Probing the Fate of Lowest-Energy Near-Infrared Metal-Centered

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Probing the Fate of Lowest-Energy Near-Infrared MetalCentered Electronic Excited States: CuCl and IrBr 42-

62-

Sergey M. Matveev, Andrey S. Mereshchenko, Maxim S. Panov, and Alexander N. Tarnovsky J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b00744 • Publication Date (Web): 03 Feb 2015 Downloaded from http://pubs.acs.org on February 9, 2015

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The Journal of Physical Chemistry

Probing the Fate of Lowest-Energy Near-Infrared MetalCentered Electronic Excited States: CuCl42- and IrBr62Sergey M. Matveev, Andrey S. Mereshchenko,† Maxim S. Panov,† Alexander N. Tarnovsky* Department of Chemistry and the Center for Photochemical Sciences, Bowling Green State University, Bowling Green, OH 43403, United States AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Present Addresses †Institute of Chemistry, Saint-Petersburg State University, 198504, Saint-Petersburg, Russian Federation

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ABSTRACT: Ultrafast transient absorption spectroscopy is used to investigate the radiationless relaxation dynamics of CuCl42- and IrBr62- complexes directly promoted into their lowest-energy excited metal-centered states upon near-infrared femtosecond excitation at 2000 nm. Both the excited CuCl42- 2E and IrBr62- 2Ug’(T2g) states undergo internal conversion to the ground electronic states, yet with significantly different lifetimes (55 fs and 360 ps, respectively) despite the fact that the 2E and the 2Ug’(T2g) states are separated by the same energy gap (~5000 cm-1) from the respective ground state. This difference likely arises from the predominance of the Jahn-Teller effect in a Cu2+ ion and the spin-orbit coupling effect in an Ir4+ ion. The approach documented in this work may be used for elucidating the role of low-energy metal-centered states in relaxation cascades of a number of coordination compounds, allowing for design of efficient light-triggered metal complexes.

KEYWORDS: transition metal complexes, femtosecond pump-probe, Jahn-Teller effect, spinorbit effect, internal conversion, conical intersection, coherence, metal-centered transitions.

ACS Paragon Plus Environment

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I.

Introduction

The last three decades have witnessed an enormous expansion in the use of metal coordination complexes where the unique photophysical and photochemical properties make them ideal for a variety of applications ranging from C–H bond activation to anticancer photodynamic therapy and energy conversion.1-6 The complexes are photoactivated in the ultraviolet-visible region of the electromagnetic spectrum, where a high density of ligand-to-metal charge-transfer (LMCT), metal-to-ligand charge-transfer (MLCT), metal-centered (MC), and intraligand excited electronic states is present.7-9 From the initially excited Franck-Condon (FC) state, the complexes can decay non-radiatively to the states of the same multiplicity (internal conversion, IC) or unlike multiplicity (intersystem crossing, ISC).8 For understanding overall photophysical and photochemical properties, the knowledge about the state-to-state relaxation dynamics is crucial. Furthermore, it is important to relate this knowledge to the ground-state structure and nature of low-lying excited states to be able to efficiently design transition metal complexes required by the applications.8-11 In many transition metal complexes, the lowest electronic excited state corresponds to the promotion of a metal d electron from an occupied to a vacant MC orbital.7,9,11 Notably, for a wide range of transition metals with d1, d2, d4, and d5 electronic configurations in octahedral-like (Oh) coordination environment where the nature of the metal and environment lifts the degeneracy of the t2g metal orbitals as well as d9 electronic configuration in tetrahedral-like (Td) environment, these electronic states occur at energies of just 5,000-10,000 cm-1 above the ground electronic state. The corresponding MC absorption bands lying in the near infrared (IR, 0.7-2.5 µm) region are called d→d transitions. Both MC orbitals9 as well as excited electronic states lying less than 10000 cm-1 above the ground electronic state “have no simple organic analogs”.


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Direct optical preparation of MC excited states is hindered by small values of the molecular decadic extinction coefficient (ε) of d→d transitions in centrosymmetric environment (Laporte selection rule).12 Significant interest exists regarding the role of the MC excited states in the overall relaxation cascade,11,13 but literature reveals only a handful of studies, namely in the Cu center of blue copper proteins.14-17 These studies, however, are not in agreement regarding the relaxation mechanism involved (ultrafast, ≤300 fs) due to difficulties to discriminate the photophysics and photochemistry arising from a population of different MC excited states. The relaxation dynamics of the lowest-energy MC excited states in Oh-like and Td-like transition metal complexes so far have received no attention. In this work, we use ~85 fs, 2000-nm excitation laser pulses (the idler output of a TOPAS-C optical parametric amplifier pumped by a Ti:sapphire laser system) to resonantly excite the lowest-energy d→d transition of CuCl42- and IrBr62-, two important prototypes of the aforementioned d9 and d5 transition metal complexes. The ensuing dynamics of the photoexcited complexes is monitored via their visible absorption into high-lying LMCT excited states by means of ultrafast transient absorption spectroscopy.

