Initial Steps of the Photodissociation of the CO Ligated Heme Group

Time-Resolved Mechanistic Depiction of Photoinduced CO2 Reduction Catalysis on a Urea-Modified Iron Porphyrin

The development of functional artificial photosynthetic devices relies on the understanding of mechanistic aspects involved in specialized photocatalysts. Modified iron porphyrins have long been explored as efficient catalysts for the light-induced reduction of carbon dioxide (CO2) towards solar fuels. In spite of the advancements in homogeneous catalysis, the development of the next generation of catalysts requires a complete understanding of the fundamental photoinduced processes taking place prior to and after activation of the substrate by the catalyst. In this work, we employ a state-of-the-art nanosecond optical transient absorption spectroscopic setup with a double excitation capability to induce charge accumulation and trigger the reduction of CO2 to carbon monoxide (CO). Our biomimetic system is composed of a urea-modified iron(III) tetraphenylporphyrin (UrFeIII) catalyst, the prototypical [Ru(bpy)3]2+ (bpy=2,2'-bipyridine) used as a photosensitizer, and sodium ascorbate as an electron donor. Under inert atmosphere, we show that two electrons can be successively accumulated on the catalyst as the fates of the photogenerated UrFeII and UrFeI reduced species are tracked. In the presence of CO2, the catalytic cycle is kick-started providing further evidence on CO2 activation by the UrFe catalyst in its formal FeI oxidation state.

NAST: Nonadiabatic Statistical Theory Package for Predicting Kinetics of Spin-Dependent Processes

We present a nonadiabatic statistical theory (NAST) package for predicting kinetics of spin-dependent processes, such as intersystem crossings, spin-forbidden unimolecular reactions, and spin crossovers. The NAST package can calculate the probabilities and rates of transitions between the electronic states of different spin multiplicities. Both the microcanonical (energy-dependent) and canonical (temperature-dependent) rate constants can be obtained. Quantum effects, including tunneling, zero-point vibrational energy, and reaction path interference, can be accounted for. In the limit of an adiabatic unimolecular reaction proceeding on a single electronic state, NAST reduces to the traditional transition state theory. Because NAST requires molecular properties at only a few points on potential energy surfaces, it can be applied to large molecular systems, used with accurate high-level electronic structure methods, and employed to study slow nonadiabatic processes. The essential NAST input data include the nuclear Hessian at the reactant minimum, as well as the nuclear Hessians, energy gradients, and spin-orbit coupling at the minimum energy crossing point (MECP) between two states. The additional computational tools included in the NAST package can be used to extract the required input data from the output files of electronic structure packages, calculate the effective Hessian at the MECP, and fit the reaction coordinate for more advanced NAST calculations. We describe the theory, its implementation, and three examples of application to different molecular systems.

Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations

Ultrafast structural dynamics with different spatial and temporal scales were investigated during photodissociation of carbon monoxide (CO) from iron(II)-heme in bovine myoglobin during the first 3 ps following laser excitation. We used simultaneous X-ray transient absorption (XTA) spectroscopy and X-ray transient solution scattering (XSS) at an X-ray free electron laser source with a time resolution of 80 fs. Kinetic traces at different characteristic X-ray energies were collected to give a global picture of the multistep pathway in the photodissociation of CO from heme. In order to extract the reaction coordinates along different directions of the CO departure, XTA data were collected with parallel and perpendicular relative polarizations of the laser pump and X-ray probe pulse to isolate the contributions of electronic spin state transition, bond breaking, and heme macrocycle nuclear relaxation. The time evolution of the iron K-edge X-ray absorption near edge structure (XANES) features along the two major photochemical reaction coordinates, i.e., the iron(II)-CO bond elongation and the heme macrocycle doming relaxation were modeled by time-dependent density functional theory calculations. Combined results from the experiments and computations reveal insight into interplays between the nuclear and electronic structural dynamics along the CO photodissociation trajectory. Time-resolved small-angle X-ray scattering data during the same process are also simultaneously collected, which show that the local CO dissociation causes a protein quake propagating on different spatial and temporal scales. These studies are important for understanding gas transport and protein deligation processes and shed light on the interplay of active site conformational changes and large-scale protein reorganization.

