Synthesis and supramolecular properties of molecular clips with anthracene sidewalls

Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.     

Ruthenium complexes of easily accessible tridentate ligands based on the 2-aryl-4,6-bis(2-pyridyl)-s-triazine motif: absorption spectra, luminescence properties, and redox behavior

A family of tridendate ligands 1 a-e, based on the 2-aryl-4,6-di(2-pyridyl)-s-triazine motif, was prepared along with their hetero- and homoleptic Ru(II) complexes 2 a-e ([Ru(tpy)(1 a-e)](2+); tpy=2,2':6',2"-terpyridine) and 3 a-e ([(Ru(1 a-e)(2)](2+)), respectively. The ligands and their complexes were characterized by (1)H NMR spectroscopy, ES-MS, and elemental analysis. Single-crystal X-ray analysis of 2 a and 2 e demonstrated that the triazine core is nearly coplanar with the non-coordinating ring, with dihedral angles of 1.2 and 18.6 degrees, respectively. The redox behavior and electronic absorption and luminescence properties (both at room temperature in liquid acetonitrile and at 77 K in butyronitrile rigid matrix) were investigated. Each species undergoes one oxidation process centered on the metal ion, and several (three for 2 a-e and four for 3 a-e) reduction processes centered on the ligand orbitals. All compounds exhibit intense absorption bands in the UV region, assigned to spin-allowed ligand-centered (LC) transitions, and moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) absorption bands in the visible region. The compounds exhibit relatively intense emissions, originating from triplet MLCT levels, both at 77 K and at room temperature. The incorporation of triazine rings and the near planarity of the noncoordinating ring increase the luminescence lifetimes of the complexes by lowering the energy of the (3)MLCT state and creating a large energy gap to the dd state.     

Luminescence properties of Pt(II) complexes containing polypyridine ligands with extended aromatic moieties

The luminescence properties of eleven Pt(ii) complexes containing polypyridine ligands with extended aromatic moieties have been studied, both in acetonitrile fluid solution at 298 K and in butyronitrile rigid matrix at 77 K. For comparison purposes, also the phosphorescence properties of three free ligands at 77 K in butyronitrile have been investigated. The absorption spectra of all the compounds exhibit intense bands (epsilon in the range 10(4)-10(5) M(-1) cm(-1)) in the UV region, which are attributed to spin-allowed ligand-centered (LC) transitions, and moderately intense bands (epsilon in the range 10(3)-10(4) M(-1) cm(-1)) in the visible region, which receive contribution from both spin-allowed LC transitions and spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. At low energy, less intense spin-forbidden MLCT bands are also present. At 77 K in rigid matrix, all the studied compounds exhibit structured and long-lived (lifetimes from 840 mus on the millisecond timescale) luminescence, which is attributed to triplet LC states in all cases. At room temperature in fluid solution the luminescence lifetime of all the compounds is largely shortened (nanosecond timescale), and most of the emission spectra are unstructured and red-shifted. For species exhibiting structured emission spectra even at room temperature, low luminescence quantum yields are always obtained (Phi < 10(4)), and their emission is assigned to triplet LC states, which mainly deactivate to the ground state by thermal-activated surface crossing to a closely-lying metal-centered (MC) triplet state. Compounds exhibiting unstructured emission show relatively high emission quantum yields (about 0.1) and their emission is assigned to a mixed LC/MLCT state.     

Wire-type ruthenium(II) complexes with terpyridine-containing [2]rotaxanes as ligands: Synthesis, characterization, and photophysical properties

[2]Rotaxanes based on the 1,2-bis(pyridinium)ethane subset[24]crown-8 ether motif were prepared that contain a terminal terpyridine group for coordination to a transition-metal ion. These rotaxane ligands were utilized in the preparation of a series of heteroleptic [Ru(terpy)(terpy-rotaxane)]2+ complexes. The compounds were characterized by 1D and 2D 1H NMR spectroscopy, X-ray crystallography, and high-resolution electrospray ionization mass spectrometry. The effect of using a rotaxane as a ligand was probed by UV/Vis/NIR absorption and emission spectroscopy of the Ru(II) complexes. In contrast with the parent [Ru(terpy)(2)]2+ complex, at room temperature the examined complexes exhibit a luminescence band in the near infrared region and a relatively long lived triplet metal-to-ligand charge-transfer (3MLCT) excited state, owing to the presence of strong-electron-acceptor pyridinium substituents on one of the two terpy ligands. Visible-light excitation of the Ru-based chromophore in acetonitrile at room temperature causes an electron transfer to the covalently linked 4,4'-bipyridinium unit and the quenching of the MLCT luminescence. The 3MLCT excited state, however, is not quenched at all in rigid matrix at 77 K. The rotaxane structure was found to affect the absorption and luminescence properties of the complexes. In particular, when a crown ether surrounds the cationic axle, the photoinduced electron-transfer process is slowed down by a factor from 2 to 3. Such features, together with the synthetic and structural advantages offered by [Ru(terpy)2]2+-type complexes compared to, for example, [Ru(bpy)3]2+-type compounds, render these rotaxane-metal complexes promising candidates for the construction of photochemical molecular devices with a wire-type structure.     

