Publications by authors named "Antonín Vlcek"

63 Publications

Solvent-Dependent Excited-State Evolution of Prodan Dyes.

J Phys Chem B 2021 12 16;125(51):13858-13867. Epub 2021 Dec 16.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-18223 Prague, Czech Republic.

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCHCHOH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CDOD, and glycerol-. In all these solvents, near-UV excitation initially produced the same S(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm occurred in polar/H-bonding solvents, slowing down on going from CDOD (1, 23 ps) to glycerol- (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar MeN → C═O intramolecular charge transfer S(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S(ππ*) state to the S(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.
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http://dx.doi.org/10.1021/acs.jpcb.1c09030DOI Listing
December 2021

Excitation-Wavelength-Dependent Photophysics of dd Di-isocyanide Complexes.

Inorg Chem 2021 Dec 14. Epub 2021 Dec 14.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-18223 Prague, Czech Republic.

Binuclear Rh(I) and Ir(I) TMB (2,5-dimethyl-2,5-diisocyanohexane) and dimen (1,8-diisocyanomenthane) complexes possess dσ*pσ and dπpσ singlet and triplet excited states that can be selectively excited in the visible and UV spectral regions. Using perturbational spin-orbit TDDFT, we unraveled the detailed character and spin mixing of these electronic transitions and found that delocalization of pσ and dπ orbitals over C≡N- groups makes C≡N stretching vibrations sensitive reporters of electron density and structural changes upon electronic excitation. Picosecond time-resolved infrared spectra measured after visible light, 375 nm, and 316 nm excitation revealed excitation-wavelength-dependent deactivation cascades. Visible light irradiation prepares the dσ*pσ state that, after one or two (sub)picosecond relaxation steps, undergoes 70-1300 ps intersystem crossing to dσ*pσ, which is faster for the more flexible dimen complexes. UV-excited dπpσ states decay with (sub)picosecond kinetics through a manifold of high-lying triplet and mixed-spin states to dσ*pσ with lifetimes in the range of 6-19 ps (316 nm) and 19-43 ps (375 nm, Ir only), bypassing dσ*pσ. Most excited-state conversion and some relaxation steps are accompanied by direct decay to the ground state that is especially pronounced for the most flexible long/eclipsed Rh(dimen) conformer.
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http://dx.doi.org/10.1021/acs.inorgchem.1c02645DOI Listing
December 2021

Photoinduced hole hopping through tryptophans in proteins.

Proc Natl Acad Sci U S A 2021 03;118(11)

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ-182 23 Prague, Czech Republic;

Hole hopping through tryptophan/tyrosine chains enables rapid unidirectional charge transport over long distances. We have elucidated structural and dynamical factors controlling hopping speed and efficiency in two modified azurin constructs that include a rhenium(I) sensitizer, Re(His)(CO)(dmp), and one or two tryptophans (W, W). Experimental kinetics investigations showed that the two closely spaced (3 to 4 Å) intervening tryptophans dramatically accelerated long-range electron transfer (ET) from Cu to the photoexcited sensitizer. In our theoretical work, we found that time-dependent density-functional theory (TDDFT) quantum mechanics/molecular mechanics/molecular dynamics (QM/MM/MD) trajectories of low-lying triplet excited states of Re(His)(CO)(dmp)-W(-W) exhibited crossings between sensitizer-localized (*Re) and charge-separated [Re(His)(CO)(dmp)/(W or W )] (CS1 or CS2) states. Our analysis revealed that the distances, angles, and mutual orientations of ET-active cofactors fluctuate in a relatively narrow range in which the cofactors are strongly coupled, enabling adiabatic ET. Water-dominated electrostatic field fluctuations bring *Re and CS1 states to a crossing where *Re(CO)(dmp)←W ET occurs, and CS1 becomes the lowest triplet state. ET is promoted by solvation dynamics around *Re(CO)(dmp)(W); and CS1 is stabilized by Re(dmp)/W electron/hole interaction and enhanced W solvation. The second hop, W ←W, is facilitated by water fluctuations near the W/W unit, taking place when the electrostatic potential at W drops well below that at W Insufficient solvation and reorganization around W make W ←W ET endergonic, shifting the equilibrium toward W and decreasing the charge-separation yield. We suggest that multiscale TDDFT/MM/MD is a suitable technique to model the simultaneous evolution of photogenerated excited-state manifolds.
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http://dx.doi.org/10.1073/pnas.2024627118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7980458PMC
March 2021

Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II).

Inorg Chem 2021 Mar 1;60(6):3514-3523. Epub 2021 Mar 1.

School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, E1 4NS London, United Kingdom.

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)(bpy)] (CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, bpy = 2,2'-bipyridine, and = 1-3, abbreviated as , , and ) undergo four () or five ( and ) successive one-electron reduction steps between -1.3 and -2.75 V versus ferrocenium/ferrocene (Fc/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in and being separated by 150-210 mV; reduction of the unsubstituted bpy ligand in and occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. -45 cm, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands , , and show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy, confirming that each reduction is localized on a single ligand. NIR spectra of , , and exhibit a prominent band attributable to the CN-Me-bpy moiety between 6000 and 7500 cm, whereas bpy-based absorption occurs between 4500 and 6000 cm; complexes , , and also exhibit a band at ca. 3300 cm due to a CN-Me-bpy → CN-Me-bpy interligand charge-transfer transition. In the UV-vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by -38 cm and 5-7-fold-enhanced relative to the ground state.
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http://dx.doi.org/10.1021/acs.inorgchem.0c03579DOI Listing
March 2021

Time-Resolved Femtosecond Stimulated Raman Spectra and DFT Anharmonic Vibrational Analysis of an Electronically Excited Rhenium Photosensitizer.

J Phys Chem A 2020 Feb 5;124(7):1253-1265. Epub 2020 Feb 5.

J. Heyrovský Institute of Physical Chemistry , Czech Academy of Sciences , Dolejškova 3 , 182 23 Prague , Czech Republic.

