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Electrochemiluminescence studies of phosphine chelated osmium(II) complexes
Inorganic Chemistry Communications 12 (2009) 378–381
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Electrochemiluminescence studies of phosphine chelated osmium(II) complexes Gonzalo Angulo a, Andrzej Kapturkiewicz a,b,*, Sheng-Yuan Chang c, Yun Chi c a b c
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Chemistry, University of Podlasie, 3 Maja 54, 08-110 Siedlce, Poland Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e
i n f o
Article history: Received 4 December 2008 Accepted 20 February 2009 Available online 3 March 2009 Keywords: Osmium(II) complexes Chelating phosphine ligands N-donors Electrochemiluminescence
a b s t r a c t The electrochemiluminescence (ECL) properties of five different yellow/orange emissive osmium(II) complexes of general formula Os(L2L’) with three different kinds of bidentate phosphine ligands L’ have been studied in acetonitrile/dioxane 1:1 solution by means of the triple-potential-step technique. The investigated complexes contain either two substituted pyrazolate or triazolate ligands L. The obtained ECL results have been interpreted in terms of the ligands nature, showing that the efficiency of ECL seems to be sensitive to the p-acidity of the phosphine ligand. To the authors’ best knowledge, the reported ECL efficiencies are the highest found for Os(II) complexes under similar conditions to date, that are at maximum five times higher than that of the famous RuðbipyÞ2þ 3 based ECL systems. Ó 2009 Elsevier B.V. All rights reserved.
The electrochemiluminescence (ECL) of transition metal complexes has found many applications in the last years [1]. The most widely used compounds are ruthenium(II) complexes with diimine chelating ligand. Ruthenium(II) forms stable red emitting compounds like the paradigmatic in ECL studies tris(2,2’bipirydyl)ruthenium(II) ion RuðbipyÞ2þ 3 , used also in organic light emitting diodes (OLEDs), imaging systems, bio-sensors and many other applications because of their relatively high ECL yield around 0.05 (defined as the amount of photons emitted per number of the þ annihilating RuðbipyÞ3þ 3 and RuðbipyÞ3 pairs). Therefore, a very active research area is committed with the search of new ECL-active luminophores. In fact, iridium based complexes have shown much higher efficiencies in a wide range of emission wavelengths [2–5]. Due to the many similarities between Ru(II) and Os(II) chelates the latter have been also investigated as ECL active compounds. The 2þ ions [6,7] clearly results obtained for OsðbipyÞ2þ 3 or OsðphenÞ3 2þ 3 or OsðphenÞ excited states are established that 3 OsðbipyÞ2þ 3 3 populated at ECL conditions but the obtained ECL efficiencies have been found to be distinctly smaller as compared to their Ru(II) counterparts, most likely due to their lower emission quantum yields in accordance with the free energy gap low (these osmium complexes emit more to the red than those with ruthenium) [8–10]. Much higher ECL intensities were observed for Os(II) complexes of Os(bipy)2L2+ or Os(phen)2L2+ type with supplementary bidentate phosphine or arsine ligands L like 1,2-bis(diphenylphosphino)ethane or 1,2-bis(diphenylarsino)ethane [6,7], ligands
* Corresponding author. Address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. Tel.: +48 22 632 3221; fax: +48 22 632 3333. E-mail address: [email protected] (A. Kapturkiewicz). 1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2009.02.021
that shift the excited MLCT to higher energies. Unlike 2þ 2+ or Os(phen)2L2+ OsðbipyÞ2þ 3 and OsðphenÞ3 , many of Os(bipy)2L complexes show very intense photoluminescence with efficiencies two to three orders of magnitude higher than their parent derivatives. Consequently the observed ECL efficiencies obtained in electron transfer annihilation between the electrochemically generated Os(phen)2L+ and Os(phen)2L3+ ions were close to that found for RuðbipyÞ2þ 3 . ECL emission, much more intense than for 2þ chelates, was the previously studied OsðbipyÞ2þ 3 and OsðphenÞ3 also observed in the ECL system based on tris(2,20 -bipyrwith the emission from azine)osmium(II) OsðbprzÞ2þ 3 3 OsðbprzÞ2þ species populated in electron transfer between the 3 3þ ions [11]. electrochemically generated OsðbprzÞþ 3 and OsðbprzÞ3 ECL systems