Cyclopalladated complexes of 4-aryl-2,1,3-benzothiadiazoles: new emitters in solution at room temperature

The cyclopalladation of the 4-aryl-2,1,3-benzothiadiazoles 1a-c with palladium acetate in acetic acid afforded the novel dimeric complexes 2a-c in good yields. These were then converted into the monomeric pyridine-, chloro-coordinated cyclometallated complexes 3a-c through reaction with lithium chloride in acetone and then pyridine in dichloromethane. All complexes were fully characterized by means of NMR, IR and elemental analysis. The X-ray structure of complex 2c revealed that it presents transoid geometry, whereas the X-ray structure of 3c shows that the pyridine ligand and the thiazole ring are mutually trans. Photophysical properties were investigated by means of UV-Vis absorption and fluorescence emission in solution. Solid-state diffuse reflectance UV-Vis spectra (DRUV) were also applied in order to better characterize the complexes photophysics in the solid state. All complexes present intense absorption at around 300 nm (λ(1)) via(1)LC transitions located in BTD ligands, and additional low energy absorption bands, higher than 450 nm (λ(2)) of (1)MLCT character. The complexes are fluorescent in solution at room temperature, where two emission bands could be observed, a high energy band (excitation @ λ(1)) ascribed to the ligand emission and an additional red shifted low intense band (excitation @ λ(2)) due to the complex emission.     

The multichromophore approach: prolonged room-temperature luminescence lifetimes in Ru(II) complexes based on tridentate polypyridine ligands

A new family of ruthenium(II) complexes with multichromophoric properties was prepared based on a "chemistry-on-the-complex" synthetic approach. The new compounds are based on tridentate chelating sites (tpy-type ligands, tpy=2,2':6',2'-terpyridine) and most of them carry appended anthryl chromophores. Complexes 2 a and 2 b were synthesized through the Pd-catalyzed Suzuki coupling reaction between 9-anthrylboronic acid and the chloro ligands on the presursor species 1 a and 1 b, respectively. The monocoupling product 2 c was also synthesized as the starting complex for a dimetallic complex under optimized Suzuki coupling conditions. The palladium(0)-catalyzed homocoupling reaction on complexes 1 a and 2 c led to dimetallic Ru(II) species 2 d and 2 e, respectively. The solid structures of complexes 2 a and 2 b were characterized by X-ray diffraction. The absorption spectra, redox behavior, luminescence properties (both at room temperature and at 77 K), and transient absorption spectra and decays of 2 a-e were investigated. The absorption spectra of all new species are dominated by ligand-centered (LC) bands in the UV region and metal-to-ligand charge-transfer (MLCT) bands in the visible region. The new compounds undergo reversible metal-centered oxidation processes and several ligand-centered reduction processes, which have been assigned to specific sites. The complexes exhibit luminescence both at room temperature in fluid solution and at 77 K in rigid matrices; the emission was attributed to (3)MLCT states at room temperature and to the lowest-lying anthracene triplet ((3)An) at low temperature, except for 2 c, which does not contain any anthryl chromophore and whose low temperature emission is also of MLCT origin. The luminescence lifetimes of complexes 2 a-d showed that multichromophoric behavior occurs in these species, allowing the luminescence lifetime of the Ru(II)-based chromophores to be prolonged to the microsecond timescale, with the anthryl groups behaving as energy-storage elements for the repopulation of the (3)MLCT state. Nanosecond transient-absorption spectroscopy confirmed the equilibration process between the triplet MLCT and An levels at room temperature. Thermodynamic and kinetic factors governing the equilibration time and the lifetime of the equilibrated excited state are discussed.     

Novel dinuclear luminescent compounds based on iridium(III) cyclometalated chromophores and containing bridging ligands with ester-linked chelating sites

