Electrogenerated chemiluminescence of platinum(II) alkynyl terpyridine complex with peroxydisulfate as coreactant

A Pt(II) alkynyl terpyridine complex containing a carbazole moiety, [Pt((t)Bu(3)tpy)(C≡C-C(6)H(4)-4-carbazole-9)](+) ((t)Bu(3)tpy = 4,4',4'-tri-tert-butyl-2,2':6',2'-terpyridine) 1, has been synthesized and characterized. The photophysical behavior has been studied, and the molecular structure has been determined by X-ray crystallography. The complex was found to exhibit intense electrogenerated chemiluminescence (ECL) using peroxydisulfate (S(2)O(8)(2-)) as coreactant in acetonitrile/water (1-25%, v/v) mixture at both glassy carbon and gold electrodes, representing the first ECL example of the Pt(II) alkynyl family. The ECL was produced at potential corresponding to the first reduction wave (-0.90 V vs SCE), significantly shifted by ～0.65 V toward more positive potential compared with that of [Ru(bpy)(3)](2+) (bpy = 2,2'-bipyridine). The ECL spectrum was found to be identical to the photoluminescence spectrum recorded in the same medium, indicating the formation of the same excited state of dπ(Pt) → π*((t)Bu(3)tpy) (3)MLCT mixed with π(C≡CR) → π*((t)Bu(3)tpy) (3)LLCT in both cases. The ECL mechanism was proposed involving the formation of the strongly oxidizing intermediate, SO(4)(?-), mainly generated during the catalytic reduction of S(2)O(8)(2-) by the electrogenerated 1(-). Chemiluminescence of 1/S(2)O(8)(2-) based on reduction with Al metal is also described.     

Ultrafast energy migration in platinum(II) diimine complexes bearing pyrenylacetylide chromophores

The ultrafast excited-state dynamics of three structurally related platinum(II) complexes has been investigated using femtosecond transient absorption spectrometry in 2-methyltetrahydrofuran (MTHF). Previous work has shown that Pt(dbbpy)(C[triple bond]C-Ph)2 (dbbpy is 4,4'-di(tert-butyl)-2,2'-bipyridine and C[triple bond]C-Ph is ethynylbenzene) has a lowest metal-to-ligand charge transfer (3MLCT) excited state, while the multichromophoric Pt(dbbpy)(C[triple bond]C-pyrene)2 (CC-pyrene is 1-ethynylpyrene) contains the MLCT state, but possesses a lowest intraligand (3IL) excited state localized on one of the CC-pyrenyl units (Pomestchenko, I. E.; Luman, C. R.; Hissler, M.; Ziessel, R.; Castellano, F. N. Inorg. Chem. 2003, 42, 1394-96). trans-Pt(PBu3)2(C[triple bond]C-pyrene)2 serves as a model system that provides a good representation of the CC-pyrene-localized 3IL state in a Pt(II) complex lacking the MLCT excited state. Following 400 nm excitation, the formation of the 3MLCT excited state in Pt(dbbpy)(C[triple bond]C-Ph)2 is complete within 200 +/- 40 fs, and intersystem crossing to the 3IL excited state in trans-Pt(PBu3)2(C[triple bond]C-pyrene)2 occurs with a time constant of 5.4 +/- 0.2 ps. Selective excitation into the low-energy MLCT bands in Pt(dbbpy)(C[triple bond]C-pyrene)2 (lambda(ex) = 480 nm) leads to the formation of the 3IL excited state in 240 +/- 40 fs, suggesting ultrafast wire-like energy migration in this molecule. The kinetic data suggest that the presence of the MLCT states in Pt(dbbpy)(C[triple bond]C-pyrene)2 markedly accelerates the formation of the triplet state of the pendant pyrenylacetylide ligand. In essence, the triplet sensitization process is kinetically faster than pure intersystem crossing in trans-Pt(PBu3)2(CC-pyrene)2 as well as vibrational relaxation in the MLCT excited state of Pt(dbbpy)(C[triple bond]C-Ph)2. These results are potentially important for the design of chromophores intended to reach their lowest excited state on subpicosecond time scales and advocate the likelihood of wire-like behavior in triplet-triplet energy transfer.     

