The influence of ligand substitution on the photochromic properties of [Ru(bpy)2(OSOR)]∙PF6 compounds (bpy = 2,2'-bipyridine, OSO = 2-methylsulfinylbenzoate) dissolved in propylene carbonate is studied by UV/VIS laser-spectroscopy as a function of temperature and exposure. The group R is either Bn (CH2C6H5), BnCl or BnMe. The photochromic properties originate from a phototriggered linkage isomerization located at the SO ligand. It is shown that the thermal stability of the studied compounds can be varied by ligand substitution in the range of 1.6 × 103 s to 3.9 × 104 s. In contrast, absorption spectra of ground and metastable states as well as the characteristic exposure of the photochromic response remain unchanged. The results are explained in the frame of photoinduced linkage isomerization located at the SO ligand.
© 2010 OSA
Small molecules with pronounced photochromic response, i.e., amplitudes of absorption changes on the sub-ps time scale, are well suited for applications in all-optical networks such as ultra-fast optical switches [1,2], logic gates  or for high-density volume data storage [4,5]. Applications in optical information and communication technology (ICT) have certain material requirements, that are 1) two or more optically distinctive states, 2) an efficient and fast photoinduced conversion between these states and 3) specific life-times of the molecule in these states. For optical ICT, the photoinduced conversion should be triggered by excitation using UV/VIS light, whereas the spectral range of the photochromic response should either be in the UV/VIS for high-density storage or in the NIR for switches and logic gates. In recent years, several coordination compounds like nitroprussides , sulfoxides  as well as organic molecules like diarylethenes  have been extensively studied with respect to their photofunctionality. These molecules seem to be good candidates for molecular photonic devices. However, there is a lack of molecules fulfilling all requirements mentioned above. Designing such molecules with the desired photofunctionality may be performed by substitution of ligands in the compound [7,8]. It also has been shown that the photofunctionality of such compounds is sensitive to the dielectric environment and can be modified by changes in the next-neighboring electrostatic or structural environment of the photoactive molecules, such as single crystals , powders , solutions , hybrid gels [11,12] or thin films .
Recently, it has been shown that the sulfoxide complex [Ru(bpy)2(OSO)]+ (bpy = 2,2'-bipyridine, OSO = 2-methylsulfinylbenzoate) offers a pronounced photochromic sensitivity in the visible spectral range in combination with high thermal stability of the metastable states . In these molecules, the photochromism is attributed to a phototriggered linkage isomerization of the sulfoxide ligand from a thermally stable S-bonded (GS) into two metastable O-bonded isomers (MS1,2) [15,16]: Exposure of the S-bonded (Ru-S-O bond) molecule by 396 nm light triggers a Ru dπ → bpy π* MLCT transition into a S-bonded excited state. Subsequently, the molecule relaxes within less than 200 ps to one of the two O-bonded (Ru-O-S bond) metastable states (MS1 or MS2) or back to GS (S-bonded) . The life-times of the metastable states at room temperature are more than 103 s . Both O-bonded structures are not perfectly symmetry-related to each other by a mirror plane, because both isomers revert back to GS at different activation energies .
In this article, we focus on the photochromic properties of [Ru(bpy)2(OSOR)]∙PF6. The properties are studied for different groups R = CH2C6H5 (Bn), CH2C6H4Cl (BnCl), or CH2C6H4CH3 (BnMe) and are compared to our results on the reference compound [Ru(bpy)2(OSO)]+ studied in Ref. 14 with CH3 at the R position. The molecular structures of the isomers in ground (GS) and metastable state (MS1,2) are schematically depicted in Fig. 1 . For clarity, only one O-bonded isomer is shown, because the two O-bonded isomers differ only in the spatial conformation, but not in atom connectivity . Therefore, both O-bonded isomers are spectroscopically and electrochemically similar , but show distinct life-times . Phototriggered isomerization from GS to MS1,2 occurs upon light exposure in the UV/blue spectral range. An optically triggered transfer from MS1,2 back to GS is not reported in these compounds. Thermal relaxation is determined by the activation energies for both relaxation processes.
