Photosensitive properties of [Ru(bpy)2(OSO)] ∙ PF6 dissolved in propylene carbonate originating from photoinduced linkage isomerism have been studied by temperature and exposure dependent transmission and UV/Vis absorption spectroscopy. An exceeding photochromic photosensitivity of S = (63 ± 10) × 103 cm l J−1 mol−1 is determined. It is attributed to a maximum population of 100% molecules in the photoinduced isomers, a unique absorption cross section per molecule and a very low light exposure of Q 0 = (0.25 ± 0.03) Ws cm−2 for isomerism. Relaxation studies of O-bonded to S-bonded isomers at different temperatures verify the existence of two distinct structures of linkage isomers determined by the activation energies of E A,1 = (0.76 ± 0.08) eV and E A,2 = (1.00 ± 0.14) eV.
© 2009 OSA
Photoswitchable molecules in crystalline or amorphous environments [1,2] are well suited for nonlinear optical applications such as optical molecular information storage [3–5]. It was recently demonstrated that the nonlinear optical properties of these molecules could be maintained in aqueous solutions . This observation indicates that a broader range of applications can be envisioned including the development of optofluidic devices . Molecules for these devices require three characteristic features of the light activated step: 1) a fast, and 2) efficient switching mechanism from ground state to metastable state, termed high photosensitivity as well as 3) an adequately long life time of the metastable state. This last feature is determined by the particular application.
We and others have found these characteristics in a class of nitrosyl compounds of which the parent, sodium nitroprusside, Na2[Fe(CN)5NO] ∙ 2H2O (SNP), is well known. Both the central metal atom (Fe) and the ligand (CN–) may be substituted to provide new compounds with designer properties: Populations of up to 76% of the metastable state have been reported  and lifetimes of the metastable state ranging from microseconds to seconds have been observed [6,9]. However, many of these compounds suffer from a poor photosensitivity due to the low absorption cross sections exhibited by this class.
Recently, photochromic ruthenium sulfoxide complexes featuring excited state linkage isomerization, similar to nitrosyl compounds, have been investigated . The photoinduced structural alteration from the S-bonded ground states to the O-bonded metastable states is accompanied by pronounced changes of the absorption spectra.
In this work, we present our first results from [Ru(bpy)2(OSO)] ∙ PF6, where bpy is 2,2’-bipyridine and OSO is 2-methylsulfinylbenzoate, a representative of this new class of compounds. Here, the linkage isomerization results in two slightly different O-bonded states, which thermally relax back to the ground state . A remarkable photochromic response in combination with 1) a strong characteristic exposure of Q 0 = (0.25 ± 0.03) Ws cm−2, 2) a switching efficiency of 100% and 3) long life times of the metastable states of τ 1 ≈1.6 × 103 s and τ 2 ≈3.5 × 104 s, reveal that photoswitchable ruthenium sulfoxide compounds may be a useful and valuable class of compounds for nonlinear optical applications.
2. Experimental details
The [Ru(bpy)2(OSO)] ∙ PF6 powder (OSO = 2-methylsulfinylbenzoate and bpy = 2,2'-bipyridine)  is dissolved in propylene carbonate (Aldrich) (c = 0.88 mmol l−1). All studies were performed using a cell of quartz glass (10 × 2 mm2) with four polished optical windows. The UV/Vis absorption spectra were taken with a Shimadzu UV-3600 spectrometer with a cell filled with pure propylene carbonate in the reference beam. Optical excitation of the sample was performed outside the spectrometer using the setup described below.
For measuring the photoinduced absorption changes during the optical excitation and the following thermal decay of the metastable states, a continuous wave pump-probe setup was designed: The cell is homogeneously illuminated by a pump beam of wavelength λ = 405 nm (Coherent Cube 405) and intensity I pump = 2.4 mW cm−2 for a duration of 10 minutes at a temperature of 45 °C yielding an exposure of Q pump = I pump ∙ t = 1.4 Ws cm−2. Afterwards the sample is heated to specific temperatures (45 °C – 110 °C) for the thermal decay measurements. During this procedure, the transmission change is detected by a probe beam of 532 nm (Coherent 315M) with I probe = 0.06 mW cm−2.
