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.     

Photochemistry and photophysics of a Pd(II) metalloporphyrin: Re(I) tricarbonyl bipyridine molecular dyad and its activity toward the photoreduction of CO2 to CO

The photochemistry and photophysics of the cationic molecular dyad, 5-{4-[rhenium(I)tricarbonylpicoline-4-methyl-2,2'-bipyridine-4'-carboxyamidyl]phenyl}-10,15,20-triphenylporphyrinatopalladium(II) ([Re(CO)(3)(Pic)Bpy-PdTPP][PF(6)]) have been investigated. The single crystal X-ray structure for the thiocyanate analogue, [Re(CO)(3)(NCS)Bpy-PdTPP], exhibits torsion angles of 69.1(9)°, 178.1(7)°, and 156.8(9)° between porphyrin plane, porphyrin-linked C(6)H(4) group, amide moiety, and Bpy, respectively. Steady-state photoexcitation (λ(ex) = 520 nm) of [Re(CO)(3)(Pic)Bpy-PdTPP][PF(6)] in dimethylformamide (DMF) results in substitution of Pic by bromide at the Re(I)Bpy core. When [Re(CO)(3)(Pic)Bpy-PdTPP][PF(6)] is employed as a photocatalyst for the reduction of CO(2) to CO in DMF/NEt(3) solution with λ(ex) > 420 nm, 2 turnovers (TNs) CO are formed after 4 h. If instead, a two-component mixture of PdTPP sensitizer and mononuclear [Re(CO)(3)(Pic)Bpy][PF(6)] catalyst is used, 3 TNs CO are formed. In each experiment however, CO only forms after a slight induction period and during the concurrent photoreduction of the sensitizer to a Pd(II) chlorin species. Palladium(II) meso-tetraphenylchlorin, the hydrogenated porphyrin analogue of PdTPP, has been synthesized independently and can be substituted for PdTPP in the two-component system with [Re(CO)(3)(Pic)Bpy][PF(6)], forming 9 TNs CO. An intramolecular electron transfer process for the dyad is supported by cyclic voltammetry and steady-state emission studies, from which the free energy change was calculated to be ΔG(ox)* = -0.08 eV. Electron transfer from Pd(II) porphyrin to Re(I) tricarbonyl bipyridine in [Re(CO)(3)(Pic)Bpy-PdTPP][PF(6)] was monitored using time-resolved infrared (TRIR) spectroscopy in the ν(CO) region on several time scales with excitation at 532 nm. Spectra were recorded in CH(2)Cl(2) with and without NEt(3). Picosecond TRIR spectroscopy shows rapid growth of bands assigned to the π-π* excited state (2029 cm(-1)) and to the charge-separated state (2008, 1908 cm(-1)); these bands decay and the parent recovers with lifetimes of 20-50 ps. Spectra recorded on longer time scales (ns, μs, and seconds) show the growth and decay of further species with ν(CO) bands indicative of electron transfer to Re(Bpy).     

CO2 photoreduction with long-wavelength light: dyads and monomers of zinc porphyrin and rhenium bipyridine

Photocatalytic CO(2) reduction has been studied for two dyads with porphyrin covalently attached to rhenium tricarbonyl bipyridine moieties, and on separate components consisting of [Re(CO)(3)(Picoline)Bpy](+) and either zinc porphyrin or zinc chlorin. TONs decrease in the order: zinc porphyrin + Re > long spacer dyad > zinc chlorin + Re > short spacer dyad.     

Synthesis, functionalization, characterization, and photophysical study of carbonyl-containing isocyano rhenium(I) diimine complexes

New classes of tunable rhenium(I) diimine luminophores with formula of [Re(CO)(CNR)(3)(N-N)]PF(6) and [Re(CO)(L(x))(CNC(6)H(4)Cl-4)(2)(1,10-phenanthroline)]PF(6), (R = C(6)H(5), 4-BrC(6)H(4), 4-ClC(6)H(4), 4-MeOC(6)H(4), 2,6-(i)Pr(2)C(6)H(3); N-N = 1,10-phenanthroline, 5,6-dibromo-1,10-phenanthroline, 4,4'-di-tert-butyl-2,2'-bipyridine; L(x) = MeCN, pyridine and PPh(3)) have been synthesized. Different synthetic routes including photo-ligand substitution and thermal carbonyl ligand substitution through the oxidative decarbonylation with trimethyl amine N-oxide, for the facial and meridional isomeric forms of [Re(CO)(CNR)(3)(N-N)]PF(6) were investigated. On the basis of these synthetic strategies, different ligand modification and functionalization of the rhenium(I) diimine luminophores with tailored excited state properties could be readily achieved. The structures of both facial and meridional conformations of [Re(CO)(CNR)(3)(N-N)]PF(6) and the complex precursors fac-[Re(CO)(3)(CNC(6)H(3)(i)Pr-2,6)(3)]OTf were determined by X-ray crystallography. These complexes display an orange to red (3)MLLCT [dπ(Re) → π*(N-N)] phosphorescence at room temperature. Detailed photophysical investigations revealed that the physical, photophysical, electrochemical, and excited state properties can be fine-tuned and tailored through the modifications of the substituents on isocyanide or diimine ligands.     

