MLCT State Research Articles

Ultrafast Relaxation Dynamics of Osmium–Polypyridine Complexes in Solution

We present steady-state absorption and emission spectroscopy and femtosecond broadband photoluminescence up-conversion spectroscopy studies of the electronic relaxation of Os(dmbp)3 (Os1) and Os(bpy)2(dpp) (Os2) in ethanol, where dmbp is 4,4′-dimethyl-2,2′-biypridine, bpy is 2,2′-biypridine, and dpp is 2,3-dipyridyl pyrazine. In both cases, the steady-state phosphorescence is due to the lowest 3MLCT state, whose quantum yield we estimate to be ≤5.0 × 10–3. For Os1, the steady-state phosphorescence lifetime is 25 ns. In both complexes, the photoluminescence excitation spectra map the absorption spectrum, pointing to an excitation wavelength-independent quantum yield. The ultrafast studies revealed a short-lived (≤100 fs) fluorescence, which stems from the lowest singlet metal-to-ligand-charge-transfer (1MLCT) state and decays by intersystem crossing to the manifold of 3MLCT states. In addition, Os1 exhibits a 50 ps lived emission from an intermediate triplet state at an energy ∼2000 cm–1 above that of the long-lived (25 ns) phosphorescence. In Os2, the 1MLCT–3MLCT intersystem crossing is faster than that in Os1, and no emission from triplet states is observed other than the lowest one. These observations are attributed to a higher density of states or a smaller energy spacing between them compared with Os1. They highlight the importance of the energetics on the rate of intersystem crossing.

Energy Dispersive XAFS: Characterization of Electronically Excited States of Copper(I) Complexes

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

Electrochemical Switching of Luminescence in Ru(II)-based Metallosupramolecular Polymer Device

Abstract A Ru(II)-based metallosupramolecular polymer RuL1 was synthesized by complexation of a Ru(II) salt and phenylenebis(terpyridine). The long polymer chains were observed on a Si substrate using atomic force microscopy. Interestingly, a RuL1 film showed luminescence from the 3MLCT state of the Ru(II) complex moieties at room temperature by excitation at the resonant wavelength of the 1MLCT absorption. In addition, we fabricated a solid-state polymer device with a RuL1 film and achieved reversible switching of the luminescence by applying voltages (3 and −3 V) between indium tin oxide electrodes, which caused electrochemical redox between Ru(II) and Ru(III).

Solvent-Induced Luminescence Quenching: Static and Time-Resolved X-Ray Absorption Spectroscopy of a Copper(I) Phenanthroline Complex

We present a static and picosecond X-ray absorption study at the Cu K-edge of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) ([Cu(dmp)2](+); dmp = 2,9-dimethyl-1,10-phenanthroline) dissolved in acetonitrile and dichloromethane. The steady-state photoluminescence spectra in dichloromethane and acetonitrile are also presented and show a shift to longer wavelengths for the latter, which points to a stronger stabilization of the excited complex. The fine structure features of the static and transient X-ray spectra allow an unambiguous assignment of the electronic and geometric structure of the molecule in both its ground and excited (3)MLCT states. Importantly, the transient spectra are remarkably similar for both solvents, and the spectral changes can be rationalized using the optimized ground- and excited-state structures of the complex. The proposed assignment of the lifetime shortening of the excited state in donor solvents (acetonitrile) to a metal-centered exciplex is not corroborated here. Molecular dynamics simulations confirm the lack of complexation; however, in both solvents the molecules come close to the metal but undergo rapid exchange with the bulk. The shortening of the lifetime of the title complex and nine additional related complexes can be rationalized by the decrease in the (3)MLCT energy. Deviations from this trend may be explained by means of the effects of the dihedral angle between the ligand planes, the solvent, and the (3)MLCT-(1)MLCT energy gap.