based on Os(II) complexes, despite some advantages like their relatively low oxidation potentials [12,13] (that make them more suitable for OLEDs because they better match the work function of the ITO anode), are pointedly less elaborated as compared to those based on Ru(II) derivatives. One can expect, however, that further development of the ligand(s) coordinating Os(II) kernel will result in new efficient ECL systems similarly as it has taken place for Ir(III) chelates [2–5]. In line with the reported efforts the synthesis of new complexes with phosphine chelating groups with very high emission yields and emission bands peaking around 600 nm, has been reported [12–14]. In this work we present results on the ECL properties (investigated by means of triple-potential step technique) of some of the compounds presented in ref. [12] (cf. Scheme 1). The complexes contain two hapto-nitrogen bidentate chelates, namely substituted (2-pyridyl)pyrazolate or substituted (2-pyridyl)triazolate anions, that are further referred as pyrazole and triazole containing ligands, respectively, and a third haptophosphine bidentate ligand
G. Angulo et al. / Inorganic Chemistry Communications 12 (2009) 378–381
379
Scheme 1. Structures of the investigated Os(L2L0 ) complexes.
of which three kinds have been studied: bis(diphenylphosphino)methane, cis-1,2-bis(diphenylphosphino)ethene, and 1,2bis(diphenyl-phosphino)benzene, that are abbreviated as dppm, dppee and dppb, respectively. ECL experiments, together with general electrochemical and photophysical characterisation of the investigated Os(II) complexes, have been performed in acetonitrile/dioxane ACN/DX – mixture (1:1 in volume) containing 0.1 M (n-C4H9)4NPF6 as supporting electrolyte. Experimental details are described in the Supporting Information. The redox potentials (referred to ferrocene/ferricenium Fc/Fc+ internal reference redox couple) obtained for the investigated complexes by means of cyclic voltammetry are collected in Table 1. All the complexes show reversible one-electron oxidation
OsðL2 L0 Þ e ¢ OsðL2 L0 Þþ
ð1Þ
located between 0.10 and 0.39 V, whereas stable reduction products have been observed (at potentials around 2.4 V) only for species containing CF3 substituted triazole OsTM and OsTE
OsðL2 L0 Þ þ e ¢ OsðL2 L0 Þ
ð2Þ
The reduction of other investigated complexes occurs at more negative potentials leading to distinctly less stable products (cf. Fig. 1). Both, reduction and oxidation potentials correlate with the nature of the N \ N chelates: the pyrazolate containing complexes OsPM and OsPE have oxidation potentials more negative than those with triazolate, OsTM, OsTE and OsTB, while the reduction is more positive for the latter group. Thus the reduction potentials correlate well with the electron withdrawing properties of these groups [12], as they are the primary electron acceptors in these complexes. The oxidation, on the other hand proceeds via the direct removal of a d electron from the Os(II) central ion. The stronger electron withdrawing character of the triazolate respect to the pyrazolate ligands, explains also the fact that the second set shows lower oxidation potentials. There is a further correlation that can be extracted from these data: within each group discussed before, those complexes containing dppm are easier to oxidize than those con-
Table 1 Electrochemical potentials (referred to Fc/Fc+) of ECL systems studied and the Gibbs free energy for the population of the excited state from the annihilation of the reduced and oxidized forms (estimated from DGes F(Ered Eox) + EMLCT relationship with EMLCT approximated as the MLCT emission maxima). The emission yield uem, the ECL yield uecl, and the excited state yield ues (as calculated from ues = uecl/uem equation). Data for 0.1 M n-(C4H9)4NPF6 ACN/DX (1:1) solutions. ECL system
Eox/V
Ered/V
DGes/eV
uem
uecl
ues
ECL single systems OsTM+/OsTM OsTE+/OsTE
+0.33 +0.39
2.44 2.39
0.72 0.70
0.28 0.35
0.11 0.25
0.39 0.71
ECL mixed systems OsTM+/CNN OsTE+/CNN OsPM+/CNN OsPE+/CNN OsTB+/CNN
+0.33 +0.39 +0.10 +0.17 +0.11
2.34 2.34 2.34 2.34 2.34
0.62 0.65 0.39 0.43 0.41
0.28 0.35 0.17 0.12 0.27
0.13 0.24 0.08 0.10 0.23
0.46 0.69 0.47 0.83 0.85
Fig. 1. Cyclic voltammograms of Os(L2L’) complexes recorded at Pt electrode in ACN/DX (1:1) solutions containing 0.1 M TBAPF6 as supporting electrolyte. Scan rate = 100 mV/s.
