Photoinduced isomerization and luminescence of fac-[Re(CO)3(ph2phen)(bpe)]+

Abstracts

The fac-[Re(CO)3(ph2phen)(trans-bpe)]PF 6 complex, ph2phen = 4,7-diphenyl-1,10-phenanthroline and trans-bpe = trans-1,2-bis(4-pyridyl)ethylene, was synthesized, characterized and its photochemical and photophysical behavior was investigated. This complex exhibits trans->cis photoisomerization of the coordinated trans-bpe ligand. The apparent quantum yields in CH3CN, determined by absorption changes, are phi313 nm = 0.19 ± 0.02, phi365 nm = 0.18 ± 0.04 and phi404 nm = 0.18 ± 0.02. Higher true quantum yields (phi365 nm = 0.40 ± 0.06) were determined by ¹H NMR spectroscopy. The coordination of the trans-bpe ligand to the rhenium(I) polypyridyl complex enables a photosensitized isomerization under a lower energy region irradiation, where the free ligand does not absorb. The increasing luminescence as the photoproduct, fac-[Re(CO)3(ph2phen)(cis-bpe)]+, is formed is ascribed to the change of the lowest lying excited state from ³IL to ³MLCT. The emission exhibits a hypsochromic shift associated with the rigidochromic effect.

photoisomerization; rhenium(I) carbonyl complexes; trans-bpe; luminescence; rigidochromic effect

O complexo fac-[Re(CO)3(ph2phen)(trans-bpe)]PF 6, ph2phen = 4,7-difenil-1,10-fenantrolina e trans-bpe = trans-1,2-bis(4-piridil)etileno, foi sintetizado, caracterizado e seu comportamento fotoquímico e fotofísico foi investigado. Esse complexo apresenta isomerização trans->cis do ligante trans-bpe coordenado. Os rendimentos quânticos aparentes em CH3CN, determinados através da variação espectral, são fi313 nm = 0.19 ± 0.02, fi365 nm = 0.18 ± 0.04 e fi404 nm = 0.18 ± 0.02. Rendimentos quânticos maiores, denominados de reais (fi365nm = 0.40 ± 0.06), foram determinados utilizando espectroscopia de ¹H RMN. A coordenação do ligante trans-bpe ao complexo polipiridínico de rênio(I) possibilita uma isomerização fotossensibilizada sob irradiação a energias menores, onde o trans-bpe livre não absorve. O aumento da luminescência com a formação do fotoproduto, fac-[Re(CO)3(ph2phen)(cis-bpe)]+, é atribuído a mudança do estado excitado de energia mais baixa de ³IL para ³MLCT. A emissão apresenta deslocamento hipsocrômico associado ao efeito rigidocrômico.

ARTICLE

Photoinduced isomerization and luminescence of fac-[Re(CO)₃(ph₂phen)(bpe)]⁺

Karina M. Frin; Neyde Y. Murakami Iha^*

Instituto de Química, Universidade de São Paulo, Av. Prof. Lineu Prestes, 748, 05508-900 São Paulo - SP, Brazil

ABSTRACT

The fac-[Re(CO)₃(ph₂phen)(trans-bpe)]PF ₆ complex, ph₂phen = 4,7-diphenyl-1,10-phenanthroline and trans-bpe = trans-1,2-bis(4-pyridyl)ethylene, was synthesized, characterized and its photochemical and photophysical behavior was investigated. This complex exhibits trans®cis photoisomerization of the coordinated trans-bpe ligand. The apparent quantum yields in CH₃CN, determined by absorption changes, are F_{313 nm} = 0.19 ± 0.02, F_{365 nm} = 0.18 ± 0.04 and F_{404 nm} = 0.18 ± 0.02. Higher true quantum yields (F_{365 nm} = 0.40 ± 0.06) were determined by ¹H NMR spectroscopy. The coordination of the trans-bpe ligand to the rhenium(I) polypyridyl complex enables a photosensitized isomerization under a lower energy region irradiation, where the free ligand does not absorb. The increasing luminescence as the photoproduct, fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺, is formed is ascribed to the change of the lowest lying excited state from ³IL to ³MLCT. The emission exhibits a hypsochromic shift associated with the rigidochromic effect.