The syntheses and study of the spectroscopic, redox, and photophysical properties of a new set of species based on Ir(III) cyclometalated building blocks are reported. This set includes three dinuclear complexes, that is, the symmetric (with respect to the bridging ligand) diiridium species [(ppy)(2)Ir(mu-L-OC(O)-C(O)O-L)Ir(ppy)(2)][PF(6)](2) (5; ppy = 2-phenylpyridine anion; L-OC(O)-C(O)O-L = bis[4-(6'-phenyl-2,2'-bipyridine-4'-yl)phenyl]-benzene-1,4-dicarboxylate), the asymmetric diiridium species [(ppy)(2)Ir(mu-L-OC(O)-L)Ir(ppy)(2)][PF(6)](2) (3; L-OC(O)-L = 4-([(6'-phenyl-2,2'-bipyridine-4'-yl)benzoyloxy]phenyl)-6'-phenyl-2,2'-bipyridine), and the mixed-metal Ir-Re species [(ppy)(2)Ir(mu-L-OC(O)-L)Re(CO)(3)Br][PF(6)] (4). Syntheses, characterization, and spectroscopic, photophysical, and redox properties of the model mononuclear compounds [Ir(ppy)(2)(L-OC(O)-L)][PF(6)] (2) and [Re(CO)(3)(L-COOH)Br] (6; L-COOH = 4'-(4-carboxyphenyl)-6'-phenyl-2,2'-bipyridine) are also reported, together with the syntheses of the new bridging ligands L-OC(O)-L and L-OC(O)-C(O)O-L. The absorption spectra of all the complexes are dominated by intense spin-allowed ligand-centered (LC) bands and by moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) bands. Spin-forbidden MLCT absorption bands are also visible as low-energy tails at around 470 nm for all the complexes. All the new species exhibit metal-based irreversible oxidation and bipyridine-based reversible reduction processes in the potential window investigated (between +1.80 and -1.70 V vs SCE). The redox behavior indicates that the metal-based orbitals are only weakly interacting in dinuclear systems, whereas the two chelating halves of the bridging ligands exhibit noticeable electronic interactions. All the complexes are luminescent both at 77 K and at room temperature, with emission originating from triplet MLCT states. The luminescence properties are temperature- and solvent-dependent, in accord with general theories: emission lifetimes and quantum yields increase on passing from acetonitrile to dichloromethane fluid solution and from room-temperature fluid solution to 77 K rigid matrix. In the dinuclear mixed-chromophore species 3 and 4, photoinduced energy transfer across the ester-linked bridging ligands seems to occur with low efficiency.     

Absorption spectra, photophysical properties, and redox behavior of ruthenium(II) polypyridine complexes containing accessory dipyrromethene-BF2 chromophores

The six multichromophoric species 1-6, containing the potentially luminescent Ru(II) polypyridine subunits and 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene fluorophores (dipyrromethene-BF(2) dyes, herein after called bodipy), have been prepared and their absorption spectra, luminescence properties (both at room temperature in fluid solution and at 77 K in rigid matrix), and redox properties have been investigated (for the structuralformulas of all the compounds, see Figure 1). For comparison purposes, also the same properties of the bodipy-based free ligands have been examined. Three of the multichromophoric species (1-3) are based on the Ru(bpy)(3)-type metal subunit, whereas 4-6 are based on the Ru(terpy)(2)-type metal subunit. Transient absorption spectroscopy at room temperature of all the compounds has also been performed. The absorption spectra of all the metal complexes show features that can be assigned to the Ru(II) polypyridine subunits and to the bodipy centers. In particular, the lowest energy spin-allowed pi-pi* transition of the bodipy groups dominates the visible region, peaking at about 530 nm. All the new complexes exhibit a rich redox behavior, with reversible processes attributed to specific sites, indicating a small perturbation of each redox center and therefore highlighting the supramolecular nature of the multichromophoric assemblies. Despite the good luminescence properties of the separated components, 1-6 do not exhibit any luminescence at room temperature; however, transient absorption spectroscopy evidences that for all of them a long-lived (microsecond time scale) excited state is formed, which is identified as the bodipy-based triplet state. Pump-probe transient absorption spectroscopy suggests that such a triplet state is formed from the promptly prepared bodipy-based (1)pi-pi* state in most cases by the intervention of a charge-separated level. At 77 K, all the complexes except complex 1 exhibit the bodipy-based fluorescence, although with a slightly shortened lifetime compared to the corresponding free ligand(s), and 4-6 also exhibit a phosphorescence assigned to the bodipy subunits. Phosphorescence of bodipy species had never been reported in the literature to the best of our knowledge: in the present cases we propose that it is an effective decay process thanks to the presence of the ruthenium heavy atom and of the closely lying (3)MLCT state of the Ru(terpy)(2)-type subunits.     