Metal–Metal Quadruple Bonds Supported by 5-Ethynylthiophene-2-carboxylato Ligands: Preparation, Molecular and Electronic Structures, Photoexcited State Dynamics, and Application as Molecular Synthons

From the reaction between M2(T(i)PB)4 and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M2(T(i)PB)2(ThCCH)2, where M = Mo (I) or W (II) and T(i)PB = 2,4,6-triisopropyl benzoate, have been isolated and characterized by (1)H NMR, IR, MALDI-TOF MS, UV-vis, steady-state emission, transient absorption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo2δ orbital. The HOMO of both I and II is the M2δ orbital, and the intense color of the compounds (I is red and II is blue) is due to the M2δ-to-ThCCH (1)MLCT transition. The S1 states for I and II are (1)MLCT. The T1 state is (3)MLCT for II, but (3)MoMoδδ* for I. The TRIR spectra of the ν(C≡C) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH ligands. Compound I is shown to be a synthon for the preparation of trans-Mo2(T(i)PB)2(ThCCPh)2 (III) and trans-Mo2(T(i)PB)2(ThCCAuPPh3)2 (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended π-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.

Mechanisms of Photoisomerization and Water-Oxidation Catalysis of Mononuclear Ruthenium(II) Monoaquo Complexes

A ligation of Ru(tpy)Cl3 (tpy = 2,2':6',2"-terpyridine) with 2-(2-pyridyl)-1,8-naphthyridine) (pynp) in the presence of LiCl gave distal-[Ru(tpy)(pynp)Cl](+) (d-1Cl) selectively, whereas the ligation gave proximal-[Ru(tpy)(pynp)OH2](2+) (p-1H2O) selectively in the absence of halide ions. (The proximal/distal isomers were defined by the structural configuration between the 1,8-naphthyridine moiety and the aquo or chloro ligand.) An aquation reaction of d-1Cl quantitatively afforded distal-[Ru(tpy)(pynp)OH2](2+) (d-1H2O) in water, and d-1H2O is quantitatively photoisomerized to p-1H2O. The mechanism of the photoisomerization was investigated by transient absorption spectroscopy and quantum chemical calculations. The temperature dependence of the transient absorption spectral change suggests existence of the thermally activated process from the (3)MLCT state with the activation energy (ΔE = 49 kJ mol(-1)), which is close to that (41.7 kJ mol(-1)) of the overall photoisomerization reaction. However, quantum chemical calculations suggest another activation process involving the conformational change of the pentacoordinated distal structure to the proximal structure. Quantum chemical calculations provide redox potentials and pK(a) values for proton-coupled electron transfer reactions from Ru(II)-OH2 to Ru(IV)═O in good agreement with experiments and provide an explanation for mechanistic differences between d-1H2O and p-1H2O with respect to water oxidation. The calculations show that water nucleophilic attack (WNA) on d-[Ru(V)-O](3+) (the ruthenyl oxo species derived from d-1H2O, calculated ΔG(‡) of 87.9 kJ/mol) is favored over p-[Ru(V)-O](3+) (calculated ΔG(‡) of 104.6 kJ/mol) for O-O bond formation. Examination of the lowest unoccupied molecular orbitals in d- and p-[Ru(V)-O](3+) indicates that more orbital amplitude is concentrated on the [Ru-O] unit in the case of d-[Ru(V)-O](3+) than in the case of p-[Ru(V)-O](3+), where some of the amplitude is instead delocalized over the pynp ligand, making this isomer less electrophilic.

Thermal and Photoinduced Electron Transfer in Directional Bis(terpyridine)ruthenium(II)–(Bipyridine)platinum(II) Complexes

AbstractMetalloligands L1 and L2 consisting of directional bis(terpyridine)ruthenium(II) units and bipyridine moieties were constructed by amide formation. From these metalloligands two Ru–Pt heterobimetallic complexes 1 and 2 were derived by a building‐block method by means of platination with [PtCl2(dmso)2]. Both bimetallic complexes 1 and 2 feature metal‐to‐ligand charge transfer (MLCT) absorptions, and emission occurs at room temperature in fluid solution from 3MLCT(Ru) states in all cases. Energy transfer from platinum to ruthenium is observed in 2 but not in 1 (light harvesting). The one‐electron‐reduced species [1]– and [2]– were prepared by reduction of 1 and 2 with decamethylcobaltocene. EPR spectra and DFT calculations reveal that the spin density is localized at the tpy–CO/Ru (tpy = terpyridine) site in [1]–, whereas it is centered at bpy–CO/Pt (bpy = 2,2′‐bipyridine) in [2]–. Efficient photoinduced electron transfer from triethanolamine to 1 and 2 is feasible by excitation at 500 nm [MLCT(Ru)].

