Microsecond MLCT excited state lifetimes in bis-tridentate Ru(ii)-complexes: significant reductions of non-radiative rate constants.

In this paper we report the photophysical properties of a series of bis-tridentate Ru-complexes, based on the dqp-ligand (dqp = 2,6-di(quinolin-8-yl)pyridine), which display several microsecond long excited state lifetimes for triplet metal-to-ligand charge transfer (MLCT) at room temperature. Temperature dependence of the excited state lifetimes for [Ru(dqp)] and [Ru(dqp)(ttpy)] (ttpy = 4'-tolyl-2,2':6',2''-terpyridine) is reported and radiative and non-radiative rate constants for the whole series are reported and discussed. We can confirm previous assumptions that the near-octahedricity of the bis-dqp complexes dramatically slows down activated decay at room temperature, as compared to most other and less long-lived bis-tridentate Ru-complexes, such as [Ru(tpy)] with τ = 0.

High extinction coefficient Ru-sensitizers that promote hole transfer on nanocrystalline TiO₂.

Two series of Ru(II) polypyridyl compounds with formulas [(bpy)2RuL](PF6)2 and [(deeb)2RuL](PF6)2, where bpy is 2,2'-bipyridine, deeb is 4,4'-diethylester-2,2'-bpy, and L is one of several substituted 9'-(1,3-dithiole-2-ylidene)-4',5'-diazafluorene ligands, were studied as potential photosensitizers for TiO2. These compounds possess notably high extinction coefficients (≥40,000 M(-1) cm(-1) @470 nm) which are shown by time-dependent density functional theory (TD-DFT) calculations to result from overlapping metal-to-ligand charge transfer (MLCT) and ligand-localized transitions. Low-temperature absorption and photoluminescence measurements were suggestive of a short-lived MLCT excited state.

Long-range electron transfer in zinc-phthalocyanine-oligo(phenylene-ethynylene)-based donor-bridge-acceptor dyads.

In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP(+) and ZnPc-OPE-C(60). A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem.

Accumulative electron transfer: multiple charge separation in artificial photosynthesis.

To achieve artificial photosynthesis it is necessary to couple the single-electron event of photoinduced charge separation with the multi-electron reactions of fuel formation and water splitting. Therefore, several rounds of light-induced charge separation are required to accumulate enough redox equivalents at the catalytic sites for the target chemistry to occur, without any sacrificial donors or acceptors other than the catalytic substrates. Herein, we discuss the challenges of such accumulative electron transfer in molecular systems.

Influence of triplet state multidimensionality on excited state lifetimes of bis-tridentate RuII complexes: a computational study.

Calculated triplet excited state potential energy surfaces are presented for a set of three bis-tridentate Ru(II)-polypyridyl dyes covering a wide range of room temperature excited state lifetimes: [Ru(II)(tpy)(2)](2+), 250 ps; [Ru(II)(bmp)(2)](2+), 15 ns; and [Ru(II)(dqp)(2)](2+), 3 μs (tpy is 2,2':6',2″-terpyridine, bmp is 6-(2-picolyl)-2,2'-bipyridine, and dqp is 2,6-di(quinolin-8-yl)pyridine). The computational results provide a multidimensional view of the (3)MLCT-(3)MC transition for the investigated complexes. Recently reported results of significantly prolonged (3)MLCT excited state lifetimes of bis-tridentate Ru(II)-complexes, for example [Ru(II)(dqp)(2)](2+), are found to correlate with substantial differences in their triplet excited state multidimensional potential energy surfaces.

Spectroscopic and structural investigations reveal the signaling mechanism of a luminescent molybdate sensor.

A heteroditopic ligand H(2)-L consisting of a dihydroxybenzene (catechol)-unit linked via an amide bond to a pyridyl-unit and its methyl-protected precursor Me(2)-L were synthesized, characterized, and their photophysical properties investigated. The three accessible protonation states of the ligand, H(3)-L(+), H(2)-L, and H-L(-), showed distinct (1)H NMR, absorption and emission spectroscopic characteristics that allow pH-sensing. The spectroscopic signatures obtained act as a guide to understand the signaling mechanism of the luminescent pH and molybdate sensor [Re(bpy)(CO)(3)(H(2)-L)](+).

Accumulative charge separation inspired by photosynthesis.

Molecular systems that follow the functional principles of photosynthesis have attracted increasing attention as a method for the direct production of solar fuels. This could give a major carbon-neutral energy contribution to our future society. An outstanding challenge in this research is to couple the light-induced charge separation (which generates a single electron-hole pair) to the multielectron processes of water oxidation and fuel generation.

Photoinduced electron transfer in Zn(II)porphyrin-bridge-Pt(II)acetylide complexes: variation in rate with anchoring group and position of the bridge.

The synthesis and photophysical characterization of two sets of zinc porphyrin platinum acetylide complexes are reported. The two sets of molecules differ in the way the bridging phenyl-ethynyl unit is attached to the porphyrin ring. One set is attached via an ethynyl unit on the β position, while the other set is attached via a phenyl unit on the meso position of the porphyrin.