Photophysical properties of 4,4'-di-tert-butyl-2,2'-bipyridine supported 6-membered 2,2'-diphenyl-X platinacycles (X = CH2, NMe, O)

The six-membered platinacycle complex, Pt((t)Bu(2)bpy)(C(6)H(4)OC(6)H(4)) (6) ((t)Bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) has been prepared from Pt((t)Bu(2)bpy)Cl(2) and 2,2'-dilithio-diphenyl ether. Platinacycle 6 and its analogs with X = CH(2) (4) and NMe (5) exhibit intense solid-state photoluminescence and nearly identical crystal structures. The photophysical properties of 4-6 in the visible range are dominated by mixed metal-ligand-to-ligand charge transfer (MLL'CT) transitions involving high-lying filled mixed metal-ligand orbitals (ML), composed primarily of platinacyclic ring-based d- and π-orbitals, and a low lying vacant π* orbital (L') of the (t)Bu(2)bpy ligand. Lone pair donation from the bridging oxygen atom and especially the NMe group increases the energy of the mixed metal-ligand orbital (ML) without altering the energy of the (t)Bu(2)bpy π* orbital. As a result, the MLL'CT state energy decreases and the absorption and emission wavelengths are red-shifted. DFT and TD-DFT calculations support the experimental results. Additional calculations on the unknown platinacycles with X = CO (7) and SO(2) (8) predict a blue-shift for the MLL'CT absorption and emission. Two nearly equal energy triplet minima were located on the DFT triplet surface for 4-6. One of these (4T-6T) has a geometry very similar to the ground-state singlet (as represented by 4-6) and is associated with the emissive (3)MLL'CT excited state. The other triplet-state (4T'-6T') has a distorted structure where the platinacycle ring is twisted out of the Pt((t)Bu(2)bpy) plane. Thermal access to this distorted triplet may be responsible for the loss of photoluminescence in room temperature solutions of 4-6.     

Picosecond isomerization in photochromic ruthenium-dimethyl sulfoxide complexes

The complexes [Ru(tpy)(bpy)(dmso)](OSO(2)CF(3))(2) and trans-[Ru(tpy)(pic)(dmso)](PF(6)) (tpy is 2,2':6',2' '-terpyridine, bpy is 2,2'-bipyridine, pic is 2-pyridinecarboxylate, and dmso is dimethyl sulfoxide) were investigated by picosecond transient absorption spectroscopy in order to monitor excited-state intramolecular S-->O isomerization of the bound dmso ligand. For [Ru(tpy)(bpy)(dmso)](2+), global analysis of the spectra reveals changes that are fit by a biexponential decay with time constants of 2.4 +/- 0.2 and 36 +/- 0.2 ps. The first time constant is assigned to relaxation of the S-bonded (3)MLCT excited state. The second time constant represents both excited-state relaxation to ground state and excited-state isomerization to form O-[Ru(tpy)(bpy)(dmso)](2+). In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.024), isomerization of [Ru(tpy)(bpy)(dmso)](2+) occurs with a time constant of 1.5 ns. For trans-[Ru(tpy)(pic)(dmso)](+), global analysis of the transient spectra reveals time constants of 3.6 +/- 0.2 and 118 +/- 2 ps associated with these two processes. In conjunction with the S-->O isomerization quantum yield (Phi(S)(-->)(O) = 0.25), isomerization of trans-[Ru(tpy)(pic)(dmso)](+) occurs with a time constant of 480 ps. In both cases, the thermally relaxed excited states are assigned as terpyridine-localized (3)MLCT states. Electronic state diagrams are compiled employing these data as well as electrochemical, absorption, and emission data to describe the reactivity of these complexes. The data illustrate that rapid bond-breaking and bond-making reactions can occur from (3)MLCT excited states formed from visible light irradiation.     

