Transition state characterization for the reversible binding of dihydrogen to bis(2,2'-bipyridine)rhodium(I) from temperature- and pressure-dependent experimental and theoretical studies

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh(I)(bpy)2]+ (bpy = 2,2'-bipyridine) to form [Rh(III)(H)2(bpy)2]+ were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh(I)(bpy)2]+ and [Rh(III)(H)2(bpy)2]+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh(III)(H)2(bpy)2]+ as a function of H2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol(-1) and -18 cal mol(-1) K(-1), respectively. The reaction enthalpy and entropy are -10.3 kcal mol(-1) and -19 cal mol(-1) K(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm(3) mol(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm(3) mol(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.     

Acid-Base Properties of the Ground and Excited States of Ruthenium(II) Bis(2,2'-bipyridine) 3-Carboxyl-2,2'-bipyridine

Acid-base properties for ruthenium(II) bis(2,2'-bipyridine) 3-carboxyl-2,2'-bipyridine reveal a ground state pK(a) of 0.82 +/- 0.07 and an excited state pK(a) of 2.31 +/- 0.05, a 1.5 pH unit increase from the ground state. The excited state pK(a) is temperature independent while the ground state pK(a)(0) increases with temperature and has DeltaH(0) and DeltaS(0) values of -990 +/- 149 cm(-)(1) and -4.57 +/- 0.48 cm(-)(1) K(-)(1), respectively. The acidic form of the complex emits at lower energy than the basic form at both 296 and 77 K. The emission energy maxima are solvent dependent and decrease in energy when the solvent changes from 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) to water and when the pH decreases. Changes in excited state lifetimes with emission energy follow the energy gap law with an intercept of 49 +/- 1 and a slope of (2.11 +/- 0.09) x 10(-)(3). Emission quantum yields for protonated and deprotonated species in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) are 0.023 +/- 0.001 and 0.110 +/- 0.002, respectively. The temperature dependence of the emission lifetimes gives energy barriers of 270 cm(-)(1) for the complex in aqueous solution at pH -0.5, and 990 cm(-)(1) in aqueous solution at pH 4.5, and 1920 cm(-)(1) in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2.)     

Changing the role of 2,2'-bipyridine from secondary ligand to protagonist in [Ru(bpy)2(N-N)]2+ complexes: low-energy, red emission from a ruthenium(II)-to-2,2'-bipyridine 3MLCT state

Two new bidentate ligands (1 and 2) with bicyclic guanidine moieties were synthesized and attached to a Ru(II)(bpy)(2) core (bpy = 2,2'-bipyridine) to afford complexes 3 and 4, which were characterized by spectroscopic and electrochemical methods. Complex 4 was further characterized by X-ray crystallography. In cyclic voltammetric studies, both complexes show a Ru(II/III) couple, which is 500 mV less positive than the Ru(II/III) couple of Ru(bpy)(3)(2+). The (1)MLCT and (3)MLCT states of 3 (560 nm/745 nm) and 4 (550 nm/740 nm) are significantly red-shifted with respect to Ru(bpy)(3)(2+) (440 nm/620 nm). Compounds 3 and 4 exhibit emission from a Ru(II)-to-bpy (3)MLCT state, which is rarely the emitting state at λ > 700 nm in [Ru(bpy)(2)(N-N)](2+) complexes.     

Heteroleptic ruthenium complexes containing uncommon 5,5'-disubstituted-2,2'-bipyridine chromophores for dye-sensitized solar cells