taining dppee or dppb. This may be due to the fact that the dppee and dppb ligands are stronger p-acids (thanks to double bond(s) connecting the P atoms) [15] and therefore interact stronger with Os(II) than with Os(III). All the complexes studied show ECL effect presenting identical spectra as recorded by photo-excitation, within the experimental error, with peaks around 600 nm (cf. Fig. 2). The fact that all compounds emit in the same range reflects that the extent of the shift in the redox potentials driven by the ligands, is similar in both the reduction and the oxidation processes. When possible, as it was in the case for OsTM and OsTE, we have performed the ECL experiment without added co-reactant (single ECL systems). Thus the neutral complex is first oxidized/reduced in the first step and then reduced/oxidized in the second, depending on the pulse sequence used, either +/ or /+, respectively. The ions annihilation reaction produces the triplet excited state of the complex and its ground state as the energy of the reaction, just the difference of reduction and oxidation potentials, is higher than the excited state triplet energy respect to the ground singlet state:
OsðL2 L0 Þþ þ OsðL2 L0 Þ ! 3 OsðL2 L0 Þ þ OsðL2 L0 Þ
ð3Þ
In these cases (OsTM and OsTE complexes) the yield of ECL reached 0.11 and 0.25, respectively (2 and 5 times the efficiency of RuðbipyÞ2þ 3 ). Correspondingly, ECL efficiencies of 0.13 (OsTM) and 0.24 (OsTE) have been found in the ECL experiments involving an auxiliary co-reactant, 1-cyanonaphthalene CNN, forming stable radical anion CNN (ECL mixed systems):
380
G. Angulo et al. / Inorganic Chemistry Communications 12 (2009) 378–381
Fig. 2. Emission spectra of Os(L2L’) complexes in ACN/DX (1:1) solutions.
OsðL2 L0 Þþ þ CNN ! 3 OsðL2 L0 Þ þ CNN
ð4Þ
where the ECL excitation proceeds via the annihilation between the cation of the given complex and the anion of the added co-reactant. To populate the excited triplet state of the complex 3*Os(L2L’) was possible because the CNN reduction potential is enough negative to provide the necessary amount of energy from the ions annihilation reaction. In the ECL experiments involving the mixed ECL system the cathodic limit potential was chosen enough negative to ensure CNN species production but enough positive to avoid electrochemical reduction of the investigated Os(II) complex. ECL processes of the other three complexes cases have been investigated only in the mixed systems because of the not enough high stability of their anionic forms Os(L2L’). OsPM led to 0.08 and OsPE to 0.10 whereas OsTB measured with the same co-reactant lead to an ECL efficiency of 0.23. As all obtained ECL yields have been found to be distinctly lower than the previously reported [12] emission quantum yields in CH2Cl2 solutions, we decided to measure them in the medium used for the ECL experiments. The obtained results are summarized in Table 2. The emission yields have been found to be lower than those previously reported in CH2Cl2 solutions. In order to check further the validity of these results, we measured as well the emission life-times sem of the complexes in ACN/DX, finding sem values between 1 and 2 microseconds. The radiative decay rates, as estimated from the emission yields and life-times, are close to 1.5 105 s1 for all the studied complexes, while their non-radia-
