Photoinduced Dynamics of a Diazidocobalt (III) Complex Studied by

Subscriber access provided by Macquarie University

B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe-Spectroscopy Steffen Straub, Luis Ignacio Domenianni, Jörg Lindner, and Peter Vöhringer J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b07210 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 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

The Journal of Physical Chemistry

1

Photoinduced Dynamics of a Diazidocobalt(III) Complex Studied by Femtosecond UV-Pump/IR-to-Vis-Probe-Spectroscopy

Steffen Straub, Luis Domenianni, Jörg Lindner, and Peter Vöhringer* Abteilung für Molekulare Physikalische Chemie Institut für Physikalische und Theoretische Chemie Rheinische Friedrich-Wilhelms-Universität Wegelerstrasse 12, 53115 Bonn, Germany [email protected]

Manuscript August 29, 2019 submitted to the Journal of Physical Chemistry A

*corresponding author

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

2 Abstract. The photochemistry of the cationic diazidocobalt(III) complex, trans-[Co(cyclam)(N3)2]+, following its ligand-to-metal charge transfer (LMCT) excitation is studied in liquid dimethyl sulfoxide (DMSO) solution using femtosecond spectroscopy with detection in a very broad spectral region covering the near-ultraviolet (near-UV) all the way to the mid-infrared (MIR) thereby enabling a combined probing of both, electronic and vibrational degrees of freedom of the dynamically evolving system. The initially prepared singlet LMCT-state decays, via the metal-centered singlet excited state, 1MC(1Eg), into the triplet ground state, 3MC (3Eg/3A1g), on a time scale shorter than 25 ps. During this time period, the vibrational spectrum demonstrates uniquely that the nature of the complex changes from a monoazidocobalt(II) species bearing a neutral azide radical ligand immediately after photon absorption to a metal-centered open-shell diazidocobalt(III) species. At the same time, the 3MC-state is characterized by a very strong absorption band centered at 710 nm, which can be assigned to a transition to the triplet LMCT-state. The 1LMCT lifetime is about 2 ps, whereas that of the excitedstate, 1MC, is defined by the primary intersystem crossing time of 6 ps. The ensuing intersystem recrossing from 3MC to the parent’s singlet ground state, 1A1g, occurs with a rate of 1 / (110 ps). The mid-infrared pump-probe spectrum after 1 ns, gives evidence for a heterolytic Co–N bond fission with a quantum yield of ~5 % leading to free azide anions and the monoazido species, trans[Co(cyclam)(N3)(OSMe2)]+, featuring an oxygen-bound DMSO ligand in its coordination sphere.

1.) Introduction The photochemical generation of coordination compounds featuring highly-oxidized transition metal centers has been widely utilized in the recent past. Pioneering work was conducted in the mid-80s by Nakamoto and coworkers1-3 on the photodecomposition of dioxygen adducts of ferrous porphyrins (por) under cryogenic conditions leading to the discovery of an oxoferryl species, (por)FeIV=O, that resembled the reactive intermediate, termed “compound-I”, of the catalytic cycle of the cytochrome P450 protein.4-5 The implication of this and related species with transition metals in extraordinarily high oxidation states for biochemistry6-7 and chemical catalysis8-9 constitutes a significant motivation for scientists to explore novel routes for their synthesis,10-12 to elucidate in detail their chemical reactivity, 13 and to reveal their molecular and electronic structures by means of high-resolution spectroscopy combined with quantum chemical calculations.14-15 In coordination compounds, a terminal oxo (O2‒) ligand is capable of contributing a total of six electrons for metal-oxygen (M‒O) binding, two of which are required for establishing a σ-bond and four can be engaged in a pair of doubly degenerate π-bonds.16 This allows in principle for the formation of a M≡O triple bond provided three empty d-orbitals of the metal with appropriate symmetry are available; e.g., in fourfold-symmetry, the e(dxz,dyz)-orbitals with π*-character. However, an increasing d-electron count at the metal causes a rising occupation of these critical orbitals and thus, reduces the M‒O bond order, making the oxo ligand increasingly basic, and hence, the coordination mode susceptible to decay processes like nucleophilic attack or protonation. Formally, in tetragonal coordination geometry, a M≡O triple bond is only feasible for d0, d1, and lowspin-d2 configured metal centers, whereas for high-spin d3 and d4 systems, the bonding situation is more appropriately described by a double bond, M=O, as in Nakamoto’s oxoferryl compound. ACS Paragon Plus Environment

Page 2 of 24

Page 3 of 24 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


3 When increasing the d-electron count beyond d4, the formation of a multiple bond in fourfoldsymmetry becomes formally prohibited. Thus, achieving such low occupations of the d-orbitals implies very high oxidation states at the metal, e.g. FeIV=O (d4), FeV O (d3) or for FeVI≡O (d3). In turn, this requirement imposes a significant constraint with respect to the choice of the metal and leads to a limit, which is commonly coined in the literature as “oxo wall” located between group 8 and 9 in the periodic table: tetragonal oxo complexes of the later 3d-transition metals (Co, Ni, and Cu) are not likely to be found.16 Interestingly, the oxo-metal moiety, MQ≡O(2‒), is isoelectronic with the coordination motif, MQ+1≡N(3‒), containing a terminal nitrido ligand attached to the same metal but at its next higher oxidation state, Q+1. This opens up an alternative photochemical route to highly oxidized transition metal complexes; namely, the photolysis of lower valent azido metal precursors.2-3, 17-19 Their irradiation was shown to result in a homolytic N–N bond cleavage and dinitrogen elimination, which leads to the desired terminal nitrido-ligand attached to the photooxidized metal center. Following early work on azidoporphyrin precursors this approach has been frequently applied and with great success to generate tetragonal nitrido complexes of the early-to-mid 3d-transition metals such as vanadium through manganese17-23 and finally, iron.24-33 Critical in these efforts is the utilization of a redoxinnocent, spectroscopically “silent” ancillary ligand such as a macrocyclic tetradentate secondary amine, which aids in preventing a spreading of spin density onto the equatorial coordination sites (not containing the photolabile azide ligand) upon optical excitation. In the case of nitridoiron(V) and iron(VI) complexes, the irradiation of the precursors were carried out either in cryogenic solid matrices24-25, 28 or in the gas phase34-37 to suppress secondary reactions such as the well-known bimolecular nitride coupling.38-39 We have recently undertaken a significant research effort aimed at unraveling the photochemical and photophysical primary processes leading to the formation of high-valent nitridoiron(V) species and to study in detail their chemical reactivities in liquid solution under ambient conditions using a combination of ultrafast mid-infrared (MIR) spectroscopy and time-resolved Fourier-transform infrared (FTIR) spectroscopy following flash photolysis.40-47 It was found that the desired dinitrogen cleavage and photooxidation of the iron center has to compete with the heterolytic cleavage of azide anions on the one hand and the homolytic loss of azide radicals and photoreduction of the metal on the other. An increase of the photolysis photon energy leads to a rising quantum yield for photooxidation. This is because the primary process immediately after photon absorption is an internal conversion, which leads effectively to an ultrafast temperature jump of the precursor complex in its electronic ground state. The actual loss of N2 occurs subsequently on the ground state surface as a thermally activated reaction and is possible only prior to vibrational cooling.41, 43-44 In this paper, we follow up on this research path and explore the elementary events induced upon impulsive electronic excitation of a complementary precursor complex that contains photolabile azide ligands but now attached to a cobalt rather than an iron center, thereby inspecting the ultrafast photochemistry and photophysics of such compounds beyond the oxo/nitrido wall. We note in passing that, inspired by earlier work of Endicott et al.,48 Henning and coworkers49-52 also studied the photochemistry of related complexes some years ago but the authors focused on square planar azidonickel(II) species and azido complexes of the heavier transition metals (Pd and Pt) and their research was limited to stationary quenching experiments or flash photolysis on microsecond time scales and longer.



