Triplet Metal-to-ligand Charge-transfer State Research Articles

Competing ultrafast photoinduced electron transfer and intersystem crossing of [Re(CO)_3(Dmp)(His124)(Trp122)]^+ in Pseudomonas aeruginosa azurin: a nonadiabatic dynamics study

We present a computational study of sub-picosecond nonadiabatic dynamics in a rhenium complex coupled electronically to a tryptophan (Trp) side chain of Pseudomonas aeruginosa azurin, a prototypical protein used in the study of electron transfer in proteins. To gain a comprehensive understanding of the photoinduced processes in this system, we have carried out vertical excitation calculations at the TDDFT level of theory as well as nonadiabatic dynamics simulations using the surface hopping including arbitrary couplings (SHARC) method coupled to potential energy surfaces represented with a linear vibronic coupling model. The results show that the initial photoexcitation populates both singlet metal-to-ligand charge transfer (MLCT) and singlet charge-separated (CS) states, where in the latter an electron was transferred from the Trp amino acid to the complex. Subsequently, a complex mechanism of simultaneous intersystem crossing and electron transfer leads to the sub-picosecond population of triplet MLCT and triplet CS states. These results confirm the assignment of the sub-ps time constants of previous experimental studies and constitute the first computational evidence for the ultrafast formation of the charge-separated states in Re-sensitized azurin.

Controlling the Lifetime of the Triplet MLCT State in Fe(II) Polypyridyl Complexes through Ligand Modification

A computational study is presented in which two strategies of ligand modifications have been explored to invert the relative energy of the metal-to-ligand charge transfer (MLCT) and metal-centered (MC) state in Fe(II)-polypyridyl complexes. Replacing the bipyridines by stronger σ donors increases the ligand-field strength and pushes the MC state to higher energy, while the use of ligands with a larger π conjugation leads to lower MLCT energies.

Mapping Vibronic Couplings in a Solar Cell Dye with Polarization-Selective Two-Dimensional Electronic-Vibrational Spectroscopy.

This study uses polarization-selective two-dimensional electronic-vibrational (2D EV) spectroscopy to map intramolecular charge transfer in the well-known solar cell dye, [Ru(dcbpy)2(NCS)2]4- (N34-), dissolved in water. A static snapshot of the vibronic couplings present in aqueous N34- is reported. At least three different initially excited singlet metal-to-ligand charge-transfer (MLCT) states are observed to be coupled to vibrational modes probed in the lowest energy triplet MLCT state, emphasizing the role of vibronic coupling in intersystem crossing. Angles between electronic and vibrational transition dipole moments are extracted from spectrally isolated 2D EV peaks and compared with calculations to develop a microscopic description for how vibrations participate with 1MLCT states in charge transfer and intersystem crossing. These results suggest that 1MLCT states with significant electron density in the electron-donating plane formed by the Ru-(NCS)2 will participate strongly in charge transfer through these vibronically coupled degrees of freedom.

How Does Peripheral Functionalization of Ruthenium(II)-Terpyridine Complexes Affect Spatial Charge Redistribution after Photoexcitation at the Franck-Condon Point?

Ruthenium polypyridine-type complexes are extensively used sensitizers to convert solar energy into chemical and/or electrical energy, and they can be tailored through their metal-to-ligand charge-transfer (MLCT) properties. Much work has been directed at harnessing the triplet MLCT state in photoinduced processes, from sophisticated molecular architectures to dye-sensitized solar cells. In dye-sensitized solar cells, strong coupling to the semiconductor exploits the high reactivity of the (hot) singlet/triplet MLCT state. In this work, we explore the nature of the (1) MLCT states of remotely substituted Ru(II) model complexes by both experimental and theoretical techniques. Two model complexes with electron-withdrawing (i.e. NO2 ) and electron-donating (i.e. NH2 ) groups were synthesized; these complexes contained a phenylene spacer to serve as a spectroscopic handle and to confirm the contribution of the remote substituent to the (1) MLCT transition. [Ru(tpy)2 ](2+) -based complexes (tpy=2,2':6',2''-terpyridine) were further desymmetrized by tert-butyl groups to yield unidirectional (1) MLCTs with large transition dipole moments, which are beneficial for related directional charge-transfer processes. Detailed comparison of experimental spectra (deconvoluted UV/Vis and resonance Raman spectroscopy data) with theoretical calculations based on density functional theory (including vibronic broadening) revealed different properties of the optically active bright (1) MLCT states already at the Franck-Condon point.

