John F. Endicott

A flash photolytic investigation of low energy homolytic processes in methylcobalamin.

Die Co-Methyl-o-Bindung in Methylcobalamin wird bereits durch Bestrahlung bei niedriger Energie gespalten.

MLCT excited states and charge delocalization in some ruthenium / ammine /polypyridyl complexes

Abstract The ab initio calculations of polypyridine π*-orbital energies are the basis for assignment of the lowest energy, highest intensity metal-to-ligand charge transfer (MLCT) transitions in simple ammine–polypyridine–ruthenium(II) complexes. A gaussian analysis of the absorption and emission spectra of these complexes enables the evaluation of reorganizational energies for the vertical MLCT transitions from component bandwidths and from apparent vibronic progressions. The observed bandwidths are about half of the widths expected in the limit of no metal–ligand mixing. The excited state-ground state mixing coefficient, α DA , is inferred to be about 0.3 in [Ru(NH 3 ) 4 bpy] 2+ based on this observation and a perturbation theory argument. These estimated reorganizational energies are combined with the observed ambient Stokes shifts to determine that the excited state electron exchange energy, K e , is small (600–1200 cm −1 for 2,2′ bipyridine complexes; ∼1500 cm −1 for 2,3-bis-(2-pyridyl)pyrazine complexes), but significant. This and the observation that the NH stretching frequency increases as the vertical MLCT energy (or α DA 2 ) decreases suggests that there is significant charge delocalization in these complexes.

Effects of Electronic Mixing in Ruthenium(II) Complexes with Two Equivalent Acceptor Ligands. Spectroscopic, Electrochemical, and Computational Studies

The lowest energy metal to ligand charge transfer (MLCT) absorption bands found in ambient solutions of [Ru(NH(3))(4)(Y-py)(2)](2+) and [Ru(L)(2)(bpy)(2)](+) complexes (Y-py a pyridine ligand and (L)(n) a substituted acetonylacetonate, halide, am(m)ine, etc.) consist of two partly resolved absorption envelopes, MLCT(lo) and MLCT(hi). The lower energy absorption envelope, MLCT(lo), in these spectra has the larger amplitude for the bis-(Y-py) complexes, but the smaller amplitude for the bis-bpy the complexes. Time-dependent density functional theory (TD-DFT) approaches have been used to model 14 bis-bpy, three bis-(Y-py), and three mono-bpy complexes. The modeling indicates that the lowest unoccupied molecular orbital (LUMO) of each bis-(Y-py) complex corresponds to the antisymmetric combination of individual Y-py acceptor orbitals and that the transition involving the highest occupied molecular orbital (HOMO) and LUMO (HOMO-->LUMO) is the dominant contribution to MLCT(lo) in this class of complexes. The LUMO of each bis-bpy complex that contains a C(2) symmetry axis also corresponds largely to the antisymmetric combination of individual ligand acceptor orbitals, while the LUMOs are more complex when there is no C(2) axis; furthermore, the energy difference between the HOMO-->LUMO and HOMO-->LUMO+1 transitions is too small (<1000 cm(-1)) to resolve in the spectra of the bis-bpy complexes in ambient solutions. Relatively weak MLCT(lo) absorption contributions are found for all of the [Ru(L)(2)(bpy)(2)](m+) complexes examined, but they are experimentally best defined in the spectra of the (L)(2) = X-acac complexes. TD-DFT modeling of the HOMO-->LUMO transition of [Ru(L)(4)bpy](m+) complexes indicates that it is too weak to be detected and occurs at significantly lower energy (about 3000-5000 cm(-1)) than the observed MLCT absorptions. Since the chemical properties of MLCT excited states are generally correlated with the HOMO and/or LUMO properties of the complexes, such very weak HOMO-->LUMO transitions can complicate the use of spectroscopic information in their assessment. As an example, it is observed that the correlation lines between the absorption energy maxima and the differences in ground state oxidation and reduction potentials (DeltaE(1/2)) have much smaller slopes for the bis-bpy than the mono-bpy complexes. However, the observed MLCT(lo) and the calculated HOMO-->LUMO transitions of bis-bpy complexes correlate very similarly with DeltaE(1/2) and this indicates that it is the low energy and small amplitude component of the lowest energy MLCT absorption band that is most appropriately correlated with excited state chemistry, not the absorption maximum as is often assumed.

Electron transfer emission in simple transition metal donor–acceptor systems

Abstract Electron transfer luminescences have been found in a simple class of covalently linked, CrIII CN RuII, transition metal complexes at 77 K in a DMSO/H2O glass. These emissions are broad, structureless and centered at about 850 nm. The emission lifetimes are on the order of 1 μs, and increase 16–26-fold upon perdeuteration of coordinated am(m)ines. The properties of the electron-transfer excited states are consistent with an inverted region, spin forbidden emission process.