4

2.) Experimental Femtosecond spectroscopy was carried out with a Ti:sapphire oscillator/regenerative amplifier frontend laser system (Newport Spectra-Physics, Solstice Ace), which is capable of providing 800 nm-laser pulses with a durations as short as 60 fs and an output power of 6 W at a repetition rate of 1 kHz. For the UV-pump/MIR-probe experiment, two beams with intensities of 1.1 W and 330 mW, respectively, were derived from the front-end’s fundamental output. The stronger beam was used to drive a commercial optical parametric amplifier (TOPAS) whereas the weaker beam was used to pump a home-built optical parametric amplifier (OPA). The signal pulses of the TOPAS were tuned to a wavelength of 1420 nm and were subsequently frequency-quadrupled to provide UV-pump-pulses centered at 355 nm with energies as high as 13 μJ. The signal pulses of the OPA were tuned to a wavelength of 1379 nm and were subsequently downconverted with the accompanying copropagating idler pulses at 1905 nm using a type-I AgGaS2 crystal cut for difference frequency generation (DFG) to provide MIR-probe pulses around 5 μm. The DFG pulses were split into probe and reference beams with about equal intensities. The probe beam was sent through a motorized delay stage to control the timing of the probe pulses relative to the pump pulses. Using a pair of gold-coated 90° off-axis parabolas both MIR beams were focused into the sample, collimated, and steered to a polychromator that was equipped with a liquid nitrogen cooled HgCdTe array detector (Infrared Associates MCT-6400) thereby enabling a referenced detection of the probe intensity. The pump pulses were focused into the sample with a fused silica lens (400 mm focal length) at a relative angle between pump and probe beam of 5°. To ensure homogeneous illumination conditions, the focus of the pump beam was located behind the sample such that its diameter was slightly larger than that of the probe beam (400 μm). For UV-pump/near-UV-to-visible-probe experiments, a commercial transient absorption spectrometer (TAS, Newport/Spectra Physics), which was interfaced to the above front-end, was used. The sample solution was circulated with a gear pump at a flow rate of ca. 100 mL min–1 through a home-built MIR sample cell that was equipped with two CaF2 windows held at a spacing of 100 μm. The synthesis of trans-diazido(1,4,8,11-tetraazacyclooctatetradecane)cobalt(III) perchlorate, trans-[Co(cyclam)(N3)2] [ClO4] (or [1][ClO4]), followed the protocol described by Bosnich et al.53 Dimethyl sulfoxide (DMSO, Sigma Aldrich, anhydrous, ≥ 99.9%) was used as the solvent and without any further purification. Fourier-transform infrared spectra and UV/Vis absorption spectra were recorded with a Nicolet 5700 FT-IR spectrometer (Thermo Electron Corp.) and a Shimadzu UV-160 spectrophotometer.

3.) Results and Discussion Stationary spectroscopy. Figure 1a displays the linear electronic absorption spectrum of [1] in room temperature DMSO solution together with its molecular structure derived from a density functional theory calculation (cf. supporting information). In the visible spectral region, a distinct ligand-field transition is observed at 574 nm, which can be assigned to the 1Eg  1A1g ligand field transition of the complex featuring a D4h-symmetrical coordination sphere. Two higher-lying bands are detected in the ultraviolet region, which peak at 345 nm and 222 nm, respectively. Their extinction coefficients are of the order of 104 M‒1 cm‒1, which is clearly indicative of ligand-to-metal charge-transfer (LMCT) resonances.54 The femtosecond spectroscopy reported here was carried out with 355 nm-excitation pulses that were tuned fully into the lower LMCT-band as indicated by the vertical arrow in Figure 1. ACS Paragon Plus Environment

Page 4 of 24

Page 5 of 24 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


5 The stationary Fourier-transform infrared (FTIR) spectrum of the complex is shown in Figure 1b. The spectrum emphasizes two spectral regions; namely, the in-plane CH2-rocking (ρ-CH2) region of the cyclam ligand from 750 cm–1 to 950 cm–1 on the left and the azide antisymmetric stretching (νas-N3) region from 1950 cm–1 to 2100 cm–1 on the right. In the former region, two separate bands can be observed at 816 cm–1 and 901 cm–1, respectively. In addition, the band at higher wavenumber features a distinct shoulder at its low-frequency side. This pattern of resonances is clear evidence for the azido ligands adopting the trans-configuration.55-57 In the azide stretching region, a very broad but slightly structured band at 2027 cm‒1 is observed. The DFT-optimized geometry of the complex is centrosymmetric and as such, one would expect the two anti-symmetric azide stretching vibrations to obey the rule of mutual exclusion, i.e. their in-phase combination becomes IR-active and Raman silent, whereas their out-of-phase combination becomes Raman-active and IR forbidden. Therefore, the substructure seen in Figure 1b implies that in room temperature liquid solution, the geometry of the complex can depart significantly from centrosymmetry. Dynamic symmetry breakage is most likely to occur either through conformational flexibility of the cyclam ligand or by torsional motions of the azide ligand about the NcyclFe̶̶NαNβ dihedral.

Figure 1. a) Electronic absorption spectrum of [1] in liquid DMSO solution at room temperature. The molecular structure of the complex is displayed in the inset. The vertical arrow indicates the photolysis wavelength used in this study. Numbers indicate peak positions in nm. B) Stationary FTIRspectrum of [1] in DMSO solution at room temperature. Numbers indicate peak positions in 1/cm.



6

Early-time pump-probe spectra. A pump-probe spectrum recorded in the antisymmetric stretching region of the azide ligands around 2000 cm–1 and at a delay of 250 fs is reproduced in Figure 2, top panel. The stationary absorption spectrum of the sample is also shown for comparison. The timeresolved spectrum, i.e. the differential optical density, ΔOD, is composed of positive and negative features that are shaded in red and blue, respectively. The negative signal, termed ground-state bleach, arises from the depletion of population in the electronic ground-state because of the resonant excitation of the complex by the 355 nm-pump-pulse at zero time delay. In the absence of any other signal contributions (and neglecting inhomogeneous broadening), the spectral shape of the bleach is expected to match the inverted stationary absorption of the sample, i.e. its inverted FTIR spectrum. As can be seen in Figure 2 the bleach peaks at 2040 cm–1 while the FTIR is maximal at 2027 cm–1. This mismatch indicates that the ground-state bleach is partially obscured by the positive signal contribution peaking at 1996 cm–1.

Figure 2. Pump-probe data recorded at a delay of 250 fs after 355 nm-excitation of [1] in liquid DMSO solution at room temperature. Top panel. UV-pump / MIR-probe spectrum in the azide antisymmetric stretching region. The FTIR spectrum of the sample is depicted in gray. An induced absorption is shaded in red while a bleach is shaded in blue. The numbers indicate peak positions in cm–1. Bottom panel. UV-pump / NUV-Vis-probe spectrum. Color shading as above. The light green component represents the ground-state bleach.58 The molar decadic extinction coefficient, ε, derived from the stationary NUV-Vis spectrum of the sample is depicted in gray (right ordinate). The two arrows emphasize the absorption modulation due to bleaching of the d-d transition. The numbers indicate peak positions in nm.


Page 6 of 24

Page 7 of 24 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


7 Positive signals, also termed induced (or transient) absorption, arise from species that are formed in response to the UV-excitation and whose absorption spectrum differs from that of the parent species, i.e. the complex in its electronic ground state. The most likely candidates for such a species is the singlet LMCT state that is directly accessed by the pump photon and/or a primary photochemical product that is formed from 1LMCT within the first 500 fs after photon absorption. In either case, the data require the antisymmetric stretching absorption of these species to be frequency downshifted with respect to that of the parent. Importantly, the area under the induced absorption is only about half as large as that of the ground-state bleach. This finding indicates that the species absorbing near 1996 cm–1 behaves IR-spectroscopically like a complex that features only one anionic azido ligand (and not two like the parent complex).59 An absorption-to-bleach ratio of 0.5 would indeed be expected if the LMCT-excitation transfers one full electron from an N3–-ligand to the cobalt(III) center thereby generating an excited state whose electronic character can be depicted by [Co•II(cyclam)(N3)(N3•)]+, i.e. a neutral azidyl radical ligand, N3•, with a spin of S = ½, which is antiferromagnetically coupled to a d7-configured (S = ½) cobalt(II) center to yield an overall spin singlet. Note that this expectation relies on assuming (i) that an openshell azidyl ligand does not absorb in the spectral region displayed in Figure 2a and (ii) that the IR cross-section of the antisymmetric stretching mode of an anionic azido ligand does not change upon electronic excitation. Previous UV-pump/MIR-probe studies41, 43, 46 on the complementary ferric complexes, trans-[Fe(cyclam)(N3)2]+ and [Fe(cyclam-ac)(N3)]+, which have relative azido stretching cross-sections of 2:1, demonstrate that assumption (ii) is a good one. In addition, the IR-absorption of the antisymmetric stretching mode of azidyl radicals is at least an order of magnitude weaker than that of azide anions and the Co-bound azido ligand.59 It is located near 1660 cm‒1,60 i.e. dramatically downshifted from the spectral window of Figure 2, and it seems highly unlikely that the radical’s absorption upshifts by more than 350 cm‒1 upon binding to the Co center without accepting an electron. The radical ligand prepared by LMCT-excitation may still be covalently bound to the metal, in which case the absorbing species represents the optically prepared electronically excited state of the structurally intact parent, [(cyclam)(N3)Co•II –•N3]+, or the radical ligand may already be detached from the metal, in which case it is better described as a correlated radical pair, [(cyclam)(N3)Co•II– □]+···N3•, where □ represents a vacancy in the metal’s coordination sphere. In the more general context of the photochemistry of cobalt(III)-containing complexes, both radical pair species have been invoked and are believed to coexist.61-65 Considering the very short delay at which the data from Figure 2 were collected it seems highly unlikely that the radical pair is a solvent separated secondary species. Finally, it should be noted that the absorption of the remaining (intact) azido ligand (the band at 1996 cm‒1) is downshifted from that of the ground state. This could be due to the formal reduction from Co(III) to Co(II) upon optical excitation. The increased electron density at the metal center gives rise to an enhanced metal-to-ligand back-bonding and thus to a slightly decreased vibrational frequency. The bottom panel of Figure 2 displays a pump-probe spectrum that was taken at the same delay of 250 fs but with detection in the near-UV-to-visible spectral region. Once again, an induced bleach is seen for wavelengths shorter than 392 nm and at the same time, an induced absorption is recorded for longer wavelengths. Strikingly, the positive signal covers the entire visible range and extends even beyond the limit of our detection window in the near-infrared at 800 nm. Importantly, the stationary UV/Vis spectrum of the complex features the distinct 1Eg(1)  1A1g ligand-field transition at 574 nm (see also supporting information) with an extinction coefficient of ~400 M–1 cm–1; however, the LMCT excitation at 355 nm does not lead to the expected net negative ΔOD in this region. Nonetheless, this ACS Paragon Plus Environment