Protein Matrix Control of Reaction Center Excitation in Photosystem II

Photosystem II (PSII) is a multisubunit pigment-protein complex that uses light-induced charge separation to power oxygenic photosynthesis. Its reaction center chromophores, where the charge transfer cascade is initiated, are arranged symmetrically along the D1 and D2 core polypeptides and comprise four chlorophyll (PD1, PD2, ChlD1, ChlD2) and two pheophytin molecules (PheoD1 and PheoD2). Evolution favored productive electron transfer only via the D1 branch, with the precise nature of primary excitation and the factors that control asymmetric charge transfer remaining under investigation. Here we present a detailed atomistic description for both. We combine large-scale simulations of membrane-embedded PSII with high-level quantum-mechanics/molecular-mechanics (QM/MM) calculations of individual and coupled reaction center chromophores to describe reaction center excited states. We employ both range-separated time-dependent density functional theory and the recently developed domain based local pair natural orbital (DLPNO) implementation of the similarity transformed equation of motion coupled cluster theory with single and double excitations (STEOM-CCSD), the first coupled cluster QM/MM calculations of the reaction center. We find that the protein matrix is exclusively responsible for both transverse (chlorophylls versus pheophytins) and lateral (D1 versus D2 branch) excitation asymmetry, making ChlD1 the chromophore with the lowest site energy. Multipigment calculations show that the protein matrix renders the ChlD1 → PheoD1 charge-transfer the lowest energy excitation globally within the reaction center, lower than any pigment-centered local excitation. Remarkably, no low-energy charge transfer states are located within the "special pair" PD1-PD2, which is therefore excluded as the site of initial charge separation in PSII. Finally, molecular dynamics simulations suggest that modulation of the electrostatic environment due to protein conformational flexibility enables direct excitation of low-lying charge transfer states by far-red light.

ONETEP + TOSCAM: Uniting Dynamical Mean Field Theory and Linear-Scaling Density Functional Theory

We introduce the unification of dynamical mean field theory (DMFT) and linear-scaling density functional theory (DFT), as recently implemented in ONETEP, a linear-scaling DFT package, and TOSCAM, a DMFT toolbox. This code can account for strongly correlated electronic behavior while simultaneously including the effects of the environment, making it ideally suited for studying complex and heterogeneous systems that contain transition metals and lanthanides, such as metalloproteins. We systematically introduce the necessary formalism, which must account for the nonorthogonal basis set used by ONETEP. In order to demonstrate the capabilities of this code, we apply it to carbon monoxide ligated iron porphyrin and explore the distinctly quantum-mechanical character of the iron 3d electrons during the process of photodissociation.

Ultrafast Structural Changes Decomposed from Serial Crystallographic Data

Direct visualization of electronic and molecular events during biochemical reactions is essential to mechanistic insights. This Letter presents an in-depth analysis of the serial crystallographic data sets collected by Barends and Schlichting et al. ( Science 2015 , 350 , 445 ) that probe the ligand photodissociation in carbonmonoxy myoglobin. This analysis reveals electron density changes caused by the formation of high-spin 3d atomic orbitals of the heme iron upon photolysis and their dynamic behaviors within the first few picoseconds. The heme iron is found popping out of and recoiling back into the heme plane in succession. These findings provide long-awaited visual validations for previous works using ultrafast spectroscopy and molecular dynamics simulations. Electron density variations are also found largely in the solvent during the first period of a low-frequency oscillation. This work demonstrates the importance of the analytical methods in detecting and isolating weak, transient signals of electronic changes arising from chemical reactions.

Ultrafast carbon monoxide photolysis and heme spin-crossover in myoglobin via nonadiabatic quantum dynamics

Light absorption of myoglobin triggers diatomic ligand photolysis and a spin crossover transition of iron(II) that initiate protein conformational change. The photolysis and spin crossover reactions happen concurrently on a femtosecond timescale. The microscopic origin of these reactions remains controversial. Here, we apply quantum wavepacket dynamics to elucidate the ultrafast photochemical mechanism for a heme-carbon monoxide (heme-CO) complex. We observe coherent oscillations of the Fe-CO bond distance with a period of 42 fs and an amplitude of ∼1 Å. These nuclear motions induce pronounced geometric reorganization, which makes the CO dissociation irreversible. The reaction is initially dominated by symmetry breaking vibrations inducing an electron transfer from porphyrin to iron. Subsequently, the wavepacket relaxes to the triplet manifold in ∼75 fs and to the quintet manifold in ∼430 fs. Our results highlight the central role of nuclear vibrations at the origin of the ultrafast photodynamics of organometallic complexes.

