Luminescence quenching of *[Ru(bpy)3]2+ by ruthenium(II) tetraphosphite complexes with different phosphite ligands

ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1260–1265

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Luminescence quenching of *[Ru(bpy)3]2+ by ruthenium(II) tetraphosphite complexes with different phosphite ligands Jose E.B. Freitas a, Diego Lomonaco a, Giuseppe Mele b, Selma E. Mazzetto a, a

Laborat´ orio de Produtos e Tecnologia em Processos-LPT, Departamento de Qu´ımica Orgˆ anica e Inorgˆ anica, Universidade Federal do Ceara , C.P. 6021, 60455-900, Fortaleza, Ceara , Brazil b Dipartimento di Ingegneria dell’Innovazione, Universita del Salento, Via Arnesano 73100, Lecce, Italy

a r t i c l e in f o

a b s t r a c t

Article history: Received 27 October 2008 Received in revised form 3 June 2009 Accepted 11 June 2009 Available online 30 June 2009

The luminescence quenching of excited Tris(2,2-bipyridine)ruthenium(II) ions by trans-[RuCl2{P(OR)3}4] complexes with different alkyl chain ligands (R ¼ C2H5, C2H5Cl, nC4H9, iC3H7 o-tolyl and tC4H9) was investigated. None of the acceptor Ru(II) phosphite complexes were luminescent, and the rate constants of the bimolecular system were determined within the range of 1.15 and 0.28  108 M1 s1 for R ¼ C2H5 and tC4H9, respectively. The results indicate a direct effect of the alkyl chains in the rate constants, showing a decrease of kq as a function of increased of the alkyl chains (R) in the ruthenium(II) tetraphosphite complexes. The greater the R group content in the phosphite ligand, the more difficult the electron transfer is. & 2009 Elsevier B.V. All rights reserved.

Keywords: Luminescence quenching Phosphite ligands Ruthenium(II) complexes

1. Introduction The chemical and photochemical properties of Ru(II)-polypyridine complexes have attracted wide interest because of their excellent catalytic and photosensitive properties [1–14]. The interest is mainly due to their use as solar energy conversion, photochemical molecular devices, photodegradation reactions and luminescent sensors. The complex is very stable to light, have desirable optical, electrochemical, and magnetic properties, exhibit intense luminescence and is very used to understand the interrelation between the composition of the coordination sphere and the electronic structure of excited state [9,10]. [Ru(bpy)3]2+ has been used in many investigations because its has very favorable photochemical properties [15–18]. For example, its absorbance in the visible and UV regions is high, in the excited state it is both a good reductant and oxidant, its reduced and oxidized forms are relatively stable to degradation reactions and its lifetime is long enough to undergo bimolecular collisions with a large variety of quenchers present in solution (bimolecular electron or energy transfer). The rate constants of bimolecular reactions are functions of a variety of molecular and solvent parameters [10–16,19–30]. The driving force and the reorganizational free energy tend to be the most important [16,19,24,25,28], but factors such as donor/ acceptor electronic coupling, distance of closest approach and reagent charge type can also be important in determining

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E-mail address: [email protected] (S.E. Mazzetto). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.06.025

variations of reaction rate constants in a series of related reactions [20,30]. As a consequence, patterns of reactivity can be altered by the choice of the ligands, even in a series of very closely related transition metal acceptor complexes. Although a large number of rate constants and activation parameters for the bimolecular quenching of transition metal complex excited states have been reported [7,9,10,20–22,28,30–39], the effects that result from changes in the transition metal complex are not totally documented. In this paper, we report the effects on the bimolecular rate constants for quenching of *[Ru(bpy)3]2+ by a homologous series of ruthenium (II) complexes, with different trialkylphosphite ligands: trans-[RuCl2{P(OR)3}4], where R ¼ C2H5, C2H4Cl, nC4H9 i C3H7, o-tolyl and tC4H9. This series of complexes should enable delineating the influence bulky acceptor ligands can have on the variations of rate constants.