3. Experimental results
The absorption spectrum of the unexposed sample (Fig. 1 , black curve) primarily shows two broad absorption bands at λ = 285 nm and λ = 400 nm. Because of the strength of the former one, the [Ru(bpy)2(OSO)]+ concentration of the solution had to be limited to c = 0.20 mmol l−1 for this measurement. During exposure to light of λ = 405 nm strong photoinduced absorption changes appear in the spectrum and yield a saturation with exposure Q pump. This absorption spectrum is shown exemplarily in Fig. 1 (blue curve) at room temperature for Q pump = 1.4 Ws cm−2. An increase and slight shift of the absorption band in the UV (λ = 285 nm) is observed. In the visible range of the spectrum two pronounced bands rise at λ = 352 nm and λ = 500 nm, whereas the absorption at λ = 400 nm is reduced, which is in accordance with Ref . We refer to the photoinduced isomers resulting in this changed absorption spectrum as metastable states (MS1,2), because the characteristic time for the relaxation back to the ground state (GS) spectrum is comparable long regarding the measurement period. The red curve depicts an absorption spectrum taken after 20 minutes of dark relaxation at room temperature from MS back to GS. The transfer reveals three isosbestic points at wavelength of 325 nm, 384 nm and 442 nm.
For characterization of photoinduced absorption changes in the metastable states, the spectral range of 270 nm to 700 nm is fitted by Gaussians to model the spectrum before (GS) and after optical excitation (MS1,2). Gaussians are used due to the thermal line broadening of the absorption bands at room temperature and in order to reduce the parameter set. The region below 270 nm shows only slight shifts and amplitude changes of the existing bands and is described with Voigt profiles. The absorption spectra and the modeled Gaussians as a function of wavenumber are shown in Fig. 2 for the visible spectral range of ground and metastable states. Comparing both spectra, one can see that the absorption band at ν = 25000 cm−1, which is prominent in the ground state (Fig. 2(a)), is completely absent after excitation (Fig. 2(b)), whereas the absorption bands, that are weak in the ground state (ν = 19900 cm−1, ν = 21500 cm−1, ν = 28600 cm−1), become stronger and undergo slight shifts in their center wavenumber and width.
Absorption spectra of the metastable states have been recorded repeatedly as a function of time at a fixed temperature, i.e. during the decay of MS1,2 back to the ground state. The analysis of the individual spectra revealed, that both relative strength and position of the bands identified with MS1,2 are not altered with the decay; no shifts of isosbestic points were determined. Thus, we restricted the further study of the decay kinetics to the probing wavelength of λ probe = 532 nm. Here, a pronounced change of the absorption coefficient from α GS = 0.23 cm−1 to α MS = 2.10 cm−1 as a function of exposure can be observed for a concentration of c = 0.20 mmol l−1.
For the characterization of the population kinetics from the ground to the metastable states, the transmitted light intensity I(Q) is converted to a relative absorption change Δα(Q) using Lambert-Beer's lawFigure 3 shows the absorption change as a function of the exposure Q exemplarily for the temperature T = 45 °C. The kinetic trace of Δα(Q) can be described by an exponential function with a characteristic population constant Q 0 = (0.25 ± 0.03) Ws cm−2 and a total change of the absorption of Δαsat = 8.22 cm−1 (c = 0.88 mmol l−1).