The surprisingly high ligation energy of CO to ruthenium porphyrins

A combined theoretical and experimental approach has been used to investigate the binding energy of a ruthenium metalloporphyrin ligated with CO, ruthenium tetraphenylporphyrin [RuII TPP], in the RuII oxidation degree. Measurements performed with VUV ionization using the DESIRS beamline at Synchrotron SOLEIL led to adiabatic ionization energies of [RuII TPP] and its complex with CO, [RuII TPP-CO], of 6.48 ± 0.03 eV and 6.60 ± 0.03 eV, respectively, while the ion dissociation threshold of [RuII TPP-CO]+ is measured to be 8.36 ± 0.03 eV using the ground-state neutral complex. These experimental data are used to derive the binding energies of the CO ligand in neutral and cationic complexes (1.88 ± 0.06 eV and 1.76 ± 0.06 eV, respectively) using a Born-Haber cycle. Density functional theory calculations, in very satisfactory agreement with the experimental results, help to get insights into the metal-ligand bond. Notably, the high ligation energies can be rationalized in terms of the ruthenium orbital structure, which is singular compared to that of the iron atom. Thus, beyond indications of a strengthening of the Ru-CO bond due to the decrease in the CO vibrational frequency in the complex as compared to the Fe-CO bond, high-level calculations are essential to accurately describe the metal ligand (CO) bond and show that the Ru-CO bond energy is strongly affected by the splitting of triplet and singlet spin states in uncomplexed [Ru TPP].

Photophysical properties and the NO photorelease mechanism of a ruthenium nitrosyl model complex investigated using the CASSCF-in-DFT embedding approach

A complete state-averaged active space self-consistent field (SA-CASSCF) calculation by means of the SA-CASSCF(18,14)-in-BP86 Miller-Manby embedding approach was performed to explore the ground and excited electronic states of the trans-[RuCl(NO)(NH₃)₄]²⁺ complex. Insights into the NO photodissociation mechanism and Ru-NO bonding properties are provided. In addition, spin-orbit (SO) interactions were taken into account to describe and characterize the spin-forbidden transitions observed at the low-energy regions of the trans-[RuCl(NO)(NH₃)₄]²⁺ UV-Vis spectrum. The SA-CASSCF(18,14)-in-BP86 electronic spectrum is in great agreement with the experimental data of Schreiner [Schreiner et al., Inorg. Chem., 1972, 11, 880].

An investigation into the photochemistry of, and the electrochemically induced CO-loss from, [(CO)5MC(OMe)Me](M = Cr or W) using low-temperature matrix isolation, picosecond infrared spectroscopy, cyclic voltammetry, and time-dependent density functional theory

The photophysics and photochemistry of [(CO)5MC(OMe)Me] (M = Cr or W) were investigated using picosecond time-resolved infrared spectroscopy (M = Cr or W), low-temperature matrix isolation techniques (M = Cr), and time-dependent density functional calculations (M = Cr or W). These studies provide unambiguous evidence for the photochemical formation of a long-lived, 18-electron metallaketene species capable of acting as a synthetically useful intermediate. For the Cr complex, an intermediate metallacyclopropanone singlet excited state was detected on the reaction path to the metallaketene species. This metallacyclopropanone excited state species has a lifetime of less than 100 ps and a characteristic bridging carbonyl band at 1770 cm(-1). The tungsten ketene species was also detected but in contrast to the chromium system, this forms directly from a low-lying triplet excited state. The electrochemical release of CO showed a greater efficiency for the chromium complex when compared to the tungsten.

Direct observation of ultrafast collective motions in CO myoglobin upon ligand dissociation

The hemoprotein myoglobin is a model system for the study of protein dynamics. We used time-resolved serial femtosecond crystallography at an x-ray free-electron laser to resolve the ultrafast structural changes in the carbonmonoxy myoglobin complex upon photolysis of the Fe-CO bond. Structural changes appear throughout the protein within 500 femtoseconds, with the C, F, and H helices moving away from the heme cofactor and the E and A helices moving toward it. These collective movements are predicted by hybrid quantum mechanics/molecular mechanics simulations. Together with the observed oscillations of residues contacting the heme, our calculations support the prediction that an immediate collective response of the protein occurs upon ligand dissociation, as a result of heme vibrational modes coupling to global modes of the protein.