Self-assembled light-harvesting systems: Ru(II) complexes assembled about Rh-Rh cores

Ru(II) polypyridine species have been assembled about dirhodium(II, II) tetracarboxylate cores. The complexes prepared have general formulas [{(terpy)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (a-type compounds: terpy = 2,2':6',2' '-terpyridine; La = 4'-(p-carboxyphenyl)-2,2':6',2' '-terpyridine; n = 1, 1a; n = 2, cis-2a and trans-2a-cis and trans refer to the arrangement of the Ru(II) species around the dirhodium core; n = 3, 3a), [{(Lb)Ru(La)}n{Rh2(CH3COO)4-n(CH3CN)2}]2n+ (b-type compounds: Lb = 6-phenyl-2,4-di(2-pyridyl)-s-triazine; n = 1, 1b; n = 2, an inseparable mixture of cis-2b and trans-2b; n = 3, 3b; n = 4, 4b), and [{(terpy)Ru(Lc)}{Rh2(CH3COO)3(CH3CN)2}]2+ (1c; Lc = 6-(p-carboxyphenyl)-2,4-di(2-pyridyl)-s-triazine). As model species, also the mononuclear [(terpy)Ru(La)]2+ (5a), [(La)Ru(Lb)]2+ (5b), and [(terpy)Ru(Lc)]2+ (5c) have been prepared. All of the complexes have been characterized by several techniques, including NMR and mass spectra, and the stability of the various species is discussed. The absorption spectra of all of the compounds are dominated by the Ru(II) polypyridine moieties, showing intense ligand-centered (LC) bands in the UV region and intense metal-to-ligand charge-transfer (MLCT) bands in the visible. The compounds exhibit several metal-centered oxidation and ligand-centered reduction processes, which have been assigned to specific subunits. Both absorption and redox data indicate a supramolecular nature of the assembled systems. Efficient energy transfer from the MLCT triplet state of the Ru-based components to the lowest-energy excited state of the dirhodium core takes place for the a-type compounds at 298 K in acetonitrile solution, whereas such a process is inefficient for the b-type and c-type species, which exhibit the typical MLCT emission. At 77 K in butyronitrile matrix, Ru-to-Rh2 energy transfer is partly efficient for both the a-type and the b-type compounds and is inefficient for 1c. The reasons for such behavior are discussed by taking into account arguments concerning the driving force and reorganization energy of the complexes.     

Synthesis, structure and spectral and redox properties of new mixed ligand monomeric and dimeric Ru(II) complexes: predominant formation of the "cis-alpha" diastereoisomer and unusual 3MC emission by dimeric complexes

The tetradentate ligands 1,8-bis(pyrid-2-yl)-3,6-dithiaoctane (pdto) and 1,8-bis(benzimidazol-2-yl)-3,6-dithiaoctane (bbdo) form the complexes [Ru(pdto)(mu-Cl)](2)(ClO(4))(2) 1 and [Ru(bbdo)(mu-Cl)](2)(ClO(4))(2) 2 respectively. The new di-mu-chloro dimers 1 and 2 undergo facile symmetrical bridge cleavage reactions with the diimine ligands 2,2'-bipyridine (bpy) and dipyridylamine (dpa) to form the six-coordinate complexes [Ru(pdto)(bpy)](ClO(4))(2) 3, [Ru(bbdo)(bpy)](ClO(4))(2) 4, [Ru(pdto)(dpa)](ClO(4))(2) 5 and [Ru(bbdo)(dpa)](ClO(4))(2) 6 and with the triimine ligand 2,2':6,2'-terpyridine (terpy) to form the unusual seven-coordinate complexes [Ru(pdto)(terpy)](ClO(4))(2) 7 and [Ru(bbdo)(terpy)](ClO(4))(2) 8. In 1 the dimeric cation [Ru(pdto)(mu-Cl)](2)(2+) is made up of two approximately octahedrally coordinated Ru(II) centers bridged by two chloride ions, which constitute a common edge between the two Ru(II) octahedra. Each ruthenium is coordinated also to two pyridine nitrogen and two thioether sulfur atoms of the tetradentate ligand. The ligand pdto is folded around Ru(II) as a result of the cis-dichloro coordination, which corresponds to a "cis-alpha" configuration [DeltaDelta/LambdaLambda(rac) diastereoisomer] supporting the possibility of some attractive pi-stacking interactions between the parallel py rings at each ruthenium atom. The ruthenium atom in the complex cations 3a and 4 exhibit a distorted octahedral coordination geometry composed of two nitrogen atoms of the bpy and the two thioether sulfur and two py/bzim nitrogen atoms of the pdto/bbdo ligand, which is actually folded around Ru(II) to give a "cis-alpha" isomer. The molecule of complex 5 contains a six-coordinated ruthenium atom chelated by pdto and dpa ligands in the expected distorted octahedral fashion. The (1)H and (13)C NMR spectral data of the complexes throw light on the nature of metal-ligand bonding and the conformations of the chelate rings, which indicates that the dithioether ligands maintain their tendency to fold themselves even in solution. The bis-mu-chloro dimers 1 and 2 show a spin-allowed but Laporte-forbidden t(2g)(6)((1)A(1g))--> t(2g)(5) e(g)(1)((1)T(1g), (1)T(2g)) d-d transition. They also display an intense Ru(II) dpi--> py/bzim (pi*) metal-to-ligand charge transfer (MLCT) transition. The mononuclear complexes 3-8 exhibit dpi-->pi* MLCT transitions in the range 340-450 nm. The binuclear complexes 1 and 2 exhibit a ligand field ((3)MC) luminescence even at room temperature, whereas the mononuclear complexes 3 and 4 show a ligand based radical anion ((3)MLCT) luminescence. The binuclear complexes 1 and 2 undergo two successive oxidation processes corresponding to successive Ru(II)/Ru(III) couples, affording a stable mixed-valence Ru(II)Ru(III) state (K(c): 1, 3.97 x 10(6); 2, 1.10 x 10(6)). The mononuclear complexes 3-7 exhibit only one while 8 shows two quasi-reversible metal-based oxidative processes. The coordinated 'soft' thioether raises the redox potentials significantly by stabilising the 'soft' Ru(II) oxidation state. One or two ligand-based reduction processes were also observed for the mononuclear complexes.     

