Metal-to-Ligand Charge Transfer Excited-State ν(CO) Shifts in Rigid Media

[Ru(bpy)3]2+* and other remarkable metal-to-ligand charge transfer (MLCT) excited states

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy)3]2+*, was shown to undergo electron transfer quenching by methylviologen dication (MV2+), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a “modular approach”. In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy)3]2+* and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated.

Ultrafast excited-state dynamics of [Re(L)(CO)3(bpy)]n complexes: involvement of the solvent

Ultrafast excited-state dynamics of [Re(L)(CO)(3)(bpy)](n) (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2'-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FlUC) and UV-vis transient absorption (TA) with approximately 100 fs time resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF(4) ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 fs (depending on L) by intersystem crossing (ISC) to a pipi* intraligand ((3)IL) and a Re(L)(CO)(3) --> bpy charge-transfer ((3)CT) excited states. A second emission decay component (1.1-1.7 ps), apparent in the red part of the spectrum, is attributed to (3)IL --> (3)CT conversion, leaving phosphorescence from the lowest (3)CT state as the only emission signal at longer time delays. The triplet conversion is slower in DMF than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are due to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV-vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to (3)IL --> (3)CT conversion as well as electronic and vibrational relaxation, and a approximately 15 ps rise of only the 373 nm pipi*(bpy(*-)) band, which slows down to approximately 1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative in other classes of metal-based molecular complexes.

An alternative synthesis method for [Os(NN)(CO)2X2] complexes (NN = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine; X = Cl, Br, I). Electrochemical and photochemical properties and behavior

A novel synthesis method is introduced for the preparation of [Os(NN)(CO)(2)X(2)] complexes (X = Cl, Br, I, and NN = 2,2'-bipyridine (bpy) or 4,4'-dimethyl-2,2'-bipyridine (dmbpy)). In the first step of this two-step synthesis, OsCl(3) is reduced in the presence of a sacrificial metal surface in an alcohol solution. The reduction reaction produces a mixture of trinuclear mixed metal complexes, which after the addition of bpy or dmbpy produce a trans(Cl)-[Os(NN)(CO)(2)Cl(2)] complex with a good 60-70% yield. The halide exchange of [Os(bpy)(CO)(2)Cl(2)] has been performed in a concentrated halidic acid (HI or HBr) solution in an autoclave, producing 30-50% of the corresponding complex. All of the synthesized trans(X)-[Os(bpy)(CO)(2)X(2)] (X = Cl, Br, I) complexes displayed a similar basic electrochemical behavior to that found in the ruthenium analog trans(Cl)-[Ru(bpy)(CO)(2)Cl(2)] studied previously, including the formation of an electroactive polymer [Os(bpy)(CO)(2)](n) during the two-electron electrochemical reduction. The absorption and emission properties of the osmium complexes were also studied. Compared to the ruthenium analogues, these osmium complexes display pronounced photoluminescence properties. The DFT calculations were made in order to determine the HOMO-LUMO gaps and to analyze the contribution of the individual osmium d-orbitals and halogen p-orbitals to the frontier orbitals of the molecules. The electrochemical and photochemical induced substitution reactions of carbonyl with the solvent molecule are also discussed.

Chemical approaches to artificial photosynthesis. 2

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Ultrafast charge separation in a photoreactive rhenium-appended porphyrin assembly monitored by picosecond transient infrared spectroscopy

