Vectorial Electron Transfer in Donor–Photosensitizer–Acceptor Triads Based on Novel Bis‐tridentate Ruthenium Polypyridyl Complexes

The first examples of rodlike donor–photosensitizer–acceptor arrays based on bis‐2,6‐di(quinolin‐8‐yl)pyridine Ru^II complexes 1 a and 3 a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time‐resolved absorption spectroscopy reveals long‐lived, photoinduced charge‐separated states (τ_CSS ( 1 a )=140 ns, τ_CSS ( 3 a )=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (≥50 % for complex 1 a and ≥95 % for complex 3 a ) are unprecedented for bis‐tridentate Ru^II polypyridyl complexes. This is attributed to the long‐lived excited state of the [Ru(dqp)₂]²⁺ complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.     

Sensing of 2,4,6‐Trinitrotoluene (TNT) and 2,4‐Dinitrotoluene (2,4‐DNT) in the Solid State with Photoluminescent RuII and IrIII Complexes

A series of metal–organic chromophores containing Ru^II or Ir^III were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long‐lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution‐based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal‐to‐ligand charge‐transfer (MLCT) excited state quenching upon addition of TNT and 2,4‐dinitrotoluene (2,4‐DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink‐jet printing and solid‐state titrations was used for the solid‐state detection of TNT; achieving a limit‐of‐quantitation (LOQ) as low as 5.6 ng cm^?2. The combined effect of a long‐lived excited state and an energetically favorable driving force for the PET process makes the Ru^II and Ir^III MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high‐energy compounds.     

Electron Storage Capability and Singlet Oxygen Productivity of a RuII Photosensitizer Containing a Fused Naphthaloylenebenzene Moiety at the 1,10-Phenanthroline Ligand

As a novel rylene type dye a diimine ligand with a fully rigid and extended π-system in its backbone was prepared by directly fusing a 1,10-phenanthroline building block with 1,8-naphthalimide. The corresponding heteroleptic ruthenium photosensitizer bearing one biipo and two tbbpy ligands was synthesized and extensively analyzed by a combination of NMR, single crystal X-ray diffraction, steady-state absorption and emission, time-resolved spectroscopy and different electrochemical measurements supported by time-dependent density functional theory calculations. The cyclic and differential pulse voltammograms revealed, that the naphthaloylenebenzene moiety enables an additional second reduction of the ligand. Moreover, this ligand possesses a very broad absorption in the visible region. In the Ru^II complex this causes an overlap of ligand-centered and metal-to-ligand charge transfer transitions. The emission of the complex is clearly redshifted compared to the ligand emission with very long-lived excited states lifetimes of 1.7 and 24.7 μs in oxygen-free acetonitrile solution. This behavior is accompanied by a surprisingly high oxygen sensitivity. Finally, this photosensitizer was successfully applied for the effective evolution of singlet oxygen challenging some of the common Ru^II prototype complexes.     

Long‐Lived,Directional Photoinduced Charge Separation in RuII Complexes Bearing Laminate Polypyridyl Ligands

Ru^II complexes incorporating both amide‐linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz), tetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐h:2′′′,3′′′‐j]phenazine (tpphz), and 9,11,20,22‐tetraazatetrapyrido[3,2‐a:2′,3′‐c:3′′,2′′‐l:2′′′,3′′′]‐pentacene (tatpp) exhibit long‐lived charge separated (CS) states, which have been analyzed using time‐resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal “bpy‐like” portion and central (or distal) “phenazine‐like” portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCT_prox or ³MLCT_dist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a Zn^II ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCT_dist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCT_dist state. In tpphz complexes, Zn^II coordination can reorder the energy of the ³MLCT_prox and ³MLCT_dist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron–hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.     

Tuning the Excited State of Photoactive Building Blocks for Metal‐Templated Self‐Assembly