Diimine-acetylide compounds of ruthenium: the structural and spectroscopic effects of oxidation

The reaction of Ru(Me(2)bipy)(PPh(3))(2)Cl(2) 1 with terminal alkynes HCCR in the presence of TlPF(6) leads to the formation of the vinylidene compounds [Ru(Me(2)bipy)(PPh(3))(2)Cl(=C=CHR)][PF(6)] (2) (2a, R = Bu(t); 2b, R = p-C(6)H(4)-Me; 2c, R = Ph). These compounds decompose in oxygenated solution to form the carbonyl compound [Ru(Me(2)bipy)(PPh(3))(2)Cl(CO)][PF(6)] (3), and may be deprotonated by K(2)CO(3) to give the ruthenium(II) terminal acetylide compounds Ru(Me(2)bipy)(PPh(3))(2)Cl(CC-R) (4) (4a, R = Bu(t); 4b, R = p-C(6)H(4)-Me; 4c, R = Ph). Cyclic voltammetry shows that 2a-c may also be reductively dehydrogenated to form 4a-c. 4a-c are readily oxidized to their ruthenium(III) analogues [4a](+)-[4c](+), and the changes seen in their UV/visible spectra upon performing this oxidation are analyzed. These show that whereas the UV/visible spectra of 4a-c show MLCT bands from the ruthenium atom to the bipyridyl ligand, those of [4a](+)-[4c](+) contain LMCT bands originating on the acetylide ligands. This is in agreement with the IR and ESR spectra of [4a](+)-[4c](+). The X-ray crystal structures of the redox pair 4a and [4a][PF(6)()] have been determined, allowing the bonding within the metal-acetylide unit to be analyzed, and an attempt is made to determine Lever electrochemical parameters (E(L)) for the vinylidene and acetylide ligands seen herein. Room temperature luminescence measurements on 4a-c show that the compounds are not strongly emissive.     

Self-assembly organometallic squares with terpyridyl metal complexes as bridging ligands

A series of novel heterometallic square complexes with the general molecular formulas [fac-Br(CO)(3)Re[mu-(pyterpy)(2)M]](4)(PF(6))(8) and [(dppf)Pd[mu-(pyterpy)(2)Ru]](4)(PF(6))(8)(OTf(8) (4), where M = Fe (1), Ru (2), or Os (3), pyterpy is 4'-(4' "-pyridyl)-2,2':6',2' '-terpyridine, dppf = 1,1'-bis(diphenylphosphino)ferrocene and OTf is trifluoromethanesulfonate, were prepared by self-assembly between BrRe(CO)(5) or (dppf)Pd(H(2)O)(2)(OTf)(2) and (pyterpy)(2)M(PF(6))(2). The obtained NMR spectra, IR spectra, electrospray ionization mass spectra, and elemental analyses are all consistent with the proposed square structures incorporating terpyridyl metal complexes as bridging ligands. Multiple redox processes were observed in all square complexes. All four complexes display strong visible absorptions in the region 400-600 nm, which are assigned as metal (Fe, Ru, or Os)-to-ligand (pyterpy) charge transfer (MLCT) bands. Square 3 exhibits an additional weak band at 676 nm, which is assigned to an Os-based (3)MLCT band. For each complex, the bands centered between 279 and 377 nm are assigned as pyterpy-based pi-pi bands and the Re-based MLCT band. Square 3 is luminescent in room-temperature solution, while squares 1, 2, and 4 do not have any detectable luminescence under identical experimental conditions.     

