Sigma-bond-to-ligand Charge Transfer Research Articles

Arylated versus Cyclometalated Platinum(II) Polypyridines: Photoluminescence from Pt(4′-R-trpy)Ph+ Systems (R = NMe2, N-Pyrrolidinyl, or 1-Pyrenyl)

Many platinum(II) polypyridine complexes are good luminophores, an enigmatic exception being Pt(trpy)Ph(+), where trpy denotes 2,2':6',2"-terpyridine. A new analysis suggests the complex is nonemissive due to (3)SBLCT (sigma-bond-to-ligand charge transfer) character in the lowest energy excited state. Bases for two distinct strategies for inducing emission from aryl derivatives become clear. The standard approach of incorporating a phenyl group into a (N(/\)N(/\)C) cyclometalating ligand relies in part on the rigidity of the ligand framework. An alternative strategy, which involves expanding the chromophore and altering the orbital parentage of the emitting state, is capable of suppressing radiationless decay even further. Indeed, the Pt(4'-pyren-1-yl-trpy)Ph(+) system emits from a low-lying (3)π-π*(pyrene) excited state that has a lifetime of 45 μs in fluid solution.

Theoretical analysis of low‐lying charge transfer states in [Ru(X) (Me)(CO)2(Me‐DAB)] (XCl, I; DAB1,4‐diaza‐1,3‐butadiene) complexes by TDDFT and CASSCF/CASPT2 methods

AbstractThe near‐ultraviolet (UV)/visible electronic spectroscopy of [Ru(X)(Me)(CO)2(Me‐DAB)] (XCl or I; DAB1,4‐diaza‐1,3‐butadiene), model complexes for a series of α‐diimine halide complexes of ruthenium is investigated by means of time‐dependent density functional theory (TDDFT) and complete active space self‐consistent field/CAS second‐order perturbation theory (CASSCF/CASPT2) methods. The convergence on the calculated transition energies to the low‐lying metal‐to‐ligand‐charge‐transfer (MLCT), X‐to‐ligand‐charge‐transfer (XLCT) with X a halide ligand) or sigma‐bond‐to‐ligand‐charge‐transfer (SBLCT) is analyzed for both methods with respect to various methodological aspects: basis set effects and functional influence on the TDDFT results versus the influence of the size and quality of the CASSCF and of the level of the perturbational treatment (single‐state or multi‐state) on the CASSCF/CASPT2 transition energies. On the basis of these accurate calculations, it is shown that whereas the lowest singlet state can be assigned to a nearly pure XLCT state in [Ru(I)(Me)(CO)2(Me‐DAB)], its character is mainly MLCT in [Ru(Cl)(Me)(CO)2(Me‐DAB)], in agreement with time‐resolved emission/infrared (IR) and resonance Raman experimental data. The experimental bands are well reproduced by the CASSCF/CASPT2 calculations despite the difficulty at converging to stable transition energies when enlarging the CASSCF. The TDDFT transitions energies are affected dramatically by the percentage of Hartree–Fock exchange in the hybrid functional. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Emission spectroscopy of metal-to-ligand-charge-transfer states of HRe(CO)3(H-dab), model system for α-diimine rhenium tricarbonyl complexes

The emission and absorption theoretical spectra of HRe(CO)3(H-dab) (H-dab = 1,4-diaza-1,3-butadiene), model system for a broad class of transition metal carbonyls α-diimine compounds, have been obtained by means of wave packet propagations on the V(q) (q = [Re–H]) CASSCF/MRCI potentials calculated for the electronic ground state and the 1MLCT (metal-to-ligand charge-transfer) absorbing state corresponding mainly to a 5dRe → π*H-dab excitation. The simulated spectra reproduce the main features of the experimental spectra of this class of complexes, namely one intense absorption band between 20 000 and 25 000 cm–1 assigned to this MLCT state and one strong emission band shifted to the red. The contribution of the low-lying bound MLCT triplet states or dissociative sigma-bond-to-ligand-charge-transfer (SBLCT) state is not taken into account in these preliminary simulations. Indeed the main goal is to understand the contribution of the absorbing MLCT state to the radiative process which may compete with the homolytic cleavage of the rhenium–hydrogen bond via the 3SBLCT state coupled to the absorbing state by spin-orbit (SO). The theoretical time-resolved emission spectrum simulated by wave packet propagation on the same potentials (absorbing 1MLCT state and electronic ground state) is also presented. To cite this article: S. Villaume, C. Daniel, C. R. Chimie 8 (2005). Les spectres d'émission et d'absorption théoriques du complexe HRe(CO)3(H-dab) (H-dab = 1,4-diaza-1,3-butadiene), système modèle pour toute une classe de composés α-diimine de métaux de transition, ont été construits par propagation de paquets d'ondes sur le potentiel V(q) (q = [Re–H]) calculé au niveau CASSCF/MRCI pour l'état fondamental et l'état absorbant 1MLCT (metal-to-ligand charge transfer) correspondant principalement à une excitation 5dRe → π*H-dab. Les spectres issus de ces simulations comportent les principales caractéristiques des spectres expérimentaux couramment observés, c'est-à-dire une bande d'absorption intense située entre 20 000 et 25 000 cm–1, attribuée à cet état MLCT, et une bande d'émission correspondante, décalée vers le rouge. La contribution des états triplets dissociants (sigma-bond-to-ligand charge transfer ou 3SBLCT) et liés (3MLCT) n'est pas prise en compte dans ces simulations préliminaires, dont le but est de comprendre la contribution de la transition 1MLCT (absorbant) vers l'état fondamental dans le spectre d'émission expérimental. Ce processus peut entrer en compétition avec la rupture homolytique de la liaison Re–H via l'état 3SBLCT couplé par spin orbite à l'état absorbant. Enfin, le spectre d'émission théorique résolu en temps accessible expérimentalement par des expériences laser pompe/sonde pour ce type de molécules est présenté. Pour citer cet article : S. Villaume, C. Daniel, C. R. Chimie 8 (2005).