Injection Kinetics and Electronic Structure at the N719/TiO2 Interface Studied by Means of Ultrafast XUV Photoemission Spectroscopy

The method of transient XUV photoemission spectroscopy is developed to investigate the ultrafast dynamics of heterogeneous electron transfer at the interface between the N719 ruthenium dye complex and TiO2 nanoparticles. XUV light from high-order harmonic generation is used to probe the electron density distribution among the ground and excited states at the interface after its exposure to a pump laser pulse of 530 nm wavelength. A spectral decomposition of the transient emission signal is used to follow the population and decay dynamics of the involved transient states individually. By comparing results obtained for the N719/TiO2 and N719/FTO interfaces, we can unambiguously reveal the kinetics of electrons injected to TiO2 from the singlet metal-to-ligand charge-transfer (MLCT) excited state of the dye. With the developed approach, we characterize both the kinetic constants and the absolute binding energies of the singlet and triplet MLCT states of the dye and the state of electrons injected to the conduction band of TiO2. The energy levels of the singlet and triplet states are found to lie 0.7 eV above and 0.2 eV below the conduction band minimum, respectively. This energetic structure gives rise to a strong driving force for injection from the singlet state and a slow electron transfer from the triplet state, the latter being possible due to a partial overlap of the triplet state band of N719 and the conduction band of TiO2.

Ultrafast Deactivation Mechanism of the Excited Singlet in the Light‐Induced Spin Crossover of [Fe(2,2′‐bipyridine)3]2+

The mechanism of the light-induced spin crossover of the [Fe(bpy)3](2+) complex (bpy=2,2'-bipyridine) has been studied by combining accurate electronic-structure calculations and time-dependent approaches to calculate intersystem-crossing rates. We investigate how the initially excited metal-to-ligand charge transfer (MLCT) singlet state deactivates to the final metastable high-spin state. Although ultrafast X-ray free-electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high-spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d(6)(t(2g)(5)e(g)(1)) configuration either directly or by means of a triplet MLCT state. This triplet ligand-field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower-lying triplet state and subsequent intersystem crossing to the high-spin state. The deactivation rate to the low-spin ground state is much smaller, which is in line with the large quantum yield reported for the process.

Photodynamic Behavior of Heteroleptic Ir(III) Complexes with Carbazole-Functionalized Dendrons Associated with Efficient Electron Transfer Processes

We prepared dendrimers of heteroleptic iridium(III) complexes, [(dfppy–Cz1)2Ir(dpq)]+ (G1) and [(dfppy–Cz2)2Ir(dpq)]+ (G2), which have the dfppy ligand connected to carbazole-functionalized dendron Czn (n = 1, 2) [dfppy–Czn = 5-Czn-2-(4,6-difluorophenyl)pyridine, dpq = 2,3-bis-(2-pyridyl)-qinoxaline, Cz1 = 4-(9-carbazolyl)benzyloxymethyl, and Cz2 = 4-[1,3-bis(9-carbazolyl)benzyloxy]benzyloxymethyl]. While parent complex [(dfppy)2Ir(dpq)]+ (G0) shows an intense emission at ∼635 nm with a lifetime of 1 μs assigned to dpq-based metal-to-ligand charge-transfer (MLCT) phosphorescence, excitation of the dendrimers at either carbazole (309 nm) or MLCT band (355 nm) resulted in markedly weaker and much shorter-lived MLCT emission (τp = 44 ns for G1 and 115 ns for G2) at room temperature. Upon exciting the carbazole chromophore of G1 and G2 at 309 nm, furthermore, both the carbazole fluorescence and the MLCT emission were very weak at room temperature. It was found that the lifetime of carbazole fluorescence is 20 ps for G1 and 62 ps for G2, shorter by 2-orders of magnitude than that of free carbazole dendron Czn′–OH (τF = 6.1 ns). These observations demonstrate that both the excited-singlet state of carbazole and the triplet MLCT state of the Ir(dpq) core are efficiently quenched in the dendrimers. At 77 K, however, the MLCT emission lifetime for both G1 and G2 is ∼7 μs that is nearly identical to that of G0 (6.8 μs), and the carbazole fluorescence lifetime is ∼11.5 ± 0.5 ns, which is again almost the same as that of Czn′–OH (11.5 ns). Since the apparent quenching of either carbazole fluorescence or MLCT emission observed at room temperature does not occur at 77 K, the temperature-dependent emission behavior of G1 and G2 for both the carbazole fluorescence and the MLCT phosphorescence was attributed to the participation of activated processes, that is, electron transfer from excited-singlet carbazole to the Ir(dpq) core as well as from the ground-state carbazole unit to the triplet MLCT Ir(dpq) core. This mechanism was supported by transient-absorption spectroscopic experiments that demonstrate the generation of the carbazole radical cation after exciting G1 and G2 by laser pulses.

