METAL COMPLEXES FOR MOLECULAR ELECTRONICS AND PHOTONICS

Probing the Excited States of d6 Metal Complexes Containing the 2,2‘-Bipyrimidine Ligand Using Time-Resolved Infrared Spectroscopy. 1. Mononuclear and Homodinuclear Systems

This paper reports time-resolved infrared (TRIR) spectroscopic studies on a series of weakly luminescent or nonluminescent 2,2'-bipyrimidine-based complexes to probe their electronic structure and the dynamic behavior of their excited states on the picosecond and nanosecond time scales. The complexes are mononuclear [Re(CO)3Cl(bpm)] (1), [Ru(CN)4(bpm)]2- (2), and [Ru(bpyam)2(bpm)]2+ (3) [bpm=2,2'-bipyrimidine; bpyam=2,2'-bipyridine-4,4'-(CONEt2)2] and their homodinuclear analogues [{Re(CO)3Cl}2(mu-bpm)] (4), [{Ru(CN)4}2(mu-bpm)]2- (5), and [{Ru(bpyam)2}2(mu-bpm)]4+ (6). Complex 1 shows the characteristic shift of the three nu(CO) bands to higher energy in the Re-->bpm triplet metal-to-ligand charge-transfer (3MLCT) state, which has a lifetime of 1.2 ns. In contrast, the dinuclear complex 4 shows nu(CO) transient bands to both higher and lower energy than the ground state indicative of, on the IR time scale, an asymmetric excited state [(OC)3ClReI(bpm*-)ReII(CO)3Cl] whose lifetime is 46 ps. The cyanoruthenate complexes 2 and 5 show comparable behavior, with a shift of the nu(CN) bands to higher energy in the excited state for mononuclear 2 but two sets of transient bands-one to higher energy and one to lower energy-in dinuclear 5, consistent with an asymmetric charge distribution [(NC)4RuII(bpm*-)RuIII(CN)4]4- in the 3MLCT state. These cyanoruthenate complexes have much longer lifetimes in D2O compared with CH3CN, viz., 250 ps and 3.4 ns for 2 and 65 ps and 1.2 ns for 5 in CH3CN and D2O, respectively. In complex 3, both higher-energy Ru-->bpyam and lower-energy Ru-->bpm 3MLCT states are formed following 400 nm excitation; the former decays rapidly (tau=6-7 ps) to the latter, and the subsequent decay of the Ru-->bpm 3MLCT state occurs with a lifetime of 60 or 97 ns in D2O or CH3CN, respectively. Similar behavior is shown by dinuclear 6 in both D2O and CH3CN, with initial interconversion from the Ru-->bpyam to the Ru-->bpm 3MLCT state occurring with tau approximately 7 ps and the resultant Ru-->bpm 3MLCT state decaying on the nanosecond time scale.

Structural and Photophysical Properties of Adducts of [Ru(bipy)(CN)4]2- with Different Metal Cations: Metallochromism and Its Use in Switching Photoinduced Energy Transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.

Interchain-Solvent-Induced Chirality Change of 1D Helical Chains: From Achiral to Chiral Crystallization

Three helical supramolecular stereoisomers of meso-2, Delta-2, and Delta-3 with the formula of cis-[Ni(f-rac-L)][Ni(CN)4] were successfully constructed based on the [Ni(f-rac-L)2+ and [Ni(CN)4]2- building blocks (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). In all three supramolecular stereoisomers, cis-[Ni(f-rac-L)]2+ cations are alternately bridged by [Ni(CN)4]2- anions through two cis (in meso-2 and Delta-2) or trans (in Delta-3) cyano groups to form one-dimensional (1D) helical chains of cis-[Ni(f-rac-L)][Ni(CN)4]. In meso-2, the right/left-handed chirality of the originally formed chain is transferred oppositely to adjacent chains through the interchain hydrogen-bonding interactions of hexameric water clusters, leading to the formation of meso-2 with a central symmetrical space group, P21/n, in which the 1D helical chains are packed in an alternating right- and left-handed chirality. In Delta-2 and Delta-3, the right/left-handed chirality of the original chain is transferred uniformly to adjacent chains through the zipper-like interchain hydrophobic interactions, resulting in the formation of Delta-2 and Delta-3 with chiral space groups of P212121 and P3121, respectively, in which all of the 1D helical chains are arranged in the same right/left-handed chirality.

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.

