Article pubs.acs.org/JPCA
Investigating the Effects of Solvent on the Ultrafast Dynamics of a Photoreversible Ruthenium Sulfoxide Complex Albert W. King, Beth Anne McClure,† Yuhuan Jin, and Jeffrey J. Rack* Nanoscale and Quantum Phenomena Institute, Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States S Supporting Information *
ABSTRACT: The photochromic complex [Ru(bpy) 2 (pySO)] 2+ [pySO is 2(isopropylsulfinylmethyl)pyridine] undergoes wavelength specific, photoreversible S → O and O → S linkage isomerizations. Irradiation of the ground state S-bonded complex with blue light produces the O-bonded isomer, while irradiation of the O-bonded isomer with green light produces the S-bonded isomer. Furthermore, isomerization time constants are solvent-dependent. Ultrafast transient absorption spectroscopy has been employed to investigate the relaxation processes that lead to S → O isomerization in 1,2-dichloroethane, propylene carbonate, and ethylene glycol. The isomerization is most rapid in 1,2dichloroethane and slowest in ethylene glycol. Photochemical reversion of the O-bonded isomer in propylene carbonate has further been investigated and indicates similar relaxation or isomerization kinetics, though the excited states that lead to isomerization are distinct between the S- and O-bonded isomers.
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INTRODUCTION Ultrafast transient absorption spectroscopy has been widely utilized to investigate dynamical processes following photoexcitation,1 particularly in transition metal complexes.2−13 Such studies serve to elucidate a variety of photophysical processes by monitoring electronic transitions in the excited state. For example, electron transfer dynamics and rates operative in photovoltaics have been studied extensively to follow charge transfer in these complex systems.14,15 Transient absorption spectroscopy has aided in the characterization of energy transfer processes, revealing excited state decay lifetimes and deactivation pathways.16,17 Furthermore, transient absorption spectroscopic studies provide crucial insights into the photochemical processes which occur during photosubstitution reactions, revealing ligand dissociation reactions on the picosecond timescale.18,19 Intramolecular linkage isomerization has also been investigated by ultrafast infrared spectroscopic methods.20−24 We are interested in utilizing ultrafast transient absorption spectroscopy to reveal the early excited state events preceding isomerization in photochromic ruthenium sulfoxide compounds.25−27 The mode of action in these complexes is a light triggered S → O and O → S isomerization. Such studies have revealed characteristics of ultrafast intersystem crossing and thermalization processes, as well as structural deformations and isomerizations in certain chromophores and photochromes. Irradiation of photochromic ruthenium sulfoxides induce a metal-to-ligand charge transfer (MLCT) from Ru(II) to a supporting polypyridyl ligand, formally oxidizing the metal center. This excitation triggers a cascade of relaxation processes from the initially prepared excited state that prompt nuclear rearrangement and subsequently isomerization. Excited state © 2014 American Chemical Society
deactivation occurs concomitant with isomerization, with the most rapid reported time constant occurring in 48 ps.28 In certain molecules, an orthogonal excited state reaction pathway exists that permits photochemical reversion from the ground state metastable O-bonded isomer to produce the ground state S-bonded isomer.29 Recently, we reported an unusual transformation in which two S → O isomerizations are triggered following a single photon absorption in a mononuclear ruthenium complex.30 These examples demonstrate the photochemical diversity of this class of compounds. To the end of optimizing the ruthenium sulfoxide isomerization, we have synthesized and investigated [Ru(bpy)2(pySO)]2+, where bpy is 2,2′-bipyridine and pySO is 2(isopropylsulfinylmethyl)pyridine.29 The pySO ligand combines flexibility in the sulfoxide moiety to permit isomerization, while the basic, chelating pyridine fragment enhances the stability of the molecule to solvent displacement, relative to nonchelating sulfoxide ligands. We have previously communicated that this ruthenium sulfoxide complex undergoes photoisomerization.29 Importantly, this study showed that MLCT excitation (370 nm) of the ground state S-bonded isomer triggered forward (S → O) isomerization, and that MLCT excitation (470 nm) of the metastable state O-bonded isomer triggered reverse (O → S) isomerization. However, that report contained no subpicosecond data and did not present a comprehensive mechanistic model to explain the reactivity. In order to further characterize and understand this unique Special Issue: Current Topics in Photochemistry Received: April 26, 2014 Revised: August 13, 2014 Published: August 13, 2014 10425
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photochemistry, we have interrogated [Ru(bpy)2(pySO)]2+ on a shorter timescale and investigated the role of solvent upon isomerization dynamics. This treatment reveals three distinct kinetic components present regardless of solvent. The shortest of these is assigned to formation of a 3MLCT state, the intermediate kinetic phase is ascribed to formation of an η2bonded species, with contributions from vibrational cooling, and the longest component is attributed to isomerization and relaxation to the S-bonded ground state isomer. Importantly, formation of the metastable O-bonded isomer is solventdependent, with observed time constants ranging from 169 to 246 ps. Photochemical reversion from the O- to S-bonded isomer is observed to occur through a similar kinetic mechanism as the forward isomerization.
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RESULTS AND DISCUSSION The visible region of the UV−vis spectrum of S-bonded [Ru(bpy)2(pySO)]2+ is dominated by a broad, moderately intense absorption band centered at 384 nm in 1,2-dichloroethane (DCE). In light of its shape, position, and in accord with literature precedent, this peak is assigned to a Ru dπ → bpy π* (MLCT) transition.27,33 There is little variation in the absorption maxima observed among the solvents investigated in this study, and the electronic absorption spectrum of the DCE sample is shown in Figure 1. Spectroscopic and
EXPERIMENTAL SECTION
The synthesis and structural characterization of the ligand and metal complex investigated in this study have been reported elsewhere.29 All solvents employed in this study were ACS- or HPLC-grade and obtained from Sigma-Aldrich or Fisher Scientific. All solvents were used as received. Electronic absorption spectra were collected on an Agilent 8453 spectrophotometer. Quantum yield calculations were determined by ferrioxalate actinometry and calculated as previously described.31,32 Femtosecond transient absorption measurements were collected on an Ultrafast Systems HELIOS transient absorption spectrometer. A Spectra Physics Solstice system that contains a Mai Tai seed laser and Empower pump laser was used to produce 3.5 W 800 nm pulses at a repetition rate of 1 kHz (150 fs pulse width), which were used to generate the pump and probe beams for the transient absorption spectrometer. The pump beam wavelength was generated with a Light Conversion TOPAS-C. A portion (2.5%) of the 800 nm beam produced by the Solstice was used to excite a CaF2 plate to generate a white light continuum (∼330−850 nm) probe beam. The spectrum was integrated for 2 s for each measurement. In a typical experiment, a bulk sample solution of ∼50 mL and absorbance of ∼0.4 AU at the excitation wavelength was flowed (∼7 mL/min) through a 2 mm path length cuvette. Variation in the flow rate from 6 to 12 mL/min showed no evidence of the O-bonded isomer from previous irradiation in the following spectrum. Flow rates smaller than 6 mL/min create artifacts due to the presence of the O-bonded isomer. Flow rates much greater than 12 mL/min suffer from poor signal-to-noise ratios. Modulation of the laser power from ∼0.4 to ∼1 mW showed no evidence of nonlinear or multiphoton effects. In order to perform transient absorption experiments on O-bonded samples, a bulk ground state (Sbonded) solution was irradiated with 355 nm from a Coherent Nd:YAG laser pulsing at 10 Hz. The bulk solution was monitored by UV−vis to ensure substantial conversion to the O-bonded isomer. Experiments on the O-bonded isomer were subsequently performed in the same manner as those on the Sbonded isomer. All transient absorption data were corrected by subtracting spectral background features that persisted from the previous pulse and appeared prepulse, as well as by applying chirp and t0 corrections using Surface Xplorer Pro 1.1.5 (Ultrafast Systems). Single-wavelength and global analysis kinetics were fit using Surface Xplorer Pro 1.1.5.