Highly Solvent Dependent Luminescence from [Ru(bpy)n(dppp2)3−n]2+ (n = 0−2)

The photophysical properties of a series of ruthenium(II) complexes possessing the dppp2 (dppp2 = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline) ligand, [Ru(bpy)(n)(dppp2)(3-n)](2+) (bpy = 2,2'-bipyridine; n = 0-3), were investigated. The dppp2-containing complexes possess a remarkable solvent dependence of the luminescence maximum, lifetime, and quantum yield. For example, the emission maximum of [Ru(bpy)(2)(dppp2)](2+) blue shifts from 752 nm in CH(3)CN to 653 nm in CH(2)Cl(2), concomitant with a 19-fold enhancement in the luminescence quantum yield. Electronic structure calculations, transient absorption spectroscopy, and electrochemistry were also used to gain insight into the photophysical properties of the dppp2 complexes. The pronounced solvent effect of the emission of these complexes is attributed to the changes in the relative stabilities of two low-lying metal-to-ligand charge transfer (MLCT) excited states on the environment, where the lowest energy MLCT state is more sensitive to the polarity of the solvent than that which lies at slightly higher energy. Transient absorption spectroscopy shows that the identity of the lowest energy MLCT state does not change as a function of solvent, however, its emission maximum and lifetime are greatly affected by the polarity of the surrounding medium. In contrast to [Ru(bpy)(2)(dppz)](2+) (dppz = dipyrido[3,2-a:2',3'-c]phenzine), the lowest energy excited state in the dppp2-containing complexes is assigned as arising from a triplet MLCT state where the charge is localized on the portion of the dppp2 ligand distal to the metal, (3)MLCT(dis).

Study of the Light‐Induced Spin Crossover Process of the [FeII(bpy)3]2+ Complex

Ab initio calculations have been performed on [Fe(II)(bpy)3](2+) (bpy = bipyridine) to establish the variation of the energy of the electronic states relevant to light-induced excited-state spin trapping as a function of the Fe-ligand distance. Light-induced spin crossover takes place after excitation into the singlet metal-to-ligand charge-transfer (MLCT) band. We found that the corresponding electronic states have their energy minimum in the same region as the low-spin (LS) state and that the energy dependence of the triplet MLCT states are nearly identical to the (1) MLCT states. The high-spin (HS) state is found to cross the MLCT band near the equilibrium geometry of the MLCT states. These findings give additional support to the hypothesis of a fast singlet-triplet interconversion in the MLCT manifold, followed by a (3)MLCT-HS ((5)T2) conversion accompanied by an elongation of the Fe-N distance.

Synthesis and photophysical properties of naphthyl-, phenanthryl-, and pyrenyl-appended bis(pyridyl)triazine ligands and their Zn(II) and Ru(II) complexes1

A family made of four bis(pyridyl)triazine ligands with appended aryl rings, including three fused aromatic rings, have been synthesized, and their corresponding homoleptic Ru(II) and Zn(II) complexes have been prepared and characterized by several means. The free ligands 2,4-di(2-pyridyl)-6-phenyl-1,3,5-triazine (L1); 2,4-di(2-pyridyl)-6-(1-naphthyl)-1,3,5-triazine (L2); 2,4-di(2-pyridyl)-6-(9-phenanthryl)-1,3,5-triazine (L3); and 2,4-di(2-pyridyl)-6-(1-pyrenyl)-1,3,5-triazine (L4) were formed in triazine ring-forming reactions from the reactions of the cyano-functionalized aromatic rings with LiNMe2 followed by the addition of 2 equiv. of 2-cyanopyridine. The metal complexes examined in this study are the homoleptic Ru(II) complexes Ru(L1)22+ (2a), Ru(L2)22+ (2b), Ru(L3)22+ (2c), and Ru(L4)22+ (2d) and Zn(II) complexes Zn(L1)22+ (3a), Zn(L2)22+ (3b), Zn(L3)22+ (3c), and Zn(L4)22+ (3d). Also, crystallographic data for the free ligands and Zn(II) and Ru(II) complexes have been obtained in some cases. The redox behaviour and absorption spectra of all the species have been investigated, together with the luminescence properties of the free ligands at room temperature in fluid solution and of the Ru(II) complexes both at room temperature in fluid solution and at 77 K in rigid matrix. The redox data indicate that the free ligands are reduced twice at relatively mild potentials (< –2.30 V vs. SCE), with the first reduction almost independent of the nature of the substituted aryl group. The UV absorption spectra of all the compounds are dominated by intense spin-allowed π–π* transitions mainly centered on the bis(pyridyl)triazine moiety; however, in L2–L4, moderately intense intraligand charge-transfer (ILCT) bands are also present. Such bands are red-shifted in the Zn(II) compounds, while they are obscured in the Ru(II) species by the more intense spin-allowed metal-to-ligand charge-transfer (MLCT) bands. The free ligands exhibit interesting emission properties, ranging from fluorescence from π–π* states to excimeric (in L4) and ILCT (in L2 and L3) emission. In the Ru(II) complexes, strong emission is found at 77 K from triplet MLCT states. For 2c and 2d, the emissive MLCT states are mixed with low-lying triplet ligand-centered states.Key words: triazine ligands, ruthenium complexes, zinc complexes, ILCT emission.