8 ground-state bleaching signal is still weakly discernible as a minimum in the broad induced absorption resulting in two peaks located at 431 nm and 660 nm. The amplitude of the buried bleaching component can be estimated to ΔOD = 3.6·10–3 as highlighted in Figure 2 by the two vertical arrows.58 From this number, we can derive an approximate extinction coefficient associated with the broad absorption of ~2400 M–1 cm–1 at a wavelength of 710 nm where ΔOD is 21.5·10–3 and the sample’s stationary absorption (and hence, any bleaching signal) is vanishingly small. An extinction coefficient in excess 103 M–1 cm–1 strongly suggests that the resonance responsible for the induced absorption cannot be a ligand-field transition and that instead, it must have strong chargetransfer character. Spectro-temporal evolution in the MIR. A series of consecutive femtosecond 355 nm-pump / MIRprobe spectra for various representative time delays between 500 fs and 1 ns is shown Figure 3. The top panel emphasizes the temporal evolution of the MIR-response for delays shorter than 20 ps. During this time interval the induced absorption initially peaking at 1996 cm–1 gradually shifts to higher wavenumbers to settle at an asymptotic peak position of 2024 cm–1. This dynamic frequency upshift is accompanied by a spectral narrowing of the absorption profile as well as by a complex modulation of the peak amplitude as indicated by the wavy arrow. At the same time, the ground state bleach initially peaking at 2040 cm–1 gradually refills and slightly upshifts to 2045 cm–1.

Figure 3. UV-Pump / MIR-probe spectra recorded in the azide antisymmetric stretching region for various delays after 355 nm-excitation of [1] in liquid DMSO solution at room temperature. The numbers indicate peak positions in cm–1. The bottom panel of Figure 3 highlights the ensuing temporal evolution for longer delays of up to 1 ns (see also the supporting information for a movie of the full data set). During this time period, a monotonous decay of the induced absorption is noticed that matches perfectly the ground-state ACS Paragon Plus Environment

Page 8 of 24

Page 9 of 24 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


9 bleach recovery thereby forming a very clear isosbestic point at an intermediate wavenumber of 2032 cm–1. The final spectrum is characterized by a quasi-permanent negative ΔOD whose peak position of 2027 cm–1 matches quantitatively that of the azide antisymmetric stretching vibration in the stationary FTIR spectrum. These results give evidence for some photochemical conversion of the complex occurring on a 1 ns-time scale; albeit with a very small primary quantum yield.

Figure 4. Top panel: UV-Pump / MIR-probe spectra (red curve) recorded in the azide antisymmetric stretching region at a delay time of 1 ns after 355 nm photolysis of [1] in liquid DMSO solution at room temperature. The blue curve represents the inverted FTIR spectrum of [1]. Bottom panel: Theoretical pump-probe spectrum (red curve) assuming the photoproducts are free azide anions and the solvolysis product, [3-O], in its singlet ground state (see text for details). The blue curve represents the inverted theoretical IR-spectrum of [1]. The asymptotic long-time spectrum is reproduced and vertically expanded in Figure 4, upper panel. It can now be noticed that apart from the bleach at 2027 cm–1, the spectrum also contains an induced absorption at 2002 cm–1, i.e. significantly downshifted from the stationary absorption of the parent’s ground state. In addition, when comparing the 1 ns-pump-probe spectrum with the inverted and properly scaled FTIR spectrum (cf. blue trace in Figure 4, upper panel), it becomes clear that the spectral width of the bleaching band is about 30% smaller than that of the stationary absorption. This observation is indicative of the presence of yet another induced absorption that is however, slightly upshifted relative the parent’s azide antisymmetric stretching band. Calculating the difference between the red and the blue trace yields a final photochemical product spectrum, which is represented by the lower trace in the upper panel of Figure 4. It consists of two distinct resonances of nearly equal amplitude that are located at 2006 cm–1 and 2054 cm–1, respectively. The former feature spectrally coincides perfectly with the antisymmetric azide stretching band of free N3–-anions in liquid solution.42, 66-67 Thus, we can confidently conclude here that the photolysis at 355 nm of complex, [1], in liquid DMSO leads with a very small quantum yield to a heterolytic bond fission from which free azide anions emerge. At the same time, a cobalt-containing fragment is formed that still ACS Paragon Plus Environment


10 retains an azide ligand since it continues to absorb in the azide stretching region; namely at 2054 cm– 1. This metal fragment can be either the coordinatively unsaturated species, [Co(cyclam)(N )(□)]2+ 3 2+ ([2]), or the solvolysis product, [Co(cyclam)(N3)(DMSO)] ([3]). The solvent molecule is known to coordinate to transition metals in two distinct binding modes: either via the terminal oxygen or through the central sulfur atom.68-70 We have conducted density functional theory (DFT) calculations on both species, [Co(cyclam)(N3)(–O=S(CH3)2)]2+ ([3-O]), and [Co(cyclam)(N3)(–S(O)(CH3)2)]2+ ([3-S]). To this end, all conceivable spin multiplicities (i.e. singlet, triplet, and quintet) have been considered for each species, their geometries have been optimized and their vibrational spectra calculated. The molecular structures of these species are given in the supporting information and are displayed in Figure 5 together with that of the parent. It turns out that the cleavage of the free azide anion from the parent, [1], and the simultaneous formation of the penta-coordinated singlet species, 1[2], requires an energy of 160 kJ / mol. However, the subsequent filling of the vacancy with a DMSO molecule releases a significant amount of energy only if the process occurs on the singlet surface and leads to the final product, 1[3-O], with the oxygen-bound DMSO. Still, the overall photoreaction, 1[1] + DMSO  1[3-O] + N3– requires an energy of 75 kJ / mol (or equivalently, 6250 cm–1), which is however easily overcome by the photon energy. The isomeric S-bound product, [3-S], is much higher up in energy regardless of the spin multiplicity and can therefore not be expected to be formed.

Figure 5. Top panel: Energetics of azide anion cleavage from the parent complex, [1], on the singlet (S = 0), triplet (S = 1), and quintet (S = 2) surfaces. Bottom panel: DFT-optimized molecular structures of the parent, [1], the penta-coordinated primary fragment, [2], and the O- and S-bound DMSOsolvolysis products, [3-O] and [3-S], respectively, all of which in their singlet state. ACS Paragon Plus Environment

Page 10 of 24

Page 11 of 24 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


11 The normal mode analyses of all putative products confirm this notion. The only monoazide species, which features an N3-asymmetric stretching resonance that is frequency-upshifted relative to that of the parent, is complex, 1[3-O]. As shown in the bottom panel of Figure 4, the theoretically predicted difference spectrum for the photoreaction, 1[1] + DMSO  1[3-O] + N3–, is in very good agreement with the experimental data recorded at 1 ns after photolysis. Thus, based on the accord between experiment and theory, we assign here the absorptive features at 2006 cm–1 and 2054 cm–1 to free azide anions and the dicationic complex, trans-[Co(cyclam)(N3)(–OS(CH3)2)]2+, in its singlet ground state, respectively. Kinetic traces recorded at representative probe wavenumbers are displayed in Figure 6, top panel. In the region of the stationary ground state absorption, i.e. at 2040 cm–1, a monotonous bleach recovery is observed. The data can be fitted phenomenologically with bi-exponential kinetics to which a very small constant offset is added. A slower component accounting for 37 % of the total recovery is associated with a time constant of 108 ps. This component fully suffices to kinetically quantify the spectro-temporal evolution shown in Figure 3, bottom panel, i.e. for delays longer than roughly 25 ps. The recovery on shorter delays is captured by the faster of the two exponential components and exhibits a time constant of 6.4 ps with a relative amplitude of 63 %.