Figure 1. Normalized absorption spectra of S-bonded (blue) and Obonded (red) [Ru(bpy)2(pySO)]2+ in 1,2-dichloroethane.
photochemical characteristics are shown in Table 1, while additional S- and O-bonded spectra pertinent to samples in propylene carbonate (PC) and ethylene glycol (EG) are shown in Figure S1 of the Supporting Information. Irradiation of the complex in DCE solution results in the growth of new absorption features at 472 and 356 nm, concomitant with a loss of intensity at 384 nm. A similar evolution of spectral features is observed in PC or EG, where irradiation of the ground state Sbonded MLCT absorption gives rise to two new absorption bands. The new absorption features are consistent with an inner sphere O-bonded coordination motif, and we have assigned the photoproduct to be the metastable O-bonded isomer of [Ru(bpy)2(pySO)]2+. Indeed, the S- (λmax = 400 nm) and O-bonded (λmax = 476 nm) isomers for [Ru(bpy)2(py)(dmso)]2+ (py is pyridine and dmso is dimethyl sulfoxide) have been reported, which were observed in the O atom transfer reaction of [(bpy)2(py)RuIVO]2+ with dimethylsulfide.34 The quantum yield for the photochemical transformation in [Ru(bpy)2(pySO)]2+ is solvent-dependent and found to be 0.17(3) in DCE, 0.11(2) in PC, and 0.022(2) in EG. These data suggest a trend in which photoisomerization is most favorable in DCE and least favorable in EG. We employed visible time-resolved spectroscopy to reveal details of this observation. Selected representative transient spectra from 370 nm irradiation of [Ru(bpy)2(pySO)]2+ in DCE are shown in Figure 2. The initial excited state spectrum formed upon excitation is characterized by an intense, narrow absorption centered at about 360 nm and a broad absorption to the red of 450 nm. Consistent with the literature analysis of excited state transient features observed in ruthenium polypyridyl chromophores, the sharp absorption at 360 nm is assigned to a π* → π* ligand-centered (LC) transition, characteristic of reduced bpy, and the absorption to the red is assigned to ligand-to-metal charge transfer (LMCT) bpy π → Ru dπ transitions arising 10426
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Table 1. Photophysical and Photochemical Properties of [Ru(bpy)2(pySO)](PF6)2 in DCE, PC, and EG and Properties of These Solvents DCE PC EG
S-λmax (nm)
O-λmax (nm)
ΦS→O
tobs (ps)a
τS→Ob (ns)
dielectricc
viscosity (cP)c,d
375 370 370
472 473 478
0.17 ± 0.03 0.11 ± 0.02 0.022 ± 0.002
168.8 ± 27 229.2 ± 17 244.5 ± 8
0.98 2.1 11.1
10.42 66.14 41.4
0.779 2.47 16.1
tobs is the longest isomerization time constant obtained from global kinetic fits, assigned to all relaxation processes. bτS→O is the isomerization time constant calculated from the isomerization time constant and tobs. cCRC Handbook of Chemistry and Physics, 82nd ed., Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2001; pp 6-151−6-186. dMuhri, P. K., Hazra, D. K. Density and Viscosity for Propylene Carbonate +1,2-Dimethoxyethane at 298.15, 308.15, and 318.15 K. J. Chem. Eng. Data 1994, 39, 375−377. a
transient spectra in PC and EG (Figures S2 and S3 of the Supporting Information, respectively) exhibit the same features as are observed in DCE, and the spectral progression is the same. However, there are temporal differences in the observed spectral features. It is notable that the position of the lowest energy absorption maximum at 440 nm formed eventually at 3000 ps does not directly correlate to that in the steady-state spectrum (472 nm). As we have noted previously,29 the photoproduct undergoes vibrational relaxation on the ground state potential energy surface. This is manifest in the narrowing and slight bathochromic shift of the absorption band of the newly formed isomer, in addition to a continuing rise in optical density throughout the experiment. In turn, this is consistent with the calculated isomerization time constants (vide infra), which are long relative to the 3 ns length of the experiment. While this relaxation causes modulation of the spectrum at longer times, the photochemical reaction has already occurred in all of the solvents investigated here. Indeed, these spectral changes are occurring on the ground state potential energy surface. A similar effect is present in the transient spectra of the O → S isomerization experiment. Both single-wavelength and global fitting techniques were employed to elucidate a kinetic model of the excited state relaxation processes leading to isomerization in [Ru(bpy)2(pySO)]2+. Single-wavelength fits were reliably obtained at 348, 440, 502, and 600 nm for the DCE data (Table 2). The 348 and 440 nm traces were chosen because they reflect not only excited state features but also formation of ground state absorption features contributed from the O-bonded ground state isomer. The 502 nm fit reflects a late-forming isosbestic point and reports exclusively on the excited state processes prior to relaxation to the ground state potential energy surface. Finally, the 600 nm fit was chosen as there is minimal contribution from either the S- or O-bonded ground state isomer at this wavelength, and thus it exclusively describes excited state contributions to the observed spectrum. Traces from 348, 440, and 600 nm were fit to triexponential functions, while the trace at 502 nm was fit to a biexponential function. Global fitting analysis results are additionally summarized in Table 2. The shortest component, ∼0.6 ps, is assigned to formation of a 3MLCT excited state from the initially formed singlet Franck−Condon (1FC) and higher lying states, consistent with previous literature reports.35 The intermediate time component, ∼6 ps, has been observed in photoisomerizing ruthenium sulfoxide complexes, and has been ascribed to formation of an intermediate (η2 bonded) structure.36,37 This assignment is based on time-resolved two-dimensional infrared (2D-IR) measurements of an analogous photochrome, [Ru(dmb)2(OSO)]+ (dmb is 4,4′-dimethyl-2,2′-dipyridyl and OSO is 2-methylsulfinylbenzoate).38,39 Notably, the spectrum at 5 ps
Figure 2. Selected transient spectra of S-bonded [Ru(bpy)2(pySO)]2+ in 1,2-dichloroethane upon photoexcitation at 370 nm. Early time traces (A; 0.4 ps, black to 10 ps, magenta) are shown above and late time traces (B; 10 ps, magenta to 3 ns, pink) are shown below. Evidence of the laser pulse has been omitted for clarity. The arrows denote the direction of spectral change.