Long‐Lived, Directional Photoinduced Charge Separation in RuII Complexes Bearing Laminate Polypyridyl Ligands

RuII complexes incorporating both amide-linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2-a:2',3'-c]phenazine (dppz), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), and 9,11,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3''']-pentacene (tatpp) exhibit long-lived charge separated (CS) states, which have been analyzed using time-resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal "bpy-like" portion and central (or distal) "phenazine-like" portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCTprox or ³MLCTdist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a ZnII ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCTdist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCTdist state. In tpphz complexes, ZnII coordination can reorder the energy of the ³MLCTprox and ³MLCTdist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron-hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.

Femtosecond optical pump–probe reflectivity studies of spin-state photo-switching in the spin-crossover molecular crystals [Fe(PM-AzA)2(NCS)2

We report here on the ultrafast photo-switching dynamics of a Fe(II) molecular material [Fe(PM-AzA)2(NCS)2]. It undergoes a thermal spin-crossover which can be detected by magnetic measurements or by optical reflectivity. We use here femtosecond optical reflectivity to study the ultrafast photo-switching dynamics. Our results indicate that the HS state is reached from the LS state within less than 100fs, through an intermediate MLCT state. This ultrafast relaxation from the electronic excited state towards the structurally relaxed HS state is followed by a vibrational cooling of the hot HS molecules within ≈1ps timescale.

Dye-Sensitized Solar Cell Studies of a Donor-Appended Bis(2,9-dimethyl-1,10-phenanthroline) Cu(I) Dye Paired with a Cobalt-Based Mediator

Bisphenanthroline complexes of Cu(I) have a number of photophysical similarities with trisbipyridineruthenium(II), including a strong MLCT absorption in the visible, a relatively long excited-state lifetime, and a thermally equilibrated MLCT state that is a potent reductant. All three of these qualities are desirable for a dye-sensitized solar cell (DSC) dye. There are also significant differences that present challenges when considering these chromophores in DSC applications. Among these are the lability of the Cu(I) complexes in solution and their long-term incompatibility with the most common mediator system, I–/I3–. We have prepared a series of [Cu(DMP)2]+-based dyes (where DMP = 2,9-dimethyl-1,10-phenanthroline) for study in DSCs paired with [CoL3]2+/3+ mediators (where L = a 2,2′-bipyridyl or 1,10-phenanthroline). In particular, we have prepared a dye having a covalently attached electron donor, the intended purpose of which is to rapidly reduce the Cu(II) generated by photoinjection. We compare the...

Cleavage of DNA by Proton-Coupled Electron Transfer to a Photoexcited, Hydrated Ru(II) 1,10-Phenanthroline-5,6-dione Complex

Visible light irradiation of a ruthenium(II) quinone-containing complex, [(phen)(2)Ru(phendione)](2+) (1(2+)), where phendione = 1,10-phenanthroline-5,6-dione, leads to DNA cleavage in an oxygen independent manner. A combination of NMR analyses, transient absorption spectroscopy, and fluorescence measurements in water and acetonitrile reveal that complex 1(2+) must be hydrated at the quinone functionality, giving [(phen)(2)Ru(phenH(2)O)](2+) (1H(2)O(2+), where phenH(2)O = 1,10-phenanthroline-6-one-5-diol), in order to access a long-lived (3)MLCT(hydrate) state (τ ∼ 360 ns in H(2)O) which is responsible for DNA cleavage. In effect, hydration at one of the carbonyl functions effectively eliminates the low-energy (3)MLCT(SQ) state (Ru(III) phen-semiquinone radical anion) as the predominant nonradiative decay pathway. This (3)MLCT(SQ) state is very short-lived (<1 ns) as expected from the energy gap law for nonradiative decay, (1) and too short-lived to be the photoactive species. The resulting excited state in 1H(2)O(2+)* has photophysical properties similar to the (3)MLCT state in [Ru(phen)(3)](2+)* with the added functionality of basic sites at the ligand periphery. Whereas [Ru(phen)(3)](2+)* does not show direct DNA cleavage, the deprotonated form of 1H(2)O(2+)* does via a proton-coupled electron transfer (PCET) mechanism where the peripheral basic oxygen sites act as the proton acceptor. Analysis of the small molecule byproducts of DNA scission supports the conclusion that cleavage occurs via H-atom abstraction from the sugar moieties, consistent with a PCET mechanism. Complex 1(2+) is a rare example of a ruthenium complex which 'turns on' both reactivity and luminescence upon switching to a hydrated state.