Photocatalytic CO2 Reductions Catalyzed by meso-(1,10-Phenanthrolin-2-yl)-Porphyrins Having a Rhenium(I) Tricarbonyl Complex

We have prepared Zn and free-base porphyrins appended with a fac-Re(phen)(CO)₃Br (where phen is 1,10-phenanthroline) at the meso position of the porphyrin, and performed photocatalytic CO₂ reduction using porphyrin–Re dyads in the presence of either triethylamine (TEA) or 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. The Zn porphyrin dyad showed a high turnover number for CO production compared with the free-base porphyrin dyad, suggesting that the central Zn ion of porphyrin plays an important role in suppressing electron accumulation on the porphyrin part and achieving high durability of the photocatalytic CO₂ reduction using both TEA and BIH. The effect of acids on the CO₂ reduction was investigated using the Zn porphyrin–Re dyad and BIH. Acetic acid, a relatively strong Brønsted acid, rapidly causes the porphyrin's color to fade upon irradiation and dramatically decreases CO production, whereas proper weak Brønsted acids such as 2,2,2-trifluoroethanol and phenol enhance the CO₂ reduction.     

New synthetic routes to cationic rhenium tricarbonyl bipyridine complexes with labile ligands

Triflate abstraction from the complex [Re(OTf)(CO)(3)(bipy)] (1) using the salt NaBAr'(4) (Ar' = 3,5-bis(trifluoromethyl)phenyl) in dichloromethane solution in the presence of L = PPh(3), NCMe, NCPh, imines, ketones, Et(2)O, THF, MeOH, and MeI affords cationic complexes [Re(L)(CO)(3)(bipy)](+) as their BAr'(4)(-) salts. The new complexes have been characterized spectroscopically and, for [Re(eta(1)-O=C(Me)R)(CO)(3)(bipy)]BAr'(4) (R = CH(3), 6a; R = Ph, 6b), and [Re(THF)(CO)(3)(bipy)]BAr'(4) (9), also by single-crystal X-ray diffraction. Compared with conventional methodologies, the route reported here allows the coordination of a broader range of weakly coordinating ligands and requires considerably milder conditions. On the other hand, the reactions of lithium acetylides with [Re(THF)(CO)(3)(bipy)]BAr'(4) (9) can be used for the high-yield syntheses of rhenium alkynyls [Re(Ctbd1;CR)(CO)(3)(bipy)] (R = Ph, 12; R = SiMe(3), 13). Complex 9 was found to catalyze the aziridination of benzylideneaniline with ethyl diazoacetate.     

Re6Te8(CN)6][(Ir(CO)(PPh3)2)6](OTf)2 : a new Re6 cluster-supported iridium(I) compound

A new hexanuclear rhenium cluster encapsulated by six iridium complexes, [Re6Te8(CN)6][(Ir(CO)(PPh3)2)6](OTf)2 (3), which is effective in catalyzing the hydrogenation of p-CH3C6H4C[triple bond]CH to p-CH3C6H4CH=CH2 has been prepared.     