Figure 6. Top panel: Kinetic traces recorded in the azide-stretching spectral region following 355 nmoptical excitation of [1] in liquid DMSO solution at room temperature. The experimental data (circles) are fitted with multi-exponential kinetics (solid curves, see text for details). Numbers indicate the probe wavenumber used for detection. Bottom panel: Absorption-to-bleach ratio in the azide asymmetric stretching region (blue circles, left ordinate) and peak position of the induced absorption (red squares, right ordinate), both as a function of the time delay after 355 nm photolysis of [1]. Note the logarithmic abscissa in both plots. Consistent with the pump-probe spectra of Figure 3, a kinetic trace taken at the isosbestic wavenumber of 2032 cm–1 remains perfectly constant (at a small ΔOD-value of –2.3·10–3) for delays ACS Paragon Plus Environment


12 in excess of 25 ps. Nonetheless, a net bleach recovery is still detected for shorter delays and requires an exponential fit with a time constant of 5.9 ps, i.e. similar to that of the fast component from above. Taken together these observations suggest that for delays longer than 25 ps (cf. dashed vertical line in Figure 6) the molecular system evolves through a simple 2-state conversion process, or equivalently, precursor-successor kinetics, with a rate constant of 1 / (108 ps) in which evidently the successor is the parent’s electronic ground state. Moreover, the precursor’s absorption spectrally overlaps with that of the parent so as to result in an apparent partial bleach recovery when the precursor itself is formed with an average rate constant of ~1 / (6 ps) (cf. Figure 3, top). The kinetic trace recorded at a probe wavenumber of 2024 cm–1, i.e. slighted downshifted from the isosbestic point and right on the peak of the induced absorption (cf. Figure 3, bottom), is fully in line with this interpretation. For delays longer than 25 ps, its decay is a perfectly scaled mirror image of the bleach recovery seen at 2040 cm–1, whereas on shorter time scales the initial bleach transforms into an induced absorption. The latter in turn, is obviously a unique spectroscopic fingerprint of the precursor responsible for replenishing the electronic ground state of [1]. As before, the 2024 cm–1trace can be fitted with biexponential kinetics and the two characteristic time constants from above (here: 112 ps and 5.8 ps) are retrieved. Having understood the kinetics leading to the repopulation of the parent’s ground state, the dynamics of spectral shifting of the induced absorption, which was observed on short time scales (cf. Figure 3, top panel), need to be examined in more detail. To this end, we again inspect the area ratio between induced absorption and induced bleach and, more importantly, its time-dependence as shown in Figure 6, bottom (filled blue circles). This ratio seems to extrapolate back for vanishingly small delay to a value of 1/2. As alluded to above this number suggests that the absorption of the UV-photon shifts a full electron from an azide anion ligand to the trivalent cobalt thereby creating an azide radical ligand attached to a cobalt(II) center. However, as the delay time increases the absorption-to-bleach ratio steadily increases to approach a value of exactly 1.0 at around 25 ps, i.e. when the ground-state recovery kinetics set in. This finding is highly suggestive that the species recovering the parent’s ground state is spectroscopically a diazido complex and that its absorption is exactly as strong as that of [1]. Thus, the formation of the ground-state precursor on a time scale below 25 ps involves the conversion of a species carrying only one anionic azide ligand to a species having two N3–-ligands and this process manifests itself in (i) a rise of a spectrally sharp induced absorption peaking at 2024 cm–1, (ii) a dynamic spectral upshifting of a broader induced absorption initially peaking at 1996 cm–1, and (iii) an apparent bleach recovery that results from an increasing spectral overlap of the dynamically upshifting absorption and the ground-state bleach. Together with the complex evolution of the peak amplitude of the induced absorption (cf. Figure 3, wavy arrow), these observations hint at the existence of another distinct elementary process in addition to the formation of the parent’s diazido precursor occurring with an inverse rate of 6 ps. The dynamics of this additional elementary process become already apparent in the kinetic trace taken near the center of the induced absorption around 1995 cm–1 (red trace in Figure 6, top), where the signal initially rises within the first few ps and subsequently fully decays to zero within 25 ps. To reproduce these data, a triple exponential fit is required whose components have time constants of 6.3 ps, 2.8 ps, and 1.8 ps. The slowest component represents a decay, it has a relative amplitude of 18 %, and is once again due to the formation of the direct ground state precursor absorbing at 2023 cm–1. In contrast, the fastest component represents an ultrafast initial rise of the absorption and has a time constant of 1.8 ps with a relative amplitude of 45 %. Finally, to mimic in a satisfactory ACS Paragon Plus Environment

Page 12 of 24

Page 13 of 24 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


13 manner the signal maximum around 2 ps (reflecting the ensuing disappearance of the dynamically upshifting induced absorption band from the detection window at 1995 cm–1) another decaying component with a time constant of 2.8 ps (37 % relative weight) is required. The spectral shifting dynamics can be isolated from the signal growth and decay by logging the peak position of the induced absorption as a function of the pump-probe delay. To this end, the absorptive signals of the fs-MIR-spectra were fitted empirically with a fifth-order polynomial and then the roots of the polynomial’s first derivatives were located numerically. The spectral peak position thus obtained is plotted in Figure 6, bottom, versus the pump-probe delay (see red squares). It can be noticed that during the first 2 ps, in which the amplitude of the 1995 cm–1-signal markedly increases, the spectral peak position barely changes. The majority of the spectral upshift occurs in the time period between 2 ps and 25 ps, i.e. when the direct precursor of the parent’s ground state is formed and the dominant absorbing species is most likely a diazido complex (see blue circles). It turns out that the time-dependent spectral position, 𝜈0(𝑡), from our peak finder cannot be fitted by simple multi-exponential kinetics. Instead, the Kubo relaxation function71 𝜈0(𝑡) ― 𝜈0(∞)

{

Δ = 𝑒𝑥𝑝 ― (𝑒 ―𝛾𝑡 + 𝛾𝑡 ― 1) 𝜈0(0) ― 𝜈0(∞) 𝛾

}

(1)

was found to provide a satisfactory fit to the experimental data as demonstrated by the solid curve in Figure 6, bottom. In eq. (1), the parameters, 𝜈0(0) = 1997.6 cm–1 and 𝜈0(∞) = 2023.9 cm–1, are the spectral position at a time delay of zero and infinity, respectively. An optimal coupling strength, Δ, of 1 / (3.7 ps) together with a relaxation rate, 𝛾, of 1 / (2.5 ps), was found through a non-linear least squares fit. Finally, we mention that the peculiar discontinuity of the peak position at a delay of 200 ps as well as the slow decay of the area ratio after 100 ps are due to photoproduct formation as discussed in the supporting information To sum up the results from the UV-pump / MIR-probe spectroscopy: The absorption of a 355 nmphoton brings about the shift of an electron from an N3–-ligand to the cobalt(III) center, which is fully in line with the notion of a singlet-LMCT excited state. As evidenced by the growth of the induced absorption at 1996 cm–1, the Franck-Condon state subsequently relaxes within the first 2 ps to an intermediate, which is predominantly a diazido species as indicated by the absorption-to-bleach ratio of 0.87 (cf. Figure 6). The intermediate then relaxes with a time constant of 6 ps to a species, which serves as a precursor repopulating the electronic ground-state of the parent. The intermediate-toprecursor conversion is accompanied by a pronounced spectral upshift of the induced absorption by about 25 cm–1 toward a value of 2024 cm–1, i.e. the peak position of the precursor’s azide asymmetric stretching absorption. The precursor has a lifetime of 110 ps and decays back to the electronic ground state of the parent, [1]. Finally, the solvolysis product, [3-O], together with free azide anions are observed after 1 ns, which gives evidence for a redox-neutral dissociation channel with a small quantum yield of ~5 % as estimated from the ratio of the bleaching signal at zero and quasi-infinite delay. Spectro-temporal evolution in the UV/Vis. Having exposed the sequential nature of the relaxation kinetics involving three consecutive elementary processes, which guide the system from the locally excited 1LMCT-state (effectively a monazido species) via two transiently populated diazido species back to the electronic ground-state, we are now confronted with the task to clarify the electronic structure of the intermediates. To this end, the full spectro-temporal evolution in the near-UV-to-



14 visible spectral region is now scrutinized in more detail. Figure 7 displays a series of corresponding pump-probe spectra that were recorded at exactly the same time delays as the MIR-spectra displayed in Figure 3. Once again, the top panel emphasizes the short-time evolution below 25 ps while the lower panel highlights the ensuing long-time evolution for delays of up to 1 ns. On the short time scale, the stronger induced absorption initially peaking in the near-UV at 431 nm is seen to rapidly rise within only 1 ps while shifting to shorter wavelengths by more than 20 nm (corresponding to an anti-Stokes shift of ca. 1200 cm–1). At the same time, the slightly weaker induced Vis-absorption initially peaking at 660 nm is also observed to grow in but in contrast to the near-UV band, it shifts to longer wavelength by 50 nm (equivalent to a Stokes shift of around 1100 cm–1) all the way to 710 nm. On the longer time scale (bottom panel), the spectro-temporal evolution is characterized by a decay of both absorption bands as well as a redshift of the near-UVband by about 15 nm.

