Photoinduced Isomerization and Luminescence of fac-[ Re ( CO ) 3 ( ph 2 phen ) ( bpe ) ]

O complexo fac-[Re(CO) 3 (ph 2 phen)(trans-bpe)]PF 6 , ph 2 phen = 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 CH 3 CN, determinados através da variação espectral, são Φ 313 nm = 0.19 ± 0.02, Φ 365 nm = 0.18 ± 0.04 e Φ 404 nm = 0.18 ± 0.02. Rendimentos quânticos maiores, denominados de reais (Φ 365nm = 0.40 ± 0.06), foram determinados utilizando espectroscopia de H 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 (ph 2 phen)(cis-bpe)], é atribuído a mudança do estado excitado de energia mais baixa de IL para MLCT. A emissão apresenta deslocamento hipsocrômico associado ao efeito rigidocrômico.


Introduction
Rhenium(I) polypyridyl complexes are well known for their interesting photochemical and photophysical properties.  The complexes, with an adequate 3 MLCT excited state and fairly long lifetimes (0.1-9.0 μs), can be used as photosensitizers for a variety of reactions including photoisomerization.
4][25][26][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. 2,27ne 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. 6,7,14,18,19,28oreover, 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 3 IL to 3 MLCT, 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.

Preparation of polymer films
The PMMA-based films were prepared in the absence of humidity and light following the procedure previously described. 11,21The fac-[Re(CO) 3 (ph 2 phen)(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 CD 3 CN as solvent.The residual CH 3 CN 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. 13,17The 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. 11,21The emission spectra were obtained using a 1.000 cm path length quartz cuvet and a front face arrangement for polymer films.

H NMR spectra
The 1 H NMR spectral data in CD 3 CN for the ligands and complexes, represented as follow, are listed in Table 1.
The α-diimine proton signals in fac-[ReCl(CO) 3 (ph 2 phen)] are shifted to high frequency in comparison to the corresponding proton of the free ph 2 phen.The same behavior has been reported for fac-[ReCl(CO) 3 (phen)] 11,13 and other complexes. 30,31After the coordination of transbpe 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 α-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.
This photochemical behavior is consistent with the IL lowest lying excited state.Time-resolved infrared (TRIR) measurements performed with fac-[Re(CO) 3 (phen)(transbpe)] + corroborate the nature of the lowest lying excited state being bpe localized ( 3 IL trans-bpe ), which is responsible for the trans→cis isomerization process. 16antum yields based on absorption spectral changes for fac-[Re(CO) 3 (ph 2 phen)(trans-bpe)] + are apparent, Φ app , since the reactant and the photoproduct absorb in the same region.In this case, the photoisomerization process can be better followed by 1 H 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 (ph 2 phen)(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 1 H NMR experiments with Φ 365nm = 0.40 ± 0.06. 22,36rans-cis photoisomerization of fac-[Re(CO) 3 (ph 2 phen)(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 reported 37 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 3 MLCT to the 3 IL excited state, which is responsible for the isomerization. 2,16,21he lower quantum yields for fac-[Re(CO) 3 (ph 2 phen)(trans-bpe)] + in comparison to fac-[Re(CO) 3 (phen)(trans-bpe)] + 21 are a result of a less efficient sensitization of the 3 IL bpe excited state due to the lower energy of the 3 MLCT which is stabilized by the two phenyl groups.Thus, the 3 MLCT Re→ph2phen decay manifold can compete with the 3 IL bpe lowest lying excited state in deactivation processes.
The emission spectra of fac-[Re(CO) 3 (ph 2 phen)(cisbpe)] + exhibit hypsochromic shift as the medium rigidity increases due to the 3 MLCT 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 3 MLCT excited state is strongly dependent on solvent organization due to its polar nature, the energy of the 3 IL state is insensitive due to its centro symmetric character. 17,21,39Thus, the observed shifts are due to the destabilization of the 3 MLCT Re→ph2phen excited state as the rigidity of the medium increases, as can be observed in the spectrum of fac-[Re(CO) 3 (ph 2 phen)(cis-bpe)] + in acetonitrile and in PMMA, Figure 6a.The emission spectrum of fac-[Re(CO) 3 (ph 2 phen)(cis-bpe)] + in EPA at 77 K resembles that of the free ph 2 phen ligand, Figure 6b, due to the contribution of the 3 IL ph2phen excited state to the 3 MLCT 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 3 IL bpe to 3 MLCT, resulting in an emissive cis-complex.Moreover, this luminescence is highly sensitive to changes in medium rigidity, by destabilizing the 3 MLCT excited state with the increase in rigidity.Such behavior can be conveniently exploited in the development of luminescent sensors.

Table 2 .
Spectral data for the rhenium(I) complexes and the free ligands in CH 3 CN a , 295(6.8),330(3.6)a,b a shoulder; b contributions of MLCT and IL transitions.

Table 1 .
1H NMR spectral data for the free ligands and the rhenium(I) complexes in CD 3 CN