Luminescent cyclometalated Rh(III), Ir(III), and (DIP)2Ru(II) complexes with carboxylated bipyridyl ligands: synthesis, X-ray molecular structure, and photophysical properties.

Luminescent cyclometalated rhodium(III) and iridium(III) complexes of the general formula [M(ppy) 2(N (wedge)N)][PF 6], with N (wedge)N = Hcmbpy = 4-carboxy-4'-methyl-2,2'-bipyridine and M = Rh ( 1), Ir ( 2) and N (wedge)N = H 2dcbpy = 4,4'-dicarboxy-2,2'-bipyridine and M = Rh ( 3), Ir ( 4), were prepared in high yields and fully characterized. The X-ray molecular structure of the monocarboxylic iridium complex [Ir(ppy) 2(Hcmbpy)][PF 6] ( 2) was also determined. The photophysical properties of these compounds were studied and showed that the photoluminescence of rhodium complexes 1 and 3 and iridium complexes 2 and 4 originates from intraligand charge-transfer (ILCT) and metal-to-ligand charge-transfer/ligand-centered MLCT/LC excited states, respectively. For comparison purposes, the mono- and dicarboxylic acid ruthenium complexes [Ru(DIP) 2(Hcmbpy)][Cl] 2 ( 5) and [Ru(DIP) 2(H 2dcbpy)][Cl] 2 ( 6), where DIP = 4,7-diphenyl-1,10-phenanthroline, were also prepared, whose emission is MLCT in nature. Comparison of the photophysical behavior of these rhodium(III), iridium(III), and ruthenium(II) complexes reveals the influence of the carboxylic groups that affect in different ways the ILCT, MLCT, and LC states.

Ligand-field excited states of hexacyanochromate and hexacyanocobaltate as sensitisers for near-infrared luminescence from Nd(iii) and Yb(iii) in cyanide-bridged d-f assemblies.

Crystallisation of [Co(CN)(6)](3-) or [Cr(CN)(6)](3-) with Ln(iii) salts (Ln = Nd, Gd, Yb) from aqueous dmf afforded the cyanide-bridged d/f systems [Ln(dmf)(4)(H(2)O)(3)(micro-CN)Co(CN)(5)] (-, discrete dinuclear species) and {[Cr(CN)(4)(micro-CN)(2)Ln(H(2)O)(2)(dmf)(4)]}(infinity) (-, infinite cyanide-bridged chains with alternating Cr and Ln centres). With Ln = Gd the characteristic long-lived phosphorescence from d-d excited states of the [M(CN)(6)](3-) units was apparent in the red region of the spectrum, with lifetimes of the order of 1 micros, since the heavy atom effect of the Gd(iii) promotes inter-system crossing at the [M(CN)(6)](3-) units to generate the phosphorescent spin-forbidden excited states. With Ln = Yb or Nd however, the d-block luminescence was completely quenched due to fast (>10(8) s(-1)) energy-transfer to the Ln(iii) centre, resulting in the characteristic sensitised emission from Yb(iii) and Nd(iii) in the near-IR region. For both - and -, calculations based on spectroscopic overlap between emission of the donor (Co) and absorption of the acceptor (Ln) suggest that the Dexter energy-transfer mechanism is responsible for the complete quenching that we observe.

Spectroscopic and computational studies of a Ru(II) terpyridine complex: the importance of weak intermolecular forces to photophysical properties.

The complex [Ru(tpy)(CO)(2)TFA]+[PF(6)]- (where tpy = 2,2':6',2' '-terpyridine and TFA = CF(3)CO(2)-) (1) has been synthesized and fully characterized spectroscopically. The X-ray structure of the complex has been determined. The photopysical properties of the ruthenium complex and the free ligand tpy have been investigated at room temperature and at 77 K in acetonitrile solution and in the solid state. Their electronic spectra are highly influenced by intermolecular stacking interactions, both in solution and in the solid state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed to characterize the electronic structure and the excited states of [Ru(tpy)(CO)(2)TFA]+[PF(6)]- and tpy. TDDFT calculations on three different conformations of free ligand have been performed as well. Absorption and emission spectra of tpy have been studied at different temperatures and concentrations in order to have a better understanding of this ruthenium derivative's properties. The absorption spectrum of 1 is characterized by metal-perturbed ligand-centered (LC) bands in the UV region. No metal-to-ligand charge transfer (MLCT) bands are observed in the visible for the complex. Only at high concentrations (10(-4) M) does a very weak band appear at 470 nm. At 77 K and low concentrations, solutions of 1 exhibit a major 3LC emission band centered at 468 nm (21.4 x 10(-3) cm(-1)). When the concentration of the complex is increased, an unstructured narrow emission at 603 nm (16.6 x 10(-3) cm(-1)), with a lifetime of 10 micros, dominates the emission spectrum in glassy acetonitrile. This emission originates from a pi-pi stacked dimeric (or oligomeric) species. TDDFT calculations performed on a tail-to-tail dimer structure, similar to that seen in the solid state, ascribe the transition to a triplet excited state, where intermolecular metal (d) --> ligand (pi*, polypyridine) charge transfer occurs. A good estimate of the transition energy is also obtained (623 nm, 1.94 eV).

