Ground- and excited-state infrared spectra of an azacrown-substituted [(bpy)Re(CO)3L]+ complex: structure and bonding in ground and excited states and effects of Ba2+ binding

Ground- and excited-state infrared spectra are reported for a [(bpy)ReI(CO)3L]+ complex (bpy = 2,2'-bipyridine) in which L contains an azacrown ether that is linked to Re via an amidopyridyl group. Ground-state band assignments are made with the aid of spectra from model complexes in which a similar electron-donating dimethylamino group replaces the azacrown, in which an electron-donor group is absent, and from the L ligands, in conjunction with DFT calculations. Picosecond time-resolved IR (TRIR) spectra in the nu(CO) region show bands characteristic of a metal-to-ligand charge-transfer (MLCT) excited state, [(bpy*-)ReII(CO)3L]+, from the complex in which an electron-donor group is absent, whereas those from the azacrown complex show bands of an MLCT state evolving into those characteristic of a ligand-to-ligand charge-transfer (LLCT) excited state, [(bpy*-)ReI(CO)3(L*+)]+, formed upon intramolecular electron transfer. Picosecond TRIR spectra of the azacrown complex in the fingerprint region show strong L ligand bands that indicate that significant charge redistribution occurs within this ligand in the MLCT state and that decay as the LLCT state forms. Picosecond TRIR spectra obtained when Ba2+ was complexed to the azacrown show bands of only an MLCT state at all times up to 2 ns, consistent with the presence of Ba2+ inhibiting electron transfer from the azacrown N atom to form the LLCT state, and the positions of the bands in the fingerprint region provide direct evidence for the proposal that charge redistribution within the L ligand induces Ba2+ release from the azacrown in the MLCT state.     

Ultrafast dynamics and excited state deactivation of [Ru(bpy)2Sq]+ and its derivatives

Femtosecond transient absorption spectroscopy has been employed to understand the excited state dynamics of [Ru(bpy)(2)Sq](+) (I; bpy is 2,2'-bipyridyl, and Sq is the deprotonated species of the semiquinone form of 1,2-dihydroxy benzene) and its derivatives, a widely studied near-infrared (NIR) active electrochromic dye. Apart from the well-defined dpi(Ru) --> pi(bpy)-based metal-to-ligand charge transfer (MLCT) transition bands at approximately 480 nm, this class of molecules generally shows another dpi(Ru) --> pi(Sq)(SOMO)-based intense MLCT band at around 900 nm, which is known to be redox active and bleaches reversibly upon a change in the oxidation state of the coordinated dioxolene moiety. To have better insight into the photoinduced electron transfer dynamics associated with this MLCT transition, detailed investigations have been carried out on exciting this MLCT band at 800 nm. Immediately after photoexcitation, bleach at 900 nm has been observed, whose recovery is found to follow a triexponential function with major contribution from the ultrafast component. This ultrafast component of approximately 220 fs has been ascribed to the S(1) to S(0) internal conversion process. In addition to the bleach, we have detected two transient species absorbing at 730 and 1000 nm with a formation time approximately 220 fs for both species. The excited state lifetimes for these two transient species have been measured to be 1.5 and 11 ps and have been attributed to excited singlet ((1)MLCT) and triplet ((3)MLCT) states, respectively. Transient measurements carried out on the different but analogous derivatives (II and III) have also shown similar recovery dynamics except that the rate for the internal conversion process has increased with the decrease in the S(1) to S(0) energy gap. The observed results are consistent with the energy gap law for nonradiative decay from S(1) to S(0).     

Photoinduced Ba2+ release and thermal rebinding by an azacrown ether linked by an alkynyl pyridine to a (bpy)Re(CO)3 group

A novel complex in which an azacrown ether is linked by an alkynyl pyridine to a (bpy)Re(CO)3 group acts as a light-controlled ion switch: Ba2+ bound to the azacrown is released in ca. 10 ns on UV excitation, and rebinds thermally from bulk solution in ca. 1 micros.     

Design and synthesis of iridium(III) azacrown complex: application as a highly sensitive metal cation phosphorescence sensor

A new metal cation probe bearing a central Ir(III) element and 1-aza-15-crown-5-ether substituted pyridyl pyrazolate as the chelate was synthesized. The octahedral molecular structure of was confirmed using single crystal X-ray diffraction analyses. Subsequent photophysical study showed yellow-green emission at approximately 560 nm in both fluid solution and solid state at room temperature. Remarkable differentiation in spectral properties upon metal cation (e.g. Ca2+) complexation makes complex a highly sensitive phosphorescence probe.     