Time-resolved femtosecond stimulated Raman spectra (FSRS) of a prototypical organometallic photosensitizer/photocatalyst ReCl(CO)(2,2'-bipyridine) were measured in a broad spectral range ∼40-2000 (4000) cm at time delays from 40 fs to 4 ns after 400 nm excitation of the lowest allowed electronic transition. Theoretical ground- and excited-state Raman spectra were obtained by anharmonic vibrational analysis using second-order vibrational perturbation theory on vibrations calculated by harmonic approximation at density functional theory-optimized structures. A good match with anharmonically calculated vibrational frequencies allowed for assigning experimental Raman features to particular vibrations. Observed frequency shifts upon excitation (ν(ReCl) and ν(CC inter-ring) vibrations upward; ν(CC, CN) and ν(Re-C) downward) are consistent with the bonding/antibonding characters of the highest occupied molecular orbital and the lowest unoccupied molecular orbital involved in excitation and support the delocalized formulation of the lowest triplet state as ReCl(CO) → bpy charge transfer. FSRS spectra show a mode-specific temporal evolution, providing insights into the intersystem crossing (ISC) mechanism and subsequent relaxation. Most of the Raman features are present at ∼40 fs and exhibit small shifts and intensity changes with time. The 1450-1600 cm group of bands due to CC, CN, and CC(inter-ring) stretching vibrations undergoes extensive restructuring between 40 and ∼150 fs, followed by frequency upshifts and a biexponential (0.38, 21 ps) area growth, indicating progressing charge separation in the course of the formation and relaxation of the lowest triplet state. Early (40-150 fs) restructuring was also observed in the low-frequency range for ν(Re-Cl) and δ(Re-C-O) vibrations that are presumably activated by ISC. FSRS experimental innovations employed to measure low- and high-energy Raman features simultaneously are described and discussed in detail.
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http://dx.doi.org/10.1021/acs.jpca.9b10840DOI Listing
February 2020

Light-Induced Nanosecond Relaxation Dynamics of Rhenium-Labeled Azurins.

J Phys Chem B 2020 02 27;124(5):788-797. Epub 2020 Jan 27.

J. Heyrovský Institute of Physical Chemistry , Czech Academy of Sciences , Dolejškova 3 , CZ-182 23 Prague , Czech Republic.

Time-resolved phosphorescence spectra of Re(CO)(dmp) and Re(CO)(phen) chromophores (dmp = 4,7-dimethyl-1,10-phenanthroline, phen = 1,10-phenanthroline) bound to surface histidines (H83, H124, and H126) of azurin mutants exhibit dynamic band maxima shifts to lower wavenumbers following 3-exponential kinetics with 1-5 and 20-100 ns major phases and a 1.1-2.5 μs minor (5-16%) phase. Observation of slow relaxation components was made possible by using an organometallic Re chromophore as a probe whose long phosphorescence lifetime extends the observation window up to ∼3 μs. Integrated emission-band areas also decay with 2- or 3-exponential kinetics; the faster decay phase(s) is relaxation-related, whereas the slowest one [360-680 ns (dmp); 90-140 ns (phen)] arises mainly from population decay. As a result of shifting bands, the emission intensity decay kinetics depend on the detection wavelength. Detailed kinetics analyses and comparisons with band-shift dynamics are needed to disentangle relaxation and population decay kinetics if they occur on comparable timescales. The dynamic phosphorescence Stokes shift in Re-azurins is caused by relaxation motions of the solvent, the protein, and solvated amino acid side chains at the Re binding site in response to chromophore electronic excitation. Comparing relaxation and decay kinetics of and suggests that electron transfer (ET) and relaxation motions in the W122 mutant are coupled. It follows that nanosecond and faster photo-induced ET steps in azurins (and likely other redox proteins) occur from unrelaxed systems; importantly, these reactions can be driven (or hindered) by structural and solvational dynamics.
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http://dx.doi.org/10.1021/acs.jpcb.9b10802DOI Listing
February 2020

Two Tryptophans Are Better Than One in Accelerating Electron Flow through a Protein.

ACS Cent Sci 2019 Jan 7;5(1):192-200. Epub 2019 Jan 7.

Beckman Institute, California Institute of Technology, Pasadena, California 91125, United States.

We have constructed and structurally characterized a azurin mutant , where two adjacent tryptophan residues (W124 and W122, indole separation 3.6-4.1 Å) are inserted between the Cu center and a Re photosensitizer coordinated to the imidazole of H126 (Re(H126)(CO)(4,7-dimethyl-1,10-phenanthroline)). Cu oxidation by the photoexcited Re label (*Re) 22.9 Å away proceeds with a ∼70 ns time constant, similar to that of a single-tryptophan mutant (∼40 ns) with a 19.4 Å Re-Cu distance. Time-resolved spectroscopy (luminescence, visible and IR absorption) revealed two rapid reversible electron transfer steps, W124 → *Re (400-475 ps, ≅ 3.5-4) and W122 → W124 (7-9 ns, ≅ 0.55-0.75), followed by a rate-determining (70-90 ns) Cu oxidation by W122 ca. 11 Å away. The photocycle is completed by 120 μs recombination. No photochemical Cu oxidation was observed in , whereas in , the photocycle is restricted to the ReH126W124 unit and Cu remains isolated. QM/MM/MD simulations of indicate that indole solvation changes through the hopping process and W124 → *Re electron transfer is accompanied by water fluctuations that tighten W124 solvation. Our finding that multistep tunneling (hopping) confers a ∼9000-fold advantage over single-step tunneling in the double-tryptophan protein supports the proposal that hole-hopping through tryptophan/tyrosine chains protects enzymes from oxidative damage.
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http://dx.doi.org/10.1021/acscentsci.8b00882DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6346393PMC
January 2019

Hole Hopping Across a Protein-Protein Interface.

J Phys Chem B 2019 02 6;123(7):1578-1591. Epub 2019 Feb 6.

J. Heyrovský Institute of Physical Chemistry , Academy of Sciences of the Czech Republic , Dolejškova 3 , CZ - 182 23 Prague , Czech Republic.