Four new heteroleptic ruthenium sensitizers [Ru(4,4'-carboxylic acid-2,2'-bipyridine)(L)(NCS)(2)] (L = 5,5'-bis(4-octylthiophen-2-yl)-2,2'-bipyridine (1), 5,5'-bis(N,N-diphenyl-4-aminophenyl)-2,2'-bipyridine (2), 5,5'-bis(5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2'-bipyridine (3) and 5,5'-bis(4-octyl-5-(N,N-diphenyl-4-aminophenyl)-thiophen-2-yl)-2,2'-bipyridine (4)) were synthesized, characterized by physicochemical and computational methods, and utilized as photosensitizers in nanocrystalline dye-sensitized solar cells (DSSCs). The λ(max) of the metal-to-ligand charge transfer (MLCT) absorption of these four ruthenium dyes (527 nm for 1, 535 nm for 2, 585 nm for 3 and 553 nm for 4) can be tuned by various structural modifications of the ancillary ligand and it was shown that increasing the conjugation length of such ligand reduces the energy as well as the molar absorption coefficient of the MLCT band. The maximum incident photon to current conversion efficiency (IPCE) of 41.4% at 550 nm, 38.6% at 480 nm, 39.4% at 470 nm and 31.1% at 480 nm for 1-, 2-, 3- and 4-sensitized solar cells were obtained. Respectable power conversion efficiencies of 3.00%, 2.51%, 2.00% and 2.03% were realized, respectively, when the sensitizers 1, 2, 3 and 4 were used in DSSCs under the standard air mass (AM) 1.5 sunlight illumination (versus 5.9% for standard N719).     

Synthesis and structural,electrochemical, and optical properties of Ru(II) complexes with azobis(2,2'-bipyridine)s

Two new ditopic ligands, 5,5"-azobis(2,2'-bipyridine) (5,5"-azo) and 5,5"-azoxybis(2,2'-bipyridine) (5,5"-azoxy), were prepared by the reduction of nitro precursors. Mononuclear and dinuclear Ru(II) complexes having one of these bridging ligands and 2,2'-bipyridine terminal ligands were also prepared, and their properties were compared with previously reported Ru(II) complexes having 4,4"-azobis(2,2'-bipyridine) (4,4"-azo). The X-ray crystal structure showed that 5,5"-azo adopts the trans conformation and a planar rodlike shape. The X-ray crystal structure of [(bpy)(2)Ru(5,5"-azo)Ru(bpy)(2)](PF(6))(4) (Ru(5,5"-azo)Ru) showed that the bridging ligand is in the trans conformation and nearly planar also in the complex and the metal-to-metal distance is 10.0 A. The azo or azoxy ligand in these complexes exhibits reduction processes at less negative potentials than the terminal bpy's due to the low-lying pi level. The electronic absorption spectra for the complexes having 5,5"-azo or 5,5"-azoxy exhibit an extended low-energy metal-to-ligand charge-transfer absorption. The ligands, 5,5"-azo and 5,5"-azoxy, and the mononuclear complex, [(bpy)(2)Ru(5,5"-azo)](2+), isomerize reversibly upon light irradiation. The low-energy MLCT state sensitizes the isomerization of the azo moiety in this complex. While [(bpy)(2)Ru(4,4"-azo)Ru(bpy)(2)](PF(6))(4) exhibits light switch properties, namely, significant electrochromism and a large luminescence enhancement, upon reduction, Ru(5,5"-azo)Ru does not show these properties. The radical anion formation upon reduction of these complexes has been confirmed by ESR spectroscopy.     

Direct observation of the fourth MLCT triplet state in ruthenium(II) tris(2,2'-bipyridine)

The luminescence properties of ruthenium(ii) tris(2,2'-bipyridine) have been recorded in butyronitrile solution and in a transparent KBr disk over a reasonable temperature range. In solution, spectral curve fitting routines indicate that emission arises solely from an ensemble of triplet states, each of which is of Metal-to-Ligand, Charge-Transfer (MLCT) character and of closely comparable energy. At ambient temperature, dual emission is observed for the KBr disk and interpreted in terms of luminescence from both the ensemble and the fourth MLCT triplet state that lies at slightly higher energy. Relative reorganisation energies, energies, Huang-Rhys factors and radiative rate constants have been calculated for the two emissive states. It is confirmed that the fourth MLCT triplet state possesses more singlet character than the ensemble.     