Cation sensors containing a (bpy)Re(CO)3 group linked to an azacrown ether via an alkenyl or alkynyl spacer: synthesis, characterisation, and complexation with metal cations in solution

Two [(bpy)Re(CO)3L]+ complexes (bpy = 2,2'-bipyridine), where L contains an aza-15-crown-5 ether which is linked to Re via an alkenyl- or alkynyl-pyridine spacer, have been synthesised along with model complexes. Solutions of the complexes in acetonitrile have been studied by UV-Vis absorption spectroscopy, and by 1D and 2D 1H NMR spectroscopy. Strong UV-Vis bands, assigned to intraligand charge-transfer transitions localised at the L ligands, blue shift on protonation of the azacrown nitrogen atom or on complexation of alkali-metal (Li+, Na+ and K+) or alkaline-earth metal (Mg2+, Ca2+ and Ba2+) cations to the azacrown; the magnitude of the blue shift is dependent on the cation, with protonation giving the largest shift of ca. 100 nm. Cation binding constants in the range of log K= 1-4 depend strongly on the identity of the metal cation. Protonation or cation complexation causes downfield shifts in the 1H NMR resonances from most of the azacrown and L ligand protons, and their magnitudes correlate with those of the blue shifts in the UV-Vis bands; shifts in the azacrown 1H NMR resonances report on how the different metal cations interact with the macrocycle. UV-Vis and 1H NMR spectra of the free L ligands enable the effect of the Re centre to be assessed. Together, the data indicate that the alkene spacer gives a more responsive sensor than the alkyne spacer by providing stronger electronic communication across the L ligand.     

Absorption,fluorescence and resonance Raman spectra of adriamycin and its complex with DNA

Absorption, fluorescence and resonance Raman spectra of adriamycin and of its complex with DNA have been measured. These results together with the analysis of the excitation profiles of the solid drug were used to interpret the complex pattern of the absorption spectra as due to the vibronic structure of a single electronic state. In addition a surplus band, depending on the concentration, was attributed to self-association.Further support was given for the intercalation nature of the drug-DNA complex. In particular it was found that the monomer form of adriamycin interacts with the nucleic acid and that interaction involves the whole chromophore framework.     

New excited state intramolecular proton transfer (ESIPT) dyes based on naphthalimide and observation of long-lived triplet excited states

A new excited state intramolecular proton transfer chromophore of naphthalimide (NI) conjugated 2-(2-hydroxyphenyl) benzothiazole () was prepared which shows red shifted absorption and long-lived triplet excited states.     

Resonance Raman and surface-enhanced resonance Raman spectra of LH2 antenna complex from Rhodobacter sphaeroides and Ectothiorhodospira sp. excited in the Qx and Qy transitions

Well-resolved vibrational spectra of LH2 complex isolated from two photosynthetic bacteria, Rhodobacter sphaeroides and Ectothiorhodospira sp., were obtained using surface-enhanced resonance Raman scattering (SERRS) exciting into the Qx and the Qy transitions of bacteriochlorophyll a. High-quality SERRS spectra in the Qy region were accessible because the strong fluorescence background was quenched near the roughened Ag surface. A comparison of the spectra obtained with 590 nm and 752 nm excitation in the mid- and low-frequency regions revealed spectral differences between the two LH2 complexes as well as between the LH2 complexes and isolated bacteriochlorophyll a. Because peripheral modes of pigments contribute mainly to the low-frequency spectral region, frequencies and intensities of many vibrational bands in this region are affected by interactions with the protein. The results demonstrate that the microenvironment surrounding the pigments within the two LH2 complexes is somewhat different, despite the fact that the complexes exhibit similar electronic absorption spectra. These differences are most probably due to specific pigment-pigment and pigment-protein interactions within the LH2 complexes, and the approach might be useful for addressing subtle static and dynamic structural variances between pigment-protein complexes from different sources or in complexes altered chemically or genetically.     

Raman excitation profiles,fluorescence spectra and excited state proton transfer of solid 1.8 dihydroxyanthraquinone at 8 K

The single crystal fluorescence spectra as well as the Raman spectra and excitation profiles of solid 1.8 dihydroxyanthraquinone have been measured. Four electronic transitions in the visible range were found: a π—π* active in absorption, a Franck—Condon absorption emission active transition and two emissions due to excited state proton and Hatom transferred configurations.     

