Light driven isomerization of coordinated ligand andmodulation of fac-[Re(CO)3(phen)(trans-bpe)]PF6 photoluminescence in rigid media

The excited state reactivity and photophysical properties of fac-[Re(CO)3(phen)(L)]PF6 (phen = 1,10-phenanthroline, L = trans- or cis-1,2-bis(4-pyridyl)ethylene, bpe) in PMMA (poly(methyl methacrylate)) films have been investigated and compared with those in acetonitrile solution.

Luminescent sensors using Re(I) metal complexes are gaining much interest because of their high sensitivity and specificity. This is due to the possibility of modulating photochemical and luminescent properties of such complexes by modifying ligands and/or solvent environment which affect the excited state properties [9][10][11][12]. These characteristics are also observed and reported in polymer matrices [13,14], for example, in the development of luminescent sensors for oxygen [15].
In previous works of our group [16][17][18][19], the redshifted absorption of the coordinated trans-bpe in fac-[ Re(CO) 3 (phen)(trans-bpe) ] + was exploited to achieve photoassisted isomerization by irradiation in the low energy region (i.e. 365 nm), where the free ligand does not absorb. The dynamics of photoisomerization in fac-[Re(CO) 3 (phen)(trans-bpe)] + have also been investigated by transient absorption (TA) and timeresolved infrared (TRIR) spectroscopies. These measurements have provided direct evidence for a photochemical transient and detailed insight into mechanism for sensitized trans → cis isomerization of bpe in rhenium carbonyl complexes.
The present work extends our study on metal complex sensitized intraligand photochemistry by the investigation of fac-[Re(CO) 3 (phen)(trans-bpe)] + † E-mail: neydeiha@iq.usp.br in rigid media aiming the possible use of the photoisomerizable properties of the ligand in the design and development of photochemical molecular devices.

Preparation.
All solvents were reagent grade (Merck, Synth or Nuclear), except for those used in polymer films preparation, photophysical and photochemical measurements, in which HPLC grade solvents (Aldrich) were employed. Trifluoromethanesulfonic acid, CF 3 SO 3 H, ammonium hexafluorophosphate, NH 4 PF 6 and poly(methyl methacrylate), PMMA (Average M W = 25, 000), from Aldrich, were used as received.
Preparation of PMMA films with the complex should be carried out in absence of humidity to obtain a clean, homogeneous and transparent film. Also, the light exposure should be avoided during the preparation and the samples kept in dark to prevent the formation of the cis-bpe complex.

Methods.
Emission experiments were performed by using an ISS Model PC1 photon-counting spectrofluorometer, with a photomultiplier-based photon counting detector. The emission spectra of polymer films were obtained by using a front face arrangement for solid samples with the light focused at the same region where the photolyses had been carried out. Lowtemperature emission experiments were made in EPA (diethyl ether-isopentane-ethanol, 5:5:2) solutions in a quartz tube at 77 K.
The electronic absorption spectra were recorded on a Hewlett-Packard 8453 diode-array spectrophotometer.
The photolyses were carried out with an Oriel 200 W Hg(Xe) arc lamp Oriel. Irradiation at 313 or 365 nm was obtained by using an appropriate interference filter. The photochemical system and photolysis procedures for solution were described in detail elsewhere [22].
Photochemical experiments in polymer films were performed in parallel to those in acetonitrile solution. The photoreaction was followed for both having the same absorbance at the irradiation wavelength (A film = A 10 mm optical path solution ).
At the beginning of the irradiation (∼ 1-6 seconds), the isomerization reaction in PMMA films presents a higher photolysis percentage than that in fluid solution. After that, the cis-complex isomer was found to form in smaller quantities in PMMA than in acetonitrile solution.
According to the literature, this behavior is typical of the photoisomerization inhibition due to a relatively small free volume in a solid matrix compared with that in a fluid solution. Imai et al. reported smaller cis isomer ratios in the photochemical stationary state for azobenzene isomerization in organic-inorganic polymer hybrids [23]. In our case, this effect is also observed, but the apparent small conversion rate is also related to the effect of a decreasing local concentration of the fac-[Re(CO) 3 (phen)(trans-bpe)] + species as the photolysis time increase.
One can see, in Figure 2, that photolyses of fac-[Re(CO) 3 (phen)(trans-bpe)] + in PMMA polymer films result in a markedly enhanced luminescence as the cis isomer is formed. The spectrum displays a broad emission profile characteristic of a MLCT excited state [24][25][26].
The distinct emissive characteristic in both bpe isomer species is due to the different lowest lying excited states in these complexes. is the ππ * state localized on the cis-bpe ligand), which results in the growth of the emission. Photolyses of the free trans-bpe were performed at 313 nm in order to further analyze the process. Irradiation of the trans-bpe ligand in PMMA also leads to the trans → cis isomerization, Figure 3, with similar spectral changes to those described in fluid solutions for absorption and emission [17,19,[27][28][29]. Although the emission intensity is greatly enhanced as a function of photolysis time, Figure 3(b), its feature is distinct of the MLCT emission previously described for the cis-bpe complex. The emission properties for the complex fac-[Re(CO) 3 (phen)(cis-bpe)] + in PMMA, in acetonitrile solution at room temperature, and in EPA at 77 K [18,30] are summarized in  film, the emission maximum is ∼ 42 nm shifted toward higher energy region in comparison to that in acetonitrile solution. A similar shift, although more pronounced (70 nm), is also observed for the emission spectrum of the complex in EPA at 77 K. The observed shifts to higher energy are a consequence of changes in the rigidity of the polymer matrix/solvent medium, so called luminescence rigidochromism. This effect, observed by Wrighton and Morse in several Re(I) complexes at 77 K, has been assigned to a difference in solvent dipole orientations between glass and fluid solution [31]. This study showed that the photoisomerization of the fac-[Re(CO) 3 (phen)(trans-bpe)] + complex can be promoted even in rigid media. The enhanced luminescence of the cis photoproduct and the strong environmental sensitivity of this Re(I) complex are characteristics that can be conveniently exploited in luminescencebased sensors or photoswitches.