Newly Designed Resorcinolate Binding for Ru(II)– and Re(I)–Polypyridyl Complexes on Oleic Acid Capped TiO2 in Nonaqueous Solvent: Prolonged Charge Separation and Substantial Thermalized 3MLCT Injection

Femtosecond pump–probe spectroscopic studies on a series of newly synthesized resorcinol-based Ru(II) and Re(I) complexes on oleic acid capped TiO2 nanoparticles have been carried out in chloroform, and the results are compared with those of the catechol analogues. The ruthenium complex shows biexponential injection; the second component arises due to injection from the thermally equilibrated 3MLCT states as a result of a weaker strength of the resorcinolate binding. Also, in comparison with catechol binding, as a result of a greater diffusion of the injected electrons into TiO2, the back electron transfer (BET) is slowed down significantly for the ruthenium complex. These are distinctive observations for any mononuclear ruthenium–polypyridyl–enediol complex reported thus far. However, the rhenium complex educes single exponential ultrafast injection (<120 fs) because of apparent injection in a high density of states and shows the most prominent results with ∼50% slowdown in the charge recombination rate ...

Effects of Sequence, Connectivity, and Counter Ions in New Amide-Linked Ru(tpy)2–Re(bpy) Chromophores on Redox Chemistry and Photophysics

New cationic metallo ligands L1-L3 based on bis(terpyridine) ruthenium(II) complexes decorated with differently substituted 2,2'-bipyridines attached via amide groups (5-NHCO-bpy, 4-CONH-bpy, 5-CONH-bpy) were prepared. Coordination of Re(I)Cl(CO)(3) fragments to the bpy unit gives the corresponding bimetallic Ru~Re complexes 1-3. Hydrogen bonds of the bridging amide groups to [PF(6)](-) counterions or to water molecules are observed both in the solid state and in solution. The impact of the amide orientation, the connecting site, and the coordination of counterions on redox and photophysical properties is explored. Both the metallo ligands L1-L3 and the bimetallic complexes 1-3 are emissive at room temperature in fluid solution. The emission originates from (3)MLCT(Ru) states in all cases. Accordingly, the first oxidation of L1-L3 and 1-3 to [L1](+)-[L3](+) and [1](+)-[3](+) is assigned to the Ru(II/III) couple, while the first reduction to [L1](-)-[L3](-) and [1](-)-[3](-) occurs at the tpy-CO ligand as shown by UV/vis, IR, and EPR spectroscopy of the chemically generated radicals. Under rapid freezing conditions, radicals [2](-) and [3](-) are stabilized as different valence isomers with the odd electron localized at the [bpy-CO](•) bridging unit instead of the [tpy-CO](•). Furthermore, in radical [3](-) this valence equilibrium is shifted from [bpy-CO](•) to [tpy-CO](•) by coordination of [PF(6)](-) counterions to the bridging amide unit and back by replacing the [PF(6)](-) counterion with [BPh(4)](-). Photoinduced electron transfer (λ(exc) = 500 nm) to L1-L3 and to 1-3 is successful using triethanolamine (TEOA) as a reducing agent. Photocatalytic reduction of CO(2) by TEOA and 1-3 is hampered by the wrong site of electron localization in the one-electron reduced species [1](-)-[3](-).