Excited state evolution towards ligand loss and ligand chelation at group 6 metal carbonyl centres

The photochemistry and photophysics of three model "half-sandwich" complexes (η(6)-benzophenone)Cr(CO)3, (η(6)-styrene)Cr(CO)3, and (η(6)-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η(6)-benzophenone)Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η(6)-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η(6)-styrene)Cr(CO)3, or (η(6)-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η(6)-allylbenzene)Cr(CO)3 and 0.43 for (η(6)-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η(6)-allylbenzene)Cr(CO)2 formed following irradiation of (η(6)-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η(6),η(2)-allylbenzene)Cr(CO)2.

Laser-driven localization of collective CO vibrations in metal-carbonyl complexes

Using the example of a cobalt dicarbonyl complex it is shown that two perpendicular linearly polarized IR laser pulses can be used to trigger an excitation of the delocalized CO stretching modes, which corresponds to an alternating localization of the vibration within one CO bond. The switching time for localization in either of the two bonds is determined by the energy gap between the symmetric and asymmetric fundamental transition frequencies. The phase of the oscillation between the two local bond excitations can be tuned by the relative phase of the two pulses. The extend of control of bond localization is limited by the anharmonicity of the potential energy surfaces leading to wave packet dispersion. This prevents such a simple pulse scheme from being used for laser-driven bond breaking in the considered example.

Probing the electronic and geometric structure of ferric and ferrous myoglobins in physiological solutions by Fe K-edge absorption spectroscopy

We present an iron K-edge X-ray absorption study of carboxymyoglobin (MbCO), nitrosylmyoglobin (MbNO), oxymyoglobin (MbO2), cyanomyoglobin (MbCN), aquomet myoglobin (metMb) and unligated myoglobin (deoxyMb) in physiological media. The analysis of the XANES region is performed using the full-multiple scattering formalism, implemented within the MXAN package. This reveals trends within the heme structure, absent from previous crystallographic and X-ray absorption analysis. In particular, the iron-nitrogen bond lengths in the porphyrin ring converge to a common value of about 2 Å, except for deoxyMb whose bigger value is due to the doming of the heme. The trends of the Fe-Nε (His93) bond length is found to be consistent with the effect of ligand binding to the iron, with the exception of MbNO, which is explained in terms of the repulsive trans effect. We derive a high resolution description of the relative geometry of the ligands with respect to the heme and quantify the magnitude of the heme doming in the deoxyMb form. Finally, time-dependent density functional theory is used to simulate the pre-edge spectra and is found to be in good agreement with the experiment. The XAS spectra typically exhibit one pre-edge feature which arises from transitions into the unoccupied dσ and dπ - πligand* orbitals. 1s → dπ transitions contribute weakly for MbO2, metMb and deoxyMb. However, despite this strong Fe d contribution these transitions are found to be dominated by the dipole (1s → 4p) moment due to the low symmetry of the heme environment.

Quantum mechanical methods for the investigation of metalloproteins and related bioinorganic compounds

It is well known that transition metal ions are often bound to proteins, conveying very specific functional properties. In fact, metalloproteins play crucial biological roles in the transport and activation of small molecules such as H2, O2, and N2, as well as in several other biochemical processes. However, even if the presence of transition metals in the active site of proteins allows a very rich biochemistry, the experimental disclosure of structure-activity relationships in metalloproteins is generally difficult exactly because of the presence of transition metals, which are intrinsically characterized by a very versatile and often elusive chemistry. For this reason, computational methods are becoming very popular tools in the characterization of metalloproteins. In particular, since computing power is becoming less and less expensive, due to the continuous technological development of CPUs, the computational tools suited to investigate metalloproteins are becoming more accessible and therefore more commonly used also in molecular biology and biochemistry laboratories. Here, we present the main procedures and computational methods based on quantum mechanics, which are commonly used to study the structural, electronic, and reactivity properties of metalloproteins and related bioinspired compounds, with a specific focus on the practical and technical aspects that must be generally tackled to properly study such biomolecular systems.