We have investigated photoinduced hole hopping in a Pseudomonas aeruginosa azurin mutant Re126WWCu, where two adjacent tryptophan residues (W124 and W122) are inserted between the Cu center and a Re photosensitizer coordinated to a H126 imidazole (Re = Re(H126)(CO)(dmp), dmp = 4,7-dimethyl-1,10-phenanthroline). Optical excitation of this mutant in aqueous media (≤40 μM) triggers 70 ns electron transport over 23 Å, yielding a long-lived (120 μs) Re(H126)(CO)(dmp)WWCu product. The Re126FWCu mutant (F124, W122) is not redox-active under these conditions. Upon increasing the concentration to 0.2-2 mM, {Re126WWCu} and {Re126FWCu} are formed with the dmp ligand of the Re photooxidant of one molecule in close contact (3.8 Å) with the W122' indole on the neighboring chain. In addition, {Re126WWCu} contains an interfacial tryptophan quadruplex of four indoles (3.3-3.7 Å apart). In both mutants, dimerization opens an intermolecular W122' → //*Re ET channel (// denotes the protein interface, *Re is the optically excited sensitizer). Excited-state relaxation and ET occur together in two steps (time constants of ∼600 ps and ∼8 ns) that lead to a charge-separated state containing a Re(H126)(CO)(dmp)//(W122)' unit; then (Cu)' is oxidized intramolecularly (60-90 ns) by (W122)', forming Re(H126)(CO)(dmp)WWCu//(Cu)'. The photocycle is closed by ∼1.6 μs Re(H126)(CO)(dmp) → //(Cu)' back ET that occurs over 12 Å, in contrast to the 23 Å, 120 μs step in Re126WWCu. Importantly, dimerization makes Re126FWCu photoreactive and, as in the case of {Re126WWCu}, channels the photoproduced "hole" to the molecule that was not initially photoexcited, thereby shortening the lifetime of Re(H126)(CO)(dmp)//Cu. Although two adjacent W124 and W122 indoles dramatically enhance Cu → *Re intramolecular multistep ET, the tryptophan quadruplex in {Re126WWCu} does not accelerate intermolecular electron transport; instead, it acts as a hole storage and crossover unit between inter- and intramolecular ET pathways. Irradiation of {Re126WWCu} or {Re126FWCu} also triggers intermolecular W122' → //*Re ET, and the Re(H126)(CO)(dmp)//(W122)' charge-separated state decays to the ground state by ∼50 ns Re(H126)(CO)(dmp) → //(W122)' intermolecular charge recombination. Our findings shed light on the factors that control interfacial hole/electron hopping in protein complexes and on the role of aromatic amino acids in accelerating long-range electron transport.
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http://dx.doi.org/10.1021/acs.jpcb.8b11982DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6384139PMC
February 2019

Vibrational Relaxation and Redistribution Dynamics in Ruthenium(II) Polypyridyl-Based Charge-Transfer Excited States: A Combined Ultrafast Electronic and Infrared Absorption Study.

J Phys Chem A 2018 Oct 28;122(40):7941-7953. Epub 2018 Sep 28.

Department of Chemistry , Michigan State University , East Lansing , Michigan 48824 , United States.

Ultrafast time-resolved electronic and infrared absorption measurements have been carried out on a series of Ru(II) polypyridyl complexes in an effort to delineate the dynamics of vibrational relaxation in this class of charge transfer chromophores. Time-dependent density functional theory calculations performed on compounds of the form [Ru(CN-Me-bpy) (bpy)] ( x = 1-3 for compounds 1-3, respectively, where CN-Me-bpy is 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine and bpy is 2,2'-bipyridine) reveal features in their charge-transfer absorption envelopes that allow for selective excitation of the Ru(II)-(CN-Me-bpy) moiety, the lowest-energy MLCT state(s) in each compound of the series. Changes in band shape and amplitude of the time-resolved differential electronic absorption data are ascribed to vibrational cooling in the CN-Me-bpy-localized MLCT state with a time constant of 8 ± 3 ps in all three compounds. This conclusion was corroborated by picosecond time-resolved infrared absorption measurements; sharpening of the CN stretch in the MLCT excited state was observed with a time constant of 3.0 ± 1.5 ps in all three members of the series. Electronic absorption data acquired at higher temporal resolution revealed spectral modulation over the first 2 ps occurring with a time constant of τ = 170 ± 50 fs, in compound 1; corresponding effects are significantly attenuated in compound 2 and virtually absent in compound 3. We assign this feature to intramolecular vibrational redistribution (IVR) within the MLCT state and represents a rare example of this process being identified from time-resolved electronic absorption data for this important class of chromophores.
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http://dx.doi.org/10.1021/acs.jpca.8b06197DOI Listing
October 2018

Photophysical Heavy-Atom Effect in Iodinated Metallocorroles: Spin-Orbit Coupling and Density of States.

J Phys Chem A 2018 Sep 6;122(37):7256-7266. Epub 2018 Sep 6.

J. Heyrovský Institute of Physical Chemistry , Czech Academy of Sciences , Dolejškova 3 , CZ-182 23 Prague , Czech Republic.

Excited-state dynamics and electronic structures of Al and Ga corrole complexes were studied as a function of the number of β-pyrrole iodine substituents. Using spectrally broad-band femtosecond-resolved fluorescence upconversion, we determined the kinetics of the Soret fluorescence decay, the concomitant rise and subsequent decay of the Q-band fluorescence, as well as of the accompanying vibrational relaxation. Iodination was found to accelerate all involved processes. The time constant of the internal conversion from the Soret to the Q states decreases from 320-540 to 70-185 fs upon iodination. Vibrational relaxation then occurs with about 15 and 0.36-1.4 ps lifetime for iodine-free and iodinated complexes, respectively. Intersystem crossing to the lowest triplet is accelerated up to 200 times from nanoseconds to 15-24 ps; its rate correlates with the iodine p(π) participation in the corrole π-system and the spin-orbit coupling (SOC) strength. TDDFT calculations with explicit SOC show that iodination introduces a manifold of low-lying singlet and triplet iodine → corrole charge-transfer (CT) states. These states affect the photophysics by (i) providing a relaxation cascade for the Soret → Q internal conversion and cooling and (ii) opening new SOC pathways whereby CT triplet character is admixed into both Q singlet excited states. In addition, SOC between the higher Q singlet and the Soret triplet is enhanced as the iodine participation in frontier corrole π-orbitals increases. Our observations that iodination of the chromophore periphery affects the whole photocycle by changing the electronic structure, spin-orbit coupling, and the density of states rationalize the "heavy-atom effect" and have implications for controlling excited-state dynamics in a range of triplet photosensitizers.
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http://dx.doi.org/10.1021/acs.jpca.8b05311DOI Listing
September 2018

Vibrational coherence transfer in the ultrafast intersystem crossing of a diplatinum complex in solution.