Excited-state absorption properties of platinum(II) terpyridyl acetylides

A comprehensive photophysical study is presented which compares the ground- and excited-state properties of four platinum(II) terpyridyl acetylide compounds of the general formula [Pt(tBu3tpy)(CCR)]+, where tBu3tpy is 4,4',4' '-tri-tert-butyl-2,2':6',2' '-terpyridine and R is an alkyl or aryl group. [Ru(tBu3tpy)3]2+ and the pivotal synthetic precursor [Pt(tBu3tpy)Cl]+ were also investigated in the current work. The latter two complexes possess short excited-state lifetimes and were investigated using ultrafast spectrometry while the other four compounds were evaluated using conventional nanosecond transient-absorption spectroscopy. The original intention of this study was to comprehend the nature of the impressive excited-state absorptions that emanate from this class of transition-metal chromophores. Transient-absorbance-difference spectra across the series contain the same salient features, which are modulated only slightly in wavelength and markedly in intensity as a function of the appended acetylide ligand. More intense absorption transients are observed in the arylacetylide structures relative to those bearing an alkylacetylide, consistent with transitions coupled to the pi system of the ancillary ligand. Reductive spectroelectrochemical measurements successfully generated the electronic spectrum of the tBu3tpy radical anion in all six complexes at room temperature. These measurements confirm that electronic absorptions associated with the tBu3tpy radical anion simply do not account for the intense optical transitions observed in the excited state of the Pt(II) chromophores. Transient-trapping experiments using the spectroscopically silent reductive quencher DABCO clearly demonstrate the loss of most transient-absorption features in the acetylide complexes throughout the UV, visible, and near-IR regions following bimolecular excited-state electron transfer, suggesting that these features are strongly tied to the photogenerated hole which is delocalized across the Pt center and the ancillary acetylide ligand.     

Excited-state properties of heteroleptic iridium(III) complexes bearing aromatic hydrocarbons with extended cores

The synthesis, complete structural characterization, electrochemistry, and excited-state dynamics of a series of four bis-heteroleptic iridium(III) charge-transfer complexes composed of a single acac-functionalized and two ortho-metalated 2-phenylpyridine ligands. The formed iodophenyl complex (2) was used as a metallosynthon to introduce extended-core ethynyltolyl (3), ethynylpyrene (4), and ethynylperylene (5) residues into these structures projecting from the acac ancillary ligand. Static and dynamic photoluminescence along with ultrafast and conventional transient absorption measurements in conjunction with cyclic voltammetry were employed to elucidate the nature of the intramolecular energy-transfer processes occurring in the excited states of polychromophores 4 and 5 and are directly compared with those of model complexes 2 and 3. Upon charge-transfer excitation of these molecules, the long-lived triplet-state metal-to-ligand charge-transfer ((3)MLCT)-based photoluminescence readily observed in 2 and 3 (τ = 1 μs) is nearly quantitatively quenched, resulting from production of the associated triplet intraligand ((3)IL) excited states in 4 and 5 through intramolecular triplet-triplet energy transfer. The respective formation of the extended-core (3)*pyrenyl and (3)*perylenyl-localized excited states in 4 and 5 is confirmed by their ultrafast excited-state evolution, which ultimately generates features associated with these (3)IL excited states and their greatly extended excited-state lifetimes with respect to the parent complexes 2 and 3.     

New Ru(II) chromophores with extended excited-state lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)(2)(PNI-phen)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(2), respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states ((3)IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.     

Design aspects for the development of mixed-metal supramolecular complexes capable of visible light induced photocleavage of DNA

Mixed-metal supramolecular complexes that couple ruthenium or osmium based light absorbers to a central rhodium(III) core have been designed which photocleave DNA upon irradiation with visible light. The complexes [[(bpy)(2)Ru(dpp)](2)RhCl(2)](PF(6))(5), [[(bpy)(2)Os(dpp)](2)RhCl(2)](PF(6))(5), and [[(tpy)RuCl(dpp)](2)RhCl(2)](PF(6))(3), where bpy = 2,2'-bipyridine, tpy = 2,2':6',2' '-terpyridine, and dpp = 2,3-bis(2-pyridyl)pyrazine, all exhibit intense metal to ligand charge transfer (MLCT) based transitions in the visible but possess lower lying metal to metal charge transfer (MMCT) excited states. These supramolecular complexes with low lying MMCT states photocleave DNA when excited into their intense MLCT transitions. Structurally similar complexes without this low lying MMCT state do not exhibit DNA photocleavage, establishing the role of this MMCT state in the DNA photocleavage event. Design considerations necessary to produce functional DNA photocleavage agents are presented herein.     

