Competition between energy transfer and interligand electron transfer in porphyrin-osmium(II) bis(2,2':6',2' '-terpyridine) dyads

Rapid intramolecular energy transfer occurs from a free-base porphyrin to an attached osmium(II) bis(2,2':6',2' '-terpyridine) complex, most likely by way of the F?rster dipole-dipole mechanism. The initially formed metal-to-ligand, charge-transfer (MLCT) excited-singlet state localized on the metal complex undergoes very fast intersystem crossing to form the corresponding triplet excited state ((3)MLCT). This latter species transfers excitation energy to the (3)pi,pi* triplet state associated with the porphyrin moiety, such that the overall effect is to catalyze intersystem crossing for the porphyrin. Interligand electron transfer (ILET) to the distal terpyridine ligand, for which there is no driving force, competes poorly with triplet energy transfer from the proximal (3)MLCT to the porphyrin. Equipping the distal ligand with an ethynylene residue provides the necessary driving force for ILET and this process now competes effectively with triplet energy transfer to the porphyrin. The rate constants for all the relevant processes have been derived from laser flash photolysis studies.     

Comparison of the photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) and the corresponding ethynylated derivative

The photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) have been recorded over a wide temperature range. An emission band is observed and attributed to radiative decay of the lowest-energy metal-to-ligand, charge-transfer (MLCT) triplet state. This triplet is coupled to two other triplet states that lie at higher energy. The second triplet, believed to be of MLCT character, is reached by crossing a barrier of only 640 cm(-1), but the highest-energy triplet, considered to be of metal-centered (MC) character, is separated from the lowest-energy MLCT triplet by a barrier of 3500 cm(-1). Analysis of the emission spectrum shows that both low- and high-frequency modes are involved in the decay process, while weak emission is seen from the second excited triplet state. The magnitude of the low- and high-frequency modes depends on temperature in fluid solution but not in a KBr disk. Apart from a substantial lowering of the triplet energy, the photophysical properties are relatively insensitive to the presence of an ethynylene substituent at the 4' position of each terpyridine ligand. However, the barrier to reaching the MC triplet is markedly reduced, and the vibrational modes become less sensitive to changes in temperature.     

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(II) bis(2,2':6',2'-terpyridine) molecular array

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2'-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).     

Photophysical properties of binuclear ruthenium(ii) bis(2,2':6',2''-terpyridine) complexes built around a central 2,2'-bipyrimidine receptor

A binuclear complex has been synthesized having ruthenium(ii) bis(2,2':6',2'-terpyridine) terminals attached to a central 2,2'-bipyrimidine unit via ethynylene groups. Cyclic voltammetry indicates that the substituted terpyridine is the most easily reduced subunit and the main chromophore involves charge transfer from the metal centre to this ligand. The resultant metal-to-ligand, charge-transfer (MLCT) triplet state is weakly emissive and has a lifetime of 60 ns in deoxygenated solution at room temperature. The luminescence yield and lifetime increase with decreasing temperature in a manner that indicates the lowest-energy MLCT triplet couples to at least two higher-energy triplets. Cations can bind to the central bipyrimidine unit, forming both 1:1 and 1:2 (ligand:metal) complexes as confirmed by electrospray MS analysis. The photophysical properties depend on the number of bound cations and on the nature of the cation. In the specific case of binding zinc(ii) cations, the 1:1 complex has a triplet lifetime of 8.0 ns while that of the 1:2 complex is 1.8 ns. The 1:1 complexes formed with Ba(2+) and Mg(2+) are more luminescent than is the parent compound while the 1:2 complexes are much less luminescent. It is shown that the coordinated cations raise the reduction potential of the central bipyrimidine unit and thereby increase the activation energy for coupling with the metal-centred state. Complexation also introduces a non-emissive intramolecular charge-transfer (ICT) state that couples to the lowest-energy MLCT triplet and provides an additional non-radiative decay route. The triplet state of the 1:2 complex formed with added Zn(2+) cations decays preferentially via this ICT state.     

Electron delocalization in a ruthenium(II) Bis(2,2':6',2' '-terpyridyl) complex

Photophysical properties have been recorded for a ruthenium(II) bis(2,2':6',2' '-terpyridine) complex bearing a single ethynylene substituent. The target compound is weakly emissive in fluid solution at room temperature, but both the emission yield and lifetime increase dramatically as the temperature is lowered. As found for the unsubstituted parent complex, the full temperature dependence indicates that the lowest-energy triplet state couples to two higher-energy triplets and to the ground state. Luminescence occurs only from the lowest-energy triplet state, but the radiative and nonradiative decay rates indicate that electron delocalization occurs at the triplet level. Comparison of the target compound with the parent complex indicates that the ethynylene group reduces the size of the electron-vibrational coupling element for nonradiative decay of the lowest-energy triplet state. Although other factors are affected by substitution, this is by far the most important feature with regard to stabilization of the triplet state.     

