Unusually Short-Lived Solvent-Dependent Excited State in a Half

Article Cite This: J. Phys. Chem. A 2018, 122, 1550−1559

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Unusually Short-Lived Solvent-Dependent Excited State in a HalfSandwich Ru(II) Complex Induced by Low-Lying 3MC States Kilian R. A. Schneider,†,‡ Philipp Traber,‡ Christian Reichardt,†,‡ Henning Weiss,§ Stephan Kupfer,‡ Helmar Görls,§ Stefanie Graf̈ e,*,‡ Wolfgang Weigand,*,§ and Benjamin Dietzek*,†,‡ †

Department Functional Interfaces, Leibniz Institute of Photonic Technology (IPHT) e. V., Albert-Einstein-Straße 9, 07745 Jena, Germany ‡ Institute of Physical Chemistry and Abbe Center of Photonics and §Institute for Inorganic and Analytical Chemistry, Friedrich-Schiller-University Jena, Helmholtzweg 4, 07743 Jena, Germany S Supporting Information *

ABSTRACT: A ruthenium complex with a half-sandwich geometry ([(p-cymene)Ru(Cl)(curcuminoid)]) was synthesized, characterized, and investigated regarding its ultrafast photophysics. These photophysical investigations of the complex revealed a weak and short-lived emission from the initially populated 1MLCT state and solvent-dependent photoinduced dynamics, where the secondarily populated 3MC state is stabilized by nonpolar solvents. Overall the decay of the 3ddMC state to the ground state is completed within picoseconds. This short excited-state lifetime is in stark contrast to the typically observed long-lived 3MLCT states with lifetimes of nanoseconds or microseconds in unstrained, octahedral ruthenium complexes but is in good agreement with the findings for distorted octahedral complexes. This is pointing to the half-sandwich geometry as a new and easy approach to study these otherwise often concealed dd states. 3

1. INTRODUCTION Ruthenium complexes attract broad interest in a variety of different fields, e.g., dye-sensitized solar cells,1−3 photocatalysts,4−6 as molecular sensors,7−9 or as photoactivated medical agents.10−15 This impressive breadth of applications roots in the variety of excited states which are accessible upon photoexcitation, including metal-to-ligand charge transfer (MLCT), metal-centered (MC), ligand-to-ligand charge transfer (LLCT), and intraligand (IL) states.16 The properties of these states can be easily tuned by ligand design.17−20 Generally, the lifetime of the excited states is (comparably) long due to efficient intersystem crossing (ISC) from the initially populated singlet to the more stable triplet states.21−23 The long-lived (τ ≥ 1 ms) and often bright triplet states in Ru(II) polypyridine complexes are typically of MLCT character. These states also play a key role for applications of these metal complexes. Therefore, they are most intensively studied. In order to achieve even longer excited-state lifetimes, low-lying 3IL states have been employed, i.e., by virtue of extend π-electron systems of the ligand, e.g., based on pyrene or oligothiophenes.24−27 On the other hand, the extension of the ligand’s π-system lowers the energy of the 3MLCT state and increases the energy gap to 3dd-MC states, which are prominent excited-state deactivation channels in photoactive Ru(II) complexes.28−32 The impact of the population of 3ddMC states in Ru(II) polypyridine complexes is manifested in temperature-dependent luminescence lifetime studies on such systems.33−35 An alternative approach to assess the impact of © 2018 American Chemical Society

dd-MC states on the lifetime of Ru(II) complexes is based on comparing the emission lifetimes of structurally similar complexes with different degrees of distortion from an ideal octahedral coordination environment. Such analysis reveals that a stronger distortion of a complex causes the energy difference between the 3MLCT and the 3dd-MC states to shrink consequently, the excited-state lifetime decreases.36−38 This observation is in line with ligand field theory, which describes a metal-centered σM * orbital of generally higher energy than the respective ligand associated molecules but can be accessed at higher temperatures or in distorted complexes, when the orbital overlap of the metal and the ligand is diminished.38 This structurally sensitive luminescence was utilized to design molecular sensors based on crown ether-extended bipyridine ligands coordinated to a Ru(II) ion: Complexation of a metal ion by the crown ether leads to structural stiffening of the ligand, and consequently the luminescence of the complex is switched on.39−41 Despite their critical importance in determining the lifetime of excited states in widely used photoactive Ru(II) complexes, 3dd-MC states have eluded a direct observation by means of time-resolved spectroscopy for a long time. Their lifetime has been considered short as the thermal activation of the 3dd-MC states from the lower lying long-lived 3MLCT states is considered the rate-limiting step of Received: November 21, 2017 Revised: January 24, 2018 Published: January 25, 2018 1550