Synthesis and spectroscopic characterization of CN-substituted bipyridyl complexes of Ru(II)

A series of ruthenium complexes having the general form [Ru(bpy)(3-n)(CN-Me-bpy)(n)](PF(6))(2) (where bpy = 2,2'-bipyridine, CN-Me-bpy = 4,4'-dicyano-5,5'-dimethyl-2,2'-bipyridine, and n = 1-3 for complexes 1-3, respectively) have been synthesized and characterized using a variety of steady-state and nanosecond time-resolved spectroscopies. Electrochemical measurements indicate that the CN-Me-bpy ligand is significantly easier to reduce than the unsubstituted bipyridine (on the order of ～500 mV), implying that the lowest energy (3)MLCT (metal-to-ligand charge transfer) state will be associated with the CN-Me-bpy ligand(s) in all three compounds. Comparison of the Huang-Rhys factors derived from spectral fitting analyses of the steady state emission spectra of complexes 1-3 suggests all three compounds are characterized by excited-state geometries that are less distorted relative to their ground states as compared to [Ru(bpy)(3)](PF(6))(2); the effect of the more nested ground- and excited-state potentials is reflected in the unusually high radiative quantum yields (13% (1), 27% (2), and 40% (3)) and long (3)MLCT-state room-temperature lifetimes (1.6 μs, 2.6 μs, and 3.5 μs, respectively) for these compounds. Coupling of the π* system into the CN groups is confirmed by nanosecond step-scan IR spectra which reveal a ～40 cm(-1) bathochromic shift of the CN stretching frequency, indicative of a weaker CN bond in the (3)MLCT excited state relative to the ground state. The fact that the shift is the same for complexes 1-3 is evidence that, in all three complexes, the long-lived excited state is localized on a single CN-Me-bpy ligand rather than being delocalized over multiple ligands.     

Rigid medium stabilization of metal-to-ligand charge transfer excited states

The salts [Ru(bpy)3](PF6)2, cis-[Ru(bpy)2(py)2](PF6)2, trans-[Ru(bpy)2(4-Etpy)2](PF6)2, [Ru(tpy)2](PF6)2, and [Re(bpy)(CO)3(4-Etpy)](PF6) (bpy=2,2'-bipyridine, py=pyridine, 4-Etpy=4-ethylpyridine, and tpy=2,2':6',2-terpyridine) have been incorporated into poly(methyl methacrylate) (PMMA) films and their photophysical properties examined by both steady-state and time-resolved absorption and emission measurements. Excited-state lifetimes for the metal salts incorporated in PMMA are longer and emission energies enhanced due to a rigid medium effect when compared to fluid CH3CN solution. In PMMA part of the fluid medium reorganization energy, lambdaoo, contributes to the energy gap with lambdaoo approximately 700 cm-1 for [Ru(bpy)3](PF6)2 from emission measurements. Enhanced lifetimes can be explained by the energy gap law and the influence of the excited-to-ground state energy gap, Eo, on nonradiative decay. From the results of emission spectral fitting on [Ru(bpy)3](PF6)2* in PMMA, Eo is temperature dependent above 200 K with partial differentialEo/ partial differentialT=2.8 cm-1/deg. cis-[Ru(bpy)2(py)2](PF6)2 and trans-[Ru(bpy)2(4-Etpy)2](PF6)2 are nonemissive in CH3CN and undergo photochemical ligand loss. Both emit in PMMA and are stable toward ligand loss even for extended photolysis periods. The lifetime of cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is temperature dependent, consistent with a contribution to excited-state decay from thermal population and decay through a low-lying dd state or states. At temperatures above 190 K, coinciding with the onset of the temperature dependence of Eo for [Ru(bpy)3](PF6)2*, lifetimes become significantly nonexponential. The nonexponential behavior is attributed to dynamic coupling between MLCT and dd states, with the lifetime of the latter greatly enhanced in PMMA with tau approximately 3 ns. On the basis of these data and data in 4:1 (v/v) EtOH/MeOH, the energy gap between the MLCT and dd states is decreased by approximately 700 cm-1 in PMMA with the dd state at higher energy by DeltaH0 approximately 1000 cm-1. The "rigid medium stabilization effect" for cis-[Ru(bpy)2(py)2](PF6)2* in PMMA is attributed to inhibition of metal-ligand bond breaking and a photochemical cage effect.     