The reaction of 2,2′:4,4′′:4′,4′′′‐quaterpyridyl (qtpy), with d⁶ ruthenium(II) (Ru^II), and rhenium(I) (Re^I) metal centers has been investigated. The pendant pyridyl groups on the products have also been methylated to produce a second series of complexes containing coordinated Meqtpy²⁺. The absorption spectra of the complexes are dominated by intraligand and charge‐transfer bands. The ruthenium(II) complexes display broad unstructured luminescence consistent with emission from a Ru(d)→diimine(π*) manifold in acetonitrile solutions. In aqueous solutions, their emissions are weaker and the lifetimes are shorter. This effect is particularly acute for complexes incorporating coordinated dipyridylpyrazine, dppz, ligands. Although the emission of the ruthenium(II) complexes containing Meqtpy²⁺ is generally shorter than their qtpy analogs, it is notable that solvent‐dependent effects are much less intense. The rhenium(I) complexes also display broad unstructured luminescence but, compared with the ruthenium(II) systems, they have a relatively short lifetime in acetonitrile. Electrochemical studies reveal that all of the Ru^II complexes display chemically reversible metal‐based oxidations. Re^I complexes only display irreversible metal‐based oxidations. In most cases, the reduction processes were not fully chemically reversible. The electrochemical and optical studies reveal that the nature of the lowest excited state of these complexes—particularly, the systems incorporating dppz—is highly dependent on the nature of the coordinated ligands. Calculations indicate that, although the excited state of most of the complexes is centered on the qtpy or Meqtpy²⁺ ligands, the excited state of the complexes containing dppz ligands is switched away from the dppz by qtpy methylation. A crystallographic study on one of the dicationic ruthenium(II) structures reveals that it forms an inclusion complex with benzene.     

On the Ligand‐to‐Metal Charge‐Transfer Photochemistry of the Copper(II) Complexes of Quercetin and Rutin

In contrast to the UV‐photoinduced ligand photoionization of the flavonoid complexes of Fe^III, redox reactions initiated in ligand‐to‐metal charge‐transfer excited states were observed on irradiation of the quercetin ( 1 ) and rutin ( 2 ) complexes of Cu^II. Solutions of complexes with stoichiometries [Cu^IIL₂] (L=quercetin, rutin) and [Cu^II₂L_n] (n=1, L=quercetin; n=3, L=rutin) were flash‐irradiated at 351 nm. Transient spectra observed in these experiments showed the formation of radical ligands corresponding to the one‐electron oxidation of L and the reduction of Cu^II to Cu^I. The radical ligands remained coordinated to the Cu^I centers, and the substitution reactions replacing them by solvent occurred with lifetimes τ<350 ns. These are lifetimes shorter than the known lifetimes (τ>1 ms) of the quercetin and rutin radical's decay.     

Ruthenium(II)–Polyimine–Coumarin Light‐Harvesting Molecular Arrays: Design Rationale and Application for Triplet–Triplet‐Annihilation‐Based Upconversion

Ru^II–bis‐pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16 000 M ^?1 cm^?1). Thus, Ru^II–polyimine complexes that show intense visible‐light absorptions are of great interest. However, no effective light‐harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible‐light‐harvesting Ru^II–coumarin arrays, which absorb at 475 nm (ε up to 63 300 M ^?1 cm^?1, 4‐fold higher than typical Ru^II–polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy‐transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady‐state and time‐resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru^II–polypyridine–chromophore light‐harvesting arrays, which states that the ¹IL energy level of the ligand must be close to the respective energy level of the metal‐to‐ligand charge‐transfer (M LCT) states. Lower energy levels of ¹IL/³IL than the corresponding ¹M LCT/³M LCT states frustrate the cascade energy‐transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light‐harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10‐diphenylanthracene as a triplet‐acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.     

Synthesis,characterization, photophysical,and redox properties of three trinuclear Ru(II) polypyridyl complexes possessing 5-amino-1,10-phenanthroline ligands

Three ruthenium(II) polypyridyl complexes with 5-amino-1,10-phenanthroline ligands have been successfully designed and synthesized. They have been fully characterized by ESI-MS, ESI-HRMS, ¹H NMR, and elemental analyses. The photophysical and electrochemical properties of the three complexes have been investigated in organic solvent. The geometrical configuration and the electron density distribution in the frontier molecular orbitals of the three complexes have been studied. The three complexes show metal-to-ligand charge transfer (¹MLCT) absorption at 445 nm, and intense triplet metal-to-ligand (³MLCT) emission at around 619 nm in fluid solution at 298 K and 580 nm in low-temperature glass. Electrochemical studies of the three complexes are consistent with one Ru^III/II reversible couple at around 1.31 V accompanied by three ligand-centered reduction couples.

     

Intercomponent and interfacial electron transfer processes in polynuclear metal complexes anchored on transparent TiO2 films

A series of polynuclear complexes based on Ru^II, Os^II, Re^I and Rh^III polypyridine moieties have been prepared in the context of intramolecular energy and electron transfer studies and of interfacial electron transfer with nanocrystalline TiO₂. The polynuclear complexes allow for the occurrence of vectorial intramolecular energy and electron transfer and have been proven to be efficient sensitizers of the wide band-gap semiconductor. The performance of photoregenerative cells based on these systems and the dynamics of the excited state intramolecular processes and of the interfacial electron transfer processes are discussed.     