Investigation of mixed oxidation state cyanide-bridged Re(V)oxo (acac(2)en/pn) and Re(I)(bipy)(CO)3 complexes

A series of cyanide-bridged complexes that combine a low-valent photoacceptor rhenium(I) metal center with an electroactive midvalent rhenium(V) complex were prepared. The synthesis involved the preparation of novel asymmetric rhenium(V) oxo compounds, cis-Re(V)O(CN)(acac(2)en) (1) and cis-Re(V)O(CN)(acac(2)pn) (2), formed by reacting trans-[Re(V)O(OH(2))(acac(2)en)]Cl or trans-Re(V)O(acac(2)pn)Cl with [NBu(4)][CN]. The μ-bridged cyanide mixed-oxidation Re(V)-Re(I) complexes were prepared by incubating the asymmetric complexes, 1 or 2, with fac-[Re(I)(bipy)(CO)(3)][OTf] to yield cis-[Re(V)O(acac(2)en)(μ-CN-1κC:2κN)-fac-Re(I)(bipy)(CO)(3)][PF(6)] (3) and [cis-Re(V)O(acac(2)pn)(μ-CN-1κC:2κN)-fac-Re(I)(bipy)(CO)(3)][PF(6)] (4), respectively.     

A general method for constructing optically active supramolecular assemblies from intrinsically achiral water-insoluble free-base porphyrins

We have developed a general method to construct optically active porphyrin supramolecular assemblies by using a simple air-water interfacial assembly process. The method involved the in situ diprotonation of the free-base porphyrins at the air-water interface and subsequent assembly under compression. We showed that two intrinsically achiral water-insoluble free-base porphyrin derivatives, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (H(2)OEP) and 5,10,15,20-tetra-p-tolyl-21H,23H-porphine (H(2)TPPMe), could be diprotonated when spread onto a 2.4 M hydrochloric acid solution surface, and the Langmuir-Schaefer (LS) films fabricated from the subphase exhibited strong circular dichroism (CD) absorption, whereas those fabricated from pure Milli-Q water subphase did not. The experimental data suggested that the helical stacking of the achiral porphyrin building blocks was responsible for the supramolecular chirality of the assemblies. Interestingly, such a method was successfully applied to a series of other intrinsically achiral free-base porphyrins such as 5,10,15,20-tetrakis(4-methoxyphenyl)-21H,23H-porphine (H(2)TPPOMe), 5,10,15,20-tetraphenyl-21H,23H-porphine (H(2)TPP), 5,10,15,20-tetrakis(4-(allyloxy)phenyl)-21H,23H-porphine (H(2)TPPOA), and 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine (H(2)TPPDOMe). A possible mechanism has been proposed. The method provides a facile way to obtain optically active porphyrin supramolecular assemblies by using intrinsically achiral water-insoluble free-base porphyrin derivatives.     

Aggregate of alkoxy-bridged Re(I)-rectangles as a probe for photoluminescence quenching

Alkoxy-bridged rhenium(I) rectangles [{(CO)(3)Re(mu-OR)(2)Re(CO)(3)}(2)(mu-bpy)(2)] (1, R = C(4)H(9); 2, R = C(8)H(17); 3, R = C(12)H(25); bpy = 4,4'-bipyridine) comprising long alkyl chains form optically transparent aggregates and exhibit luminescence enhancement in the presence of water. The aggregation of Re(I)-rectangle was followed using a light-scattering technique. Presumably, the enhanced luminescence efficiency resulted from restriction of torsional molecular motion in the aggregates. In addition, the rate of bimolecular quenching of Re(I)-aggregates in the triplet excited state by various electron donors (amines) and acceptors (quinones) was efficient. These results indicate that the excited state of aggregated Re(I) surfactants with an electron acceptor and donor facilitate the electron-transfer quenching process after they became preassociated inside the Re(I)-aggregated species. These synthesized compounds may be useful fluorescent materials in optoelectronic applications.     

Directed assembly of transition-metal-coordinated molecular loops and squares from salen-type components. Examples of metalation-controlled structural conversion

A series of transition-metal-containing molecular "loops" and "squares" has been prepared via a directed-assembly approach and characterized. The molecular loops were prepared from the reaction of cis-(PEt(3))(2)Pt(OTf)(2) with bis(4-pyridyl)-functionalized free-base salen-type ligands. Zn(II)-metalation of the salen-type ligands in the molecular loops converts the loops to molecular squares. Alternatively, the squares can be obtained by the directed assembly of cis-(PEt(3))(2)Pt(OTf)(2) and bis(4-pyridyl)-functionalized Zn(II)-salen-type ligands. A concentration-dependent dynamic equilibrium between cyclic species was observed when bis(3-pyridyl)-functionalized free-base salen-type ligand was employed in the reaction. Zn(II) or Cr(III) metalation of the free-base ligand shifted the equilibrium to the single dimeric species. The incorporation of multiple reactive metal sites into a single, cavity-containing supramolecular structure points toward catalytic applications for these new assemblies.     