Proc Natl Acad Sci U S A 2018 07 25;115(28):E6396-E6403. Epub 2018 Jun 25.

Laboratoire de Spectroscopie Ultrarapide, Lausanne Centre for Ultrafast Science, Institut des Sciences et Ingénierie Chimiques, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;

We investigate the ultrafast transient absorption response of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt(μ-POH)] [hereafter abbreviated Pt(pop)], in acetonitrile upon excitation of its lowest singlet A state. Compared with previously reported solvents [van der Veen RM, Cannizzo A, van Mourik F, Vlček A, Jr, Chergui M (2011) 133:305-315], a significant shortening of the intersystem crossing (ISC) time (<1 ps) from the lowest singlet to the lowest triplet state is found, allowing for a transfer of vibrational coherence, observed in the course of an ISC in a polyatomic molecule in solution. Density functional theory (DFT) quantum mechanical/molecular mechanical (QM/MM) simulations of Pt(pop) in acetonitrile and ethanol show that high-lying, mostly triplet, states are strongly mixed and shifted to lower energies due to interactions with the solvent, providing an intermediate state (or manifold of states) for the ISC. This suggests that the larger the solvation energies of the intermediate state(s), the shorter the ISC time. Because the latter is smaller than the pure dephasing time of the vibrational wave packet, coherence is conserved during the spin transition. These results underscore the crucial role of the solvent in directing pathways of intramolecular energy flow.
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http://dx.doi.org/10.1073/pnas.1719899115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6048523PMC
July 2018

Ultrafast Wiggling and Jiggling: Ir(1,8-diisocyanomenthane).

J Phys Chem A 2017 Dec 29;121(48):9275-9283. Epub 2017 Nov 29.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic , Dolejškova 3, CZ-182 23 Prague, Czech Republic.

Binuclear complexes of d metals (Pt, Ir, Rh,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M-M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir(1,8-diisocyanomenthane) (Ir(dimen)), investigated by fs-ns time-resolved IR spectroscopy (TRIR) in the region of C≡N stretching vibrations, ν(C≡N), 2000-2300 cm. The ν(C≡N) IR band of the singlet and triplet dσ*pσ excited states is shifted by -22 and -16 cm relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the dσ*pσ state depend on the initially excited Franck-Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir-Ir contraction and 5 ps Ir-Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(C≡N) IR bandwidth oscillates with the frequency of the ν(Ir-Ir) wave packet, ca. 80 cm, indicating that the dephasing time of the high-frequency (16 fs) C≡N stretch responds to much slower (∼400 fs) Ir-Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal-metal unit and that the coherent Ir-Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.
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http://dx.doi.org/10.1021/acs.jpca.7b10215DOI Listing
December 2017

Electronic Structures of Reduced and Superreduced Ir(1,8-diisocyanomenthane) Complexes.

Inorg Chem 2017 Mar 20;56(5):2874-2883. Epub 2017 Feb 20.

J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences , Dolejškova 3, 182 23 Prague, Czech Republic.

Molecular and electronic structures of Ir(1,8-diisocyanomenthane) (Ir(dimen)) complexes have been investigated by DFT for n = 2, 1, 0 (abbreviated 2+, 1+, 0). Calculations reproduced the experimental structure of 2+, ν(C≡N) IR, and visible absorption spectra of all three oxidation states, as well as the EPR spectrum of 1+. We have shown that the two reduction steps correspond to successive filling of the Ir-Ir pσ orbital. Complexes 2+ and 1+ have very similar structures with 1+ having a shorter Ir-Ir distance. The unpaired electron density in 1+ is delocalized along the Ir-Ir axis and over N atoms of the eight C≡N- ligands. The second reduction step 1+ → 0 changes the Ir(CN-) coordination geometry at each Ir site from approximately planar to seesaw whereby one -N≡C-Ir-C≡N- moiety is linear and the other bent at the Ir (137°) as well as N (146°) atoms. Although complex 0 is another example of a rare (pσ) dimetallic species (after [Pt(μ-PO(BF))], J. Am. Chem. Soc. 2016, 138, 5699), the redistribution of lower lying occupied molecular orbitals increases electron density predominantly at the bent C≡N- ligands whose N atoms are predicted to be nucleophilic reaction centers.
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http://dx.doi.org/10.1021/acs.inorgchem.6b03001DOI Listing
March 2017

Reduced and Superreduced Diplatinum Complexes.

J Am Chem Soc 2016 05 18;138(17):5699-705. Epub 2016 Apr 18.

Beckman Institute, California Institute of Technology , Pasadena, California 91125, United States.