Mixed-valence properties of Ruthenium–Polypyridine dimers bridged by Imidazolate and Triazolate Ligands

The dinuclear complex cis,cis-[(bpy)₂ClRu(μ-bim)RuCl(bpy)₂] ^{n +} (bpy = 2,2′-bipyridine; bim = benzimidazolate; n = 1, 2, or 3) was synthesized, isolated as a hexafluorophosphate salt, and investigated in organic solutions by cyclic voltammetry and UV/visible/NIR spectroelectrochemistry. The mixed-valent species (n = 2) displays significant metal–metal electronic coupling in the ground state but exhibits localized Ru(III) and Ru(II) oxidation states, as deduced from its intervalence charge transfer (IVCT) band and redox parameters. On the basis of the resonance energy (H _AB) estimated in the context of Hush's semiclassical theory, the extent of intermetallic communication was found to be larger than that recently reported for the bta-bridged analog (bta = benzotriazolate). Some differences between the IVCT features of these systems have been rationalized in terms of the degree of σ,π-basic character of the bridging ligands, according to an electron superexchange mechanism of the “hole-transfer” type. The stabilization of the mixed-valent complexes is attributed mainly to cooperative metal-to-ligand/ligand-to-metal charge-transfer effects. The combined π-acceptor and σ,π-donor abilities of the ancillary (bpy) and bridging (bim or bta) ligands, respectively, are also responsible for the high stability of the fully oxidized (Ru^III–L–Ru^III) and fully reduced (Ru^II–L–Ru^II) isovalent species.     

Theoretical investigation of charge transfer excitation and charge recombination in acenaphthylene–tetracyanoethylene complex

Ab initio calculations were performed to investigate the charge separation and charge recombination processes in the photoinduced electron transfer reaction between tetracyanoethylene and acenaphthylene. The excited states of the charge‐balanced electron donor–acceptor complex and the singlet state of ion pair complex were studied by employing configuration interaction singles method. The equilibrium geometry of electron donor–acceptor complex was obtained by the second‐order Møller–Plesset method, with the interaction energy corrected by the counterpoise method. The theoretical study of ground state and excited states of electron donor–acceptor complex in this work reveals that the S₁ and S₂ states of the electron donor–acceptor complexes are excited charge transfer states, and charge transfer absorptions that corresponds to the S₀ → S₁ and S₀ → S₂ transitions arise from π–π* excitations. The charge recombination in the ion pair complex will produce the charge‐balanced ground state or excited triplet state. According to the generalized Mulliken–Hush model, the electron coupling matrix elements of the charge separation process and the charge recombination process were obtained. Based on the continuum model, charge transfer absorption and charge transfer emission in the polar solvent of 1,2‐dichloroethane were investigated. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 23–35, 2003     

Integrating proton coupled electron transfer (PCET) and excited states

In many of the chemical steps in photosynthesis and artificial photosynthesis, proton coupled electron transfer (PCET) plays an essential role. An important issue is how excited state reactivity can be integrated with PCET to carry out solar fuel reactions such as water splitting into hydrogen and oxygen or water reduction of CO₂ to methanol or hydrocarbons. The principles behind PCET and concerted electron–proton transfer (EPT) pathways are reasonably well understood. In Photosystem II antenna light absorption is followed by sensitization of chlorophyll P₆₈₀ and electron transfer quenching to give P₆₈₀⁺. The oxidized chlorophyll activates the oxygen evolving complex (OEC), a CaMn₄ cluster, through an intervening tyrosine–histidine pair, Y_Z. EPT plays a major role in a series of four activation steps that ultimately result in loss of 4e^?/4H⁺ from the OEC with oxygen evolution. The key elements in photosynthesis and artificial photosynthesis – light absorption, excited state energy and electron transfer, electron transfer activation of multiple-electron, multiple-proton catalysis – can also be assembled in dye sensitized photoelectrochemical synthesis cells (DS-PEC). In this approach, molecular or nanoscale assemblies are incorporated at separate electrodes for coupled, light driven oxidation and reduction. Separate excited state electron transfer followed by proton transfer can be combined in single semi-concerted steps (photo-EPT) by photolysis of organic charge transfer excited states with H-bonded bases or in metal-to-ligand charge transfer (MLCT) excited states in pre-associated assemblies with H-bonded electron transfer donors or acceptors. In these assemblies, photochemically induced electron and proton transfer occur in a single, semi-concerted event to give high-energy, redox active intermediates.     