Figure 7. UV-Pump / near-UV-to-Vis-probe spectra recorded at various delays after 355 nmexcitation of [1] in liquid DMSO solution at room temperature. The color coding is identical to that of Figure 3. The dotted spectrum in the upper panel corresponds to the spectrum recorded at a delay of 250 fs (also shown in Figure 2). The numbers indicate peak positions in nm. Representative kinetic traces in the near-UV-to-Vis region are reproduced in Figure 8. When probing the short-wavelength induced absorption at 410 nm, an ultrafast rise of the signal is observed within only 1 ps followed by a biphasic decay. Fitting triple-exponential kinetics to the data returns time constants of 122 ps and 5.7 ps for the two decaying components and 370 fs for the rising component. Choosing a detection wavelength of 574 nm, which is at the peak of the 1Eg(1)  1A1g ligand-field transition of [1], results in a pure decay containing three exponentials with time constants of 117 ps, 13 ps, and 1.1 ps. Finally, tuning the probe window to 710 nm, i.e. into the center of the Vis-band, produces again a rise followed by a decay. The former has a time constant of 1.5 ps whereas the ACS Paragon Plus Environment

Page 14 of 24

Page 15 of 24 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


15 latter exhibits a time constant of 120 ps. Thus, we can conclude here that the two distinctive time constants from the infrared spectral region of around 110 ps and 6 ps also show up in the near-UV to Vis spectral region. The longer of which specified the lifetime of the direct precursor to the parent’s electronic ground state (i.e. the ground-state recovery time), whereas the former was the characteristic lifetime of the first intermediate from which the ground-state precursor is formed. The ultrafast component below 2 ps agrees also well with the relaxation time of the Franck-Condon state derived from the MIR data. Finally, we also examine the dynamics of spectral shifting of both transient electronic resonances using the peak finder from above. This analysis demonstrates that the time-dependent anti-Stokes shift of the induced near-UV absorption is complete within the 25 ps-window that was already identified from the MIR data. In contrast, the dynamic Stokes shift of the induced VIS-absorption is complete within the first 2-3 ps, i.e. within the relaxation of the Franck-Condon state.

Figure 8. Top panel: Kinetic traces recorded in the near-UV-to-Vis spectral region following 355 nmoptical excitation of [1] in liquid DMSO solution at room temperature. The experimental data (circles) are fitted with multi-exponential kinetics (solid curves, see text for details). Numbers indicate the detection wavelengths. Bottom panel: Spectral peak position of the induced near-UV-absorption (blue symbols, left ordinate) and the induced Vis-absorption (blue symbols, right ordinate). Primary light-induced processes. Before interpreting the combined body of time-resolved spectroscopic data from the near-UV to the NIR, it is instructive to inspect the energetics of the parent complex. In Figure 9, this information is compiled from our own stationary spectroscopic data and accompanying CASSCF/NEVPT2 calculations, (described in the supporting information), as well as from data available in the literature. As mentioned above, the d-d band of [1] at 574 nm is due to a transition to the 1Eg-state, which also represents the first excited metal centered state, 1MC. According to the experimental UV/Vis spectrum, 1MC is located at 208 kJ / mol (corresponding to 17422 cm–1) above the 1A1g ground state (green arrow in Figure 9). Moreover, a spin-forbidden band ACS Paragon Plus Environment


16 of the closely related complex, [Co(NH3)4(N3)2]+, has been observed by Fujita and Shimura72 at a wavelength of 943 nm. The authors assigned this band to the 3T1g  1A1g transition in the Ohsymmetry approximation (brown arrow in Figure 9). These findings thus locate the lowest triplet state at an energy of 127 kJ / mol (or 10600 cm–1) above 1A1g. According to our ab initio calculations, the triplet ground state is made up of the 3Eg(1)-pair of states (in D4h-symmetry), which is also quasidegenerate with 3A2g. The theory predicts these states to be located at an energy of (110 ± 3) kJ / mol (i.e. (9161 ± 189) cm–1), which is still in very good agreement with the experiment. In Figure 9, these three lowest triplet states are collectively abbreviated by 3MC and are included so as to be energetically consistent with the experimentally measured spin-forbidden band of the tetraammine model complex from Ref. [72]. Note that the lowest metal-centered quintet state, 5MC(5B2g), is expected to be higher up in energy;73 according to our theory it lies between 1MC(1Eg) and 3MC(3E /3A ) at 159 kJ / mol (or 13326 cm–1). g 2g

Figure 9. Energy level scheme (right) of complex, [1], derived from the experimental electronic absorption spectrum (left). The very weak singlet-triplet absorption was taken from data reported in Ref. [72]. Note the axis break and the switching from linear to logarithmic scaling of the extinction coefficient in the left panel. The gray shaded ovals on the right emphasize the electronic configuration of the various states with the five metal d-orbitals in blue and one ligand-centered orbital in red. For simplicity, the d-orbital splitting is sketched for a complex having an octahedral ligand field with axial compression (point group D4h). With the energetics of the ligand-field states established one can now move on to build a model for the primary relaxation processes of [1] after 355 nm-excitation (cf. Figure 10). Absorption of the pump photon prepares the same 1LMCT-state that is also responsible for the low-energy chargetransfer band seen in the parent’s stationary UV/Vis spectrum at 345 nm (or 28985 cm‒1). The absorption-to-bleach ratio in the MIR region gives compelling evidence that already during the first three picoseconds after the interaction with the pump pulse, the complex recovers its IRspectroscopic diazido character.59 With the Franck-Condon state having largely monoazido character, its decay is apparently accompanied by a metal-to-ligand (Co(II)  N3•) back-electron-transfer.


Page 16 of 24

Page 17 of 24 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


17 Assuming this back-transfer preserves the spin multiplicity, a singlet metal-centered (i.e. ligand-field) state of trivalent cobalt will be formed. In light of the energetics, it seems indeed highly plausible that the deactivation of the optically prepared 1LMCT-state creates an intermediate population in 1MC(1Eg). This process is formally an internal conversion as it conserves the overall spin multiplicity of the system but at the same time, it is also an electron transfer from a d(eg)-orbital of the metal back into the hole that was originally created in a ligand-centered orbital by the ultraviolet pump pulse. From the above energy level diagram, one would then expect the Franck-Condon state to give rise to a transient absorption around 622 nm (orange arrow in Figure 9) to a higher lying charge transfer state of unknown character (thus, 1CT*, presumably LMCT involving electronic promotions from lower-lying ligand orbitals or into the a1g-orbital of the cobalt). This upper CT-state shows up in the stationary UV/Visspectrum as a very intense and broad absorption band around 220 nm (or ~45000 cm–1). A deactivation path, 1LMCT  1MC(1Eg), would manifest itself as a rapid decay of this visible induced absorption together with an appearance of two new transient absorptions. One of which is expected in the near infrared around 865 nm and is due to the 1LMCT  1MC(1Eg) charge transfer resonance involving an electronic promotion from a ligand-centered orbital into the eg/b2g-orbitals of the cobalt. The other absorption is expected in the near-UV around 360 nm and corresponds to the 1CT*  1MC(1E ) transition. Importantly, all signals predicted in the near-UV, the visible, and the NIR g are due to charge-transfer resonances and as such, they are at least an order of magnitude stronger than a typical ligand-field transition. This rationalizes why the expected bleaching of the 1Eg(1)  1A1g stationary absorption band at 574 nm cannot be observed cleanly. Interestingly, when correcting the UV-pump / Vis-probe spectrum at a delay of 250 fs for this relatively small bleaching component, the induced absorption is indeed seen to peak at 630 nm (cf. Figure 2, green curve in the bottom panel), i.e. remarkably close to the wavelength anticipated for the 1CT*  1LMCT-transition.