from the unreduced bpy ligand as well as additional, weak bpy π* → π* LC transitions. The weak absorbance in the region near 402 nm is due to the overlap of these excited state absorption features with a ground state bleach (negative peak). Over the following 10 ps, the absorption at 360 nm reduces in intensity and is eventually obscured by the ground state bleach. This feature becomes more negative and appears to shift slightly toward the blue. The visible spectrum continues to change as time evolves from 10 to 3000 ps (Figure 2B). Intensity again grows in the blue, with the appearance of an absorption band near 345 nm, while the LMCT region loses all intensity and reaches zero. Meanwhile, a distinct absorption band forms about 450 nm then grows in intensity and shifts to 440 nm. The 3000 ps transient spectrum represents the end of the experiment and features two absorption maxima at 345 and 440 nm and a shallow bleach near 390 nm. The change of absorption at wavelengths >550 nm corresponds directly to loss of the excited state, as there is no significant ground state absorption contribution from either the S- or O-bonded isomers in this region. The new absorption features at 345 and 440 nm develop as the excited state decays and so are assigned to formation of a new electronically relaxed state, namely, the O-bonded isomer. This assignment is in accord with bulk photolysis results as well as previous investigations. In general, 10427
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(FSRS) suggests this process may be as fast as 30 fs.42 Picosecond time-correlated single photon counting experiments in methylene chloride and quantum chemical calculations by Siddique et al. reveal relatively slow (∼13 ps) 1 MLCT to 3MLCT conversion in [Cu(dmp)2]+.43 This assignment was later supported by Meyer, Chen, and coworkers, who reported ultrafast transient absorption spectroscopy and fluorescence upconversion experiments, revealing an ∼13 ps relaxation time constant in acetonitrile, ethylene glycol, and toluene solutions.44 Interestingly, a short time component of ∼100 fs is observed upon excitation of [Cu(dmp)2]+ and assigned to structural distortion of the tetrahedral ground state geometry to a flattened tetrahedral geometry in the excited state. In this flattened state, coupling between the occupied singlet MLCT state and lower energy triplet states is ligand based and very weak (∼30 cm−1), thus leading to the slow intersystem crossing (ISC) time constant. Both geometric distortion and ISC rates were found to be independent of solvent for [Cu(dmp)2]+ and related complexes.44 However, as noted by Chergui, the ISC time constants are not simply a function of spin−orbit coupling but also of nuclear motion.7 As the large Stokes’ shifts attest, there is a large excited state distortion from 1FC to 3MLCT for these compounds.45 If solvent is slow to respond to this change in nuclear configuration then this may explain why this short time constant features a solvent dependence in contrast to [Ru(bpy)3]2+ and [Cu(dmp)2]+. We have previously suggested that the intermediate time component may correspond to the formation of a distinct state with an intermediate (η2, or SO side-on bonded) coordinate geometry, which has some literature precedence.46 In addition to 2D-IR data,38,39 computational results suggest large nuclear structural changes in the excited state, as may be expected in the formation of such a configuration. For example, in [Ru(bpy)2(bpte)]2+ and [Ru(bpy)2(bete)]2+ [bpte is 1,2bis(phenylthio)ethane and bete is 3,6-dithiaoctane], an elongation of the Ru−S bond distance in the 3MLCT state is found.47 Accordingly, we expect a similar lengthening of the Ru−S bond and anticipate that this effect may be comparatively larger with the sulfoxide ligands as the S atom in the sulfoxide group has less electron density than the S atom in the thioether group. Some reports have assigned solvent-dependent vibrational cooling (VC) to time constants in this general temporal regime in complexes that undergo large amplitude nuclear rearrangements upon photoexcitation.48,49 As there is little qualitative or structural change to the transient absorption spectrum within this time regime, and significant nuclear distortions occur during linkage isomerization, VC may contribute to this time component. Both structural reorganization and VC effects may contribute to the slight variation observed in the intermediate time component (Table 2), as this kinetic phase is generally longer in EG and PC than that observed in DCE. If the sulfoxide moiety were to completely decoordinate in the course of the isomerization, recoordination would be wholly dependent upon Brownian motion and one may expect to see much slower time constants, in addition to substantial changes in the transient spectrum and distinct kinetic phases corresponding to the relaxation of a plurality of species, none of which are observed in the photochemistry of this molecule. These observations support the plausibility of an η2 structure occurring at some point in the ruthenium sulfoxide isomerization, and possibly concomitant with vibrational
Table 2. Observed Time Constants (in ps) from SingleWavelength Kinetic Fits Following 370 nm Excitation in DCE and EG and 360 nm Excitation in PC DCE 348 nm t1 t2 t3
t1 t2 t3
0.60 2.7 183.4 440
± 0.1 ± 0.5 ± 22 nm
PC
EG
370 nm
395 nm
0.97 5.2 226.6 460
± 0.2 ± 0.7 ± 26 nm
1.1 5.8 245.1 430
± 0.3 ±1 ±7 nm
t1 t2 t3
0.60 ± 0.09 3.5 ± 0.4 218.4 ± 10 502 nm
1.3 ± 0.1 8.6 ± 1 223.0 ± 15 493 nm
0.76 ± 0.06 6.0 ± 0.3 273.9 ± 7.6 490 nm
t1 t2
0.57 ± 0.06 5.3 ± 0.7 600 nm
0.8 ± 0.08 6.5 ± 0.5 600 nm
0.82 ± 0.05 7.6 ± 0.4 600 nm
t1 t2 t3
0.594 ± fixed 6.0 ± 0.8 191.4 ± 13 global fitting analysis
1.1 ± 0.2 11.1 ± 3.7 237.7 ± 33.8 global fitting analysis
1.8 ± 0.1 35.4 ± 7.6 289.0 ± 18.3 global fitting analysis
0.62 ± 0.1 2.7 ± 0.4 168.8 ± 26
1.1 ± 0.2 6.2 ± 1 229.2 ± 17
1.0 ± 0.09 7.6 ± 0.9 244.5 ± 8
displays features of an excited state and is dissimilar to that of the O-bonded isomer (vide infra). This state immediately precedes isomerization, and we and others have suggested that it is comprised of contributions from ligand field (LF) states as well as the 3MLCT excited state.40 The geometry of this state is poised for isomerization, and we term it the triplet S′ excited state, 3ESS′. The longest time component, ∼200 ps, occurs with the loss of all excited state transient features and with the formation of new electronically relaxed absorption features, which are generally consistent with the ground state absorption spectrum of the O-bonded isomer. Accordingly, this time component is ascribed to the formation of the electronically relaxed O-bonded isomer. Because the isomerization quantum yield is not unity, this observed time constant also corresponds to the reformation of the electronically relaxed ground state Sbonded isomer. Time constants obtained from global fitting analysis are in reasonably good agreement with those derived from single-wavelength kinetics, corroborating kinetic components of 0.62 (0.1), 2.7 (0.4), and 168 (26) ps. Applying this combination of single-wavelength kinetics and global fitting analysis reveals that this kinetic model is qualitatively conserved in PC and EG (Table 2). There is a fast time component, approximately one picosecond or less, in all cases, and an intermediate (3−8 ps) time component which varies slightly with solvent. A long time component, on the order of hundreds of picoseconds, is also found to be solventdependent. The variation observed in the shortest time component, which is ∼0.6 ps in DCE and ∼1 ps in PC and EG, is unusual for 3MLCT formation. In contrast, solventindependent time constants for the formation of 3MLCT states have been reported in the classical examples [Ru(bpy)3]2+ and [Cu(dmp)2]+ (dmp is 2,9-dimethyl-1,10-phenanthroline). In [Ru(bpy)3]2+, the formation of the vibrationally relaxed 3 MLCT excited state occurs with an observed time constant of ∼100 fs in nitrile, methanolic, and formamide solutions, despite their significantly varying viscosities and dielectric constants.41 Femtosecond stimulated Raman spectroscopy 10428