After reaching saturation in the relative absorption change during the pump phase, i.e. after reaching the maximum population of the metastable states MS1 and MS2, the thermal decay is studied. The absorption change at 532 nm detected at temperatures in the range of 45 °C to 110 °C is identified with the relaxation of the metastable states. Its kinetics is shown in Fig. 4 exemplarily for a temperature of 64 °C: The absorption change was normalized to its saturation value Δαsat and its logarithm is plotted linearly in time. The kinetics show a biexponential decay which is assigned to the two metastable states. The two decay constants (τ 1 and τ 2) are illustrated with linear functions (solid lines I, II) in the logarithmic plot; the data set with the result of the fitting procedure is shown in the inset within a linear plot. On long time scales a deviation from the description via the sum of two exponential decays is observed (III). This regime shifts towards lower time scales at higher temperatures and can be assigned to a possible thermal degradation of the molecule.
Influences of the probing light intensity on the relaxation times τ 1,2 could not be detected even by an intensity increase I probe by a factor of 20.
These temperature dependent measurements of the decay constants allow for the determination of the activation energy for each metastable state MS1,2 (see Fig. 5 ) using Arrhenius' law
E A,1 = (0.76 ± 0.08) eV and E A,2 = (1.00 ± 0.14) eV. The corresponding frequency factors are Z 1 = (3.68 ± 0.85) × 109 s−1 and Z 2 = (1.79 ± 0.61) × 1012 s−1. Here we note that an experimental error of the frequency factor in the order of one order of magnitude has to be considered taking the limited temperature interval for the Arrhenius plot into account. At temperatures higher than 90 °C both decay times become nearly independent of the temperature, which we assign to the degradation of the molecule. These data points were therefore neglected in this analysis.
For the determination of the photochromic sensitivity S(λ) of the [Ru(bpy)2(OSO)]+ compound, it is first inevitable to assign the results of our time-dependent optical study to the photoinduced structural mechanisms at the molecular level: McClure et al. have recently proposed an energy level diagram for the isomerization of [Ru(bpy)2(OSO)]+ . Therein the optical excitation by 396 nm light from the S-bonded ground state to an S-bonded excited state, that relaxes within less than 200 ps to O-bonded metastable states, is described: The linkage isomerization of the SO ligand results in a change of the Ru-S-O bond to a Ru-O-S bond (O-bonded structure), thus forming either the molecular structure related to MS1 or MS2 depicted in Fig. 6 . From our study it follows, that these two molecular structures are not perfectly symmetry-related to each other by a mirror plane because of the determined two different activation energies. The exponential exposure dependence of the population kinetics of MS1,2 (Fig. 3) gives experimental evidence for a first order reaction of the excitation process as expected from the model described above.
Using the determined activation energies E A,1 and E A,2, the energy level diagram from Ref . can be completed with the energy barriers for the thermal decay process as displayed in Fig. 7 ; for simplicity, both barriers of the two distinct molecular structures are shown within the same diagram. The respective life times at 300 K of both O-bonded states are determined using Eq. (2) to be about τ 1 ≈1.6 × 103 s and τ 2 ≈3.5 × 104 s. The independence of the relaxation process on the probing intensity reveals that the probability for backswitching from the metastable states to the ground state is too low to be verified in our measurements. According to the energy level diagram in Fig. 7, this means that the probability for a transition from an excited O-bonded state by light of 532 nm to MS1,2 is much higher than for a transition to GS.
The photoinduced changes of the absorption spectra of [Ru(bpy)2(OSO)] ∙ PF6 dissolved in propylene carbonate shown in Fig. 2 present a complete disappearance of the absorption band at wavelength λ = 396 nm after exposure; this band corresponds to the Ru dπ → bpy π* Metal-to-Ligand-Charge-Transfer (MLCT) transition  and can therefore be taken as indicator for the number of molecules in the ground state (GS). The complete disappearance of this absorption band denotes a population of p MS1,2 = 100% of the metastable states MS1,2; this means all molecules are transferred from GS to MS1,2. Moreover, the entire absorption spectrum from 200 – 900 nm of the exposed sample depicted as grey line in Fig. 1, including the appearance of the two prominent new MLCT maxima at λ = 352 nm and λ = 500 nm , is assigned to the generated O-bonded states. By comparison of the absorbance before and after exposure (Fig. 2(a,b) a small fraction (≈10%) of molecules transferred to the metastable states prior the initial measurement can be identified within the spectrum of the unexposed sample. This residual population fraction can be explained by the tremendous photosensitivity of the substance and is assigned to the excitation by ambient light during the sample preparation prior to the measurement.