Light-induced charge separation in a donor–chromophore–acceptor nanocomposite poly[TPA-Ru(tpy)2]@ZnO

The synthesis and characterisation of a new donor–chromophore–acceptor system based on poly(vinyltriphenylamine) as the electron donor and a glycine-functionalised bis(2,2′;6′,2′′-terpyridine)ruthenium(II) complex acting both as a chromophore and as an anchor group attached to ZnO nanorods as the electron acceptor are described. The TPA-containing block copolymer was synthesised by Reversible Addition Fragmentation Chain Transfer (RAFT) polymerisation and the ruthenium complex glycine conjugates prepared by Solid Phase Peptide Synthesis (SPPS) were attached via post-polymerisation esterification. GPC, NMR, IR and UV-Visible spectroscopy were used to characterise the multifunctional chromophore–donor polymer. Zinc oxide nanorods were functionalised with the block copolymer by multisite adsorption via the glycine COOH groups. The functionalised nanoparticles were well dispersible in organic solvents. Photoluminescence studies showed a complete quenching of the phosphorescence of the ruthenium chromophore (3MLCT state). Kelvin probe force microscopy (KPFM) was used to confirm that under continuous excitation of the RuII complex (1MLCT) the polymer corona develops a positive charge and thus efficient charge separation between ZnO and the polymer is achieved.

X-ray transient absorption structural characterization of the 3MLCT triplet excited state of cis-[Ru(bpy)2(py)2]2+

The excited state dynamics and structure of the photochemically active complex cis-[Ru(bpy)2(py)2](2+) have been investigated using optical transient absorption (OTA) and X-ray transient absorption (XTA) spectroscopy, and density functional theory (DFT). Upon light-excitation in aqueous solution cis-[Ru(bpy)2(py)2](2+) undergoes ultrafast dissociation of one pyridine ligand to form cis-[Ru(bpy)2(py)(H2O)](2+). OTA measurements highlighted the presence of two major time components of 1700 ps and 130 ps through which the system decays to the ground-state and evolves towards the photoproduct. XTA data were acquired after 150 ps, 500 ps, and 3000 ps from laser excitation (λexc = 351 nm) and provided the transient structure of the (3)MLCT state corresponding to the longer time component in the OTA experiment. In excellent agreement with DFT, XTA shows that the (3)MLCT geometry is characterized by an elongation of the dissociating Ru-N(py) bond and a shortening of the trans Ru-N(bpy) bond with respect to the ground state. Conversely, calculations show that the (3)MC state has a highly distorted structure with Ru-N(py) bonds between 2.77-3.05 Å.

Understanding the redox properties of dinuclear ruthenium(ii) complexes by a joint experimental and theoretical analysis

A combined experimental and theoretical approach has been used to investigate the redox properties of two dinuclear Ru(II) complexes, 2a and 2b, containing the planar dpt-ph-dpt bridging ligand (dpt-ph-dpt = 1'',4''-bis(2,4-dipyrid-2'-yl-1,3,5-triazin-6-yl)benzene). The redox properties of the free bridging ligand and the X-ray structure of 2a have also been reported, together with the X-ray structure of a related mononuclear compound, for comparison purposes. The photophysical processes of 2a and 2b have also been studied by pump-probe transient absorption spectroscopy. Compounds 2a and 2b are able to reversibly collect six and eight electrons, respectively, upon electrochemical reduction at mild potentials (>-2.0 V vs. SCE). A detailed assignment of the various reduction processes to specific subunits of the dinuclear arrays has been made possible by calculation of the HOMOs and LUMOs of native and bireduced species. For example, computation allowed us to clarify the redox behavior of 2b: the first reduction processes of this compound occur at almost coincident potentials, with successive electrons added on the same subunit (namely, the bridging ligand). Charge redistribution towards the other subunits of the molecular framework upon second reduction, revealed by calculation performed on the bireduced species, is the key to interpreting this peculiar behavior. Inter-ligand electron hopping interconverts the MLCT state involving the peripheral ligand to that of the (lower-lying) MLCT state that involves the bridging ligand. This process is faster than 350 fs in 2a, where its driving force is higher than 0.2 eV, whereas it occurs with a time constant of about 6 ps in 2b, having a smaller driving force for the process. Both compounds decay to the ground state, with MLCT emission on the nanosecond time scale, however a faster component of such decay is kinetically evidenced, indicating a process of about 200-250 ps in both cases, which is tentatively assigned to relatively slow diffusive solvent dynamics.