Second-sphere interaction of anions with a weakly binding metal complex host: probing the effect of counteranions

The reaction of [Re(OTf)(CO)5] with N-methylimidazole (MeIm) afforded [Re(CO)3(MeIm)3]OTf (1). The reactions of 1 with KPF6, NaBPh4 and NaBAr'4 (Ar' = 3,5-bis(trifluoromethyl)phenyl) afforded [Re(CO)3(MeIm)3]PF6 (2) [Re(CO)3(MeIm)3]BPh4 (3) and [Re(CO)3(MeIm)3]BAr'4 (4) respectively. An analogous reaction using N-phenylimidazole (PhIm) yielded [Re(CO)3(PhIm)3]BAr'4 (7). These new compounds were characterized by IR and NMR, and the structures of 1 and 2 were determined by X-ray diffraction. Compounds [Re(CO)3(MeIm)3]2[PtCl6] (5), [Re(CO)3(MeIm)3][HSO4] (6), [Re(CO)3(PhIm)3][Br] (8) and [Re(CO)3(PhIm)3][NO3] (9) were crystallized from equimolar mixtures of either 4 or 7 and the tetrabutylammonium salt of the corresponding anion, and their structures were determined by X-ray diffraction. The solution behavior of 1-4, 7 toward several anions was studied spectroscopically, including the quantitative determination of binding constants by 1H NMR. The cationic tris(imidazole)complexes are stable against imidazole-by-anion substitution, and the main hydrogen bonding interactions involve the imidazole NC(H)N groups. The binding constants for compounds 1-4 with several external anions follow the order 1<2<3<4, indicating that the strength of the cationic complex-counteranion interaction follows the order OTf(-) > PF6(-) > BPh4(-) > BAr'4(-).     

Insertion of unsaturated organic electrophiles into molybdenumbond-alkoxide and rheniumbond-alkoxide bonds of neutral,stable carbonyl complexes

Alkoxo complexes [Re(OR)(CO)(3)(N-N)] (R=Me, Et, tBu; N-N=2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'bipyridine (bipy'), 1,10-phenanthroline (phen)) and [M(OMe)(eta(3)-allyl)(CO)(2)(phen)] (M=Mo, W) have been synthesized in good yields and using mild conditions by the reaction of sodium alkoxides with [Re(OTf)(CO)(3)(N-N)] and [MCl(eta(3)-allyl)(CO)(2)(phen)] precursors. These have been characterized by IR and NMR spectroscopy as well as by X-ray diffraction for [W(OMe)(eta(3)-allyl)(CO)(2)(phen)] (10). The reactions of the molybdenum and rhenium alkoxo complexes with isocyanates, R'NCO, yield [L(n)M[N(R')C(O)OR]] complexes; the carbamate ligand, which results from an R'NCO insertion into the Mbond;OR bond, is monodentate through the nitrogen atom. The solid-state structures of Mo and Re examples have been determined by X-ray diffraction. The geometry around the carbamate nitrogen of these compounds is planar, and the distances indicate delocalization of the nitrogen lone pair involving mainly the carbonyl groups. Experiments carried out with the Re complexes showed that aryl isocyanates are more reactive than their alkyl counterparts, and that bulky R' groups led to slow rates of insertion. Insertion reactions were also observed with isothiocyanates, although here it is the Sbond;C bond that inserts into the Mbond;OR bond, and the resulting ligand is bound to the metal by sulfur. Competition experiments with the Re compounds indicate that isocyanates are more reactive than isothiocyanates towards the Rebond;OR bonds. Tetracyanoethylene inserts into the Rebond;OMe bond of [Re(OMe)(CO)(3)(bipy')], forming a complex with a 2-methoxytetracyanoethyl ligand; the structure of which was determined by X-ray diffraction. The formation of the xanthato complex [Re(SC(S)OtBu)(CO)(3)(bipy)] (20) by reaction of [Re(OTf)(CO)(3)(bipy)] with CS(2) and NaOtBu, but not by the reaction of CS(2) and [Re(OtBu)(CO)(3)(bipy)] (5 a), suggests that the insertion reactions do not take place by ionization of the alkoxo complexes to give the free alkoxide ion.     