2. Experimental details Ethanol and acetone (Aldrich) were distilled under reduced pressure before use. Trifluoroacetic acid (Merck, spectroscopic grade) was used as purchased. The aqueous solutions were prepared using doubly distilled water. All solvents used were purified by standard methods and distilled before use. [Ru(bpy)3]2+ (Aldrich) was used as received and the reagents were purchased commercially in the highest available purity and used as received. The trans-[RuCl2{P(OR)3}4] complexes (R ¼ C2H5, C2H4Cl, nC4H9 i C3H7, o-tolyl) were prepared according to the methods described in the literature [22,31,40,41]. For the preparation of

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Fig. 1. ORTEP representation of (a) trans-[RuCl2{P(OC2H5)3}4] [22], (b) [Ru(bpy)3]2+ [42] and (c) cone angle representation for symmetric ligands [43].

trans-[RuCl2{P(OtC4H9)3}4] complex, the best conditions were obtained with 0.01 mol of ruthenium chloride trihydrate, 0.60 mol of phosphite ligand and 0.18 mol of NaBH4. The mixture of ruthenium-phosphite ligand was stirred for 48 h at room temperature, under argon atmosphere and absence of light. Then, NaBH4 was added slowly in an ice bath and the solution was stirred for 48 h to obtain a yellow-brown solution. The volume was reduced by 90–95% and kept in a freezer for around two to three months for partial precipitation. The results of elemental analysis were: C, 49.51%, H, 9.42%; C48H108Cl2O12P4Ru calc.: C, 49.29% H, 9.39%; yield ffi12%. The spectroscopic properties showed small changes compared to the others trans-[RuCl2{P(OR)3}4] complexes studied: an absorption band at l ¼ 254 nm (e ¼ 3.24  103 M1 cm1), a shoulder at around l ¼ 287 nm (e ¼ 0.99  102 M1 cm1) and another band at l ¼ 412 (e ¼ 0.21 102 M1 cm1) [22,40,41]. All experiments and manipulations were carried out under an Ar atmosphere. All-glass connections and routine techniques of air-sensitive material transfer were employed. UV–vis absorption measurements were performed at room temperature with an HP 8453 diode array spectrophotometer. Electrochemical measurements were performed on milimolar solutions of the complex in a BAS 100A instrument. The working electrode was a vitreous carbon, the reference electrode was Ag/AgCl and the auxiliary electrode was Pt wire. The formal potential, E1/2 for the Ru(III)/Ru(II) couple in the phosphite complex, was calculated as the arithmetic mean of the anodic and cathodic peak potentials. Half-wave potentials were obtained for the complexes in an aqueous solution of 1 mM trifluoroacetic acid/ethanol

[ethanol–water (1:1 v/v) dissolved in an aqueous solution of 0.1% trifluoroacetic acidic (TFA)]. The solvent mixture used in the electrochemical measurements was the same as that employed with the samples of ruthenium complexes in the presence of [Ru(bpy)3]2+ in the luminescence quenching experiments. The luminescence lifetime (t0) of the [Ru(bpy)3]2+ was determined using an Edinburgh CD 900 microsecond timeresolved phosphorescence spectrometer. Corrected steady-state emission spectra of the 105 M [Ru(bpy)3]2+ solutions were recorded at 298 K using an Edinburgh CDD 900 photon counting spectrofluorimeter. The luminescence quenching measurements were calculated from the integration of the most intense emission band centered at 612 nm. All the solutions ([Ru(bpy)3]2+ and quenchers [Q]) were degassed under a flow of N2 for around 20 min before measurement and kept under an N2 atmosphere to prevent quenching by O2. The quencher additions were made without interfering with the N2 atmosphere by injection with a 50 mL syringe through a rubber septum attached to the sample cuvette. The concentration of the stock quencher solution was 1 mM and small volume additions (around 10–50 mL) were made to avoid altering the initial [Ru(bpy)3]2+ concentration. The excitation wavelength was 450 nm and the emission wavelength was scanned between 500 and 700 nm.

3. Results and discussion The ORTEP representation of the reactants employed in the present work: trans-[RuCl2{P(OR)3}4] [22], [Ru(bpy)3]2+ [42]

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Table 1 Absorption and emission spectral data, redox potentials of ruthenium complexes, cone angle, excited state lifetime and the experimental bimolecular quenching rate constants for the *[Ru(bpy)3]2+ by trans-[RuCl2{P(OR)3}4] complexes in trifluoroacetic acid/ethanol at 298 K. Complex

Absorption maximum (nm) (emax, 104 M1 cm1)

Emission maximum (nm)

Excited state lifetime s (ns)

Redox potential (V)

[Ru(bpy)3]2+

285 (6.47), 344 (0.69) 425 (1.21), 450 (1.33)

612a 595c 613e

498a 600d

Trans-[RuCl2{P(OR)3}4]g R ¼ C2H5 R ¼ C2H5Cl R ¼ nC4H9 R ¼ iC3H7 R ¼ o-tolyl R ¼ tC4H9