Rhenium(bipyridine)(tricarbonyl)(picoline) units have been linked covalently to tetraphenylmetalloporphyrins of magnesium and zinc via an amide bond between the bipyridine and one phenyl substituent of the porphyrin. The resulting complexes, abbreviated as [Re(CO)(3)(Pic)Bpy-MgTPP][OTf] and [Re(CO)(3)(Pic)Bpy-ZnTPP][OTf], exhibit no signs of electronic interaction between the Re(CO)(3)(bpy) units and the metalloporphyrin units in their ground states. However, emission spectroscopy reveals solvent-dependent quenching of porphyrin emission on irradiation into the long-wavelength absorption bands localized on the porphyrin. The characteristics of the excited states have been probed by picosecond time-resolved absorption (TRVIS) spectroscopy and time-resolved infrared (TRIR) spectroscopy in nitrile solvents. The presence of the charge-separated state involving electron transfer from MgTPP or ZnTPP to Re(bpy) is signaled in the TRIR spectra by a low-frequency shift in the nu(CO) bands of the Re(CO)(3) moiety similar to that observed by spectroelectrochemical reduction. Long-wavelength excitation of [Re(CO)(3)(Pic)Bpy-MTPP][OTf] results in characteristic TRVIS spectra of the S(1) state of the porphyrin that decay with a time constant of 17 ps (M = Mg) or 24 ps (M = Zn). The IR bands of the CS state appear on a time scale of less than 1 ps (Mg) or ca. 5 ps (Zn) and decay giving way to a vibrationally excited (i.e., hot) ground state via back electron transfer. The IR bands of the precursors recover with a time constant of 35 ps (Mg) or 55 ps (Zn). The short lifetimes of the charge-transfer states carry implications for the mechanism of reaction in the presence of triethylamine.

Excited-State Dynamics of Structurally Characterized [Re I (CO) 3 (phen)(HisX)] + (X = 83, 109) Pseudomonas a eruginosa Azurins in Aqueous Solution

The triplet metal-to-ligand charge transfer ((3)MLCT) dynamics of two structurally characterized Re(I)(CO)(3)(phen)(HisX)-modified (phen = 1,10-phenanthroline; X = 83, 109) Pseudomonas aeruginosa azurins have been investigated by picosecond time-resolved infrared (TRIR) spectroscopy in aqueous (D(2)O) solution. The (3)MLCT relaxation dynamics exhibited by the two Re(I)-azurins are very different from those of the sensitizer [Re(I)(CO)(3)(phen)(im)](+) (im = imidazole). Whereas the Re(I)(CO)(3) intramolecular vibrational relaxation in Re(I)(CO)(3)(phen)(HisX)Az (4 ps) is similar to that of [Re(I)(CO)(3)(phen)(im)](+) (2 ps), the medium relaxation is much slower ( approximately 250 vs 9.5 ps); the 250-ps relaxation is attributable to reorientation of D(2)O molecules as well as structural reorganization of the rhenium chromophore and nearby polar amino acids in each of the modified proteins.

pH-Dependent Electron Transfer from Re-bipyridyl Complexes to Metal Oxide Nanocrystalline Thin Films

Photoinduced interfacial electron transfer (ET) from molecular adsorbates to semiconductor nanoparticles has been a subject of intense recent interest. Unlike intramolecular ET, the existence of a quasicontinuum of electronic states in the solid leads to a dependence of ET rate on the density of accepting states in the semiconductor, which varies with the position of the adsorbate excited-state oxidation potential relative to the conduction band edge. For metal oxide semiconductors, their conduction band edge position varies with the pH of the solution, leading to pH-dependent interfacial ET rates in these materials. In this work we examine this dependence in Re(L(P))(CO)3Cl (or ReC1P) [L(P) = 2,2'-bipyridine-4,4'-bis-CH2PO(OH)2] and Re(L(A))(CO)3Cl (or ReC1A) [L(A) = 2,2'-bipyridine-4,4'-bis-CH2COOH] sensitized TiO2 and ReC1P sensitized SnO2 nanocrystalline thin films using femtosecond transient IR spectroscopy. ET rates are measured as a function of pH by monitoring the CO stretching modes of the adsorbates and mid-IR absorption of the injected electrons. The injection rate to TiO2 was found to decrease by 1000-fold from pH 0-9, while it reduced by only a factor of a few to SnO2 over a similar pH range. Comparison with the theoretical predictions based on Marcus' theory of nonadiabatic interfacial ET suggests that the observed pH-dependent ET rate can be qualitatively accounted for by considering the change of density of electron-accepting states caused by the pH-dependent conduction band edge position.