Control of photochemical, photophysical, electrochemical, and photocatalytic properties of rhenium(I) complexes using intramolecular weak interactions between ligands

Intramolecular interactions between ligands have been successfully applied as a novel tool for controlling various properties of a series of cis,trans-[Re(dmb)(CO)(2)(PR(3))(PR'(3))](+)-type complexes (dmb = 4,4'-dimethyl-2,2'-bipyridine), in the ground state and in the excited state and in the one-electron reduced form. For rhenium complexes with two triarylphosphine ligands, P(p-XPh)(3), the dmb ligand was sandwiched by four aryl rings having CH(aryl)-pi(pyridine)-pi(aryl) interactions. On the other hand, complexes with one triarylphosphine ligand and one trialkylphosphite ligand, P(OR)(3), had pi-pi and CH-pi interactions between each pyridine ring in the dmb ligand and the aryl group in the P(p-XPh)(3). Various properties of these two series of rhenium complexes were compared with those of complexes having two trialkylphosphite ligands, which do not interact through space with the dmb ligand. Properties of the complexes associated mainly with the dmb ligand are strongly affected by the intramolecular interactions: (1) UV/vis absorptions to the pi-pi and (1)MLCT excited states were both red-shifted, but (2) emission from the (3)MLCT excited state was blue-shifted; (3) the lifetime of the (3)MLCT excited state was prolonged up to 3-fold; (4) the reduction potential in the ground state was positively shifted by 110 mV with pi-pi and CH-pi interactions and by 180-200 mV with the CH-pi-pi interactions. (5) In the excited states, the oxidation power of the complex was also enhanced by the intramolecular interactions. (6) In the corresponding one-electron-reduced species cis,trans-[Re(dmb(-.)(CO)(2)(PR(3))(PR'(3))], the intramolecular interactions are maintained and strongly affected their UV/vis spectra. (7) Photocatalysis for CO(2) reduction was significantly enhanced only by the CH-pi-pi interaction.     

Bridge‐Localized HOMO‐Binding Character of Divinylanthracene‐Bridged Dinuclear Ruthenium Carbonyl Complexes: Spectroscopic,Spectroelectrochemical, and Computational Studies

The electronic properties of four divinylanthracene‐bridged diruthenium carbonyl complexes [{RuCl(CO)(PMe₃)₃}₂(μ? CH?CHArCH?CH)] (Ar=9,10‐anthracene ( 1 ), 1,5‐anthracene ( 2 ), 2,6‐anthracene ( 3 ), 1,8‐anthracene ( 4 )) obtained by molecular spectroscopic methods (IR, UV/Vis/near‐IR, and EPR spectroscopy) and DFT calculations are reported. IR spectroelectrochemical studies have revealed that these complexes are first oxidized at the noninnocent bridging ligand, which is in line with the very small ν(C?O) wavenumber shift that accompanies this process and also supported by DFT calculations. Because of poor conjugation in complex 1 , except oxidized 1⁺ , the electronic absorption spectra of complexes 2⁺ , 3⁺ , and 4⁺ all display the characteristic near‐IR band envelopes that have been deconvoluted into three Gaussian sub‐bands. Two of the sub‐bands belong mainly to metal‐to‐ligand charge‐transfer (MLCT) transitions according to results from time‐dependent DFT calculations. EPR spectroscopy of chemically generated 1⁺ – 4⁺ proves largely ligand‐centered spin density, again in accordance with IR spectra and DFT calculations results.     

The effects of the substitution on the imidazole ligand on the photochemical properties of fac-[Mn(CO)3(phen)(Imidazole)](SO3CF3) complexes

Photochemical and photophysical data are reported for a series of fac-[Mn(CO)(3)(phen)(Im-R)](SO(3)CF(3)) complexes, where phen is 1,10-phenanthroline and Im is imidazole. Intraligand and metal-to-ligand charge transfer (MLCT) transitions are observed in the electronic absorption spectra of these complexes and are sensitive to the nature of the ligand substituent. At room temperature the emission spectra show a clear progression from broad structureless MLCT to highly structured pi-pi* emission on going from R = -H, -CH(3), -C(6)H(5), to -Metro, where Metro is 2-methyl-5-nitroimidazole. Even at low temperatures the latter complexes show only the pi-pi* emission. The trend in the photophysical properties found in the emission spectra parallels the changes in the photochemical properties with the electron-donating or electron-withdrawing power of the substituent on the imidazole ligand. Although MLCT irradiation of the complexes with R = -H, -CH(3) leads to the mer-[Mn(CO)(3)(phen)(Im-R)](+) isomers, the complexes with the imidazole ligand substituted by -C(6)H(5) or -Metro release the Im-R ligand and produce the stereoretentive fac-[Mn(CO)(3)(phen)(S)](+) complexes. The stereochemical fate and mechanistic implications of the photolysis reactions are discussed in terms of the nature of ligand substitution.     