Figure 10. Sketch of the photochemical processes following 355 nm-LMCT-excitation of [1] in DMSO solution at room temperature. Note that 5% of the initially excited complexes do not return to the ground state but instead generate the solvolysis product, [3-O]. Moreover, because of the ultrafast nature of the primary internal conversion / electron transfer (IC/ET) process, these dynamics cannot be understood in terms of a simple kinetic transfer of population, 1LMCT  1MC(1Eg), between the two states involved. Rather, these dynamics give rise to ACS Paragon Plus Environment


Page 18 of 24

18 a highly complex spectro-temporal evolution throughout the visible-to-near-UV region. The ultrafast Stokes-shift documented in Figure 8 (red symbols in the bottom panel) reflects a continuous motion of the complex, very much like an overdamped wavepacket, from the optically prepared FranckCondon state initially absorbing at 622 nm to the metal centered state eventually absorbing at 865 nm. Concurrently with the Stokes-shift brought about by the wavepacket’s motion away from the Franck-Condon state, an electron is transferred back from the cobalt center to the N3•-ligand, thereby restoring the diazido character of the complex and establishing an apparent absorption-tobleach ratio in the azide stretching region of 1.59 All of these dynamics occur within the first 2-3 ps after electronic excitation. More, these primary IC/ET wavepacket dynamics should prepare the 1MC(1Eg) state in a vibrationally excited fashion, thereby causing the near-UV resonance 1CT*  1MC(1Eg) to not only build up very rapidly (cf. Figure 8, blue trace in the top panel) but also to dynamically blue-shift as the first excited ligand-field singlet state vibrationally cools (blue symbols in the bottom panel). However, it appears as if the 1MC(1Eg)-state begins to decay even before its vibrational cooling has completed. Evidence for a competition between vibrational and electronic relaxation within 1MC(1Eg) can be derived from the peculiar behavior of the peak amplitude of the induced MIR-absorption near 2015 cm–1 (wavy arrow in Figure 3, top). The above energetics suggest a depopulation of 1MC(1Eg) via intersystem crossing to the 3MC(3Eg/3A2g)-states. It is also conceivable on energetic grounds that the 5MC(5B2g)state (gray levels in Figures 9 and 10) serves as a gateway that funnels the population from the singlet into the triplet manifold. From the time-resolved data, we can then conclude that the triplet state is the above mentioned direct precursor to the parent’s electronic ground state and the precursor’s formation via the intersystem crossing, 1MC(1Eg)  3MC(3Eg/3A2g), occurs with a rate constant of 1/(6 ps). The parent’s triplet state is characterized by an antisymmetric azide stretching frequency of 2024 cm–1 and thus, its corresponding MIR-resonance is frequency downshifted relative to that of the parent 1A1g-ground-state; in good agreement with our DFT-calculations. The photoinduced primary events are finally concluded by the slower intersystem re-crossing from the triplet ground-state to the singlet ground state occurring with a rate constant of 1/(110 ps). 4. Conclusions In summary, we have revealed the photophysical and photochemical processes following an ultrafast LMCT-excitation of cyclamdiazidocobalt(III) in liquid DMSO solution using a combination of femtosecond-time and frequency-resolved spectroscopies in the mid-infrared and near-ultravioletto-visible spectral regions. The primary dynamics can be succinctly written according to the sequence 1

𝐺𝑆(1𝐴1𝑔)

355 𝑛𝑚 1

𝐿𝑀𝐶𝑇

~ 2.5 𝑝𝑠 1

𝑀𝐶(1𝐸𝑔)

6.0 𝑝𝑠 3

𝑀𝐶(3𝐸𝑔/3𝐴2𝑔)

110 𝑝𝑠 1

𝐺𝑆(1𝐴1𝑔).

The initially prepared excited state is deactivated by a primary internal conversion, which is brought about by an ultrafast back-electron-transfer to the metal. The succeeding processes are metalcentered spin-flip dynamics, which are composed of a very efficient intersystem crossing from the first singlet-excited state to the triplet-ground state followed by slower intersystem re-crossing back to the singlet ground state. Each of the intermediates of the above reaction sequence has been identified through its spectroscopic UV/Vis and NIR signatures and through the consistent spectrochemical kinetics recorded independently in the near-UV-to-Vis and MIR spectral regions. A minor fraction of complexes undergoes redox-neutral photochemistry, which preserves the oxidation state of +3 at the cobalt center and generates the solvolysis product; i.e. the monoazido complex ACS Paragon Plus Environment

Page 19 of 24 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


19 featuring an oxygen-bound DMSO solvent molecule. A similar reactivity, which effectively substitutes an azido ligand by a solvent molecules while preserving the oxidation state at the metal has also been observed in more polar solvents like acetonitrile for the complementary ferric complex, trans[Fe(cyclam)(N3)2]+.45 The solvolysis of our diazido-cobalt(III) species could not be studied in acetonitrile because its solubility was too low in this solvent. Surprisingly, experimental evidence for a homolytic Co‒N bond cleavage and formation of azide radicals with an appreciable quantum yield has not been found in this work. We will however follow up on this reaction channel by exploring in more detail the pump wavelength dependence of the spectro-temporal evolution from the near-UV all the way to the MIR. It may very well be that a formation of the solvent (S) separated radical pair, [(cyclam)(N3)Co•–□]+···S···N3•, from the initially prepared LMCT-state requires simply a larger amount of excess energy. If successful, we are planning to further engage in step-scan and rapid-scan FTIR experiments to investigate the kinetics of this well-known photoreduction channel as well as the lifetime of the azidocobalt(II) species on microsecond to millisecond time scale. Supporting Information DFT-optimized structures of the solvolysis products; movie of the spectro-temporal UV-pump/MIRto-VIS probe response; electronic structure of [1] from CASSCF/NEVPT2 calculations; mid-IR-peak shift and absorption-to-bleach area ratio Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft through the Priority Program SPP 2102, “Light-controlled reactivity of metal complexes” (project VO 593/8-1) is gratefully acknowledged. References (1) Bajdor, K.; Nakamoto, K., Formation of Ferryltetraphenylporphyrin by Laser Irradiation. J. Am. Chem. Soc. 1984, 106, 3045-3046. (2) Nakamoto, K., Structure, Spectra and Biological Significance of High-Valent Iron(IV,V) Porphyrins. J. Mol. Struct. 1997, 408, 11-16. (3) Nakamoto, K., Resonance Raman Spectra and Biological Significance of High-Valent Iron(IV,V) Porphyrins. Coord. Chem. Rev. 2002, 226, 153-165. (4) Ortiz de Montellano, P. R., Cytochrome P-450: Structure, Mechanism, and Biochemistry. Kluwer Academic/Plenum Publishers: New York, 2005. (5) Rittle, J.; Green, M. T., Cytochrome P450 Compound I: Capture, Characterization, and C-H Bond Activation Kinetics. Science 2010, 330, 933-937. (6) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que Jr., L., Dioxygen Activation at Mononuclear Nonheme Iron Active Sites: Enzymes, Models, and Intermediates. Chem. Rev. 2004, 104, 939986. (7) Krebs, C.; Fujimori, D. G.; Walsh, C. T.; Bollinger, J. M., Non-Heme Fe(IV)-Oxo Intermediates. Acc. Chem. Res. 2007, 40, 484-492. (8) Groves, J. T., High-Valent Iron in Chemical and Biological Oxidations. J. Inorg. Biochem. 2006, 100, 434-447. (9) Nam, W., High-Valent Iron(IV)-Oxo Complexes of Heme and Non-Heme Ligands in Oxygenation Reactions. Acc. Chem. Res. 2007, 40, 522-531. (10) Bukowski, M. R.; Koehntop, K. D.; Stubna, A.; Bominaar, E. L.; Halfen, J. A.; Munck, E.; Nam, W.; Que, L., A Thiolate-Ligated Nonheme Oxoiron(IV) Complex Relevant to Cytochrome P450. Science 2005, 310, 1000-1002. ACS Paragon Plus Environment