Temperature-induced switching of the mechanism for intramolecular energy transfer in a 2,2':6',2' '-Terpyridine-based Ru(II)-Os(II) trinuclear array

The synthesis and photophysical properties of a linear 2,2':6',2' '-terpyridine-based trinuclear Ru(II)-Os(II) nanometer-sized array are described. This array comprises two bis(2,2':6',2' '-terpyridine) ruthenium(II) terminals connected via alkoxy-strapped 4,4'-diethynylated biphenylene units to a central bis(2,2':6',2' '-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the "synthesis at metal" approach, and the Ru(II)-Ru(II) separation is ca. 50 A. Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures. Forster-type energy transfer prevails in a glassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at higher temperatures. This latter process, which is weakly activated, involves long-range superexchange interactions between the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer. Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. The CT triplet does not undergo direct charge recombination to form the ground state but transfers energy, possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains the geometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occurs across a particular torsion angle of 37 degrees . The probability of triplet energy transfer depends on temperature but always exceeds 75%.     

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

Synthesis and properties of the elusive ruthenium(II) complexes of 4'-cyano-2,2':6',2' '-terpyridine

The heteroleptic and homoleptic ruthenium(II) complexes of 4'-cyano-2,2':6',2' '-terpyridine are synthesized by palladium catalyzed cyanation of the corresponding Ru(II) complexes of 4'-chloro-2,2':6',2' '-terpyridine. The introduction of the strongly electron-withdrawing cyano group into the Ru(tpy)(2)(2+) moiety dramatically changes its photophysical and redox properties as well as prolongs its room temperature excited-state lifetime.     

4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl (R = H, OEt, Ph, Cl, CN) platinum(II) phenylacetylide complexes: synthesis and photophysics

A series of new square-planar 4'-(5'-R-pyrimidyl)-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complexes ( 1a- 5a) bearing different substituents (R = H, OEt, Ph, Cl, CN) on the pyrimidyl ring have been synthesized and characterized. The electronic absorption, photoluminescence, and triplet transient difference absorption spectra were investigated. All of the complexes exhibit broad, moderately strong absorption between 400 and 500 nm that can be tentatively assigned to the metal-to-ligand charge transfer ( (1)MLCT) transition, possibly mixed with some ligand-to-ligand charge transfer ( (1)LLCT) character. Photoluminescence arising from the (3)MLCT state was observed both in fluid solutions at room temperature and in a rigid matrix at 77 K. The (1)MLCT/ (1)LLCT absorption bands and the (3)MLCT emission bands for 1a- 5a red-shift in comparison to those of the corresponding 4'-toly-2,2':6',2'-terpyridyl platinum(II) phenylacetylide complex. In addition, the energies of the (1)MLCT/ (1)LLCT absorption and the (3)MLCT emission bands exhibit a linear correlation with the Hammett constant (sigma p) of the 5'-substituent on the pyrimidyl ring. The lifetime of the (3)MLCT emission at room temperature is governed by the energy gap law. The triplet transient difference absorption spectra of 1a- 5a exhibit a broad absorption band from 500 to 800 nm, and a bleaching band between 420 and 500 nm. Complex 5a, which contains the -CN substituent, exhibits a lower-energy triplet absorption band at 785 nm and a shorter lifetime (130 ns) in CH 3CN than 2a, which has the -OEt substituent, does (lambda T1-Tn (max) = 720 nm, tau T = 660 ns). The triplet excited-state absorption coefficients at the band maxima for 1a- 5a vary from 36 600 L.mol (-1).cm (-1) to 115 090 L.mol (-1).cm (-1), and the quantum yields of the triplet excited-state formation range from 0.19 to 0.66. All complexes exhibit a moderate nonlinear transmission for nanosecond laser pulses at 532 nm. Moreover, these complexes can generate singlet oxygen efficiently in air-saturated CH 3CN solutions, with the singlet oxygen generation quantum yield (Phi Delta) varying from 0.24 to 0.46.     

