Conformationally gated photoinduced processes within photosensitizer-acceptor dyads based on osmium(II) complexes with triarylpyridinio-functionalized terpyridyl ligands: insights from theoretical analysis

A theoretical analysis, based on density functional theory, has been carried out to study photoinduced processes within a recently experimentally characterized (Lainé, P. P.; Bedioui, F.; Loiseau, F.; Chiorboli, C.; Campagna, S. J. Am. Chem. Soc. 2006, http://dx.doi.org/10.1021/ja058357w.) series of Os(II) bis-tpy complexes (tpy = 2,2':6'2' '-terpyridine) functionalized by 2,4,6-triarylpyridinium groups, TP+. These dyad systems, designed to produce a charge-separated state (CSS) upon light excitation, are made up of a photosensitizer unit (P, the metal complex) and a tunable acceptor unit (A, the TP+). A full ab initio characterization of the electronic and structural properties of the lowest-lying triplet excited states, as well as of the CSS, allowed for a complete rationalization of the photoinduced processes taking place within the dyads. Among salient insights, theory allowed (i) substantiation of the inner P structural planarization as the relaxation mode of the MLCT states, (ii) confirmation of the existence of a ligand-centered triplet excited state (3LC) shown to essentially involve the nitro substituent of A (TP+-NO2) and lying very close in energy to the P-centered 3MLCT state, and (iii) a demonstration that the energy of the 3LC level is independent of intercomponent tilt angle (theta1). On this basis, the occurrence of a reversible electronic energy transfer between the 3MLCT and the 3LC states could be substantiated and shown to depend on the intramolecular conformation represented by theta1, which actually governs their electronic coupling (essentially via the degree of intercomponent conjugation). These computational issues were checked against experimental data already available and the results of a specifically undertaken photophysical study. Finally, CSS formation has been confirmed by studying the spin density patterns of reduced nitro-derivatized dyads. Taken together, these findings accurately account for the different excited-state behaviors of the dyads as a function of the level of structural restriction of their intercomponent conformation (and related amplitude for torsional fluctuations), thus providing theoretical evidence of conformationally gated photoinduced electron- and energy-transfer processes.     

Primary photoinduced processes in bimetallic dyads with extended aromatic bridges. tetraazatetrapyridopentacene complexes of ruthenium(II) and osmium(II)

The photophysics of the binuclear complexes [(phen)2M(tatpp)M(phen)2]4+, where M = Ru or Os, phen = 1,10-phenanthroline, and tatpp = 9,11,20,22-tetraazatetrapyrido[3,2-a:2'3'-c:3',2'-l:2',3']pentacene, has been studied in acetonitrile and dichloromethane by femtosecond and nanosecond time-resolved techniques. The results demonstrate that complexes of different metals have different types of lowest excited state: a tatpp ligand-centered (LC) triplet in the case of Ru(II); a metal-to-ligand charge-transfer (MLCT) triplet state in the case of Os(II). The excited-state kinetics is strongly solvent-dependent. In the Ru(II) system, the formation and decay of the LC state take place, respectively, in 25 ps and ca. 5 ns in CH3CN and in 0.5 ps and 1.3 micros in CH2Cl2. These solvent effects can be rationalized on the basis of a thermally activated decay of the LC state through the upper MLCT state. In the Os(II) system, the formation and decay of the MLCT state take place, respectively, in 3.8 and 60 ps in CH3CN and in 0.5 and 4 ps in CH2Cl2. These effects are consistent with the solvent sensitivity of the MLCT energy, in terms of driving force and energy-gap law arguments. The relevance of these results for the use of ladder-type aromatic bridges as potential molecular wires is discussed.     

Highly coupled dyads based on phthalocyanine-ruthenium(II) tris(bipyridine) complexes. Synthesis and photoinduced processes