Catalytic properties of the dioxomolybdenum siloxide MoO2(OSiPh3)2 and its 2,2'-bipyridine adduct MoO2(OSiPh3)2(bpy)

The tetrahedral triphenylsiloxy complex MoO(2)(OSiPh(3))(2) (1) and its Lewis base adduct with 2,2'-bipyridine, MoO(2)(OSiPh(3))(2)(bpy) (2), were prepared and characterised by IR/Raman spectroscopy, and thermogravimetric analysis. Both compounds catalyse the epoxidation of cis-cyclooctene at 55 degrees C using tert-butylhydroperoxide (t-BuOOH) is decane as the oxidant, giving 1,2-epoxycyclooctane as the only product. The best results were obtained in the absence of a co-solvent (other than the decane) or in the presence of 1,2-dichloroethane, while much lower activities were obtained when hexane or acetonitrile were added. With no co-solvent, catalyst 1 (initial activity 272 mol x molMo(-1) x h(-1)for a catalyst:substrate: oxidant molar ratio of 1:100:150) is much more active than 2(initial activity 12 mol x molMo(-1) x h(-1)). The initial reaction rates showed first order dependence with respect to the initial concentration of olefin. With respect to the initial amount of oxidant, the rate order dependence for 1 (1.9) was higher than that for 2 (1.6).The dependence of the initial reaction rate on reaction temperature and initial amount of catalyst was also studied for both catalysts. The lower apparent activation energy of 1 (11 kcal x mol(-1)) as compared with 2 (20 kcal x mol(-1)) is in accordance with the higher activity of the former.     

Photophysical properties of ruthenium(II) tris(2,2'-bipyridine) complexes bearing conjugated thiophene appendages

A small series of ruthenium(II) tris(2,2'-bipyridine) complexes has been synthesized in which ethynylated thiophene residues are attached to one of the 2,2'-bipyridine ligands. The photophysical properties depend on the conjugation length of the thiophene-based ligand, and in each case, dual emission is observed. The two emitting states reside in thermal equilibrium at ambient temperature and can be resolved by emission spectral curve-fitting routines. This allows the properties of the two states to be evaluated in both fluid butyronitrile solution and a transparent KBr disk. It is concluded that both emitting states are of metal-to-ligand charge-transfer (MLCT) character, and despite the presence of conjugated thiophene residues, there is no indication for a low-lying pi,pi*-triplet state that promotes nonradiative decay of the excited-state manifold. A key feature of these systems is that the conjugation length imposed by the thiophene-based ligand helps to control the rate constants for both radiative and nonradiative decay from the two MLCT triplet states.     

trans-cis Photoisomerization of the styrylpyridine Ligand in [Re(CO)3(2,2'-bipyridine)(t-4-styrylpyridine)]+: role of the metal-to-ligand charge-transfer excited states

The trans-cis isomerization of the styrylpyridine carbon-carbon double bond induced by visible light irradiation in fac-[Re(CO)(3)(bpy)(stpy)](+) (bpy = 2,2'-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)(3)(bpy)(stpy)](+) have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol(-1) each other. The energy difference between the cis and trans isomers is 27.0 kJ mol(-1). The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d(Re)-->pi*(bpy)) transitions calculated at about 25,000 cm(-1) and by a strong intraligand (1)IL (pi(stpy)-->pi*(stpy)) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the C=C bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the (1)IL (S(1)) and electronic ground (S(0)) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the (3)IL state. From the (3)IL(t) (t = trans) the system evolves to the perpendicular intermediate (3)IL(p) (p = perpendicular) following a 90 degrees rotation around the styrylpyridine C=C bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the (3)IL and electronic ground state PEC determines the quantum yield of the isomerization process.     

Isotope effects on the picosecond time-resolved emission spectroscopy of tris(2,2'-bipyridine)ruthenium (II)

The excited-state properties of the transition metal complexes tris(2,2'-bipyridine) ruthenium(II) and tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II) are examined using picosecond time-resolved luminescence spectroscopy. For both complexes, direct observation of a short-lived high-energy emission with a lifetime of less than 4 ps is reported. Upon deuteriation of the complexes the lifetime of the high-energy emission shows a marked increase with a biexponential decay (20 and approximately 300 ps components). Examination by time-resolved excited-state resonance Raman shows that for the perprotio complexes features attributable to the 3MLCT excited state are formed within 4 ps, while for the perdeuterio a rise time of approximately 20 ps is observed in the 3MLCT features. The results indicate that the emission in both cases may be 1MLCT in origin and are discussed with respect to heterogeneous electron transfer.     