The photochemistry of transition metal complexes using density functional theory

The use of density functional theory (DFT) and time-dependent DFT (TD-DFT) to study the photochemistry of metal complexes is becoming increasingly important among chemists. Computational methods provide unique information on the electronic nature of excited states and their atomic structure, integrating spectroscopy observations on transient species and excited-state dynamics. In this contribution, we present an overview on photochemically active transition metal complexes investigated by DFT. In particular, we discuss a representative range of systems studied up to now, which include CO- and NO-releasing inorganic and organometallic complexes, haem and haem-like complexes dissociating small diatomic molecules, photoactive anti-cancer Pt and Ru complexes, Ru polypyridyls and diphosphino Pt derivatives.

Following ligand migration pathways from picoseconds to milliseconds in type II truncated hemoglobin from Thermobifida fusca

CO recombination kinetics has been investigated in the type II truncated hemoglobin from Thermobifida fusca (Tf-trHb) over more than 10 time decades (from 1 ps to ∼100 ms) by combining femtosecond transient absorption, nanosecond laser flash photolysis and optoacoustic spectroscopy. Photolysis is followed by a rapid geminate recombination with a time constant of ∼2 ns representing almost 60% of the overall reaction. An additional, small amplitude geminate recombination was identified at ∼100 ns. Finally, CO pressure dependent measurements brought out the presence of two transient species in the second order rebinding phase, with time constants ranging from ∼3 to ∼100 ms. The available experimental evidence suggests that the two transients are due to the presence of two conformations which do not interconvert within the time frame of the experiment. Computational studies revealed that the plasticity of protein structure is able to define a branched pathway connecting the ligand binding site and the solvent. This allowed to build a kinetic model capable of describing the complete time course of the CO rebinding kinetics to Tf-trHb.

Potential energy mapping of the excited-states of (η6-arene)Cr(CO)3 complexes: the evolution toward CO-loss or haptotropic shift processes

The potential energy profiles of the optically accessible excited states of two model (η(6)-arene)Cr(CO)(3) systems were explored using Time-Dependent Density Functional Theory. Two photochemical reactions were investigated, CO-loss and the haptotropic or ring-slip of the arene ligand. In both cases the photochemical reaction requires the surmounting of a small thermal barrier in the lowest energy excited state. In the case of (η(6)-benzene)Cr(CO)(3) only one excited state is populated following 400 nm excitation and this leads to the release of CO. The calculated energy barrier to this process is 13 kJ mol(-1). In the case of (η(6)-thiophenol)Cr(CO)(3) two excited states are accessible one leading to CO-loss while the other results in the ring-slip process. The calculated barrier to the ring-slip process is 11 kJ mol(-1). The calculations are consistent with the results of picosecond time-resolved infrared studies.

Electronic structure, spin-states, and spin-crossover reaction of heme-related Fe-porphyrins: a theoretical perspective

The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

Equilibrium, photophysical, photochemical and quantum chemical examination of anionic mercury(I) porphyrins

Hg22+ ion and 5,10,15,20-tetrakis(parasulphonato-phenyl)porphyrin anion can form 2:1 (2 clusters:1 porphyrin) and 2:2 complexes, while the formation of the 1:1 species is not observable: it is only an intermediate, similarly to the cases of other large metal ions of small charge-density. The differences between mercury(I) and mercury(II) porphyrins in the composition of monoporphyrins (2:1 vs. 1:1), in the stability and the Soret absorption based on the arrangement of 2:2 complexes (asymmetric vs. probably symmetric sandwich-structure), in the kinetic behavior (molecularities and the special dimerization of HgIIP4-), in the product of the photoinduced dissociations of 2:2 bisporphyrins (free-base ligand vs. 1:1 complex) can prove that no mercury(II) porphyrins can form due to the possible disproportion of dimercury(I) ions. However, the similarities in the absorption, photophysical and photochemical features (also to other out-of-plane metalloporphyrins) suggest that the out-of-plane position of metal center and the distorted structure of complexes may be responsible for these common properties, the so-called sitting-atop characteristics. Moreover, the calculated structural data of the theoretically studied 1:1 mercury(I) porphyrin are very similar to those of Hg II P as a consequence of the charge separation in the cluster based on the strength of metal-nitrogen bonds. In the case of the 2:2 species, neither the increased distance (because of the Hg-Hg bond), nor the absence of 45° rotation of the two ligands can significantly modify the π-π interaction because its both measured and calculated absorption spectra are similar to those of Hg II2 P 2.