Keywords: photoisomerization, rhenium(I) carbonyl complexes, trans-bpe, luminescence, rigidochromic effect

RESUMO

O complexo fac-[Re(CO)₃(ph₂phen)(trans-bpe)]PF ₆, ph₂phen = 4,7-difenil-1,10-fenantrolina e trans-bpe = trans-1,2-bis(4-piridil)etileno, foi sintetizado, caracterizado e seu comportamento fotoquímico e fotofísico foi investigado. Esse complexo apresenta isomerização trans®cis do ligante trans-bpe coordenado. Os rendimentos quânticos aparentes em CH₃CN, determinados através da variação espectral, são F_{313 nm} = 0.19 ± 0.02, F_{365 nm} = 0.18 ± 0.04 e F_{404 nm} = 0.18 ± 0.02. Rendimentos quânticos maiores, denominados de reais (F_365nm = 0.40 ± 0.06), foram determinados utilizando espectroscopia de ¹H RMN. A coordenação do ligante trans-bpe ao complexo polipiridínico de rênio(I) possibilita uma isomerização fotossensibilizada sob irradiação a energias menores, onde o trans-bpe livre não absorve. O aumento da luminescência com a formação do fotoproduto, fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺, é atribuído a mudança do estado excitado de energia mais baixa de ³IL para ³MLCT. A emissão apresenta deslocamento hipsocrômico associado ao efeito rigidocrômico.

Introduction

Rhenium(I) polypyridyl complexes are well known for their interesting photochemical and photophysical properties.^1-22 These complexes, with an adequate ³MLCT excited state and fairly long lifetimes (0.1–9.0 µs), can be used as photosensitizers for a variety of reactions including photoisomerization.

A well known model of photoisomerizable molecules are the stilbene-like ligands.^2,23-27 They provide a classic example of one of the outstanding advantages of photochemistry – the possibility of selective preparation of a less stable higher energy isomer. Photoinduced isomerization of these molecules can be achieved by direct irradiation or by sensitization via inter- or intramolecular energy transfer processes.^2,27

One interesting intramolecular sensitization approach is the coordination of a stilbene-like ligands to a metal center such as rhenium(I). These complexes are especially attractive over their free organic counterparts because of the additional advantage of their MLCT state tunability. This feature allows the use of visible light in the photoisomerization process, which can be conveniently exploited in the development of molecular devices, such as photoswitches.^{6,7,14,18,19,28}

Moreover, the photoisomerization process of rhenium(I) polypyridyl complexes with a photoisomerizable stilbene-like ligand results in a change of the lowest excited state nature, from ³IL to ³MLCT, leading to the formation of emissive complexes. Their emission profiles can be tuned by changing the polypyridine ligand as well as by modifying the rigidity of the medium, an important feature for sensing applications.

In our previous works^{10-13,16,17,21,22} the photoisomerization of trans-L in fac-[Re(CO)₃(NN)(trans-L)]⁺, NN = polypyridynes and L = 1,2-bis(4-pyridyl)ethylene (bpe), or 4-styrylpyridine (stpy), and the luminescence of their photoproducts were investigated. Here, we extend our investigations by using 4,7-diphenyl-1,10-phenanthroline (ph₂phen) as a polypyridyl ligand in the photoisomerization of trans-bpe in fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺ in order to observe the tunability of the photochemical and photophysical behaviors of trans- and cis-isomer complexes.