Participation of co-ligands in electronic transitions of platinum(II) diazabutadiene complexes.

The low-lying electronic transitions and photochemical reactions of a series of [((i)Pr-DAB)Pt(R)(2)] (where the co-ligand R = CH(3), CD(3), adme, neop, neoSi, C(triple bond)C(t)Bu, C(triple bond)CPh, Ph, Mes) compounds were studied using both experimental (electronic absorption and resonance Raman spectroscopy) and theoretical (density functional theory, DFT) techniques. The high-lying filled orbitals were revealed to have a significant co-ligand contribution in the case of alkyl complexes, while this contribution is predominant for the complexes with unsaturated co-ligands. Because the electronic transition removes electron density from the sigma(Pt-C) bond in the former complexes, it is best described as a metal-to-ligand charge transfer transition (MLCT) with partial sigma-bond-to-ligand charge transfer (SBLCT) character. Because the sigma(Pt-C) orbital is not involved in the HOMOs of the latter complexes, the low-lying transitions were characterized as mixed MLCT/L'LCT, where L'LCT stands for ligand-to-ligand charge transfer from the pi system of the unsaturated co-ligand to the pi((i)Pr-DAB) orbital. The alkyl complexes are photoreactive on visible light irradiation with Pt-C bond homolysis as the primary step. The efficiency of the photoreaction increases with increasing sigma donor strength of the alkyl ligand. The absolute quantum yield is quite low. The other complexes are virtually photostable, except when irradiated at relatively high energies.

Photodissociation and electronic spectroscopy of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene): quantum wavepacket dynamics based on ab initio potentials.

The photodissociation dynamics of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene) were studied by means of wavepacket propagations on CASSCF/MR-CCI potentials calculated for the electronic ground state and low-lying excited states as a function of two coordinates, q(a) and q(b), that correspond to the Re-H bond homolysis and to the axial CO loss, respectively. The theoretical absorption spectrum is characterized by two bands, one intense peak centered at lambda=500 nm (21,000 cm(-1)) and one broad band centered at 310 nm (32,500 cm(-1)). The visible band was assigned to the low-lying metal-to-ligand charge-transfer (MLCT) states with a main contribution of the a(1)A'-->c(1)A' transition corresponding to the 3d(xz)-->pi*(dab) excitation. The second band calculated in the UV energy domain was assigned to the d(1)A' (sigma(Mn-H)-->pi*(dab)) state corresponding to a sigma-bond-to-ligand charge-transfer (SBLCT) state. The photodissociation dynamics of the low-lying (1)MLCT and (3)SBLCT states following irradiation in the visible energy domain was simulated by wavepacket propagation on the two-dimensional diabatic potentials V(q(a), q(b)) coupled by the spin-orbit. In contrast to what was found for the manganese analogue, the (1)MLCT state is nonreactive and a rather slow (beyond the ps time scale), nontotal and indirect homolysis of the Re-H bond occurs through (1)MLCT-->(3)SBLCT intersystem crossing.