Synthesis, luminescence, and electrochemical studies of tris(homoleptic) ruthenium(II) and osmium(II) complexes of 6'-tolyl-2,2':4',2'-terpyridine

The synthesis and photophysical and electrochemical properties of tris(homoleptic) complexes [Ru(tpbpy)3](PF6)2 (1) and [Os(tpbpy)3](PF6)2 (2) (tpbpy = 6'-tolyl-2,2':4',2' '-terpyridine) are reported. The ligand tpbpy is formed as the side product during the synthesis of 4'-tolyl-2,2':6',2' '-terpyridine (ttpy) and characterized by single-crystal X-ray diffraction: monoclinic, P21/c. The tridentate tpbpy coordinates as a bidentate ligand. The complexes 1 and 2 exhibit two intense absorption bands in the UV region (200-350 nm) assignable to the ligand-centered (1LC) pi-pi* transitions. The ruthenium(II) complex exhibits a broad absorption band at 470 nm while the osmium(II) complex exhibits an intense absorption band at 485 nm and a weak band at 659 nm assignable to the MLCT (dpi-pi*) transitions. A red shifting of the dpi-pi* MLCT transition is observed on going from the Ru(II) to the Os(II) complex as expected from the high-lying dpi Os orbitals. These complexes exhibit ligand-sensitized emission at 732 and 736 nm, respectively, upon light excitation onto their MLCT band through excitation of higher energy LC bands at room temperature. The MLCT transitions and the emission maxima of 1 and 2 are substantially red-shifted compared to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2. The emission of both the complexes in the presence of acid is completely quenched indicating that the emission is not due to the protonation of the coordinated ligands. Our results indicate the occurrence of intramolecular energy transfer from the ligand to the metal center. Both the complexes undergo quasi-reversible metal-centered oxidation, and the E1/2 values for the M(II)/M(III) redox couples (0.94 and 0.50 V versus Ag/Ag+ for 1 and 2, respectively) are cathodically shifted with respect to that of [Ru(bpy)3](PF6)2 and [Os(bpy)3](PF6)2 (E1/2 = 1.28 and 1.09 V versus Ag/Ag+, respectively). The tris(homoleptic) Ru(II) and Os(II) complexes 1 and 2 could be used to construct polynuclear complexes by using the modular synthetic approach in coordination compounds by exploiting the coordinating ability of the pyridine substituent. Furthermore, these complexes offer the possibility of studying the influence of electron-withdrawing and electron-donating substituents on the photophysical properties of Ru(II) and Os(II) polypyridine complexes.     

Cyclometalated ruthenium(II) complexes with a bis-carbene CCC-pincer ligand

The first series of cyclometalated ruthenium complexes with a CCC-pincer bis-carbene ligand have been obtained as bench-stable compounds. Single-crystal X-ray analysis of one of these complexes with 4'-di-p-anisylamino-2,2':6',2'-terpyridine is presented. The Ru(II/III) redox potentials and MLCT absorptions of these complexes can be varied by attaching an electron-donating or -withdrawing group on the noncyclometalating ligand.     

Tuning the gas phase redox properties of copper(II) ternary complexes of terpyridines to control the formation of nucleobase radical cations

Electrospray ionization (ESI) tandem mass spectrometry (MS/MS) of ternary copper(II) complexes of [Cu(terpyX)(M)]2+ (where terpyX = is a substituted 2,2':6',2'-terpyridine ligand; M = the nucleobases: adenine, guanine, thymine and cytosine) was examined as a means of forming radical cations of nucleobases in the gas phase. The following substituents were examined: 4'-NMe2-2,2':6',6'-terpyridine; 4'-OH-2,2':6',6'-terpyridine; 4'-F-2,2':6',6'-terpyridine; 2,2':6',6'-terpyridine; 4'-Cl-2,2':6',6'-terpyridine; 4'-Br-2,2':6',6'-terpyridine; 4'-CO2H-2,2':6',6'-terpyridine; 4'-NO2-2,2':6',6'-terpyridine and 6,6'-dibromo-2',2:6',2'-terpyridine. Each of the ternary complexes [Cu(terpyX)(M)]2+ was mass selected and subjected to collision induced dissociation (CID) in a quadrupole ion trap. The types of fragmentation reactions observed for these complexes depend on the nature of the substituent on the terpyridine ligand, while the yields of the radical cations of the nucleobases follow the order of their ionization energies (IEs): G (lowest IE) > A > C > T (highest IE). In general, radical cation formation is favoured for electron withdrawing substituents (e.g. NO2) while loss of the neutral nucleobase is favoured for electron donating substituents (e.g. NMe2). Loss of the protonated nucleobase is a major fragmentation pathway for the OH substituted terpyridine system, consistent with its ability to bind to a metal centre as a deprotonated ligand. Crystal structure determinations of (6,6'-dibromo-2',2:6',2'-terpyridine)bis(nitrato)copper(II) and diaqua(4'-oxo-2,2':6',6'-terpyridine)copper(II) nitrate monohydrate were performed and correlated with the ESI results.     