Our results reveal a modification of the thermal stability of the O-bonded isomers with only slightly influencing the absorption spectra and characteristic exposure of the compound. These results are important for the application of such compounds, because they reveal the possibility of a selective tuning of specific properties of the compounds. The results are discussed in the frame of substitution of other ligands on such polypyridine sulfoxide complexes [7,17].
2. Experimental details
The sulfoxide powder, prepared as described in Refs. 16 and 18, is dissolved in propylene carbonate (Aldrich) ( for pump-probe experiments, for UV/VIS spectroscopy). All studies were performed using a cell of quartz glass () with four polished optical windows. The UV/VIS absorption spectra were taken with a Shimadzu UV-3600 two-beam spectrometer with a cell filled with pure propylene carbonate as reference. Optical excitation of the sample was performed outside the spectrometer using the setup described below.
The photoinduced absorption changes during the optical excitation and the following thermal relaxation of the metastable states are studied in an orthogonal geometry setup: The sample is homogeneously illuminated through the large windows of the cell by a continuous wave pump beam of wavelength (Coherent Cube 405) and intensity at a temperature of 45 °C. Afterwards the sample is heated to specific temperatures (45 °C – 110 °C) for studying the thermal relaxation. During the whole procedure, the transmission change is detected by a probe beam of 532 nm (Coherent 315M) with orthogonally directed with respect to the pump beam.
3. Experimental results
3.1 Absorption spectroscopy
The ground state absorption spectra of the four compounds in propylene carbonate at room temperature are shown in Fig. 2 . The lower concentration of was used to resolve the pronounced absorption band at . For clarity, the spectra are shifted by against each other. All spectra are dominated in the visible range by an absorption band at ( for OSOBnMe) that is assigned to the metal-to-ligand charge transfer (MLCT) excitation . The absorption in the UV range with the pronounced absorption band at is assigned to ligand centered (LC) transitions located at the bipyridine [14,19].
Despite small differences in the amplitudes, which can be attributed to minor deviations in the concentration of the solution, the visible spectra for all compounds are nearly identical to each other. OSOBn and OSOBnMe show a slight and broad absorption at , which is ascribed to a Ru3+ impurity of the sulfoxide powder due to the preparation of the compounds. In the UV spectral range, for the Bn containing compounds the shoulder at is more pronounced as for the reference OSO compound.
As for the ground state, the absorption bands of metastable states, i.e., upon optical excitation up to saturation, in the visible range differ only slightly in their amplitudes, but the positions of the absorption bands are unaffected. Figure 3 depicts the absorption spectra of the solutions after exposure to the pump beam (, ), again the spectra are shifted against each other for clarity. All spectra show two new prominent absorption maxima at and , respectively. These bands are assigned to MLCT excitations of the O-bonded isomers . In the UV, for all compounds an increase of the absorption at 245 nm can be observed. During the photoexcitation three isosbestic points appear in the UV/blue spectral range.
The wavelengths of the characteristic absorption bands of the ground and metastable states together with the ones of the isosbestic points are listed in Table 1 .
3.2 Population kinetics
The kinetics trace of the phototriggered generation of the metastable isomers is determined by analysis of the absorption change Δα as a function of exposure Q at the probe wavelength (). This wavelength is near to one of the appearing absorption bands features at 501 nm. Figure 4 shows the absorption change Δα normalized to the saturated absorption change Δα sat as a function of exposure to the pump light of exemplarily for a temperature of 45 °C. The normalized absorption change is saturated for the maximum conversion of the molecules to the metastable states MS1,2.