Recognition and binding of a helix-loop-helix peptide to carbonic anhydrase occurs via partly folded intermediate structures.

We have studied the association of a helix-loop-helix peptide scaffold carrying a benzenesulfonamide ligand to carbonic anhydrase using steady-state and time-resolved fluorescence spectroscopy. The helix-loop-helix peptide, developed for biosensing applications, is labeled with the fluorescent probe dansyl, which serves as a polarity-sensitive reporter of the binding event. Using maximum entropy analysis of the fluorescence lifetime of dansyl at 1:1 stoichiometry reveals three characteristic fluorescence lifetime groups, interpreted as differently interacting peptide/protein structures.

Extension of the charge separated-state lifetime by supramolecular association of a tetrathiafulvalene electron donor to a zinc/gold bisporphyrin.

Supramolecular triads were prepared by self-assembly of 4'-pyridyl-2-tetrathiafulvalene axially bound on ZnP-spacer-AuP(+) dyads; the lifetime of the charge separated state ((+)TTF-ZnP-Spacer-AuP ) formed upon light excitation of the triad is greatly increased with respect to that found in the parent dyad.

Vectorial electron transfer in donor-photosensitizer-acceptor triads based on novel bis-tridentate ruthenium polypyridyl complexes.

The first examples of rodlike donor-photosensitizer-acceptor arrays based on bis-2,6-di(quinolin-8-yl)pyridine Ru(II) complexes 1a and 3a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time-resolved absorption spectroscopy reveals long-lived, photoinduced charge-separated states (tau(CSS) (1a)=140 ns, tau(CSS) (3a)=200 ns) formed by stepwise electron transfer.

Hydrated and dehydrated tertiary interactions--opening and closing--of a four-helix bundle peptide.

The structural heterogeneity and thermal denaturation of a dansyl-labeled four-helix bundle homodimeric peptide was studied with steady-state and time-resolved fluorescence spectroscopy and with circular dichroism (CD). At room temperature the fluorescence decay of the polarity-sensitive dansyl, located in the hydrophobic core region, can be described by a broad distribution of fluorescence lifetimes, reflecting the heterogeneous microenvironment. However, the lifetime distribution is nearly bimodal, which we ascribe to the presence of two major conformational subgroups.

A class of fluorescent heterocyclic dyes revisited: photophysics, structure, and solvent effects.

The photophysical behavior of a series of 2-methylthio-5-(Z-carbonyl)thieno-[3,4-e]-3,4-dihydro-1,2,3-triazine-4-ones was investigated by absorption and emission spectroscopy in a range of solvents representing a systematic variation in polarity, polarizability, as well as hydrogen bond donating and accepting capabilities. In all the investigated compounds, the heterocyclic 6-membered ring of the chromophore is involved in interactions between electron donor and acceptor substituents on the thiophene ring. Throughout the series, the charge-donating methylthio group is kept constant, while the acceptor (Z-carbonyl) is varied between amide, ester, and ketone functionalities.

Size- and solvent-dependent kinetics for cis-trans isomerization in donor-acceptor systems.

We have investigated, using time-resolved and steady-state optical spectroscopy, the cis-trans isomerization dynamics in a series of charge transfer, donor-acceptor compounds. The number of donor (dithiafulvene) and acceptor (p-nitrophenyl) moieties as well as their spatial arrangement around a central ethynylethene core has been varied in a systematic way. All compounds in the series are weakly fluorescent.

Very fast single-step photoinduced charge separation in zinc porphyrin bridged to a gold porphyrin by a bisethynyl quaterthiophene.

A new heterometallic dyad composed of a zinc porphyrin linked by bisethynyl quaterthiophene to a gold porphyrin was synthesized according to a stepwise modular approach. The latter dyad and the parent reference compounds (porphyrin-ethynylquaterthiophene) were characterized by electrochemistry, spectroelectrochemistry, and femtosecond transient absorption spectrocopy. We showed that light excitation of the zinc or the gold porphyrin induces a very fast and quantitative charge separation over a distance of 25 A which occurs through a superexchange mechanism.

Bistridentate ruthenium(II)polypyridyl-type complexes with microsecond 3MLCT state lifetimes: sensitizers for rod-like molecular arrays.

A series of bistridentate ruthenium(II) polypyridyl-type complexes based on the novel 2,6-di(quinolin-8-yl)pyridine (dqp) ligand have been synthesized and their photophysical properties have been studied. The complexes are amenable to substitution in the 4-position of the central pyridine with conserved quasi-C2v symmetry, which allows for extension to isomer-free, rod-like molecular arrays for vectorial control of electron and energy transfer. DFT calculations performed on the parent [Ru(dqp) 2](2+) complex (1) predicted a more octahedral structure than in the typical bistridentate complex [Ru(tpy)2](2+) (tpy is 2,2':6',2"-terpyridine) thanks to the larger ligand bite angle, which was confirmed by X-ray crystallography.