A new series of multicomponent ZnPc-Ru(bpy)(3) systems, 1a-c, consisting of a zinc-phthalocyanine linked through conjugated and/or nonconjugated connections to a ruthenium(II) tris(bipyridine) complex, has been synthesized. The ruthenium complexes 1a-c were prepared from phthalocyanines 2a-c, bearing a 4-substituted-2,2'-bipyridine ligand by treatment with [Ru(bpy)2Cl2].2H2O. Different synthetic strategies have been devised to prepare the corresponding dyad precursors (2a-c). Compound 2a, for example, with an ethenyl bridge, was synthesized by statistical condensation of 4-tert-butylphthalonitrile and 5-[(E)-2-(3,4-dicyanophenyl)ethenyl]-2,2'-bipyridine (3) in the presence of zinc chloride. Compounds 2b and 2c, having, respectively, an amide or an ethynyl bridge, were prepared following a different synthetic approach. The method involves the coupling of an appropriate 5-substituted-2,2'-bipyridine to an unsymmetrical phthalocyanine suitably functionalized with an amino (4) or an ethynyl group (5). The photophysical properties of the dyads that are ZnPc-Ru(bpy)3 1a-c and related model compounds have been determined by a variety of steady-state (i.e., fluorescence) and time-resolved methods (i.e., fluorescence and transient absorption). Clearly, intramolecular electronic interactions between the two subunits dominate the photophysical events following the initial excitation of either chromophore. These intramolecular interactions lead, in the case of photoexcited ZnPc, to faster intersystem crossing kinetics compared to a ZnPc reference, while photoexcited [Ru(bpy)3]2+) undergoes a rapid and efficient transduction of triplet excited-state energy to the Pc.     

Photoinduced processes within compact dyads based on triphenylpyridinium-functionalized bipyridyl complexes of ruthenium(II)

As an alternative to conventional charge-separation functional molecular models based on long-range ET within redox cascades, a "compact approach" has been examined. To this end, spacer elements usually inserted between main redox-active units within polyad systems have been removed, allowing extended rigidity but at the expense of enhanced intercomponent electronic communication. The molecular assemblies investigated here are of the P-(theta (1))-A type, where the theta (1) twist angle is related to the degree of conjugation between the photosensitizer (P, of {Ru(bpy)(3)}(2+) type) and the electron-acceptor (A). 4-N- and 4-N-,4'-N-(2,4,6-triphenylpyridinio)-2,2'-bipyridine ligands (A(1)-bpy and A(2)-bpy, respectively) have been synthesized to give complexes with Ru(II), 1-bpy and 2-bpy, respectively. Combined solid-state analysis (X-ray crystallography), solution studies ((1)H NMR, cyclic voltammetry) and computational structural optimization allowed verifying that theta (1) angle approaches 90 degrees within 1-bpy and 2-bpy in solution. Also, anticipated existence of strong intercomponent electronic coupling has been confirmed by investigating electronic absorption properties and electrochemical behavior of the compounds. The capability of 1-bpy and 2-bpy to undergo PET process was evaluated by carrying out their photophysical study (steady state emission and time-resolved spectroscopy at both 293 and 77 K). The conformational dependence of photoinduced processes within P-(theta (1))-A systems has been established by comparing the photophysical properties of 1-bpy (and 2-bpy) with those of an affiliated species reported in the literature, 1-phen. A complementary theoretical analysis (DFT) of the change of spin density distribution within model [1-bpy(theta (1))](-) mono-reduced species as a function of theta (1) has been undertaken and the possibility of conformationally switching emission properties of P was derived.     

Chemiluminescence from osmium(II) complexes with phenanthroline, diphosphine and diarsine ligands

The reaction of various [Os(L)(2)(L')](2+) complexes (where L and L' are phenanthroline, diphosphine or diarsine ligands) and organic reducing agents after chemical or electrochemical oxidation of the reactants produces an emission of light corresponding to MLCT transitions. In certain instances, the emission was greater than that of [Ru(bipy)(3)](2+), but the relative signals were dependent on many factors, including reagent concentration, mode of oxidation, reducing agent and the sensitivity of the photodetector over the wavelength range.     

Long-lived emission from platinum(II) terpyridyl acetylide complexes

Photoluminescence with high quantum yield and long lifetime from a triplet metal-to-ligand charge transfer (MLCT) excited state in fluid solution at room temperature has been observed for a series of platinum(II) 4'-p-tolyl-terpyridyl acetylide complexes.     