II.

Experimental and Computational Details

Materials. Copper(II) perchlorate hexahydrate (98%), tetraethylammonium chloride (>98%), and acetonitrile (> 99.5%) were purchased from Sigma Aldrich. Chloroform (HPLC grade) was used as received from EMD Millipore Chemicals. A mixture of Cu(ClO4)2 (5mM) and NEt4Cl (200mM) was used in the ultrafast experiments to ensure that the vast majority of copper(II) in a form of CuCl42-. Under these conditions, 99% of copper(II) form CuCl42- whereas the remaining fraction forms CuCl3- in acetonitrile solutions, as we estimated using the “Medusa” software18


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with the following overall stability constants: β1= 3.4 × 108, β2= 4.3 × 1015, β3= 2.8 × 1022, and β4= 5.1 × 1025.19 Absorption spectra of the Cu(ClO4)2 and NEt4Cl mixture are similar in both solvents and chloroform is a poor ligand compared to acetonitrile, and therefore, CuCl42- also predominates in chloroform solutions. K2IrBr6 was purchased from Surepure Chemetals. To be soluble in acetonitrile and chloroform, potassium cations in K2IrBr6 were substituted by tetrabutylammonium cations (tBu4NBr precursor, Sigma-Aldrich). In the ultrafast experiments, 6 mM solutions of IrBr62- were utilized. Near-IR (800 to 2900 nm) and UV-vis absorption spectra of CuCl42- and IrBr62- solutions were measured using a Perkin-Elmer LAMBDA 750 and a Varian Cary 50 spectrophotometer, respectively. The spectra showed no decomposition of the solutions exposed to 2000-nm pulsed irradiation for several days (1-kHz train of ~90-fs, 14-μJ pulses focused into a 200-μm diameter spot). Ultrafast Transient Absorption. Our set-up based on a regeneratively amplified Ti:sapphire laser (85 fs, 800 nm, and 1 kHz) is described in detail elsewhere.20,21 The amplified beam was 50:50 split with the first half sent to a TOPAS-C optical parametric amplifier to generate signal (1333 nm) and idler (2000 nm) pulse pairs. The signal beam was filtered out. The idler pulses, mechanically chopped at half of the 1-kHz repetition rate of the amplified system such that only every second pulse reaches the sample, were used for excitation (9.5 μJ pulse-1). The second half of the amplified output was attenuated, sent through a computer-controlled optical stage to adjust the time delay with respect to the excitation pulse, and then focused onto a 3-mm CaF2 window to produce a white-light continuum spanning from 335 to 760 nm. The white-light continuum beam was split into a reference beam made to bypass the sample, and a probe beam, which, at the sample position, was focused to a 60-μm diameter spot and overlapped at an 6°-angle with the excitation beam focused to a 200-μm diameter spot. The probe (after the sample) and


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reference beams were dispersed by a spectrograph and recorded using a dual diode array synchronized to the 1-kHz repetition rate. The difference between the decadic logarithms of a probe-to-reference intensity ratio measured at a specific position of the optical stage for excitation on and off represents the change in the sample absorbance (ΔA) at the corresponding delay time. The ΔA values were averaged for 300 “on-off” pairs at each of ~120 delay times along the stage scan, and the whole procedure was repeated for ~70-100 times for averaging. The solutions were circulated through a Spectrosil UV quartz flow cell with a 0.2-mm path length. The linearity of the ΔA signals with excitation energy was ensured. The polarization of the excitation beam was set at the magic angle (54.7o) with respect to that of the probe beam to cancel the effect of the rotational diffusion on the recorded ΔA data. The ΔA signals measured for neat solvents (CH3CN and CHCl3) as well as NEt4Cl (200mM) in CH3CN were observed to be smaller and much smaller around zero and 100-200 fs delay times, respectively, than the corresponding ΔA signals measured for CuCl42- and IrBr62- solutions at the identical excitation conditions. This makes it possible to subtract accurately the solvent ΔA signals, after proper scaling to account for solute absorption, from the solution ΔA signals. The short-time (-100 to 200 fs) ΔA spectra of CuCl42- and IrBr62- were thus obtained. At delay times > 200 fs, the ΔA signals arise completely from excitation of CuCl42- and IrBr62-. All ΔA spectra were corrected for group-velocity dispersion in the probe path with an accuracy of 25 fs using the solvent signals around zero delay time.22,23 The ΔA kinetic traces were modelled as a sum of exponential functions convoluted with the 125-fs (fwhm, Gaussian) response function centered at zero delay time using software package Spectra-Solve™, version 1.5. The fit residuals, R(t), were analyzed using fast Fourier transform (FFT) and exponentially damped cosine function fits, R(t) = A·exp(-