Pt(ii) diimine complexes bearing carbazolyl-capped acetylide ligands: synthesis, tunable photophysics and nonlinear absorption

A series of new Pt(II) diimine complexes with different carbazolyl-capped acetylide ligands (Pt-1–Pt-5) were synthesized and characterized. Their photophysical properties were investigated systematically via spectroscopic and theoretical methods. All complexes exhibit ligand-centered 1π,π* transitions in the UV region, and broad, structureless metal-to-ligand charge transfer (1MLCT)/ligand-to-ligand charge transfer (1LLCT) absorption bands in the visible spectral region. All complexes are emissive in solution at room temperature, with the emitting state being tentatively assigned to the 3MLCT/3LLCT states for Pt-1–Pt-4, and the emitting state of Pt-5 exhibiting a switch from the 3π,π* state in high-polarity solvents to the 3MLCT state in low-polarity solvents. Complexes Pt-1–Pt-5 all exhibit moderate triplet transient absorption (TA) from the visible to the NIR region, where reverse saturable absorption (RSA) could occur. The spectroscopic studies and theoretical calculations indicate that the photophysical properties of these Pt complexes can be tuned drastically by the carbazolyl-capped acetylide ligand, which would be useful for rational design of transition-metal complexes with high emission quantum yield, long excited-state lifetime, broadband excited-state absorption, and strong nonlinear transmittance for organic light-emitting and/or broadband nonlinear transmission applications.

Photoswitchable alkoxy-bridged binuclear rhenium(i) complexes – a potential probe for biomolecules and optical cell imaging

We describe the solvothermal synthesis, structural characterization, photophysics and potential applications as probes of two alkoxy-bridged binuclear Re(I) complexes, [{Re(CO)3(1,4-NVP)}2(μ2-OR)2] (1, R = C4H9; 2, R = C10H21; 1,4-NVP = 4-(1-naphthylvinyl)pyridine). Irradiation of 1 and 2 at 365 nm leads to an interesting trans-cis photoisomerization process, which was examined by 1H NMR, UV-vis, emission and time-resolved techniques. Compounds 1 and 2 exhibit photoswitchable luminescence enhancement arising from photoinduced intramolecular energy transfer from the 3MLCT state of the Re(I) chromophore to the triplet excited state of the ligand 1,4-NVP. In addition, these Re(I) complexes serve as excellent probes for the ultrasensitive detection of biological molecules, including bovine serum albumin (BSA) and a platelet derived growth factor (PDGF) binding aptamer. Our results also suggest that, since these Re(I) complexes have low cytotoxicity and fluorescence properties under physiological conditions, they could be a useful probe for optical imaging of cancer cells by confocal laser scanning microscopy.

時間分解XAFSによる水溶液中の [Ru&lt;sup&gt;II&lt;/sup&gt;(bpy)&lt;sub&gt;3&lt;/sub&gt;]&lt;sup&gt;2+&lt;/sup&gt;の&lt;sup&gt;3&lt;/sup&gt;MLCT状態の構造観測

時間分解XAFSによる水溶液中の [Ru&lt;sup&gt;II&lt;/sup&gt;(bpy)&lt;sub&gt;3&lt;/sub&gt;]&lt;sup&gt;2+&lt;/sup&gt;の&lt;sup&gt;3&lt;/sup&gt;MLCT状態の構造観測

Water detection by “turn on” fluorescence of the quinone-containing complexes [Ru(phen)2(1,10-phenanthroline-5,6-dione)2+] and [Ru(phenanthroline-5,6-dione)3]2+

Addition of water to the quinone functions in [Ru(phen)2(pdn)](2+) (1) and [Ru(pdn)3](2+) (2) (where phen = 1,10-phenanthroline and pdn = 1,10-phenanthroline-5,6-dione) turns on fluorescence at 605 nm, as formation of the geminal diol eliminates the predominant quinone-based non-radiative decay pathway and gives rise to a long-lived (3)MLCT state similar in nature to that seen in [Ru(phen)3](2+). Using NMR, the equilibrium constant for the hydration reaction of 1 in acetonitrile was determined to be 0.0253. From this data and experimental fitting of the luminescent titration data, the equilibrium constant for 2 of 1.62 × 10(-5) and emission yields for hydrated 1 and 2 were determined. Interestingly, all three quinone functions must be hydrated in 2 for luminescence, which is why the equilibrium constants vary so much. The 'turn on' luminescence allows for a very sensitive detection of water in aprotic solvents such as acetonitrile.