Ruthenium(II) complexes with new large-surface ligands based on electron-accepting expanded pyridiniums: insights from density functional theory

With the aim of designing new inorganic photosensitizers for photovoltaic applications, the structural and electronic properties of two Ru(II) complexes containing terpyridine-based ligands derived from expanded pyridiniums both branched—polyphenyl—and fused—polycyclic—were investigated by the means of density functional theory (DFT) and time-dependent DFT (TD-DFT). In particular, the structure and electronic absorption of the fused architectures—including the isolated ligand and its complex—were compared with those of their respective branched precursors with the aim to account for the their enhanced electronic features in the visible spectral region. The theoretical insights gained into the “large-surface” ligand and its associated complex open the route for a joint experimental and theoretical design of new inorganic photosensitizers based on fused expanded pyridiniums.     

Photophysical and anion sensing properties of platinum(II) terpyridyl complexes with phenolic ethynyl ligands

A series of platinum(ii) terpyridyl complexes with phenolic ethynyl ligands have been synthesized and characterized. Their photophysical and sensing properties towards anions such as fluoride, acetate and dihydrogenphosphate have been investigated. These complexes show a colorimetric response and fluorescence quenching in the presence of anions including fluoride, acetate and dihydrogenphosphate, and selective sensing towards fluoride in some cases. The sensing mechanism has been investigated by spectrophotometric and (1)H NMR titration.     

Synthesis and characterization of luminescent osmium(II) carbonyl complexes based on chelating dibenzoylmethanate and halide ligands

Octahedral Os(II) complexes 1-5 with formula [Os(CO)3X(dbm)] are prepared through utilization of both solid-state pyrolysis and ligand exchange reactions. These complexes exhibit prominent 3pi-pi* phosphorescence with unusually long lifetimes (29-64 micros) and high quantum yields (0.08-0.13).     

Organoplatinum(II) complexes with phosphite ligands

The phosphite complexes cis-[PtMe₂L(SMe₂)] in which L = P(OⁱPr)₃, 1a, or L = P(OPh)₃, 1b, were synthesized by the reaction of cis,cis-[Me₂Pt(μ-SMe₂)₂PtMe₂] with 2 equiv. of L. If 4 equiv. of L was used the bis-phosphite complexes cis-[PtMe₂L₂] in which L = P(OⁱPr)₃, 2a, or L = P(OPh)₃, 2b, were obtained. The reaction of cis-[Pt(p-MeC₆H₄)₂(SMe₂)₂] with 2 equiv. of L gave the aryl bis-phosphite complexes cis-[Pt(p-MeC₆H₄)₂L₂] in which L = P(OⁱPr)₃, 2a′, or L = P(OPh)₃, 2b′. Use of 1 equiv. of L in the latter reaction gave the bis-phosphite complex along with the starting complex in a 1:1 ratio.The complexes failed to react with MeI. The reaction of cis,cis-[Me₂Pt(μ-SMe₂)₂PtMe₂] with 2 equiv. of the phosphine PPh₃ gave cis-[PtMe₂(PPh₃)₂] and cis-[PtMe₂(PPh₃)(SMe₂)] along with unreacted starting material. Reaction of cis-[PtMe₂L(SMe₂)], 1a and 1b with the bidentate phosphine ligand bis(diphenylphosphino)methane, dppm = Ph₂PCH₂PPh₂, gave [PtMe₂(dppm)], 8, along with cis-[PtMe₂L₂], 2. The reaction of cis-[PtMe₂L(SMe₂)] with 1/2 equiv. of the bidentate N-donor ligand NN = 4,4′-bipyridine yielded the binuclear complexes [PtMe₂L(μ-NN)PtMe₂L] in which L = P(OⁱPr)₃, 3a, or L = P(OPh)₃, 3b.The complexes were fully characterized using multinuclear NMR (¹H, ¹³C, ³¹P, and ¹⁹⁵Pt) spectroscopy.     

Helical racemate architecture based on osmium(II)-polypyridyl complexes: Synthesis and structural characterisation

New polypyridyl osmium(II) complexes [Os(κ³-tptz)(EPh₃)₂Cl]BF₄ (E = P, 1; As, 2) with group 15 donor ligands are reported. Structural studies on the representative complex [Os(κ³-tptz)(PPh₃)₂Cl]BF₄ revealed formation of helical racemates with sidewise stacking of right and left-handed anti-parallel helical strands. Salient structural features and DNA binding studies along with binding constant [6.6 × 10³ M⁻¹] and site size [0.12] of the complex 1 with calf thymus (ct) DNA by absorption spectroscopy are described.     

