420
and 430 nm, respectively. 25 The strong fluorescence shown by naphthalimides 2a and 2b
indicated that fluorescence quenching observed in 1a was not caused by the intervention of
the triazole ring connector.
The interaction of the pyridyl derivative 2a with the symmetric Zn-salophen 1b was probed
also by emission spectroscopy. For practical reasons, we performed a reverse titration, where
Zn-salophen 1b was added to a 10-7 M chloroform solution of naphthalimide 2a. We observed
an intense quenching of the emission band of 2a as the amount of 1b in solution was
increased. Conversely, the incremental addition of Zn-salophen 1b to a solution of the
reference naphthalimide 2b had a negligible quenching effect on its emission. (Figure S18 SI).
Taken together, these results indicated that the quenching of the emission of the 1,8naphthalimide unit by the Zn-salophen required a spatio-temporal proximity of the two

chromophores, naphthalimide and Zn-salophen. Using steady-state fluorescence experiments
is not possible to unequivocally assign the nature of the quenching process that takes place in
the 1b•2a supramolecular complex. Interestingly, we observed that the emission spectrum of
a solution containing 1b and 2a showed a slight increase in the emission band of the Znsalophen (λ = 550 nm) compared to an analogous one containing 1b and 2b (Figure 8). In
addition, the absorption band of the Zn-salophen 1b centered at 420 nm perfectly overlaps the
emission band of the 1,8-naphthalimide 2a (Figure S19 SI). Taken in concert, these
observations suggested that the nature of the quenching process can be related to an energy
transfer process. However, based on our results we cannot exclude the existence of a
photoinduced electron transfer process as responsible of the quenching.25
The strong overlap that existed between the absorption spectra of the two chromophores did
not allow the selective excitation of one of them. This limitation prevents the possibility to
unequivocally demonstrate the energy transfer process.
Figure 8.

In close analogy to the results obtained in the supramolecular 1b•2a complex, we assigned the
unexpected emission properties featured by the naphthalimide unit of the receptor 1a in
chloroform solution to the existence of a photoinduced quenching process (ET or PET) with
the Zn-salophen moiety.
Sensing studies
Although the emission properties of receptor 1a were not the expected ones, we decided to
test its behavior in the presence of an anion using emission spectroscopy. Our expectation was
that the binding of a coordinating anion to receptor 1a could be transduced in changes in the
emission properties of the naphthalimide unit (e.g. fluorescence restore). As shown in Figure
9a, the addition of 10 equivalents of TBA(AcO) to a micromolar solution of 1a caused a
hypsochromic shift of the emission band corresponding to the salophen unit. Unfortunately,
the photoinduced process responsible for the quenching of the naphthalimide fluorescence in
1a (i.e. electron or energy transfer) was not strongly influenced by acetate binding.
Figure 9.

Finally, we studied the behavior of the supramolecular complex 1b•2a as a “turn-on”fluorescent sensor-ensemble for the detection of anions. In this case, the addition of the
acetate anion (0 to 10 eq) to the solution containing the partially assembled 1b•2a complex
caused an increase in the emission of the 1,8-naphthalimide unit (Figure 9b). This observation
is in complete agreement with the displacement of the pyridine-functionalized naphthalimide
2a in the 1b•2a complex by the acetate anion yielding the 1b•AcO- complex. The reported
association constant for Zn-salophen 1b•AcO- complex is one order of magnitude higher than
the one we calculated for the 1b•2a complex.19 Therefore, acetate effectively competes with

the pyridine-functionalized 1,8-naphthalimide bound to the Zn-salophen. The free
naphthalimide 2b that is released to the bulk solution recovers its emission properties.