tive decay rates are of about 10 105 s1 for OsPM and OsPE and about 3 105 s1 for OsTM, OsTE and OsTB, revealing a quite coherent picture of the obtained photophysical results. The invariance of the radiative rate constant suggests that as all complexes emit in the same region, the matrix coupling element for the charge transfer (MLCT emission) is also almost invariant, as it is to be expected from the close nature of the different emitters. Due to the fact that the energy gaps between the states involved in the MLCT transition of all the complexes under study are nearly exactly the same, one can also expect that the non-radiative rate constants should be comparable, which is not exactly the case. It is likely that the non-radiative decay of the excited 3*MLCT state in the investigated complexes is partly governed by the thermally activated population of close lying metal-centred excited d–d dark states [9]. It can be thought that for the pyrazole ligand the 3*MLCT is closer in energy to these dark states, thence the faster non-radiative deactivation. In any case, as the phosphine chelating agents are high crystal field ligands, the Os(II) dd excited states are shifted to higher energies as compared to 3*MLCT states what results in relatively high emission yields of the complexes under study. Another reason would be a difference in the Franck–Condon factors for the different complexes. The scarcity of the data does not allow, however, preferring one of the above explanations over the others. With data for the ECL efficiencies uecl and for the emission yield uem, one can calculate the excited state population yield ues in the performed ECL experiments according to the uecl = uem ues relationship. As it can be seen in Table 1 compounds OsPE and OsTB exhibit the highest ues yields, correspondingly 0.83 and 0.85. OsTE shows a yield of 0.69, while OsPM and OsTM reached both ues about 0.46. In other words, the complexes with dppee or dppb have somewhat higher yields than those with dppm. The reason for this behaviour remains unclear and we will not further discuss this fact since it is very difficult to asses any conclusion based on the not too big differences found for the ues yields. In summary, the studied Os(II) complexes show ECL effect with quite high yields, all of them higher than the paradigmatic and than any other Os(II) complex so far reported in RuðbipyÞ2þ 3 the literature [9,16,17]. Besides the effects of the hapto-nitrogen ligands, one can conclude that among the phosphine ligands those with a higher intra-ligand conjugation may lead to higher excited state efficiencies in ECL processes. Further work seeking for other Os(II) complexes guided by the trends here outlined could lead to still more ECL-efficient compounds. One can expect that it could be possible to prepare similar Os(II) complexes for ECL application in aqueous solution and/or polymer matrices. This promising opportunity needs, however, further experimental work. Acknowledgements GA and YC thank for financial supports from European Commission and National Science Council of Taiwan, respectively. Appendix A. Supplementary material
Table 2 Photophysical characteristics of the investigated Os(L2L’) complexes in ACN/DX (1:1) oxygen-free solutions. Emission maxima kem, emission quantum yields uem, emission life-times sem, radiative krad and non-radiative deactivation knr constants (obtained from krad = uem/sem and knr = (1 uem)/sem relationships, respectively). 5
Complex
kem /nm
uem
sem/ls
krad/10 s
OsPM OsPE OsTM OsTE OsTB
605 605 600 595 605
0.17 0.12 0.28 0.35 0.27
1.01 0.80 1.92 2.35 1.84
1.7 1.5 1.5 1.4 1.5
1
5
knr/10 s 8.2 11.0 3.7 2.8 3.4
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2009.02.021. References
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[1] A.J. Bard (Ed.), Electrogenerated Chemiluminescence, Marcell Dekker, New York, 2004. [2] A. Kapturkiewicz, G. Angulo, Dalton Trans. (2003) 3907. [3] A. Kapturkiewicz, T.-M. Chen, I.R. Laskar, J. Nowacki, Electrochem. Commun. 6 (2004) 827. [4] A. Kapturkiewicz, J. Nowacki, P. Borowicz, Electrochim. Acta 50 (2005) 3395. [5] A. Kapturkiewicz, J. Nowacki, P. Borowicz, Z. Phys. Chem. 220 (2006) 525.