Electropolymerization of Vinylbipyridine Complexes of Ruthenium(II) and Osmium(II) in SiO2 Sol−Gel Films

PF(6)(-) salts of the complexes [Ru(vbpy)(3)](2+) and [Os(vbpy)(3)](2+) (vbpy = 4-methyl-4'-vinyl-2,2'-bipyridine) have been electropolymerized into the pores of SiO(2) sol-gel films deposited on conductive Tin(IV)-doped indium oxide-coated glass slides (ITO, In(2)O(3):Sn). The resulting transparent composites represent a new class of materials of general formulas ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) and ITO/SG-poly-[Os(vbpy)(3)](PF(6))(2). The composites are stable with respect to loss of complexes to the external solution and demonstrate several interesting phenomena: (1) Sol-gel pores, serving as diffusion channels for the vbpy complexes and counterions, play a key role in the formation of the polymer and dictate the electrochemical properties of the resulting composite. (2) Dynamic polymer growth occurs within individual diffusion channels creating parallel structures of filled and unfilled channels. (3) Unidirectional charge transfer and a "bilayer" effect have been shown to operate in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) films exposed to [Os(vbpy)(3)](PF(6))(2) in the external solution. (4) Photophysical properties of the metal-to-ligand charge transfer (MLCT) excited states in ITO/SG-poly-[Ru(vbpy)(3)](PF(6))(2) composites are significantly modified compared to electropolymerized films on ITO or model monomeric complexes in solution.

A high-sensitivity femtosecond to microsecond time-resolved infrared vibrational spectrometer

We describe an apparatus that provides, for the first time, a seamless bridge between femtosecond and microsecond time-resolved Raman and infrared vibrational spectroscopy. The laser system comprises an actively Q-switched sub-nanosecond pulsed kilohertz laser electronically synchronized to an ultrafast titanium sapphire regenerative amplifier to within 0.2 ns. The ultrafast amplifier provides the stable probe light source enabling high-sensitivity infrared vibrational spectroscopy of transients. Time-resolved infrared spectra of the excited-state relaxation dynamics of metal carbonyl compounds are presented to illustrate the capability of the apparatus, and transient data is resolved from 1 picosecond to over 100 microseconds. The results are compared to conventional nanosecond Fourier transform infrared (FT-IR) and laser based flash photolysis time-resolved infrared technology.

Excited-State Dynamics of fac-[ReI(L)(CO)3(phen)]+ and fac-[ReI(L)(CO)3(5-NO2-phen)]+ (L = Imidazole, 4-Ethylpyridine; Phen = 1,10-Phenanthroline) Complexes

The nature and dynamics of the lowest excited states of fac-[Re(I)(L)(CO)(3)(phen)](+) and fac-[Re(I)(L)(CO)(3)(5-NO(2)-phen)](+) [L = Cl(-), 4-ethyl-pyridine (4-Etpy), imidazole (imH); phen = 1,10-phenanthroline] have been investigated by picosecond visible and IR transient absorption spectroscopy in aqueous (L = imH), acetonitrile (L = 4-Etpy, imH), and MeOH (L = imH) solutions. The phen complexes have long-lived Re(I) --> phen (3)MLCT excited states, characterized by CO stretching frequencies that are upshifted relative to their ground-state values and by widely split IR bands due to the out-of-phase A'(2) and A"nu(CO) vibrations. The lowest excited states of the 5-NO(2)-phen complexes also have (3)MLCT character; the larger upward nu(CO) shifts accord with much more extensive charge transfer from the Re(I)(CO)(3) unit to 5-NO(2)-phen in these states. Transient visible absorption spectra indicate that the excited electron is delocalized over the 5-NO(2)-phen ligand, which acquires radical anionic character. Similarly, involvement of the -NO(2) group in the Franck-Condon MLCT transition is manifested by the presence of an enhanced nu(NO(2)) band in the preresonance Raman spectrum of [Re(I)(4-Etpy)(CO)(3)(5-NO(2)-phen)](+). The Re(I) --> 5-NO(2)-phen (3)MLCT excited states are very short-lived: 7.6, 170, and 43 ps for L = Cl(-), 4-Etpy, and imH, respectively, in CH(3)CN solutions. The (3)MLCT excited state of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) is even shorter-lived in MeOH (15 ps) and H(2)O (1.3 ps). In addition to (3)MLCT, excitation of [Re(I)(imH)(CO)(3)(5-NO(2)-phen)](+) populates a (3)LLCT (imH --> 5-NO(2)-phen) excited state. Most of the (3)LLCT population decays to the ground state (time constants of 19 (H(2)O), 50 (MeOH), and 72 ps (CH(3)CN)); in a small fraction, however, deprotonation of the imH.+ ligand occurs, producing a long-lived species, [Re(I)(im.)(CO)(3)(5-NO(2)-phen).-]+.