A d(8)-d(8) complex [Pt2(μ-P2O5(BF2)4](4-) (abbreviated Pt(pop-BF2)(4-)) undergoes two 1e(-) reductions at E1/2 = -1.68 and Ep = -2.46 V (vs Fc(+)/Fc) producing reduced Pt(pop-BF2)(5-) and superreduced Pt(pop-BF2)(6-) species, respectively. The EPR spectrum of Pt(pop-BF2)(5-) and UV-vis spectra of both the reduced and the superreduced complexes, together with TD-DFT calculations, reveal successive filling of the 6pσ orbital accompanied by gradual strengthening of Pt-Pt bonding interactions and, because of 6pσ delocalization, of Pt-P bonds in the course of the two reductions. Mayer-Millikan Pt-Pt bond orders of 0.173, 0.268, and 0.340 were calculated for the parent, reduced, and superreduced complexes, respectively. The second (5-/6-) reduction is accompanied by a structural distortion that is experimentally manifested by electrochemical irreversibility. Both reduction steps proceed without changing either d(8) Pt electronic configuration, making the superreduced Pt(pop-BF2)(6-) a very rare 6p(2) σ-bonded binuclear complex. However, the Pt-Pt σ bonding interaction is limited by the relatively long bridging-ligand-imposed Pt-Pt distance accompanied by repulsive electronic congestion. Pt(pop-BF2)(4-) is predicted to be a very strong photooxidant (potentials of +1.57 and +0.86 V are estimated for the singlet and triplet dσ*pσ excited states, respectively).
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http://dx.doi.org/10.1021/jacs.6b02559DOI Listing
May 2016

Thermally Tunable Dual Emission of the d(8)-d(8) Dimer [Pt2(μ-P2O5(BF2)2)4](4).

Inorg Chem 2016 Mar 24;55(5):2441-9. Epub 2016 Feb 24.

Institut für Physikalische und Theoretische Chemie, Universität Regensburg , Universitätstrasse 31, D-93040 Regensburg, Germany.

High-resolution fluorescence, phosphorescence, as well as related excitation spectra, and, in particular, the emission decay behavior of solid [Bu4N]4[Pt2(μ-P2O5(BF2)2)4], abbreviated Pt(pop-BF2), have been investigated over a wide temperature range, 1.3-310 K. We focus on the lowest excited states that result from dσ*pσ (5dz(2)-6pz) excitations, i.e., the singlet state S1 (of (1)A2u symmetry in D4h) and the lowest triplet T1, which splits into spin-orbit substates A1u((3)A2u) and Eu((3)A2u). After optical excitation, an unusually slow intersystem crossing (ISC) is observed. As a consequence, the compound shows efficient dual emission, consisting of blue fluorescence and green phosphorescence with an overall emission quantum yield of ∼ 100% over the investigated temperature range. Our investigation sheds light on this extraordinary dual emission behavior, which is unique for a heavy-atom transition metal compound. Direct ISC processes in Pt(pop-BF2) are largely forbidden due to spin-, symmetry-, and Franck-Condon overlap-restrictions and, therefore, the ISC time is as long as 29 ns for T < 100 K. With temperature increase, two different thermally activated pathways, albeit still relatively slow, are promoted by spin-vibronic and vibronic mechanisms, respectively. Thus, distinct temperature dependence of the ISC processes results and, as a consequence, also of the fluorescence/phosphorescence intensity ratio. The phosphorescence lifetime also is temperature-dependent, reflecting the relative population of the triplet T1 substates Eu and A1u. The highly resolved phosphorescence shows a ∼ 220 cm(-1) red shift below 10 K, attributable to zero-field splitting of 40 cm(-1) plus a promoting vibration of 180 cm(-1).
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http://dx.doi.org/10.1021/acs.inorgchem.5b02839DOI Listing
March 2016

Anharmonicity Effects in IR Spectra of [Re(X)(CO)3(α-diimine)] (α-diimine = 2,2'-bipyridine or pyridylimidazo[1,5-a]pyridine; X = Cl or NCS) Complexes in Ground and Excited Electronic States.

J Phys Chem A 2015 Oct 28;119(40):10137-46. Epub 2015 Sep 28.

J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences , Dolejškova 3, 182 23 Prague 8, Czech Republic.

Infrared spectra of [Re(X)(CO)(3)(α-diimine)] (α-diimine = 2,2'-bipyridine, X = Cl, NCS, or pyridylimidazo[1,5-a]pyridine, X = Cl) in the ground and the lowest triplet electronic states were calculated by a global hybrid density functional going beyond the harmonic level by means of second-order vibrational perturbation theory (VPT2) and including bulk solvent effects by the polarizable continuum model (PCM). The full-dimensionality (FD) VPT2 is compared with the reduced-dimensionality (RD) model, where only selected vibrational modes are calculated anharmonically. The simulated difference IR spectra (excited state minus ground state) in the ν(CO) region closely match experimental time-resolved infrared (TRIR) spectra. Very good agreement was also obtained for ground-state spectra in the fingerprint region. In comparison with the harmonic simulated spectra, the calculated anharmonic frequencies are closer to experimental values and do not require scaling when the B3LYP functional is used. Several spectral features due to combination bands have been identified by VPT2 simulations in the ν(CO) spectral region, which are of importance for a correct interpretation of TRIR experiments.
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http://dx.doi.org/10.1021/acs.jpca.5b07585DOI Listing
October 2015

Electronic Excited States of Tungsten(0) Arylisocyanides.

Inorg Chem 2015 Sep 12;54(17):8518-28. Epub 2015 Aug 12.

J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences , Dolejškova 3, CZ-182 23 Prague, Czech Republic.

W(CNAryl)6 complexes containing 2,6-diisopropylphenyl isocyanide (CNdipp) are powerful photoreductants with strongly emissive long-lived excited states. These properties are enhanced upon appending another aryl ring, e.g., W(CNdippPh(OMe2))6; CNdippPh(OMe2) = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide (Sattler et al. J. Am. Chem. Soc. 2015, 137, 1198-1205). Electronic transitions and low-lying excited states of these complexes were investigated by time-dependent density functional theory (TDDFT); the lowest triplet state was characterized by time-resolved infrared spectroscopy (TRIR) supported by density functional theory (DFT). The intense absorption band of W(CNdipp)6 at 460 nm and that of W(CNdippPh(OMe2))6 at 500 nm originate from transitions of mixed ππ*(C≡N-C)/MLCT(W → Aryl) character, whereby W is depopulated by ca. 0.4 e(-) and the electron-density changes are predominantly localized along two equatorial molecular axes. The red shift and intensity rise on going from W(CNdipp)6 to W(CNdippPh(OMe2))6 are attributable to more extensive delocalization of the MLCT component. The complexes also exhibit absorptions in the 300-320 nm region, owing to W → C≡N MLCT transitions. Electronic absorptions in the spectrum of W(CNXy)6 (Xy = 2,6-dimethylphenyl), a complex with orthogonal aryl orientation, have similar characteristics, although shifted to higher energies. The relaxed lowest W(CNAryl)6 triplet state combines ππ* excitation of a trans pair of C≡N-C moieties with MLCT (0.21 e(-)) and ligand-to-ligand charge transfer (LLCT, 0.24-0.27 e(-)) from the other four CNAryl ligands to the axial aryl and, less, to C≡N groups; the spin density is localized along a single Aryl-N≡C-W-C≡N-Aryl axis. Delocalization of excited electron density on outer aryl rings in W(CNdippPh(OMe2))6 likely promotes photoinduced electron-transfer reactions to acceptor molecules. TRIR spectra show an intense broad bleach due to ν(C≡N), a prominent transient upshifted by 60-65 cm(-1), and a weak down-shifted feature due to antisymmetric C≡N stretch along the axis of high spin density. The TRIR spectral pattern remains unchanged on the femtosecond-nanosecond time scale, indicating that intersystem crossing and electron-density localization are ultrafast (<100 fs).
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http://dx.doi.org/10.1021/acs.inorgchem.5b01203DOI Listing
September 2015