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previously reported this transformation in a bulk PC solution and calculated a quantum yield of 0.027(6).29 The excited state dynamics of the reverse (O → S) isomerization have now been investigated by transient absorption spectroscopy and are reported here. Similar to the S-bonded complex, the transient spectra of the O-bonded isomer excited at 472 nm (Figure 3)
cooling. Further studies will attempt to further clarify this assignment. Variation is observed among the longest (isomerization and S-bonded ground state formation) time constants with respect to the solvents investigated in this study. The shortest of these relaxation time constants is 168.8 ± 26 ps in DCE and extends to 229.2 ± 17 ps in PC and 244.5 ± 8 ps in EG based on the global fitting analysis results. This observation is in accord with the quantum yield measurements, indicating that the photoisomerization is least efficient in EG. Since the observed time constant corresponds to all relaxation processes, the isomerization quantum yield must be utilized to explicitly calculate the isomerization time constant (see the Supporting Information for further details). Accounting for the isomerization quantum yields, the isomerization time constants (τS→O) are 0.98, 2.1, and 11.1 ns in DCE, PC and EG, respectively. As the isomerization time constant slows with viscosity (but does not trend with dielectric; see Table 1), viscous effects must predominately contribute in this series. Curiously, the trend in time constants is in contrast to previous studies of electrochemical isomerization in ruthenium polpyridyl sulfoxide complexes. In one case, isomerization rates for [Ru(bpy)2(OSO)]+ and [Ru(bpy)(biq)(OSO)]+ (biq is 2,2′biquinoline) were obtained in PC, acetonitrile, and methylene chloride.50 The rates were found to be solvent-dependent, with the greatest isomerization rate observed in PC, relative to acetonitrile (intermediate) and DCM (smallest). As the oxidized RuIII isomerizing species are not expected to exhibit radically different dipole moments between the two compounds, these data were rationalized by assigning the isomerization rate difference to solute−solvent interactions predominated by solvent viscosity rather than dielectric constant. Feringa and co-workers have reported similar observations of viscous solvent effects facilitating nuclear rearrangements.51 Rapid, unidirectional cis-trans isomerization in sterically overcrowded alkenes occurred more rapidly in highly viscous media and was rationalized as a consequence of the metastable species occupying less space than the ground state isomer. The nuclear distortions that preempt isomerization produce a void in viscous solvents, which would otherwise not occur in less viscous solvents and thus decrease solvent friction to enhance isomerization rates. A similar analysis was applied to the ruthenium sulfoxide electrochemical isomerization. As in the OSO complexes, both dielectric and viscous effects are surely operative in the photoisomerization of [Ru(bpy)2(pySO)]2+. Moreover, the variation observed in the fast and intermediate time components in the various solvents demonstrates that the excited state potential energy surface is also influenced by characteristics of the solvent. In light of these data, variation in the isomerization time constants with respect to solvent likely reveals differences in the coupling of the excited state to the ground state, beyond simple contributions from the friction of solvent viscosity. Computational studies and recent experimental work indicate that the relevant excited and ground state potential surfaces are strongly coupled and suggest the presence of a conical intersection or conical intersection seams that facilitate isomerization.36,40 In such a model, solvent may affect structural dynamics and the electronic dipole of the excited state molecule, perturbing the coupling of the excited and ground state surfaces. A most intriguing feature of the [Ru(bpy)2(pySO)]2+ complex is the observation of photochemical reversion from the O-bonded isomer to the S-bonded isomer. We have
Figure 3. Select transient spectra of O-bonded [Ru(bpy)2(pySO)]2+ in propylene carbonate photoexcitation at 472 nm. Early time traces (A; 0.7 ps, black to 10 ps, teal) (top) and late time traces (B; 10 ps, teal to 3 ns, burgundy) (bottom). Evidence of the laser pulse has been omitted for clarity.
reveal an intense, narrow absorption centered about 380 nm and a broad, featureless absorption to the red of 575 nm in the initial excited state spectrum. As in the excited state of the Sbonded isomer, the narrow 380 nm absorption is assigned to π* → π* transitions on the reduced bpy ligand, while the absorption to the red of the spectrum is assigned to LMCT transitions from the unreduced bpy, with contributions from the LC transition. The negative feature centered near 470 nm is a consequence of ground state bleach and the pump laser pulse, which has been omitted for clarity. As time progresses, the excited state absorption features universally decay, and the bleach at 470 nm decreases in intensity. A mild bathochromic shift of the 380 nm band is observed. Ultimately, all absorption features in the red region of the spectrum decay to zero ΔOD, while a new absorption band is evident near 390 nm with a persistent, shallow bleach at 470 nm. The 3000 ps spectrum at the end of the experiment agrees with previous measurements of this complex and is interpreted as the difference between the spectra of the S- and O-bonded ground state isomers. Thus, the final transient spectrum is consistent with formation of the Sbonded isomer produced from excitation of the initial Obonded isomer. Global fitting analysis reveals a biexponential relaxation with time constants of 7.2 (0.9) ps and 162 (30) ps. A subpicosecond time component, attributable to the formation of the 3MLCT excited state from the initially formed 1FC and higher lying excited states, is not observed, and is presumably faster than the temporal resolution afforded by our 10429
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instrumental setup. As in the S → O isomerization kinetics, the
Investigating the Effects of Solvent on the Ultrafast Dynamics of a Photoreversible Ruthenium Sulfoxide Complex Albert W. King, Beth Anne McClure,† Yuhuan Jin, and Jeffrey J. Rack* Nanoscale and Quantum Phenomena Institute, Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States S Supporting Information *
ABSTRACT: The photochromic complex [Ru(bpy) 2 (pySO)] 2+ [pySO is 2(isopropylsulfinylmethyl)pyridine] undergoes wavelength specific, photoreversible S → O and O → S linkage isomerizations. Irradiation of the ground state S-bonded complex with blue light produces the O-bonded isomer, while irradiation of the O-bonded isomer with green light produces the S-bonded isomer. Furthermore, isomerization time constants are solvent-dependent. Ultrafast transient absorption spectroscopy has been employed to investigate the relaxation processes that lead to S → O isomerization in 1,2-dichloroethane, propylene carbonate, and ethylene glycol. The isomerization is most rapid in 1,2dichloroethane and slowest in ethylene glycol. Photochemical reversion of the O-bonded isomer in propylene carbonate has further been investigated and indicates similar relaxation or isomerization kinetics, though the excited states that lead to isomerization are distinct between the S- and O-bonded isomers.