According to Reference  the ground state absorption band at λ = 258 nm is assigned to the π → π* intraligand transition at the bpy ligands. Further peaks in the UV can be assigned to similar intraligand transitions. In the metastable states, the absorption bands in the UV show only slight changes in comparison with the ground state spectrum. Hence, it can be concluded, that bpy intraligand transitions are at the origin of these bands as well which are not directly affected by the linkage isomerization.
As stated in Ref , the O-bonded isomers are spectroscopically similar, which is confirmed by our time-dependent decay measurements of the absorption spectra, that reveal no wavelength dependence of the decay constants τ 1,2. Hence, the allocation of the absorption bands to each of the metastable isomers is prevented, and a possible population of MS1 on expense of MS2 or vice versa could not be detected with the used optical techniques.
Nevertheless, MLCT peaks at 355 nm, 469 nm and at 352 nm, 480 nm have been determined from time-dependent DFT for the two structure shown in Fig. 6 . Thus, the observation of two absorption peaks, which can be fitted by a sum of four Gaussians only, is in full accordance with the presence of the O-bonded isomers.
The amplitude of absorption changes related to the O-bonded states, the maximum population and the exposure for reaching saturation of the photochromic response now allow us to determine the photosensitivity S(λ) of [Ru(bpy)2(OSO)]+: We define S(λ) referring to Ref . as absorption change per incident energy per unit amount of substance. Therefore, the maximum absorption change in the visible spectral range Δα(λ max) is divided by the product of the characteristic exposure Q 0 and the concentration c of the molecules in the solution:
For comparison, a related photosensitivity for the photochromic response of sodium nitroprusside (SNP) at room temperature can be calculated: As reported in Ref . at room temperature only MS2 could be populated in SNP single crystals and aqueous solutions. Using the characteristic exposure and the maximum photoinduced absorption change determined for λ probe = 532 nm from that reference, a photosensitivity of S SNP(λ = 532 nm) = 0.47 cm l J−1 mol−1 can be calculated. This highlights the tremendous photosensitivity of [Ru(bpy)2(OSO)] ∙ PF6, that is four orders of magnitude higher than the one of SNP. This can be explained by a unique absorption cross section of the [Ru(bpy)2(OSO)]+ compound.
In conclusion, [Ru(bpy)2(OSO)] ∙ PF6 dissolved in propylene carbonate fulfills the requirements, that have been initially stated: The extremely high photosensitivity yields a fast response characteristic with a pronounced photochromic signal. In combination with the long life times, the compound is suited for e.g. molecular data storage applications. According to the nitrosyl class, it can be expected, that the characteristic properties of the ruthenium-sulfoxide complex can be tuned to an even higher photosensitivity or to defined life times by ligand substitution [9,14]. Moreover, changes of the refractive-index must be connected with the linkage isomerism according to Lorentz-Lorenz relation and results for SNP [3,4] and can be roughly estimated from the absorption change (see Fig. 1) using the Kramers-Kronig relation to be larger than 10−5 at the probing wavelength (532 nm) for a [Ru(bpy)2(OSO)] ∙ PF6 concentration of 0.88 mmol l−1. This will allow for photoinduced refractive-index changes, which are the key for controlling light by light for instance in optofluidic devices.
Financial support from the Deutsche Forschungsgemeinschaft (projects GRK 695, WO618/8-1) is gratefully acknowledged. We thank Beth Anne McClure for preparation of the sample employed in these studies. JJR thanks the National Science Foundation (CHE 0809699) for financial support.
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