On-line version ISSN 1678-4790
J. Braz. Chem. Soc. vol.17 no.8 São Paulo Nov./Dec. 2006
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
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 F313 nm = 0.19 ± 0.02, F365 nm = 0.18 ± 0.04 and F404 nm = 0.18 ± 0.02. Higher true quantum yields (F365 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
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 F313 nm = 0.19 ± 0.02, F365 nm = 0.18 ± 0.04 e F404 nm = 0.18 ± 0.02. Rendimentos quânticos maiores, denominados de reais (F365nm = 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.
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 (0.19.0 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 works10-13,16,17,21,22 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.
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: Mw = 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 described11,13,17 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.
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 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, dpRe®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 (lmax = 290 nm) shows a characteristic intraligand transition (IL) band which is red shifted (lmax = 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 (ILbpe) 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, dpRe®p*ph2phen) based on comparison with the spectra of fac-[ReCl(CO)3 (ph2phen)].
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 (3ILtrans-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, Fapp, 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 F365nm = 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 ILbpe 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 3ILbpe excited state due to the lower energy of the 3MLCT which is stabilized by the two phenyl groups. Thus, the 3MLCTRe®ph2phen decay manifold can compete with the 3ILbpe 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 complexes1,5,8,11,17,21,22,39 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 3MLCTRe®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 3ILph2phen excited state to the 3MLCT emission.
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 3ILbpe 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.
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.
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Received: June 12, 2006
Published on the web: December 6, 2006
FAPESP helped in meeting the publication costs of this article.