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t/c)·cos(2vt + ), with the amplitude (A), the frequency (v), the phase (), and the decay time constant (c) set as free parameters, using Origin versions 5.0 and 7.0. Computational Methodologies. Geometry optimization of ground-state IrBr62- and CuCl42species and vibrational frequency calculations were carried out using density functional theory (DFT) with the B3LYP24 density functional, as implemented in Gaussian 09 program package.25 The 6-311G26,27 basis set and the Def2-tzvp28 basis set with effective core potential were utilized for CuCl42- and IrBr62-, respectively. In these and other computations mentioned below, the polarized continuum model (PCM)29 with the relevant solvent parameters was used for the description of the solvent. To examine the energetics of ionic dissociation of IrBr62-, we scanned the ground-state potential energy surface of this complex along the Ir-Br bond elongation coordinate with subsequent geometry optimization of the fragments in different environments such gas phase, chloroform, acetonitrile, and water. The computations were performed using B3LYP/Def2-tzvp/PCM, Gaussian 09. To evaluate the Ir-Br bond dissociation energy for radical photoreductive cleavage of IrBr62- in acetonitrile, we carried out single-point coupled-cluster single and double excitations (CCSD)30-33 computations on a IrBr52- and Br· separated pair computed using B3LYP/Def2-tzvp/PCM at a separation distance of 36 Å. For vibration energylevel density calculations, the Tardy, Rabinovitch, and Whitten approach was used, see eq. (3) in their work,34 with the total energy set to be = 5000 cm-1.

III.

Results and Discussion

CuCl42- and IrBr62- exhibit weak MC transitions in the IR region (ε ~ 100 M-1 cm-1, Fig. 1) and intense LMCT transitions in the UV-visible spectral range, Fig. 1. In acetonitrile, CuCl42- has D2d


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point group symmetry, resulting in d-orbitals with splitting shown in Fig. 1A, and, as a result, a Cu2+ ion has the ground electronic 2B2 state.19,35-37 Excitation at 2000 nm predominantly promotes CuCl42- into the 2E excited state,36 where an electron from one of the e d-orbitals is promoted to the single-occupied b2 d-orbital. The five t2g electrons of an Ir4+ ion occupy two degenerate doubly-occupied ug’ orbitals and a singly occupied eg” orbital in the IrBr62- Oh complex, resulting in the ground electronic 2Eg”(T2g) state, Fig. 1B.38-41 The first excited 2Ug’(T2g) state is formed via promotion of an electron from the ug’ to the eg” orbital. Energy splitting of the ug’ and eg” orbitals is due to spin-orbit coupling in Ir4+. Experimentally, the splitting between the 2

Eg”(T2g) and 2Ug’(T2g) states in IrBr62- is observed to be 5000 cm-1,39,40 corresponding to the first

absorption band of this complex at 2 µm.

A

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Figure 1. Near-IR steady-state absorption spectra of CuCl42- (A) and IrBr62- (B) in acetonitrile, spectral assignments of the contributing MC transitions, and one-electron orbital approximation of the CuCl42- 2B2 and IrBr62- 2Eg” (in Griffith double group notation40) ground state. The bottom panels show the UV-visible absorption spectra of CuCl42- and IrBr62- in acetonitrile. Within the first 100 fs following 2000-nm excitation of CuCl42- into the lowest-excited 2E state, we observe the induced absorption (positive ΔA signal) extending to ~620 nm with significant strength from 480 to 550 nm and the induced absorption between 340 and 380 nm, Fig. 2. These transient absorption signals are modulated by damped oscillations and superimposed on the negative ΔA signals (maximum strength, 405 nm), which arise due to bleaching of the ground state. From 100 to 400 fs, the ground-state bleach recovers by about half of its initial amplitude. In the same time interval, the transient absorption in the 520-620 nm range weakens strongly whereas the ΔA signals between 340 and 380 nm weaken less strongly. Multiexponential fits (convoluted with a 125-fs response function) of ΔA kinetic traces were obtained, Fig. 2 and Fig. 1S in Supporting Information, which result in a 55-fs and a 240-fs time constant for the initial decay in the 540-560 and 350-370 nm regions, respectively. Fast Fourier transform analysis (FFT) of the fit residues yields an oscillation frequency of 238 ± 26 cm-1, Fig. 2 and Fig. 2S. Fits of the residuals to a damped cosine function yield the oscillation frequencies that closely agree with those obtained by FFT and the coherence decay time constant of ~300 fs, Fig. 2S and Table 1S. In the blue and red wings of the long-wavelength transient absorption (395-415 and 540-560 nm) the 238-cm-1 oscillation exhibits a π-phase shift, Fig. 2. Following the initial decay phase, a differently shaped transient absorption is observed, which continues to decay, but on a slower time scale. The ground-state bleach peak (probe wavelength, 405 nm) recovers with a 2-ps time constant. The induced absorption decay faster, the more a probe wavelength is tuned towards the