Influence of the variation of the co-ligands on the electronic transitions and emission properties of [Pt(I)(CH3)3(iPr-DAB)], [Pt(CH3)4(α-diimine)] and [Pt(SnPh3)2(CH3)2(iPr-DAB)]: an experimental and theoretical study

This paper reports the results of a combined spectroscopic (UV/Vis, resonance Raman, emission) and theoretical study of [Pt(I)(CH3)3(iPr-DAB)] (iPr-DAB = N,N′-diisopropyl-1,4-diaza-1,3-butadiene), [Pt(CH3)4(R-DAB)] (R = alkyl or aryl), [Pt(CH3)4(α-diimine)] (α-diimine = pyridine-2-carbaldehyde-N-tert-butylimine or 3,4,7,8-tetramethyl-1,10-phenanthroline), and [Pt(SnPh3)2(CH3)2(iPr-DAB)]. The difference in character between the halide-to-ligand charge transfer (XLCT; X = I) transition of [Pt(I)(CH3)3(iPr-DAB)] and the sigma-bond-to-ligand charge transfer (SBLCT) transitions of the other complexes, is clearly established by resonance Raman (rR) spectroscopy. DFT MO calculations confirm the assignment of the frontier orbitals and lowest-energy electronic transitions, and support the interpretation of the rR spectra. All complexes emit at low temperatures, with excited state lifetimes strongly depending on the character and reactivity of the lowest excited state.

Novel complexes cis, cis-[Rh(R) 2(I)(CO)(dmb)] (R=Me, iPr; dmb=4,4′-dimethyl-2,2′-bipyridine): synthesis, structure and photoreactivity

The synthesis, structure and photochemistry of the novel rhodium(III) complexes cis,cis-[Rh(R) 2(I)(CO)(dmb)] (R=Me ( 1), i Pr ( 2)) are reported. Although many di- and trimethyl–rhodium(III) complexes are known, cis,cis-[Rh( i Pr) 2(I)(CO)(dmb)] ( 2) is the first diisopropyl–rhodium(III) compound. Single-crystal X-ray diffraction studies revealed the structure of 1. Resonance Raman spectra were obtained by excitation into the lowest-energy allowed electronic transition of the complexes. These spectra show that this transition has halide-to-ligand charge transfer (XLCT) character. Upon irradiation in solution, both 1 and 2 give rise to Rh–R bond homolysis as evidenced by IR, UV–vis and spin-trap EPR investigations. The photoreaction is proposed to occur after crossing from the XLCT state to the reactive sigma-bond-to-ligand charge transfer (SBLCT) state. For the i Pr-complex homolysis is observed at longer wavelength irradiation than for the methyl derivative, indicating that in the former case the reactive state is lower in energy.

A time-resolved infrared spectroscopic study of [M(SnR3)2(CO)2(α-diimine)] (M = Ru, Os; R = Ph, Me): evidence of charge redistribution in the lowest-excited state

According to previous electronic absorption, resonance Raman and DFT studies the lowest-energy electronic transition of the RuII and OsII complexes trans,cis-[M(SnR3)2(CO)2(α-diimine)] (α-diimine = bpy, etc.) has a σ(Sn–M–Sn) → π*(α-diimine) or sigma-bond-to-ligand charge-transfer (SBLCT) character. Nanosecond time-resolved step-scan IR (s2-TRIR) spectra of a series of these complexes are reported which indicate that the initial SBLCT excitation is followed by a redistribution of the electron density if this transition has an appreciable charge transfer character. This effect is virtually the same for the Ru and Os compounds, but different for the SnMe3 and SnPh3 complexes. s2-TRIR spectra of [Os(SnPh3)2(CO)2(dmb)] showed the occurrence of an infrared rigidochromic effect in a low-temperature glass.

ChemInform Abstract: Mechanisms of Ultrafast Metal-Ligand Bond Splitting upon MLCT Excitation of Carbonyl-Diimine Complexes

Abstract Using CO dissociation from [Cr(CO) 4 (bpy)], alkyl homolysis from [Re(R)(CO) 3 (dmb)] and dissociative isomerisation of [Mn(Br)(CO) 3 ( i Pr DAB)] as characteristic examples, it is shown how excitation into charge transfer excited states can lead to ultrafast metal–ligand bond splitting. The overall course of organometallic photochemical reactions is clearly determined by excited state dynamics, which occur at the earliest times after excitation. Branching of the evolution of the optically prepared Franck–Condon excited state between reactive and relaxation pathways seems to be a general mechanism that limits photochemical quantum yields. CO dissociation and alkyl homolysis can be described as an adiabatic evolution on potential energy surfaces of metal-to-ligand charge transfer (MLCT) or sigma-bond-to-ligand charge transfer (SBLCT) excited states, respectively. These states acquire a dissociative character from upper states with which they interact along the reaction coordinate. Coupling of an excited state with the dissociative continuum of the electronic ground state provides an alternative reaction mechanism. This was demonstrated for the dissociative isomerisation of fac -[Mn(Br)(CO) 3 ( i Pr DAB)], which occurs from ligand-to-ligand charge transfer (LLCT) excited state. In general, it follows that an understanding of organometallic photochemistry requires a knowledge of the energies and characters of the relevant electronic states as a function of possible reaction coordinates.

