Chantal Daniel

Electronic spectroscopy and photoreactivity in transition metal complexes

The recent developments in quantum chemistry providing the theoretical tools to determine the properties of transition metal complexes in their electronic excited states are presented. The contrast between the impressive fast evolution of the computational strategies adapted to the treatment of ground state molecular properties and the gradual improvement of the methods that are more specifically turned towards the study of electronic excited states is discussed. Recent applications in transition metal coordination chemistry are selected to outline the degree of methodological maturity in electronic spectroscopy and photo-induced reactivity, illustrating the necessity for a strong interplay between theory and experiment.

Analysis and control of laser induced fragmentation processes in CpMn(CO)3

Abstract In this paper we present experimental and theoretical findings about the dynamics of ultrafast fragmentation processes which occur during resonant multiphoton ionization of CpMn(CO) 3 with femtosecond laser pulses. Employing a two-color pump and probe scheme, it was possible to retrieve lifetimes of the electronically excited parent molecule and its first fragment CpMn(CO) 2 . The observed time of 66 fs for the loss of the first CO-ligand is in good agreement with the results of one-dimensional quantum dynamical model simulations based on three relevant adiabatic ab initio potentials and the related components of the transition dipole matrix elements, in C s symmetry. Subsequently, smaller fragments appear somewhat later during approximately 100 fs. Based on these findings we performed feedback control experiments on the system in order to optimize individual fragmentation/ionization paths. With the routine a considerable increase of – for example – the CpMn(CO) + /CpMn(CO) + 3 intensity ratio was achieved. The obtained optimized laser pulses correlate well with the fast dynamics of the photoinduced preparation of CpMn(CO) + 3 versus CpMn(CO) + product ions, respectively.

A theoretical study of Ru(II) polypyridyl DNA intercalators structure and electronic absorption spectroscopy of [Ru(phen)2(dppz)]2+ and [Ru(tap)2(dppz)]2+ complexes intercalated in guanine-cytosine base pairs.

The structural and spectroscopic properties of [Ru(phen)(2)(dppz)](2+) and [Ru(tap)(2)(dppz)](2+) (phen=1,10-phenanthroline; tap=1,4,5,8-tetraazaphenanthrene; dppz=dipyridophenazine ) have been investigated by means of density functional theory (DFT), time-dependent DFT (TD-DFT) within the polarized continuum model (IEF-PCM) and quantum mechanics/molecular mechanics (QM/MM) calculations. The model of the Delta and Lambda enantiomers of Ru(II) intercalated in DNA in the minor and major grooves is limited to the metal complexes intercalated in two guanine-cytosine base pairs. The main experimental spectral features of these complexes reported in DNA or synthetic polynucleotides are better reproduced by the theoretical absorption spectra of the Delta enantiomers regardless of intercalation mode (major or minor groove). This is especially true for [Ru(phen)(2)(dppz)](2+). The visible absorption of [Ru(tap)(2)(dppz)](2+) is governed by the MLCT(tap) transitions regardless of the environment (water, acetonitrile or bases pair), the visible absorption of [Ru(phen)(2)(dppz)](2+) is characterized by transitions to metal-to-ligand-charge-transfer MLCT(dppz) in water and acetonitrile and to MLCT(phen) when intercalated in DNA. The response of the IL(dppz) state to the environment is very sensitive. In vacuum, water and acetonitrile these transitions are characterized by significant oscillator strengths and their positions depend significantly on the medium with blue shifts of about 80 nm when going from vacuum to solvent. When the complex is intercalated in the guanine-cytosine base pairs the (1)IL(dppz) transition contributes mainly to the band at 370 nm observed in the spectrum of [Ru(phen)(2)(dppz)](2+) and to the band at 362 nm observed in the spectrum of [Ru(tap)(2)(dppz)](2+).

Spin-orbit induced radiationless transitions in organometallics: Quantum simulation of the intersystem crossing processes in the photodissociation of HCo(CO)4

A theoretical description of the “fast” (<50 ps) intersystem crossing (ISC) processes occurring during the photodissociation of HCo(CO)4 is presented. The radiationless transitions are simulated by wave packet propagations on spin-orbit coupled two-dimensional potential energy surfaces (CASSCF/CCI) calculated along two reaction coordinates (qa=[Co–H] and qb=[Co–COax]). A mechanism of deactivation of the singlet excited state of HCo(CO)4 initially populated on UV excitation has been proposed. This mechanism differs slightly from the one deduced from a one-dimensional simulation performed separately, either along the Co–H bond or along the Co–COax bond: (i) in a very short time scale (<20 fs) 35% of the system dissociates toward the primary products H+Co(CO)4 (1E), whereas the 1E→3A1 and 1E→3E intersystem crossings occur within 50 ps; (ii) as soon as the lowest triplet states are populated, the system dissociates either to H+Co(CO)4 (3A1) or to H+Co(CO)4 (3E) on the corresponding potential energy surfaces; ...