Ruthenium-terpyridine complexes with multiple ethynylpyrenyl or ethynyltoluyl subunits: X-ray structure, redox, and spectroscopic properties.

Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units.

cis-trans photoisomerization in [Ru(DIP)2(MeOH)2][OTf]2: synthesis, NMR, X-ray structure of the trans-isomer and photophysical properties.

The first trans-ruthenium complex trans-[Ru(DIP)2(MeOH)2][OTf]2 (1b, where DIP = 4,7-diphenyl-1,10-phenanthroline) incorporating a pi-extended ligand was prepared via two methods: either photolysis of cis-[Ru(DIP)2(OTf)2] in MeOH-Et(2)O or via crystallization from MeOH-Et(2)O in direct sunlight. The X-ray molecular structure of is reported and confirmed the trans geometry of the title compound. The cis-trans isomerization process was monitored by 1H-NMR and showed that 1b reverts back to cis-[Ru(DIP)2(MeOH)2][OTf]2 (1a) in methanol-d4 after 15 h at 55 degrees C or several days at room temperature. The absorption spectra recorded in MeOH showed a bathochromic shift of the MLCT band of the trans-isomer 1b relative to that of the cis complex 1a. Interestingly at 77 K the emission spectrum of 1b is red shifted compared to the cis analog 1a. A rational explanation in terms of the energy of the excited states in the cis- and trans-isomers is proposed to explain this behavior.

Fast, through-bond mediated energy transfer from Ir(III) to Ru(II) in di- and tetranuclear heterometallic assemblies: elucidation of a two-step Ir --> Ir --> Ru energy transfer process.

Energy transfer processes triggered by light excitation (lambda(exc) = 278 nm) have been investigated in a heterometallic complex, Ir-Ru, and in a tetranuclear assembly, ((F4))(2)-Ir-Ru, containing iridium(III) and ruthenium(II) centres {Ir represents the unit [(ppy)(2)Ir(bpy)](+), Ru is [Ru(bpy)(3)](2+) and Ir(F4) is [Ir(F(2)ppy)(2)(4-bpy-C(6)H(4)-)](+) (ppy = 2-phenylpyridyl, bpy = 2,2'-bipyridyl)}. In the dinuclear species, the two metallic components are linked by a biphenylene bridge connected between the 4-positions of the bpy of Ir and one of the three bpy's of Ru. The tetrametallic species comprises the dimeric unit appended with Ir(F4) units at the 4'-positions of the ppy ligands. For both the dinuclear and tetranuclear complexes, the metal centers are held at fixed distances by the interposed phenylene bridges. We have obtained the optical (absorption and emission) properties for the mononuclear species Ir(F4), Ir and Ru and for the polymetallic species Ir-Ru and ((IrF4))(2)-Ir-Ru. For Ir-Ru, the Ir --> Ru energy transfer step is exothermic by ca. 0.22 eV, based on the emission energies of the respective mononuclear components at 77 K. In turn, within ((F4))(2)-Ir-Ru, the excited state energy of Ir(F4) is ca. 0.26 eV higher than Ir. By using lambda(exc) = 278 nm, it is possible to predominantly excite the iridium-based units of both Ir-Ru and (Ir(F4))(2)-Ir-Ru. For both cases, energy transfer is found to be fast and efficient with k(en) > 2 x 10(8) s(-1), leading to detectable emission only from the Ru component both at room temperature and at 77 K. In particular, for ((F4))(2)-Ir-Ru, based on the evaluated intermetallic distances (e.g. 28.5 A for the shortest Ir(F4)-Ru distance) and the overlap integrals of donor and acceptor units, the observed energy transfer is too fast to be accounted for by through-space Förster transfer. In this species, energy transfer probably occurs in a two-step process, Ir(F4)-->Ir-->Ru, both steps involving a through-bond Dexter mechanism.

Spectroscopic and redox properties of novel d6-complexes engineered from all Z-ethenylthiophene-bipyridine ligands.