Synergistic Effects of Metals in a Promising RuII−PtII Assembly for a Combined Anticancer Approach: Theoretical Exploration of the Photophysical Properties

Ru^II?Pt^II complexes are a class of bioactive molecules of interest as anticancer agents that combine a light‐absorbing chromophore with a cisplatin‐like unit. The results of a DFT and TDDFT investigation of a Ru^II complex and its conjugate with a cis‐PtCl₂ moiety reveal that a synergistic effect of the metals makes the assembly a promising multitarget anticancer drug. Inspection of type I and type II photoreactions and spin–orbit coupling computations reveals that the cis‐PtCl₂ moiety improves the photophysical properties of the Ru^II chromophore, ensuring efficient singlet oxygen generation and making the assembly suitable for photodynamic therapy. At the same time, the Ru^II chromophore promotes a new alternative activation mechanism of the Pt^II ligand via a triplet metal‐to‐ligand charge transfer (³M LCT) state, before reaching the biological target. The importance of the supramolecular architecture is accurately derived, opening interesting new perspectives on the use of bimetallic Ru^II?Pt^II assemblies in a combined anticancer approach.     

A Ruthenium Complex–Porphyrin–Fullerene‐Linked Molecular Pentad as an Integrative Photosynthetic Model

A ruthenium complex, porphyrin sensitizer, fullerene acceptor molecular pentad has been synthesized and a long‐lived hole–electron pair was achieved in aqueous solution by photoinduced multistep electron transfer: Upon irradiation by visible light, the excited‐state of a zinc porphyrin (¹ZnP*) was quenched by fullerene (C₆₀) to afford a radical ion pair, ^1,3(ZnP^.+‐C₆₀^.−). This was followed by the subsequent electron transfer from a water oxidation catalyst unit (Ru^II) to ZnP^.+ to give the long‐lived charge‐separated state, Ru^III‐ZnP‐C₆₀^.−, with a lifetime of 14 μs. The ZnP worked as a visible‐light‐harvesting antenna, while the C₆₀ acted as an excellent electron acceptor. As a consequence, visible‐light‐driven water oxidation by this integrated photosynthetic model compound was achieved in the presence of sacrificial oxidant and redox mediator.     

Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited-State Decay

Square-planar Ni^II complexes are interesting as cheaper and more sustainable alternatives to Pt^II luminophores widely used in lighting and photocatalysis. We investigated the excited-state behavior of two Ni^II complexes, which are isostructural with two luminescent Pt^II complexes. The initially excited singlet metal-to-ligand charge transfer (¹MLCT) excited states in the Ni^II complexes decay to metal-centered (³MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the ³MC states to the electronic ground state within 9–21 ps. This contrasts with the population of an emissive triplet ligand-centered (³LC) excited state upon excitation of the Pt^II analogues. Structural distortions of the Ni^II complexes are responsible for this discrepant behavior and lead to dark ³MC states far lower in energy than the luminescent ³LC states of Pt^II compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited-state relaxation in four-coordinate Ni^II complexes could be decelerated such that luminescent ³LC or ³MLCT excited states become accessible. These insights are relevant to make Ni^II fit for photophysical and photochemical applications that relied on Pt^II until now.     

Coordination Chemistry of N‐Heterocyclic Nitrenium‐Based Ligands

Comprehensive studies on the coordination properties of tridentate nitrenium‐based ligands are presented. N‐heterocyclic nitrenium ions demonstrate general and versatile binding abilities to various transition metals, as exemplified by the synthesis and characterization of Rh^I, Rh^III, Mo⁰, Ru⁰, Ru^II, Pd^II, Pt^II, Pt^IV, and Ag^I complexes based on these unusual ligands. Formation of nitrenium–metal bonds is unambiguously confirmed both in solution by selective ¹⁵N‐labeling experiments and in the solid state by X‐ray crystallography. The generality of N‐heterocyclic nitrenium as a ligand is also validated by a systematic DFT study of its affinity towards all second‐row transition and post‐transition metals (Y–Cd) in terms of the corresponding bond‐dissociation energies.     