Electrochemical deposition of poly(trans-[RuCl2(4-vinylpyridine)4]) and its reductive desorption: cyclic voltammetry and electrochemical quartz crystal microbalance studies

The electropolymerization of trans-[RuCl(2)(vpy)(4)](vpy = 4-vinylpyridine) on Au or Pt electrodes was studied by cyclic voltammetry and the electrochemical quartz crystal microbalance (EQCM) technique. Cyclic voltammetry of the monomer in DMSO on Au shows reductions at -2.0 and -2.2 V. Potential cycling over the first wave leads to polymer formation; however, scanning over the second wave leads to desorption of the polymer. These observations were confirmed by EQCM measurements which also revealed a high polymerization efficiency. Electrolysis, EQCM and XPS measurements showed that desorption was associated with substitution of chloride ligands by DMSO when the polymer was in a highly reduced state. The film also showed reversible mass changes due to the oxidation and accompanying ingress of charge-balancing anions and solvent into the film. Measurements on the dried films revealed that large quantities of solvent are trapped in the film during the electropolymerization process.

Binuclear wirelike dimers based on ruthenium(II)-bipyridine units linked by ethynylene-oligothiophene-ethynylene bridges

The syntheses, structural characteristics, electrochemical behavior, ground-state spectra, photophysical properties, and transient absorption (TA) spectra in CH(3)CN solvent are reported for binuclear [(bpy)(2)Ru(bpy-E(T)(n)()E-bpy)Ru(bpy)(2)](4+) complexes, Ru(bpyT(n)bpy)Ru, where the Ru-based units are connected by alternating 3,4-dibutylthiophene (DBT')/thiophene (T') fragments linked via ethynyl groups (E) to bpy ligands at the 5-position (bpy is 2,2'-bipyridine). The ligand bpyT(3)bpy represents a module containing DBT'/T'/DBT' subunits, and bpyT(5)bpy accounts for a DBT'/T'/DBT'/T'/DBT' pattern. The syntheses and electrochemical and spectroscopic (emission and TA) properties in CH(2)Cl(2) solvent of the bpyT(n)()bpy ligands are likewise reported. The behavior of the Ru(bpyT(n)bpy)Ru dimers has been compared to that of the bpyT(n)bpy ligands and to that of a related mononuclear complex, [(bpy)(2)Ru(bpy-E-DBT')](2+), Ru(bpyDBT'). For the dimeric complexes, the electrochemical results show that the first reduction step takes place at the bpy ligand(s) bearing an ethynylene group, the first oxidation step is thiophene-centered, and further oxidation involves the metal centers, which are only weakly interacting. The photophysical and TA results for the Ru(bpyT(n)bpy)Ru dimers account for the presence of low-lying oligothiophene-centered (3)pi,pi levels, while higher-lying metal-ligand charge transfer ((3)MLCT) levels are thermally accessible only for the case of Ru(bpyT(3)bpy)Ru; the possible role of charge separation (CS) levels (from oxidation at the thiophene bridge and reduction at one of the coordinated bpy's) is also discussed.

Construction of New Heteroselenometallic Clusters: Formation of Crownlike [Et4N]4[(μ5-WSe4)(CuI)5(μ-I)2] and Octahedral Polymeric [(μ6-WSe4)Cu6I4(py)4]n from Planar [Et4N]4[(μ4-WSe4)Cu4I6] with Additional Faces

The coplanar cluster compound [Et4N]4[(mu4-WSe4)Cu4I6] (1) was prepared from reaction of [Et4N]2[WSe4] with 4 equiv of CuI in N,N-dimethylformamide (DMF) solution in the presence of [Et(4)N]I. Treatment of 1 with pyridine (py) in dry MeCN gave the neutral cluster [(mu4-WSe4)Cu4(py)6I2] (2) in good yield. Recrystallization of 1 from py/i-PrOH resulted in the reorganization of the coplanar WSe4Cu4 core and the formation of a neutral polymeric cluster [(mu3-WOSe3)Cu3(py)3(mu-I)]n (3) containing a nest-shaped OWSe3Cu3 core and a terminal W=O bond. The interaction of cluster 1 with excess PPh3 in CH3Cl3 gave [(mu3-WSe4)Cu3(PPh3)3(mu3-I)] (4) which has a cubanelike SeWSe3Cu3I core. Treatment of 1 with 1 equiv of CuI in dimethyl sulfoxide (DMSO) yielded [Et4N]4[(mu5-WSe4)(CuI)5(mu-I)2] (5) which has a crown-like core structure. Treatment of 1 in DMF with 2 equiv of CuI in the presence of py resulted in the formation of a two-dimensional polymeric cluster, [(mu6-WSe4)Cu6I4(py)4]n (6), consisting of an octahedral WSe4Cu6 repeating unit. The solid-state structures of clusters 3, 5, and 6 have been further established by X-ray crystallography. The nonlinear optical properties of 6 have been also investigated. Cluster 6 was found to exhibit good photostability and a large optical limiting effect with the limiting threshold being ca. 0.3 J cm(-2).