Experimental

All solvents were reagent grade, except those for film preparation and photochemical and photophysical measurements, when HPLC grade solvents from Aldrich were employed. [ReCl(CO)₅] from Strem and ph₂phen, trifluoromethanesulfonic acid (CF₃SO₃H), trans-1,2-bis(4-pyridyl)ethylene (trans-bpe) and poly(methyl methacrylate) (PMMA: M_w = 101,000) from Aldrich were used as received. Potassium tris(oxalato)ferrate(III) was prepared and purified according to the literature procedure²⁹ from reagent grade potassium oxalate (Merck) and iron(III) chloride hexahydrate (Reagen).

Synthesis of fac-[ReCl(CO)₃(ph₂phen)]

The fac-[ReCl(CO)₃(ph₂phen)] complex was synthesized according to the procedure previously described for fac-[ReCl(CO)₃(phen)].^13,21 The [ReCl(CO)₅] complex (0.27 g, 0.75 mmol) and an excess of ph₂ phen (0.5 g, 1.5 mmol) were suspended in 30 mL of xylene (Nuclear) and heated to reflux for 6 hours. After cooling, the resulting solid was separated by filtration and washed with xylene. The crude product was recrystallized from dichloromethane (Merck) by addition of n-pentane (Nuclear). Yield 69%. Anal. Calc. for ReC₂₇H₁₆O₃N₂ Cl: C, 50.82; H, 2.53; N, 4.39%. Found: C, 51.11; H, 2.47; N, 4.42%.¹H NMR for fac-[ReCl(CO)₃(ph₂ phen)] d(CD₃CN): 9.45 (d, 2H), 8.11 (s, 2H), 7.93 (d, 2H), 7.65 (s, 10H).

Synthesis of fac-[Re(CF₃SO₃)(CO)₃ (ph₂phen)]

The fac-[Re(CF₃SO₃)(CO)₃(ph ₂phen)] complex was synthesized following the procedure previously described.^11,13,21 CF₃SO₃H (0.50 mL, 5.6 mmol) was added to a 35 mL dichloromethane suspension of fac-[ReCl(CO)₃(ph₂ phen)] (0.30 g, 0.47 mmol) under argon. The complex was precipitated by slow addition of diethyl ether. Yield 79%. Anal. Calc. for ReC₂₈H₁₆ N₂O₆F₃S: C, 39.11; H, 2.18; N, 3.04%. Found: C, 39.21; H, 2.45; N, 3.22%. ¹H NMR for fac-[Re(CF₃SO₃) (CO)₃(ph₂phen)] d(CD₃CN): 9.46 (d, 2H), 8.16 (s, 2H), 8.01 (d, 2H), 7.68 (s, 10H).

Synthesis of fac-[Re(CO)₃(ph₂phen)(trans-bpe)]PF₆

The fac-[Re(CO)₃(ph₂phen)(trans-bpe)]PF ₆ complex was prepared using the procedure previously described^11,13,17 with slight modifications. The fac-[Re(CF₃SO₃)(CO)₃(ph ₂phen)] complex (0.20 g, 0.27 mmol) and trans-bpe (0.35 g, 1.9 mmol) were dissolved in 45 mL of methanol (Synth) and heated to reflux for 9 hours. Solid NH₄PF₆ was added to a room temperature solution to precipitate the yellow complex, which was subsequently collected by filtration and washed with water to eliminate excess of NH₄PF₆ and then with diethyl ether. Yield 50%. Anal. Calc. for ReC₃₉ H₂₆N₄O₃F₆ P: C, 50.37; H, 2.82; N, 6.03%. Found: C, 50.07; H, 2.77; N, 5.96%. ¹H NMR for fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺d(CD₃CN): 9.64 (d, 2H), 8.55 (d, 2H), 8.31 (d, 2H), 8.08 (s, 2H), 8.06 (d, 2H), 7.63 (s, 10H), 7.40 (d, 2H), 7.36 (d, 2H), 7.34 (d, 1H), 7.23 (d, 1H).