Rhenium-to-Benzoylpyridine and Rhenium-to-Bipyridine MLCT Excited States of fac-[Re(Cl)(4-benzoylpyridine)2(CO)3] and fac-[Re(4-benzoylpyridine)(CO)3(bpy)]+: A Time-Resolved Spectroscopic and Spectroelectrochemical Study

The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.

Transient Mixed-Valence Character of ReI4(CO)12(4,4‘-bpy)4Cl4

This study addresses, in detail, the orbital nature and the extent of metal-metal communication in the lowest emitting triplet state of Re(4)(CO)(12)(4,4'-bpy)(4)Cl(4) (where 4,4'-bpy = 4,4'-bipyridine) as well as the symmetry of the lowest (3)MLCT manifold in comparison to that of the ground state. All spectral evidence points to (1). a (3)MLCT excited manifold localized between a single Re(I) corner and an adjacent bridging ligand, (2). a transient mixed-valence state that is completely localized between a single transiently oxidized Re center and the adjacent metals, and (3). a second-order charge transfer from a localized transiently reduced bridging ligand to the adjacent Re(I) center to which it is attached, effectively lowering its oxidation state. The orbital nature of the lowest (3)MLCT manifold is fully corroborated by a molecular orbital diagram derived from quantum chemical modeling studies, while the existence of the localization, localized mixed valency, and second-order charge transfer rely on spectral evidence alone. This work makes use of low-temperature time-resolved infrared (TRIR) techniques as well as a luminescence study. Many of the nuances of the luminescence and TRIR data interpretation are extracted from statistical analysis and quantum chemical modeling studies. The relative concentrations of the dominant conformers that exist for Re(4)(CO)(12)(4,4'-bpy)(4)Cl(4) have also been estimated from Boltzmann statistics.

Solvent-Dependent Dynamics of the MQ•→ReII Excited-State Electron Transfer in [Re(MQ+)(CO)3(dmb)]2+

The Re-->MQ(+) MLCT excited state of [Re(MQ(+))(CO)(3)(dmb)](2+) (MQ(+) = N-methyl-4,4'-bipyridinium, dmb = 4,4'-dimethyl-2,2'-bipyridine), which is populated upon 400-nm irradiation, was characterized by picosecond time-resolved IR and resonance Raman spectroscopy, which indicate large structural differences relative to the ground state. The Re-->MQ(+) MLCT excited state can be formulated as [Re(II)(MQ*)(CO)(3)(dmb)](2+). It decays to the ground state by a MQ*-->Re(II) back-electron transfer, whose time constant is moderately dependent on the molecular nature of the solvent, instead of its bulk parameters: formamides approximately DMSO approximately MeOH (1.2-2.2 ns) < THF, aliphatic nitriles (3.2-3.9 ns) << ethylene-glycol approximately 2-ethoxyethanol (4.2-4.8 ns) < pyridine (5.7 ns) < MeOCH(2)CH(2)OMe (6.9 ns) < PhCN (7.5 ns) < MeNO(2) (8.6 ns) <<< CH(2)Cl(2), ClCH(2)CH(2)Cl (25.9-28.9 ns). An approximate correlation was found between the back-reaction rate constant and the Gutmann donor number. Temperature dependence of the decay rate measured in CH(2)Cl(2), MeOH, and BuCN indicates that the inverted MQ*-->Re(II) back-electron transfer populates a manifold of higher vibrational levels of the ground state. The solvent dependence of the electron transfer rate is explained by solvent effects on inner reorganization energy and on frequencies of electron-accepting vibrations, by interactions between the positively charged MQ(+) pyridinium ring and solvent molecules in the electron-transfer product, that is the [Re(MQ(+))(CO)(3)(dmb)](2+) ground state.