Ground-state properties of ruthenium(II) and osmium(II) tin trihydride complexes: A DFT study

DFT methods have been applied for the calculation of several ground-state properties of neutral and charged ruthenium(II) and osmium(II) tin trihydride complexes bearing N-donor, P-donor and C-donor ancillary ligands in their coordination sphere. Complexes of the type M(SnH₃)(Tp)(PPh₃)P(OMe)₃, M(SnH₃)(Cp)(PPh₃)P(OMe)₃ and [M(SnH₃)(Bpy)₂P(OMe)₃]⁺ (M = Ru, Os; Tp = tris(pyrazol-1-yl)borate; Cp = cyclopentadienyl ion; Bpy = 2,2′-bipyridine) have been studied using the EDF2 and B3PW91 functionals. The same calculations have been carried out also on the corresponding [M]-CH₃ and [M]-H compounds, to compare the electronic features of the different reactive ligands coordinated to the same metal fragments. Charge distribution analyses were used to give insight into the roles of the transition metal centres and the ancillary ligands on the properties of the coordinated SnH₃ group. The molecular orbitals of the methyl- and trihydrostannyl-complexes were compared to understand the nature of the [M]-SnH₃ bond and the electronic transitions of these species.     

QTAIM study of transition metal complexes with cyclophosphazene-based multisite ligands I: Zinc(II) and nickel(II) complexes

The metal–ligand bonds in [(ZnCl₂)₂L], [NiLCl]⁺ and two isomers of [NiLCl₂] complexes with multimodal ligand L = hexakis(2-pyridyloxy)cyclotriphosphazene, cyclo-[NP(NC₅H₄O)₂]₃, are investigated at DFT level of theory using hybrid B3LYP functional. Electron density is evaluated in terms of QTAIM (quantum theory of atoms-in-molecule) topological analysis of electron density. The bonds of central transition metal atoms with phosphazene nitrogens are shorter, stronger and more polar than with the aromatic pyridine nitrogens. The atomic charges of phosphazene nitrogens are ca twice more negative than at the pyridine ones. The higher mechanical strain in the five-coordinate metal complexes than in the six-coordinate ones may be concluded.     

Porphyrinic dyads and triads assembled around iridium(III) bis-terpyridine: photoinduced electron transfer processes

Multicomponent arrays based on a central iridium(III) bis-terpyridine complex (Ir) used as assembling metal and free-base, zinc(II) or gold(III) tetraaryl-porphyrins (PH(2), PZn, PAu) have been designed to generate intramolecular photoinduced charge separation. The rigid dyads PH(2)-Ir, PZn-Ir, PAu-Ir, and the rigid and linear triads PH(2)-Ir-PAu, PZn-Ir-PAu, as well as the individual components Ir, PH(2), PZn, PAu have been synthesized and characterized by various techniques including electrochemistry. Their photophysical properties either in acetonitrile or in dichloromethane and toluene have been determined by steady-state and time-resolved methods. In acetonitrile, excitation of the triad PH(2)-Ir-PAu leads to a charge separation with an efficiency of 0.5 and a resulting charge-separated (CS) state with a lifetime of 3.5 ns. A low-lying triplet localized on PH(2) and the presence of the heavy Ir(III) ion offer the CS state an alternative deactivation path through the triplet state. The behavior of the triad PZn-Ir-PAu in dichloromethane is rather different from that of PH(2)-Ir-PAu in acetonitrile since the primary electron transfer to yield PZn(+)()-Ir(-)-PAu is not followed by a secondary electron transfer. In this solvent, both unfavorable thermodynamic and electronic parameters contribute to the inefficiency of the second electron-transfer reaction. In contrast, in toluene solutions, the triad PZn-Ir-PAu attains a CS state with a unitary yield and a lifetime of 450 ns. These differences can be understood in terms of ground-state charge-transfer interactions as well as different stabilization of the intermediate and final CS states by solvent.     

QTAIM study of transition metal complexes with cyclophosphazene-based multisite ligands II. Cobalt(II) complexes

[(CoLCl)CoCl₃], [(CoMeLCl)CoCl₃], [(CoLBr)CoBr₃], [CoLBr]⁺ and [CoMeLBr₂] complexes, L = hexakis(2-pyridyloxy)cyclotriphosphazene, MeL = hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene, in the most stable high spin states are investigated at DFT level of theory using hybrid B3LYP functional. The exchange coupling parameter evaluated using a broken symmetry treatment increases with the ligands mass. Electron density is evaluated in terms of QTAIM (Quantum Theory of Atoms-in-Molecule) topological analysis of electron density. The bonds of central Co atoms with phosphazene nitrogens are shorter, stronger and more polar than with the aromatic pyridine nitrogens and their higher ellipticities may be explained by the π contribution from the phosphazene ring. The atomic charges of phosphazene nitrogens are ca. twice more negative than at the pyridine ones.     