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blue or red end of the spectrum (1.75 and 1.1 ps for the 350-370 and 540-560 nm wavelength regions, respectively). This effect manifests itself as spectral narrowing and shift of the ΔA spectra with time, for instance, the 450-nm maximum at 400 fs shifts to 440 nm at 1 ps, and then to 429 nm at 3 ps. Within 10 ps after excitation, the entire ΔA spectrum decays to the background noise level, suggesting complete recovery of the ground state.

l, nm

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0.8

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550-nm residuals

Figure 2. Transient absorption (ΔA) spectra of CuCl42- in acetonitrile for a series of time delays between the 2000-nm excitation and probe pulses. The dash line is the normalized 0.4-ps ΔA


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spectrum of CuCl42- in chloroform. Two lower insets display the 550- and 405-nm ΔA kinetic traces (symbols), dominant time constants obtained from multiexponential fits (blue) convoluted with the 125-fs fwhm, Gaussian-shaped cross-correlation function (red), residuals and their fits to damped cosine functions. The oscillations in the 405- and 550-nm residuals are out of phase (a ~π shift), bottom. The upper inset shows a single 238 ± 25 cm-1 band resulting from FFT analysis of the residuals of the multiexponential fits of the 360-, 405-, 450-, and 550-nm ΔA kinetic traces. We note that the ~340-380 and 430-620 nm transient absorption cannot be attributed to a CuIICl3-· product (absorption maximum, 460 nm) of ionic dissociation of CuCl42- (a Cl- byproduct absorbs below 220 nm). According to high level ab initio computations, the Cu-Cl homolytic bond dissociation energy is 23780 cm-1; as this process does not involve the charge separation, it is only modestly affected by polar solvation.42 Consistent with this is our finding of quantitative photostability of CuCl42- upon prolonged (> 24 hrs.) irradiation at 2000 nm (energy density, 0.44 mJ cm-2). Moreover, radical photoreductive dissociation of CuCl42- is unfavorable even upon 470-nm (21270 cm-1) irradiation of this complex.43 We rule out overcoordinated LF exciplexes44 as the species responsible for the transient absorption observed because the ΔA spectra measured in chloroform and acetonitrile are similar, Fig. 2. Furthermore, promotion of an electron to a partially occupied orbital preserves the multiplicity of the electronic states involved. As a result, ISC in Cu(II) complexes is not possible. Therefore, the only relaxation channel available following the 2E←2B2 d-d transition is vibrational relaxation within the 2E, internal conversion from the 2E to the 2B2, and vibrational relaxation within the 2B2 ground state. In the UV-vis absorption spectrum of CuCl42- in Fig. 1, two overlapped bands at 290 and 294 nm, the 345-nm band, and the 405-nm band with the sub-