Ligand-selective photodissociation from [Ru(bpy)(4AP)4]2+: a spectroscopic and computational study

The new complex [Ru(bpy)(4AP)(4)](2+) (1), where bpy = 2,2'-bipyridine and 4AP = 4-aminopyridine, undergoes selective photodissociation of two 4APs upon light excitation of the metal-ligand-to-ligand charge-transfer (MLLCT) band at 510 nm. The photoproducts of the reaction are mer-[Ru(bpy)(4AP)(3)(H(2)O)](2+) (2a) and trans-(4AP)[Ru(bpy)(4AP)(2)(H(2)O)(2)](2+) (3a). Photodissociation occurs in two consecutive steps with quantum yields of phi(1) = (6.1 +/- 1.0) x 10(-3) and phi(2) = (1.7 +/- 0.1) x 10(-4), respectively. Complex 1 was characterized by combined spectroscopic and theoretical techniques. EXAFS experiments at the Ru K-edge (22 117 eV) of 1 in an aqueous solution gave a Ru-N distance of 2.09 +/- 0.01 A. Photoproducts were characterized by electronic spectroscopy, 1D and 2D NMR, and mass spectrometry. Singlet and triplet excited states of 1 were studied by density functional theory (DFT) and time-dependent DFT for characterizing the optical properties of the complex. In the singlet state, (1)MC (metal-centered) dissociative states lie 0.65 eV above the main (1)MLLCT transition in the visible region of the UV-vis absorption spectrum. In the triplet state, the energy difference between these states is not reduced. However, potential energy curves of singlet and triplet excited states of 1 along the Ru-N(axial 4AP) and Ru-N(equatorial 4AP) stretching coordinates show that the release of the first 4AP may occur from the triplet state by mixing of (3)MLLCT and (3)MC dissociative states. This mixing is favored when the Ru-N(equatorial 4AP) bond is elongated, explaining the formation of the photoproduct 2a.

Synthesis, characterisation and photochemistry of Pt(IV) pyridyl azido acetato complexes

Pt(II) azido complexes [Pt(bpy)(N(3))(2)] (1), [Pt(phen)(N(3))(2)] (2) and trans-[Pt(N(3))(2)(py)(2)] (3) incorporating the bidentate diimine ligands 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) or the monodentate pyridine (py) respectively, have been synthesised from their chlorido precursors and characterised by X-ray crystallography; complex 3 shows significant deviation from square-planar geometry (N(3)-Pt-N(3) angle 146.7 degrees ) as a result of steric congestion at the Pt centre. The novel Pt(IV) complexes trans, cis-[Pt(bpy)(OAc)(2)(N(3))(2)] (), trans, cis-[Pt(phen)(OAc)(2)(N(3))(2)] (), trans, trans, trans-[Pt(OAc)(2)(N(3))(2)(py)(2)] (), were obtained from via oxidation with H(2)O(2) in acetic acid followed by reaction of the intermediate with acetic anhydride. Complexes 4-6 exhibit interesting structural and photochemical properties that were studied by X-ray, NMR and UV-vis spectroscopy and TD-DFT (time-dependent density functional theory). These Pt(IV) complexes exhibit greater absorption at longer wavelengths (epsilon = 9756 M(-1) cm(-1) at 315 nm for 4 ; epsilon = 796 M(-1) cm(-1) at 352 nm for 5 ; epsilon = 16900 M(-1) cm(-1) at 307 nm for 6 , in aqueous solution) than previously reported Pt(IV) azide complexes, due to the presence of aromatic amines, and 4-6 undergo photoactivation with both UVA (365 nm) and visible green light (514 nm). The UV-vis spectra of complexes 4-6 were calculated using TD-DFT; the nature of the transitions contributing to the UV-vis bands provide insight into the mechanism of production of the observed photoproducts. The UV-vis spectra of 1-3 were also simulated by computational methods and comparison between Pt(II) and Pt(IV) electronic and structural properties allowed further elucidation of the photochemistry of 4-6.