Synthesis, structures and properties of Cu(II) and Mn(II) complexes with 1,10-phenanthroline-2-carboxylic acid and 2,2'-bipyridine ligands

Four new 2,2'-bipyridine and 1,10-phenanthroline complexes, namely [Mn(phenca)(2)]·(H(2)O)(2) (1), [Cu(4)(phen)(4)(OH-)(4)(H(2)O)(2)](DMF)(4)(ClO(4)-)(4)(H(2)O) (2), [Cu(2)(2,2-bipy)(2)(C(2)O(4)2-)(H(2)O)(2)(NO(3))(2)] (3) and [Cu(2,2-bipy)(2)(ClO(4)-)](ClO(4)-) (4) (2,2'-bpy = 2,2'-bipyridine, Hphenca = 1,10-phenanthroline-2-carboxylic acid) have been synthesized by a hydrothermal reaction. The products were characterized by elemental analysis, infrared spectroscopy and X-ray crystal diffraction. While strong hydrogen bonds play central roles in the formation of the 3D structure, the combined influence of the weak interactions such as π···π interactions is also evident in the structures. A preliminary investigation on the ion exchange properties of the complexes is presented.     

Thermal and photo control of the linkage isomerism of bis(thiocyanato)(2,2'-bipyridine)platinum(II)

Three linkage isomers, bis(thiocyanato-S)(2,2'-bipyridine)platinum(II) ([Pt(SCN)(2)(bpy)]), (thiocyanato-S)(thiocyanato-N)(2,2'-bipyridine)platinum(II) ([Pt(SCN)(NCS)(bpy)]), and bis(thiocyanato-N)(2,2'-bipyridine)platinum(II) ([Pt(NCS)(2)(bpy)]) were isolated, and their structures were elucidated. The crystal data are as follows: for [Pt(SCN)(2)(bpy)], C(12)H(8)N(4)S(2)Pt, orthorhombic, P2(1)2(1)2(1) (No. 19), a = 12.929(9) A, b = 18.67(1) A, c = 5.497(4) A, Z = 4; for [Pt(SCN)(NCS)(bpy)], C(12)H(8)N(4)S(2)Pt, monoclinic, P2(1)/n (No. 14), a = 10.909(7) A, b = 7.622(4) A, c = 16.02(1) A, beta = 102.323(7) degrees, Z = 4; for [Pt(NCS)(2)(bpy)], C(12)H(8)N(4)S(2)Pt, orthorhombic, Pbcm (No. 57), a = 10.3233(8) A, b = 19.973(2) A, c = 6.4540(5) A, Z = 4. The stacking structures of the isomers were found to be different depending on the coordination geometries based on the N- and S-coordination of the thiocyanato ligands, which control the color and luminescence of the crystals sensitively. The isomerization behaviors of the complex have been investigated both in solution and in the solid state. In solution, stepwise thermal isomerization from [Pt(SCN)(2)(bpy)] to [Pt(NCS)(2)(bpy)] by way of [Pt(SCN)(NCS)(bpy)] was observed using (1)H NMR spectroscopy. Reverse isomerization, from [Pt(NCS)(2)(bpy)] to [Pt(SCN)(NCS)(bpy)] and [Pt(SCN)(2)(bpy)], occurred when irradiated with near ultraviolet (UV) light. In contrast, the [Pt(SCN)(2)(bpy)] yellow crystals exhibited thermal isomerization directly to red crystals of [Pt(NCS)(2)(bpy)], as detected by changes in the emission spectrum, which indicates that the flip of two SCN(-) ligands correlatively occurred in the solid state. The yellow crystals of [Pt(SCN)(NCS)(bpy)] were also converted to the thermodynamically stable red crystal of [Pt(NCS)(2)(bpy)] though the reverse isomerization has never been observed to occur by photoirradiation in the solid state.     