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INTRODUCTION Ultrafast transient absorption spectroscopy has been widely utilized to investigate dynamical processes following photoexcitation,1 particularly in transition metal complexes.2−13 Such studies serve to elucidate a variety of photophysical processes by monitoring electronic transitions in the excited state. For example, electron transfer dynamics and rates operative in photovoltaics have been studied extensively to follow charge transfer in these complex systems.14,15 Transient absorption spectroscopy has aided in the characterization of energy transfer processes, revealing excited state decay lifetimes and deactivation pathways.16,17 Furthermore, transient absorption spectroscopic studies provide crucial insights into the photochemical processes which occur during photosubstitution reactions, revealing ligand dissociation reactions on the picosecond timescale.18,19 Intramolecular linkage isomerization has also been investigated by ultrafast infrared spectroscopic methods.20−24 We are interested in utilizing ultrafast transient absorption spectroscopy to reveal the early excited state events preceding isomerization in photochromic ruthenium sulfoxide compounds.25−27 The mode of action in these complexes is a light triggered S → O and O → S isomerization. Such studies have revealed characteristics of ultrafast intersystem crossing and thermalization processes, as well as structural deformations and isomerizations in certain chromophores and photochromes. Irradiation of photochromic ruthenium sulfoxides induce a metal-to-ligand charge transfer (MLCT) from Ru(II) to a supporting polypyridyl ligand, formally oxidizing the metal center. This excitation triggers a cascade of relaxation processes from the initially prepared excited state that prompt nuclear rearrangement and subsequently isomerization. Excited state © 2014 American Chemical Society
deactivation occurs concomitant with isomerization, with the most rapid reported time constant occurring in 48 ps.28 In certain molecules, an orthogonal excited state reaction pathway exists that permits photochemical reversion from the ground state metastable O-bonded isomer to produce the ground state S-bonded isomer.29 Recently, we reported an unusual transformation in which two S → O isomerizations are triggered following a single photon absorption in a mononuclear ruthenium complex.30 These examples demonstrate the photochemical diversity of this class of compounds. To the end of optimizing the ruthenium sulfoxide isomerization, we have synthesized and investigated [Ru(bpy)2(pySO)]2+, where bpy is 2,2′-bipyridine and pySO is 2(isopropylsulfinylmethyl)pyridine.29 The pySO ligand combines flexibility in the sulfoxide moiety to permit isomerization, while the basic, chelating pyridine fragment enhances the stability of the molecule to solvent displacement, relative to nonchelating sulfoxide ligands. We have previously communicated that this ruthenium sulfoxide complex undergoes photoisomerization.29 Importantly, this study showed that MLCT excitation (370 nm) of the ground state S-bonded isomer triggered forward (S → O) isomerization, and that MLCT excitation (470 nm) of the metastable state O-bonded isomer triggered reverse (O → S) isomerization. However, that report contained no subpicosecond data and did not present a comprehensive mechanistic model to explain the reactivity. In order to further characterize and understand this unique Special Issue: Current Topics in Photochemistry Received: April 26, 2014 Revised: August 13, 2014 Published: August 13, 2014 10425
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photochemistry, we have interrogated [Ru(bpy)2(pySO)]2+ on a shorter timescale and investigated the role of solvent upon isomerization dynamics. This treatment reveals three distinct kinetic components present regardless of solvent. The shortest of these is assigned to formation of a 3MLCT state, the intermediate kinetic phase is ascribed to formation of an η2bonded species, with contributions from vibrational cooling, and the longest component is attributed to isomerization and relaxation to the S-bonded ground state isomer. Importantly, formation of the metastable O-bonded isomer is solventdependent, with observed time constants ranging from 169 to 246 ps. Photochemical reversion from the O- to S-bonded isomer is observed to occur through a similar kinetic mechanism as the forward isomerization.
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RESULTS AND DISCUSSION The visible region of the UV−vis spectrum of S-bonded [Ru(bpy)2(pySO)]2+ is dominated by a broad, moderately intense absorption band centered at 384 nm in 1,2-dichloroethane (DCE). In light of its shape, position, and in accord with literature precedent, this peak is assigned to a Ru dπ → bpy π* (MLCT) transition.27,33 There is little variation in the absorption maxima observed among the solvents investigated in this study, and the electronic absorption spectrum of the DCE sample is shown in Figure 1. Spectroscopic and
EXPERIMENTAL SECTION
The synthesis and structural characterization of the ligand and metal complex investigated in this study have been reported elsewhere.29 All solvents employed in this study were ACS- or HPLC-grade and obtained from Sigma-Aldrich or Fisher Scientific. All solvents were used as received. Electronic absorption spectra were collected on an Agilent 8453 spectrophotometer. Quantum yield calculations were determined by ferrioxalate actinometry and calculated as previously described.31,32 Femtosecond transient absorption measurements were collected on an Ultrafast Systems HELIOS transient absorption spectrometer. A Spectra Physics Solstice system that contains a Mai Tai seed laser and Empower pump laser was used to produce 3.5 W 800 nm pulses at a repetition rate of 1 kHz (150 fs pulse width), which were used to generate the pump and probe beams for the transient absorption spectrometer. The pump beam wavelength was generated with a Light Conversion TOPAS-C. A portion (2.5%) of the 800 nm beam produced by the Solstice was used to excite a CaF2 plate to generate a white light continuum (∼330−850 nm) probe beam. The spectrum was integrated for 2 s for each measurement. In a typical experiment, a bulk sample solution of ∼50 mL and absorbance of ∼0.4 AU at the excitation wavelength was flowed (∼7 mL/min) through a 2 mm path length cuvette. Variation in the flow rate from 6 to 12 mL/min showed no evidence of the O-bonded isomer from previous irradiation in the following spectrum. Flow rates smaller than 6 mL/min create artifacts due to the presence of the O-bonded isomer. Flow rates much greater than 12 mL/min suffer from poor signal-to-noise ratios. Modulation of the laser power from ∼0.4 to ∼1 mW showed no evidence of nonlinear or multiphoton effects. In order to perform transient absorption experiments on O-bonded samples, a bulk ground state (Sbonded) solution was irradiated with 355 nm from a Coherent Nd:YAG laser pulsing at 10 Hz. The bulk solution was monitored by UV−vis to ensure substantial conversion to the O-bonded isomer. Experiments on the O-bonded isomer were subsequently performed in the same manner as those on the Sbonded isomer. All transient absorption data were corrected by subtracting spectral background features that persisted from the previous pulse and appeared prepulse, as well as by applying chirp and t0 corrections using Surface Xplorer Pro 1.1.5 (Ultrafast Systems). Single-wavelength and global analysis kinetics were fit using Surface Xplorer Pro 1.1.5.