Mechanism of Ligand Photodissociation in Photoactivable [Ru(bpy)2L2]2+ Complexes: A Density Functional Theory Study

A series of four photodissociable Ru polypyridyl complexes of general formula [Ru(bpy)2L2](2+), where bpy = 2,2'-bipyridine and L = 4-aminopyridine (1), pyridine (2), butylamine (3), and gamma-aminobutyric acid (4), was studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). DFT calculations (B3LYP/LanL2DZ) were able to predict and elucidate singlet and triplet excited-state properties of 1-4 and describe the photodissociation mechanism of one monodentate ligand. All derivatives display a Ru --> bpy metal-to-ligand charge transfer (MLCT) absorption band in the visible spectrum and a corresponding emitting triplet (3)MLCT state (Ru --> bpy). 1-4 have three singlet metal-centered (MC) states 0.4 eV above the major (1)MLCT states. The energy gap between the MC states and lower-energy MLCT states is significantly diminished by intersystem crossing and consequent triplet formation. Relaxed potential energy surface scans along the Ru-L stretching coordinate were performed on singlet and triplet excited states for all derivatives employing DFT and TDDFT. Excited-state evolution along the reaction coordinate allowed identification and characterization of the triplet state responsible for the photodissociation process in 1-4; moreover, calculation showed that no singlet state is able to cause dissociation of monodentate ligands. Two antibonding MC orbitals contribute to the (3)MC state responsible for the release of one of the two monodentate ligands in each complex. Comparison of theoretical triplet excited-state energy diagrams from TDDFT and unrestricted Kohn-Sham data reveals the experimental photodissociation yields as well as other structural and spectroscopic features.

Extending the Light‐Harvesting Properties of Transition‐Metal Dendrimers

We report a study of the electronic energy-transfer dynamics within the transition-metal polypyridine complex OsRu3pyr6 (Os[(dpp)Ru(bpy{pyrene})2]3(8+), where dpp=2,3-bis(2'-pyridyl)pyrazine and bpy=2,2'-bipyridine) after excitation with UV light. By using a broadband visible femtosecond probe, we are able to simultaneously detect both the energy transfer from the peripheral aromatic ligands to the Os center and the sub-picosecond energy transfer from the initially excited Ru-bpy ligand-centered state to the Os triplet metal-to-ligand charge-transfer (MLCT) state. Pyrene energy transfer occurs from both the nonrelaxed and the relaxed S(1) states on timescales of 6 and 45 ps, respectively. In both cases, the energy transfer is described by means of Förster energy transfer theory. Sub-picosecond energy transfer within the OsRu3 metal-ligand core most likely includes a direct energy transfer between the higher-lying ligand-centered states on Ru and Os, in addition to the transfer between the MLCT states. The absorption cross-sections in both the UV and the visible spectral regions are enhanced by attaching the aromatic pyrene ligands. Furthermore, energy transfer is directed only towards the Os core, which ultimately leads to an exclusive population of the Os-based triplet MLCT state, thus making the OsRu3pyr6 transition metal-polypyridine dendrimer an obvious candidate for artificial light-harvesting systems.