Theoretical studies of phosphorescence spectra of tris(2,2'-bipyridine) transition metal compounds

Phosphorescence spectra of tris(2,2'-bipyridine) metal compounds, [M(bpy)3]n+, where M = Zn(II), Ru(II), Os(II), Rh(III), and Ir(III), were calculated using a harmonic oscillator approximation of adiabatic potential surfaces obtained by density functional theory (DFT). Using the Huang-Rhys (S) factors calculated by theoretical Franck-Condon analysis of T1 and S0 geometries, we successfully reproduced the emission spectra observed under various conditions by nonempirical calculations. The simulations of well-structured spectra of the Zn(II), Rh(III), and Ir(III) compounds confirmed that the emission originated from localized ligand-centered excited states with considerably distorted geometries of C2 symmetry. The spectrum simulation revealed that the phosphorescence state of [Ru(bpy)3]2+ was localized 3MLCT both in a solution and a glass matrix. Furthermore, a highly resolved phosphorescence spectrum observed for [Ru(bpy)3]2+ doped in a [Zn(bpy)3](ClO4)2 crystal was reproduced well using the geometry of the localized 3MLCT by assuming mode-specific broadening of low-frequency intramolecular vibrational modes. The deuterium effects of the electronic origins of the doped crystal observed by Riesen et al. were in excellent agreement with those predicted for the localized 3MLCT. However, the calculated satellite structures of the localized 3MLCT involving bpy-h8 in [Ru(bpy-h8)(3-x)(bpy-d8)x]2+ (x = 1,2) exhibited only the bpy-h8 vibrational modes, inconsistent with the simultaneous appearance of both bpy-h8 and bpy-h8 modes in the observed spectra. A simulation on the basis of the geometry of the delocalized 3MLCT was in reasonable agreement with an unresolved spectrum observed for a neat crystal of [Ru(bpy)3](PF6)2, which is inconsistent with the assignments of localized 3MLCT on the basis of the electronic origins. The inconsistency of the assignment on the basis of the adiabatic model is discussed in terms of vibronic coupling between the localized 3MLCT states. The 3MLCT state in [Os(bpy)3]2+ seems to vary with the environment: a fully localized 3MLCT in a solution, partially localized in a glass matrix, and delocalized in PF6 salts.     

Molecular Orbital Calculation and Spectroscopic Study of the Photochemical Generation of Bis(2,2'-bipyridine)rhodium(I) from Bis(2,2'-bipyridine)(oxalato)rhodium(III)

A planar complex, [Rh(bpy)(2)](+) (bpy = 2,2'-bipyridine), was obtained from [Rh(ox)(bpy)(2)](+) (ox = oxalato) by photoirradiation. A rate constant k for the photoreaction was evaluated as 1 x 10(8) s(-1) in simple first-order kinetics, whereas a ligand dissociation, a reorganization of the coordinated bpy, and a two-electron transfer were involved in the reaction. The process of the Rh(I) complex generation was investigated in terms of a discrete variational (DV)-Xalpha molecular orbital calculation on [Rh(ox)(HN=CHCH=NH)(2)](+) instead of [Rh(ox)(bpy)(2)](+). From the calculation, using the transition-state method, it was predicted that a transition of the ox pi orbital to the metal 4d(z)()2 orbital caused the ligand dissociation and the reorganization of the coordinated bpy occurred in the ox pi to Rh 4d(x)()2(-y)2 excited state stabilized by the ox elimination. Upon release of the ligand and a change from octahedral to square-planar geometry, the electron density on the metal increased and the Rh 4d orbital acquired a d(8) electronic configuration.     

六氟磷酸二(2,2'-联吡啶)·(4,4'-二甲基-2,2'-联吡啶) 合钌(Ⅱ)的二维核磁共振谱分析

用一维NMR方法研究了电化学发光物质六氟磷酸二(2,2'-联吡啶)·(4,4'-二甲基-2,2'-联吡啶)合钌(Ⅱ)的立体结构,借助二维1H-1H COSY和1H-13C COSY实验技术对其氢谱和碳谱进行了完全的归属,并给出了其氢谱和碳谱的化学位移值.     