Figure 1. Normalized absorption spectra of S-bonded (blue) and Obonded (red) [Ru(bpy)2(pySO)]2+ in 1,2-dichloroethane.
photochemical characteristics are shown in Table 1, while additional S- and O-bonded spectra pertinent to samples in propylene carbonate (PC) and ethylene glycol (EG) are shown in Figure S1 of the Supporting Information. Irradiation of the complex in DCE solution results in the growth of new absorption features at 472 and 356 nm, concomitant with a loss of intensity at 384 nm. A similar evolution of spectral features is observed in PC or EG, where irradiation of the ground state Sbonded MLCT absorption gives rise to two new absorption bands. The new absorption features are consistent with an inner sphere O-bonded coordination motif, and we have assigned the photoproduct to be the metastable O-bonded isomer of [Ru(bpy)2(pySO)]2+. Indeed, the S- (λmax = 400 nm) and O-bonded (λmax = 476 nm) isomers for [Ru(bpy)2(py)(dmso)]2+ (py is pyridine and dmso is dimethyl sulfoxide) have been reported, which were observed in the O atom transfer reaction of [(bpy)2(py)RuIVO]2+ with dimethylsulfide.34 The quantum yield for the photochemical transformation in [Ru(bpy)2(pySO)]2+ is solvent-dependent and found to be 0.17(3) in DCE, 0.11(2) in PC, and 0.022(2) in EG. These data suggest a trend in which photoisomerization is most favorable in DCE and least favorable in EG. We employed visible time-resolved spectroscopy to reveal details of this observation. Selected representative transient spectra from 370 nm irradiation of [Ru(bpy)2(pySO)]2+ in DCE are shown in Figure 2. The initial excited state spectrum formed upon excitation is characterized by an intense, narrow absorption centered at about 360 nm and a broad absorption to the red of 450 nm. Consistent with the literature analysis of excited state transient features observed in ruthenium polypyridyl chromophores, the sharp absorption at 360 nm is assigned to a π* → π* ligand-centered (LC) transition, characteristic of reduced bpy, and the absorption to the red is assigned to ligand-to-metal charge transfer (LMCT) bpy π → Ru dπ transitions arising 10426
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Table 1. Photophysical and Photochemical Properties of [Ru(bpy)2(pySO)](PF6)2 in DCE, PC, and EG and Properties of These Solvents DCE PC EG
S-λmax (nm)
O-λmax (nm)
ΦS→O
tobs (ps)a
τS→Ob (ns)
dielectricc
viscosity (cP)c,d
375 370 370
472 473 478
0.17 ± 0.03 0.11 ± 0.02 0.022 ± 0.002
168.8 ± 27 229.2 ± 17 244.5 ± 8
0.98 2.1 11.1
10.42 66.14 41.4
0.779 2.47 16.1
tobs is the longest isomerization time constant obtained from global kinetic fits, assigned to all relaxation processes. bτS→O is the isomerization time constant calculated from the isomerization time constant and tobs. cCRC Handbook of Chemistry and Physics, 82nd ed., Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 2001; pp 6-151−6-186. dMuhri, P. K., Hazra, D. K. Density and Viscosity for Propylene Carbonate +1,2-Dimethoxyethane at 298.15, 308.15, and 318.15 K. J. Chem. Eng. Data 1994, 39, 375−377. a
transient spectra in PC and EG (Figures S2 and S3 of the Supporting Information, respectively) exhibit the same features as are observed in DCE, and the spectral progression is the same. However, there are temporal differences in the observed spectral features. It is notable that the position of the lowest energy absorption maximum at 440 nm formed eventually at 3000 ps does not directly correlate to that in the steady-state spectrum (472 nm). As we have noted previously,29 the photoproduct undergoes vibrational relaxation on the ground state potential energy surface. This is manifest in the narrowing and slight bathochromic shift of the absorption band of the newly formed isomer, in addition to a continuing rise in optical density throughout the experiment. In turn, this is consistent with the calculated isomerization time constants (vide infra), which are long relative to the 3 ns length of the experiment. While this relaxation causes modulation of the spectrum at longer times, the photochemical reaction has already occurred in all of the solvents investigated here. Indeed, these spectral changes are occurring on the ground state potential energy surface. A similar effect is present in the transient spectra of the O → S isomerization experiment. Both single-wavelength and global fitting techniques were employed to elucidate a kinetic model of the excited state relaxation processes leading to isomerization in [Ru(bpy)2(pySO)]2+. Single-wavelength fits were reliably obtained at 348, 440, 502, and 600 nm for the DCE data (Table 2). The 348 and 440 nm traces were chosen because they reflect not only excited state features but also formation of ground state absorption features contributed from the O-bonded ground state isomer. The 502 nm fit reflects a late-forming isosbestic point and reports exclusively on the excited state processes prior to relaxation to the ground state potential energy surface. Finally, the 600 nm fit was chosen as there is minimal contribution from either the S- or O-bonded ground state isomer at this wavelength, and thus it exclusively describes excited state contributions to the observed spectrum. Traces from 348, 440, and 600 nm were fit to triexponential functions, while the trace at 502 nm was fit to a biexponential function. Global fitting analysis results are additionally summarized in Table 2. The shortest component, ∼0.6 ps, is assigned to formation of a 3MLCT excited state from the initially formed singlet Franck−Condon (1FC) and higher lying states, consistent with previous literature reports.35 The intermediate time component, ∼6 ps, has been observed in photoisomerizing ruthenium sulfoxide complexes, and has been ascribed to formation of an intermediate (η2 bonded) structure.36,37 This assignment is based on time-resolved two-dimensional infrared (2D-IR) measurements of an analogous photochrome, [Ru(dmb)2(OSO)]+ (dmb is 4,4′-dimethyl-2,2′-dipyridyl and OSO is 2-methylsulfinylbenzoate).38,39 Notably, the spectrum at 5 ps
Figure 2. Selected transient spectra of S-bonded [Ru(bpy)2(pySO)]2+ in 1,2-dichloroethane upon photoexcitation at 370 nm. Early time traces (A; 0.4 ps, black to 10 ps, magenta) are shown above and late time traces (B; 10 ps, magenta to 3 ns, pink) are shown below. Evidence of the laser pulse has been omitted for clarity. The arrows denote the direction of spectral change.