Multiple Charge-Transfer Emissions from Different Metal−Ligand Pairs in Ruthenium Diimines

Ruthenium diimines are unique in their emissivity. Optical excitation with light of less than 500 nm leads to a strong emission in the 600-700 nm range. All emissive ruthenium complexes appear to undergo intersystem crossing from the absorptive singlet metal-to-ligand charge-transfer (MLCT) state to an emissive triplet MLCT state localized on the lowest-energy metal-ligand pair. In contrast to this currently accepted model, in which a single emissive state is populated and then equilibrates among other states based on a particular set of conditions, the excitation-wavelength dependence of the [(bpy)2RudppH]3+ emission suggests two emissive pathways. One populates an emissive MLCT state localized on a bpy-Ru pair, and the other populates a lower-energy MLCT state localized on the dpp-Ru pair.

A New Heptanuclear Dendritic Ruthenium(II) Complex Featuring Photoinduced Energy Transfer Across High‐Energy Subunits

The new heptanuclear ruthenium(II) dendron, [Cl(2)Ru{(micro-2,3dpp)Ru[(micro-2,3-dpp)Ru(bpy)2]2}2](PF6)12 (1; 2,3-dpp=2,3-bis(2'-pyridyl)pyrazine; bpy = 2,2'-bipyridine), was prepared by means of the "complexes as ligands/complexes as metals" synthetic strategy, and its absorption spectrum, redox behavior, and luminescence properties were investigated. Compound 1 is a multicomponent species, which contains three different types of chromophores (namely, the {Cl(2)Ru(micro-2,3-dpp)2} core, the {Ru(micro-2,3dpp)3}2+ intermediate, and the {(bpy)2Ru(micro-2,3-dpp)}2+ peripheral subunits) and several redox-active sites. The new species exhibits very intense absorption bands in the UV region (epsilon value in the 10(5)-10(6) M(-1) cm(-1) range) as a result of spin-allowed ligand-centered (LC) transitions, and intense bands in the visible region (epsilon value in the 10(4)-10(5) M(-1) cm(-1) range) as a result of the various spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. The redox investigation (accomplished by cyclic and differential pulse voltammetry) indicates that 1 undergoes a series of reversible metal-centered oxidation and ligand-centered reduction processes within the potential window investigated (+1.90 / -1.40 V vs. the standard calomel electrode, SCE). The assignment of each absorption bond and redox process to specific subunits of 1 was achieved by comparison with the properties of smaller multinuclear species of the same family, namely [Cl(2)Ru{(micro-2,3-dpp)Ru(bpy)2}2]4+ (2), [(bpy)2Ru(u-2,3-dpp)Ru(bpy)2]4+ (4), and [Ru{(micro-2,3-dpp)Ru(bpy)2}3]4+ (5). The title compound exhibits luminescence both at room temperature in acetonitrile fluid solution and at 77 K in butyronitrile rigid matrix. The emission is attributed to the triplet MLCT (3MLCT) state involving the core {Cl(2)Ru(micro-2,3-dpp)2} subunit. Interestingly, the 3MLCT levels involving the peripheral {(bpy)2Ru(micro-2,3-dpp)}2+ subunits are deactivated by energy transfer to the emitting level, in spite of the presence of interposed high-energy (Ru(micro-2,3-dpp)3}2+ components, which, in other dendrimers, acted as "isolating" subunits toward energy-transfer processes. Ultrafast experiments on 1 and on the parent species 2 and 5 allowed us to rationalize this behavior and highlight that a sequential two-step electron-transfer process can be held responsible for the efficient overall energy transfer, which offers a way to overcome a limitation in antenna metal dendrimers.

Rapid intersystem crossing in highly phosphorescent iridium complexes

Femtosecond time-resolved absorption measurements on Ir complexes are performed. On excitation at wavelength 400 nm, singlet metal-to-ligand charge-transfer (MLCT) states are excited. On probing at 580 nm, a transient absorption appears with time constant 70–100 fs. This transient state rising rapidly but decaying very slowly is assigned to be the lowest triplet MLCT state. Accordingly intersystem crossing in these Ir complexes is rapid. Measured with a nanosecond laser, emission from the lowest triplet MLCT state decays with lifetimes 1.55, 1.65 and 1.7 μs for Ir(ppy) 3, Ir(DBQ) 2(acac) and Ir(MDQ) 2(acac), respectively. On 266-nm excitation with a femtosecond laser, high-energy states with mostly π–π * character (ligand-centered states) are accessed; they relax to the lowest triplet MLCT state with time constant 100–350 fs. The efficient flow of energy from high-energy electronic states to 3MLCT results in high quantum yield in electroluminescent phosphorescence in organic light-emitting diodes.