Cone angle(y)h (deg.) 109 110 112 130 141 172

E1/2 (V vs. SCE) 0.72 0.69 0.68 0.67 0.66 0.66

kobsd,i (M1 s1 108) 1.15 1.09 1.05 0.80 0.45 0.28

+0.84b 0.84b 1.28f +1.26f DGo (eV) 0.12 0.15 0.16 0.17 0.18 0.18

a

This work. Reduction and oxidation potential to *[Ru(bpy)3]2+. In water at pH47 [25]. d In aqueous medium at pH 12.5 [26]. e In MeCN [27]. f Reduction and oxidation potential to [Ru(bpy)3]2+. g The acceptors trans-[RuCl2{P(OR)3}4] complexes did not present emission intensities. h [24]. i Errors are estimated to be r10% of the rate constant reported here. b c

complexes and the characteristics of phosphorus (III) ligands [43] based on the cone angle1 are illustrated in the Fig. 1. The versatility of the Ru(II)–P(III) complexes is particularly related to the characteristics of phosphorus ligands coordinated to the metal, where the s-donor, p-acceptor properties and steric hindrance can be controlled by changing the R substituent. These trialkylphosphane ligands (phosphite or phosphine) are commonly classified by the cone angle [43], which is an important parameter, to determinate their stability. The absorption and emission spectral data, electrochemical parameters, cone angles, excited state lifetimes, free energies and experimental bimolecular quenching rate constants are given in Table 1. The electronic absorption spectra, X-ray data, RMN, kinetic and photochemical behavior of trans-[RuCl2{P(OR)3}4] complexes was discussed previously [22,31,38,44–46]. These acceptors’ Ru(II) phosphite complexes are not luminescent. The excited state of (dp)6 [RuII(bpy)3]2+ has been extensively investigated [16,20,29,30,39,47–49] and is well formulated as [RuIII(bpy)2(bpy)]2+, with an excited electron located on a single bipyridine ligand. The ground state has a moderate molar absorptivity (e10,000 M1 cm1) and the Stokes shift is relatively large (absorption and emission are maximal at 450 and 610 nm, respectively). Besides this, it shows a moderate luminescence quantum yield (fF40.1), is little self-quenching, has a long excited state lifetime (ffi0.6 ms) and has useful redox properties: *[RuII(bpy)3]2++e ¼ [RuII(bpy)2(bpy)]+-E1/2(r)ffi0.84 V

(1)

[RuIII(bpy)3]3++e ¼ *[RuII(bpy)3]2+-E1/2(o)ffi+0.84 V

(2)

complexes: [Ru(bpy)3]2++[RuIICl2{P(OR)3}4][RuIII(bpy)3]3++[RuICl2{P(OR)3}4]

(3)

[Ru(bpy)3]2++[RuIICl2{P(OR)3}4]-[RuII(bpy)2 (bpy)]++[RuIIICl2{P(OR)3}4]+

(4)

[Ru(bpy)3]2++[RuIICl2{P(OR)3}4]-[Ru(bpy)3]2++ *[(3LF)RuIICl2{P(OR)3}4]

(5)

Of these possibilities, the reductions of the trans-[RuCl2 {P(OR)3}4] complexes, Eq. (3), are not thermodynamically feasible, and the ligand field excited states tend to have energies that are significantly larger than the 3MLCT excited state of [Ru(bpy)3]2+. On the other hand, the oxidative quenching process, Eq. (4), is thermodynamically favorable and most likely accounts for the observations. The *[Ru(bpy)3]2+ luminescence with maximum intensity at l ¼ 612 nm was generated by means of irradiations at 450 nm. The excited state lifetime of [Ru(bpy)3]2+ decreased systematically with increasing concentrations of the trans-[RuCl2{P(OR)3}4] complexes, characterizing the direct contribution of steric effects on the kq values. The variations in luminescence emission intensity with changing concentrations of the quenching species were analyzed by the Stern–Volmer2 relationship, assuming predominately dynamic quenching: Io =I ¼ 1 þ Kq to ½Q 

The excited state of *[Ru(bpy)3]2+ can function as an electron donor, Eq. (3), an electron acceptor, Eq. (4) and possibly an energy donor, Eq. (5) with the trans-[RuIICl2{P(OR)3}4]

1 According to Tolman [43], the cone angle for ligands is the apex angle of a cylindrical cone, centered at 228 ppm from the center of the P atom, which just touches the Van der Waals radii of the outermost atoms of the ligand.