Raman spectra and structure of biphenyl isoatomers (the S0, S1, T1 states and the cation and anion radicals)

The Raman and transient resonance Raman spectra of biphenyl (BP) and its perdeuterated analogue (BP-d₁₀) in three different electronic states (S₀, S₁ and T₁) and in two different ionized states (cation and anion) have been recorded in solution. The S₀ Raman spectra have also been measured for the crystalline state. The obtained set of spectra are analysed on the basis of the established vibrational assignments for the ground state of the planar (crystal) and the non-planar (solution) structures. The analysis suggests that BP in solution exists as a twisted structure in the S₁ state, but that it takes planar or nearly planar structures in the T₁, the cationic and the anionic states.     

Oxide- and zeolite-supported molecular metal complexes and clusters: physical characterization and determination of structure, bonding, and metal oxidation state

This article is a review of the physical characterization of well-defined site-isolated molecular metal complexes and metal clusters supported on metal oxides and zeolites. These surface species are of interest primarily as catalysts; as a consequence of their relatively uniform structures, they can be characterized much more precisely than traditional supported catalysts. The properties discussed in this review include metal nuclearity, oxidation state, and ligand environment, as well as metal-support interactions. These properties are determined by complementary techniques, including transmission electron microscopy; X-ray absorption, infrared, Raman, and NMR spectroscopies; and density functional theory. The strengths and limitations of these techniques are assessed in the context of results characterizing samples that have been investigated thoroughly and with multiple techniques. The depth of understanding of well-defined metal complexes and metal clusters on supports is approaching that attainable for molecular analogues in solution. The results provide a foundation for understanding the more complex materials that are typical of industrial catalysts.     

Rhenium-to-benzoylpyridine and rhenium-to-bipyridine MLCT excited states of fac-[Re(Cl)(4-benzoylpyridine)(2)(CO)(3)] and fac-[Re(4-benzoylpyridine)(CO)(3)(bpy)](+): a time-resolved spectroscopic and spectroelectrochemical study

The lowest allowed electronic transition of fac-[Re(Cl)(CO)(3)(bopy)(2)] (bopy = 4-benzoylpyridine) has a Re --> bopy MLCT character, as revealed by UV-vis and stationary resonance Raman spectroscopy. Accordingly, the lowest-lying, long-lived, excited state is Re --> bopy (3)MLCT. Electronic depopulation of the Re(CO)(3) unit and population of a bopy pi orbital upon excitation are evident by the upward shift of nu(CO) vibrations and a downward shift of the ketone nu(C=O) vibration, respectively, seen in picosecond time-resolved IR spectra. Moreover, reduction of a single bopy ligand in the (3)MLCT excited state is indicated by time-resolved visible and resonance Raman (TR(3)) spectra that show features typical of bopy(*)(-). In contrast, the lowest allowed electronic transition and lowest-lying excited state of a new complex fac-[Re(bopy)(CO)(3)(bpy)](+) (bpy = 2,2'-bipyridine) have been identified as Re --> bpy MLCT with no involvement of the bopy ligand, despite the fact that the first reduction of this complex is bopy-localized, as was proven spectroelectrochemically. This is a rare case in which the localizations of the lowest MLCT excitation and the first reduction are different. (3)MLCT excited states of both fac-[Re(Cl)(CO)(3)(bopy)(2)] and fac-[Re(bopy)(CO)(3)(bpy)](+) are initially formed vibrationally hot. Their relaxation is manifested by picosecond dynamic shifts of nu(C(triple bond)O) IR bands. The X-ray structure of fac-[Re(bopy)(CO)(3)(bpy)]PF(6).CH(3)CN has been determined.     

Assigning the resonance Raman spectral features of rhodopsin, isorhodopsin and bathorhodopsin in bovine photostationary state spectra.

Abstract—High resolution resonance Raman spectra of rhodopsin. isorhodopsin and photostationary state mixtures containing a high percentage of bathorhodopsin arc presented. New spectral features are detected which were not obsei-ved in lower resolution studies by other workers. All of the hands in the photostationary state spcctra arc assigned based on pure rhodopsin and isorhodopsin resonance Raman results and alterations in the photostationary state mixture. The spectral features in these spectra are invariant from 20 to 150K indicating that retinal and protein structural alteration, consistent with a model of excitation proposed by Lewis, occurs in steady-state spectra even at 20 K. In addition, the relative intensity of certain features in the photostationary state spectra are altered upon D₂O suspension. One explanation for these alterations is that the contributions of various intcrmediates to the photostationary state mixture are changed when membrane fragments are suspended in D₂O.     