DOI: 10.1021/acs.jpca.7b11470 J. Phys. Chem. A 2018, 122, 1550−1559

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The Journal of Physical Chemistry A

multiconfigurational methods, such as the second-order perturbational treatment of a complete active space selfconsistent field (CASSCF)49 or a restricted active space selfconsistent field (RASSCF)50,51 reference wave function (CASPT2 and RASPT2),52−54 and density functional theory (DFT) are widely applied in the literature to address the photophysics of transition metal complexes incorporating scalar relativistic (SR) effects.55−57 Although the latest advancements in the field of multiconfigurational methods, e.g., by means of the generalized active space self-consisted field (GASSCF)58 methodology or the density matrix renormalization group (DMRG),59,60 the computational demand of such calculations is still significant. A computationally less demanding alternative is given by the scalar-relativistic zeroth-order regular approximation (SR-ZORA) that allows to assess spin−orbit coupling at DFT and time-dependent DFT (TD-DFT) levels of theory. Molecular dynamical (MD) simulations have been successfully coupled to on-the-fly SR calculations for medium-sized organic compounds as well as (highly symmetry) transition metal complexes to yield an unambiguous rationalization of the excited-state relaxation process leading, i.e., to energy and/or electron transfer.61−63 In the present contribution DFT and TD-DFT simulationsaddressing SR effects by means of SRZORAare employed to elucidate the excited-state relaxation channels of 1 along an effective energy dissipation coordinate within the coupled singlet−triplet manifold.

excited-state decay in Ru(II)−polypyridine complexes. Few attempts to spectroscopically characterize such states were realized, e.g., in solid phase experiments or indirect lifetime calculations from time-resolved experiments within certain kinetic models.42,43 The first direct observation of dd-states was made by Hauser et al. in geometrically constrained octahedral ruthenium complexes with e.g. 6-methylbipyridene as ligand. Hauser et al. concluded that a constrained ligand sphere leads to weakened ligand-field strength as well as a longer N−Ru bond length. Thereby the MC-dd state gets energetically favorable and thus populated.44 However, the pool of ligands to achieve MC population is limited due to the trade-off of lowering the dd-MC state versus a too fast depopulation.45 Therefore, a new approach to get hand on and modulate these MC states is postulated in this paper. For the studies presented here, a new complex ([(pcymene)Ru(Cl)(curcuCF3)], shown in Figure 1) was synthe-

2. EXPERIMENTAL SECTION The synthesis of the complex is described in the Supporting Information. 2.1. Crystal Structure Determination. Crystals suitable for X-ray crystallography were grown by slow diffusion of nhexane into a solution of 1 in chloroform. The molecule crystallizes in the monoclinic space group P21/n together with one molecule of chloroform per unit cell. The isopropyl group and one CF3 group are distorted due to rotation. The ruthenium atom displays the expected half-sandwich geometry (see Figure 2). Both the backbone and the phenyl rings were found to be twisted. The angle between the planes formed of each phenyl ring is 9.9°, while the dihedral angle C(13)− C(1)−C(2)−C(3) is approximately 10.4°. The intensity data were collected on a Nonius KappaCCD diffractometer, using graphite-monochromated Mo Kα radia-

Figure 1. Structures of compound 1 and its precursors 1a and 1b.