ð6Þ

2 I ¼ luminescence emission intensity, Io ¼ emission intensity in the absence of the quenching species, kq ¼ bimolecular quenching constant, to ¼ luminescence lifetime in the absence of the quencher, [Q] ¼ quenching species concentration, g ¼ quenching efficiency, N ¼ Avogadro’s number, Rf and Rq ¼ molecular radii of the luminescence species and quencher, Df and Dq ¼ diffusion coefficients of the luminescence species and quencher, k ¼ Boltzmann’s constant, T ¼ temperature, Z ¼ viscosity and R ¼ collision radius of the molecule [50,51].

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force for the reaction: kobsd ¼ Ko

! ðws þ DGoRP Þ2 1 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi exp or kobsd HRP ‘ 4ws kb T 4pws kb T

2p

¼ Ko kET ¼ Ko Ae½g

Fig. 2. Profile of Stern-Volmer plot for a bimolecular quenching of *[Ru(bpy)3]2+ by trans-[RuCl2{P(OR)3}4] complexes; [(’) R ¼ P(C2H5) y ¼ 1091 and () R ¼ P(tC4H9) y ¼ 1721].

kq ¼ g

4pN kT ðR þ Rq ÞðDf þ Dq Þ and D ¼ 1000 f 6pZR

ð7Þ

The luminescence lifetime to of the [Ru(bpy)3]2+ measured in the working solution (ethanol) was 498 ns in the absence of added quencher. Using the experimentally determined luminescence lifetime, we converted the Stern–Volmer (KSV) values for a series of trans-[RuCl2{P(OR)3}4] complexes into the respective quenching rate constants (Kq), Table 1. The simplified form of Eqs. (6) and (7) are: Io =I ¼ 1 þ Ksv ½Q 

ð8Þ

The experimental bimolecular quenching of *[Ru(bpy)3]2+ by Ru(II) complexes with different P(III) ligands was monitored by measuring the decrease in the emission intensity at 612 nm as a function of the concentration of the trans-[RuCl2{P(OR)3}4] complexes. Each experiment yielded a value of Io/I applicable to the particular concentration conditions (Fig. 2 and Table 1). The results showed a direct effect of the alkyl chains on the rate constant, i.e., a decrease of kq as a function of increasing alkyl chains in the phosphite ligands. A good linear fit of the Stern–Volmer equation was obtained with the intercept unity for all systems, Fig. 2, indicating that the quenching process is predominantly dynamic and the reaction is said to be adiabatic, i.e. the electronic interaction is sufficiently strong to make the electronic transmission coefficient equal to one [10].

3.1. Quenching by electron transfer According to the Marcus theory,3 the electron transfer rate constant can be expressed by Eq. (9) [24,25,54], and its value allows making several predictions concerning electron transfer, where the most salient points are the reorganization energy (related to the activation energy) and the thermodynamic driving 3 Ko is an equilibrium constant for forming a reactive D/A encounter complex, KET is the rate of electron transfer, HAB is the electronic coupling between the initial and final states, l is the reorganization energy, DGo is the total Gibbs free energy change for the electron transfer reaction and kb is the Boltzmann constant.

