Versatile Schiff-base hydrazone fluorescent receptors: Synthesis, spectroscopy and complexation studies

Inorganica Chimica Acta 380 (2012) 40–49

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Versatile Schiff-base hydrazone fluorescent receptors: Synthesis, spectroscopy and complexation studies Cristina Núñez a,b,⇑, Javier Fernández Lodeiro b, Mario Diniz a, Marco Galesio a a b

REQUIMTE-CQFB, Departamento de Química, FCT-Universidade NOVA de Lisboa, 2829 Monte de Caparica, Portugal Grupo BIOSCOPE, Departamento de Química-Física, Facultade de Ciencias, Universidade de Vigo, Campus de Ourense, 32004 Ourense, Spain

a r t i c l e

i n f o

Article history: Available online 8 September 2011 Young Investigator Award Special Issue Keywords: Fluorescence chemosensor Acyclic ligands Metal recognition Copper(II) Zinc(II)

a b s t r a c t A family of emissive acyclic ligands (1–4) and their Cu2+ and Zn2+ metal complexes has been synthesized and characterised by microanalysis, IR, MALDI mass spectrometry, UV–Vis and fluorescence emission. For comparative purposes, the coordination ability of all chemosensors (1–4) towards the same metal ions was explored in solution (with the help of the UV–Vis and emission spectroscopy) and in gas phase (by MALDI mass spectrometry). The formation of mononuclear complexes with two or three units of ligands was observed during the entire process. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Schiff bases (SB) are currently prepared in high yield using onestep procedures via condensation of common aldehydes with amines [1,2]. Taking into account the highly desirable attributes of this type of ligands, vast families of bidentate, tridentate and tetradentate Schiff base-ligated metal complexes, have been designed and prepared. SB ligands and their complexes have been studied extensively with the aim of shedding light on various aspects of catalytic activity [3–7], magnetic, spectroscopic and anticancer properties [8–13] as well as on the role of metal ions in biological systems [14–16]. The SB have also been used to develop multifunctional fluorescent chemosensors (FC) [17]. FC can be divided into a fluorophore, a spacer and a receptor unit; the receptor being the central processing unit (CPU) of a chemosensor. FC have been developed to be a useful tool to sense in vitro and in vivo biologically important species such as metal ions and anions because of the simplicity and high sensitivity as fluorescent assays [18]. The C@N isomerization is the predominant decay process of the excited states for compounds with an unbridged C@N structure so that those compounds are often nonfluorescent. In contrast to this, the fluorescence of its analogues with a covalently bridged C@N structure increases dramatically due to the suppression of C@N isomerization in the excited states [19]. Therefore the C@N isomer⇑ Corresponding author at: REQUIMTE-CQFB, Departamento de Química, FCTUniversidade NOVA de Lisboa, 2829 Monte de Caparica, Portugal. E-mail address: [email protected] (C. Núñez). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.08.041

ization can be applied to the design of chemosensors for metal ions. The binding of metal ions within the C@N group would stop the isomerization, and a significant fluorescence enhancement could be achieved. In addition, copper(II) and zinc(II) Schiff base complexes have also aroused a wide interest because they possess a diverse spectrum of biological and pharmaceutical activities. Copper(II) is known to form complexes with a variety of molecular geometries (e.g., tetrahedral, square planar, square pyramidal, and octahedral) [20]. Furthermore, copper is a bioelement and an active site in several metalloenzymes and proteins [21–23]. Zinc(II) has attracted a great deal of attention ascribing to the biological significance. Zinc is the second most abundant transition metal ion in the human body after iron [24]. Zinc(II) is believed to be an essential factor in many biological processes such as brain function and pathology, gene transcription, immune function, and mammalian reproduction [25], as well as some pathological processes, such as Alzheimer’s disease, epilepsy, ischaemic stroke, and infantile diarrhoea [26]. Non-destructive techniques based on fluorescence emission spectroscopy, using fluorescent sensors to detect these metal ions, form an important research area which has developed considerably in the last few years [27]. As a part of our ongoing research into the design and synthesis of new fluorescence chemosensors [28], a family of acyclic ligands is being presented (1–4), a family which contains containing naphthalene, b-naphtol, anthracene and pyrene chromophores groups as well as Cu2+ and Zn2+ metal complexes (see Scheme 1).

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C. Núñez et al. / Inorganica Chimica Acta 380 (2012) 40–49 2 2

2

2

Scheme 1. Schematic representation of ligands 1–4.

2. Results and discussion 2.1. Design and synthesis of chemosensors 1–4 The synthesis of compounds 1 was carried out following the method described by Asís et al. [29]. The synthesis of ligand 2 was carried out at room temperature in absolute ethanol by adding the emissive precursor over a cold solution of the hydrated hydrazine precursor. The reaction mixture was stirred overnight under argon atmosphere. The final product characterised as the compound 2 was purified by successive cleaning steeps using hexane and diethyl ether. The yield obtained was of 69% (2). The synthesis of ligands 3 and 4 were carried out following the method described by us [30]. All compounds were characterised by elemental analysis, 1H and 13C NMR, infrared spectroscopy (KBr pellets), MALDI MS spectrometry, UV–Vis and emission spectroscopy. The infrared spectra (KBr disc) of compounds 1–4 show bands at ca. 1646 cm1 corresponding to the imine bond. Peaks attributable to unreacted amine or carbonyl groups were not present. The absorption bands corresponding to m(C@C)ar vibrations of the aromatic groups appear in their expected positions at ca. 1617 and 1595 cm1. The MALDI mass spectra of sensors 1–4 present intense peaks corresponding to the protonaned monomeric forms. Peaks corresponding to the dimeric forms of these compounds (10 –40 ) are also observed, confirming the formation of these species in gas phase. In order to explore the potential uses in the synthesis of new multifunctional emissive materials, many solid metal complexes were synthesized in a 2:1 ligand-to-metal molar ratios. The coordination ability of chemosensors 1–4 with hydrated tetrafluoroborate salts of Cu2+ and Zn2+ is being studied. The reaction of sensors 1–4 with metal salts in a 1:2 metal-to-ligand ratio in ethanol led to compounds of the general formula [M(1– 4)3](BF4)2xH2O and [M(1–4)3](BF4)2xC2H6O (M = Cu2+ and Zn2+) in good yield (46–69%). Chemosensors 1–4 reacted quickly with the aforementioned metal ions to produce air-stable coloured solids. All of them were soluble in DMSO and acetone, partially soluble in absolute ethanol and methanol but insoluble in other common organic solvents. The complexes were characterised by elemental analysis, IR and MALDI-TOF-MS spectrometry. The IR spectra of the complexes were recorded as KBr discs. The band due to the imine bond is shifted to higher frequencies in the spectra of the metal complexes. This has suggested that the Nimine atom is involved with the coordination amongst the metal ion [31]. The bands due to the m(C@C)ar stretching modes of aromatic rings on the complexes appear in the same position when compared with the spectrum of the free ligands. The broad absorption band presented in the region 3450–3380 cm1 is probably due to the existence of lattice and/or coordinated water in the molecule. Therefore, making it difficult to see the bands due to m(N–H) stretching vibrations, which would appear in this region. The IR spectra of the tetrafluoroborate complexes show the presence of several bands in the region associated with the BF4

vibrations. This suggests some interactions with the metal ion or due to the hydrogen bond interactions [32]. MALDI-mass spectra of most compounds registered in acetone display peaks corresponding to [ML]+, [ML(C2H6O)]+, [ML(C2H6O)2]+, [M(L)2]+, [M(L)2(C2H6O)]+, [M(L)3]+ and to [M(L)3(C2H6O)]+ (M = Cu2+ and Zn2+, L = 1–4) fragments, which indicated the integrity of the ligand, and the presence of the metal ions in the complexes. 2.2. Spectrophotometric studies 2.2.1. Effect on protonation of sensors 1–4 The reductive quenching of hydrocarbon fluorophores by aliphatic amines can be used to control the energy transfer process. On this basis, it is possible to switch from energy transfer to electron transfer by changing the pH or by metal ion coordination. In the absence of protons or metal coordination, the electron transfer is the dominant process and no fluorescence emission can be observed. Protonation of the polyamine bridge or metal complexation prevents the electron transfer and allows the energy transfer to take place [33]. Ligands 1–4 have a low solubility in water. In addition in water–absolute ethanol solutions (50:50, v/v) those ligands precipitated. For the aforementioned reasons all the spectroscopic studies have been done in absolute ethanol. Fig. 1A–D shows the absorption and the emission spectra of 1 and 2 in ethanol as a function of increasing amounts of HBF4 at room temperature. The absorption spectra of sensor 1 in absolute ethanol solution shows two bands centered at ca. 230 and 313 nm. The absorption spectra of sensor 2 in the same solvent shows a big number of bands centered at ca. 237, 302, 313, 344 and 358 nm corresponding to the beta-naphtol cromophore. The non-fluorescence observed for compounds 1 and 2 is probably due to the fact that the amino groups are not protonated in both cases, and that the photoinduced electron transfer (PET) takes place from the amine lone pair to the excited state of the cromophore [33]. The absorption spectra of typical chromophore containing naphthalene units, show an absorption maximum at ca. 280 nm, and are pH independent [34]. In the case of sensors 1 and 2, the absorption spectrum is slightly affected by protonation. The inset of Fig. 1A and C shows how, at 313 nm and 358 nm, the absorbance decreases with the number of equivalents of protons added to the solution of sensors 1 and 2, stabilizing after the addition of two and one protons, respectively. There is little broadening (or extended tail) of the naphthalene absorption (above 300 nm). This presumably means little formation of a pre-formed charge-transfer complex (CT complex, also commonly known as an electron donor–acceptor complex) [35], which would result from the interaction between the electron-donating moiety (amine) and the electron-accepting species (naphthalene). Fig. 1B and D show the protonation effect in the fluorescence intensity of sensor 1 and 2. These bands increase in intensity with protonation until the second and the first proton added to sensors 1 and 2, respectively.

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(A) 0.5

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[H+] / [2]

300

320

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380

420

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500

Wavelength / nm

Fig. 1. Spectrophometric (A and C) and spectrofluorimetric (B and D) titrations of sensor 1 and 2 in ethanol as a function of added HBF4. The insets show the absorption at 313 and 358 nm; and the normalised fluorescence intensity at 394 and 402 nm ([1] = 1  105 M, [2] = 1.50  105 M; kexc1 = 313 nm, kexc2 = 358 nm).

The fully protonated forms are the most emissive species due to the fact that quenching by electron transfer, from protonated amines to the excited naphthalene, is not thermodynamically favourable [36]. One important issue of this study is to address the origin of the 394 and 402 nm bands, for sensors 1 and 2, respectively. In theory, this has two possible origins: (1) an excimer formation between the two naphthalene units of adjacent molecules; (2) the formation of a charge transfer (CT) excited-state involving the amines and naphthalene groups. In that case, the assignation to a naphthalene–naphthalene intermolecular excimer is not possible [37]. The emission spectrum displayed by 1 and 2 is related to the presence of a charge transfer (CT) excited state generated by a transition from a nitrogen lone pair nN to a p⁄ naphthalene orbital. Such a nNp⁄ CT excited state lies at a lower energy than the luminescent pp⁄ excited state of naphthalene, whose fluorescence if thereby quenched. The same CT excited state, of course, is also expected for compound 2, which indeed shows an emission spectrum quite similar to that of 1. Fig. 2A–D shows the absorption and emission spectra of 3 and 4 in ethanol as a function of increasing amounts of HBF4 at room temperature. The absorption spectrum of sensor 3 in absolute ethanol solution shows one band above 330 nm with maxima at 331, 350, 369, and 384 nm, and the absorption spectrum of sensor 4 in the same solvent shows a great number of bands centered at ca. 243, 277, 289, 356 and 365 nm. Similar to the case of sensors 1 and 2, chemosensors 3 and 4 are not emissive because of the photoinduced electron transfer mechanism from the amine lone pair to the excited state of the anthracene and pyrene cromophores.

Fig. 2A and C show that the absorption spectra for ligands 3 and 4 are not affected by protonation. Fig. 2B and D show the protonation effect in the fluorescence intensity. In the inset of Fig. 2B and D, the emission bands at 483 and 436 nm, assigned to the anthracene and pyrene emission, respectively, increase in intensity until the addition of two equivalents of acid. This happens due to the protonation of the amine groups and consequently blocking of the PET quenching of the chromophores emission. Two protonation constants were predicted during the interaction of sensors 1–4 in the presence of H+, using HypSpec software and are summarised in Table 1 [38]. 2.2.2. Metal ions titrations Aliphatic amines closely bonded with fluorophore moieties are involved in the electron-transfer quenching, which makes it possible to signal the presence of metal cations in polyamine systems. The stronger the involvement of these nitrogen atoms in the complexation is, the stronger the effect on the luminescence of the compound [39]. Metal ions coordinated to such nitrogen atoms prevent photoinduced electron transfer (PET) quenching from the lone pair of electrons of each nitrogen atom to the fluorophore moieties. It was reported that d10 transition-metal ions such as Zn2+ enhance the fluorescence of this kind of ligands because these ions usually do not introduce low-energy metal centered or chargeseparated excited states into the molecules. Therefore so that the electron-transfer or energy-transfer phenomena cannot usually occur [40]. On the other hand, transition metal ions like Cu2+ possess empty or half-filled orbitals of convenient energy, which can be involved in an energy-transfer mechanism of the Dexter type, quenching excited states [41]. In order to explore the behaviour of sensors 1–4 towards these different types of metal ions (Cu2+ and Zn2+), several titrations followed by absorption and emission were performed. These sensing studies were developed using absolute ethanol as solvent at 298 K.

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600

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Fig. 2. Spectrophometric (A and B) and spectrofluorimetric (B and C) titrations of sensor 3 and 4 in ethanol as a function of added HBF4. The inset show the normalised fluorescence intensity at 483 and 436 nm for sensors 3 and 4, respectively. ([3] = 1.83  105 M, [4c] = 1  105 M, [4d] = 1  106 M, kexc3 = 383 nm, kexc4 = 365 nm).

Table 1 Stability constants for chemosensors 1–4 in the presence of Cu2+ and Zn2+ in EtOH. Ligand

Interaction (M:L) L = 1–4

Rlog b (Emission)

1

Zn2+ (1:3) Cu2+ (1:3) Zn2+ (1:3) Cu2+ (1:3) Zn2+ (1:3) Cu2+ (1:3) Zn2+ (1:3) Cu2+ (1:3)

12.43 ± 1.78  102 14.39 ± 3.02  102 13.17 ± 1.65  102 15.78 ± 1.02  102 – 6.90 ± 4.71  102 – 14.67 ± 2.35  102

2 3 4

Fig. 3A–D show the absorption and emission spectra of sensor 1 and 2 in the presence of increasing amounts of Cu(BF4)2 and Zn(BF4)2, respectively. Fig. 3A shows the absorption spectrum of 1 upon complexation with Cu2+. A decrease in the band centered at 313 nm and assigned to a ligand-to-metal charge transfer (CT), is observed [42]. The same behaviour is observed in the absorption spectrum of sensor 1 after titration with Zn2+. Titration of chemosensor 2 with Zn(BF4)2 in ethanol solution at 298 K (Fig. 3C), can be followed by the formation of a new band centered at ca. 400 nm assigned to a CT process. A decrease in the band centered at 358 nm and assigned to the p–p⁄ transition of the chromophore, is also observed. A well-defined isosbestic point is observed at 368 nm, suggesting the presence of two species in solution, which are the free sensor 2 and the mononuclear metal complex. Similar effect is observed after adding of Cu2+ to the chemosensor 2. Insets in plots 3B and 3D show the normalised fluorescence intensity at 380 nm, after the addition of Cu2+ to sensor 1, and at 402 nm after the addition of Zn2+ to sensor 2. An increase of the fluorescence is observed in both cases. Similar behaviour is observed after adding Zn2+ and Cu2+ to chemosensors 1 and 2, respectively.

These chelation enhanced fluorescence effects (CHEF) can take place when the nitrogen lone pairs are bound to the metal ion. This prevents the PET processes from the amino group to the adjacent naphthalene moiety and favours the fluorescence emission [43]. Strong changes were observed in the absorption and the emission spectra of ethanolic solutions at 298 K of 3 and 4, when they were titrated with Cu(BF4)2 or Zn(BF4)2. Parts A and C of Fig. 4 show the absorption titrations of 3 and 4 with Cu2+, respectively. Adding increasing amounts of anhydrous copper tetrafluoroborate to an ethanolic solution of 3 (1.00  105), at 298 K, leads to a decrease in the absorption band centered at 383 nm. Titration of chemosensor 4 with Cu(BF4)2 in ethanol solution (Fig. 4C), can be followed by the formation of a new band centered at ca. 397 nm assigned to the CT process. A decrease in the band centered at 366 nm assigned to the p–p⁄ transition of the chromophore is also observed. A well-defined isosbestic point is observed at 384 nm, suggesting the presence of two species in solution, which are the free sensor 4 and the metal complex. No changes are observed in the absorption spectrum of sensor 3 and 4 after titration with Zn2+. Parts C and D of Fig. 4 show the fluorescence titrations of 3 and 4 with Cu(BF4)2, respectively. The addition of Cu2+ increases the fluorescence of ethanol solutions of 3 and 4 [44]. Similar behaviour is observed after titrations of 3 and 4, with Zn(BF4)2, but the addition of a larger number of equivalents of Zn2+ is necessary to achieve the equilibrium. This could be due to the fact that the protonated forms of these ligands compete effectively with the metal ions and prevent the formation of the complexes. The different behaviour of Cu2+ and Zn2+ metal ions is attributed to the different stabilities of their complexes, since the Zn2+ complexes of these ligands are probably less stable than those of Cu2+. This explains that in case of complexation of sensors 3 and 4 with Zn2+ no values of the stability constants are listed in Table 2.

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Fig. 3. Spectrophometric (A and C) and spectrofluorimetric (B and D) titrations of sensors 1 and 2 in ethanol as a function of added Cu(BF4)2 (sensor 1) and Zn(BF4)2 (sensor 2). In Fig. 3A and C, the insets show the absorption at 313 and at 358 and 400 nm, respectively. The insets shows the normalised fluorescence intensity at 380 nm (B) and 402 nm (D) ([1 = 2] = 1  105 M; kexc1 = 313 nm, kexc2 = 358 nm).

(A) 0.15

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Fig. 4. Spectrophometric (A and C) and spectrofluorimetric (B and D) titrations of sensors 3 and 4 in ethanol as a function of added Cu(BF4)2. In plots 4A and 4C, the insets show the absorption at 383 (sensor 3) and at 366 and 393 nm (sensor 4), respectively. The insets show the normalised fluorescence intensity at 416 nm (B) and 439 nm (D) ([3] = 1.83  105 M, [4] = 1  105 M; kexc3 = 383 nm, kexc4 = 366 nm).

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45

Metal complex

kmax (nm); log e

kem (nm)

Dk (nm)

/

DI-TOF-MS appears at 309.13 m/z. This peak with 100% of intensity can be unambiguously attributed to the protonated dimeric form (10 ) formed in gas phase. Another peak corresponding to the protonated monomeric form (1) with 172.06 m/z is detected (10% of intensity). After adding 0.5 equiv of Zn2+, peaks at 256.34 m/z (20% of intensity) and at 371.10 m/z (65% of intensity) are observed, corresponding to [Zn1]+ and [Zn10 ]+, respectively. But the more intense peak (100% of intensity) is assigned to the protonated dimeric 10 . After adding 1 equivalent of Zn2+, an intense peak at 371.09 m/z (100% of intensity) corresponding to [Zn10 ]+ is observed. In this case, the peak assigned to the protonated dimeric specie presents low intensity (40%). Similar results were observed after MALDITOF-MS titration of sensor 1 with Cu2+. Fig. 6 summarises the MALDI-TOF-MS titration of sensor 3 with Zn(BF4)2. Before adding the metal salt, the sensor peak in the MALDI-TOF-MS always appears at 221.09 m/z. This peak with 100% of intensity can be without a doubt attributed to the protonated form of sensor 3. Another peak corresponding to the protonated dimeric form (30 ) formed in gas phase with 409.18 m/z is detected (35% of intensity). In the presence of 0.5 equiv of Zn2+, a new peak with low intensity appears at 503 m/z (25% of intensity) corresponding to the mononuclear specie [Zn(3)2] (see Fig. 6). By adding 1 equivalent of the metal salt, a peak at 471.12 m/z with 100% of intensity, attributed to the dimeric complex [Zn30 ] shows higher intensity. No peaks attributed to the other species were observed. Similar results were observed after MALDI-TOF-MS titration of sensor 3 with Cu2+. This result suggests that the formation and the complexation of the dimmer forms (10 and 30 ) in gas phase are more favourable. This result is different from the behaviour observed in solution and in solid state. In all cases, adding increased amounts of metal ion did not produce any peaks attributable to the formation of dinuclear complexes. Unfortunately, unfavourable results were observed in the MALDI-TOF-MS metal ion titrations of chemosensors 2 and 4.

[Cu(1)3](BF4)22C2H6O (5) [Zn(1)3](BF4)25H2O (6) [Cu(2)3](BF4)2C2H6O (7)

358; 360; 291; 318; 364; 381; 400; 317; 346; 364; 389; 409; 429; 333; 352; 371; 391; 413; 334; 352; 372; 392;

4.42 4.31 4.46 4.25 4.06 4.05 3.99 4.46 4.39 4.42 4.46 4.60 4.50 3.70 3.93 4.11 4.18 4.15 4.08 4.34 4.56 4.68

438 438 441

80 78 41

<0.001 <0.001 <0.001

3. Conclusions

421; 291; 337; 357; 394; 430; 291; 337; 357; 394; 430;

4.76 4.95 4.54 4.76 4.98 5.09 4.69 4.28 4.52 4.73 4.85

The stability constants of the mononuclear species formed by the interaction of sensors 1–4 in the presence of Cu2+ and Zn2+ are calculated using HypSpec software and are summarised in Table 2 [38]. All data fit to a stoichiometry 3:1 ligand-to-metal. Taking into account the values summarised in Table 2, the strongest interaction expected for sensors 1, 2 and 4 is with Cu2+. In general, the interaction with Zn2+ is low, and the stability constant values of the interaction of the sensors 3 and 4 with this metal ion can not be calculated. The sensing behaviour of chemosensors 1–4 towards Cu2+ and Zn2+ has been studied in solution, using absolute ethanol as the solvent. All the synthesized metal complexes are partially soluble in ethanol and all of them have been optically characterised in DMSO. The data are summarised in Table 3. The relative fluorescence quantum yields for all metal complexes were determined using a 0.1 M solution of quinine sulfate in 0.5 M H2SO4 as standard (/ = 0.546); the values obtained were always below 1  103 [45]. 2.3. Spectrometric studies by MALDI-TOF-MS spectrometry In order to explore the application of chemosensor 1–4 as a molecular probe for metal ions in gas phase, several MALDI-TOFMS titrations were performed. Compounds 1–4 dissolved in ethanol without any additional MALDI matrix were titrated with Cu2+ and Zn2+ ions in 2:1 and 1:1 ligand-to-metal molar ratios. To perform the metal titrations, a strategy called ‘‘a dried droplet solution’’ was explored: two solutions containing the ligand (1– 4) (1 lL) and the metal salt (1 lL) were mixed and shaked and then applied to the MALDI-TOF-MS sample holder. Fig. 5 summarises the MALDI-TOF-MS titration of sensor 1 with Zn(BF4)2. Before adding the metal salt, the sensor peak in the MAL-

Table 2 Optical data for the metal complexes of compounds 1–4 in DMSO at 298 K.

[Zn(2)3](BF4)2 (8)

[Cu(3)3](BF4)22C2H6O (9)

[Zn(3)3](BF4)2.H2O (10)

[Cu(4)3](BF4)2C2H6O (11)

[Zn(4)3](BF4)23H2O (12)

519

90

<0.001

448

35

<0.001

443

22

<0.001

477

47

<0.001

468

38

<0.001

In summary, four naphthalene, b-naphtol, anthracene and pyrene-based chemosensors (1–4) have been developed to investigate their protonation behaviour and sensing capability towards divalent Cu2+ and Zn2+ metal ions. These systems present a range of different and interesting properties in solid state, in solution and in gas phase. Several metal ion titrations of sensors 1–4 with Cu2+ and Zn2+ followed by absorption and emission are performed. In all cases, the stability constant values suggest a stoichiometry 3:1 ligandto-metal. In order to compare the behaviour of sensors 1–4 in solid state, solid metal complexes with divalent Cu2+ and Zn2+ metal ions have been synthesized and characterised. The compounds have been isolated as mononuclear complexes, responding to the same stoichiometry 3:1 ligand-to-metal observed in solution. To explore the application of chemosensor 1–4 as molecular probe for metal ions in gas phase, several MALDI-TOF-MS titrations were performed. In that case, mononuclear complexes with the dimeric forms of chemosensors are observed. In the photophysical characterisation of chemossensors 1–4, all fluorescence emission bands are observed because the photoinduced electron transfer (PET) takes place from the amine lone pair to the excited state of the cromophore. Protonation or complexation of the aliphatic nitrogen atom prevents the PET mechanism and allows an increase in the fluorescence.

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Fig. 5. MALDI-TOF-MS spectra of chemosensor 1 in the presence of 0, 0.5 and 1 equivalent of Zn2+.

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Fig. 6. MALDI-TOF-MS spectra of chemosensor 2 in the presence of 0, 0.5 and 1 equivalent of Zn2+.

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3.1. Experimental 3.1.1. Physical measurements Elemental analyses were performed in a Fisons EA-1108 analyser at the CACTI, University of Vigo Elemental analyses Service. Infra-red spectra were recorded as KBr discs on a Bio-Rad FTS 175-C spectrophotometer. 1H, 13C, COSY, DEPT and HMQC NMR spectra were recorded on a Bruker AMX-500 spectrometer and DMSO was used as the solvent in all cases. MALDI-TOF-MS analysis were performed in a MALDI-TOF-MS model voyager DE-PRO biospectrometry workstation equipped with a nitrogen laser radiating at 337 nm from Applied Biosystems (Foster City, United States) at the REQUIMTE DQ, Universidade Nova de Lisboa. The acceleration voltage was of 2.0  104 kV with a delayed extraction (DE) time of 200 ns. The spectra represent accumulations of 5  100 laser shots. The reflectron mode was used. The ion source and flight tube pressures were less than 1.80  107 and 5.60  108 Torr, respectively. The MALDI mass spectra of the soluble samples (1 or 2 mg/mL), such as the ligand and metal complexes were recorded using the conventional sample preparation method for MALDI-MS. 3.1.2. Spectrophotometric and spectrofluorimetric measurements UV–Vis absorption spectra (220–900 nm) were performed using a JASCO-650 UV–Vis spectrophotometer and fluorescence spectra on a HORIBA JOVIN-IBON Spectramax 4 at the BIOSCOPE Group in the University of Vigo. The linearity of the fluorescence emission vs. the concentration was checked out within the concentration range used (105 to 106 M). A correction of the absorbed light was performed when necessary. All spectrofluorimetric titrations were performed as follows: a stock solution of the ligand (ca. 1.00  103 M) was prepared by dissolving an appropriate amount of the ligand in a 50 mL volumetric flask and diluting it to the mark with absolute ethanol. The titration solutions ([1–4] = 1.00  106 and 1.00  105 M) were prepared by appropriate dilution of the stock solution. Titrations were carried out by adding microliter amounts of standard solutions of the ions dissolved in ethanol. All the measurements were performed at 298 K. 3.1.3. Chemicals and starting materials Hydrazine hydrate, 1-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, 9-anthraldehyde, pyrene-1-carboxaldehyde and hydrated tetrafluoroborate salts were commercial products (from Alfa and Aldrich). The used solvents were of reagent grade and purified by the usual methods. The synthesis of compound 1 was carried out following the method described by Asís et al. [29]. The synthesis of ligands 3 and 4 was carried out following the method described previously [30]. 3.1.4. Experimental synthesis of chemosensor 2 3.1.4.1. Synthesis of compound 2. At 0 °C, 8 mL of hydrazine hydrate (Sigma-Aldrich) were stirred for about 15 minutes. During this period, an ethanolic solution of 2-hydroxy-1-naphthaldehyde (0.258 mg; 1.5 mmol) was prepared in 15 mL of ethanol and afterwards added to the previous. The mixture was stirred overnight under argon atmosphere. The resulting greenish brown solution was concentrated in vacuum to 1/3 of its volume. Hexane was added to form a brown precipitate that was filtered off and dried over vacuum. 3.1.4.2. Compound 2. Anal. Calc. for C11H10N2O (MW:186.08): C, 70.9; H, 5.4; N, 15.0. Found: C, 70.7; H, 5.0; N, 14.7%. Yield: 69%. Melting point: 150–152 °C. IR (KBr, cm1): 3290 [m(NH)], 1646 [m(C@N)imine], 1617, 1595 [m(C@C)ar]. MALDI-MS (m/z): 187.07 [2+H]+. Colour: brown. 1H NMR (400 MHz, DMSO): 8.81 (s, 1H); 8.04–7.12 (m, 6H); 5.51 (s, 1H); 1.65 (s, 2H). 13C NMR (400 MHz, DMSO): 159.3; 145.1; 127.1–123.5; 116.1; 109.3; 105.4.

3.1.5. Synthesis of the metal complexes of chemosensors 1–4 The syntheses of metal complexes were carried out in a 2:1 ligand-to-metal molar ratio. The appropriate metal salt (0.02 mmol) in absolute ethanol (5 mL) was added dropwise to a stirred solution of the ligands 1–4 (0.04 mmol) in the same solvent (25 mL). The resulting solutions were heated and stirred for 2 h, and the solvent was partially removed to ca. 5 mL. The precipitates formed were filtered off and dried over vacuum, yielding the metal complexes of chemosensors 1–4. 3.1.6. [Cu(1)3](BF4)22C2H6O (5) Anal. Calc. for C34H42B2CuF8N6O2 (MW: 839.27): C, 52.9; H, 5.0; N, 10.0. Found: C, 52.8; H, 5.5; N, 10.4%. Yield: 48%. IR (KBr, cm1): 1652 [m(C@N)imin], 1619, 1572 [m(C@C)ar], 1134, 1108, 1086, 1035, 804 [m(BF4-)]. MALDI-MS (m/z): 281.16 [Cu1(C2H6O)]+. Colour: brown. UV–Vis (DMSO) k = 358 nm (e = 26673 M1 cm1). 3.1.7. [Zn(1)3](BF4)25H2O (6) Anal. Calc. for C33H40B2F8N6O5Zn (MW: 838.24): C, 47.2; H, 4.8; N, 10.0. Found: C, 47.4; H, 4.4; N, 10.1%. Yield: 46%. BB IR (KBr, cm1): 1646 [m(C@N)imin], 1608 [m(C@C)ar], 1126, 1089, 1037, 803 [m(BF4)]. MALDI-MS (m/z): 283.19 [Zn1(C2H6O)]+. Colour: yellow. UV–Vis (DMSO) k = 360 nm (e = 20545 M1 cm1). 3.1.8. [Cu(2)3](BF4)2C2H6O (7) Anal. Calc. for C35H36B2CuF8N6O4 (MW: 841.22): C, 49.9; H, 4.3; N, 9.9. Found: C, 50.2; H, 4.2; N, 9.3%. Yield: 52%. IR (KBr, cm1): 1667 [m(C@N)imin], 1622, 1608 [m(C@C)ar], 1134, 1108, 1086, 1035, 804 [m(BF4-)]. MALDI-MS (m/z): 249.25 [Cu2]+, 295.23 [Cu2(C2H6O)]+, 435.83 [Cu(2)2]+, 664.86 [Cu(2)3(C2H6O)]+. Colour: green. UV–Vis (DMSO) k = 291 nm (e = 29005 M1 cm1); k = 318 nm (e = 17923 M1 cm1); k = 364 nm (e = 11550 M1 cm1); k = 381 nm (e = 11333 M1 cm1); k = 400 nm (e = 9967 M1 cm1). 3.1.9. [Zn(2)3](BF4)2 (8) Anal. Calc. for C33H30B2F8N6O3Zn (MW: 796.17): C, 49.7; H, 3.8; N, 10.5. Found: C, 49.6; H, 4.1; N, 10.9%. Yield: 63%. IR (KBr, cm1): 1646 [m(C@N)imin], 1607 [m(C@C)ar], 1130, 1110, 1089, 1038, 815 [m(BF4)]. MALDI-MS (m/z): 252.15 [Zn2]+, 296.37 [Zn2(C2H6O)]+, 437.23 [Zn(2)2]+, 484.00 [Zn(2)2(C2H6O)]+, 624.53 [Zn(2)3]+. Colour: yellow. UV–Vis (DMSO) k = 317 nm (e = 28851 M1 cm1); k = 346 nm (e = 24730 M1 cm1); k = 364 nm (e = 26603 M1 cm1); k = 389 nm (e = 29229 M1 cm1); k = 409 nm (e = 39874 M1 cm1) ; k = 429 nm (e = 32074 M1 cm1). 3.1.10. [Cu(3)3](BF4)2.2C2H6O (9) Anal. Calc. for C49H48B2CuF8N6O2 (MW: 989.32): C, 59.4; H, 4.9; N, 8.5. Found: C, 59.9; H, 4.3; N, 8.5%. Yield: 48%. IR (KBr, cm1): 1667 [m(C@N)imin], 1622, 1608 [m(C@C)ar], 1146, 1084, 1039, 817 [m(BF4-)]. MALDI-MS (m/z): 282.74 [Cu3]+, 549.03 [Cu(3)2(C2H6O)]+, 722.45 [Cu(3)3]+, 770.13 [Cu(3)3(C2H6O)]+. Colour: brown. UV–Vis (DMSO) k = 333 nm (e = 5046 M1 cm1); k = 352 nm (e = 8636 M1 cm1); k = 371 nm (e = 13020 M1 cm1); k = 391 nm (e = 15181 M1 cm1); k = 413 nm (e = 14250 M1 cm1). 3.1.11. [Zn(3)3](BF4)2H2O (10) Anal. Calc. for C45H38B2F8N6OZn (MW: 916.25): C, 58.9; H, 4.2; N, 9.2. Found: C, 58.9; H, 5.0; N, 9.5%. Yield: 64%. IR (KBr, cm1): 1646 [m(C@N)imin], 1622, 1607 [m(C@C)ar], 1127, 1109, 1084, 10 42, 807 [m(BF4-)]. MALDI-MS (m/z): 331.14 [Zn(3)(C2H6O)]+, 507.66 [Zn(3)2]+, 722.84 [Zn(3)3]+. Colour: orange.