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band at 435 nm are due to 2A1(σ)←2B2 and 2E(σ,π)←2B2, 2E(π,σ)←2B2, 2E(non-bonding)←2B2, and 2A2←2B2 LMCT transitions, respectively.36 Photoexcitation depletes the ground-state population causing the negative 405-nm band in the ΔA spectrum. CuCl42- in the excited 2E state undergoes excited-state absorption (ESA) due electric-dipole allowed transitions into the aforementioned excited states. As the 2E is located 5150 cm-1 above the 2B2,36 these transitions should be situated at 340, 346, 420, 511, and 560 nm. Consequently, we expect, noting that the 413-nm transition may be cancelled by the bleach, the appearance of three distinct ESA signals at ~345, ~510, and ~560 nm. These signals indeed have been observed in the short-time spectra, but not after 400 fs, suggesting that internal conversion from the 2E to the 2B2 is complete within the first ~400 fs after excitation. This IC process populates upper regions of the ground-state potential well from which several LMCT states are resonant for 340-380 nm absorption transitions. In contrast, 540-560 nm photons do not reach the lowest-energy LMCT 2A2 excited state from the upper region of the 2B2 state because of the insufficient photon energy.45 This suggests that population relaxation from the 2E excited state (55 fs) is predominantly probed between 540 and 560 nm, whereas the excited 2E state decay entangled with vibrational relaxation through the upper regions of the ground-state well (~250 fs) is probed between 340 and 380 nm. The π-phase shift observed suggests coherent vibrational motion on the product potential energy surface,46,47 and the oscillation frequency matches the b2 out-of-phase stretch of ground-state CuCl42- (low-temperature IR/Raman spectra: 246-269 cm-1

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; B3LYP: 210 cm-1,

Table 2S). The spectral narrowing and blue shift of the transient absorption observed at times longer than 400 fs is the manifestation of vibrational relaxation of molecules in the lower regions of the ground-state 2B2 potential well. Indeed, the absorption band of a vibrationally excited molecule is wider, has less center absorption, and shifted to longer wavelengths as compared to


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the absorption band of a vibrationally relaxed molecule.49 Close to the 2B2 surface bottom, vibrational relaxation is slow and occurs with a ~2-ps time constant. For IrBr62- in acetonitrile, when the 2000-nm excitation pulse temporarily overlaps with the probe pulse, the ΔA spectra have the shape close to the second derivative of the IrBr62- absorption line shape, Fig. 3 and Fig. 3S, as expected from the excitation-induced dynamic Stark effect.50,51 Starting from ~50 fs, the Stark effect fades away, and we observe negative ΔA signals due to depletion of the ground-state LMCT absorption, namely in the transitions from 2Eg”(T2g) to the 2

Uu’(T2u) and 2Uu’(T2u) from 525 to 550 nm, 2Eu”(T2u) at 600 nm, and 2Eg”(T1g) and 2Uu’(T1u)

states from 685 to 710 nm, Fig. 1B and Fig. 3. Also, two prominent transient absorption bands at 420 and 760 nm are observed at 100 fs. Following a minor spectral reshaping to 200 fs, these ΔA bands slowly decay monoexponentially with a time constant of 360 ± 50 ps. The same time constant is observed for recovery of the ground-state bleach features. All ΔA signals decay to zero at ~1.5 ns after excitation, suggesting that all excited 2Ug’(T2g) IrBr62- complexes returned back to the bottom of the ground 2Eg”(T2g) state. Similar transient absorption and long decay time constant were observed in chloroform solutions, Fig. 3. What is the nature of these transient absorption signals? Because the FC excited 2Ug’(T2g) state is situated 5000 cm-1 above the ground 2Eg”(T2g) state, the LMCT(π,eg) excited state and the 2

Uu’(T2u) and 2Uu’(T2u) excited states reachable from 2Eg”(T2g) state at 330 nm and from 525 to

550 nm, respectively, become accessible from the 2Ug’(T2g) state at ~420 and ~760 nm. These spectral positions are consistent with the ΔA bands observed. Therefore, we conclude that IrBr62in the lowest-energy excited 2Ug’(T2g) state undergoes slow, sub-nanosecond IC into the ground 2

Eg”(T2g) state. Other candidate mechanisms can be dismissed as follows. The low-energy

excited states of IrBr62- are not prone to radical photoreductive cleavage into IrBr52- and Br· beca-


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∆A, 602 nm

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700

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Figure 3. Transient absorption (ΔA) spectra upon 2000-nm excitation of IrBr62- in acetonitrile. Delay times between the excitation and probe pulses are given in the legends. The dash line is the normalized 50-ps ΔA spectrum of IrBr62- in chloroform. The upper inset shows the 602-nm ΔA kinetic traces for IrBr62- solutions in acetonitrile and chloroform (square and diamond symbols, respectively) and their single exponential fits with a 360-ps time constant. For IrBr62- in acetonitrile, the short-time ΔA spectra (-50 to 50 fs, lower inset) are attributable to the excitationinduced dynamic Stark effect because their spectral shape is close to that of the second derivative of the steady-state absorption spectrum. use these states lie at significantly lower energies compared to the corresponding bond dissociation energy (~11530 cm-1). We computed this value using the coupled-cluster singledouble (CCSD) method30-33 with solvent effects simulated using the polarizable continuum model (PCM)29 with acetonitrile parameters. Next, ionic dissociation of IrBr62- into IrBr5- and Br-