Application of time-resolved infrared spectroscopy to electronic structure in metal-to-ligand charge-transfer excited states

Infrared data in the nu(CO) region (1800-2150 cm(-1), in acetonitrile at 298 K) are reported for the ground (nu(gs)) and polypyridyl-based, metal-to-ligand charge-transfer (MLCT) excited (nu(es)) states of cis-[Os(pp)2(CO)(L)](n)(+) (pp = 1,10-phenanthroline (phen) or 2,2'-bipyridine (bpy); L = PPh3, CH(3)CN, pyridine, Cl, or H) and fac-[Re(pp)(CO)3(4-Etpy)](+) (pp = phen, bpy, 4,4'-(CH3)2bpy, 4,4'-(CH3O)2bpy, or 4,4'-(CO2Et)2bpy; 4-Etpy = 4-ethylpyridine). Systematic variations in nu(gs), nu(es), and Delta(nu) (Delta(nu) = nu(es) - nu(gs)) are observed with the excited-to-ground-state energy gap (E(0)) derived by a Franck-Condon analysis of emission spectra. These variations can be explained qualitatively by invoking a series of electronic interactions. Variations in dpi(M)-pi(CO) back-bonding are important in the ground state. In the excited state, the important interactions are (1) loss of back-bonding and sigma(M-CO) bond polarization, (2) pi(pp*-)-pi(CO) mixing, which provides the orbital basis for mixing pi(CO)- and pi(4,4'-X(2)bpy)-based MLCT excited states, and (3) dpi(M)-pi(pp) mixing, which provides the orbital basis for mixing pipi- and pi(4,4'-X(2)bpy*-)-based MLCT states. The results of density functional theory (DFT) calculations on the ground and excited states of fac-[Re(I)(bpy)(CO)3(4-Etpy)](+) provide assignments for the nu(CO) modes in the MLCT excited state. They also support the importance of pi(4,4'-X2bpy*-)-pi(CO) mixing, provide an explanation for the relative intensities of the A'(2) and A' ' excited-state bands, and provide an explanation for the large excited-to-ground-state nu(CO) shift for the A'(2) mode and its relative insensitivity to variations in X.     

Tuning the emissive triplet excited states of platinum(II) Schiff base complexes with pyrene, and application for luminescent oxygen sensing and triplet-triplet-annihilation based upconversions

Pt(II) Schiff base complexes containing pyrene subunits were prepared using the chemistry-on-complex approach. This is the first time that supramolecular photochemical approach has been used to tune the photophysical properties of Schiff base Pt(II) complexes, such as emission wavelength and lifetimes. The complexes show intense absorption in the visible region (ε = 13100 M(-1) cm(-1) at 534 nm) and red phosphorescence at room temperature. Notably, much longer triplet excited state lifetimes (τ = 21.0 μs) were observed, compared to the model complexes (τ = 4.4 μs). The extension of triplet excited state lifetimes is attributed to the establishment of equilibrium between the metal-to-ligand charge-transfer ((3)MLCT) state (coordination centre localized) and the intraligand ((3)IL) state (pyrene localized), or population of the long-lived (3)IL triplet excited state. These assignments were fully rationalized by nanosecond time-resolved difference absorption spectra, 77 K emission spectra and density functional theory calculations. The complexes were used as triplet sensitizers for triplet-triplet-energy-tranfer (TTET) processes, i.e. luminescent O(2) sensing and triplet-triplet annihilation (TTA) based upconversion. The O(2) sensitivity (Stern-Volmer quenching constant) of the complexes was quantitatively evaluated in polymer films. The results show that the O(2) sensing sensitivity of the pyrene containing complex (K(SV) = 0.04623 Torr(-1)) is 15-fold of the model complex (K(SV) = 0.00313 Torr(-1)). Furthermore, significant TTA upconversion (upconversion quantum yield Φ(UC) = 17.7% and the anti-Stokes shift is 0.77 eV) was observed with pyrene containing complexes being used as triplet sensitizers. Our approach to tune the triplet excited states of Pt(II) Schiff base complexes will be useful for the design of phosphorescent transition metal complexes and their applications in light-harvesting, photovoltaics, luminescent O(2) sensing and upconversion, etc.     