Preparation of polymer films

The PMMA-based films were prepared in the absence of humidity and light following the procedure previously described.^11,21 The fac-[Re(CO)₃(ph₂phen)(trans-bpe)]PF ₆ complex was dissolved in acetonitrile, added to an acetonitrile solution of PMMA and left to dry.

Methods

Electronic absorption spectra were recorded on a Hewlett Packard 8453 spectrophotometer with quartz cuvets of 1.000 or 0.100 cm optical length.

NMR spectra were recorded on a Bruker AC-200 (200 MHz) or a DPX-300 (300 MHz) spectrometer at 300 K using CD₃CN as solvent. The residual CH₃CN signals were employed as internal standard.

Photolyses at 313, 365 or 404 nm were carried out with an Oriel 200 W Hg(Xe) arc lamp powered by an Oriel universal power supply model 68700 by selecting the wavelength using appropriate interference filters. Light intensities were determined by tris(oxalato)ferrato(III) actinometry before and after each photolysis experiment. The photochemical system and photolysis procedure have been described in detail elsewhere.^13,17 The irradiations were performed in a 1.000 cm optical length quartz cuvet connected to another 0.100 cm optical length quartz cuvet for measurements of the absorption spectra.

Emission experiments were performed by using an ISS photon counting spectrofluorometer, model PC1, with a photomultiplier-based photon counting detector. The photophysical system and procedures have been described in detail elsewhere.^11,21 The emission spectra were obtained using a 1.000 cm path length quartz cuvet and a front face arrangement for polymer films.

Results and Discussion

¹H NMR spectra

The ¹H NMR spectral data in CD₃CN for the ligands and complexes, represented as follow, are listed in Table 1.

The signals of aromatic protons for fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺ were assigned by comparison of ¹H NMR spectra of the free ligands and fac-[ReCl(CO)₃(ph₂phen)]. The H-H COSY spectrum of fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺ (see Figure 1) corroborated the assignments.

The a-diimine proton signals in fac-[ReCl(CO)₃ (ph₂phen)] are shifted to high frequency in comparison to the corresponding proton of the free ph₂phen. The same behavior has been reported for fac-[ReCl(CO)₃(phen)]^11,13 and other complexes.^30,31 After the coordination of trans-bpe to the metal center, the proton signals of the bound pyridyl ring are shifted to low frequency in comparison to its unbound pyridyl ring, leading to a higher electronic perturbation in Ha', Hb' and Hc' signals due to the anisotropic effect of the a-diimine ligand.^13,17,32

Electronic absorption spectra

The spectral data of the rhenium(I) polypyridyl complexes and of the free ligands in acetonitrile solution are summarized in Table 2. The absorption spectra in acetonitrile are shown in Figure 2.

The higher energy bands of fac-[ReCl(CO)₃(ph₂phen)] can be associated with electronic transitions localized mainly in the ligand and assigned to ph₂phen intraligand transitions (IL, p®p*). The lower energy absorption bands (300-450 nm) are ascribed to a metal to ligand charge transfer transition (MLCT, dp_Re®p*_ph2phen). Such spectral characteristics are typical of fac-tricarbonyl diimine compounds and the assignments were made in comparison to the literature.^{1,3,11,13,17,21,22,33,34}

The absorption spectrum of trans-bpe (l_max = 290 nm) shows a characteristic intraligand transition (IL) band which is red shifted (l_max = 312 nm) upon protonation. A similar shift is observed upon coordination of trans-bpe to the rhenium tricarbonyl polypyridyl complexes. Molar absorptivity of 10⁴ L mol^-1 cm^-1 in the 300-365 nm region for the fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺ complex is much higher than the 10³ L mol^-1 cm^-1 commonly observed for rhenium diimine MLCT transitions.^34,35 The higher molar absorptivity for fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺ and the presence of a similar low energy absorption band in protonated free trans-bpe indicate a contribution of the intraligand transition of the coordinated trans-bpe. Therefore, the absorption bands in the 300-365 nm region for fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺ have the contributions of the intraligand (IL_bpe) p®p*_bpe and MLCT transitions, Figure 2.^{6,7,11-14,17,18,20} The tail extending to ca. 425 nm for fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺ is ascribed to a metal to ligand charge transfer transition (MLCT, dp_Re®p*_ph2phen) based on comparison with the spectra of fac-[ReCl(CO)₃ (ph₂phen)].