Temperature dependence of X- and Q-band EPR spectra of the dinuclear manganese(II) complex [(NO2Bpmp)Mn2(mu-OAc)2]+: determination of the exchange constant and of the spin parameters for the S=1, 2, and 3 spin states

A new dinuclear manganese(II) complex was synthesised with the biscompartimental ligand 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-nitrophenol (NO(2)BpmpH) and characterised by X-ray crystallography. Magnetic susceptibility measurements revealed that the two high-spin Mn(II) ions are antiferromagnetically coupled with a singlet-to-triplet separation of 7.2 cm(-1). The powder EPR spectra were recorded for both X- and Q-bands between 1.8 K and 35 K. A detailed analysis of these spectra led to the determination of three out of five individual spin-state zero-field splitting parameters. From the proposed simulations, the exchange coupling constant J and the intermetallic distance have been computed.     

Relevance of the ligand exchange rate and mechanism of fac-[(CO)3M(H2O)3]+ (M = Mn, Tc, Re) complexes for new radiopharmaceuticals

The water exchange process on fac-[(CO)3Mn(H2O)3]+ and fac-[(CO)3Tc(H2O)3]+ was kinetically investigated by 17O NMR as a function of the acidity, temperature, and pressure. Up to pH 6.3 and 4.4, respectively, the exchange rate is not affected by the acidity, thus demonstrating that the contribution of the monohydroxo species fac-[(CO)3M(OH)(H2O)2] is not significant, which correlates well with a higher pKa for these complexes compared to the homologue fac-[(CO)3Re(H2O)3]+ complex. The water exchange rate K298ex/s(-1) (DeltaHex double dagger/kJ mol(-1); DeltaSex double dagger/J mol(-1) K(-1); DeltaV double dagger/cm3 mol-1) decreases down group 7 from Mn to Tc and Re: 23 (72.5; +24.4; +7.1) > 0.49 (78.3; +11.7; +3.8) > 5.4 x 10(-3) (90.3; +14.5; -). For the Mn complex only, an O exchange on the carbonyl ligand could be measured (K338co = 4.3 x 10(-6) s(-1)), which is several orders of magnitude slower than the water exchange. In the case of the Tc complex, the coupling between 17O (I = 5/2) and 99Tc (I = 9/2) nuclear spins has been observed (1J99Tc,17O = 80 +/- 5 Hz). The substitution of water in fac-[(CO)3M(H2O)3]+ by dimethyl sulfide (DMS) is slightly faster than that by CH3CN: 3 times faster for Mn, 1.5 times faster for Tc, and 1.2 times faster for Re. The pressure dependence behavior is different for Mn and Re. For Mn, the change in volume to reach the transition state is always clearly positive (water exchange, CH3CN, DMS), indicating an Id mechanism. In the case of Re, an Id/Ia changeover is assigned on the basis of reaction profiles with a strong volume maximum for pyrazine and a minimum for DMS as the entering ligand.     

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

Theoretical determination of the excited states and of g-factors of the Creutz-Taube ion, [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+

The Creutz-Taube complex [(NH3)5-Ru-pyrazine-Ru-(NH3)5]5+ is studied using wave function-based methods, namely the CASSCF/MS-CASPT2 method. Spin-orbit effects have been calculated with the SO-RASSI program. The nature of the ground state is analyzed, and all the excited states up to 50,000 cm(-1) are calculated. They form a quasi continuum from 25,000 cm(-1), theoretical bands are assigned to UV-visible spectra, and MCD bands are assigned by calculating transition moments from first principles for both spectroscopies. g-Factors are calculated from first principles and modeled by a model Hamiltonian: they compare well to experimental values and are shown to be the same as in the monomeric species.     

Theoretical study on vibrational circular dichroism spectra of tris(acetylacetonato)metal(III) complexes: anharmonic effects and low-lying excited states

The open-shell density functional theory calculations with M06 exchange-correlation functional and all-electron Douglas-Kroll second order scalar relativistic correction were performed to interpret the vibrational circular dichroism (VCD) spectra of four kinds of tris(acetylacetonato)metal(III), [M(III)(acac)(3)] (acac = acetylacetonato, M = Ru, Cr, Co, and Rh). It was deduced that the experimental spectra were well reproduced by the calculation with harmonic approximation in case of [Co(III)(acac)(3)] (d(6); S = 0), [Rh(III)(acac)(3)] (d(6); S = 0), and [Ru(III)(acac)(3)] (d(5); S = 1/2). In case of [Cr(III)(acac)(3)] (d(3); S = 3/2), anharmonic effects should be taken into account to predict the accurate vibrational frequencies of closely located modes. Time-dependent density functional theory calculations were performed to estimate the contribution of excited states in the VCD spectra. As a consequence, the presence of the low-lying excited states was predicted for [Ru(III)(acac)(3)] alone, which agreed with the experimental observation.