Molecular engineering for optical properties of 5-substituted-1,10-phenanthroline-based Ru(II) complexes.

A series of homo- and heteroleptic Ru(ii) complexes [Ru(phen)3-n(phen-X)n](PF6)2 (n = 0-3, X = CN, epoxy, H, NH2) were prepared and characterized. The influence of electron-withdrawing or electron-releasing substituents of the 1,10-phenanthroline ligands on the photo-physical properties was evaluated. It reveals fundamental interests in the fine tuning of redox potentials and photo-physical characteristics, depending both on the nature of the substitution of the ligand, and on the symmetry of the related homo- or heteroleptic complex. These complexes exhibit linear absorption and two-photon absorption (2PA) cross-sections over a broad range of wavelength (700-900 nm) due to absorption in the intra-ligand charge transfer (ILCT) and the metal-to-ligand charge transfer (MLCT) bands. These 2PA properties were more particularly investigated in the 700-1000 spectral range for a family of complexes bearing electro-donating ligands (phen-NH2).

Iron(ii) complexes with diazinyl-NHC ligands: impact of π-deficiency of the azine core on photophysical properties.

Ligand field enhancing N-heterocyclic carbene (NHC) ligands were recently shown to prevent photo-induced spin crossover in Fe(ii) complexes due to their intricate effects on the electronic excited state structure. Due to their pico- to nanosecond lifetimes, these complexes are now good candidates for photo-sensitizing applications. Herein we report the synthesis and photophysical characterization of a new family of homoleptic Fe(ii) complexes with C^N^C ligands involving diazines as the central N-heteroaromatic ligand. For these four carbene bond complexes, ultrafast transient absorption spectroscopy revealed a significant improvement of the excited-state lifetime. A record 32 ps lifetime was measured for a complex bearing a ligand combining a π-deficient pyrazine nucleus and a benzimidazolylidene as NHC. When compared to other azine-based ligands investigated, we argue that the lifetimes are modulated by a small excited state barrier expressing the ability of the ligand to reach the Fe-N distance needed for internal conversion to the ground state.

Impact of the fac/ mer Isomerism on the Excited-State Dynamics of Pyridyl-carbene Fe(II) Complexes.

The control of photophysical properties of iron complexes and especially of their excited states decay is a great challenge in the search for sustainable alternatives to noble metals in photochemical applications. Herein we report the synthesis and investigations of the photophysics of mer and fac iron complexes bearing bidentate pyridyl-NHC ligands, coordinating the iron with three ligand-field-enhancing carbene bonds. Ultrafast transient absorption spectroscopy reveals two distinct excited state populations for both mer and fac forms, ascribed to the populations of the T1 and the T2 states, respectively, which decay to the ground state via parallel pathways. We find 3-4 ps and 15-20 ps excited-state lifetimes, with respective amplitudes depending on the isomer. The longer lifetime exceeds the one reported for iron complexes with tridentate ligands analogues involving four iron-carbene bonds. By combining experimental and computational results, a mechanism based on the differential trapping of the triplet states in spin-crossover regions is proposed for the first time to explain the impact of the fac/ mer isomerism on the overall excited-state lifetimes. Our results clearly highlight the impact of bidentate pyridyl-NHC ligands on the photophysics of iron complexes, especially the paramount role of fac/ mer isomerism in modulating the overall decay process, which can be potentially exploited in the design of new Fe(II)-based photoactive compounds.

Synthesis and Computational Study of a Pyridylcarbene Fe(II) Complex: Unexpected Effects of fac/ mer Isomerism in Metal-to-Ligand Triplet Potential Energy Surfaces.

The synthesis and the steady-state absorption spectrum of a new pyridine-imidazolylidene Fe(II) complex (Fe-NHC) are presented. A detailed mechanism of the triplet metal-to-ligand charge-transfer states decay is provided on the basis of minimum energy path (MEP) calculations used to connect the lowest-lying singlet, triplet, and quintet state minima. The competition between the different decay pathways involved in the photoresponse is assessed by analyzing the shapes of the obtained potential energy surfaces. A qualitative difference between facial ( fac) and meridional ( mer) isomers' potential energy surface (PES) topologies is evidenced for the first time in iron-based complexes. Indeed, the mer complex shows a steeper triplet path toward the corresponding 3MC minimum, which lies at a lower energy as compared to the fac isomer, thus pointing to a faster triplet decay of the former. Furthermore, while a major role of the metal-centered quintet state population from the triplet 3MC region is excluded, we identify the enlargement of iron-nitrogen bonds as the main normal modes driving the excited-state decay.