Conclusions
In conclusion, we have synthesized a Zn-salophen receptor, 1a, covalently decorated with a
1,8-naphthalimide unit. We have shown that the attachment of the Zn-salophen to a 1,8naphthalimide provokes a significant quenching in the emission of the latter. We have also
designed and characterized a supramolecular system based on the symmetric Zn-salophen 1b
and the 1,8-naphthalimide pyridyl derivative 2a. In chloroform the two components form a 1:1
complex in which the pyridyl residue is axially coordinated to the Zn-salophen. By means of
UV-Vis titrations, we determined the association constant of the 1b•2a complex to be Ka =
(4.66± 0.41)×105 M-1. Interestingly, the emission of the naphthalimide 2a is also quenched
upon formation of the 1b•2a complex 19. The results of the investigation of the emission
properties of the supramolecular system 1b•2a suggested that a photoinduced energy transfer
process could be responsible for the quenching of the naphthalimide unit. We ascribed the
existence of a similar photoinduced process in the covalent structure of receptor 1a to explain
the observed quenching of the fluorophore component. Unfortunately, the performed steadystate fluorescent experiments were not suitable to discard the existence of a photoinduced
electron transfer between chromophores as an alternative pathway for emission deactivation.
The Zn-salophen receptor 1a equipped with the naphthalimide unit transduces the binding of
acetate anions to the Zn-center in significant changes of the UV-Vis absorption spectrum.
Conversely, the emission properties of 1a are almost unaffected by the recognition event.
Finally, we used the supramolecular system 1b•2a as a “turn-on”-fluorescent sensor ensemble
in which the presence of the anion was signaled through a typical indicator displacement
assay.

Experimental Section
General Methods and Instrumentations
Reagents and solvents were obtained from commercial suppliers and used without further
purification. Spectrophotometric grade chloroform containing amylene as stabilizer was
purchased from Sigma Aldrich and freshly deacidified with basic aluminum oxide before each
UV-Vis measurement. The 1H and 13C NMR spectra were recorded at 300MHz, 400 MHz or 500
MHz for 1H or at 75 MHz, 100 MHz and 125 MHz for 13C, respectively. The chemical shifts (δ)
for 1H and 13C are given in ppm relative to residual signals of the solvents (CHCl3, δ = 7.26 ppm
for 1H NMR, δ = 77.16 ppm for 13C NMR or DMSO, δ = 2.50 ppm for 1H NMR, δ = 39.52 ppm for
13C NMR). When necessary, 1H and 13C signals were assigned by means of COSY, HSQC, NOESY
and ROESY 2D-NMR sequences. High-resolution mass spectra (HRMS) were obtained on
MicroTOF II from Bruker Daltonics (HPLC-MS-TOF) with positive ionization mode (ESI+). UV-Vis
measurements were carried out on a Shimadzu UV-2401PC spectrophotometer equipped with
a photomultiplier detector, double beam optics, and D2 and W light sources. Fluorescence

measurements were performed in a Spectrofluorimeter Fluorolog Horiba Jobin Yvon. Dilute
solutions (A < 0.05) were prepared in order to minimize inner filter effects.
Fluorescence quantum yield for naphthalimide 2a was determined by using quinine sulphate in
0.1 N (0.05 M) sulphuric acid solution as reference standard (Φ = 0.53).28
The determination of the unknown quantum yields was performed by comparing the
absorption and the integrated fluorescence intensity of solutions whose absorbance is
maintained below 0.05 at the excitation wavelength (350 nm). The unknown quantum yield
was then calculated applying Eq 1.

Acknowledgements
A.D.C, M.P. and F.Y.M. acknowledge “Ricerca scientifica di Ateneo 2013” and MIUR “PRIN
2010CX2TLM”. M.P., G.A. and P. B. thank Gobierno de España MINECO (project CTQ201123014), Severo Ochoa Excellence Accreditation 2014-2018 (SEV-2013-0319), and the ICIQ
Foundation for funding. We also acknowledge the support and sponsorship provided by COST
Action CM1005.
Keywords: zinc-salophen • 1,8-naphthalimide • fluorescence quenching • energy transfer •
indicator-displacement assay