Synthesis, characterization, and preliminary intramolecular energy transfer studies of rigid, emissive, rhenium-linked porphyrin dimers

The reaction of pyridyl functionalized porphyrins with Re(CO)(5)Cl in THF results in the formation of porphyrin dimers which, despite incorporation of rhenium into the assemblies, remain fluorescent. The rigid compounds provide an efficient geometry and/or orbital pathway for singlet energy transfer, rendering these compounds suitable, in principle, for the study of both through-bond and through-space energy transfer. Derivatives containing both metallated and freebase porphyrins connected via the metal corners display efficient porphyrin-porphyrin energy transfer. The photophysical properties of the assemblies have been studied by both steady-state and time-resolved fluorescence techniques, yielding approximate rates and efficiencies for porphyrin-porphyrin energy transfer.     

fac-[Re(CO)3(dmso-O)3](CF3SO3): a new versatile and efficient Re(I) precursor for the preparation of mono and polynuclear compounds containing fac-[Re(CO)3]+ fragments

We show here that the new complex fac-[Re(CO)3(dmso-O)3](CF3SO3) (1), efficiently prepared in one step from [ReBr(CO)5] and featuring a broad range of solubility, is, in general, a better precursor for the one-step synthesis of mono- and polynuclear inorganic compounds containing fac-[Re(CO)3]+ fragments compared to the commonly used (NEt4)2fac-[ReBr3(CO)3] and fac-[Re(CO)3(CH3CN)3](Y) (Y = PF6, BF4, ClO4) species. Compound 1 is the first example of a Re(I)-dmso complex structurally characterized and confirms the rule that dmso is always O-bonded when trans to CO. The reactivity of 1 was tested in the one-step preparation of several new and known complexes. The O-bonded sulfoxides of 1 are replaced under mild conditions by tri- (L3) and bidentate ligands (L2) to produce fac-[Re(CO)3(L3)]+ and fac-[Re(CO)3(L2)(dmso-O)]+ compounds, respectively. An excess of monodentate ligands (L) and more forcing conditions are needed to prepare fac-[Re(CO)3(L)3]+ compounds. The new compounds include fac-[Re(CO)3(bipy)(dmso-O)](CF3SO3) (4), that turned out to be an excellent precursor for binding the luminescent fac-[Re(CO)3(bipy)]+ fragment to polytopic ligands for the construction of more elaborate assemblies. One example reported here is the two-step preparation of fac-[{Re(CO)3(bipy)}(mu-4,4'-bipy){Ru(TPP)(CO)}](CF3SO3) (8) (TPP = tetraphenylporphyrin). The X-ray structures of the new compounds 1, 4, of the bis-porphyrin complex fac-[Re(CO)3Cl(4'MPyP)2] (13) (4'MPyP = 5-(4'pyridyl)-10,15,20-triphenylporphyrin), and of the rhenium-cyclophane [{(CO)3Re(mu-OH)2Re(CO)3}2(micro-4,4'-bipy)2] (15), among others, are described. Compound 1 might find useful applications in supramolecular chemistry (metal-mediated assembly of large architectures), in the in situ preparation of stable Re compounds to be used in nuclear medicine, and for the labeling of biomolecules.     

Kinetics and mechanism of rhenium-catalyzed oxygen atom transfer from pyridine N-oxides to phosphines

The oxygen atom transfer (OAT) reaction cited does not occur on its own in >10 h. Oxorhenium(V) compounds having the formula MeReO(dithiolate)PZ(3) catalyze the reaction; the catalyst most studied was MeReO(mtp)PPh(3), 1, where mtpH(2) = 2-(mercaptomethyl)thiophenol. The mechanism was studied by multiple techniques. Kinetics (initial-rate and full-time-course methods) established this rate law: v = k(c)[1][PyO](2)[PPh(3)](-1). Here and elsewhere PyO symbolizes the general case XC(5)H(4)NO and PicO that with X = 4-Me. For 4-picoline, k(c) = (1.50 +/- 0.05) x 10(4) L mol(-1) s(-1) in benzene at 25.0 degrees C; the inverse phosphine dependence signals the need for the removal of phosphine from the coordination sphere of rhenium prior to the rate-controlling step (RCS). The actual entry of PPh(3) into the cycle occurs in a fast step later in the catalytic cycle, after the RCS; its relative rate constants (k(4)) were evaluated with pairwise combinations of phosphines. Substituent effects were studied in three ways: for (YC(6)H(4))(3)P, a Hammett correlation of k(c) against 3sigma gives the reaction constant rho(c)(P) = +1.03, consistent with phosphine predissociation; for PyO rho(c)(N) = -3.84. It is so highly negative because PyO enters in three steps, each of which is improved by a better Lewis base or nucleophile, and again for (YC(6)H(4))(3)P as regards the k(4) step, rho(4) = -0.70, reflecting its role as a nucleophile in attacking a postulated dioxorhenium(VII) intermediate. The RCS is represented by the breaking of the covalent N-O bond within another intermediate inferred from the kinetics, [MeReO(mtp)(OPy)(2)], to yield the dioxorhenium(VII) species [MeRe(O)(2)(mtp)(OPy)]. A close analogue, [MeRe(O)(2)(mtp)Pic], was identified by (1)H NMR spectroscopy at 240 K in toluene-d(8). The role of the "second" PyO in the rate law and reaction scheme is attributed to its providing nucleophilic assistance to the RCS. Addition of an exogenous nucleophile (tetrabutylammonium bromide, Py, or Pic) caused an accelerating effect. When Pic was used, the rate law took on the new form v = k(NA)[1][PicO][Pic][PPh(3)](-1); k(NA) = 2.6 x 10(2) L mol(-1) s(-1) at 25.0 degrees C in benzene. The ratio k(c)/k(NA) is 58, consistent with the Lewis basicities and nucleophilicities of PicO and Pic.     