from the unreduced bpy ligand as well as additional, weak bpy π* → π* LC transitions. The weak absorbance in the region near 402 nm is due to the overlap of these excited state absorption features with a ground state bleach (negative peak). Over the following 10 ps, the absorption at 360 nm reduces in intensity and is eventually obscured by the ground state bleach. This feature becomes more negative and appears to shift slightly toward the blue. The visible spectrum continues to change as time evolves from 10 to 3000 ps (Figure 2B). Intensity again grows in the blue, with the appearance of an absorption band near 345 nm, while the LMCT region loses all intensity and reaches zero. Meanwhile, a distinct absorption band forms about 450 nm then grows in intensity and shifts to 440 nm. The 3000 ps transient spectrum represents the end of the experiment and features two absorption maxima at 345 and 440 nm and a shallow bleach near 390 nm. The change of absorption at wavelengths >550 nm corresponds directly to loss of the excited state, as there is no significant ground state absorption contribution from either the S- or O-bonded isomers in this region. The new absorption features at 345 and 440 nm develop as the excited state decays and so are assigned to formation of a new electronically relaxed state, namely, the O-bonded isomer. This assignment is in accord with bulk photolysis results as well as previous investigations. In general, 10427
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(FSRS) suggests this process may be as fast as 30 fs.42 Picosecond time-correlated single photon counting experiments in methylene chloride and quantum chemical calculations by Siddique et al. reveal relatively slow (∼13 ps) 1 MLCT to 3MLCT conversion in [Cu(dmp)2]+.43 This assignment was later supported by Meyer, Chen, and coworkers, who reported ultrafast transient absorption spectroscopy and fluorescence upconversion experiments, revealing an ∼13 ps relaxation time constant in acetonitrile, ethylene glycol, and toluene solutions.44 Interestingly, a short time component of ∼100 fs is observed upon excitation of [Cu(dmp)2]+ and assigned to structural distortion of the tetrahedral ground state geometry to a flattened tetrahedral geometry in the excited state. In this flattened state, coupling between the occupied singlet MLCT state and lower energy triplet states is ligand based and very weak (∼30 cm−1), thus leading to the slow intersystem crossing (ISC) time constant. Both geometric distortion and ISC rates were found to be independent of solvent for [Cu(dmp)2]+ and related complexes.44 However, as noted by Chergui, the ISC time constants are not simply a function of spin−orbit coupling but also of nuclear motion.7 As the large Stokes’ shifts attest, there is a large excited state distortion from 1FC to 3MLCT for these compounds.45 If solvent is slow to respond to this change in nuclear configuration then this may explain why this short time constant features a solvent dependence in contrast to [Ru(bpy)3]2+ and [Cu(dmp)2]+. We have previously suggested that the intermediate time component may correspond to the formation of a distinct state with an intermediate (η2, or SO side-on bonded) coordinate geometry, which has some literature precedence.46 In addition to 2D-IR data,38,39 computational results suggest large nuclear structural changes in the excited state, as may be expected in the formation of such a configuration. For example, in [Ru(bpy)2(bpte)]2+ and [Ru(bpy)2(bete)]2+ [bpte is 1,2bis(phenylthio)ethane and bete is 3,6-dithiaoctane], an elongation of the Ru−S bond distance in the 3MLCT state is found.47 Accordingly, we expect a similar lengthening of the Ru−S bond and anticipate that this effect may be comparatively larger with the sulfoxide ligands as the S atom in the sulfoxide group has less electron density than the S atom in the thioether group. Some reports have assigned solvent-dependent vibrational cooling (VC) to time constants in this general temporal regime in complexes that undergo large amplitude nuclear rearrangements upon photoexcitation.48,49 As there is little qualitative or structural change to the transient absorption spectrum within this time regime, and significant nuclear distortions occur during linkage isomerization, VC may contribute to this time component. Both structural reorganization and VC effects may contribute to the slight variation observed in the intermediate time component (Table 2), as this kinetic phase is generally longer in EG and PC than that observed in DCE. If the sulfoxide moiety were to completely decoordinate in the course of the isomerization, recoordination would be wholly dependent upon Brownian motion and one may expect to see much slower time constants, in addition to substantial changes in the transient spectrum and distinct kinetic phases corresponding to the relaxation of a plurality of species, none of which are observed in the photochemistry of this molecule. These observations support the plausibility of an η2 structure occurring at some point in the ruthenium sulfoxide isomerization, and possibly concomitant with vibrational
Table 2. Observed Time Constants (in ps) from SingleWavelength Kinetic Fits Following 370 nm Excitation in DCE and EG and 360 nm Excitation in PC DCE 348 nm t1 t2 t3
t1 t2 t3
0.60 2.7 183.4 440
± 0.1 ± 0.5 ± 22 nm
PC
EG
370 nm
395 nm
0.97 5.2 226.6 460
± 0.2 ± 0.7 ± 26 nm
1.1 5.8 245.1 430
± 0.3 ±1 ±7 nm
t1 t2 t3
0.60 ± 0.09 3.5 ± 0.4 218.4 ± 10 502 nm
1.3 ± 0.1 8.6 ± 1 223.0 ± 15 493 nm
0.76 ± 0.06 6.0 ± 0.3 273.9 ± 7.6 490 nm
t1 t2
0.57 ± 0.06 5.3 ± 0.7 600 nm
0.8 ± 0.08 6.5 ± 0.5 600 nm
0.82 ± 0.05 7.6 ± 0.4 600 nm
t1 t2 t3
0.594 ± fixed 6.0 ± 0.8 191.4 ± 13 global fitting analysis
1.1 ± 0.2 11.1 ± 3.7 237.7 ± 33.8 global fitting analysis
1.8 ± 0.1 35.4 ± 7.6 289.0 ± 18.3 global fitting analysis
0.62 ± 0.1 2.7 ± 0.4 168.8 ± 26
1.1 ± 0.2 6.2 ± 1 229.2 ± 17
1.0 ± 0.09 7.6 ± 0.9 244.5 ± 8
displays features of an excited state and is dissimilar to that of the O-bonded isomer (vide infra). This state immediately precedes isomerization, and we and others have suggested that it is comprised of contributions from ligand field (LF) states as well as the 3MLCT excited state.40 The geometry of this state is poised for isomerization, and we term it the triplet S′ excited state, 3ESS′. The longest time component, ∼200 ps, occurs with the loss of all excited state transient features and with the formation of new electronically relaxed absorption features, which are generally consistent with the ground state absorption spectrum of the O-bonded isomer. Accordingly, this time component is ascribed to the formation of the electronically relaxed O-bonded isomer. Because the isomerization quantum yield is not unity, this observed time constant also corresponds to the reformation of the electronically relaxed ground state Sbonded isomer. Time constants obtained from global fitting analysis are in reasonably good agreement with those derived from single-wavelength kinetics, corroborating kinetic components of 0.62 (0.1), 2.7 (0.4), and 168 (26) ps. Applying this combination of single-wavelength kinetics and global fitting analysis reveals that this kinetic model is qualitatively conserved in PC and EG (Table 2). There is a fast time component, approximately one picosecond or less, in all cases, and an intermediate (3−8 ps) time component which varies slightly with solvent. A long time component, on the order of hundreds of picoseconds, is also found to be solventdependent. The variation observed in the shortest time component, which is ∼0.6 ps in DCE and ∼1 ps in PC and EG, is unusual for 3MLCT formation. In contrast, solventindependent time constants for the formation of 3MLCT states have been reported in the classical examples [Ru(bpy)3]2+ and [Cu(dmp)2]+ (dmp is 2,9-dimethyl-1,10-phenanthroline). In [Ru(bpy)3]2+, the formation of the vibrationally relaxed 3 MLCT excited state occurs with an observed time constant of ∼100 fs in nitrile, methanolic, and formamide solutions, despite their significantly varying viscosities and dielectric constants.41 Femtosecond stimulated Raman spectroscopy 10428