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[12] Y.-M. Cheng, G.-H. Lee, P.-T. Chou, L.-S. Chen, Y. Chi, C.-H. Yang, Y.-H. Song, S.Y. Chang, P.-I. Shih, C.-F. Shu, Adv. Funct. Mater. 18 (2008) 183. [13] Y. Chi, P.-T. Chou, Chem. Soc. Rev. 36 (2007) 1421. [14] P.-T. Chou, Y. Chi, Chem. Eur. J. 13 (2007) 380. [15] B. Carlson, G.D. Phelan, W. Kaminsky, L. Dalton, X. Jiang, S. Liu, A.K.-Y. Jen, J. Am. Chem. Soc. 124 (2002) 14162. [16] M.M. Richter, Chem. Rev. 104 (2004) 3003. [17] R. Pyati, M.M. Richter, Ann. Rep. Prog. Chem, Sec. C 103 (2007) 12.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Electrochemiluminescence studies of phosphine chelated osmium(II) complexes Gonzalo Angulo a, Andrzej Kapturkiewicz a,b,*, Sheng-Yuan Chang c, Yun Chi c a b c
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland Institute of Chemistry, University of Podlasie, 3 Maja 54, 08-110 Siedlce, Poland Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
a r t i c l e
i n f o
Article history: Received 4 December 2008 Accepted 20 February 2009 Available online 3 March 2009 Keywords: Osmium(II) complexes Chelating phosphine ligands N-donors Electrochemiluminescence
a b s t r a c t The electrochemiluminescence (ECL) properties of five different yellow/orange emissive osmium(II) complexes of general formula Os(L2L’) with three different kinds of bidentate phosphine ligands L’ have been studied in acetonitrile/dioxane 1:1 solution by means of the triple-potential-step technique. The investigated complexes contain either two substituted pyrazolate or triazolate ligands L. The obtained ECL results have been interpreted in terms of the ligands nature, showing that the efficiency of ECL seems to be sensitive to the p-acidity of the phosphine ligand. To the authors’ best knowledge, the reported ECL efficiencies are the highest found for Os(II) complexes under similar conditions to date, that are at maximum five times higher than that of the famous RuðbipyÞ2þ 3 based ECL systems. Ó 2009 Elsevier B.V. All rights reserved.
The electrochemiluminescence (ECL) of transition metal complexes has found many applications in the last years [1]. The most widely used compounds are ruthenium(II) complexes with diimine chelating ligand. Ruthenium(II) forms stable red emitting compounds like the paradigmatic in ECL studies tris(2,2’bipirydyl)ruthenium(II) ion RuðbipyÞ2þ 3 , used also in organic light emitting diodes (OLEDs), imaging systems, bio-sensors and many other applications because of their relatively high ECL yield around 0.05 (defined as the amount of photons emitted per number of the þ annihilating RuðbipyÞ3þ 3 and RuðbipyÞ3 pairs). Therefore, a very active research area is committed with the search of new ECL-active luminophores. In fact, iridium based complexes have shown much higher efficiencies in a wide range of emission wavelengths [2–5]. Due to the many similarities between Ru(II) and Os(II) chelates the latter have been also investigated as ECL active compounds. The 2þ ions [6,7] clearly results obtained for OsðbipyÞ2þ 3 or OsðphenÞ3 2þ 3 or OsðphenÞ excited states are established that 3 OsðbipyÞ2þ 3 3 populated at ECL conditions but the obtained ECL efficiencies have been found to be distinctly smaller as compared to their Ru(II) counterparts, most likely due to their lower emission quantum yields in accordance with the free energy gap low (these osmium complexes emit more to the red than those with ruthenium) [8–10]. Much higher ECL intensities were observed for Os(II) complexes of Os(bipy)2L2+ or Os(phen)2L2+ type with supplementary bidentate phosphine or arsine ligands L like 1,2-bis(diphenylphosphino)ethane or 1,2-bis(diphenylarsino)ethane [6,7], ligands