Ultrafast excited-state processes in inorganic systems.

Acc Chem Res 2015 May;48(5):1207-8

Michigan State University.

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http://dx.doi.org/10.1021/acs.accounts.5b00235DOI Listing
May 2015

Spin-orbit TDDFT electronic structure of diplatinum(II,II) complexes.

Inorg Chem 2015 Apr 16;54(7):3491-500. Epub 2015 Mar 16.

†J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague, Czech Republic.

[Pt2(μ-P2O5H2)4](4-) (Pt(pop)) and its perfluoroborated derivative [Pt2(μ-P2O5(BF2)2)4](4-) (Pt(pop-BF2)) are d(8)-d(8) complexes whose electronic excited states can drive reductions and oxidations of relatively inert substrates. We performed spin-orbit (SO) TDDFT calculations on these complexes that account for their absorption spectra across the entire UV-vis spectral region. The complexes exhibit both fluorescence and phosphorescence attributable, respectively, to singlet and triplet excited states of dσ*pσ origin. These features are energetically isolated from each other (∼7000 cm(-1) for (Pt(pop-BF2)) as well as from higher-lying states (5800 cm(-1)). The lowest (3)dσ*pσ state is split into three SO states by interactions with higher-lying singlet states with dπpσ and, to a lesser extent, pπpσ contributions. The spectroscopically allowed dσ*pσ SO state has ∼96% singlet character with small admixtures of higher triplets of partial dπpσ and pπpσ characters that also mix with (3)dσ*pσ, resulting in a second-order (1)dσ*pσ-(3)dσ*pσ SO interaction that facilitates intersystem crossing (ISC). All SO interactions involving the dσ*pσ states are weak because of large energy gaps to higher interacting states. The spectroscopically allowed dσ*pσ SO state is followed by a dense manifold of ligand-to-metal-metal charge transfer states, some with pπpσ (at lower energies) or dπpσ contributions (at higher energies). Spectroscopically active higher states are strongly spin-mixed. The electronic structure, state ordering, and relative energies are minimally perturbed when the calculation is performed at the optimized geometries of the (1)dσ*pσ and (3)dσ*pσ excited states (rather than the ground state). Results obtained for Pt(pop) are very similar, showing slightly smaller energy gaps and, possibly, an additional (1)dσ*pσ - (3)dσ*pσ second order SO interaction involving higher (1)dπpσ* states that could account in part for the much faster ISC. It also appears that (1)dσ*pσ → (3)dσ*pσ ISC requires a structural distortion that has a lower barrier for Pt(pop) than for the more rigid Pt(pop-BF2).
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http://dx.doi.org/10.1021/acs.inorgchem.5b00063DOI Listing
April 2015

Electron-transfer acceleration investigated by time resolved infrared spectroscopy.

Acc Chem Res 2015 Mar 20;48(3):868-76. Epub 2015 Feb 20.

‡J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague, Czech Republic.

Ultrafast electron transfer (ET) processes are important primary steps in natural and artificial photosynthesis, as well as in molecular electronic/photonic devices. In biological systems, ET often occurs surprisingly fast over long distances of several tens of angströms. Laser-pulse irradiation is conveniently used to generate strongly oxidizing (or reducing) excited states whose reactions are then studied by time-resolved spectroscopic techniques. While photoluminescence decay and UV-vis absorption supply precise kinetics data, time-resolved infrared absorption (TRIR) and Raman-based spectroscopies have the advantage of providing additional structural information and monitoring vibrational energy flows and dissipation, as well as medium relaxation, that accompany ultrafast ET. We will discuss three cases of photoinduced ET involving the Re(I)(CO)3(N,N) moiety (N,N = polypyridine) that occur much faster than would be expected from ET theories. [Re(4-N-methylpyridinium-pyridine)(CO)3(N,N)](2+) represents a case of excited-state picosecond ET between two different ligands that remains ultrafast even in slow-relaxing solvents, beating the adiabatic limit. This is caused by vibrational/solvational excitation of the precursor state and participation of high-frequency quantum modes in barrier crossing. The case of Re-tryptophan assemblies demonstrates that excited-state Trp → *Re(II) ET is accelerated from nanoseconds to picoseconds when the Re(I)(CO)3(N,N) chromophore is appended to a protein, close to a tryptophan residue. TRIR in combination with DFT calculations and structural studies reveals an interaction between the N,N ligand and the tryptophan indole. It results in partial electronic delocalization in the precursor excited state and likely contributes to the ultrafast ET rate. Long-lived vibrational/solvational excitation of the protein Re(I)(CO)3(N,N)···Trp moiety, documented by dynamic IR band shifts, could be another accelerating factor. The last discussed process, back-ET in a porphyrin-Re(I)(CO)3(N,N) dyad, demonstrates that formation of a hot product accelerates highly exergonic ET in the Marcus inverted region. Overall, it follows that ET can be accelerated by enhancing the electronic interaction and by vibrational excitation of the reacting system and its medium, stressing the importance of quantum nuclear dynamics in ET reactivity. These effects are experimentally accessible by time-resolved vibrational spectroscopies (IR, Raman) in combination with quantum chemical calculations. It is suggested that structural dynamics play different mechanistic roles in light-triggered ET involving electronically excited donors or acceptors than in ground-state processes. While TRIR spectroscopy is well suitable to elucidate ET processes on a molecular-level, transient 2D-IR techniques combining optical and two IR (or terahertz) laser pulses present future opportunities for investigating, driving, and controlling ET.
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http://dx.doi.org/10.1021/ar5004048DOI Listing
March 2015

Ultrafast electronic and vibrational relaxations in mixed-ligand dithione-dithiolato Ni, Pd, and Pt complexes.