A quantum chemical investigation of the metal-to-ligand-charge-transfer photochemistry

According to a number of recent experiments reported for a class of α-diimine mono- and di-nuclear transition metal carbonyls, these molecules may either photodissociate leading to highly reactive intermediates or manifest the photophysics of metal-to-ligand-charge-transfer (MLCT) complexes. The duality between these two extremes of behavior may be used to promote different applications like catalytic activity or energy/electrons transfers. The potential energy surfaces (PESs) associated with the low-lying singlet and triplet excited states of Mn(H)(CO) 3(DAB) (DAB=1,4-diaza-1,3-butadiene) calculated for the Mn–H bond homolysis and for the dissociation of an axial carbonyl ligand illustrate the complexity and the richness of the photochemistry and photophysics of this class of molecules. It is shown that the presence of significant energy barriers on the reaction path corresponding to the homolysis of the Mn–H bond are responsible for the low efficiency of this process in related complexes. The simulation of the dynamics of the 1MLCT excited state, calculated at 26 000 cm −1 and contributing mainly to the visible absorption spectrum, indicates that the most probable primary reaction is an ultrafast dissociation (<200 fs) of the carbonyl ligand. The relative positions of the quasi-bound MLCT states with respect to the sigma bond-to-ligand-charge-transfer (SBLCT) excited state corresponding to the σ Mn–H →π ∗ DAB excitation will govern the photoreactivity of the molecules. A comparative study of the sequence, the nature of the low-lying excited states and of the associated state correlation diagrams for the hydride complex and the ethyl analog Mn(Et)(CO) 3(DAB) points to a better reactivity of the ethyl-substituted complex in agreement with the experimental data. This is a consequence of a lowering of the excited states, an increase in the density of states in the visible energy domain and a mixing of the triplet SBLCT and MLCT states on going from the hydride to the ethyl complex. The influence of the metal center on going from the manganese to the rhenium compound is expressed by an important mixing of the MLCT and SBLCT states and a lowering of the excited states due to a relativistic destabilization of the d shells of the metal center and an indirect stabilization of the π ∗ DAB orbital through its interaction with the 6p of the rhenium. These heavy atom effects should induce a better photoreactivity for the third-row transition metal complexes than for the first-row analogs.

Widely different luminescence lifetimes of the .DELTA.RRR, .LAMBDA.SSS and the .DELTA.RRS, .LAMBDA.SSR diastereomers of fac-tris[(8-quinolyl)phenylmethylsilyl]iridium(III): exciplex formation with solvents by distinct .sigma.-donor and .pi.-acceptor binding mechanisms

Luminescence lifetimes ([tau][sub m]) of the [sigma]-bond-to-ligand charge-transfer (SBLCT) excited states of two diastereomers of fac-tris[(8-quinolyl)phenylmethylsilyl]iridium(III) differ by about a factor of 2 and are strongly solvent dependent. The [tau][sub m] values of the more symmetric [Delta]RRR, [Lambda]SSS diastereomer (A) are generally longer than those of the less symmetric [Delta]RRS, [Lambda]SSR diastereomer (B); [tau][sub m]'s of both diastereomers are substantially shortened relative to their values in aliphatic hydrocarbons by exciplex formation with a variety of weakly coordinating solvents including aromatic hydrocarbons, olefins, ethers, ketones, alcohols, and nitriles. Quenching constants (k[sub q]) due to exciplex formation are found to be much larger for B than they are for A in the [sigma]-donor solvents (cyclic ethers, ketones, alcohols, and nitriles); however, k[sub q] values of B are slightly smaller than those of A in [pi]-acceptor solvents (aromatic hydrocarbons, olefins). The results suggest that [sigma]-donor solvents form exciplexes by binding at the metal center, whereas [pi]-acceptor solvents bind at a quinolyl radical anion ligand site. A and B may prove useful as luminescent environmental probes which can distinguish between [sigma]-donor and [pi]-acceptor binding sites. 19 refs., 1 fig., 1 tab.

Excited states of nitrogen, silicon-chelated transition metal complexes: characteristics of .sigma.-bond-to-ligand charge-transfer (SBLCT) excited states in fac-Ir[(6-isopropyl-8-quinolyl)diorganosilyl

Excited states of nitrogen, silicon-chelated transition metal complexes: characteristics of .sigma.-bond-to-ligand charge-transfer (SBLCT) excited states in fac-Ir[(6-isopropyl-8-quinolyl)diorganosilyl