The kinetic trace of the absorption change can be described by an exponential function yielding the characteristic population constants Q 0 listed in Table 2 . As shown in the insert of Fig. 4 only the kinetic trace of OSOBnMe deviates from the other three traces. This results in a slightly higher characteristic exposure for this compound, which is still comparable to the others within the experimental error. The pronounced photochromic sensitivity S, i.e., the maximum absorption change divided by the product of characteristic exposure Q 0 and the concentration c of the molecules in the solution , is determined for , where the maximum absorption change in the visible spectral range is observed for all substances. The sensitivity S is comparable within the experimental error for all substances. The error for S is due to the error in the concentration of the solutions and the error in the characteristic exposure.
3.3 Thermal stability
The metastable isomers MS1,2 undergo a thermally activated relaxation to the ground state, which has been studied in detail for the OSO ligand in Ref. 14: The kinetics trace of the relaxation process can be described by a biexponential function, with two characteristic relaxation times τ 1,2(T). Each relaxation time can be best related to one metastable isomer. Their temperature dependence allows for derivation of the energy barrier E A for the thermally activated relaxation. The temperature dependence of these relaxation times for the three Bn containing compounds is depicted in Fig. 5 . As previously reported for the OSO compound, the relaxation times follow Arrhenius' law for temperatures below . The deviation from Arrhenius' law at higher temperatures is ascribed to thermal degradation of the molecules () . Therefore, data points for temperatures above are neglected for fitting with Arrhenius' law.
The fits yield activation energies E A, i and frequency factors Zi () listed in Table 3 together with the relaxation times τ 1,2 at . Because of the small temperature range available for the fitting procedure, the frequency factors cannot be determined more accurate than one order of magnitude of the value.
In contrast to the absorption spectra and the characteristic exposure, the thermal stability is influenced by exchanging the group R. For instance, at the fast relaxation (τ 1) is decelerated from to , whereas the slow relaxation (τ 2) is accelerated from to . As a consequence, the ratio τ 2/τ 1 has been reduced from about 10 for OSO to 4 – 5 for the Bn containing compounds.
Using the Arrhenius' law, the activation energies and frequency factors from Table 3, life-times at 300 K can be calculated for all compounds: The time constants τ 1 for the fast relaxation process for all compounds are , , and for R = Bn, BnCl, and BnMe ( ). The constants τ 2 for the slow relaxation process are , , and for R = Bn, BnCl, and BnMe ( ).
The substitution of the group R with several benzyl groups results in a variation of thermal stabilities of the metastable isomers MS1,2 with life-times modified by a factor of up to 4. It is accompanied neither by influences on the VIS absorption spectra nor changed population constants of the compounds.
The addition of the group R does not affect the absorption spectra presented in Figs. 2 and 3 in the visible spectral range. For wavelengths below 300 nm, the differences in the spectra seem to occur due to changes in the amplitudes of absorption bands in the range of 235 nm to 245 nm. The absorption of the compounds in this spectral region is mostly defined by LC transitions at the bipyridine ligands . For a detailed correlation of the changed absorption spectra in the UV range with the substitution of R, knowledge about the molecular structure and the electron density distribution of these four compounds is required.
The kinetic trace of the optically induced generation of the metastable isomers shows an exponential behavior for all substances. Therefore, the underlying process can be regarded as first order photoreaction as reported for the OSO compound . Since the process is triggered by the photoinduced MLCT transition to the bpy ligands, it is expected that the efficiency of the electronic excitation process is not influenced by R. However, by exchange of R, the ground state energy surface will be modified. According to the model reported by Rack et al. , this might change the necessary conditions for a light-induced transfer from GS to MS1,2  and, therefore, result in an altered quantum efficiency for the transfer. Based on the characteristic exposures Q 0 listed in Table 2, as expected, only minor influence on the generation process by the group R can be derived. Since the kinetic traces observed in our studies do not only depend on the quantum efficiency of the transfer, but also on the relaxation times for the thermally activated isomerization from MS1,2 to GS at the population temperature and on experimental parameters like the concentration of the solution or the experimental geometry, we cannot determine absolute quantum efficiencies from our measurements. We can conclude that at this temperature the quantum efficiency is not affected by the exchange of the group R, because the experimental parameters are not changed for the different compounds and the relaxation times are in all cases large compared to the population time at the given pump intensity.