Dalton Trans 2014 Dec;43(47):17666-76

Institute of Applied Physics, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland.

Ultrafast excited-state dynamics of planar Pt, Pd, and Ni dithione-dithiolato complexes were investigated by transient absorption spectroscopy on the femtosecond-picosecond timescale. All studied complexes show a common photobehaviour, although individual kinetics parameters and quantum yields vary with the metal, the dithione ligand and, namely the solvent (DMF, MeCN). Laser pulse irradiation at 800 nm populates the lowest singlet excited state of a dithiolato → dithione charge transfer character, (1)LL'CT. The optically excited state undergoes a solvation-driven sub-picosecond electronic relaxation that enhances the dithione/dithiolato charge separation. The (1)LL'CT state decays with a 1.9-4.5 ps lifetime by two simultaneous pathways: intersystem crossing (ISC) to the lowest triplet state (3)LL'CT and non-radiative decay to the ground state. ISC occurs on a ∼6 ps timescale, virtually independent of the metal, whereas the rate of the non-radiative decay to the ground state decreases on going from Ni (2 ps) to Pd (3 ps) and Pt (∼10 ps). (3)LL'CT is initially formed as a vibrationally excited state. Its equilibration (cooling) takes place on a picosecond timescale and is accompanied by a competitive decay to the ground state. Equilibrated (3)LL'CT is populated with a quantum yield of less than 50%, depending on the metal: Pt > Pd > Ni. (3)LL'CT is long-lived for Pt and Pd (≫500 ps) and short-lived for Ni (∼15 ps). Some of the investigated complexes also exhibit spectral changes due to vibrational cooling of the singlet (2-3 ps, depending on the solvent). Rotational diffusion occurs with lifetimes in the 120-200 ps range. Changing the dithione (Bz2pipdt/(i)Pr2pipdt) as well as dithiolate/diselenolate (dmit/dsit) ligands has only small effects on the photobehavior. It is proposed that the investigated dithione-dithiolato complexes could act as photooxidants (*E(o) ≈ +1.2 V vs. NHE) utilizing their lowest excited singlet ((1)LL'CT), provided that the excited-state electron transfer is ultrafast, competitive with the picosecond decay. On the other hand, the efficiency of any triplet-based processes would be severely limited by the low quantum yield of the triplet population.
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http://dx.doi.org/10.1039/c4dt01955eDOI Listing
December 2014

Fluorescence quenching of (dimethylamino)naphthalene dyes Badan and Prodan by tryptophan in cytochromes P450 and micelles.

J Phys Chem B 2014 Aug 14;118(34):10085-91. Epub 2014 Aug 14.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic , Dolejškova 3, CZ-182 23 Prague, Czech Republic.

Fluorescence of 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700-800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 10(8) s(-1) (CYP102) and 3.7 × 10(8) s(-1) (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at -1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at -0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics.
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http://dx.doi.org/10.1021/jp504625dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4148165PMC
August 2014

Photophysics of singlet and triplet intraligand excited states in [ReCl(CO)3(1-(2-pyridyl)-imidazo[1,5-α]pyridine)] complexes.

J Am Chem Soc 2014 Apr 11;136(16):5963-73. Epub 2014 Apr 11.

School of Biological and Chemical Sciences, Queen Mary University of London , Mile End Road, London E1 4NS, United Kingdom.

Excited-state characters and dynamics of [ReCl(CO)3(3-R-1-(2-pyridyl)-imidazo[1,5-α]pyridine)] complexes (abbreviated ReGV-R, R = CH3, Ph, PhBu(t), PhCF3, PhNO2, PhNMe2) were investigated by pico- and nanosecond time-resolved infrared spectroscopy (TRIR) and excited-state DFT and TD-DFT calculations. Near UV excitation populates the lowest singlet state S1 that undergoes picosecond intersystem crossing (ISC) to the lowest triplet T1. Both states are initially formed hot and relax with ∼20 ps lifetime. TRIR together with quantum chemical calculations reveal that S1 is predominantly a ππ* state localized at the 1-(2-pyridyl)-imidazo[1,5-α]pyridine (= impy) ligand core, with impy → PhNO2 and PhNMe2 → impy intraligand charge-transfer contributions in the case of ReGV-PhNO2 and ReGV-PhNMe2, respectively. T1 is predominantly ππ*(impy) in all cases. It follows that excited singlet and corresponding triplet states have to some extent different characters and structures even if originating nominally from the same preponderant one-electron excitations. ISC occurs with a solvent-independent (CH2Cl2, MeCN) 20-30 ps lifetime, except for ReGV-PhNMe2 (10 ps in CH2Cl2, 100 ps in MeCN). ISC is 200-300 times slower than in analogous complexes with low-lying MLCT states. This difference is interpreted in terms of spin-orbit interaction and characters of orbitals involved in one-electron excitations that give rise to S1 and T1 states. ReGV-R present a unique case of octahedral heavy-metal complexes where the S1 lifetime is long enough to allow for separate spectroscopic characterization of singlet and triplet excited states. This study provides an insight into dynamics and intersystem crossing pathways of low-lying singlet and triplet excited states localized at bidentate ligands bound directly to a heavy metal atom. Rather long (1)IL lifetimes indicate the possibility of photonic applications of singlet excited states.
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http://dx.doi.org/10.1021/ja413098mDOI Listing
April 2014

Tryptophan-accelerated electron flow across a protein-protein interface.