Photoinduced isomerization

Irradiation of trans-bpe at 313 nm can be followed by changes in absorption spectra, which show clear and well-defined isosbestic points and are in accordance with those reported in the literature.^13,23-25,27

The trans®cis isomerization of the coordinated trans-bpe ligand can be followed by absorption changes observed for fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺, Figure 3, upon 313, 365 or 404 nm irradiation.

The quantum yields, determined by absorption spectral changes, are 0.19 ± 0.02, 0.18 ± 0.04 or 0.18 ± 0.02, respectively for 313, 365 or 404 nm irradiation and constant within experimental error.

This photochemical behavior is consistent with the IL lowest lying excited state. Time-resolved infrared (TRIR) measurements performed with fac-[Re(CO)₃(phen)(trans-bpe)]⁺ corroborate the nature of the lowest lying excited state being bpe localized (³IL_trans-bpe ), which is responsible for the trans®cis isomerization process.¹⁶

Quantum yields based on absorption spectral changes for fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺ are apparent, F_app, since the reactant and the photoproduct absorb in the same region. In this case, the photoisomerization process can be better followed by ¹H NMR spectroscopy. For both isomers, chemical shifts and coupling constants of the protons, specially for the olefinic ones, are fairly different. Upon irradiation of fac-[Re(CO)₃(ph₂phen)(trans-bpe)] ⁺, the trans-isomer signals at 7.50 - 7.20 ppm decrease, while the cis-isomer signals at 7.00 - 6.50 ppm gradually build up in intensity, as can be observed in Figure 4. The quantum yields are determined using the integral of the olefinic proton signals of the cis-isomer. In this way, much higher quantum yields, the true ones, are determined by ¹H NMR experiments with F_365nm = 0.40 ± 0.06.^22,36

Trans-cis photoisomerization of fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺ can be achieved even under 404 nm irradiation due to the efficient sensitization of the stilbene-like ligand via intramolecular energy transfer from the MLCT to the IL_bpe excited state.

It has been reported³⁷ that the singlet and triplet mechanisms are always present in the photoisomerization process of stilbene-like compounds. However, a functional group increases the coupling between the singlet and triplet manifolds, and the triplet mechanism becomes more competitive. For the rhenium(I) complexes, the metal centers enhance the intersystem crossing, either through the heavy atom effect or as a functional group, favoring the triplet mechanism by transferring the energy from the ³MLCT to the ³IL excited state, which is responsible for the isomerization.^2,16,21

The lower quantum yields for fac-[Re(CO)₃ (ph₂phen)(trans-bpe)]⁺ in comparison to fac-[Re(CO)₃ (phen)(trans-bpe)]⁺²¹ are a result of a less efficient sensitization of the ³IL_bpe excited state due to the lower energy of the ³MLCT which is stabilized by the two phenyl groups. Thus, the ³MLCT_Re®ph2phen decay manifold can compete with the ³IL_bpe lowest lying excited state in deactivation processes.

The photoisomerization process can also be followed by the increasing luminescence as the photoproduct, fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺, is formed upon irradiation of the trans-complex, Figure 5. The broad and structureless band in acetonitrile solution at room temperature is typically a ³MLCT emission of rhenium(I) compounds.^{1,3,5,8,11,13,17,21,33,34,38,39}

In order to understand the nature of the emissive state, the investigation was also carried out in different media, e.g., PMMA at 298 K and EPA (diethyl ether-isopentane-ethanol, 5:5:2) at 77 K, Figure 6.