2

=4ws RT

ð9Þ

Thus, the observed rate constants for bimolecular reactions are expected to increase with increasing free energy until they reach the diffusional limit, and they should eventually decrease as the driving force increases. The value of ws for the [Ru(bpy)3]3+,2+ self-exchange reaction in aqueous solution is estimated to be about 0.8 eV [45], while the values in the ethanolic solutions used here may be somewhat smaller, since |DGRPo|5ws for all of the reactions examined here. For the purpose of comparing the observations with the theoretical expectation it is useful to put Eq. (9) in the form, sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   ffi Ko A ð10Þ 4ws kb T ln ¼ ws þ DGoRP kobsd The observed parameters in Table 1 are reasonably consistent with AE6  10!2 s1, KoE0.5 M1, RT ¼ 0.0255 eV and wsE0.96 eV. The three reactions with the smallest driving forces have almost identical rate constants, suggesting that these reactions are near the diffusion limit in the solvent used (2). Although all of these reactions are in the normal region (|DGoRP|5ws), the reactions with the largest driving forces have the smallest quenching rate constants. The latter are also the reactions with the bulkiest ligands (or largest cone angles). This is consistent with smaller values of Ko for these systems. Since the acceptor complex shapes can be approximated as oblate spheroids, this suggests that the electron transfer probability (or HDA) is largest for close approach of the electron donor along the Cl–Ru–Cl axis and that this approach is inhibited in the complexes with very large cone angles. With reference to Eq. (9), the distance between the donor and acceptor can alter the rates of electron transfer through the distance dependence of: (1) HRP [58–60] (2) wRP [52] or (3) some factor related to the departure of the molecule from a spherical shape and its incorporation into Ko [52]. One generally expects HAB to decrease [53–55] and wRP to increase [52] with increasing rDA (if the separation distance is the only parameter changed), and this should result in corresponding decreases in kET. However, these reductions in rate constant are opposed by increases in the encounter (or collision) probability (and Ko), and in the systems considered here by decreases in solvent reorganizational energy as the complex dimensions increase. It is very rare to find systems in which such effects significantly alter the observed rate constants. Furthermore, since rDA is not fixed in bimolecular reactions, the observed rate constants probably correspond to an averaging of its opposing effects on these different parameters. Thus, even though the effects observed here are very small, they are systematic and are a very rare demonstration of stereochemical constraints on bimolecular electron transfer reactions. The literature contains several studies contributing to a better understanding of the factors that affect the rates of photoinduced electron transfer reactions of Ru(II) complexes [10,15,35,56–62]. Thangamuthy et al. [35], for example, reported the importance of meta- and para-substituted for the excited state ruthenium(II)polypyridine complexes. He also mentions that the electrochemical data are not very sensitive to the bulkiness of the alkyl group and that the steric interactions between two reactant molecules may become a

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The present study clearly established the relations between the role of the steric effect in the photoinduced electron transfer reactions between phosphite complexes of Ru(II) with different trialkylphosphite ligands and Tris(2,2-bipyridine)ruthenium(II) ions. The magnitudes of kq obtained in this work (108 M1 cm1) are consistent with the findings of other authors regarding electron transfer quenching. Acknowledgements We gratefully acknowledge financial support from the National Research Council (CNPq). We also thank Professor John F. Endicott for reviewing the manuscript. References

Fig. 3. Plot of the luminescence quenching rate constants (kq) as a function of the free energy of *[Ru(bpy)3]2+ by trans-[RuCl2{P(OR)3}4] complexes [R ¼ (1) C2H5, (2) C2H4Cl, (3) nC4H9, (4) iC3H7, (5) o-tolyl and (6) tC4H9].

dominant factor when large ligands and bulky quenchers are involved. The effects in the photoinduced electron transfer reactions of ruthenium(II)-polypyridine complexes with 2,6-disubstituted phenolate ions showed that an increase in the size of the alkyl group may heighten the importance of the steric effect and thereby the decrease in the kq values. Kitamura [62] also observed that the change in the internuclear distance between two reactants is the dominant factor when bulky ligands are involved. He reported results of quenching of *[Ru(bpy)3]2+ by aromatic amines, where small changes in the spectroscopic and redox properties was observed, even with the addition of bulky ligands like an isobutyl and neopentyl groups. However, the quenching rate constants are highly influenced by the introduction of alkyl groups while the reduction potential is no longer affected by the introduction of a bulkier substituent on the ligands. Thus, the Kq decreases. These and other reports in the literature agree with those presented here, where the decrease of kq is a function of the increasing volume in the substituent R of the phosphate ligands in trans-[RuCl2{P(OR)3}4] complexes. The spectroscopic and electrochemical parameters undergo small changes with increasing R. To understand the effect of DGo on the kq, log kq is plotted against DGo, Fig. 3. The rate decreases with the introduction of alkyl groups with high cone angle, and the reaction becomes more exergonic (DGo more negative). The introduction of bulky ligands affects the rate of electron transfer by increasing the distance between donor and acceptor. The figure clearly indicates the decrease of kq in the sequence: C2H54C2H4Cl4nC4H94iC3H74o-tolyl4tC4H9. The nonlinear plot shows the correlation between steric bulk (cone angle) and ease of electron transfer. Generally, the rates of electron transfer are expected to increase with decreasing donor–acceptor distances, and the maximum rate should be observed when the reaction is activation less, i.e., when DG1 is negative, preferably when DG1 ¼ l. In other words, the rate is optimized when the standard free energy change for the reaction is matched exactly by the energy required for reorganization of the donor, acceptor and solvent molecules. This behavior is exactly what we observed for the trans[RuCl2{P(OR)3}4] complexes studied: the smaller the distance between donor and acceptor (governed by y), the greater the rate constant was.

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