J Am Chem Soc 2013 Oct 2;135(41):15515-25. Epub 2013 Oct 2.

Beckman Institute, California Institute of Technology , Pasadena, California 91125, United States.

We report a new metallolabeled blue copper protein, Re126W122Cu(I) Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: Re(I)(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt Cu(I) oxidation (<50 ns), followed by slow back ET to regenerate Cu(I) and ground-state Re(I) with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122Cu(I))2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←Cu(I), where // denotes a protein-protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the Cu(II) form shows two Re126W122Cu(II) molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122Cu(I))2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes.
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http://dx.doi.org/10.1021/ja406830dDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3855362PMC
October 2013

Re and Br X-ray absorption near-edge structure study of the ground and excited states of [ReBr(CO)3(bpy)] interpreted by DFT and TD-DFT calculations.

Inorg Chem 2013 May 30;52(10):5775-85. Epub 2013 Apr 30.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague, Czech Republic.

X-ray absorption spectra of fac-[ReBr(CO)3(bpy)] near the Re L3- and Br K-edges were measured in a steady-state mode as well as time-resolved at 630 ps after 355 nm laser pulse excitation. Relativistic spin-orbit time-dependent density functional theory (TD-DFT) calculations account well for the shape of the near-edge absorption (the ″white line″) of the ground-state Re spectrum, assigning the lowest-lying transitions as core-to-ligand metal-to-ligand charge transfer from Re 2p(3/2) into predominantly π*(bpy) molecular orbitals (MOs) containing small 5d contributions, followed in energy by transitions into π* Re(CO)3 and delocalized σ*/π* MOs. Transitions gain their intensities from Re 5d and 6s participation in the target orbitals. The 5d character is distributed over many unoccupied MOs; the 5d contribution to any single empty MO does not exceed 29%. The Br K-edge spectrum is dominated by the ionization edge and multiple scattering features, the pre-edge electronic transitions being very weak. Time-resolved spectra measured upon formation of the lowest electronic excited state show changes characteristic of simultaneous Re and Br electronic depopulation: shifts of the Re and Br edges and the Re white line to higher energies and emergence of new intense pre-edge features that are attributed by TD-DFT to transitions from Re 2p(3/2) and Br 1s orbitals into a vacancy in the HOMO-1 created by electronic excitation. Experimental spectra together with quantum chemical calculations provide a direct evidence for a ReBr(CO)3 → bpy delocalized charge transfer character of the lowest excited state. Steady-state as well as time-resolved Re L3 spectra of [ReCl(CO)3(bpy)] and [Re(Etpy)(CO)3(bpy)](+) are very similar to those of the Br complex, in agreement with similar (TD) DFT calculated transition energies as well as delocalized excited-state spin densities and charge changes upon excitation.
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http://dx.doi.org/10.1021/ic3025843DOI Listing
May 2013

Electron hopping through proteins.

Coord Chem Rev 2012 Nov 5;256(21-22):2478-2487. Epub 2012 Apr 5.

Beckman Institute, California Institute of Technology, Mail Code 139-74, Pasadena, CA 91125, USA.

Biological redox machines require efficient transfer of electrons and holes for function. Reactions involving multiple tunneling steps, termed "hopping," often promote charge separation within and between proteins that is essential for energy storage and conversion. Here we show how semiclassical electron transfer theory can be extended to include hopping reactions: graphical representations (called hopping maps) of the dependence of calculated two-step reaction rate constants on driving force are employed to account for flow in a rhenium-labeled azurin mutant as well as in two structurally characterized redox enzymes, DNA photolyase and MauG. Analysis of the 35 Å radical propagation in ribonucleotide reductases using hopping maps shows that all tyrosines and tryptophans on the radical pathway likely are involved in function. We suggest that hopping maps can facilitate the design and construction of artificial photosynthetic systems for the production of fuels and other chemicals.
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http://dx.doi.org/10.1016/j.ccr.2012.03.032DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3570191PMC
November 2012

Structural control of 1A2u-to-3A2u intersystem crossing in diplatinum(II,II) complexes.

J Am Chem Soc 2012 Aug 16;134(34):14201-7. Epub 2012 Aug 16.

Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA.

Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt(2)(μ-P(2)O(5)(BF(2))(2))(4)](4-), abbreviated Pt(pop-BF(2))), yields a radiative decay rate (k(r) = 1.7 × 10(8) s(-1)) an order of magnitude greater than that of the parent complex, Pt(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt(pop) [ISC activation energies: 2230 cm(-1) for Pt(pop-BF(2)); 1190 cm(-1) for Pt(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin-orbit coupling between the (1)A(2u) state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in E(a) is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt(2)(P-O-P)(4) core that would allow direct (1)A(2u)-(3)A(2u) spin-orbit coupling.
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http://dx.doi.org/10.1021/ja305666bDOI Listing
August 2012

Spin-orbit treatment of UV-vis absorption spectra and photophysics of rhenium(I) carbonyl-bipyridine complexes: MS-CASPT2 and TD-DFT analysis.

J Phys Chem A 2012 Nov 17;116(46):11319-29. Epub 2012 Aug 17.

J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-182 23 Prague, Czech Republic.

The lowest-lying spectral transitions in [ReX(CO)(3)(bpy)] (X = Cl, Br, I; bpy = 2,2'-bipyridine) complexes were calculated by means of spin-orbit time-dependent density functional theory (SO-TD-DFT) and spin-orbit multistate complete active space second-order perturbation theory (SO-MS-CASPT2). Computational results are compared with absorption spectra measured in different solvents and used to qualitatively explain the temperature dependence of the phosphorescence decay parameters that were measured for the whole series of complexes. Spin-orbit excited-state calculations interpret their electronic absorption spectra as arising from a bunch of spin mixed states with a singlet component of only 50-90% (depending on the halide), and attribute the phosphorescence decay to thermal population of spin-mixed states with a substantial singlet character.
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http://dx.doi.org/10.1021/jp305461zDOI Listing
November 2012
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