sized. The ruthenium carries an η6-bound cymene, a chloride, and a curcuminoid ligand, whose extended π-system can take part in charge transfer transitions and increases the extinction coefficients in the visible spectral range, while the photostability of the complex is enhanced by the substitution of the aryl part of the curcuminoid.46 This compound can be considered as a model for examining the photophysics of half-sandwich structured ruthenium complexes and by virtue of the cymene ligand contains a strongly strained coordination environment of the Ru(II) ion. Thus, this geometry is expected to lower the 3 dd-MC states and makes them accessible for direct spectroscopic investigation. The photophyiscal studies presented combine steady-state absorption and emission (and emission lifetime) spectroscopy with (scalar-relativistic) quantum chemical calculations and femtosecond transient absorption spectroscopy (fs-TA). The quantum chemical calculations allow us to unravel the nature of the excited states within the Franck−Condon (FC) region as well as to identify photorelaxation channels, e.g., correlated to 3 dd-MC states. To investigate the initial photoexcitation as well as the subsequent excited-state processes such as ISC and relaxation within the triplet manifold (scalar), relativistic effects need to be addressed computationally.47,48 Both state-of-the-art

Figure 2. ORTEP drawing of 1. Hydrogen atoms, atoms created by distortion, and solvent molecule are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ru(1)−C(min): 2.176(2); Ru(1)− C(max): 2.204(3); Ru(1)−O(1): 2.0756(15); Ru(1)−O(2): 2.0760(15); Ru(1)−Cl(1): 2.4274(6); O(1)−Ru(1)−O(2): 88.02(6); O(1)−C(1)−C(2) 126.0(2). The ellipsoids represent a probability of 40%. 1551


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Figure 3. Left: steady-state absorption spectra in different solvents (blue: DCM; black: ACN; red: DMSO) and simulated spectrum (black dashed line, SR-ZORA-B3LYP/def2-SV(P)) in ACN, broadening with Gaussian functions with 0.20 eV fwhm. Right: charge density differences for prominent spin−orbit states.

tion. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semiempirical basis using multiple scans.64−66 The structure was solved by direct methods (SHELXS67) and refined by full-matrix least-squares techniques against Fo2 (SHELXL-9767). All hydrogen atom positions were included at calculated positions with fixed thermal parameters. MERCURY68 was used for structure representations. Crystal Data for 1: C31H27ClF6O2Ru*CHCl3, Mr = 801.41 g mol−1, red-brown prism, size 0.120 × 0.098 × 0.088 mm3, monoclinic, space group P21/n, a = 10.4075(2), b = 11.2696(3), c = 28.2897(6) Å, β = 95.544(1)°, V = 3302.53(13) Å3, T = −140 °C, Z = 4, ρcalcd = 1.612 g cm−3, μ(Mo Kα) = 8.61 cm−1, multiscan, transmin: 0.6869, transmax: 0.7456, F(000) = 1608, 21 113 reflections in h(−13/13), k(−14/14), l(−36/34), measured in the range 2.31° ≤ Θ ≤ 27.10°, completeness Θmax = 99.3%, 7249 independent reflections, Rint = 0.0295, 6571 reflections with Fo > 4σ(Fo), 457 parameters, 0 restraints, R1obs = 0.0324, wR2obs = 0.0690, R1all = 0.0376, wR2all = 0.0718, GOOF = 1.069, largest difference peak and hole: 0.991/−0.573 e Å−3. 2.2. Sample Preparation for Spectroscopic Experiments. Compound 1 was investigated spectroscopically by steady-state absorption, emission, and femtosecond timeresolved transient absorption spectroscopy. The respective experiments were performed at room temperature in airequilibrated solutions of acetonitrile (ACN), dichloromethane (DCM) and dimethyl sulfoxide (DMSO). The solvents were obtained in spectroscopic-grade from Sigma-Aldrich and used without further purification. All measurements were executed in quartz glass cuvettes with a path length of 1 mm and with a concentration of approximately 1.2 mmol/L, yielding an optical density of 0.4 at 400 nm for transient absorption measurements. Emission measurements were performed in a 1 cm cuvette with an optical density of 0.1 at 400 nm; a Jasco V-670 spectrophotometer was used for measuring the steady-state absorption spectra before and after each fs-TA measurement to ensure sample integrity. Steady-state emission experiments were performed using a Jasco FP-6200 spectrofluorometer (λex