The emission spectra of fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺ exhibit hypsochromic shift as the medium rigidity increases due to the ³MLCT destabilization. This behavior has already been reported for other rhenium polypyridyl complexes^{1,5,8,11,17,21,22,39} and is known as the rigidochromic effect. This effect arises from variations in the dipolar interactions between the excited molecule and the solvent dipoles of the surrounding medium. While the ³MLCT excited state is strongly dependent on solvent organization due to its polar nature, the energy of the ³IL state is insensitive due to its centro symmetric character.^17,21,39 Thus, the observed shifts are due to the destabilization of the ³MLCT_Re®ph2phen excited state as the rigidity of the medium increases, as can be observed in the spectrum of fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺ in acetonitrile and in PMMA, Figure 6a. The emission spectrum of fac-[Re(CO)₃(ph₂phen)(cis-bpe)] ⁺ in EPA at 77 K resembles that of the free ph₂phen ligand, Figure 6b, due to the contribution of the ³IL_ph2phen excited state to the ³MLCT emission.

Conclusion

The coordination of the trans-bpe unit to the rhenium(I) tricarbonyl polypyridyl complex is an interesting approach to photosensitize an isomerization to visible light, e.g. at 404 nm, where the free ligand does not absorb. This feature can be exploited in designing photoresponsive species capable of performing light induced functions, which are useful in the development of photochemical molecular devices.

The photoisomerization of coordinated bpe leads to a change of the lowest excited state nature, from ³IL_bpe to ³MLCT, resulting in an emissive cis-complex. Moreover, this luminescence is highly sensitive to changes in medium rigidity, by destabilizing the ³MLCT excited state with the increase in rigidity. Such behavior can be conveniently exploited in the development of luminescent sensors.

Acknowledgments

We are thankful to the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

36. Frin, K. M.; Itokazu, M. K.; Murakami Iha, N. Y.; unpublished results.

Received: June 12, 2006

Published on the web: December 6, 2006

FAPESP helped in meeting the publication costs of this article.