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is controlled by the Coulomb barrier arising from the repulsive interaction between the two likecharged fragments.52,53 A 2000-nm photon has energy smaller than the height of the Coulomb barrier computed for IrBr62- in different solvents (a factor of 1.32 and 1.15 in chloroform and acetonitrile, respectively; density functional theory combined with a PCM description of the solvent, Fig. 4S), and therefore, is unlikely to lead to permanent ionic dissociation. Partial ligand dissociation and re-coordination is not likely as this would result in absorption of pentabromoiridate anion fragments in the visible region of the ground-state bleach54 and recoordination would be expected to occur differently in acetonitrile and chloroform solvents because of their different polarity, and hence, a different degree of stabilization of charge separation. Finally, ISC is not possible in IrBr62-, for the same reason as previously considered for CuCl42-. Thus, CuCl42- and IrBr62- in lowest-energy excited MC states, the 2E and 2Ug’, respectively, relax via internal conversion to the respective electronic ground state, albeit with a remarkable difference (a factor of 7,000) in the relaxation rate constants. How can such a large difference in the internal conversion rate constants be understood? The traditional approach is to use the Fermi golden rule in the form: k = (4π2/h)|Vel|2FCρ, where h is the Planck constant, Vel is an electronic matrix element defining the coupling strength between the initial and final electronic states, FC a Franck-Condon factor between the vibrational states involved, ρ is the density of accepting vibrational states in the final electronic state, and k is the radiationless rate constant.55-57 This model has been successful in describing the energy gap dependence of the radiationless relaxation rate in coordination compounds.58-60 However, the energy gap is the same (5000 cm-1) for the 2B2 ← 2E IC in CuCl42- and the 2E”g ← 2U’g IC in IrBr62-. At 5000 cm-1, the density of the accepting vibrational states34 can be evaluated to be 3.4 109 cm-1 for CuCl42- and 3.6 1014 cm-1


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for IrBr62-, which hugely favors IC for IrBr62-, contrary to our observations. To resolve this contradiction, we recall that the k = (4π2/h)|Vel|2FCρ expression is derived using certain approximations to the matrix element involving initial and final state combined electronic and vibrational wavefunctions.55-57 The Born-Oppenheimer approximation is that the wavefunctions are separable into electronic and nuclear parts. Another one is that Vel can be approximated (with the strength of electronic coupling varying slowly with the nuclear coordinate) as energy and geometry independent. Coupling of the electronic and nuclear motions in the JT effect is the classic example of the violation of the aforementioned approximations.60,61 Cu2+ ions are among the strongest JT centers, and CuCl42- has been regarded as a representative example of a series of tetragonal systems.37,63 It is well known that an E-state at D2d geometry represents JT D2d conical intersection (CI1), and the degeneracy of E electronic states in the tetragonal symmetry groups is split by vibrational modes of b1 and b2 symmetry, giving rise to the E ⊗ (b1 + b2) JT effect.63,64 These nuclear displacements lift the electronic degeneracy of 2E into B1 + B2 in the D2d → C2v descent of symmetry, carrying the nuclei over into distorted configurations in which the resulting potential energy surfaces are horizontally displaced, Fig. 4. It is worth noting the resemblance of this picture to a vibronic distortion of emissive states in copper(II) porphyrins.65,66 In the case of pronounced JT effects as in CuCl42-, the relative horizontal displacement is substantial, and these energy surfaces are expected to cross not far from the respective well bottom with the electronic ground-state surface,67 forming a conical intersection topology (CI2).57 The relaxation between the excited and ground states within 100 fs and vibrational wavepacket motion observed are consistent with the presence of these CIs in the relaxation pathway.54,68,69 Impulsive photoexcita-


16

2A 2E

1

340-346 nm ESA

2E

420 nm ESA

2E 2A

2A 2B

360 nm ESA

t1u to eg

534 nm ESA

t2g to eg

697 nm

2U ’(T ) u 2u 2E ”(T ) u 2u

511 nm ESA 560 nm ESA

2

ESA

742 nm

1

ESA

835 nm

1

2E(b

ESA

1 +b2)

CuCl42Energy

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Energy

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2E ”(T ) u 1u

2U ’ (T ) g 2g

2B

2E ”(2T ) g 2g

2

IrBr62 -

Nuclear coordinate

CI1

2B

2

b1+b2

CI2

IC

2E ”(2T ) g 2g

Nuclear coordinate

Figure 4. Generalized potential energy surface diagram of the ground and MC lowest-energy excited states. The Franck-Condon energy separation of the MC excited states is estimated from experimental data. A 2000-nm excitation photon directly populates the 2E CuCl42- and the 2Ug’ IrBr62-. Absorption transitions from these MC states (ESA) and vibrationally hot ground state to high-energy LMCT states are probed. Schematic dynamics following initial excitation of the lowest-energy MC states are shown, see text for further details.