Triplet excited state distortions in a pyrazolate bridged platinum dimer measured by X-ray transient absorption spectroscopy

The excited-state structure of a dinuclear platinum(II) complex with tert-butyl substituted pyrazolate bridging units, [Pt(ppy)(μ-(t)Bu(2)pz)](2) (ppy = 2-phenylpyridine; (t)Bu(2)pz = 3,5-di-tert-butylpyrazolate) is studied by X-ray transient absorption (XTA) spectroscopy to reveal the transient electronic and nuclear geometry. DFT calculations predict that the lowest energy triplet excited state, assigned to a metal-metal-to-ligand charge transfer (MMLCT) transition, has a contraction in the Pt-Pt distance. The Pt-Pt bond length and other structural parameters extracted from fitting the experimental XTA difference spectra from full multiple scattering (FMS) and multidimensional interpolation calculations indicates a metal-metal distance decrease by approximately 0.2 ? in the triplet excited state. The advantages and challenges of this approach in resolving dynamic transient structures of nonbonding or weak-bonding dinuclear metal complexes in solution are discussed.     

Characterization of low energy charge transfer transitions in (terpyridine)(bipyridine)ruthenium(II) complexes and their cyanide-bridged bi- and tri-metallic analogues

The lowest energy metal-to-ligand charge transfer (MLCT) absorption bands found in ambient solutions of a series of [Ru(tpy)(bpy)X](m+) complexes (tpy = 2,2':3',2'-terpyridine; bpy = 2,2'-bipyridine; and X = a monodentate ancillary ligand) feature one or two partly resolved weak absorptions (bands I and/or II) on the low energy side of their absorption envelopes. Similar features are found for the related cyanide-bridged bi- and trimetallic complexes. However, the weak absorption band I of [(bpy)(2)Ru{CNRu(tpy)(bpy)}(2)](4+) is missing in its [(bpy)(2)Ru{NCRu(tpy)(bpy)}(2)](4+) linkage isomer demonstrating that this feature arises from a Ru(II)/tpy MLCT absorption. The energies of the MLCT band I components of the [Ru(tpy)(bpy)X](m+) complexes are proportional to the differences between the potentials for the first oxidation and the first reduction waves of the complexes. Time-dependent density functional theory (TD-DFT) computational modeling indicates that these band I components correspond to the highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) transition, with the HOMO being largely ruthenium-centered and the LUMO largely tpy-centered. The most intense contribution to a lowest energy MLCT absorption envelope (band III) of these complexes corresponds to the convolution of several orbitally different components, and its absorption maximum has an energy that is about 5000 cm(-1) higher than that of band I. The multimetallic complexes that contain Ru(II) centers linked by cyanide have mixed valence excited states in which more than 10% of electronic density is delocalized between the nearest neighbor ruthenium centers, and the corresponding stabilization energy contributions in the excited states are indistinguishable from those of the corresponding ground states. Single crystal X-ray structures and computational modeling indicate that the Ru-(C≡N)-Ru linkage is quite flexible and that there is not an appreciable variation in electronic structure or energy among the conformational isomers.     

Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re I(CO)3(2,2'-bipyridine)} chromophore

Two multifunctional photoactive complexes [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+)=N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy=2,2'-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans-->cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3)-->MeDpe(+ 3)MLCT (MLCT=metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximately 42 (73 %) and approximately 430 ps (27 %). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3)-->MeDpe(+) and Re(CO)(3)-->bpy (3)MLCT states, from which a MeDpe(+) localized intraligand (3)pipi* excited state ((3)IL) is populated with lifetimes of approximately 0.6 and approximately 10 ps, respectively. The (3)IL state undergoes a approximately 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe(+))(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.     