Electrochemical and spectroscopic manifestations of donor-acceptor coupling in cyanide bridged transition metal complexes: contrasts between RuCNRu, CoCNRu and RhCNRu systems

Abstract An issue of contrasting donor-acceptor electronic coupling in cyano-bridged transition metal complexes containing ruthenium, rhodium or cobalt acceptors has been addressed by means of spectroscopic and electrochemical comparisons of several series of M(CNRu(NH3)5)2 complexes where M is RuII(bpy)2, CoIII(MCL), RhIII(bpy)2 or RhIII(MCL) and MCL is a tetraazamacrocyclic ligand. All the complexes exhibit metal-to-metal (MII→RuIII(NH3)5 or RuII(NH3)5→MIII) charge transfer (MMCT) absorptions. These are highest in energy for M=Rh(III) and lowest for M=Ru(II). The electrochemical half-wave potentials of the Ru(NH3)3+,2+5 couples are strongly correlated with the energies of the MMCT transitions and span a 300 mV range. The M=CoIII(MCL), RhIII(bpy)2 and RhIII(MCL) complexes, in which RuII(NH3)5→MIIIMMCT transitions are observed, have E 1 2 values greater than 0.22 V versus SCE, while the M=RuII(bpy)2 complexes have values of E 1 2 which are less than 0.0 V. The results suggest that common perturbational treatments of the MMCT transitions in RuCNRu complexes are inadequate owing to the exceptionally strong electronic coupling, and that the perturbational MMCT band shape analysis underestimates the electronic coupling. This work indicates that the donor-acceptor electronic coupling is greater in RuCNRu complexes than in RuCNCo complexes, qualitatively in accord with reports of contrasting MMCT excited state back electron transfer behavior.

Computational modeling of the triplet metal-to-ligand charge-transfer excited-state structures of mono-bipyridine-ruthenium(II) complexes and comparisons to their 77 K emission band shapes

A computational approach for calculating the distortions in the lowest energy triplet metal to ligand charge-transfer ((3)MLCT = T(0)) excited states of ruthenium(II)-bipyridine (Ru-bpy) complexes is used to account for the patterns of large variations in vibronic sideband amplitudes found in the experimental 77 K emission spectra of complexes with different ancillary ligands (L). Monobipyridine, [Ru(L)(4)bpy](m+) complexes are targeted to simplify analysis. The range of known emission energies for this class of complexes is expanded with the 77 K spectra of the complexes with (L)(4) = bis-acetonylacetonate (emission onset at about 12,000 cm(-1)) and 1,4,8,11-tetrathiacyclotetradecane and tetrakis-acetonitrile (emission onsets at about 21,000 cm(-1)); no vibronic sidebands are resolved for the first of these, but they dominate the spectra of the last two. The computational modeling of excited-state distortions within a Franck-Condon approximation indicates that there are more than a dozen important distortion modes including metal-ligand modes (low frequency; lf) as well as predominately bpy modes (medium frequency; mf), and it simulates the observed 77 K emission spectral band shapes of selected complexes very well. This modeling shows that the relative importance of the mf modes increases very strongly as the T(0) energy increases. Furthermore, the calculated metal-centered SOMOs show a substantial bpy-π-orbital contribution for the complexes with the highest energy T(0). These features are attributed to configurational mixing between the diabatic MLCT and the bpy (3)ππ* excited states at the highest T(0) energies.

Nearest- and next-nearest-neighbor Ru(II)/Ru(III) electronic coupling in cyanide-bridged tetra-ruthenium square complexes.

Electrochemical properties of cyanide-bridged metal squares, [Ru(4)](4+) and [Rh(2)-Ru(2)](6+), clearly demonstrate the role of the nearest (NN) metal moiety in mediating the next-nearest neighbor (NNN) metal-to-metal electronic coupling. The differences in electrochemical potentials for successive oxidations of equivalent Ru(II) centers in [Ru(4)](4+) are ΔE(1/2) = 217 mV and 256 mV and are related to intense, dual metal-to-metal-charge-transfer (MMCT) absorption bands. This contrasts with a small value of ΔE(1/2) = 77 mV and no MMCT absorption bands observed to accompany the oxidations of [Rh(2)-Ru(2)](6+). These observations demonstrate NN-mediated superexchange mixing by the linker Ru of NNN Ru(II) and Ru(III) moieties and that this mixing results in a NNN contribution to the ground state stabilization energy of about 90 ± 20 meV. In contrast, the classical Hush model for mixed valence complexes with the observed MMCT absorption parameters predicts a NNN stabilization energy of about 6 meV. The observations also indicate that the amount of charge delocalization per Ru(II)/Ru(III) pair is about 4 times greater for the NN than the NNN couples in these CN-bridged complexes, which is consistent with DFT modeling. A simple fourth-order secular determinant model is used to describe the effects of donor/acceptor mixing in these complexes.