1

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In order to compare the emission intensity of all the molecules under study (1a, 1b and 2a or 2b), we selected an excitation wavelength
at which all the molecules are excited.
1
The relative energy levels of the frontier HOMO and LUMO orbitals of 1b and 2a were calculated at the B3LYP/6-31G theory level using
a chloroform continuum solvent model PCM. The computed energy values were HOMO(1b): -5.32 eV, LUMO(1b): -2.03 eV,
HOMO(2a): -6.39 eV and LUMO(2a): -2.70 eV. These energy values suggested that a photoinduced electron transfer process
occurring from the HOMO of 1b to the SOMO-1 of 2a is thermodynamically viable. However, the results of these calculations were
1

not sufficient to rule out the existence of an energy transfer process as an alternative mechanism of the emission quenching of 2a
in the 1b2a complex.
1
The excitation wavelength was fixed at 310 nm in order to have a minimum direct excitation of the 1,8-naphthalimide units (2a or 2b).
This allowed to observe the emission band of the Zn-salophen 1b, otherwise covered by the strong emission of the 1,8naphthalimides.
1 M. J. Adams, J. G. Highfield, G. F. Kirkbright, Anal. Chem. 1977, 49, 1850-1852.

Figure 1. Line-drawing structures of the molecules prepared and studied in this work.

Scheme 1. Synthesis of non-symmetrical salophen 1a. Reagents and conditions: a) [Cu(CH3CN)4]PF6, TBTA, DMSO, room temperature,
48 h, 69%; b) ZnCl2, Et3N, DCM/MeOH 2:1, room temperature, 18 h, 75%.

Figure 2. Top (a) and side (b) views of the MM3 energy minimized structure of salophen 1a shown in stick representation. Non-polar
hydrogen atoms are omitted for clarity. A sphere centered on 1a and with a radii of 8 Å that corresponds to the value determined from the
DOSY experiment is shown in (b).

Figure 3. Selected region of the 1H-NMR of 1a in CDCl3 (4.49×10-3 M) upon addition of 0 (a), 0.5 (b), 1 (c) and 1.5 (d) equivalent of
TBA(AcO).

Figure 4. Absorption spectra in molar absorption coefficient scale of 1a (solid line), 1b (dotted line) and 2b (dashed line) in chloroform.

.

-5

-

Figure 5. Absorption spectra of 1a (3.69×10 M) upon addition of incremental amounts of AcO (0-6 eq) in chloroform. Inset: fit of the
experimental data at 405 nm and 370 nm to the calculated theoretical binding curve for a 1:1 binding model considering two coloured
species (free 1a and 1a•AcO complex).

Figure 6. Emission spectra of 1a (solid line, 1×10-6 M), 1b (dotted line, 1×10-6 M) and 2b (dashed line, 1×10-7 M) in chloroform λex = 360
nm. Please notice the break in the scale of normalized emission intensity.

Figure 7. Selected area of the 2D NOESY spectrum of the 1:1 complex 1b•2a in CDCl3 solution . The MM3 energy minimized structure
of the 1b•2a complex is shown as an inset.

-7

Figure 8. Comparison of the emission spectra at the final point of the titration of a solution of 2a (red line, 6.62×10 M ) and 2b (green
-7
i
line, 6.84×10 M) with 11 eq of 1b. λex = 310 nm.

Figure 9. (a) Changes observed in the emission spectra of 1a (1.51×10-6 M) in chloroform solution upon addition of acetate (0-10 eq). λex
= 380 nm. (b) Recovery of fluorescence emission upon addition of acetate anion (as tetrabutylammonium salt, 0-10 eq) to a chloroform
solution of 2a (8.29 ×10-6 M) and 1b (1.75 ×10-5 M). λex = 360 nm.

(1)
Where X and ST are the sample and the standard, respectively; Φis the fluorescence quantum yield; grad is the gradient of the
plot of the integrated fluorescence intensity vs absorbance; and n is the refractive index of the solvent (X = CHCl3 n = 1.45, ST =
H2SO4 0.1 N, n = 1.35).