Luminescent charge-transfer platinum(II) metallacycle

The photophysical and electrochemical properties of a platinum(II) diimine complex bearing the bidentate diacetylide ligand tolan-2,2'-diacetylide (tda), Pt(dbbpy)(tda) [dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine] (1), are compared with two reference compounds, Pt(dbbpy)(C[triple bond]CPh)(2) (2) and Pt(dppp)tda [dppp = 1,3-bis(diphenylphosphino)propane] (3), respectively. The X-ray crystal structure of 1 is reported, which illustrates the nearly perfect square planarity exhibited by this metallacycle. Chromophore 2 possesses low-lying charge-transfer excited states analogous to 1, whereas structure 3 lacks such excited states but features a low-lying platinum-perturbed tda intraligand triplet manifold. In CH(2)Cl(2), 1 exhibits a broad emission centered at 562 nm at ambient temperature, similar to 2, but with a higher photoluminescence quantum yield and longer excited-state lifetime. In both instances, the photoluminescence is consistent with triplet-charge-transfer excited-state parentage. The rigidity imposed by the cyclic diacetylide ligand in 1 leads to a reduction in nonradiative decay, which enhances its room-temperature photophysical properties. By comparison, 3 radiates highly structured tda-localized triplet-state phosphorescence at room temperature. The 77 K emission spectrum of 1 in 4:1 EtOH/MeOH becomes structured and is quantitatively similar to that measured for 3 under the same conditions. Because the 77 K spectra are nearly identical, the emissions are assigned as (3)tda in nature, implying that the charge-transfer states are raised in energy, relative to the (3)tda levels in 1 in the low-temperature glass. Nanosecond transient absorption spectrometry and ultrafast difference spectra were determined for 1-3 in CH(2)Cl(2) and DMF at ambient temperature. In 1 and 2, the major absorption transients are consistent with the one-electron reduced complexes, corroborated by reductive spectroelectrochemical measurements performed at room temperature. As 3 does not possess any charge-transfer character, excitation into the pipi* transitions of the tda ligand generated transient absorptions in the relaxed excited state assigned to the ligand-localized triplet state. In all three cases, the excited-state lifetimes measured by transient absorption are similar to those measured by time-resolved photoluminescence, suggesting that the same excited states giving rise to the photoluminescence are responsible for the absorption transients. ESR spectroscopy of the anions 1- and 2- and reductive spectroelectrochemistry of 1 and 2 revealed a LUMO based largely on the pi* orbital of the dbbpy ligand. Time-dependent density functional theory calculations performed on 1-3 both in vacuum and in a CH(2)Cl(2) continuum revealed the molecular orbitals, energies, dipole moments, and oscillator strengths for the various electronic transitions in these molecules. A DeltaSCF-method-derived shift applied to the calculated transition energies in the solvent continuum yielded good agreement between theory and experiment for each molecule in this study.     

Influence of a gold(I)-Acetylide subunit on the photophysics of Re(phen)(CO)3Cl

The synthesis and photophysical properties of two new Re(I) complexes are reported: fac-Re(phenC triple-bond CH)(CO)(3)Cl (where phenC triple bond CH is 5-ethynyl-1,10-phenanthroline) and its Au(I)-acetylide analogue (fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl). Also reported are the photophysical measurements obtained for the benchmark fac-Re(phen)(CO)(3)Cl chromophore, as well as the phenC triple-bond CAuPPh(3) and phenC triple-bond CH ligands. The unstable nature of the precursor gold-containing ligand illustrates the advantage of using the "chemistry on the complex" approach, which facilitated preparation of the Re-Au binuclear complex. Where possible, all compounds were studied by static and transient absorption (TA), as well as steady-state and time-resolved photoluminescence (TRPL), at room temperature (RT) and 77 K, as well as nanosecond time-resolved infrared (TRIR) spectroscopy. The spectroscopic information provided by these techniques enabled a thorough evaluation of excited-state decay in most cases. In fac-Re(phenC triple bond CH)(CO)(3)Cl, the RT excited-state decay is most consistent with a metal-to-ligand charge transfer (MLCT) assignment, whereas at 77 K, the lowest excited state is dominated by the triplet intraligand ((3)IL) state, localized within the diimine ligand. The lowest excited state in fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl seems to result from an admixture of Re-based MLCT and (3)IL states resident on the phenC triple-bond CAuPPh(3) moiety. TA and TRIR methods indicate that these excited states are thermally equilibrated at room temperature. At 77 K, the MLCT energy of fac-Re(phenC triple-bond CAuPPh(3))(CO)(3)Cl is increased as a result of the glassy medium and the resulting excited state can be considered to be ligand-localized.     