Structures, spectroscopic properties and redox potentials of quaterpyridyl Ru(II) photosensitizer and its derivatives for solar energy cell: a density functional study

Scalar relativistic density functional theory (DFT) has been used to explore the spectroscopic and redox properties of Ruthenium-type photovoltaic sensitizers, trans-[Ru((R)L)(NCS)(2)] ((R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-2,2' : 6',2' : 6',2'-quaterpyridine, R = H (1), Me (2), (t)Bu (3) and COOH (4); (R)L = 4,4'-di-R-4',4'-bis(carboxylic acid)-cycloquaterpyridine, R = COOH (5)). The geometries of the molecular ground, univalent cationic and triplet excited states of 1-5 were optimized. In complexes 1-4, the quaterpyridine ligand retains its planarity in the molecular, cationic and excited states, although the C≡N-Ru angle representing the SCN → Ru coordination approaches 180° in the univalent cationic and triplet excited states. The theoretically designed complex 5 displays a curved cycloquaterpyridine ligand with significantly distorted SCN → Ru coordination. The electron spin density distributions reveal that one electron is removed from the Ru/NCS moieties upon oxidation and the triplet excited state is due to the Ru/NCS → polypyridine charge transfer (MLCT/L'LCT). The experimental absorption spectra were well reproduced by the time-dependent DFT calculations. In the visible region, two MLCT/L'LCT absorption bands were calculated to be at 652 and 506 nm for 3, agreeing with experimental values of 637 and 515 nm, respectively. The replacement of the R- group with -COOH stabilizes the lower-energy unoccupied orbitals of π* character in the quaterpyridine ligand in 4. This results in a large red shift for these two MLCT/L'LCT bands. In contrast, the lower-energy MLCT/L'LCT peak of 5 nearly disappears due to the introduction of cycloquaterpyridine ligand. The higher energy bands in 5 however become broader and more intense. As far as absorption in the visible region is concerned, the theoretically designed 5 may be a very promising sensitizer for DSSC. In addition, the redox potentials of 1-5 were calculated and discussed, in conjunction with photosensitizers such as cis-[Ru(L(1))(2)(X)(2)] (L(1) = 4,4'-bis(carboxylic acid)-2,2'-bipyridine; X = NCS(-) (6), Cl(-) (7) and CN(-) (8)), cis-[Ru(L(1)')(2)(NCS)(2)] (L(1)' = 4,7-bis(carboxylic acid)-1,10-phenanthroline, 9), [NH(4)][Ru(L(2))(NCS)(3)] (L(2) = 4,4',4'-tris(carboxylic acid)-2,2' : 6',2'-terpyridine, 10) and [Ru(L(2))(NCS)(3)](-) (11).     

Water-soluble alkylated bis{4'-(4-pyridyl)-2,2':6',2'-terpyridine}ruthenium(II) complexes for use as photosensitizers in water oxidation: a complementary experimental and TD-DFT investigation