Ultrafast excited-state dynamics preceding a ligand trans-cis isomerization of fac-[Re(Cl)(CO)3(t-4-styrylpyridine)2] and fac-[Re(t-4-styrylpyridine)(CO)3(2,2'-bipyridine)]+

UV-vis absorption and resonance Raman spectra of the complexes fac-[Re(Cl)(CO)3(stpy)2] and fac-[Re(stpy)(CO)3(bpy)]+ (stpy = t-4-styrylpyridine, bpy = 2,2'-bipyridine) show that their lowest absorption bands are dominated by stpy-localized intraligand (IL) pi pi* transitions. For the latter complex a Re --> bpy transition contributes to the low-energy part of the absorption band. Optical population of the 1IL excited state of fac-[Re(Cl)(CO)3(stpy)2] is followed by an intersystem crossing (< or =0.9 ps) to an 3IL state with the original planar trans geometry of the stpy ligand. This state undergoes a approximately 90 degrees rotation around the stpy C=C bond with a 11 ps time constant. An electronically excited species with an approximately perpendicular orientation of the phenyl and pyridine rings of the stpy ligand is formed. Conversion to the ground state and isomerization occurs in the nanosecond range. Intraligand excited states of fac-[Re(stpy)(CO)3(bpy)]+ show the same behavior. Moreover, it was found that the planar reactive 3IL excited state is rapidly and efficiently populated after optical excitation into the Re --> bpy 1MLCT excited state. A 1MLCT --> 3MLCT intersystem crossing takes place first with a time constant of 0.23 ps followed by an intramolecular energy transfer from the ReI(CO)3(bpy) chromophore to a stpy-localized 3IL state with a 3.5 ps time constant. The fast rate ensures complete conversion. Coordination of the stpy ligand to the ReI center thus switches the ligand trans-cis isomerization mechanism from singlet to triplet (intramolecular sensitization) and, in the case of fac-[Re(stpy)(CO)3(bpy)]+, opens an indirect pathway for population of the reactive 3IL excited state via MLCT states.     

Mechanochromic luminescence switch of platinum(II) complexes with 5-trimethylsilylethynyl-2,2'-bipyridine

Planar platinum(II) complexes Pt(bpyC≡CSiMe(3))(C≡CC(6)H(4)R-4)(2) (R = H (1), Bu(t) (2)) with 5-trimethylsilylethynyl-22'-bipyridine show an unusual, reversible, and reproducible mechanical stimuli-responsive color and luminescence switch. When crystalline 1 or 2 is ground, bright yellow-green emitting is immediately converted to red luminescence with an emission red shift of 121-155 nm for 1 or 53-89 nm for 2. Meanwhile, the crystalline state is transformed to an amorphous phase that can be reverted to the original crystalline state by organic vapor adsorbing or heating, along with red luminescence turning back to yellow-green emitting. The reversibility and reproducibility of luminescence mechanochromic properties have been dynamically monitored by the variations in emission spectra and X-ray diffraction patterns. The drastic grinding-triggered emission red shift is likely involved in the formation of a dimer or an aggregate through Pt-Pt interaction, resulting in a conversion of the (3)MLCT/(3)LLCT emissive state in the crystalline state into the (3)MMLCT triplet state in the amorphous phase. Compared with the drastic grinding-triggered emission red shift in 1 (121-155 nm), the corresponding response shift in 2 (53-89 nm) is much smaller since a bulky tert-butyl in C≡CC(6)H(4)bu(t)-4 induces the planar platinum(II) molecules to stack through a longer Pt-Pt distance and less intermetallic contact compared with that in 1, as suggested from EXAFS studies.     