Equilibrium, photophysical, photochemical, and quantum chemical examination of anionic mercury(II) mono- and bisporphyrins

Mercury(II) ion and 5,10,15,20-tetrakis(parasulfonato-phenyl)porphyrin anion can form 1:1, 2:2, and 3:2 (metal ion/porphyrin) out-of-plane (OOP) complexes, from which Hg2P2(8-) has not been identified until now. Identification of this species significantly promoted the confirmation of the composition and the precise elucidation of the equilibrium of Hg3P2(6-). Since the formation of each complex is too fast, their kinetic behavior was studied from the side of dissociation. The rate-determining step in dissociations, as well as in the formation of the 2:2 complex, that is, the dimerization of 1:1 complex, proved to be virtually first-order under these conditions, while the consecutive formations of HgP(4-) and Hg3P2(6-) are second-order reactions. The equilibria can be spectrophotometrically investigated because the Soret- as well as the Q-absorption bands of the free-base ligand are more and more red-shifted in the series of 1:1, 2:2, and 3:2 complexes, and the split of Q-bands disappears as the singlet-1 excited states become degenerate; in the case of bisporphyrins, the bands broaden, especially in the longer-wavelength region of the spectra. The quantum yield and the lifetime of S1-fluorescence from the macrocycle is decreased by the insertion of a mercury(II) ion due to distortion, and in bisporphyrins the luminescence totally ceases because their more complicated structure promotes other ways of energy dissipation. The lifetime of the triplet excited-state is also reduced by metalation. The transient absorption measured upon excitation of Hg3P2(6-) probably originates from Hg2P2(8-) formed by efficient photodissocation during the laser pulse. This photoinduced dissociation is characteristic to out-of-plane complexes, but in metallo-monoporphyrins it needs the energetically higher Soret-excitation; in bisporphyrins, it can take place during irradiation at the longer Q-wavelengths. Investigation of the intramolecular photoredox reactions has proved that for the increased efficiency of the indirect photoinduced LMCT, not the redox potential, but the position of the metal center is responsible. The two orders of magnitude higher photoredux quantum yield for the 3:2 complex, compared to that of the 2:2 species, can be explained by the repulsive effect of the inner mercury(II) ion pushing the other two farther out of the ligand cavity. In bisporphyrins the second excited states are photochemically more reactive than the first ones, while most of the photochemical processes of HgP(4-) originate from the first excited state. According to our quantum chemical calculations, the mercury(II) ion causes the expansion of the porphyrin-cavity; therefore its out-of-plane position is smaller than the value expected based on its ionic radius. In the hitherto unknown 2:2 dimer two 1:1 saucer-shaped monomers are kept together by secondary forces, mostly by pi-pi interaction, but their relative arrangement was not unequivocally determined by the two DFT functionals used. The arrangements with a symmetry axis or plane perpendicular to both rings are not favored; instead, the two monomers are shifted along the porphyrin planes, either in a Hg-P-Hg-P or a Hg-P-P-Hg order. Our time-dependent density functional theory (TD-DFT) calculations indicate that the electronic spectra are not very sensitive to the structure of the dimer, even though the environment of the porphyrin rings is quite different if one of the metal ions is between or outside of both macrocycles. The calculated spectral shifts agree only partially with the experimental data. The TD-DFT calculations suggest that the chromophores are not fully independent in the bisporphyrins and that the observed spectral shift cannot be uniquely assigned to the geometrical distortion of the porphyrin macrocyle.

Air-stable, heme-like water-soluble iron(II) porphyrin: in situ preparation and characterization

Preparation of the water-soluble, kinetically labile, high-spin iron(II) tetrakis(4-sulfonatophenyl)porphyrin, Fe(II)TPPS(4-), has been realized in neutral or weakly acidic solutions containing acetate buffer. The buffer played a double role in these systems: it was used for both adjusting pH and, via formation of an acetato complex, trapping trace amounts of iron(III) ions, which would convert the iron(II) porphyrins to the corresponding iron(III) species. Fe(II)TPPS(4-) proved to be stable in these solutions even after saturation with air or oxygen. In the absence of acetate ions, however, iron(II) ions play a catalytic role in the formation of iron(III) porphyrins. While the kinetically inert iron(III) porphyrin, Fe(III)TPPS(3-), is a regular one with no emission and photoredox properties, the corresponding iron(II) porphyrin displays photoinduced features which are typical of sitting-atop complexes (redshifted Soret absorption and blueshifted emission and Q absorption bands, photoinduced porphyrin ligand-to-metal charge transfer, LMCT, reaction). In the photolysis of Fe(II)TPPS(4-) the LMCT process is followed by detachment of the reduced metal center and an irreversible ring-opening of the porphyrin ligand, resulting in the degradation of the complex. Possible oxygen-binding ability of Fe(II)TPPS(4-) (as a heme model) has been studied as well. Density functional theory calculations revealed that in solutions with high acetate concentration there is very little chance for iron(II) porpyrin to bind and release O(2), deviating from heme in a hydrophobic microenvironment in hemoglobin. In the presence of an iron(III)-trapping additive that is much less strongly coordinated to the iron(II) center than the acetate ion, Fe(II)TPPS(4-) may function as a heme model.