Dealing with the thermal stability of the metastable states of the sulfoxide compounds, the group R influences the life-times of the metastable states by about a factor of 4, but does not influence τ 1 and τ 2 in the same direction. An increase of τ 1 and the decrease of τ 2 for all Bn containing compounds compared to the reference is observed. Without detailed knowledge about the molecular structure of the compounds and their three isomers each, it can be suggested that the R group is built-in in a way that the two metastable isomers are more symmetric in their structure than for the OSO-compound. From DFT calculation for the latter compound, the differences between the structures of MS1 and MS2 can be reduced to a non symmetry equivalent mirroring of the ambidentate ligand OSO on the O-Ru-O plane [14,16] as a first order approximation. Symmetry equivalent structures of MS1,2 would result in equal relaxation times for both relaxations to the GS.
Our results show that the ratio τ 2/τ 1 that is about 10 for the OSO compound  is reduced to values of 4 to 5 in the three other compounds. Also the activation energies E A, i, i.e., the energy barriers for the thermally activated transfer from MS1,2 to GS, as well as the frequency factors Zi that can be regarded as a measure for the interaction between the molecule and its environment, are strongly dependent on the group R. Both ratios—the one of the activation energies and the one of the frequency factors—are smaller for the Bn-compounds as for OSO, which underlines the suggestion that the structures of MS1,2 are more near to be symmetrically equivalent in this compounds. We’d like to stress that the performed evaluation of the activation barrier allows for first insights into the different structures (GS, MS1,2) in such photoswitchable molecules. A verification of such insight can be done by complicated exposure-dependent X-ray diffraction experiments.
Besides the modification of the photofunctionality by ligand substitution as it is studied in this article, the influence of the direct environment of the compounds has to be considered when dealing with such compounds: Except for several biophotonic applications, the compounds need to be embedded into solid environments for addressing stationary photoactive molecules. It has been shown that embedding into hybrid gels [11,12], single crystals  or thin films  influences the photofunctionality of photoactive nitroprussides. Therefore, the photofunctionality of our compounds needs to be verified in such environments.
Our results show that modification of R in this sulfoxide compounds allows for a tuning of the thermal stability of the optically generated isomers MS1,2 without influencing the photochromic sensitivity  of the compound. Studies on similar sulfoxide compounds by Rack et al. report exchanging L2 in [Ru(tpy)(L2)(DMSO)]2+ (tpy = 2,2':6',2”-terpyridine, DMSO = dimethylsulfoxide) to result in: 1) A pronounced shift of the MLCT absorption band of the GS by about 100 nm, 2) tuning of the quantum efficiency up to a complete suppression of the light-induced isomerization, and 3) only a minor effect on the thermal stability [7,21]. This offers a possibility for selective tuning of the molecular properties by exchanging specific ligands to yield molecules featuring optimized properties for regarded applications in molecular photonics like (WORM) volume data storage [4,5]. Also recently, a sulfoxide compound [Ru(bpy)2(pySO)]2+ has been reported allowing for phototriggered transfer from MS to GS . Our results on the selective tuning in sulfoxide compounds can be transferred to this new compound, so that a combination of high thermal stability in both optically distinctive states is combined with a fast phototriggered for- and backward switching and the selective tunability yield a tunable optically bistable system like it is required for applications in ultra-fast optical switches [1,2] or logic gates .
Financial support from the Deutsche Forschungsgemeinschaft (project GRK 695) is gratefully acknowledged. We thank Beth Anne McClure for preparation of the samples employed in these studies. JJR acknowledges the US National Science Foundation for funding (CHE 0809699).
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