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previously reported this transformation in a bulk PC solution and calculated a quantum yield of 0.027(6).29 The excited state dynamics of the reverse (O → S) isomerization have now been investigated by transient absorption spectroscopy and are reported here. Similar to the S-bonded complex, the transient spectra of the O-bonded isomer excited at 472 nm (Figure 3)
cooling. Further studies will attempt to further clarify this assignment. Variation is observed among the longest (isomerization and S-bonded ground state formation) time constants with respect to the solvents investigated in this study. The shortest of these relaxation time constants is 168.8 ± 26 ps in DCE and extends to 229.2 ± 17 ps in PC and 244.5 ± 8 ps in EG based on the global fitting analysis results. This observation is in accord with the quantum yield measurements, indicating that the photoisomerization is least efficient in EG. Since the observed time constant corresponds to all relaxation processes, the isomerization quantum yield must be utilized to explicitly calculate the isomerization time constant (see the Supporting Information for further details). Accounting for the isomerization quantum yields, the isomerization time constants (τS→O) are 0.98, 2.1, and 11.1 ns in DCE, PC and EG, respectively. As the isomerization time constant slows with viscosity (but does not trend with dielectric; see Table 1), viscous effects must predominately contribute in this series. Curiously, the trend in time constants is in contrast to previous studies of electrochemical isomerization in ruthenium polpyridyl sulfoxide complexes. In one case, isomerization rates for [Ru(bpy)2(OSO)]+ and [Ru(bpy)(biq)(OSO)]+ (biq is 2,2′biquinoline) were obtained in PC, acetonitrile, and methylene chloride.50 The rates were found to be solvent-dependent, with the greatest isomerization rate observed in PC, relative to acetonitrile (intermediate) and DCM (smallest). As the oxidized RuIII isomerizing species are not expected to exhibit radically different dipole moments between the two compounds, these data were rationalized by assigning the isomerization rate difference to solute−solvent interactions predominated by solvent viscosity rather than dielectric constant. Feringa and co-workers have reported similar observations of viscous solvent effects facilitating nuclear rearrangements.51 Rapid, unidirectional cis-trans isomerization in sterically overcrowded alkenes occurred more rapidly in highly viscous media and was rationalized as a consequence of the metastable species occupying less space than the ground state isomer. The nuclear distortions that preempt isomerization produce a void in viscous solvents, which would otherwise not occur in less viscous solvents and thus decrease solvent friction to enhance isomerization rates. A similar analysis was applied to the ruthenium sulfoxide electrochemical isomerization. As in the OSO complexes, both dielectric and viscous effects are surely operative in the photoisomerization of [Ru(bpy)2(pySO)]2+. Moreover, the variation observed in the fast and intermediate time components in the various solvents demonstrates that the excited state potential energy surface is also influenced by characteristics of the solvent. In light of these data, variation in the isomerization time constants with respect to solvent likely reveals differences in the coupling of the excited state to the ground state, beyond simple contributions from the friction of solvent viscosity. Computational studies and recent experimental work indicate that the relevant excited and ground state potential surfaces are strongly coupled and suggest the presence of a conical intersection or conical intersection seams that facilitate isomerization.36,40 In such a model, solvent may affect structural dynamics and the electronic dipole of the excited state molecule, perturbing the coupling of the excited and ground state surfaces. A most intriguing feature of the [Ru(bpy)2(pySO)]2+ complex is the observation of photochemical reversion from the O-bonded isomer to the S-bonded isomer. We have
Figure 3. Select transient spectra of O-bonded [Ru(bpy)2(pySO)]2+ in propylene carbonate photoexcitation at 472 nm. Early time traces (A; 0.7 ps, black to 10 ps, teal) (top) and late time traces (B; 10 ps, teal to 3 ns, burgundy) (bottom). Evidence of the laser pulse has been omitted for clarity.
reveal an intense, narrow absorption centered about 380 nm and a broad, featureless absorption to the red of 575 nm in the initial excited state spectrum. As in the excited state of the Sbonded isomer, the narrow 380 nm absorption is assigned to π* → π* transitions on the reduced bpy ligand, while the absorption to the red of the spectrum is assigned to LMCT transitions from the unreduced bpy, with contributions from the LC transition. The negative feature centered near 470 nm is a consequence of ground state bleach and the pump laser pulse, which has been omitted for clarity. As time progresses, the excited state absorption features universally decay, and the bleach at 470 nm decreases in intensity. A mild bathochromic shift of the 380 nm band is observed. Ultimately, all absorption features in the red region of the spectrum decay to zero ΔOD, while a new absorption band is evident near 390 nm with a persistent, shallow bleach at 470 nm. The 3000 ps spectrum at the end of the experiment agrees with previous measurements of this complex and is interpreted as the difference between the spectra of the S- and O-bonded ground state isomers. Thus, the final transient spectrum is consistent with formation of the Sbonded isomer produced from excitation of the initial Obonded isomer. Global fitting analysis reveals a biexponential relaxation with time constants of 7.2 (0.9) ps and 162 (30) ps. A subpicosecond time component, attributable to the formation of the 3MLCT excited state from the initially formed 1FC and higher lying excited states, is not observed, and is presumably faster than the temporal resolution afforded by our 10429
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instrumental setup. As in the S → O isomerization kinetics, the