Establishment of structure-conductivity relationship for tris(2,2'-bipyridine) ruthenium ionic C(60) salts

Three ionic C(60) salts with [Ru(bpy)(3)](m+) (bpy = 2,2'-bipyridine) as cations were synthesized. The UV-vis-NIR spectra, XPS spectra, and elemental analysis have demonstrated their compositions: [Ru(bpy)(3)](2)(C(60)) (1), [Ru(bpy)(3)](C(60)) (2), and [Ru(bpy)(3)](C(60))(2) (3). Single crystals of polycrystalline compounds 1 and 2 were obtained as solvates. At room temperature, all three salts are semiconducting with the highest four-probe conductivity observed for compound 1 at ～10 S m(-1). The electronic conduction mechanisms can be described appropriately by an electron hopping model in this immobilized polyvalent redox system.     

Ultrafast excited state dynamics controlling photochemical isomerization of N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium coordinated to a {Re I(CO)3(2,2'-bipyridine)} chromophore

Two multifunctional photoactive complexes [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) and [Re(MeDpe(+))(CO)(3)(bpy)](2+) (MeDpe(+)=N-methyl-4-[trans-2-(4-pyridyl)ethenyl]pyridinium, bpy=2,2'-bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet-visible-infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time-resolved absorption spectroscopy in the visible and infrared regions; and time-resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about -1.1 V versus Fc/Fc(+) and is localized at MeDpe(+). Reduction alone does not induce a trans-->cis isomerization of MeDpe(+). [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) is photostable, while [Re(MeDpe(+))(CO)(3)(bpy)](2+) and free MeDpe(+) isomerize under near-UV irradiation. The lowest excited state of [Re(Cl)(CO)(3)(MeDpe(+))(2)](2+) has been identified as the Re(Cl)(CO)(3)-->MeDpe(+ 3)MLCT (MLCT=metal-to-ligand charge transfer), decaying directly to the ground state with lifetimes of approximately 42 (73 %) and approximately 430 ps (27 %). Optical excitation of [Re(MeDpe(+))(CO)(3)(bpy)](2+) leads to population of Re(CO)(3)-->MeDpe(+) and Re(CO)(3)-->bpy (3)MLCT states, from which a MeDpe(+) localized intraligand (3)pipi* excited state ((3)IL) is populated with lifetimes of approximately 0.6 and approximately 10 ps, respectively. The (3)IL state undergoes a approximately 21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited-state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe(+))(CO)(3)(bpy)](2+) reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe(+))(CO)(3)(bpy)](2+) emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.     

Ultrafast fluorescence detection in tris(2,2'-bipyridine)ruthenium(II) complex in solution: relaxation dynamics involving higher excited states

The excited-state dynamics of a transition metal complex, tris(2,2'-bipyridine)ruthenium(II), [Ru(bpy)(3)](2+), has been investigated using femtosecond fluorescence upconversion spectroscopy. The relaxation dynamics in these molecules is of great importance in understanding the various ultrafast processes related to interfacial electron transfer, especially in semiconductor nanoparticles. Despite several experimental and theoretical efforts, direct observation of a Franck-Condon singlet excited state in this molecule was missing. In this study, emission from the Franck-Condon excited singlet state of [Ru(bpy)(3)](2+) has been observed for the first time, and its lifetime has been estimated to be 40 +/- 15 fs. Biexponential decays with a fast rise component observed at longer wavelengths indicated the existence of more than one emitting state in the system. From a detailed data analysis, it has been proposed that, on excitation at 410 nm, crossover from higher excited (1)(MLCT) states to the vibrationally hot triplet manifold occurs with an intersystem crossing time constant of 40 +/- 15 fs. Mixing of the higher levels in the triplet state with the singlet state due to strong spin-orbit coupling is proposed. This enhances the radiative rate constant, k(r), of the vibrationally hot states within the triplet manifold, facilitating the upconversion of the emitted photons. The vibrationally excited triplet, which is emissive, undergoes vibrational cooling with a decay time in the range of 0.56-1.3 ps and relaxes to the long-lived triplet state. The results on the relaxation dynamics of the higher excited states in [Ru(bpy)(3)](2+) are valuable in explaining the role of nonequilibrated higher excited sensitizer states of transition metal complexes in the electron injection and other ultrafast processes.