• 1. Wrighton, M.; Morse, D. L.; J. Am. Chem. Soc. 1974, 96, 998.
• 2. Wrighton, M. S.; Morse, D. L.; Pdungsap, L.; J. Am. Chem. Soc. 1975, 97, 2073.
• 3. Giordano, P. J.; Wrighton, M. S.; J. Am. Chem. Soc. 1979, 101, 2888.
• 4. Lees, A. J.; Chem. Rev. 1987, 87, 711.
• 5. Stufkens, D. J.; Comments Inorg. Chem. 1992, 13, 359.
• 6. Yam, V. W. W. ; Lau, V. C. Y.; Cheung, K. K.; J. Chem. Soc., Chem. Commun. 1995, 259.
• 7. Yam, V. W. W.; Lau, V. C. Y.; Wu, L. X.; J. Chem. Soc., Dalton Trans. 1998, 1461.
• 8. Lees, A. J.; Coord. Chem. Rev. 1998,177, 3.
• 9. Stufkens, D. J.; Vlcek Jr., A.; Coord. Chem. Rev. 1998, 177, 127.
• 10. Garcia, C. G.; Lima, J. F.; Murakami Iha, N. Y.; Coord. Chem. Rev. 2000, 196, 219.
• 11. Itokazu, M. K.; Polo, A. S.; Murakami Iha, N. Y.; Int. J. Photoenergy 2001, 3, 143.
• 12. Argazzi, R. A.; Bertolasi, E.; Chiorboli, C.; Bignozzi, C. A.; Itokazu, M. K.; Murakami Iha, N. Y.; Inorg. Chem. 2001, 40, 6885.
• 13. Itokazu, M. K.; Polo, A. S.; Araújo de Faria, D. L.; Bignozzi, C. A.; Murakami Iha, N. Y.; Inorg. Chim. Acta 2001, 313, 6885.
• 14. Yam, V. W. W.; Lau, V. C. Y.; Zhang, J.; Chu, B. W. K.; Zhu, N.; Organometallics 2001, 20, 4911.
• 15. Sun, S. S.; Lees, A. J.; Coord. Chem. Rev. 2002, 230, 171.
• 16. Dattelbaum, D. M.; Itokazu, M. K.; Murakami Iha, N. Y.; Meyer, T.J.; J. Phys. Chem. A. 2003, 107, 4092.
• 17. Itokazu, M. K.; Polo, A. S.; Murakami Iha, N. Y.; J. Photochem. Photobiol. A. 2003, 160, 27.
• 18. Wenger, O. S.; Henling, L. M.; Day, M. W.; Winkler, J. R.; Gray, H. B.; Inorg. Chem. 2004, 43, 2043.
• 19. Wenger, O. S.; Henling, L. M.; Day, M. W.; Winkler, J. R.; Gray, H. B.; Polyhedron 2004, 23, 2955.
• 20. Busby, M.; Matousek, P.; Towrie, M.; Vlcek Jr., A.; J. Phys. Chem. A. 2005, 109, 3000.
• 21. Polo, A. S.; Itokazu, M. K.; Murakami Iha, N. Y.; J. Photochem. Photobiol. A. 2006, 181, 73.
• 22. Polo, A. S.; Itokazu, M. K.; Frin, K. M.; de Patrocínio, A. O. T.; Murakami Iha, N. Y.; Coord. Chem. Rev. 2006, 250, 1669.
• 23. Whitten, D. G.; McCall, M. T.; J. Am. Chem. Soc. 1969, 91, 5097.
• 24. Whitten, D. G.; Lee, Y. J.; J. Am. Chem. Soc. 1970, 92, 415.
• 25. Lee, Y. J.; Whitten, D. G.; Pedersen, L.; J. Am. Chem. Soc. 1971, 93, 6330.
• 26. Bartocci, G.; Bortolus, P.; Mazzucato, U.; J. Phys. Chem. 1973, 77, 605.
• 27. Görner, H.; Kuhn, H. J.; Advances in Photochemistry, John Wiley & Sons: New York,1995, vol.19.
• 28. Sans, M. Q.; Belser, P.; Coord. Chem. Rev. 2002, 229, 59.
• 29. Hatchard, C. G.; Parker, C. A.; Proc. R. Soc. Lond. A 1958, 235, 518.
• 30. Sarneski, J. E.; Rickson, L. E.; Reilley, C. N.; Inorg. Chem. 1981, 20, 2137.
• 31. Kamath, S. S.; Uma, V.; Srivastava, S.; Inorg. Chim. Acta 1989, 161, 49.
• 32. Orrell, K. G.; Osborne, A. G.; Silva, M. W. ; Hursthouse, M. B.; Coles, S. J.; Polyhedron 1997, 16, 3003.
• 33. Fredericks, S. M.; Luong, J. C.; Wrighton, M. S.; J. Am. Chem. Soc. 1979, 101, 7415.
• 34. Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A.; Inorg. Chem. 1990, 29, 4335.
• 35. Shaw, J. R.; Schmehl, R. H.; J. Am. Chem. Soc. 1991, 113, 389.
• 37. Waldeck, D. H.; Chem. Rev. 1991, 91, 415.
• 38. Leasure, R. M.; Sacksteder, L.; Nesselrodt, D.; Reitz, G. A.; Demas, J. N.; DeGraff, B.A.; Inorg. Chem. 1991, 30, 3722.
• 39. Chen, P.; Meyer, T. J.; Chem. Rev. 1998, 98, 1439.

Publication Dates

• Publication in this collection
  07 Feb 2007
• Date of issue
  Dec 2006

History

• Accepted
  06 Dec 2006
• Received
  12 June 2006