= 400 nm), while the emission lifetime was investigated using a Streak camera (Hamamatsu, C4792-01) with a temporal resolution of approximately 10 ps and laser excitation at 400 nm. The fs-TA experiments were performed using a custom-built setup which is described in detail elsewhere.69 The measurements were performed upon excitation at either 403 or 470 nm. The excited-state dynamics were probed with a white-light supercontinuum generated by focusing a minor part of the amplifier output into a CaF2 plate. The data were chirpcorrected and subsequently analyzed using a sum of exponential functions for a global fit. The temporal resolution of the experiment is limited to 300 fs due to strong contributions from coherent artifact signals to the data,70,71 which impair the reliable analysis of the pump−probe data by means of multiexponential fitting at shorter delay times. To study the solvent dependence of the light-induced dynamics, different solvents were used, which vary in their polarity and dynamic viscosity: ACN (ε = 35.7, μ = 0.39 mPa s), DCM (ε = 8.93, μ = 0.42 mPa s), DMSO (ε = 46.7, μ = 2.14 mPa s). 2.3. Computational Methods. All quantum chemical calculations on 1 were performed using Gaussian 09.72 The (singlet) ground-state structures and electronic properties were obtained at the DFT level of theory by means of the exchange correlation functional B3LYP73,74 in combination with the def2SV(P)75,76 basis set, as implemented in the EMSL basis set exchange library.77,78 A subsequent vibrational analysis performed for all optimized structures (solvent: ACN, DCM, and DMSO) revealed minima on the 3N − 6 dimensional potential energy hypersurface (PES). For the excited-state calculations, TD-DFT was applied with the aforementioned computational setup. Excited-state properties, namely excitedstate characters, excitation energies, and transition dipole moments, were calculated for the 100 lowest singlet and triplet excited states. Furthermore, geometry optimizations were performed at the TD-DFT level of theory for selected triplet states of 3MLCT and 3MC character. Solvent effects (ACN, DCM, and DMSO) were treated by the integral equation formalism of the polarizable continuum model (IEFPCM)79 for 1552


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The steady-state emission measurement (λex = 400 nm) yielded very weak emission (see Figure 4), which impaired the

all ground- and excited-state properties. The nonequilibrium procedure of solvation was applied for the calculation of excitation energies within the FC region, which is well adapted for processes where only the fast reorganization of the electronic distribution of the solvent is important. In contrast, the equilibrium procedure of solvation was used for excitedstate geometry optimizations. Spin−orbit coupled singlet−singlet and singlet−triplet excited states on the previously optimized equilibrium structures (Gaussian 09) were calculated by means of the scalar-relativistic zeroth-order regular approximation (SRZORA)80−82 TD-DFT as implemented in the Amsterdam Density Functional (ADF 2017)83 software package. The B3LYP (VWN5 version) exchange-correlation functional was used with a polarized triple-ζ slater type orbital (STO) basis set for ruthenium, ZORA-TZP, and a polarized double-ζ STO basis set, ZORA-DZP, for the remaining atoms.84 Spin−orbit couplings (SOCs) between the 20 lowest singlet excited and triplet states were obtained based on ref 47 using the perturbative SOC approach.85,86 Solvent effects were addressed using the Conductor-like Screening Model (COSMO) for ACN.87,88 Molecular graphics were performed with UCSF Chimera;89 cube files were created using either Multiwfn 3.490 or ADFview.

Figure 4. Emission spectra (λex = 400 nm) of compound 1 in different solvents (blue: DCM; black: ACN; red: DMSO). Inset: the respective excitation spectra (∗ indicating Raman bands).