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations.

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.

Absorption and Electroabsorption Spectra of [(NH3)5Ru(4,4‘-bipyridine)Ru(NH3)5]4+in Water by ab Initio Calculations

The absorption spectrum in the visible of [(NH3)5Ru(4,4‘-bipyridine)Ru(NH3)5]4+ in water has been studied by extensive multireference configuration interaction calculations. Solvent effects were included by the polarizable continuum model. Size and shape of the cavity surrounding the solute molecule were chosen according to a method that we have previously developed for systems in which solute−solvent hydrogen bonds occur. The dependence of the ground state, as well as that of the first singlet and triplet excited states, has been investigated as a function of the torsion between the two pyridine rings of 4,4‘-bipyridine. The line shape profile of the metal-to-ligand charge transfer (MLCT) band has thus been obtained. The modifications of the band due to a static electric field have then been studied, and the possible role of the triplet MLCT state in determining the observed Stark spectrum has been investigated.

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 and Emission Properties of Di- and Trinuclear Ruthenium(II) Rack-Type Complexes

The absorption spectra and the luminescence properties of three dinuclear RuII complexes and one trinuclear RuII complex have been investigated. All the complexes have rack-type structures. The dinuclear complexes 1, 2, and 3 incorporate a bis-tridentate bridging ligand made up of a pyrimidine and four pyridine moieties, as well as two 2,2′:6′,2′-terpyridyl (tpy) ligands. The trinuclear complex 4 incorporates a tris-tridentate bridging ligand made up of two pyrimidine and five pyridine moieties, as well as three tpy ligands. The absorption spectra of the complexes show a large number of ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) bands. All the complexes exhibit emission from a triplet MLCT state, with maxima in the spectral range 840–950 nm (lifetimes between 40 and 80 ns) at 298 K in fluid solution, and in the spectral range 760–810 nm (lifetimes between 2 and 3 μs) at 77 K in rigid matrices. A fine tuning of the absorption and luminescence properties of complexes 1–3can be achieved by changing the substituents on the pyrimidine ring of the bridging ligand. Efficient energy transfer within the rack structure 4 occurs from the (upper-lying) central metal-based chromophore to the (lower-lying) peripheral ones.

A quantum chemical investigation of the metal-to-ligand-charge-transfer photochemistry

According to a number of recent experiments reported for a class of α-diimine mono- and di-nuclear transition metal carbonyls, these molecules may either photodissociate leading to highly reactive intermediates or manifest the photophysics of metal-to-ligand-charge-transfer (MLCT) complexes. The duality between these two extremes of behavior may be used to promote different applications like catalytic activity or energy/electrons transfers. The potential energy surfaces (PESs) associated with the low-lying singlet and triplet excited states of Mn(H)(CO) 3(DAB) (DAB=1,4-diaza-1,3-butadiene) calculated for the Mn–H bond homolysis and for the dissociation of an axial carbonyl ligand illustrate the complexity and the richness of the photochemistry and photophysics of this class of molecules. It is shown that the presence of significant energy barriers on the reaction path corresponding to the homolysis of the Mn–H bond are responsible for the low efficiency of this process in related complexes. The simulation of the dynamics of the 1MLCT excited state, calculated at 26 000 cm −1 and contributing mainly to the visible absorption spectrum, indicates that the most probable primary reaction is an ultrafast dissociation (<200 fs) of the carbonyl ligand. The relative positions of the quasi-bound MLCT states with respect to the sigma bond-to-ligand-charge-transfer (SBLCT) excited state corresponding to the σ Mn–H →π ∗ DAB excitation will govern the photoreactivity of the molecules. A comparative study of the sequence, the nature of the low-lying excited states and of the associated state correlation diagrams for the hydride complex and the ethyl analog Mn(Et)(CO) 3(DAB) points to a better reactivity of the ethyl-substituted complex in agreement with the experimental data. This is a consequence of a lowering of the excited states, an increase in the density of states in the visible energy domain and a mixing of the triplet SBLCT and MLCT states on going from the hydride to the ethyl complex. The influence of the metal center on going from the manganese to the rhenium compound is expressed by an important mixing of the MLCT and SBLCT states and a lowering of the excited states due to a relativistic destabilization of the d shells of the metal center and an indirect stabilization of the π ∗ DAB orbital through its interaction with the 6p of the rhenium. These heavy atom effects should induce a better photoreactivity for the third-row transition metal complexes than for the first-row analogs.