Mechanism of visible-light photoisomerization of a rhenium(I) carbonyl-diimine complex

The mechanism of photoisomerization of a Re(I) carbonyl-diimine complex under visible-light irradiation is deciphered by means of ab initio calculations. By highlighting the key role of triplet states as well as spin-orbit and vibronic couplings, we provide a clear picture of this complicated multi-step process.

Spin–orbit induced radiationless transitions in organometallics: Quantum simulation of the 1E→3A1 intersystem crossing process in HCo(CO)4

A theoretical description of the ‘‘fast’’ (<50 ps) intersystem crossing processes occurring at critical geometries during the photodissociation of HCo(CO)4 is presented. The radiationless transitions are simulated by wave packet propagations along one‐dimensional reaction coordinate on the spin–orbit coupled potential energy surfaces. The propagation are performed separately, either along the Co–H bond or along the Co–COax bond. This original approach has enabled us to understand the mechanism of desactivation of the initially populated singlet excited state in this molecule which should be considered as a model for other organometallics. We propose the following mechanism: (i) in a very short time scale (<20 fs) 40% of the system dissociates towards the primary products H+Co(CO)4 (1E), whereas the 1E→3A1 intersystem crossing along the Co–H bond elongation occurs within 50 ps; (ii) the dissociation of an axial carbonyl ligand occurs in a larger time scale (200 fs) and only 2% of the system dissociates alo...

Spin–orbit coupled excited states in transition metal complexes: A configuration interaction treatment of HCo(CO)4

A direct spin–orbit coupled configuration interaction method is presented. The basis functions are Slater determinants expressed as combinations of alpha and beta strings. The one and two electron operators are expressed in second quantization form. Spin–orbit coupling is described as a one electron effective operator. The structure of the configuration expansion is single and double excitations from a limited full CI space. Calculations of the Ms=0 component of the three lowest excited states 1,3E and 3A1 in HCo(CO)4 have been performed in order to evaluate the spin–orbit interactions responsible for the 1E→3E and the 1E→3A1 radiationless transitions. A large difference is found in favor of the 1E→3E path.

Electronic spectroscopy of HRe(CO)5: a CASSCF/CASPT2 and TD-DFT study

Abstract The low-lying excited states of HRe(CO) 5 have been calculated at the CASSCF/CASPT2 and TD-DFT level of theory using relativistic effective core potentials (ECP) or ab initio model potentials (AIMP). The theoretical absorption spectrum is compared to the experimental one. Despite the similarity between the experimental absorption spectra of HMn(CO) 5 and HRe(CO) 5 in the UV/visible energy domain it is shown that the assignment differs significantly between the two molecules. The low-lying excited states of HRe(CO) 5 correspond to 5d→π * CO excitations whereas the spectrum of HMn(CO) 5 consists mainly of 3d→3d and 3d→ σ * Mn–H excitations. If the CASPT2 and TD-DFT results are quite comparable for the lowest excited states, the upper part assignment is more problematic with the TD-DFT method.

A CASSCF/MR-CCI study of the excited states of Fe(CO)5

Abstract CASSCF/MR-CCI calculations have been performed to determine excited states of Fe(CO)5 in the VUV region as well as the corresponding dipole moments. In spite of the high density of states between 25 000 and 45 000 cm−1, only three allowed transitions, with rather high oscillator strengths, 1 A 1 ′ → a 1 E ′, 1 A 1 ′ → 1 A 2 ″, 1 A 1 ′ → b 1 E ′ are present. The a 1 E ′ state is metal-centered (3d to 3d excitation) whereas the 1 A 2 ″ and b 1 E ′ correspond to 3d to πCO∗ excitations. The importance of these states for dissociation processes is discussed.

A CASSCF/CASPT2 study of the low-lying excited states of Mn2(CO)10

Abstract High-level ab initio quantum chemical methods are used to assign the low energetic part of the gas phase UV spectrum of dimanganese decacarbonyl. It is characterized by two bands at 26 700 and 29 740 cm−1 corresponding to the a 1 E 1 (3dπ→σ*Mn–Mn) and a 1 B 2 (σMn–Mn→σ*Mn–Mn) transitions which are calculated at 26 370 and 27 460 cm−1, respectively. In obtaining these values, the CASPT2 method has been pushed to its limits using the level shift correction in order to reduce the intruder states problem. It is shown that this method cannot be used safely without careful analysis of the excitation energies as a function of the level shift values. Our results are compared with recent density functional calculations.