determination of quantum yields. Nonetheless, a solventdependent emission band shift is apparent with the most hypsochromic emission in DCM at approximately 480 nm. The emission lifetime was measured to be 7 ps in ACN (Supporting Information Figure S1). The maximum of the excitation spectra is at about 400 nm in all solvents, significantly shifted bathochromically compared to the absorption spectrum (λmax = 360 nm). The evident difference between excitation and absorption spectrum will be addressed later, since a photophysical model is needed for the argument. The weak emission of 1 significantly differs from the emission of (prototypically considered) octahedral ruthenium complexes which quite generally show a rather strong (φem ≈ 0.04) and long-lived (τem = 600 ns) emission in the range of 600−700 nm.29,91 Though 1 differs significantly from the prototypical [Ru(bpy)3]2+, these numbers are helpful to get a general idea of typical ruthenium complex parameters. Therefore, we conclude that the emission characteristics of 1 point to fluorescence, which would be assigned to a bright transition of the initially populated singlet state before ISC to the triplet manifold proceeds, thereby explaining the comparable small Stokes shift and the short lifetime of the emission. This will be further discussed later on with the time-resolved data. 3.2. Excited-State Relaxation Scheme. As shown in section 3.1, the initial photoexcitation at 403 nm (3.08 eV) leads to the population of the bright spin−orbit states SO39 and SO40 at 3.02 eV, which mainly comprise an 1IL π−π* excitation localized on the curcuminoid ligand, while excitation at 470 nm (2.64 eV) predominantly populates SO16 and SO17 at 2.47 eV. Subsequently, excited-state relaxation cascades may lead to a population transfer to the lowest excited SO state. This doubly degenerate 3MLCT state (SO1/SO2) is predicted at 1.99 eV in the FC region, while its electronic character changes within the adiabatic picture upon structural equilibration (see Figure 5A): In the FC region, this state is of 3MLCT character, while upon structural relaxation it is converted into a 3MC state (schematically shown in Figure 5B). The underlying photoinduced exited-state relaxation processes are further investigated along a linear interpolated Cartesian coordinate (LICC) connecting the FC region with the 3MC equilibrium, illustrated in Figure 5D by virtue of displacement vectors. Prominent structural variations, i.e., with respect to the coordination sphere of the ruthenium between the FC region and the 3MC

3. RESULTS AND DISCUSSION 3.1. Steady-State Absorption and Emission Spectroscopy. For a basic understanding of the optical properties, the UV−vis steady-state absorption spectra are reported in Figure 3. The absorption spectra of 1 in different solvents (DMSO, ACN, and DCM) show qualitatively similar behavior with three prominent bands at 460, 360, and 295 nm. The nature of the electronic transitions underlying the experimental spectra is revealed by quantum chemical calculations. The most prominent band is located at 360 nm and is accompanied by a broad band in the blue part of the spectrum, i.e., at 460 nm. SR-ZORA TD-DFT assigns the absorption at 460 nm to the degenerate spin−orbit (SO) states SO16 and SO17 (at 2.47 eV) of mixed electronic character. Both spin− orbit states feature considerable singlet−triplet mixing (SO16: 42% singlet, 58% triplet, SO17: 52% singlet, 48% triplet). The singlet and triplet states are predominantly of 1MLCT character, involving a charge transfer from the ruthenium to the curcuminoid ligand, and of 3MC character, respectively. The broad UV band at 360 nm is mainly associated with a 1ππ* transition involving the bright IL state SO40 ( f = 0.90) at 3.02 eV, which is centered on the curcuminoid; in addition, the less absorbing 1MLCT SO39 (f = 0.33, 3.02 eV) contributes to the absorption feature. Both SO states can be associated with (almost) pure singlet states with 88% and 94% singlet contributions. Interestingly, all four excitations populate the same curcuminoid π*-orbital (LUMO, MO 173 in Table S1). The respective charge density differences for SO16, SO17, SO39, and SO40 are illustrated along the simulated absorption spectrum in Figure 3. The absorption feature at 295 nm can be assigned to several transitions of MLCT (toward the curcuminoid ligand) and IL character (see Table S6 for more details). It should be noted that neither of the bright excitations in the absorption spectrum involves the p-cymene ligand, which apparently does not actively take part in the ground-state absorption of the complex but merely influences the ligand field of the ruthenium center. 1553


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Figure 5. (A) Singlet−singlet (dashed lines) and singlet−triplet (straight lines) excited states along the first eight LICC steps obtained by ADF. Contributions of the 3MC state from the Franck−Condon region to its equilibrium is visualized by red dots. Diabatic PES of the 3MC is highlighted in red. (For the complete 20 steps of the LICC, see Figure S4.) (B) Thereof derived Jablonski scheme. (C) Relevant spin−orbit couplings from initiated S7 to T5 and T6. (D) Displacement vectors visualizing the reaction coordinate (LICC). (E) Comparison of transient spectra of 1 in DMSO between excitation at 470 (solid line) and 403 nm (dashed line) after 0.5 (black) and 10 ps (gray).