Observations implicating vibronic coupling in covalently linked transition metal electron transfer systems

Abstract Experimental observations on the properties and photophysical behavior of several bimetallic and trimetallic donor-acceptor (D-A) complexes are considered. Photoinduced charge separation followed by back electron transfer seems to conform well to the behavior inferred from Born-Oppenheimer-based perturbation theory models when D-A coupling is weak, β DA −1 , but not when D-A coupling is very strong, β DA > 10 3 cm − . The best documented examples of deviations from the expected behavior are drawn from CNR-bridge transition metal systems, and this same class of D-A complexes also exhibits anomalous ground state properties: (a) a shift to lower energy, which is a function of D-A coupling, of the bridging CN − stretching frequency only when a donor and an acceptor are bridged; (b) a much larger than expected, (based on D-A charge transfer absorption) thermodynamic stabilization. Related behavior of strongly coupled D-A complexes with larger bridging ligands has also been reported. It is pointed out that substitution of a vibronic perturbation for the purely electronic perturbation of Born-Oppenheimer-based models can at least partially account for these observations. The vibronic model employed involves ligand to metal charge transfer (LMCT) and metal to ligand (MLCT) charge transfer interactions with nearest neighbor donor and acceptor respectively. The pertinent LMCT and MLCT parameters are, in principle, measurable, and their magnitudes seem compatible with the observed behavior. Some further implications and extensions are considered.

Photoinduced electron transfer in linked transition metal donor—acceptor complexes

Abstract Several complexes of the general type D − LA have been synthesized in which D − L and AL are well characterized transition metal complexes, and the strength of the donoracceptor electronic coupling has been inferred from measurements of: (a) the oscillators strengths of adjacent metal-to-metal charge transfer (MMCT) absorptions; (b) the shifts in half-wave potentials of Ru(NH 3 ) 5 3+,2+ couples; and (c) the back electron transfer (BET) behavior observed to result from irradiation of the MMCT absorption band. A special emphasis of this work has been complexes containing degenerate pairs of acceptors (or donors). In a typical complex, a ruthenium(II) center functions as the donor and the acceptor may be of any of several substitution inert metal complexes (Ru 3+ , Co 3+ , Rh 3+ or Cr 3+ ). When L1,2-bis(2,2′-bipyridyl-4-yl)ethane, electronic coupling is weak and BET in the photogenerated Ru III LCo II intermediate is non-adiabatic and nearly identical to that of the outer-sphere Ru(bpy) 3 2+ /Co(bpy) 3 3+ (bpy, 2,2′-bipyridine) couple, indicating no special role for the aliphatic linker. When LCN − , the donor—acceptor electronic coupling is very strong and gives rise to intense MMCT absorption bands and to substantial shifts in the D/D − and A/A − electrochemical half-wave potentials. The electronic coupling inferred from the electrochemical shifts is consistently found to be much larger than that inferred from a simple pertubational interpretation of the MMCT absorption bands when the complex contains degenerate acceptors (or donors). Possible origins of this effect are discussed. Picosecond flash photolysis experiments indicate that the electron transfer intermediates (DCNA − ) are much longer lived when A is a cobalt(III) complex than when A is a ruthenium(III) complex. The lifetime of the intermediate is further increased when the lowest energy electronic configuration of the cobalt(II) center has quartet spin multiplicity. The kinetic data imply that the electronic coupling matrix element ( H kin DA ) appropriate for the BET process in an order of magnitude smaller than the matrix element ( H op DA inferred from MMCT spectroscopy. It is proposed that this is simple symmetry effect: the dπ—dσ electronic transitions are “ x, y -allowed”, while A − -to-D BET is a “ z -allowed” process. Such considerations suggest that orbital symmetries play an important role in this strongly coupled limit.

Metal-to-metal electron-transfer emission in cyanide-bridged chromium-ruthenium complexes: effects of configurational mixing between ligand field and charge transfer excited states.

Irradiations of the transition metal-to-transition metal charge transfer (MMCT) absorption bands of a series of cyanide-bridged chromium(III)-ruthenium(II) complexes at 77 K leads to near-infrared emission spectra of the corresponding chromium(II)-ruthenium(III) electron transfer excited states. The lifetimes of most of the MMCT excited states increase more than 10-fold when their am(m)ine ligands are perdueterated. These unique emissions have weak, low frequency vibronic sidebands that correspond to the small excited-state distortions in metal-ligand bonds that are characteristic of transition metal electron transfer involving only the non-bonding metal centered d-orbitals suggesting that the excited-state Cr(II) center has a triplet spin configuration. However, most of the electronically excited complexes probably have overall doublet spin multiplicity and exhibit an excitation energy dependent dual emission with the near in energy Cr(III)-centered and MMCT doublet excited states forming an unusual mixed valence pair.