Influence of triplet state multidimensionality on excited state lifetimes of bis-tridentate RuII complexes: a computational study

Calculated triplet excited state potential energy surfaces are presented for a set of three bis-tridentate Ru(II)-polypyridyl dyes covering a wide range of room temperature excited state lifetimes: [Ru(II)(tpy)(2)](2+), 250 ps; [Ru(II)(bmp)(2)](2+), 15 ns; and [Ru(II)(dqp)(2)](2+), 3 μs (tpy is 2,2':6',2″-terpyridine, bmp is 6-(2-picolyl)-2,2'-bipyridine, and dqp is 2,6-di(quinolin-8-yl)pyridine). The computational results provide a multidimensional view of the (3)MLCT-(3)MC transition for the investigated complexes. Recently reported results of significantly prolonged (3)MLCT excited state lifetimes of bis-tridentate Ru(II)-complexes, for example [Ru(II)(dqp)(2)](2+), are found to correlate with substantial differences in their triplet excited state multidimensional potential energy surfaces. In addition to identification of low-energy transition paths for (3)MLCT-(3)MC conversion associated with simultaneous elongation of two or more Ru-N bonds for all investigated complexes, the calculations also suggest significant differences in (3)MLCT state volume in the multidimensional reaction coordinate space formed from various combinations of Ru-N bond distance variations. This is proposed to be an important aspect for understanding the large differences in experimentally observed (3)MLCT excited state lifetimes. The results demonstrate the advantage of considering multidimensional potential energy surfaces beyond the Franck-Condon region in order to predict photophysical and photochemical properties of bis-tridentate Ru(II)-polypyridyl dyes and related metal complexes.     

Mechanism of ligand photodissociation in photoactivable [Ru(bpy)2L2]2+ complexes: a density functional theory study

A series of four photodissociable Ru polypyridyl complexes of general formula [Ru(bpy)2L2](2+), where bpy = 2,2'-bipyridine and L = 4-aminopyridine (1), pyridine (2), butylamine (3), and gamma-aminobutyric acid (4), was studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). DFT calculations (B3LYP/LanL2DZ) were able to predict and elucidate singlet and triplet excited-state properties of 1-4 and describe the photodissociation mechanism of one monodentate ligand. All derivatives display a Ru --> bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet (3)MLCT state (Ru --> bpy). 1-4 have three singlet metal-centered (MC) states 0.4 eV above the major (1)MLCT states. The energy gap between the MC states and lower-energy MLCT states is significantly diminished by intersystem crossing and consequent triplet formation. Relaxed potential energy surface scans along the Ru-L stretching coordinate were performed on singlet and triplet excited states for all derivatives employing DFT and TDDFT. Excited-state evolution along the reaction coordinate allowed identification and characterization of the triplet state responsible for the photodissociation process in 1-4; moreover, calculation showed that no singlet state is able to cause dissociation of monodentate ligands. Two antibonding MC orbitals contribute to the (3)MC state responsible for the release of one of the two monodentate ligands in each complex. Comparison of theoretical triplet excited-state energy diagrams from TDDFT and unrestricted Kohn-Sham data reveals the experimental photodissociation yields as well as other structural and spectroscopic features.     

Preparation, characterization, and photophysical properties of Pt-M (M = Ru, Re) heteronuclear complexes with 1,10-phenanthrolineethynyl ligands

When 3-ethynyl-1,10-phenanthroline (HCCphen) or 3,8-diethynyl-1,10-phenanthroline (HCCphenCCH) is utilized as a bifunctional bridging ligand via stepwise molecular fabrication, a series of Pt-Ru and Pt-Re heteronuclear complexes composed of both platinum(II) terpyridyl acetylide chromophores and a Ru(phen)(bpy)2/Re(phen)(CO)3Cl subunit were prepared by complexation of one or two Pt((t)Bu3tpy)(2+) units to the mononuclear Ru(II) or Re(I) precursor through platinum acetylide sigma coordination. These Pt-Ru and Pt-Re complexes exhibit intense low-energy absorptions originating from both Pt- and Ru (Re)-based metal-to-ligand charge-transfer (MLCT) states in the near-visible region. They are strongly luminescent in both solid states and fluid solutions with a submicrosecond range of lifetimes and 0.27-6.58% of quantum yields in degassed acetonitrile. For the Pt-Ru heteronuclear complexes, effective intercomponent Pt --> Ru energy transfer takes place from the platinum(II) terpyridyl acetylide chromophores to the ruthenium(II) tris(diimine)-based emitters. In contrast, dual emission from both Pt- and Re-based (3)MLCT excited states occurs because of less efficient intercomponent Pt --> Re energy transfer in the Pt-Re heteronuclear complexes.