* Corresponding author. Address: Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland. Tel.: +48 22 632 3221; fax: +48 22 632 3333. E-mail address: [email protected] (A. Kapturkiewicz). 1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2009.02.021
that shift the excited MLCT to higher energies. Unlike 2þ 2+ or Os(phen)2L2+ OsðbipyÞ2þ 3 and OsðphenÞ3 , many of Os(bipy)2L complexes show very intense photoluminescence with efficiencies two to three orders of magnitude higher than their parent derivatives. Consequently the observed ECL efficiencies obtained in electron transfer annihilation between the electrochemically generated Os(phen)2L+ and Os(phen)2L3+ ions were close to that found for RuðbipyÞ2þ 3 . ECL emission, much more intense than for 2þ chelates, was the previously studied OsðbipyÞ2þ 3 and OsðphenÞ3 also observed in the ECL system based on tris(2,20 -bipyrwith the emission from azine)osmium(II) OsðbprzÞ2þ 3 3 OsðbprzÞ2þ species populated in electron transfer between the 3 3þ ions [11]. electrochemically generated OsðbprzÞþ 3 and OsðbprzÞ3 ECL systems based on Os(II) complexes, despite some advantages like their relatively low oxidation potentials [12,13] (that make them more suitable for OLEDs because they better match the work function of the ITO anode), are pointedly less elaborated as compared to those based on Ru(II) derivatives. One can expect, however, that further development of the ligand(s) coordinating Os(II) kernel will result in new efficient ECL systems similarly as it has taken place for Ir(III) chelates [2–5]. In line with the reported efforts the synthesis of new complexes with phosphine chelating groups with very high emission yields and emission bands peaking around 600 nm, has been reported [12–14]. In this work we present results on the ECL properties (investigated by means of triple-potential step technique) of some of the compounds presented in ref. [12] (cf. Scheme 1). The complexes contain two hapto-nitrogen bidentate chelates, namely substituted (2-pyridyl)pyrazolate or substituted (2-pyridyl)triazolate anions, that are further referred as pyrazole and triazole containing ligands, respectively, and a third haptophosphine bidentate ligand
G. Angulo et al. / Inorganic Chemistry Communications 12 (2009) 378–381
379
Scheme 1. Structures of the investigated Os(L2L0 ) complexes.
of which three kinds have been studied: bis(diphenylphosphino)methane, cis-1,2-bis(diphenylphosphino)ethene, and 1,2bis(diphenyl-phosphino)benzene, that are abbreviated as dppm, dppee and dppb, respectively. ECL experiments, together with general electrochemical and photophysical characterisation of the investigated Os(II) complexes, have been performed in acetonitrile/dioxane ACN/DX – mixture (1:1 in volume) containing 0.1 M (n-C4H9)4NPF6 as supporting electrolyte. Experimental details are described in the Supporting Information. The redox potentials (referred to ferrocene/ferricenium Fc/Fc+ internal reference redox couple) obtained for the investigated complexes by means of cyclic voltammetry are collected in Table 1. All the complexes show reversible one-electron oxidation
OsðL2 L0 Þ e ¢ OsðL2 L0 Þþ
ð1Þ
located between 0.10 and 0.39 V, whereas stable reduction products have been observed (at potentials around 2.4 V) only for species containing CF3 substituted triazole OsTM and OsTE
OsðL2 L0 Þ þ e ¢ OsðL2 L0 Þ
ð2Þ