A series of quasilinear dinuclear complexes incorporating ruthenium(II)- and osmium(II)-tris(2,2'-bipyridine) units has been prepared in which the individual metal-containing moieties are separated by 3,4-dibutyl-2,5-diethenylthiophene spacers and end-capped by 3,4-dibutyl-2-ethenylthiophene subunits; related ruthenium(II) and osmium(II) mononuclear complexes have also been prepared where one bpy unit is likewise end-capped by 3,4-dibutyl-2-ethenylthiophene subunits [bpy = 2,2'-bipyridine]. Overall, mononuclear species, labeled here Ru and Os, and dinuclear species, RuRu, OsOs, and RuOs, have been prepared and investigated. Their electrochemical behavior has been studied in CH3CN solvent and reveals ethenylthiophene-centered oxidations (irreversible steps at > +1.37 V vs SCE), metal-centered oxidations (reversible steps at +1.30 V vs SCE for Ru(II/III) and +0.82 V vs SCE for Os(II/III)), and successive reduction steps localized at the substituted bpy subunits. The spectroscopic studies performed for the complexes in CH3CN solvent provided optical absorption spectra associated with transitions of ligand-centered nature (LC, from the bpy and ethenylthiophene subunits) and metal-to-ligand charge-transfer nature (MLCT), with the former dominating in the visible region (400-600 nm). While the constituent ethenylthiophene-bpy ligands are strong fluorophores (fluorescence efficiency in CH2Cl2 solvent, phi em = 0.49 and 0.39, for the monomer and the dimer, respectively), only weak luminescence is observed for each complex in acetonitrile at room temperature. In particular, (i) the complexes Ru and RuRu do not emit appreciably, and (ii) the complexes Os, OsOs, and RuOs exhibit triplet emission of 3Os --> L CT character, with phi em in the range from 10-3 to 10-4. These features are rationalized on the basis of the role of nonemissive triplet energy levels, 3Th, centered on the ethenylthiophene spacer. These levels appear to lie lower in energy than the 3Ru --> L CT triplet levels, and in turn higher in energy than the 3Os --> L CT triplet levels, along the sequence 3Ru --> L CT > 3Th > 3Os --> L CT.

Dinuclear iridium(III) complexes consisting of back-to-back tpy-(ph)n-tpy bridging ligands (n = 0, 1, or 2) and terminal cyclometallating tridentate N-C-N ligands.

Three dinuclear iridium(III) complexes consisting of a conjugated bis-tpy type bridging ligand and cyclometallating capping tridentate ligands of the 1,3-di-2-pyridylbenzene family have been prepared (tpy, 2,2',6',2' '-terpyridine). The two tpy units of the bridge are connected via their back-positions (4') either directly or with a p-phenylene or p-biphenylene spacer. The synthesis relies on the reaction between the dinuclear [Ir(dpb)Cl2]2 complex (dpb-H =1,3-dipyridyl-4,6-dimethylbenzene) and the corresponding bis-tpy ligand. Electrochemical measurements afford metal-centered oxidation and ligand-centered reduction potentials; from the oxidation steps, no evidence is obtained for a strong coupling between the two iridium(III) subunits of the dinuclear species. For all complexes, ground-state absorption data in the 380 nm to visible region show a trend which is consistent with the presence of charge-transfer (CT) transitions involving different degrees of electronic delocalization at the bridging ligands. (dpb)Ir(tpy-tpy)Ir(dpb)4+ exhibits an appreciable luminescence at room temperature (phi = 3.0 x 10(-3); tau = 3.3 ns), whereas no emission from the other binuclear complexes is detected. All binuclear complexes luminesce at 77 K, and a metal-to-ligand CT nature for (dpb)Ir(tpy-tpy)Ir(dpb)4+ is suggested, whereas a ligand-centered (LC) emission is proposed for (dpb)Ir(tpy-(ph)2-tpy)Ir(dpb)4+ on the basis of the comparison with the phosphorescence properties of the free bridging ligand, tpy-(ph)2-tpy. Transient absorbance experiments at room temperature afford the absorption spectra and lifetimes of the non-emissive excited states. For (dpb)Ir(tpy-ph-tpy)Ir(dpb)4+ and (dpb)Ir(tpy-(ph)2-tpy)Ir(dpb)4+, the spectra exhibit a broad profile peaking around 780 nm, quite intense in the case of (dpb)Ir(tpy-(ph)2-tpy)Ir(dpb)4+, and lifetimes of 160 and 440 ps, respectively.

Red-shifted luminescence from naphthalene-containing ligands due to pi-stacking in self-assembled coordination cages.