most favorable state. These processes are considered to occur rapidly in or at least in close proximity to the FC region, therefore spanning a time window of presumably only few hundreds of femtoseconds. To further evaluate this model and elucidate the following dynamics, the excited-state physics of compound 1 was examined using fs-TA. 3.3. Femtosecond Transient Absorption Spectroscopy. The excited-state dynamics of 1 was monitored upon excitation both at 403 and 470 nm, i.e., exciting the red flank of the MLCT transition (at 470 nm) and a mixed IL/MLCT band (at 403 nm). According to the calculations discussed before, both excitations are expected to yield the same 3MC state. This theoretical prediction is in line with the experimental observation that the transient spectra and kinetic traces (even at short time scales) recorded upon excitation at 403 and 470 nm (see Figure 5E) show qualitatively identical excited-state relaxation. The dissipation of excess energy upon excitation at 403 nm and the ISC and the charge migration from MLCT to MC state are apparently extremely rapid and cannot be resolved in the experiments presented here due to strong contributions of coherent artifact signals adjacent to the pump−probe overlap. The transient absorption spectra and kinetic traces of 1 in all solvents are shown in Figure 6. The spectrum in DMSO reveals a double-peak structure below 500 nm with an energy difference between the peaks at 465 and 490 nm of approximately 2000 cm−1 (see Figure 6C). This doublemaximum peak is accompanied by a broad excited-state absorption at longer wavelengths, i.e., peaking beyond the experimentally accessible spectral range. In ACN the maximum of the strong visible band appears shifted toward 400 nm, but its double-peak structure remains visible, in particular in the spectra recorded with 1.5 and 15 ps delay time. Also in ACN an unstructured and broad red-to-far-red absorption feature is

equilibrium, are collected in Figure S5, while the charge density differences of the 3MC state within both equilibrium structures are visualized in Figure S6. The structural variations along the LICC can be mainly associated with a twist of the cymene ligand with respect to the ruthenium−curcuminoid moiety, which results directly from the population of the 3MC state and induced alteration of the electronic structure at the ruthenium. For reasons of clarity, the adiabatic PESs calculated along the LICC, depicted in Figure 5A, will be discussed by virtue of the leading excited singlet and triplet states underlying the respective spin−orbit states. SO39 and SO40 originate from linear combinations of the dark S6, the bright S7, and the triplet state T11 (see Table S3 for more details). Upon ISC the population is transferred from the initially populated bright 1IL state S7 (3.02 eV) to the 3MLCT state T11 (2.94 eV) by means of a SOC of 91 cm−1. T11 is coupled to the 3MC states T6, T7, and T8 (at 2.50, 2.51, and 2.57 eV) by SOCs of 173, 173, and 238 cm−1; furthermore, T5 (2.44 eV in FC region) is accessible upon vibrational cooling and internal conversion from T7 coupled by 342 cm−1 as well as from T8 with 363 cm−1 (see Figure 5C for more details). Subsequently, the 3MC state, T5, relaxes to the triplet ground state along the LICC. SO16 and SO17accessible upon excitation at 470 nmoriginate from mixing of S1 MLCT state (2.46 eV)degenerate with the 3MC T5and the triplet states T6 and T7 of 3MLCT/3MC character (2.50 and 2.51 eV) (see Tables S1−S3). Thus, excitation at 470 and 403 nm leads in consequence to the population of the 3MC (ground) state along the LICC. The proposed relaxation process on the adiabatic PESs (singlet and triplet) along the LICC is highlighted in Figure 5A. The calculations give insight into the excited states landscape of 1, where after a primarily population of a 1MLCT state subsequent relaxation processes like ISC and charge redistribution take place, yielding a 3MC state as an energetically 1554


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Figure 6. Temporal evolution of transient absorption spectra for 1 (right) in different solvents (A: DCM; B: ACN; C: DMSO) at 0.5 (red), 1.5 (orange), 10 (green), 100, and 1500 ps (purple) after pumping at 403 nm. After 50 ps (ACN and DCM) and 400 ps (DMSO) no further changes were observed. Kinetic traces (left) of compound 1 at different wavelengths in different solvents (D: DCM; E: ACN; F: DMSO).

Figure 7. Decay associated spectra of compound 1 in DCM (A), ACN (B), and DMSO (C) with two or three time constants.