The reduction of other investigated complexes occurs at more negative potentials leading to distinctly less stable products (cf. Fig. 1). Both, reduction and oxidation potentials correlate with the nature of the N \ N chelates: the pyrazolate containing complexes OsPM and OsPE have oxidation potentials more negative than those with triazolate, OsTM, OsTE and OsTB, while the reduction is more positive for the latter group. Thus the reduction potentials correlate well with the electron withdrawing properties of these groups [12], as they are the primary electron acceptors in these complexes. The oxidation, on the other hand proceeds via the direct removal of a d electron from the Os(II) central ion. The stronger electron withdrawing character of the triazolate respect to the pyrazolate ligands, explains also the fact that the second set shows lower oxidation potentials. There is a further correlation that can be extracted from these data: within each group discussed before, those complexes containing dppm are easier to oxidize than those con-
Table 1 Electrochemical potentials (referred to Fc/Fc+) of ECL systems studied and the Gibbs free energy for the population of the excited state from the annihilation of the reduced and oxidized forms (estimated from DGes F(Ered Eox) + EMLCT relationship with EMLCT approximated as the MLCT emission maxima). The emission yield uem, the ECL yield uecl, and the excited state yield ues (as calculated from ues = uecl/uem equation). Data for 0.1 M n-(C4H9)4NPF6 ACN/DX (1:1) solutions. ECL system
Eox/V
Ered/V
DGes/eV
uem
uecl
ues
ECL single systems OsTM+/OsTM OsTE+/OsTE
+0.33 +0.39
2.44 2.39
0.72 0.70
0.28 0.35
0.11 0.25
0.39 0.71
ECL mixed systems OsTM+/CNN OsTE+/CNN OsPM+/CNN OsPE+/CNN OsTB+/CNN
+0.33 +0.39 +0.10 +0.17 +0.11
2.34 2.34 2.34 2.34 2.34
0.62 0.65 0.39 0.43 0.41
0.28 0.35 0.17 0.12 0.27
0.13 0.24 0.08 0.10 0.23
0.46 0.69 0.47 0.83 0.85
Fig. 1. Cyclic voltammograms of Os(L2L’) complexes recorded at Pt electrode in ACN/DX (1:1) solutions containing 0.1 M TBAPF6 as supporting electrolyte. Scan rate = 100 mV/s.
taining dppee or dppb. This may be due to the fact that the dppee and dppb ligands are stronger p-acids (thanks to double bond(s) connecting the P atoms) [15] and therefore interact stronger with Os(II) than with Os(III). All the complexes studied show ECL effect presenting identical spectra as recorded by photo-excitation, within the experimental error, with peaks around 600 nm (cf. Fig. 2). The fact that all compounds emit in the same range reflects that the extent of the shift in the redox potentials driven by the ligands, is similar in both the reduction and the oxidation processes. When possible, as it was in the case for OsTM and OsTE, we have performed the ECL experiment without added co-reactant (single ECL systems). Thus the neutral complex is first oxidized/reduced in the first step and then reduced/oxidized in the second, depending on the pulse sequence used, either +/ or /+, respectively. The ions annihilation reaction produces the triplet excited state of the complex and its ground state as the energy of the reaction, just the difference of reduction and oxidation potentials, is higher than the excited state triplet energy respect to the ground singlet state:
OsðL2 L0 Þþ þ OsðL2 L0 Þ ! 3 OsðL2 L0 Þ þ OsðL2 L0 Þ
ð3Þ
In these cases (OsTM and OsTE complexes) the yield of ECL reached 0.11 and 0.25, respectively (2 and 5 times the efficiency of RuðbipyÞ2þ 3 ). Correspondingly, ECL efficiencies of 0.13 (OsTM) and 0.24 (OsTE) have been found in the ECL experiments involving an auxiliary co-reactant, 1-cyanonaphthalene CNN, forming stable radical anion CNN (ECL mixed systems):
380
G. Angulo et al. / Inorganic Chemistry Communications 12 (2009) 378–381
Fig. 2. Emission spectra of Os(L2L’) complexes in ACN/DX (1:1) solutions.