20 (11) Wang, B.; Lee, Y. M.; Tcho, W. Y.; Tussupbayev, S.; Kim, S. T.; Kim, Y.; Seo, M. S.; Cho, K. B.; Dede, Y.; Keegan, B. C.; Ogura, T.; Kim, S. H.; Ohta, T.; Baik, M. H.; Ray, K.; Shearer, J.; Nam, W., Synthesis and Reactivity of a Mononuclear Non-Haem Cobalt(Iv)-Oxo Complex. Nat. Commun. 2017, 8. (12) McDonald, A. R.; Que, L., High-Valent Nonheme Iron-Oxo Complexes: Synthesis, Structure, and Spectroscopy. Coord. Chem. Rev. 2013, 257, 414-428. (13) de Visser, S. P.; Rohde, J. U.; Lee, Y. M.; Cho, J.; Nam, W., Intrinsic Properties and Reactivities of Mononuclear Nonheme Iron-Oxygen Complexes Bearing the Tetramethylcyclam Ligand. Coord. Chem. Rev. 2013, 257, 381-393. (14) Decker, A.; Rohde, J. U.; Que, L.; Solomon, E. I., Spectroscopic and Quantum Chemical Characterization of the Electronic Structure and Bonding in a Non-Heme FeIV=O Complex. J. Am. Chem. Soc. 2004, 126, 5378-5379. (15) Decker, A.; Clay, M. D.; Solomon, E. I., Spectroscopy and Electronic Structures of Mono- and Binuclear High-Valent Non-Heme Iron-Oxo Systems. J. Inorg. Biochem. 2006, 100, 697-706. (16) Winkler, J. R.; Gray, H. B., Electronic Structures of Oxo-Metal Ions. Springer: Berlin, 2012; Vol. 142, p 17-28. (17) Groves, J. T.; Takahashi, T.; Butler, W. M., Synthesis and Molecular-Structure of a Nitrido(Porphyrinato)Chromium(V) Complex. Inorg. Chem. 1983, 22, 884-887. (18) Buchler, J. W.; Dreher, C., Metal-Complexes with Tetrapyrrole Ligands .33. Preparation of Azidochromium(III)Porphyrins, Azidomanganese(III)Porphyrins, and Azidoiron(III) Porphyrins and Their Photolysis to Terminal or Bridged Nitridometal Porphyrins. Z. Naturforsch. B 1984, 39, 222230. (19) Hill, C. L.; Hollander, F. J., Structural Characterization of a Complex of Manganese(V) Nitrido[Tetrakis(Para-Methoxyphenyl)Porphinato]-Manganese(V). J. Am. Chem. Soc. 1982, 104, 7318-7319. (20) Meyer, K.; Bendix, J.; Metzler-Nolte, N.; Weyhermuller, T.; Wieghardt, K., Nitridomanganese(V) and -(VI) Complexes Containing Macrocyclic Amine Ligands. J. Am. Chem. Soc. 1998, 120, 72607270. (21) DuBois, J.; Hong, J.; Carreira, E. M.; Day, M. W., Nitrogen Transfer from a Nitridomanganese(V) Complex: Amination of Silyl Enol Ethers. J. Am. Chem. Soc. 1996, 118, 915-916. (22) DuBois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M.; Day, M. W., Synthesis and Structure of Novel Mn-Iii and Mn-V Complexes: Development of a New, Mild Method for Forming Mn Equivalent to N Bonds. Angew. Chem. Int. Ed. 1997, 36, 1645-1647. (23) Niemann, A.; Bossek, U.; Haselhorst, G.; Wieghardt, K.; Nuber, B., Synthesis and Characterization of Six-Coordinate Nitrido Complexes of Vanadium(V), Chromium(V), and Manganese(V). Isolation of a Dinuclear, Mixed-Valent Mu-Nitrido Chromium(Iii)/Chromium(V) Species. Inorg. Chem. 1996, 35, 906-915. (24) Wagner, W. D.; Nakamoto, K., Resonance Raman Spectra of Nitridoiron(V) Porphyrin Intermediates Produced by Laser Photolysis. J. Am. Chem. Soc. 1989, 111, 1590-1598 (25) Wagner, W. D.; Nakamoto, K., Formation of Nitridoiron(V) Porphyrins Detected by Resonance Raman-Spectroscopy. J. Am. Chem. Soc. 1988, 110, 4044-4045 (26) Meyer, K.; Bill, E.; Mienert, B.; Weyhermüller, T.; Wieghardt, K., Photolysis of Cis- and Trans[FeIII(Cyclam)(N3)2]+ Complexes: Spectroscopic Characterization of a Nitridoiron(V) Species. J. Am. Chem. Soc. 1999, 121, 4859-4876. (27) Grapperhaus, C. A.; Mienert, B.; Bill, E.; Weyhermüller, T.; Wieghardt, K., Mononuclear (Nitrido)Iron(V) and (Oxo)Iron(IV) Complexes Via Photolysis of [(Cylam-Acetato)FeIII(N3)]+ and Ozonolysis of [(Cylam-Acetato)FeIII(O3SCF)]+ in Water/Acetone Mixtures. Inorg. Chem. 2000, 39, 5306-5317. (28) Aliaga-Alcalde, N.; DeBeer George, S.; Mienert, B.; Bill, E.; Wieghardt, K.; Neese, F., The Geometric and Electronic Structure of [(Cyclam-Acetato)Fe(N)]+: A Genuine Iron(V) Species with a Ground-State Spin S = 1/2. Angew. Chem. Int. Ed. 2005, 44, 2908-2912.


Page 20 of 24

Page 21 of 24 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


21 (29) Berry, J. F.; Bill, E.; Bothe, E.; Weyhermüller, T.; Wieghardt, K., Octahedral Non-Heme Non-Oxo Fe(IV) Species Stabilized by a Redox-Innocent N-Methylated Cyclam-Acetate Ligand. J. Am. Chem. Soc. 2005, 127, 11550-11551. (30) Berry, J. F.; Bill, E.; Bothe, E.; DeBeer George, S.; Mienert, B.; Neese, F.; Wieghardt, K., An Octahedral Coordination Complex of Iron(VI). Science 2006, 312, 1937-1941. (31) Berry, J. F.; Bill, E.; Bothe, E.; Neese, F.; Wieghardt, K., Octahedral Non-Heme Oxo and Non-Oxo Fe(IV) Complexes: An Experimental/Theoretical Comparison. J. Am. Chem. Soc. 2006, 128, 13515-13528. (32) Berry, J. F.; Bill, E.; Garcia-Serres, R.; Neese, F.; Weyhermuller, T.; Wieghardt, K., Effect of NMethylation of Macrocyclic Amine Ligands on the Spin State of Iron(III): A Tale of Two Fluoro Complexes. Inorg. Chem. 2006, 45, 2027-2037. (33) Petrenko, T.; George, S. D.; Aliaga-Alcalde, N.; Bill, E.; Mienert, B.; Xiao, Y.; Guo, Y.; Sturhahn, W.; Cramer, S. P.; Wieghardt, K.; Neese, F., Characterization of a Genuine Iron(V)-Nitrido Species by Nuclear Resonant Vibrational Spectroscopy Coupled to Density Functional Calculations. J. Am. Chem. Soc. 2007, 129, 11053-11060. (34) Andris, E.; Navratil, R.; Jasik, J.; Sabenya, G.; Costas, M.; Srnec, M.; Roithova, J., Detection of Indistinct Fe-N Stretching Bands in Iron(V) Nitrides by Photodissociation Spectroscopy. Chem. Eur. J. 2018, 24, 5078-5081. (35) Andris, E.; Navratil, R.; Jasik, J.; Sabenya, G.; Costas, M.; Srnec, M.; Roithova, J., Spin-StateControlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex. Angew. Chem. Int. Ed. 2017, 56, 14057-14060. (36) Sabenya, G.; Lazaro, L.; Gamba, I.; Martin-Diaconescu, V.; Andris, E.; Weyhermuller, T.; Neese, F.; Roithova, J.; Bill, E.; Lloret-Fillol, J.; Costas, M., Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform. J. Am. Chem. Soc. 2017, 139, 9168-9177. (37) Krahe, O.; Neese, F.; Engeser, M., Iron Azides with Cyclam-Derived Ligands: Are They Precursors for High-Valent Iron Nitrides in the Gas Phase? ChemPlusChem 2013, 78, 1053-1057. (38) Betley, T. A.; Peters, J. C., A Tetrahedrally Coordinated L3Fe-Nx Platform That Accommodates Terminal Nitride (FeIV:N) and Dinitrogen (FeI-N2-FeI) Ligands. J. Am. Chem. Soc. 2004, 126, 62526254. (39) Krahe, O.; Bill, E.; Neese, F., Decay of Iron(V) Nitride Complexes by a N-N Bond-Coupling Reaction in Solution: A Combined Spectroscopic and Theoretical Analysis. Angew. Chem. Int. Ed. 2014, 53, 8727-8731. (40) Torres-Alacan, J.; Vöhringer, P., Photolysis of a High-Spin Azidoiron(III) Complex Studied by TimeResolved Fourier-Transform Infrared Spectroscopy. Chem. Eur. J. 2017, 23, 6746-6751. (41) Torres-Alacan, J.; Lindner, J.; Vöhringer, P., Probing the Primary Photochemical Processes of Octahedral Iron(V) Formation with Femtosecond Mid-Infrared Spectroscopy. ChemPhysChem 2015, 16, 2289-2293. (42) Czurlok, D.; Torres-Alacan, J.; Vöhringer, P., Ultrafast 2DIR Spectroscopy of Ferric Azide Precursors for High-Valent Iron. Vibrational Relaxation, Spectral Diffusion, and Dynamic Symmetry Breaking. J. Chem. Phys. 2015, 142, 212402. (43) Vennekate, H.; Schwarzer, D.; Torres-Alacan, J.; Vöhringer, P., The Photochemical Route to Octahedral Iron(V). Primary Processes and Quantum Yields from Ultrafast Mid-Infrared Spectroscopy. J. Am. Chem. Soc. 2014, 136, 10095-10103. (44) Torres-Alacan, J.; Vöhringer, P., Generating High-Valent Iron with Light: Photochemical Dynamics from Femtoseconds to Seconds. Int. Rev. Phys. Chem. 2014, 33, 521-553. (45) Torres-Alacan, J.; Das, U.; Filippou, A. C.; Vöhringer, P., Observing the Formation and the Reactivity of an Octahedral Iron(V) Nitrido Complex in Real Time. Angew. Chem. Int. Ed. 2013, 52, 12833-12837. (46) Vennekate, H.; Schwarzer, D.; Torres-Alacan, J.; Krahe, O.; Filippou, A. C.; Neese, F.; Vöhringer, P., Ultrafast Primary Processes of an Iron-(III) Azido Complex in Solution Induced with 266 Nm Light. Phys. Chem. Chem. Phys. 2012, 14, 6165-6172.