Photoexcitation in Cu(I) and Re(I) complexes containing substituted dipyrido[3,2-a:2',3'-c]phenazine: a spectroscopic and density functional theoretical study

Copper(I) and rhenium(I) complexes [Cu(PPh(3))(2)(dppz-11-COOEt)]BF(4), [Cu(PPh(3))(2)(dppz-11-Br)]BF(4), [Re(CO)(3)Cl(dppz-11-COOEt)] and [Re(CO)(3)Cl(dppz-11-Br)] (dppz-11-COOEt = dipyrido-[3,2a:2',3'c]phenazine-11-carboxylic ethyl ester, dppz-11-Br = 11-bromo-dipyrido[3,2a:2',3'c]-phenazine) have been studied using Raman, resonance Raman, and transient resonance Raman (TR(2)) spectroscopy, in conjunction with computational chemistry. DFT (B3LYP) frequency calculations with a 6-31G(d) basis set for the ligands and copper(I) centers and an effective core potential (LANL2DZ) for rhenium in the rhenium(I) complexes show close agreement with the experimental nonresonance Raman spectra. Modes that are phenazine-based, phenanthroline-based, and delocalized across the entire ligand structure were identified. The nature of the absorbing chromophores at 356 nm for ligands and complexes was established using resonance Raman spectroscopy in concert with vibrational assignments from calculations. This analysis reveals that the dominant chromophore for the complexes measured at 356 nm is ligand-centered (LC), except for [Re(CO)(3)Cl(dppz-11-Br)], which appears to have additional chromophores at this wavelength. Calculations on the reduced complexes, undertaken to model the metal-to-ligand charge transfer (MLCT) excited state, show that the reducing electron occupies a ligand MO that is delocalized across the ligand structure. Resonance Raman spectra (lambda(exc) = 514.5 nm) of the reduced rhenium complexes show a similar spectral pattern to that observed in [Re(CO)(3)Cl(dppz)](*-); the measured bands are therefore attributed to ligand radical anion modes. These bands lie at 1583-1593 cm(-1) for [Re(CO)(3)Cl(dppz-11-COOEt)] and 1611 cm(-1) for [Re(CO)(3)Cl(dppz-11-Br)]. The thermally equilibrated excited states are examined using nanosecond-TR(2) spectroscopy (lambda(exc) = 354.7 nm). The TR(2) spectra of the ligands provide spectral signatures for the (3)LC state. A band at 1382 cm(-1) is identified as a marker for the (3)LC states of both ligands. TR(2) spectra of the copper and rhenium complexes of dppz-11-Br show this (3)LC band, but it is not prominent in the spectra of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) and [Re(CO)(3)Cl(dppz-11-COOEt)]. Calculations suggest that the lowest triplet states of both of the rhenium(I) complexes and [Cu(PPh(3))(2)(dppz-11-Br)](+) are metal-to-ligand charge transfer in nature, but the lowest triplet state of [Cu(PPh(3))(2)(dppz-11-COOEt)](+) appears to be LC in character.     