A new record excited state (3)MLCT lifetime for metalorganic iron(ii) complexes.

Herein we report the synthesis and time-resolved spectroscopic characterization of a homoleptic Fe(ii) complex exhibiting a record (3)MLCT lifetime of 26 ps promoted by benzimidazolylidene-based ligands. Time dependent density functional molecular modeling of the triplet excited state manifold clearly reveals that, at equilibrium geometries, the lowest (3)MC state lies higher in energy than the lowest (3)MLCT one. This unprecedented energetic reversal in a series of iron complexes, with the stabilization of the charge-transfer state, opens up new perspectives towards iron-made excitonic and photonic devices, hampering the deactivation of the excitation via metal centered channels.

Ground and excited state properties of new porphyrin based dyads: a combined theoretical and experimental study.

The properties of the ground and excited states of several porphyrins appended with external chelates coordinated to ruthenium-bisbipyridine units are reported. The important modification of the absorption spectrum upon coordination with the ruthenium complex showed that a significant electronic communication between the two subunits was present in the ground state. Experimental results were compared with quantum chemistry calculations performed at density functional theory and time-dependent density functional theory level. The influence of the exchange-correlation functional on the quality of the computed absorption spectrum is shown, and the better behavior of hybrid functionals over long-range corrected ones was rationalized. The excited states topology analysis, performed using natural transition orbitals, gave a more evident confirmation of the communication between the subunits and showed that these new compounds can be promising as dyes in dye-sensitized solar cells.

New dyads using (metallo)porphyrins as ancillary ligands in polypyridine ruthenium complexes. Synthesis and electronic properties.

Porphyrins bearing enaminoketones at their periphery have been used as ancillary ligands in ruthenium complexes. Free base, nickel and zinc porphyrins were successfully coordinated to Ru(bpy)(2)Cl(2) under microwave irradiation. The positive contribution of the ruthenium complex was demonstrated by the complexes' wide absorption domains that covered the 500-600 nm region where the parent porphyrins did not absorb. Electrochemical as well as computational data revealed an efficient electronic communication between the porphyrins and the ruthenium cation in the dyads.

Strong π-delocalization and substitution effect on electronic properties of dithienylpyrrole-containing bipyridine ligands and corresponding ruthenium complexes.

The first dithienylpyrrole (DTP)-based bipyridine ligands has been prepared and coordinated with ruthenium to give the corresponding homoleptic complexes. Bipyridine was bound at pyrrole (DTP(1)) or thiophene (DTP(2)) ring. A strong bathochromic effect was obtained by switching from pyrrole to thiophene for ligands and complexes. Interestingly the DTP(2) series offered a wide absorption window from UV to visible domain with an almost constant absorbance. These effects are due to a larger extent of delocalization as supported by DFT calculations and photophysical measurements.

Theoretical study of new ruthenium-based dyes for dye-sensitized solar cells.

Two relevant, recently reported, ruthenium-based complexes to be used as sensitizers in Grätzel photovoltaic cells are theoretically studied. The UV/vis absorption spectra have been computed within the time-dependent density functional theory formalism. The obtained excitation energies are compared with the experimental results, and the nature of the transition is analyzed in terms of the electronic density. A preliminary study on the performance of different functionals against the equation of motion coupled cluster is performed on a smaller model system.

Combination of cobalt and iron polypyridine complexes for improving the charge separation and collection in Ru(terpyridine)(2)-sensitised solar cells.

Mixtures of polypyridine Fe(II) and Co(II) complexes are used as electron mediators in Ru-thienyltpy-sensitised solar cells (tpy=terpyridine). The use of the metalorganic redox couples allows for improved charge-collection efficiency with respect to the classical iodide/iodine couple which, when associated to Ru-tpy(2) dyes, usually produces poor performance. The improved charge collection is explained by a combination of effective dye regeneration and decreased recombination with the oxidised electrolyte on the basis of data obtained by transient spectroscopy and photoelectrochemical measurements. The efficiency of the regeneration cascade is also critically dependent upon the ability of the Co(II) complex to intercept Fe(III) centres, as clearly indicated by chronocoulometry experiments.

Homoleptic ruthenium complex bearing dissymmetrical 4-carboxy-4'-pyrrolo-2,2'-bipyridine for efficient sensitization of TiO(2) in solar cells.