How close can you get? Studies of ultrafast light-induced processes in ruthenium-[60] fullerene dyads with short pyrazolino and pyrrolidino links.

Two pyrazoline- and one pyrrolidine-bridged Ru(II)bipyridine-[60]fullerene dyads have been prepared and studied by ultrafast time-resolved spectroscopy. A silver-assisted synthesis route, in which Ag(I) removes the chlorides from the precursor complex Ru(bpy) 2Cl 2 facilitates successful coordination of the [60]fullerene-substituted third ligand. Upon light excitation of the ruthenium moiety, the emission was strongly quenched by the fullerene.

Steric influence on the excited-state lifetimes of ruthenium complexes with bipyridyl-alkanylene-pyridyl ligands.

The structural effect on the metal-to-ligand charge transfer (MLCT) excited-state lifetime has been investigated in bis-tridentate Ru(II)-polypyridyl complexes based on the terpyridine-like ligands [6-(2,2'-bipyridyl)](2-pyridyl)methane ( 1) and 2-[6-(2,2'-bipyridyl)]-2-(2-pyridyl)propane ( 2). A homoleptic ([Ru( 2) 2] (2+)) and a heteroleptic complex ([Ru(ttpy)( 2)] (2+)) based on the new ligand 2 have been prepared and their photophysical and structural properties studied experimentally and theoretically and compared to the results for the previously reported [Ru( 1) 2] (2+). The excited-state lifetime of the homoleptic Ru (II) complex with the isopropylene-bridged ligand 2 was found to be 50 times shorter than that of the corresponding homoleptic Ru (II) complex of ligand 1, containing a methylene bridge.

Single-step electron transfer on the nanometer scale: ultra-fast charge shift in strongly coupled zinc porphyrin-gold porphyrin dyads.

The synthesis, electrochemical properties, and photoinduced electron transfer processes of a series of three novel zinc(II)-gold(III) bisporphyrin dyads (ZnP--S--AuP(+)) are described. The systems studied consist of two trisaryl porphyrins connected directly in the meso position via an alkyne unit to tert-(phenylenethynylene) or penta(phenylenethynylene) spacers. In these dyads, the estimated center to center interporphyrin separation distance varies from 32 to 45 A.

Strongly coupled zinc phthalocyanine-tin porphyrin dyad performing ultra-fast single step charge separation over a 34 A distance.

A zinc(II) phthalocyanine-tin(IV) porphyrin dyad with a strong electronic coupling was synthesized and upon light excitation shown to exhibit ultra-fast, long-range electron transfer in a single step.

Six-membered ring chelate complexes of Ru(II): structural and photophysical effects.

The structural and photophysical properties of Ru(II)-polypyridyl complexes with five- and six-membered chelate rings were studied for two bis-tridentate and two tris-bidentate complexes. The photophysical effect of introducing a six-membered chelate ring is most pronounced for the tridentate complex, leading to a room-temperature excited-state lifetime of 810 ns, a substantial increase from 180 ns for the five-membered chelate ring model complex. Contrasting this, the effect is the opposite in tris-bidentate complexes, in which the lifetime decreases from 430 ns to around 1 ns in going from a five-membered to six-membered chelate ring.

Bio-inspired, side-on attachment of a ruthenium photosensitizer to an iron hydrogenase active site model.

The first ruthenium-diiron complex [(mu-pdt)Fe2(CO)5{PPh2(C6H4CCbpy)}Ru(bpy)2]2+ 1 (pdt = propyldithiolate, bpy = 2,2'-bipyridine) is described in which the photoactive ruthenium trisbipyridyl unit is linked to a model of the iron hydrogenase active site by a ligand directly attached to one of the iron centers. Electrochemical and photophysical studies show that the light-induced MLCT excited state of the title complex is localized towards the potential diiron acceptor unit. However, the relatively mild potential required for the reduction of the acetylenic bipyridine together with the easily oxidized diiron portion leads to a reductive quenching of the excited state, instead.

A 3.0 micros room temperature excited state lifetime of a bistridentate RuII-polypyridine complex for rod-like molecular arrays.

A bistridentate RuII-polypyridine complex [Ru(bqp)2]2+ (bqp = 2,6-bis(8'-quinolinyl)pyridine) has been prepared, which has a coordination geometry much closer to a perfect octahedron than the typical Ru(terpyridine)2-type complex. Thus, the complex displays a 3.0 mus lifetime of the lowest excited metal-to-ligand charge transfer (3MLCT) state at room temperature.

The RNA-protein complex: direct probing of the interfacial recognition dynamics and its correlation with biological functions.

The N protein from bacteriophage lambda is a key regulator of transcription antitermination. It specifically recognizes a nascent mRNA stem loop termed boxB, enabling RNA polymerase to read through downstream terminators processively. The stacking interaction between Trp-18 of WT N protein and A7 of boxB RNA is crucial for efficient antitermination.