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tion of CuCl42- to 2E creates a vibrational wavepacket in the Franck-Condon state. There, the b1 and b2 vibrational modes, which are Raman-active in D2d environment, cause immediate nuclear distortion. The concomitant Jahn-Teller stabilization of energy and horizontal displacement of the resulting potential energy surface propels the wavepacket towards the crossing with the ground state. After a partial oscillation on the excited state surface and passing through the CI2 (a 55-fs lifetime), the wavepacket makes a couple of oscillations within the ground-state well before becoming delocalized. Either combination (b1 twisting and b2 asymmetric bending, or b1 twisting and b2 out-of-phase stretching, Table 2S) may split the electronic degeneracy of the 2E state. Coherent motion is observed in the 238 cm-1 out-of-phase stretch, which then is either the b2 splitting mode itself or coupled to the b2 twisting splitting mode (nuclear displacements of these modes resemble each other, and therefore, they may be coupled, see Supporting Information for the animation of the molecular motion). The essence of the JT effect is coupling of the electronic angular momentum with the vibrational angular momentum. On the other hand, spin-orbit coupling is the coupling of the electronic and spin angular momenta.61 The spin-orbit coupling interaction in IrBr62- splits the 2

T2g state of Ir4+ into the ground 2Eg” and lowest-excited 2Ug’ states, forming a Kramers doublet.40

For this complex, spin-orbit interaction is much larger that JT interaction,70 and because these interactions compete for the electronic angular momentum, the spin-orbit effect quenches the JT effect. The shapes of the potential energy surfaces relevant to this case were thoroughly discussed.62 The curves obtained by slicing through the potential energy surfaces of a JT active normal mode do no longer intersect each other. Instead, the two potential energy surfaces have the minima separated vertically by the strength of spin-orbit coupling, but their relative horizontal displacement remains small, Fig. 4. As a result, the surfaces do not cross low enough


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to allow a radiationless transition, resulting in a 360-ps lifetime of the FC excited 2Ug’ state of IrBr62-. To conclude, relaxation dynamics of Franck-Condon lowest-energy metal-centered excited states is observed for the first time, to the best of our knowledge, for CuCl42- and IrBr62-, which are archetypes of Td and Oh d5 and d9 transition metal complexes. The excited CuCl42- 2E and IrBr62- 2Ug’ states relax to the respective ground electronic state via internal conversion, but with a factor of 7,000 difference in the rate constants despite being located at about the same energy above the ground state (5000 cm-1). Whereas the 55-fs radiationless decay of the excited CuCl42complex and observation of vibrational wavepacket motion manifests the presence of two conical intersections (CI1 and CI2: JT induced and ground-state crossings), the excited 2Ug’ and ground 2Eg” states of IrBr62- are separated by spin-orbit coupling, and as a result, the 2Ug’ → 2Eg” IC in IrBr62- becomes unexpectedly slow (360 ps). As far as we know, no attempts have been made prior to the present work to study the relaxation dynamics between the same-multiplicity electronic states born out through spin-orbit coupling in odd-electron coordination compounds. When the spin-orbit coupling quenches the JT effect in metal complexes,61,62 our work shows that IC, even between the states separated by a small energy gap, may occur in a sub-nanosecond range and not necessarily ultrafast.71 The present work underlines one more time that the same energy gap does not warrant similar radiationless rate constants, and that the nature of the states involved, including vibronic and relativistic effects, should be taken into account.58-60 Finally, a detailed understanding of the dynamics of low-lying MC states is needed for elucidating their role in the relaxation cascades of transition metal complexes,11,13,72 which in applications typically originate from one of high-energy CT states formed by absorption in the visible range. In this regard, we expect that selective near-IR excitation of MC states will be useful for Oh


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metal complexes in which the degeneracy of T2g orbitals is lifted, for instance, Fe3+, Rh4+, Ru3+, Ru4+, Rh4+, and Os4+, and Td metal complexes of d9 electronic configuration, where Au and Ag may serve as examples.