Ruthenium(II) coordination chemistry of a fused donor-acceptor ligand: synthesis, characterization, and photoinduced electron-transfer reactions of [{Ru(bpy)2}(n)(TTF-ppb)](PF6)(2n) (n = 1, 2)

A pi-extended, redox-active bridging ligand 4',5'-bis(propylthio)tetrathiafulvenyl[i]dipyrido[2,3-a:3',2'-c]phenazine (L) was prepared via direct Schiff-base condensation of the corresponding diamine-tetrathiafulvalene (TTF) precursor with 4,7-phenanthroline-5,6-dione. Reactions of L with [Ru(bpy)(2)Cl(2)] afforded its stable mono- and dinuclear ruthenium(II) complexes 1 and 2. They have been fully characterized, and their photophysical and electrochemical properties are reported together with those of [Ru(bpy)(2)(ppb)](2+) and [Ru(bpy)(2)(mu-ppb)Ru(bpy)(2)](4+) (ppb = dipyrido[2,3-a:3',2'-c]phenazine) for comparison. In all cases, the first excited state corresponds to an intramolecular TTF --> ppb charge-transfer state. Both ruthenium(II) complexes show two strong and well-separated metal-to-ligand charge-transfer (MLCT) absorption bands, whereas the (3)MLCT luminescence is strongly quenched via electron transfer from the TTF subunit. Clearly, the transient absorption spectra illustrate the role of the TTF fragment as an electron donor, which induces a triplet intraligand charge-transfer state ((3)ILCT) with lifetimes of approximately 200 and 50 ns for mono- and dinuclear ruthenium(II) complexes, respectively.     

A novel heteroditopic terpyridine-pincer ligand as building block for mono- and heterometallic Pd(II) and Ru(II) complexes

A palladium-catalyzed Stille coupling reaction was employed as a versatile method for the synthesis of a novel terpyridine-pincer (3, TPBr) bridging ligand, 4'-{4-BrC6H2(CH2NMe2)2-3,5}-2,2':6',2' '-terpyridine. Mononuclear species [PdX(TP)] (X = Br, Cl), [Ru(TPBr)(tpy)](PF6)2, and [Ru(TPBr)2](PF6)2, synthesized by selective metalation of the NCNBr-pincer moiety or complexation of the terpyridine of the bifunctional ligand TPBr, were used as building blocks for the preparation of heterodi- and trimetallic complexes [Ru(TPPdCl)(tpy)](PF6)2 (7) and [Ru(TPPdCl)2](PF6)2 (8). The molecular structures in the solid state of [PdBr(TP)] (4a) and [Ru(TPBr)2](PF6)2 (6) have been determined by single-crystal X-ray analysis. Electrochemical behavior and photophysical properties of the mono- and heterometallic complexes are described. All the above di- and trimetallic Ru complexes exhibit absorption bands attributable to (1)MLCT (Ru --> tpy) transitions. For the heteroleptic complexes, the transitions involving the unsubstituted tpy ligand are at a lower energy than the tpy moiety of the TPBr ligand. The absorption bands observed in the electronic spectra for TPBr and [PdCl(TP)] have been assigned with the aid of TD-DFT calculations. All complexes display weak emission both at room temperature and in a butyronitrile glass at 77 K. The considerable red shift of the emission maxima relative to the signal of the reference compound [Ru(tpy)2]2+ indicates stabilization of the luminescent 3MLCT state. For the mono- and heterometallic complexes, electrochemical and spectroscopic studies (electronic absorption and emission spectra and luminescence lifetimes recorded at room temperature and 77 K in nitrile solvents), together with the information gained from IR spectroelectrochemical studies of the dimetallic complex [Ru(TPPdSCN)(tpy)](PF6)2, are indicative of charge redistribution through the bridging ligand TPBr. The results are in line with a weak coupling between the {Ru(tpy)2} chromophoric unit and the (non)metalated NCN-pincer moiety.     

Absorption Spectra and Luminescence Properties of Isomeric Platinum (II) and Palladium (II) Complexes containing 1,1′-biphenyldiyl, 2-phenylpyridine,and 2,2′-bijpyridine as ligands

The absorption spectra, luminescence spectra, and luminescence lifetimes of the isomeric [M(bph)(bpy)] and [M(phpy)₂] complexes M = Pt(II) or Pd(II), bph^2? = 1,1′-biphenyl-2,2′-diyl dianion, phpy^? = 2-phenylpyridine-2′-yl anion, and bpy = 2,2′-bipyridine have been investigated and compared with those of [M(bpy)₂]²⁺ complexes and of the free protonated ligands H₂bph, Hbpy⁺, and Hphpy. In the absorption spectra, the region below 320 mm is dominated by ligand-centered (LC) transitions, whereas metal-to-ligand charge transfer (MLCT) transitions are responsible for the bands present in the near UV/VIS region. The MLCT bands move to higher energies on replacing Pt with Pd and in going from [M(bph)(bpy)] to the [M(phpy)₂] isomer. For the mixed-ligand complexes, evidence for both M → bph^2? (at higher energies) and M → bpy bands is found. The structured luminescence observed at 77 K shows lifetimes of 4.0 and 1.1 μs for [Pt(phpy)₂] and [Pt(bph)(bpy)], respectively, and 480 and 250 μs for the analogous Pd complexes. On the basis of the energy and lifetime data, the luminescence of the Pt(II) complexes is assigned to the lowest triplet MLCT excited state, whereas for the Pd complexes the luminescent state is thought to result from a mixture of MLCT and LC triplet levels.     