An easily accessible homoleptic complex [Ru(L3)(3)](2+) containing a dissymmetrical bipyridine (L3) substituted by a pyrrole and a carboxylic group has been evaluated in a dye-sensitized solar cell. The new dye displayed extended absorption domain, high absorbance, and a promising 65% IPCE value. Higher scores were reached using a cobalt-iron mediator instead of the usual LiI/I(2) couple for regeneration of the Ru(II) state. Transient absorption spectroscopy was used to explain the mediator effect.

Ruthenium complexes bearing pi-extended pyrrolo-styryl-bipyridine ligand: electronic properties and evaluation as photosensitizers.

A new Ru(L)(dcbpy)(NCS)2 dye containing a pyrrole-based pi-extended ligand has been prepared and showed very good light harvesting ability with near 70% IPCE in the visible region. The performance is very close to those of the standard N3 with a wider absorption range.

Apoptosis/necrosis switch in two different cancer cell lines: influence of benzoquinone- and hydrogen peroxide-induced oxidative stress intensity, and glutathione.

Depending on the strength of oxidative stress, cells exhibit proliferative, apoptotic or necrotic responses. We have investigated whether the severity of glutathione (GSH) depletion could determine the type of cell death using 1,4-benzoquinone (BQ) and H(2)O(2) in two different tumor cell lines (human mammary gland carcinoma MCF-7 and rat hepatoma H5-6). BQ-treated surviving cells showed an increase in GSH, but no detectable oxidized glutathione (GSSG) nor reactive oxygen species (ROS) augmentation. Alternatively, H(2)O(2) depressed GSH. BQ induced mostly apoptosis, up to 90% cell elimination, while necrosis was prominent in H(2)O(2)-treated cultures. The resistance of BQ-treated cells to necrosis could be due to increased cellular GSH and formation of BQ-GSH conjugates which are less toxic than free BQ, minimal toxicity being provided by GS4-BHQ. This ability of certain cancer cells to tightly keep the apoptotic pathway may have therapeutic applications for oxidation-based drugs.

Tuning of ruthenium complex properties using pyrrole- and pyrrolidine-containing polypyridine ligands.

The effect of pyrrole- and pyrrolidine-containing ligands (L) on the properties of heteroleptic [RuL2dcbpy]2+ complexes has been investigated. TiO2 electrodes modified with the new complexes exhibited extended absorption domains and high absorbances. Providing that a cobalt-based mediator was used for regeneration of the RuII state, good incident photon-to-current efficiency (near 80%) values were obtained in the pyrrole series.

Pyrrolidine-containing polypyridines: new ligands for improved visible light absorption by ruthenium complexes.

[reaction: see text] A range of new electron-releasing pyrrolidine-containing bipyridines and terpyridines has been prepared via selective metalation-cross-coupling sequences. The obtained ligands have been involved in microwave-assisted ruthenium complexation leading to homoleptic complexes in high yield. The electron-donor effect of the pyrrolidine nucleus led to a notable improvement of visible light absorption and strong changes in the electrochemical behavior, opening new opportunities for the design of photovoltaic devices.

Energy Transfer in Rigid Ru(II)/Os(II) Dinuclear Complexes with Biscyclometalating Bridging Ligands Containing a Variable Number of Phenylene Units.

We have prepared rodlike cyclometalated Ru(II)/Os(II) dinuclear complexes, (ttp)Ru(dpb-(ph)(n)()-dpb)Os(ttp)(2+), where the biscyclometalating bridging ligands contain dipyridylbenzene fragments, dbp, separated by a variable number, n, of phenylene spacers, and the terminal ligand is a terpyridine derivative [dpbH is di-2-pyridyl-1,3-benzene, ttp is 4'-p-tolyl-2,2':6',2"-terpyridine, and n = 0-2]. The rigid bridging ligands keep the metal centers at a distance r(MM) = 11, 15.5, and 20 Å, depending on n. Photoinduced energy transfer has been investigated by luminescence spectroscopy in nitrile solvents at room temperature and at 77 K (i.e., in frozen medium). According to a classical description of the process, the energy transfer occurs in a nearly activationless regime, is governed by electronic factors, and can be described in terms of the Dexter-type mechanism. The obtained energy transfer rates roughly span 3 orders of magnitude and indicate (i) that the temperature (i.e., the state of the solvent) has a small influence on the process and (ii) that the interposed phenylene spacers are weak attenuators of intercenter electronic coupling, H [H = H(0) exp(-betar(MM)), with beta approximately 0.33 Å(-)(1)].