22 (47) Torres-Alacan, J.; Krahe, O.; Filippou, A. C.; Neese, F.; Schwarzer, D.; Vöhringer, P., The Photochemistry of [FeIIIN3(Cyclam-Ac)]PF6 at 266 Nm. Chem. Eur. J. 2012, 18, 3043-3055. (48) Ferraudi, G. J.; Endicott, J. F.; Barber, J. R., Contrasting Photochemical Processes in Azidopentaamminecobalt and Isothiocyanatopentaamminecobalt(III) Complexes - Implications for Excited-State Processes. J. Am. Chem. Soc. 1975, 97, 6406-6415. (49) Hennig, H.; Ritter, K.; Knoll, H.; Vogler, A., Spectrophotometric Studies of the Photolysis of Diazido Bis(Phosphine) Metal(II) Complexes. Inorg. Chim. Acta. 1993, 211, 117-120. (50) Hennig, H.; Hofbauer, K.; Handke, K.; Stich, R., Unusual Reaction Pathways in the Photolysis of Diazido(Phosphane)Nickel(II) Complexes: Nitrenes as Intermediates in the Formation of Nickel(0) Complexes. Angew. Chem. Int. Ed. 1997, 36, 408-410. (51) Kurz, D.; Hennig, H.; Reinhold, J., Nickelatetrazoles as Intermediates in the Course of the Photolysis of Nickel(II) Azido Complexes? A Theoretical Study. Z Anorg Allg Chem 2000, 626, 354361. (52) Hennig, H.; Ritter, K.; Chibisov, A. K.; Gorner, H.; Grevels, F. W.; Kerpen, K.; Schaffner, K., Comparative Time-Resolved Ir and Uv Spectroscopic Study of Monophosphine and Diphosphine Platinum(II) Azido Complexes. Inorg. Chim. Acta. 1998, 271, 160-166. (53) Bosnich, B.; Poon, C. K.; Tobe, M. L., Complexes of Cobalt(III) with a Cyclic Tetradentate Secondary Amine. Inorg. Chem. 1965, 4, 1102-1108. (54) The 345 nm-band is undoubtedly an azide-to-cobalt charge transfer resonance as it is absent in the complex, [Co(cyclam)(Cl)2]+. The 222 nm-band is most likely a Ncyclam-to-Co charge-transfer transition. Such an interpretation is fully in line with an assignment of the UV-absorption bands (appearing between 240 nm and 220 nm) of a variety of different (alkylamine)cobalt(III) complexes by Weit et al. (see Ref. [61]). (55) Hughes, M. N.; McWhinnie, W. R., Infra-Red Spectra (667-222 cm-1) of Some Cobalt (III) Bis (Ethylenediamine) Complexes. J. Inorg. Nucl. Chem. 1966, 28, 1659-1665. (56) Kipp, E. B.; Haines, R. A., Infrared Studies of Cis-Bis(Halogenoacetato)Bis(Ethylenediamine)Cobalt(III) and Trans-Bis(Halogenoacetato)Bis(Ethylenediamine)-Cobalt(III) Complexes. Can. J. Chem. 1969, 47, 1073-1075. (57) Poon, C. K., The Infrared Spectra of Some Cis- and Trans- Isomers of Octahedral Cobalt(III) Complexes with a Cyclic Quadridentate Secondary Amine. Inorg. Chim. Acta. 1971, 5, 322-324. (58) Note that the local minimum in the induced absorption spectrally coincides exactly with the maximum of the d-d-band in the stationary UV/Vis-spectrum of the parent. If the extinction coefficient of the induced absorption at 574 nm were exactly as large as that of the stationary absorption at the same wavelength, induced bleach and induced absorption would perfectly cancel each other and as a result, the differential optical density would be zero at that wavelength. Thus, the extinction coefficent of the induced absorption must be clearly much larger than 400 L / mol cm. The bleach component can be estimated as follows: The concentration of the parent was chosen to yield an absorbance at the pump wavelength of 0.3 over an effective thickness of 100 mm. With a beam waist of 350 μm inside the 1 mm-thick sample, the pump focal volume was 20 nL. Together with a pump pulse energy of 7 μJ these numbers suggets a concentration of photoexcited molecules in the pump region of 1 mM. Given the extinction coefficient of the parent at 547 nm of 400 L / (mol cm), one then arrives at a bleaching signal of 3.6 mOD over the sample's effective thickness as indicaed by the vertical arrows in Figure 1. (59) The IR-oscillator strength of the antisymmetric stretching vibration of free azide anions is similar to that of the N3‒-ligand in a coordination compound (see also Ref. [45] where this is demonstrated for the complementary ferric complex trans-[Fe(Cyclam)(N3)2]+). In contrast, the IR-oscillator strength of the same vibration in the neutral azidyl radical is at least an order of magnitude smaller. Indeed, it is so small that in the IR, the radical must be detected indirectly by using the azide anion generated through an electron transfer quenching reaction of the type N3• + D‒  N3‒ + D (see also Ref. [40], where the reaction with excess azide anions according to N3• + N3‒  N6‒ was used and N6‒ was detected).


Page 22 of 24

Page 23 of 24 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


23 (60) Tian, R.; Facelli, J. C.; Michl, J., Vibrational and Electronic Spectra of Matrix-Isolated N3 and N3‒. J. Phys. Chem. 1988, 92, 4073-4079. (61) Weit, S. K.; Ferraudi, G.; Grutsch, P. A.; Kutal, C., Charge-Transfer Spectroscopy and Photochemistry of Alkylamine Cobalt(III) Complexes. Coord. Chem. Rev. 1993, 128, 225-243. (62) Balzani, V.; Moggi, L.; Scandola, F.; Carassiti, V., Photochemistry of Cobalt(III) Complexes. Inorg. Chim. Acta Rev. 1967, 1, 7-34. (63) Jyotsna, T.; Kumar, C. V.; Jockusch, S.; Turro, N. J., Steady-State and Time-Resolved Studies of the Photocleavage of Lysozyme by Co(III) Complexes. Langmuir 2010, 26, 1966-1972. (64) Walker, L. A.; Jarrett, J. T.; Anderson, N. A.; Pullen, S. H.; Matthews, R. G.; Sension, R. J., TimeResolved Spectroscopic Studies of B-12 Coenzymes, the Identification of a Metastable Cob(Iii)Alamin Photoproduct in the Photolysis of Methylcobalamin. J. Am. Chem. Soc. 1998, 120, 3597-3603. (65) Walker, L. A.; Shiang, J. J.; Anderson, N. A.; Pullen, S. H.; Sension, R. J., Time-Resolved Spectroscopic Studies of B-12 Coenzymes: The Photolysis and Geminate Recombination of Adenosylcobalamin. J. Am. Chem. Soc. 1998, 120, 7286-7292. (66) Sando, G. M.; Dahl, K.; Owrutsky, J. C., Vibrational Spectroscopy and Dynamics of Azide Ion in Ionic Liquid and Dimethyl Sulfoxide Water Mixtures. J. Phys. Chem. B 2007, 111, 4901-4909. (67) Olschewski, M.; Knop, S.; Lindner, J.; Vöhringer, P., Vibrational Relaxation of Azide Ions in Liquidto-Supercritical Water. J. Chem. Phys. 2011, 134, 214504. (68) Cotton, F. A.; Francis, R.; Horrocks, W. D., Sulfoxides as Ligands .2. The Infrared Spectra of Some Dimethyl Sulfoxide Complexes. J. Phys. Chem. 1960, 64, 1534-1536. (69) Wayland, B. B.; Schramm, R. F., Evidence for Both Sulphur and Oxygen Co-Ordination Sites in Tetrakis(Dimethyl Sulphoxide)Palladium(II) Cation. Chem. Commun. 1968, 1465-1466. (70) Senoff, C. V.; Jr..E, M.; Goel, R. G., Dimethylsulfoxidepentaammineruthenium(II) Hexafluorophosphate - Spectroscopic Study. Can. J. Chem. 1971, 49, 3585-3589. (71) Kubo, R., A Stochastic Theory of Line-Shape and Relaxation. In Fluctuation, Relaxation and Resonance in Magnetic Systems, ter Haar, D., Ed. Oliver & Boyd: Edinburgh, 1962; pp 23-68. (72) Fujita, J.; Shimura, Y., The Absorption Spectra of Cobalt(III) Complexes. III. The Spin-Forbidden Bands. Bull. Chem. Soc. Jpn. 1963, 36, 1281-1285. (73) Hollebone, B. R.; Langford, C. H.; Malkhasian, A. Y. S., Magnetic Circular-Dichroism Spectra of Cobalt(III) Amine Complexes of Photochemical Importance. Can. J. Chem. 1985, 63, 1918-1921.



TOC graphic 85x47mm (300 x 300 DPI)


Page 24 of 24

Photoinduced Dynamics of a Diazidocobalt (III) Complex Studied by

Recommend Documents