A series of N-alkylated derivatives [RuL(2)][PF(6)](4) has been prepared from [Ru(pytpy)(2)][PF(6)](2) (N-alkyl substituent = 4-cyanobenzyl, 4-nitrobenzyl, ethyl, cyanomethyl, allyl, octyl). Solution NMR spectroscopic, electrochemical and photophysical properties are reported, along with the single crystal structure of [Ru(4)(2)][PF(6)](4)·H(2)O (4 = 4'-(4-(1-ethylpyridinio))-2,2':6',2'-terpyridine). Anion exchange leads to the water-soluble [RuL(2)][HSO(4)](4) salts (N-alkyl substituent = benzyl, 4-cyanobenzyl, 4-nitrobenzyl, ethyl, cyanomethyl, allyl, octyl) and the NMR spectroscopic signatures of pairs of hexafluoridophosphate and hydrogensulfate salts are compared. The change in anion has little effect on the energies of absorptions in the electronic spectra, although for all complexes, decreases in extinction coefficients are observed. The emission spectra and lifetimes for the hexafluoridophosphate and hydrogensulfate salts show similar trends; all exhibit an emission close to 720-730 nm (λ(ex) = 510 nm). For a given ligand, L, the emission lifetime decreases on going from [RuL(2)][PF(6)](4) to [RuL(2)][HSO(4)](4). However, trends are the same for both salts, i.e. the longest lived emitters are observed for N-ethyl, N-octyl and N-benzyl derivatives, and the shortest lived emitters are those containing cyano or nitro groups. Significantly, in the absorption spectra of the complexes, there is little variation in the energy of the MLCT band, suggesting that the character of the ligand orbital involved in the transition contains no character from the N-substituent. We have addressed this by carrying out a complementary DFT and TD-DFT study. Calculated absorption spectra predict a red shift in λ(max) on going from [Ru(pytpy)(2)](2+) to [RuL(2)](4+), and little variation in λ(max) within the series of [RuL(2)](4+) complexes; these results agree with experimental observations. Analysis of the compositions of the MOs involved in the MLCT transitions explain the experimental observations, showing that there is no contribution from orbitals on the N-alkyl substituents, consistent with the fact that the nature of the N-substituents has little influence on the energy of the MLCT band. The theoretical results also reveal satisfactory agreement between calculated and crystallographic data for [Ru(1)(2)](4+) (1 = 4'-(4-(1-benzylpyridinio))-2,2':6',2'-terpyridine) and [Ru(4)(2)](4+).     

Photophysics and optical limiting of platinum(II) 4'-arylterpyridyl phenylacetylide complexes

A series of platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl phenylacetylide complexes (5-8) with 4'-naphthyl, 4'-phenanthryl, 4'-anthryl, and 4'-pyrenyl substituents have been synthesized and characterized. The emission properties of these complexes and their corresponding platinum(II) 4'-aryl-2,2':6',2' '-terpyridyl chloride complexes (1-4) at room temperature and 77 K have been systematically investigated. Except for the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex that emits from an admixing state consisting of metal-to-ligand charge-transfer (3MLCT), intraligand charge-transfer (3ILCT), and 3pi,pi characters, emissions of 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl phenylacetylide complexes all originate from a 3MLCT-dominant state. The emission lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl phenylacetylide complex (8) is longer than 2 mus at room temperature, and more than 300 mus at 77 K, while the other three complexes possess an emission lifetime of 200-400 ns at room temperature and tens of microseconds at 77 K. Replacing the chloride ligand in the 4'-naphthyl, 4'-phenanthryl, and 4'-anthryl-2,2':6',2' '-terpyridyl chloride complexes by a phenylacetylide ligand significantly increases the emission efficiency by an order of magnitude, and the emission lifetimes become longer. In contrast, such an alternation has no pronounced effect on the emission efficiency and lifetime of the 4'-pyrenyl-2,2':6',2' '-terpyridyl complexes. In the transient difference absorption (TA) spectra of 5 and 6, a moderately intense absorption band from 470 to 830 nm and a bleaching band between 400 and 470 nm were observed. For 7, the TA spectrum features a narrow, weak bleaching band at approximately 380 nm and a strong, narrow band at approximately 420 nm, as well as a broad, structureless band from 470 to 750 nm. In addition, a fourth, positive band appears above 800 nm. Complex 8 exhibits a strong, narrow bleaching band at approximately 340 nm and a broad, positive band extending from 370 to 830 nm, with the band maximum appearing at approximately 520 nm. The lifetimes obtained from the kinetic transient absorption measurement coincide with those from the kinetic emission measurement, indicating that the transient absorption originates from the same excited state that emits or, alternatively, from a state that is in equilibrium with the emitting state. All complexes exhibit optical limiting for 4.1 ns laser pulses at 532 nm, with 8 giving rise to the strongest optical limiting, presumably because of the much longer triplet excited-state lifetime and the stronger transient absorption at 532 nm.     