Incorporation of ligands containing substituted naphthalene cores into coordination cages results in extensive aromatic pi-stacking between ligands; this results in a red-shifted 'excimer-like' luminescence component from the naphthyl groups compared to the free ligands, which is diagnostic of, and can be used to monitor, cage assembly.

Energy transfer in hybrids based on a thiophene-substituted ethynylbipyridine dimer decorated with Re(I), Ru(II), and Os(II) units.

The preparation, structural features, electrochemical behavior, and optical properties (at room temperature and at 77 K) are reported for a series of thiophene-containing hybrids based on the bent conjugated backbone of a rigid ditopic ligand, the dimeric moiety 3,4-dibutyl-2,5-bis{5'-[(3,4-dibutylthien-2-ylethynyl)-2,2'-bipyridin-5-yl]ethynyl}thiophene (TBTBT). Within the dimer, the diethynyl-2,2'-bipyridine units (bpy, the coordination sites) alternate with three 3,4-dibuthylthiophene units and coordination of the [Re(CO)3Cl], [Ru(bpy)2]2+, and [Os(bpy)2]2+ centers results in the mononuclear species RuTBTBT and OsTBTBT and the binuclear species RuTBTBTRu, OsTBTBTOs, RuTBTBTOs, and ReTBTBTOs. At room temperature, the emitting states obtained by photoexcitation are of 3MLCT nature, and vibronic analysis of the emission spectra indicates that they are largely delocalized over the TBTBT ligand. In the binuclear species, the intermetal separation is ca. 17 A, and for RuTBTBTOs, an efficient Ru --> Os excitation transfer takes place, resulting solely in an Os-based emission. The process is ascribed to double-electron transfer (Dexter), as mediated by the TBTBT ligand; a similar conclusion holds for the case of ReTBTBTOs. For RuTBTBTOs, the process is discussed in some detail also with regard to the possibility of disentangling the constituent hole and electron-transfer events.

The electron transfer rate of large TPA based compounds: a joint theoretical and electrochemical approach.

A series of triphenylamine (TPA) based compounds is investigated by means of density functional theory and cyclic voltammetry. Using the Nicholson's formalism, the measured deltaE(p) are correlated with B3LYP/6-31G* calculated reorganisation energies (lambda), elucidating the trend followed by the electron transfer rate of these compounds. Besides the direct dependency upon the dimension of the cationic fragment contributing to the hole stabilisation, the lambdas are tuned by the symmetry local to the TPA units, as evidenced by the structural relaxation of the cations. MDTAB shows the interesting combination of low ionisation potential (IP) and low lambda. This can make this compound interesting for practical applications in organic light emitting diode (OLEDs) devices, due to the direct correlation of the IP and lambda with the hole transfer efficiency to the anode, along with the hole mobility.

Eilatin complexes of ruthenium and osmium. synthesis, electrochemical behavior, and near-IR luminescence.

The synthesis and characterization of new Ru(II) and Os(II) complexes of the ligand eilatin (1) are described. The new complexes [Ru(bpy)(eil)(2)](2+) (2), [Ru(eil)(3)](2+) (3), and [Os(eil)(3)](2+) (4) (bpy = 2,2'-bipyridine; eil = eilatin) were synthesized and characterized by NMR, fast atom bombardment mass spectrometry, and elemental analysis. In the series of complexes [Ru(bpy)(x)(eil)(y)()](2+) (x + y = 3), the effect of sequential substitution of eil for bpy on the electrochemical and photophysical properties was examined. The absorption spectra of the complexes exhibit several bpy- and eil-associated pi-pi and metal-to-ligand charge-transfer (MLCT) transitions in the visible region (400-600 nm), whose energy and relative intensity depend on the number of ligands bound to the metal center (x and y). On going from [Ru(bpy)(2)(eil)](2+) (5) to 2 to 3, the d(pi)(Ru) --> pi(eil) MLCT transition undergoes a red shift from 583 to 591 to 599 nm, respectively. Electrochemical measurements performed in dimethyl sulfoxide reveal several ligand-based reduction processes, where each eil ligand can accept up to two electrons at potentials that are significantly anodically shifted (by ca. 1 V) with respect to the bpy ligands. The complexes exhibit near-IR emission (900-1100 nm) of typical (3)MLCT character, both at room temperature and at 77 K. Along the series 5, 2, and 3, upon substitution of eil for bpy, the emission maxima undergo a blue shift and the quantum yields and lifetimes increase. The radiative and nonradiative processes that contribute to deactivation of the excited level are discussed in detail.