Using a global fit routine (see Figure 7), the decay is characterized by one fast (τ1 = 0.4, 0.5, and 0.2 ps in DMSO, ACN, and DCM, respectively) and a slower (τ2 = 8.0, 8.0, and 5.6 ps in DMSO, ACN, and DCM, respectively) kinetic component. After approximately 50 ps, the transient absorption signal of 1 in DCM and ACN has essentially decayed to zero, indicating that the system has fully relaxed back to the ground state. In DMSO a third, slower kinetic component, τ3 = 67 ps, is identified (see Figure 6F). The presence of this comparably slow decay explains the fact that the ΔOD signal of 1 in DMSO only vanished after more than 350 ps. Notably, the transient absorption kinetic traces in DCM show a negative ΔOD signal at around 475 nm, which builds up within 1 ps and

visible (see Figure 6B). In DCM the general shape of the transient-absorption spectrum seems to be preserved, but the bands are shifted to much shorter wavelengths (see Figure 6A). This spectral shift of the differential absorption features can be correlated with solvent polarity: The spectrum in the most polar solvent DMSO shows a strong double band at 465 and 490 nm, whereas in ACN this can be seen at 412 and 450 nm, and in the least polar solvent DCM a strong rise at around 410 nm is evident. The long-wavelength excited-state absorption in DCM is at around 600 nm, while in ACN and DMSO it appears significantly shifted to longer wavelengths. Irrespective of the solvent, a global decay of the initially visible excited-state absorption is prominent (see Figure 6D,E). 1555


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shifts in the ESA band at short time scales in DMSO indicates that in the higher polar solvent the 3MC state is barely stabilized, and hence no vibrational cooling is observed (see Figure S2). However, the high polarity of DMSO stabilizes MLCT states. This leads to the population of a stabilized 3 MLCT state, which is only accessible from the nonstabilized 3 MC state in DMSO. This process is manifested in the third characteristic time constant τ3 only observed upon dissolution of 1 in DMSO. The spectral changes associated with τ3 reveal a change from the double-band feature to a single broad banda feature which is not observed, e.g., in ACN. Here, an overall decay of the double-peak structure is apparent (see Figure S3).

subsequently decays within 10 ps (see Figure 6D). This band might be associated with emission of 1. In DMSO and ACN, no negative differential absorption signal (indicative of emission) is visible, which can be rationalized considering that the strong excited-state absorption is shifted bathochromatically in ACN and DMSO compared to DCM. Thus, in the more polar solvents, the excited-state absorption spectrally overlaps with the weak emission band, rendering it nondetectable in the pump−probe experiment. The first, fast component τ1 observed in the TA data describes the formation of the structurally relaxed 3MC state, which is populated via ISC from the initially excited 1MLCT state. Such low-lying 3MC states being accessible by downhill processes from initially excited MLCT states seem to be characteristic for Ru(II) half-sandwich processes; i.e., their presence was also inferred by Sadler et al. upon working with [(p-cymene)Ru(2,2′-bipyrimidine)(pyridine)]2+.92 This result can be rationalized by considering a half-sandwich complex as a strongly distorted octahedron due to geometric constraints. Thereby, the respective MLCT state gets destabilized compared to the MC states.36,38,44,45 Our results show that in DCM and ACN the MC state is depopulated to the electronic ground state with the second slower time constant τ2. The fast ground-state decay observed for the half-sandwich complex of Ru(II) is in good agreement with the results of Hauser et al.44 of several picoseconds to several hundreds of picoseconds, depending on the distortion of the octahedral geometry. The findings pointing to a direct observation of the formation and decay of a dd-MC show that half-sandwich Ru(II) complexes might present an intriguing new route to directly study dd-MC states via a new structure motif. As mentioned above, very weak fluorescence was observed from the complexes. While the emission spectrally overlaps with the lowest energy ground-state absorption band, it is most efficiently excited in the long-wavelength flank of the intense 360 nm absorption. The latter one was assigned to a mixture of 1 ππ* and 1MLCT transitions. These experimental findings in context of the photophysical model put forward here indicate that ISC (quenching the emission) occurs most favorably from vibrationally hot excited states. In contrast, molecules excited near the vibrational ground state of the 1MLCT state have a higher propensity to decay via fluorescence as they cannot decay via ISC into the triplet manifold and get trapped in emissive singlet states. This model is supported by the steadystate spectra, which show a significant red-shift of 0.344 eV of the excitation maximum compared to the absorption spectrum, thereby reaffirming that more fluorescence photons are created by exciting near the vibrationally cold excited state. To explain the third component τ3, which is only apparent in DMSO, the influence of the solvent needs to be considered. Assuming spectrally similar excited-state absorption in all solvents (i.e., intense double-band structure and broad bathochromic absorption), a model is suggested in which the electronic states underlying the excited-state absorption features are spectrally shifted as the polarity of the solvent varies. The MC state can be considered nonpolar; thereby, stabilization by nonpolar solvents (i.e., DCM, noticeably less in ACN) explains the blue-shift of the ESA in DCM and ACN compared to DMSO. The 3MC state appears to be most stabilized in DCM, therefore leading to a larger energy gap between the initially populated 1MLCT and this state in DCM than in DMSO. Consequently, a vibrational cooling during the first picosecond is visible in DCM. The absence of any spectral