Ruthenium(II) complexes with improved photophysical properties based on planar 4'-(2-pyrimidinyl)-2,2':6',2' '-terpyridine ligands

A series of new tridentate polypyridine ligands, made of terpyridine chelating subunits connected to various substituted 2-pyrimidinyl groups, and their homoleptic and heteroleptic Ru(II) complexes have been prepared and characterized. The new metal complexes have general formulas [(R-pm-tpy)Ru(tpy)]2+ and [Ru(tpy-pm-R)2]2+ (tpy = 2,2':6',2' '-terpyridine; R-pm-tpy = 4'-(2-pyrimidinyl)-2,2':6',2' '-terpyridine with R = H, methyl, phenyl, perfluorophenyl, chloride, and cyanide). Two of the new metal complexes have also been characterized by X-ray analysis. In all the R-pm-tpy ligands, the pyrimidinyl and terpyridyl groups are coplanar, allowing an extended delocalization of acceptor orbital of the metal-to-ligand charge-transfer (MLCT) excited state. The absorption spectra, redox behavior, and luminescence properties of the new Ru(II) complexes have been investigated. In particular, the photophysical properties of these species are significantly better compared to those of [Ru(tpy)2]2+ and well comparable with those of the best emitters of Ru(II) polypyridine family containing tridentate ligands. Reasons for the improved photophysical properties lie at the same time in an enhanced MLCT-MC (MC = metal centered) energy gap and in a reduced difference between the minima of the excited and ground states potential energy surfaces. The enhanced MLCT-MC energy gap leads to diminished efficiency of the thermally activated pathway for the radiationless process, whereas the similarity in ground and excited-state geometries causes reduced Franck Condon factors for the direct radiationless decay from the MLCT state to the ground state of the new complexes in comparison with [Ru(tpy)2]2+ and similar species.     

Theoretical Study of Electron Density Delocalization in the Excited States of Transition Metal Complexes

This paper presents a theoretical study of electron density delocalization effects over an electron-accepting ligand in metal-to-ligand charge-transfer (MLCT) complexes in the excited states, where the ligand is 4,4'-X₂-2,2'-bpy (X = H, NH₂, CH₃, Ph, Cl, CO₂Et, NO₂, bpy = 2,2'-bipyridine) or terpy (2,2':6',2'-terpyridine). Optimal geometry calculations are performed for neutral ligand molecules and their radical anions modeling the state of the ligands during MLCT excitations. Spin density distribution over atoms in the radical anions is used as a measure of the degree of delocalization. The role of spin density distribution in excitation-induced changes of geometrical parameters of the ligands is considered.     

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.     

Direct measurement of excited-state intervalence transfer in [(tpy)Ru(III)(tppz(*-))Ru(II)(tpy)](4+) by time-resolved near-infrared spectroscopy

Extension of time-resolved infrared (TRIR) measurements into the near-infrared region has allowed the first direct measurement of a mixed-valence band in the metal-to-ligand charge transfer (MLCT) excited state of a symmetrical ligand-bridged complex. Visible laser flash excitation of [(tpy)Ru(tppz)Ru(tpy)]4+ (tppz is 2,3,5,6-tetrakis(2-pyridyl)pyrazine; tpy is 2,2':6',6' '-terpyridine) produces the mixed-valence, MLCT excited state [(tpy)RuIII(tppz*-)RuII(tpy)]4+* with the excited electron localized on the bridging tppz ligand. A mixed-valence band appears at numax = 6300 cm-1 with a bandwidth-at-half- maximum, Deltanu1/2 = 1070 cm-1. In the analogous ground-state complex, [(tpy)Ru(tppz)Ru(tpy)]5+, a mixed-valence band appears at numax = 6550 cm-1 with Deltanu1/2 = 970 cm-1 which allows a comparison to be made of electronic coupling across tppz0 and tppz*- as bridging ligands.     

Electronic structures and absorption spectra of linkage isomers of trithiocyanato (4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) ruthenium(II) complexes: a DFT study

Black dye (BD), isomer 1 ([Ru(II)(H3-tctpy)(NCS)3](-1), where H3-tctpy = 4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) is known to be an excellent sensitizer for dye-sensitized solar cells and exhibits a very good near-IR photo response, compared to other ruthenium dyes. Because isothiocyanate is a linear ambidentate ligand, BD has three other linkage isomers, [Ru(H3-tctpy)(NCS)2(SCN)](-1), isomer 2 and 2', and [Ru(H3-tctpy))(SCN)3](-1), isomer 3. In this study, we have calculated the geometry of BD and its isomers by DFT. Further, we have analyzed the bonding in these isomers using NBO methods. TDDFT calculations combined with scalar relativistic zero-order regular approximations (SR-ZORA) have been carried out to simulate the absorption spectra. Calculations have been performed for the isomers both in vacuo and in solvent (ethanol). The inclusion of the solvent is found to be important to obtain spectra in good agreement with the experiment. The first absorption bands are dominated by the metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT).