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978-1-4244-5747-2/10/$26.00 ©2009 IEEE 1

Engineering of Cyclometallated Platinum(II) Complexes Incorporating Acetylide Ligands for Chemosensing

Jean-Luc Fillaut*a, Pierre-Henri Lanoë a, Véronique Guerchais a, Hubert Le Bozec a J.A. Gareth Williams b

a Sciences Chimiques de Rennes UMR 6226 CNRS-Université de Rennes 1 Avenue du Général Leclerc, 35042 Rennes Cedex France

E-mail: jean-luc.fillaut@univ-rennes1.fr b Department of Chemistry, University of Durham, South Road, Durham, DH1 3LE, Unit Kingdom

ABSTRACT A series of new cyclometalated phenylbipyridyl platinum(II) acetylide complexes was investigated for their photophysical and heavy metal binding properties. Both the absorption and emission properties of platinum(II) complexes with functionalized acetylide ligands as specific receptors for heavy metal ions can be strongly affected upon binding to specific metal ions so that they can be used as models of highly sensitive chemosensors. Keywords: chemical sensor, phosphorescence, heavy metals, platinum alkynyl.

1. INTRODUCTION Heavy metal ions play a vital role in various domains such as analytical chemistry and biological and environment sciences. Zn2+, Fe3+, and Cu2+ are the three most abundant essential heavy metal ions in the human body and participate in diverse biological processes. Pb2+ Ni2+ and Cd2+ are also considered as significant pollutants, which can cause serious environmental and health problems. Therefore, the design and development of potential chemosensors for the detection of heavy metal ions are very important.

Aiming at exploring the responses toward various heavy metal ions, we designed novel platinum(II) complexes (Scheme 1), where the cyclometalated [Pt(tBu2-C^N^N)(C≡C-R)] (tBu2-C^N^N = 4,4’-di(tert-butyl)-6-phenyl-2,2’-bipyridine) chromophore served as a signal emitter. Indeed complexes of the type [{C^N^N }Pt(C≡CR)] show intense phosphorescence from green/yellow to red, with µs lifetimes, depending on the substituents on the N,N,C-donor ligand and the acetylide [1]. Absorption and luminescence sensing behavior of these platinum(II) complexes can be modulated by variations in the binding sites of the acetylide ligands [2]. We now show that upon binding to specific metal ions both the absorption and emission properties of platinum(II) complexes with functionalized acetylide ligands as specific receptors for heavy metal ions can be strongly affected so that they can be used as models of highly sensitive chemosensors.

1 2 3 Scheme 1. Structures of the studied cyclometalated [Pt(tBu2-C^N^N)(C≡C-R)] complexes

(tBu2-C^N^N = 4,4’-di(tert-butyl)-6-phenyl-2,2’-bipyridine)

2. ELECTRONIC ABSORPTION AND EMISSION SPECTROSCOPY Cyclometalated phenylbipyridyl platinum(II) acetylide complexes [Pt(tBu2-C^N^N)(C≡C-R)] (tBu2-C^N^N = 4,4’-di(tert-butyl)-6-phenyl-2,2’-bipyridine) were chosen in order to form neutral Pt(II) species that are advantageous over cationic complexes in terms of affinity for Mn+ ions [1, 2]. The presence of the tert-butyl groups sterically inhibits undesirable intermolecular interactions that frequently lead to concentration-dependent self-quenching of the luminescence [1]. The free alkynes were first constructed and subsequently reacted with the [Pt(tBu2-C^N^N)Cl] precursor [1].

The electronic absorption spectra of platinum(II) phenylbipyridyl complexes 1-3, in acetonitrile solution exhibit intense absorption bands at 320 – 380 nm and less intense bands at 390 – 550 nm. With reference to previous spectroscopic work on platinum(II) phenylbipyridyl complexes [1a], the high-energy intense absorption bands are assigned to intraligand (IL) transitions of the phenyl-bipyridine and alkynyl ligands (IL [C^N^N] + IL’ (-C≡C-Ar), π → π*. The bands at 390 – 550 nm are accordingly assigned as a [d π (Pt)→ π*(C^N^N)] metal-to

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ligand charge transfer (MLCT) transitions with mixing of some [π(C≡C)→π*(C^N^N)] ligand-to-ligand charge-transfer (LLCT) transition. Worthy to note, the low-energy absorption, arising from mixed LLCT + MLCT transitions from both the metallic fragment Pt and the acetylide -C≡C-C6H4-R segments to the LUMO (π*) localized on the phenyl-bipyridyl (C^N^N) is highly sensitive to changes in the acetylide ligands. In particular, the strong electron donor effect of the functional group (DPA for instance) results in a red shift of the LLCT + MLCT based absorption bands in 1 compared to 2-3.

Table 1. Photophysical data for complexes 1-3 in acetonitrile solution at 298 K. Compounds Absorption

λmax (nm) (ε, L.mol-1.cm-1) Emission

λem (nm ) τ [degassed MeCN] (oxygenated) φa

1 295 (36900); 425 (5300); 475 (5000) 580 [nd] (-) 0.003 2 280 (27100); 322 (23100); 360 (12500) 410 (7100) 566 [600 ns] ; (105 ns) 0.073 3 275 (26400); 330 (27900); 368 (37100); 420 (15800) 567 [17,5 µs] (213 ns) 0.027

a. Measured in degassed MeCN using [Ru(bpy)3]Cl2 in air-equilibrated water as the standard.

Unlike most of the other platinum(II) phenylbipyridyl alkynyl complexes that were found to exhibit intense luminescence at 550 – 700 nm, complex 1 shows very weak emission (φ = 0.003) at 550 nm in acetonitrile solution at 298 K. This quasi non-emissive behaviour may be ascribed to the quenching of the emissive state by photoinduced electron transfer (PET), [3] in which the electron is transferred from the electron-rich amino group to the platinum phenylbipyridyl unit to quench the emissive 3MLCT excited state.

The room-temperature emission spectrum of complex 2 upon excitation into the MLCT band (λexc = 450 nm) exhibits a structurally unresolved spectrum with a maximum at 566 nm, and emission quantum efficiencies of the order of 7%. The lifetime of emission from the excited state is of the order of 600 nanoseconds, that is typical of emission origins with triplet parentage. The emission energy is slightly blue-shifted from related [Pt(C^N^N)(C≡CAr)] derivatives (Ar = Ph: λem = 582 nm) [1a]. This is in agreement with the tunability of the emission energies, depending on the nature of the acetylide substituent. The electron-withdrawing pyridine, via electronic conjugation of the C≡C triple bond, may stabilize the Pt-based HOMO leading to blue-shifted emission. In line with previous studies, [1] the emission of 2 is thus assigned to 3MLCT excited states to which acetylide-to-diimine L’LCT transitions may also contribute.

Complex 3 emits intense luminescence, with a maximum at 567 nm, in degassed acetonitrile solution, upon excitation into the MLCT band (λexc = 420 nm). Worthy to note, the observed emission lifetime of ∼20 µs is much longer than those of related complexes without the flavone unit, despite the emission being in the same region; e.g. for [Pt(tBu2-phbpy)(–C≡C–Ph)], τ = 0.8 µs [1a]. This suggests that the emissive excited state has substantial 3IL(C≡C-FLV-3-OR) character, with the metal making a smaller contribution to the HOMO than it does in the related [Pt(C^N^N)(C≡CAr)] compounds. Clearly, the contribution of the metal—with its high spin–orbit coupling constant—is sufficient to promote intersystem crossing to the 3IL(C≡C-FLV) triplet and its subsequent formally forbidden radiative decay.

3. CATION-BINDING PROPERTIES The cation-binding properties of platinum (II) phenylbipyridyl alkynyl complexes 1-3 have been studied by electronic absorption and luminescence spectrophotometries. A range of metal ions were screened in the form of their perchlorate salts: Mg2+, Ca2+, Ni2+, Zn2+, Cd2+ and Pb2+.

Complex 1 was designed to be largely non-fluorescent itself due to the photoinduced electron transfer (PET) from the electron-donating binding site (dipicolylamine site as shown in Scheme 1a) to the excited platinum phenylbipy luminophore. Binding specific cations would decrease the electron-donating ability of the alkynyl ligand and turn on the luminescence of the 3MLCT excited state.

Effects on the absorption spectrum of 1 upon addition of metal cations (Ba2+, Mg2+, Ni2+, Zn2+, Cd2+, Pb2+) as their perchlorate salts were thus studied. Addition of Ba2+, Mg2+, and Ca2+, up to 50 equiv., to the solution of 1 in acetonitrile does not lead to any change in its absorption spectrum. However, addition of Ni2+, Zn2+, Cd2+, and Pb2+ cations results in significant changes of the absorption spectrum (Figure 3a : addition of Pb2+). The broad band at 450 – 550 nm strongly decreases while a new band centred at 500 nm concomitantly grows in monotonically throughout the addition. Well-defined isosbestic points are observed, suggestive of a ground-state equilibrium of the free 1 and cation-complexed species. Binding constants (log K) in the range from 5.7 (± 0.1) (Ni2+) to 6.7 (± 0.3) (Pb2+) were determined from these plots corresponding to 1:1 stoichiometries.

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Scheme 2 Complex 1 shows an emissive off−on effect in presence of zinc, cadmium and lead (II) ions perchlorate salts: upon addition of Zn2+, Cd2+, and Pb2+ cations in CH3CN at 298 K the luminescence emission intensity of 1 gradually increases (see figure 3b). The large Stokes shift, relatively long lifetime and susceptibility to quenching by oxygen suggest that the photoluminescence emanates from a triplet charge transfer excited state. After the addition of 0–40 equiv of each Zn2+, Cd2+, and Pb2 the phosphorescence emission intensity of 1 exhibited enhancement. Conversely, addition of Ni2+ ends in a further decrease of the luminescence of 1. Thus, 1 shows different PET effects according to the binded metal ions.

The luminescence enhancement observed upon addition of Zn2+, Cd2+, and Pb2+ cations is induced by a PET inhibition process in which electron transfer from the nitrogen lone pair electrons of the dipicolylamine unit to the platinum phenylbipyridyl unit was blocked upon the binding of the sensor to these metal salts. The specific behaviour observed upon addition of Ni2+ could be due to the presence of deactivating dd transitions of this d8 ion lying between the π*( C^N^N) and the ground state, in the resulting species.

Binding constants (log K) corresponding to 1:1 stoichiometries with Zn2+, Cd2+, and Pb2+ were determined to in the range from 2.8 (± 0.1) (Ni2+) to 6.7 (± 0.3) (Pb2+). The significantly lower affinity (by three orders of magnitude) of 1 for these ions can be attributed to a weaker electron density in the DPA unit in the excited state compared to that of the ground state. It provides thus a quite direct evidence of the [π(C≡C)→π*( C^N^N)] LLCT contribution in Pt(imine)acetylide complexes.

Addition of Ba2+, Mg2+, Ni2+, Zn2+, and Cd2+, up to 50 equiv., to the solution of 2 in acetonitrile does not lead to any change in its absorption spectrum. However, addition of Pb2+ cations results in significant changes of the absorption spectrum of 2. The broad band at 370 – 450 nm strongly decreases monotonically throughout the addition while a new band centred at 500 nm concomitantly grows in with increasing Pb2+ concentration. A well-defined isosbestic point at 450 nm is shown, suggestive of a ground-state equilibrium of the free 2 and lead-complexed 2-Pb2+ species. In addition, a remarkable luminescence decrease at 570 nm (λexc = 450 nm) was observed upon addition of lead cations, while the shape, the lifetime and the energy of the emission band did not change.