Wavelength-dependent electron and energy transfer pathways in a side-to-face ruthenium porphyrin/perylene bisimide assembly

A new side-to-face supramolecular array of chromophores, where a pyridyl-substituted perylene bisimide dye axially binds to two ruthenium porphyrin fragments, has been prepared by self-assembly. The array is formulated as DPyPBI[Ru(TPP)(CO)](2), where DPyPBI = N,N'-di(4-pyridyl)-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-tetracarboxylic acid bisimide and TPP = 5,10,15,20-tetraphenylporphyrin. The photophysical behavior of DPyPBI[Ru(TPP)(CO)](2) has been studied by fast (nanoseconds) and ultrafast (femtoseconds) time-resolved techniques. The observed behavior sharply changes with excitation wavelength, depending on whether the DPyPBI or Ru(TPP)(CO) units are excited. After DPyPBI excitation, the strong fluorescence typical of this unit is completely quenched, and time-resolved spectroscopy reveals the occurrence of photoinduced electron transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 5.6 ps) followed by charge recombination (tau = 270 ps). Upon excitation of the Ru(TPP)(CO) fragments, on the other hand, ultrafast (tau < 1 ps) intersystem crossing is followed by triplet energy transfer from the ruthenium porphyrin to the perylene bisimide dye (tau = 720 ps). The perylene-based triplet state decays to the ground state on a longer time scale (tau = 9.8 micros). The photophysics of this supramolecular array provides remarkable examples of (i) wavelength-dependent behavior (a small change in excitation wavelength causes a sharp switch from electron to energy transfer) and (ii) intramolecular sensitization (the triplet state of the perylene bisimide, inaccessible in the free dye, is efficiently populated in the array).     

Involvement of a binuclear species with the Re-C(O)O-Re moiety in CO2 reduction catalyzed by tricarbonyl rhenium(I) complexes with diimine ligands: strikingly slow formation of the Re-Re and Re-C(O)O-Re species from Re(dmb)(CO)3S (dmb = 4,4'-dimethyl-2,2'-bipyridine, S = solvent)

Excited-state properties of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6), [Re(dmb)(CO)(3)](2) (where dmb = 4,4'-dimethyl-2,2'-bipyridine), and other tricarbonyl rhenium(I) complexes were investigated by transient FTIR and UV-vis spectroscopy in CH(3)CN or THF. The one-electron reduced monomer, Re(dmb)(CO)(3)S (S = CH(3)CN or THF), can be prepared either by reductive quenching of the excited states of fac-[Re(dmb)(CO)(3)(CH(3)CN)]PF(6) or by homolysis of [Re(dmb)(CO)(3)](2). In the reduced monomer's ground state, the odd electron resides on the dmb ligand rather than on the metal center. Re(dmb)(CO)(3)S dimerizes slowly in THF, k(d) = 40 +/- 5 M(-1) s(-1). This rate constant is much smaller than those of other organometallic radicals which are typically 10(9) M(-1) s(-1). The slower rate suggests that the equilibrium between the ligand-centered and metal-centered radicals is very unfavorable (K approximately 10(-4)). The reaction of Re(dmb)(CO)(3)S with CO(2) is slow and competes with the dimerization. Photolysis of [Re(dmb)(CO)(3)](2) in the presence of CO(2) produces CO with a 25-50% yield based on [Re]. A CO(2) bridged dimer, (CO)(3)(dmb)Re-CO(O)-Re(dmb)(CO)(3) is identified as an intermediate. Both [Re(dmb)(CO)(3)](2)(OCO(2)) and Re(dmb)(CO)(3)(OC(O)OH) are detected as oxidation products; however, the previously reported formato-rhenium species is not detected.     

Fluorescence-based nitric oxide detection by ruthenium porphyrin fluorophore complexes

The ruthenium(II) porphyrin fluorophore complexes [Ru(TPP)(CO)(Ds-R)] (TPP = tetraphenylporphinato dianion; Ds = dansyl; R = imidazole (im), 1, or thiomorpholine (tm), 2) were synthesized and investigated for their ability to detect nitric oxide (NO) based on fluorescence. The X-ray crystal structures of 1 and 2 were determined. The Ds-im or Ds-tm ligand coordinates to an axial site of the ruthenium(II) center through a nitrogen or sulfur atom, respectively. Both exhibit quenched fluorescence when excited at 368 or 345 nm. Displacement of the metal-coordinated fluorophore by NO restores fluorescence within minutes. These observations demonstrate fluorescence-based NO detection using ruthenium porphyrin fluorophore conjugates.