ASSOCIATED CONTENT Supporting Information Figures of multiexponential fits of transient absorption data, FFT and damped cosine function fits of exponential fit residuals for CuCl42-, Stark effect spectra for IrBr62-, the assignment of vibrational modes of CuCl42- and IrBr62-, and the animation of the molecular motion along the b2 modes for CuCl42-. Supporting Information for the material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Author Contributions S. M. M. conceived the study. A. S. M. and S. M. M. designed the experiments on Cu and Ir complexes, respectively. M. S. P. gave technical support and conceptual advice. S. M. M., A. S. M., and A. N. T. interpreted the data and wrote sections of the paper.

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was supported by the NSF (CAREER CHE-0847707 and CHE-0923360). An allocation of computer time from the Ohio Supercomputer Center (CHE130073) is gratefully acknowledged.

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(69) Polli, D.; Altoè, P.; Weingart, O.; Spillane, K. M.; Manzoni, C.; Brida, D.; Tomasello, G.; Orlandi, G.; Kukura, P.; Mathies, R. A.; et al. Conical Intersection Dynamics of the Primary Photoisomerization Event in Vision. Nature 2010, 467, 440-443. (70) Keiderling, T. A.; Stephens, P. J.; Piepho S. B. Slater, J. L.; Schatz, P. N. Infrared Absorption and Magnetic Circular Dichroism of Cs2ZrCl6:Ir4+. Chem. Phys. 1975, 11, 343-348. (71) Chergui, M. On the Interplay between Charge, Spin and Structural Dynamics in Transition Metal Complexes. Dalton Trans. 2012, 41, 13022-13029. (72) Juban, E. A,; McCusker, J. K. Ultrafast Dynamics of 2E State Formation in Cr(acac)3. J. Am. Chem. Soc. 2005, 127, 6857-6865.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 35

TOC GRAPHICS

55 fs IrBr62-

360 ps CuCl42-


30

Page 31 of 35

A

2E←2B 2B

2A

100

CuCl42-/D2d

l, µm

1←

1←

2.5

2B

2Ug’ (2T

2g) ←

2Eg”(2T

2g)

2.0

ug’

e

Metal orbitals

2

1.5

b1

1.0

a1

1.5

2

ε, M-1•cm-1

0

50

Metal orbitals

1.0

t1g

Ligand orbitals 40000

6000

ε, cm-1 M-1

eg”

B

l, µm

2.5

t2g

2.0

2

2B

b2

IrBr62-/Oh

ε, M-1•cm-1 50

0

100

4000

30000

3000

20000

2000

10000

1000

4000

2000

0

300

400

l, nm

500

600

0 200

300

400

500

600

l, nm

700

800

ε, cm-1 M-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 900 1000

Figure 1. Matveev et al. “Probing the Fate…”


1


l, nm

400

500

delay time, ps: 0 0.1 0.4 0.4, CHCl3

600 FFT Amplitude (a.u.)

5 4 3 2

1 3 20

t1 = 55 fs

-1

0.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

405-nm residuals

0.4

Time, ps

-1

238 cm

250 500 750 1000 -1

Wavenumber, cm

t1 = 400 fs t2 = 1.7 ps

-∆A, 405 nm

0 ∆A, 550 nm

∆A, mO∆

0

Time, ps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 35

0.8

0.0

0.5

Time, ps

4 5

550-nm residuals



2

Page 33 of 35

∆A, 602 nm

1

∆A, mO∆

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


t = 360 ps 0

500

0 500

-1

l, nm 600

1000

Time, ps

700

delay time, ps: 0.1 0.5 3 50 200 1800 50, CHCl3

-50 fs -10 fs 10 fs 50 fs nd

2 derivative

400

500

1500

600

l, nm

700



3


Energy

2A 2E

1

340-346 nm ESA

2E

420 nm ESA

2E 2A

2A 2B

360 nm ESA

t1u to eg

534 nm ESA

t2g to eg

697 nm

2U ’(T ) u 2u 2E ”(T ) u 2u

511 nm ESA 560 nm ESA

2

ESA

742 nm

1

ESA

835 nm

1

2E(b

ESA

1 +b2)

CuCl42Energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 35

2E ”(T ) u 1u

2U ’ (T ) g 2g

2B

2E ”(2T ) g 2g

2

IrBr62 -

Nuclear coordinate

CI1

2B

b1+b2

CI2

IC

2E ”(2T ) g 2g

2

Nuclear coordinate



4

Page 35 of 35

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


55 fs IrBr62-

360 ps CuCl42-

TOC Matveev et al. “Probing the Fate…”


5

Probing the Fate of Lowest-Energy Near-Infrared Metal-Centered

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