4. CONCLUSION In this work the complex ([(p-cymene)Ru(Cl)(curcuCF3)] was synthesized, characterized crystallographically, and investigated spectroscopically with regard to its ultrafast photophysics, using steady-state as well as time-resolved techniques and scalarrelativistic TD-DFT simulations. To our knowledge, this is the first systematic study of the excited-state dynamics of any halfsandwich (or arene) structured ruthenium complex with steady-state and transient absorption measurements as well as quantum chemical calculations. The joint experimental− theoretical approach pursued shows that low-lying 3MC states, which are stabilized in nonpolar solvents, dominate the excitedstate relaxation pathway in the complex: The initially populated 1 MLCT state is depopulated through emission as well as intersystem crossing to a 3MC state. In DCM and ACN a direct decay to the ground state is visible. In DMSO a third state of MLCT character is accessible through the high solvent polarity with a significantly longer ground-state recovery. The presence of the characteristic 3MC states can be rationalized by considering the half-sandwich structure as highly distorted octahedral. This reinforces the idea by Hauser et al.44 that reordering of the vital states can be realized by altering the ligand sphere, e.g., introducing strained ligands or a geometrically distorted coordination environment. However, we were able to show a different approach for reaching suitable energy levels by using an arene complex as a border case of distorted octahedral complexes. Though the first results seem promising, systematic studies on the impact of structural variations of the complex and the ligands, i.e., the examination of an octahedral Ru−curcurminoid and a ruthenium half-sandwich complex, remain to be performed. This work will also have to generalize the reaction coordinate, which was assigned here by the scalarrelativistic calculations.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b11470. Synthesis of precursors and compound 1; wavelengths (λ), excitation energies (E), oscillator strengths ( f), leading transitions, and involved MOs for several transitions; integrated photon count trace of ultrafast streak camera measurement of compound 1 in ACN; transient spectra at short respectively long time period (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.G.) E-mail [email protected]. 1556


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ORCID

Philipp Traber: 0000-0003-3392-2573 Stephan Kupfer: 0000-0002-6428-7528 Stefanie Gräfe: 0000-0002-1747-5809 Benjamin Dietzek: 0000-0002-2842-3537 Author Contributions

K.R.A.S. and P.T. contributed equally. Funding

K.R.A.S. thanks the Carl-Zeiss-Stiftung for support. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K. and S.G. thank the Thuringian State Government for financial support within the ACP Explore project. All calculations have been performed at the Universitätsrechenzentrum of the Friedrich Schiller University.

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ABBREVIATIONS MLCT, metal-to-ligand charge transfer; MC, metal-centered; LLCT, ligand-to-ligand charge transfer; IL, intraligand; ISC, intersystem crossing; ACN, acetonitrile; DCM, dichloromethane; DMSO, dimethyl sulfoxide; SOC, spin−orbit coupling; fs, femtosecond; ps, picosecond; FC, Franck− Condon; CASSCF, complete active space self-consistent field; RASSCF, restricted active space self-consistent field; DFT, density functional theory; SR, scalar relativistic; GASSCF, generalized active space self-consisted field; DMRG, density matrix renormalization group; SR-ZORA, scalar-relativistic zeroth-order regular approximation; TD-DFT, time-dependent DFT; MD, molecular dynamical.

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