Photosensitized generation of singlet oxygen from ruthenium(II) and osmium(II) bipyridyl complexes

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, kq, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1-7.7) x 10(9) dm3 mol(-1) s(-1). Efficiencies of singlet oxygen production, fDeltaT, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10-0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, kq, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, kq were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, kq1, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, kq3, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin-orbit coupling constant of the central metal.     

Copper(II) chloride complexes with multimodal ligands based on the cyclotriphosphazene platform

The reaction of hexakis(2-pyridyloxy)cyclotriphosphazene (L) and hexakis(4-methyl-2-pyridyloxy)cyclotriphosphazene (MeL) with copper(ii) chloride afford the complexes [CuLCl(2)], [(CuCl(2))(2)(MeL)], [CuLCl]PF(6) and [Cu(MeL)Cl]PF(6). The single-crystal X-ray structure of [CuLCl(2)] shows the copper ion to be in a square based pyramidal distorted trigonal bipyramidal (SBPDTBP) environment (tau= 0.47) with L acting as a kappa(3)N donor, coordinating via the nitrogen atoms from two non-geminal pyridyloxy pendant arms, a nitrogen atom in the phosphazene ring and two chloride ions. In the dimetallic complex, [(CuCl(2))(2)(MeL)], the geometry about both (symmetry related) copper(ii) centres is also SBPDTBP (tau= 0.57) with a 'N(3)Cl(2)' donor set. In the monocation of [CuLCl]PF(6), L acts as a kappa(5)N donor, bonding to the copper(ii) centre through the nitrogen atoms of four pyridyloxy pendant arms, a phosphazene ring nitrogen atom and a chloride ion to give an elongated rhombic octahedral coordination sphere. The phosphazene ring atoms remain virtually coplanar in all three structures as a consequence of the phenoxy-hinge, which links the pyridine pendant donors to the cyclotriphosphazene platform, allowing the formation of six-membered chelate rings. The spectroscopic (mass spectral, EPR and electronic) and magnetic properties of the complexes are discussed. The EPR and variable temperature magnetic susceptibility results for the dicopper complex, [(CuCl(2))(2)(MeL)], point to a very weak electronic interaction between the metal atoms.     

DFT/TDDFT study on the electronic structures and optoelectronic properties of several red-emitting osmium(II) complexes with different P^P ancillary ligands

The ground and excited state geometries of several red-emitting phosphors (N^N)(2)Os(P^P) [where N^N = 5-(1-isoquinolyl)-1,2,4-triazoles, P^P = bis(dimethylphosphino)methylene(dmpm) (1); P^P = cis-1,2-bis-(dimethylphosphino)ethene(dmpe) (2); P^P = 1,2-bis(dimethylphosphino)benzene(dmpb) (3); P^P = 1,2-bis(dimethylphosphino)naphthalene(dmpn) (4); P^P = 1,2-bis(dimethylphosphino)-4-cyano-benzene(dmpcb) (5)] have been investigated by using the density functional theory (DFT) methods. The calculated results indicate that, for the studied complexes, the electron-transporting performance is better than the hole-transporting performance. The alteration of cis-P^P ancillary ligands with different conjugation lengths and substituents has an impact on the optoelectronic properties of these complexes, especially the electron-withdrawing group -CN in 5. The calculated energy gaps are nearly the same for complexes 1 to 4 (3.34 eV), while for 5, the HOMO and LUMO energies are lowered and the energy gap increases (3.42 eV). The absorption of 1 is red shifted, while that of 5 is blue shifted compared with the absorptions of 2, 3, and 4, which have similar absorptions. Complexes 2, 3, and 4 have almost identical emission wavelength 699 nm, while 1 (715 nm) and 5 (735 nm) are red shifted. The calculated electron affinities and reorganization energies indicate that complex 5 is the easiest for electron injection and has the best electron-transporting performance.