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UV–Vis (DMSO) k = 334 nm (e = 12264 M1 cm1); k = 352 nm (e = 22189 M1 cm1); k = 372 nm (e = 36653 M1 cm1); k = 392 nm (e = 47879 M1 cm1);k = 421 nm (e = 58044 M1 cm1z). 3.1.12. [Cu(4)3](BF4)2C2H6O (11) Anal. Calc. for C53H42B2CuF8N6O (MW: 1015.28): C, 62.6; H, 4.2; N, 8.3. Found: C, 62.8; H, 4.2; N, 8.4%. Yield: 69%. IR (KBr, cm1): 1648 [m(C@N)imin], 1603 [m(C@C)ar], 1141, 1113, 1096, 1030, 802 [m(BF4)]. MALDI-MS (m/z): 350.92 [Cu(4)(C2H6O)]+, 400.47 [Cu(4) (C2H6O)2]+, 555.99 [Cu(4)2]+, 602.70 [Cu(4)2(C2H6O)]+, 796.53 [Cu (4)3]+, 846.80 [Cu(4)3(C2H6O)]+. Colour: yellow. UV–Vis (DMSO) k = 291 nm (e = 90838 M1 cm1); k = 337 nm (e = 35394 M1 cm1); k = 357 nm (e = 58826 M1 cm1); k = 394 nm (e = 95662 M1 cm1); k = 430 nm (e = 124532 M1 cm1). 3.1.13. [Zn(4)3](BF4)23H2O (12) Anal. Calc. for C51H42B2F8N6O3Zn (MW: 1024.27): C, 59.7; H, 4.1; N, 8.2. Found: C, 59.4; H, 3.9; N, 8.4%. Yield: 61%. IR (KBr, cm1): 1647 [m(C@N)imin], 1598 [m(C@C)ar], 1185, 1112, 1087, 1049, 802 [m(BF4)]. MALDI-MS (m/z): 798.25 [Zn(4)3]+. Colour: yellow. UV–Vis (DMSO) k = 291 nm (e = 49155 M1 cm1); k = 337 nm (e = 19403 M1 cm1); k = 357 nm (e = 33219 M1 cm1); k = 394 nm (e = 54626 M1 cm1); k = 430 nm (e = 71979 M1 cm1). Acknowledgements We are grateful to Xunta de Galiza (Spain) for the project 09CSA043383PR and 10CSA383009PR (Biomedicine) for financial support. The authors thank the Scientific Association Proteomass for financial support. C.N. and M.G. thank the Fundação para a Ciência e a Tecnologia/FEDER (Portugal/EU) programme postdoctoral contracts (SFRH/BPD/65367/2009 and SFRH/BPD/73939/ 2010, respectively). J.F.L. thanks Xunta de Galiza (Spain) for a research contract achieved by project 09CSA043383PR in Biomedicine. We are grateful to Dr. Carlos Lodeiro and Dr. Jose Luis Capelo from the BIOSCOPE, Physical Chemistry Department, University of Vigo, Spain for their help with the photophysical and MALDI-MS studies and the scientific discussions. References [1] P. Guerriero, S. Tamburini, P.A. Vigato, Coord. Chem. Rev. 139 (1995) 17. [2] P.A. Vigato, S. Tamburini, Coord. Chem. Rev. 248 (2004) 1717. [3] R. Drozdzak, B. Allaert, N. Ledoux, I. Dragutan, V. Dragutan, F. Verpoort, Coord. Chem. Rev. 249 (2005) 3055. [4] E. Tsuchida, K. Oyaizu, Coord. Chem. Rev. 237 (2003) 213. [5] S. Foster, A. Rieker, K. Maruyama, K. Murata, A. Nishinaga, J. Org. Chem. 61 (1996) 3320. [6] J.S. Fossey, C.J. Richards, Tetrahedron Lett. 44 (2003) 8773. [7] H. Kwong, L. Cheng, W. Lee, J. Mol. Catal. A Chem. 150 (1999) 23. [8] L. El-Sayed, H.A.M. Al-Gwidi, Energy Fuels 14 (2000) 179. [9] P.S. Subramanian, E. Suresh, D. Srinivas, Inorg. Chem. 39 (2000) 2053. [10] Z. Yang, R. Yang, F. Li, K. Yu, Polyhedron 19 (2000) 2599. [11] M.R.A. Pillai, G. Samuel, S. Banerjee, B. Mathew, H.D. Sarma, S. Jurisson, Nucl. Med. Biol. 26 (1999) 69. [12] M.R.A. Pillai, K. Kothari, B. Mathew, N.K. Pilkwal, S. Jurisson, Nucl. Med. Biol. 26 (1999) 233. [13] M.R.A. Pillai, K. Kothari, Sh. Banerjee, G. Samuel, M. Suresh, H.D. Sarma, S. Jurisson, Nucl. Med. Biol. 26 (1999) 555. [14] M.L. Golden, C.M. Whaley, M.V. Rampersad, J.H. Reibenspies, R.D. Hancock, M.Y. Darensbourg, Inorg. Chem. 44 (2005) 875. [15] M. Cushmam, D. Yang, J.T. Mihalic, J. Chen, S. Gerhardt, R. Huber, M. Fischer, K. Kis, A. Bacher, J. Org. Chem. 67 (2002) 6871. [16] P.A.N. Ready, M. Nethaji, A.R. Chakravarty, Inorg. Chem. 41 (2002) 450.

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