Tridentate Complexes of Palladium(II) and Platinum(II) Bearing bis‐Aryloxide Triazole Ligands: A Joint Experimental and Theoretical Investigation

A novel class of palladium(II) and platinum(II) complexes bearing tridentate bis‐aryloxide triazole ligands was prepared by using straightforward and high‐yielding synthetic routes. The complexes were fully characterized and the molecular structures of four derivatives were unambigously determined by single‐crystal X‐ray diffractometric analyses. For the most promising luminescent Pt^II derivatives, further experimental investigations were carried out to characterize their photophysical features and to ascertain the nature of the emitting excited state by means of electronic absorption, steady‐state, and time‐resolved emission techniques in different conditions. In degassed fluid solution the complexes displayed broad and featureless photoluminescence with λ_em=522–585 nm, excited‐state lifetime up to few microseconds and quantum yield (PLQY) up to 17 %, depending on the nature of both ancillary ligand and substituent on the tridentate ligand. Computational investigation using density functional theory and time‐dependent DFT were performed to gain insight into the electronic processes responsible for optical transitions and structure–photoluminescence relationship. Jointly, experimental and theoretical characterization indicated that the radiative transition arises from an excited state with admixed triplet‐manifold metal‐to‐ligand charge transfer and ligand‐centered (³MLCT/³LC) character. We elucidated the modulation of the photophysical properties upon variation of substituents for this new family of complexes.     

Unravelling the Quenching Mechanisms of a Luminescent RuII Probe for CuII

We have investigated the photophysical and photochemical features of a luminescent heteroleptic Ru^II‐polypyridyl probe and of its corresponding Ru^II‐Cu^II dinuclear complex formed upon the analyte binding through extensive density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations. The molecular probe contains the tailored imidazo[4,5‐f]‐1,10‐phenanthroline (IIP) ligand for simultaneously binding the Ru^II core and the target metal ion in aqueous solution. We have rationalized the static photoluminescence quenching observed upon the Cu^II coordination, on the grounds of distinct excited state deactivation mechanisms which are absent in the free Ru^II complex probe. Additionally, the emission quenching found upon increasing the solution pH has also been investigated. When coordinated IIP deprotonates, the nature of the lowest excited state of its complex changes from ³MLCT to ³LLCT/³IL. The strong base‐induced emission quenching can be understood in terms of both the energy‐gap law, since the ³LLCT/³IL states lie at a significantly lower energy than the ³MLCT state increasing the contribution of non‐radiative mechanisms, and the expected slower radiative rates from such ³LLCT/³IL states. After Cu^II binding, the lowest triplet excited state is similar to the analyte‐free probe in both energy and electronic nature. However, Cu‐centered non‐radiative excited states, populated after photoinduced electron transfer and intersystem crossing processes, are responsible for the population drainage of the emissive state.     

Tetrathiafulvalene‐Fused Porphyrins via Quinoxaline Linkers: Symmetric and Asymmetric Donor–Acceptor Systems

A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P–TTF–P triads 1 a – c and asymmetric P–TTF dyads 2 a , b in good yields. These planar and extended π‐conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge‐transfer excitations within the donor–acceptor assemblies. Quantum‐chemical calculations elucidate the nature of the electronically excited states. The compounds are electrochemically amphoteric and primarily exhibit low oxidation potentials. Cyclic voltammetric and spectroelectrochemical studies allow differentiation between the TTF and porphyrin sites with respect to the multiple redox processes occurring within these molecular assemblies. Transient absorption measurements give insight into the excited‐state events and deliver corresponding kinetic data. Femtosecond transient absorption spectra in benzonitrile may suggest the occurrence of fast charge separation from TTF to porphyrin in dyads 2 a , b but not in triads 1 a – c . Clear evidence for a photoinduced and relatively long lived charge‐separated state (385 ps lifetime) is obtained for a supramolecular coordination compound built from the ZnP–TTF dyad and a pyridine‐functionalized C₆₀ acceptor unit. This specific excited state results in a (ZnP–TTF)^?+ ??? (C₆₀py)^?? state. The binding constant of Zn^II ??? py is evaluated by constructing a Benesi–Hildebrand plot based on fluorescence data. This plot yields a binding constant K of 7.20×10⁴ M ^?1, which is remarkably high for bonding of pyridine to ZnP.     

A trimetallic mixed Ru(II)/Fe(II) terpyridyl complex with a long-lived excited state in solution at room temperature

The photophysical behavior of a series of mono- and trimetallic Ru(II) and mixed Ru(II)/Fe(II) bis-terpyridyl complexes was examined. The complexes have bridging terpyridyl ligands linked by phenylene-vinylene substituents on the terpyridyl. For the complexes bridged by a single phenylene-vinylene, the lowest-energy excited state is metal-to-ligand charge transfer (MLCT), and excited-state decay is on the 1-10 ns time scale. The complexes with two phenylene-vinylene groups have thermally equilibrated excited states that are localized on the phenylene-vinylene bridge and have much longer lifetimes (>200 ns). Remarkably, the trimetallic complex having an Fe(II) terpyridyl center also has a long-lived excited state, despite the fact that low-energy iron-localized excited states exist on the complex.