Scheme 3 The above observations demonstrate that the pyridine-based macrocycle in 2 is valuable to specifically bind lead cations. We assume that the 1:1 complexation of Pb2+ cations occurs via the nitrogen atom of the pyridine ring and leads to a stronger electron-acceptor capability of the pyridine ring (Scheme 2). Such electronic effect may lead to a blue-shifted MLCT/L’LCT transition band, instead a new low-energy band (centered at ca. 500 nm) is generated. We suggest that this new low-energy band due to lead-complexed 2-Pb2+ could be assigned to an ML'CT [d π(Pt)→π*(C≡C-py)] transition. The residual emission, the characteristics (λem and τ) of which are unchanged, can be attributed to the remaining unbounded Pt complex 2, still present in solution. Interestingly, upon irradiation into the new lower-energy absorption band (500 nm, CH3CN) attributed to complex 2-Pb2+, no emission is observed. This non-emissive state becomes the lower-lying excited state, as the result of the lowering of the energy of the π* orbitals of the C≡C-py fragment upon lead binding. In other words, the sensitivity of 2 to Pb2+ results in a switch of CT to the opposite direction (Scheme 3).

Compound 3 also displays a high selectivity for Pb2+: the IL band in the UV region decreases with increasing [Pb2+] (along with a slight hypsochromic shift), while the lower-energy band at 420 nm increases in intensity throughout the addition. In contrast to this response to Pb2+, addition of Mg2+, Ca2+, Cd2+, Ni2+ and Zn2+, even in concentrations up to 10-3 M, does not lead to any detectable spectral changes. The complexation of Pb2++on 3

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might occur via the formation of a chelate involving the oxygen atom of the C=O group of the benzopyrone moiety and the terminal oxygen atom of the polyether arm.

Scheme 4 The sensitivity of 3 to Pb2+ results in an unprecedented system that switches from triplet to singlet emission in response to metal binding (Scheme 4). Addition of increasing concentrations of Pb2+to 3 in CH3CN led to the appearance of a new, blue-shifted, structured band centred at ca. 450 nm, while the intensity of the initial band centred at 567 nm concomitantly decreased. This change in emission was triggered only by Pb2+ ions and not by the presence of other cations (Mg2+, Ca2+, Cd2+, Ni2+ and Zn2+).

The dramatic change in the emission spectrum of 3 upon binding of Pb2+ evidences a profound change in the excited state properties of the molecule. A clue to its origin is provided by the observation that the emission in the 400 – 475 nm region has a lifetime of only 1.7 ± 0.5 ns in degassed solution (compared to 18 µs for the initial luminescence band at 567 nm in the absence of Pb2+), and resembles very closely the emission profile displayed by the free flavone acetylide HC≡C-FLV (This uncomplexed terminal alkyne displays fluorescence upon UV excitation at room temperature, centred at 437 nm in MeCN). The new emission band in the 400 – 475 nm region registered in the presence of an excess of Pb2+ can be attributed to fluorescence from the singlet state of the flavone. On the other hand, a second, weak component with a lifetime of ∼700 ns, superimposed on the tail of the fluorescence is detected, which is not detectable in aerated solution. This component may be due to emission from the 3MLCT state associated with the “Pt(C N N)(−C≡C)” moiety. These changes can be tentatively rationalised in terms of binding of Pb2+ leading to a decrease in the energy of the highest-occupied flavone-localised orbitals. This results in the decoupling of the flavone orbitals from those of the “Pt(C^N^N)(−C≡C)” unit, leading to fluorescence from the singlet state of the flavone and 3MLCT emission from the Pt centre.

4. CONCLUSION This paper summarizes the design, syntheses and photophysical studies of selected platinum(II) complexes [Pt(tBu2-C^N^N)(C≡C-R) 1−3 which contain functionalized acetylide ligands with specific binding units R for heavy and transition metal cations. These studies demonstrated that the absorption properties of [Pt(tBu2-C^N^N)(C≡C-R)] complexes can be modulated through modification of the acetylide ligands and binding of cations. Complexes 1−3 display a low-energy absorptions arising from mixed LLCT/MLCT transitions from the metallic fragment and the aceylide ligand Pt-C≡C-C6H4-R. The luminescence origin is suggested to be derived from the excited state of [dπ(Pt)→π*( tBu2-C^N^N)] 3MLCT, mixed with some [π (C≡C)→π*( tBu2-C^N^N)] 3LLCT character. In presence of cations binded to the receptors, strong modifications of the energy of the HOMO and LUMO result in large spectral changes in absorption and emission in response to metal binding

ACKNOWLEDGMENT We thank the COST D035-0010-05 and the Alliance programme (12115YC). for financial support.

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