OsðL2 L0 Þþ þ CNN ! 3 OsðL2 L0 Þ þ CNN
ð4Þ
where the ECL excitation proceeds via the annihilation between the cation of the given complex and the anion of the added co-reactant. To populate the excited triplet state of the complex 3*Os(L2L’) was possible because the CNN reduction potential is enough negative to provide the necessary amount of energy from the ions annihilation reaction. In the ECL experiments involving the mixed ECL system the cathodic limit potential was chosen enough negative to ensure CNN species production but enough positive to avoid electrochemical reduction of the investigated Os(II) complex. ECL processes of the other three complexes cases have been investigated only in the mixed systems because of the not enough high stability of their anionic forms Os(L2L’). OsPM led to 0.08 and OsPE to 0.10 whereas OsTB measured with the same co-reactant lead to an ECL efficiency of 0.23. As all obtained ECL yields have been found to be distinctly lower than the previously reported [12] emission quantum yields in CH2Cl2 solutions, we decided to measure them in the medium used for the ECL experiments. The obtained results are summarized in Table 2. The emission yields have been found to be lower than those previously reported in CH2Cl2 solutions. In order to check further the validity of these results, we measured as well the emission life-times sem of the complexes in ACN/DX, finding sem values between 1 and 2 microseconds. The radiative decay rates, as estimated from the emission yields and life-times, are close to 1.5 105 s1 for all the studied complexes, while their non-radia-
tive decay rates are of about 10 105 s1 for OsPM and OsPE and about 3 105 s1 for OsTM, OsTE and OsTB, revealing a quite coherent picture of the obtained photophysical results. The invariance of the radiative rate constant suggests that as all complexes emit in the same region, the matrix coupling element for the charge transfer (MLCT emission) is also almost invariant, as it is to be expected from the close nature of the different emitters. Due to the fact that the energy gaps between the states involved in the MLCT transition of all the complexes under study are nearly exactly the same, one can also expect that the non-radiative rate constants should be comparable, which is not exactly the case. It is likely that the non-radiative decay of the excited 3*MLCT state in the investigated complexes is partly governed by the thermally activated population of close lying metal-centred excited d–d dark states [9]. It can be thought that for the pyrazole ligand the 3*MLCT is closer in energy to these dark states, thence the faster non-radiative deactivation. In any case, as the phosphine chelating agents are high crystal field ligands, the Os(II) dd excited states are shifted to higher energies as compared to 3*MLCT states what results in relatively high emission yields of the complexes under study. Another reason would be a difference in the Franck–Condon factors for the different complexes. The scarcity of the data does not allow, however, preferring one of the above explanations over the others. With data for the ECL efficiencies uecl and for the emission yield uem, one can calculate the excited state population yield ues in the performed ECL experiments according to the uecl = uem ues relationship. As it can be seen in Table 1 compounds OsPE and OsTB exhibit the highest ues yields, correspondingly 0.83 and 0.85. OsTE shows a yield of 0.69, while OsPM and OsTM reached both ues about 0.46. In other words, the complexes with dppee or dppb have somewhat higher yields than those with dppm. The reason for this behaviour remains unclear and we will not further discuss this fact since it is very difficult to asses any conclusion based on the not too big differences found for the ues yields. In summary, the studied Os(II) complexes show ECL effect with quite high yields, all of them higher than the paradigmatic and than any other Os(II) complex so far reported in RuðbipyÞ2þ 3 the literature [9,16,17]. Besides the effects of the hapto-nitrogen ligands, one can conclude that among the phosphine ligands those with a higher intra-ligand conjugation may lead to higher excited state efficiencies in ECL processes. Further work seeking for other Os(II) complexes guided by the trends here outlined could lead to still more ECL-efficient compounds. One can expect that it could be possible to prepare similar Os(II) complexes for ECL application in aqueous solution and/or polymer matrices. This promising opportunity needs, however, further experimental work. Acknowledgements GA and YC thank for financial supports from European Commission and National Science Council of Taiwan, respectively. Appendix A. Supplementary material
Table 2 Photophysical characteristics of the investigated Os(L2L’) complexes in ACN/DX (1:1) oxygen-free solutions. Emission maxima kem, emission quantum yields uem, emission life-times sem, radiative krad and non-radiative deactivation knr constants (obtained from krad = uem/sem and knr = (1 uem)/sem relationships, respectively). 5
Complex
kem /nm
uem
sem/ls
krad/10 s
OsPM OsPE OsTM OsTE OsTB
605 605 600 595 605
0.17 0.12 0.28 0.35 0.27
1.01 0.80 1.92 2.35 1.84
1.7 1.5 1.5 1.4 1.5
1
5
knr/10 s 8.2 11.0 3.7 2.8 3.4
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2009.02.021. References
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