Excitation spectra of dibenzoborole containing pi-electron systems: controlling the electronic spectra by changing the p(pi)-pi* conjugation

We report time-dependent density functional theory calculations of the vertical excitation energies for the singlet states of three-coordinate 5H-dibenzoborole (DBB) derivatives and four-coordinate 5-fluoro-5H-dibenzoborole ion (FDBB) derivatives. These molecules show remarkable hypsochromic (blue) shifts in their fluorescence spectra and bathochromic (red) shifts in their absorption spectra when the bridging boron atoms change their coordination number from three to four. We constructed a series of derivatives of DBB and FDBB and studied how the energies of the electronic excitations change. The states with prominent oscillator strength in all of the DBB and FDBB derivatives show similar shifts of their excitation energies upon coordination. The three-coordinate DBB derivative 5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bis[p-(N,N-diphenylamino)phenyl]-5H-dibenzo[d,b]borole has an intense absorption at 3.25 eV, which shifts in the four-coordinate FDBB derivative 5-fluro-5-(2,4,6-triisopropylphenyl)-2,8-dimethoxy-3,7-bis[p-(N,N-dip henylamino)phenyl]-5H-dibenzo[d,b]borole ion to 3.17 eV. The experimental absorption peaks are 3.43 and 3.31 eV, respectively. In addition, we investigated and analyzed the nature of these electronic excitations using attachment/detachment density plots, with which we characterized the changes in electron density that arose from the excitations.

A computational study of oxidation of ruthenium porphyrins via ORuIV and ORuVIO species

An unrestricted density functional theory (UDFT) was applied to study the oxidation of ruthenium porphyrins, [RuP], via an interaction with molecular oxygen. The important role of dimeric [RuP] complexes, i.e. [RuP]-O2-[RuP], in the oxidation mechanism and particular in the cleavage of O-O bond of molecular oxygen has been studied. Geometries and relative Gibbs free energies of the intermediate Ru-complexes, i.e. dimeric oxo-Ru-porphyrins and O2Ru(II)-(or O2- Ru(III))-, ORu(IV)- and ORu(VI)O-porphyrins, were evaluated along the proposed reaction pathway. The detailed thermodynamic data of the oxidation reaction [Ru(II)P] --> O[Ru(IV)P] --> O[Ru(VI)P]O and important aspects of the vibrational spectra of an oxo-[RuP] has been presented.

Nonlinear optical studies of heme protein dynamics: Implications for proteins as hybrid states of matter

Protein structure is fundamentally related to function. However, static structures alone are insufficient to understand how a protein works. Dynamics play an equally important role. Given that proteins are highly associated aperiodic systems, it may be expected that protein dynamics would follow glass-like dynamics. However, protein functions occur on time scales orders of magnitude faster than the time scales typically associated with glassy systems. It is becoming clear that the reaction forces driving functions do not sample entirely the large number of configurations available to a protein but are highly directed along an optimized pathway. Could there be any correlation between specific topological features in protein structures and dynamics that leads to strongly correlated atomic displacements in the dynamical response to a perturbation? This review will try to provide an answer by focusing upon recent nonlinear optical studies with the aim of directly observing functionally important protein motions over the entire dynamic range of the protein response function. The specific system chosen is photoinduced dynamics of ligand dissociation at the active site in heme proteins, with myoglobin serving as the simplest model system. The energetics and nuclear motions from the very earliest events involved in bond breaking on the femtosecond time scale all the way out to ligand escape and bimolecular rebinding on the microsecond and millisecond time scale have been mapped out. The picture that is emerging is that the system consists of strongly coupled motions from the very instant the bond breaks at the active site that cascade into low frequency collective modes specific to the protein structure. It is this coupling that imparts the ability of a protein to function on time scales more commensurate with liquids while simultaneously conserving structural integrity akin to solids.