Luminescent alkynylplatinum(II) complexes of 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine-type ligands with ready tunability of the nature of the emissive states by solvent and electronic property modulation

A new class of luminescent alkynylplatinum(II) complexes of tridentate bis(N-alkylbenzimidazol-2'-yl)pyridines (bzimpy), [Pt(R,R'-bzimpy)(C[triple chemical bond]C-R')]X (X=PF(6), OTf), and one of their chloro precursor complexes, [Pt(R,R'-bzimpy)Cl]PF(6), have been synthesized and characterized; one of the alkynyl complexes has also been structurally characterized by X-ray crystallography. Electrochemical studies showed that the oxidation wave is alkynyl ligand-based in nature with some mixing of the metal center-based contribution, whereas the two quasi-reversible reduction couples are mainly bzimpy-based reductions. The electronic absorption and luminescence properties of the complexes have also been investigated. In solution, the high-energy and intense absorption bands are assigned as the pi-pi* intraligand (IL) transitions of the bzimpy and alkynyl ligands, whereas the low-energy and moderately intense absorptions are assigned to an admixture of metal-to-ligand charge-transfer (MLCT) (dpi(Pt)-->pi*(R,R'-bzimpy)) and ligand-to-ligand charge-transfer (LLCT) (pi(C[triple chemical bond]C-R')-->pi*(R,R'-bzimpy)) transitions. Upon variation of the electronic effects of the arylalkynyl ligands, vibronic-structured or structureless emission bands, originating from triplet metal-perturbed intraligand (IL) or an admixture of triplet metal-to-ligand charge-transfer (MLCT) and ligand-to-ligand charge-transfer (LLCT) excited states respectively, were observed in solution. Interestingly, two of the complexes showed a dual luminescence that was sensitive to the polarity of the solvents. Upon cooling from 298 K to 155 K, drastic color, UV/Vis, and luminescence changes were observed in a butyronitrile solution of 1, and were ascribed to the formation of aggregate species through PtPt and pi-pi stacking interactions. DFT and time-dependent DFT (TD-DFT) calculations have been performed to verify and elucidate the results of the electrochemical and photophysical properties.     

Switching between ligand-to-ligand charge-transfer, intraligand charge-transfer, and metal-to-ligand charge-transfer excited states in platinum(II) terpyridyl acetylide complexes induced by pH change and metal ions

A series of platinum(II) terpyridyl alkynyl complexes, [Pt{4'-(4-R1-C6H4)terpy}(C[triple chemical bond]C-C6H4-R(2)-4)]ClO4 (terpy=2,2':6',2'-terpyridyl; R1=R2=N(CH3)2 (1); R1=N(CH3)2, R2=N-[15]monoazacrown-5 (2); R1=CH3, R2=N(CH3)2 (3); R1=N(CH3)2, R2=H (4); R1=CH3, R2=H (5)), has been synthesized and the photophysical properties of the complexes have been examined through measurement of their UV/Vis absorption spectra, photoluminescence spectra, and transient absorptions. Complex 3 shows a lowest-energy absorption corresponding to a ligand-to-ligand charge-transfer (LLCT) transition from the acetylide to the terpyridyl ligand, whereas 4 shows an intraligand charge-transfer (ILCT) transition from the pi orbital of the 4'-phenyl group to the pi* orbital of the terpyridyl. Upon protonation of the amino groups in 3 and 4, their lowest-energy excited states are switched to dpi(Pt)-->pi*(terpy) metal-to-ligand charge-transfer (MLCT) states. The lowest-energy absorption for 1 and 2 may be attributed to an LLCT transition from the acetylide to the terpyridyl. Upon addition of an acid to a solution of 1 or 2, the amino group on the acetylide is protonated first, followed by the amino group on the terpyridyl. Thus, the lowest excited state of 1 and 2 can be successively switched from the LLCT state to the ILCT state and then to the MLCT state by controlling the amount of the acid added. Such switches in the excited state are fully reversible upon subsequent addition of a base to the solution. Sequential addition of alkali metal or alkaline earth metal ions and then an acid to a solution of 2 also leads to switching of its lowest excited state from the LLCT state, first to the ILCT state and then to the MLCT state. All of the complexes exhibit a transient absorption of the terpyridyl anion radical, which is present in all of the LLCT, ILCT, and MLCT states. However, the shape of the transient absorption spectrum depends on both the substitution pattern on the terpyridyl moiety and the nature of the excited state.