Niels Harrit

Time-Resolved X-ray Scattering of an Electronically Excited State in Solution. Structure of the 3A2u State of Tetrakis-μ-pyrophosphitodiplatinate(II)

The structure of the (3)A(2u) excited state of tetrakis-mu-pyrophosphitodiplatinate(II) in aqueous solution is investigated by time-resolved X-ray scattering on a time scale from 100 ps to 1 micros after optical pumping. The primary structural parameter, the Pt-Pt distance, is found to be 2.74 A, which is 0.24 A shorter than the ground-state value. The contraction is in excellent agreement with earlier estimates based on spectroscopic data in solution and diffraction data in the crystalline state. As a second structural parameter, the distance between the P planes in the (3)A(2u) excited state was determined to be 2.93 A, i.e., the same as that in the ground state. This result implies that a slight lengthening of the Pt-P bond occurs following excitation.

Bond shortening (1.4 Å) in the singlet and triplet excited states of [Ir2(dimen)4]2+ in solution determined by time-resolved X-ray scattering.

Ground- and excited-state structures of the bimetallic, ligand-bridged compound Ir2(dimen)4(2+) are investigated in acetonitrile by means of time-resolved X-ray scattering. Following excitation by 2 ps laser pulses at 390 nm, analysis of difference scattering patterns obtained at eight different time delays from 250 ps to 300 ns yields a triplet excited-state distance between the two Ir atoms of 2.90(2) Å and a triplet excited-state lifetime of 410(70) ns. A model incorporating the presence of two ground-state structures differing in Ir–Ir separation is demonstrated to fit the obtained data very well, in agreement with previous spectroscopic investigations. Two ground-state isomers with Ir–Ir separations of 3.60(9) and 4.3(1) Å are found to contribute equally to the difference scattering signal at short time delays. Further studies demonstrate the feasibility of increasing the effective time resolution from the 100 ps probe width down to the 10 ps regime by positioning the laser pump pulse at selected points in the X-ray probe pulse. This approach is used to investigate the structures of both the singlet and the triplet excited states of Ir2(dimen)4(2+).

2,6,10-Tris(dialkylamino)trioxatriangulenium salts: a new promising fluorophore. Ion-pair formation and aggregation in non-polar solvents

The D3h symmetric tris(dialkylamino)trioxatriangulenium ((R2N)3TOTA+) ions are structurally related to classical stains and fluorophores such as triphenylmethane dyes and rhodamines. New derivatives of (R2N)3TOTA+, in which long flexible alkyl chains surround the planar and rigid aromatic core, have been synthesized and isolated as hexafluorophosphate salts. In contrast to short-chain derivatives of triphenylmethane dyes, the new compounds described here are soluble in hydrocarbon solvents. The photophysical properties of these carbenium salts are investigated. Characteristic alterations of the photophysical properties as a function of solvent polarity, concentration and temperature are observed. A model to rationalize the phenomena is presented comprising three states of aggregation, i.e. freely solvated ions, tight ion-pairs, and dimers of ion-pairs. Reduced symmetry in the tight ion-pairs is held responsible for the observed splitting of the long wavelength absorption band. Exciton coupling in the ion-pair dimers leads to reduced oscillator strength of the S0-S1 transition. The model is supported by semi-empirical calculations of the structures and electronic transitions of the free cation and its ion-pair(s).

Columnar Self-Assembly and Alignment of Planar Carbenium Ions in Langmuir-Blodgett Films

Structural and optical properties of multilayer Langmuir-Blodgett (LB) films of two amphiphilic carbenium salts 2-didecylamino-6,10-bis(dimethylamino)-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-1) and 2,6-bis(decylmethylamino)-10-dimethylamino-4,8,12-trioxatriangulenium hexafluorophosphate (ATOTA-2) are described. The LB films were prepared on lipophilic glass by standard vertical dipping. Grazing incidence X-ray diffraction (GIXD) measurements show that the planar organic cores, in spite of their positive charge, form closely packed columns with a repeating distance of ∼3.45 Å. Specular X-ray reflectivity (SXR) reveals the LB multilayers to consist of Y-type bilayers with thickness 31 Å for ATOTA-1 and 41 Å for ATOTA-2. This significant difference is ascribed to the different packing motifs of the alkyl chains in the two LB films. GIXD and polarized UV-vis absorption and emission spectroscopy show that the columnar aggregates in the LB films are oriented along the dipping direction. This alignment is attributed to shear effects during LB transfer. The main absorption band of the LB films is blue-shifted compared to that in solution, while the fluorescence is red-shifted by more than 100 nm. These findings suggest the presence of H-aggregates in agreement with the cofacial packing derived from the X-ray measurements. Polarized absorption spectroscopy with variable angle of incidence was used to resolve two perpendicular optical transitions in the visible range, one at 460 nm polarized perpendicular to the columnar direction, in the plane of the film, and one at 420 nm polarized along the film normal.

Two different Ca2+ ion binding sites in factor VIIa and in des(1–38) factor VIIa

The Ca2+ ion binding of factor VIIa and the derivative lacking the γ‐carboxyglutamic acid domain, des(1–38) factor VIIa, was investigated using intrinsic protein fluorescence and Tb3+ ion phosphorescence methods. Binding of Ca2+ ions giving rise to a decrease in the intrinsic protein fluorescence (approximately 50% at saturating conditions) is seen with both proteins. Each of the saturation curves is in accordance with the formation of a 1:1 complex of factor VIIa‐Ca2+ (K D∼30 μM) and des(1–38) factor VIIa‐Ca2+ (K D∼40 μM)). Yet another Ca2+ ion binding site reveals itself in each protein in Tb3+ ion phosphorescence experiments. Ca2+ ion competition studies have showed 1:1 complexes (K D′s∼2 mM). The results are interpreted in terms of two different Ca2+ ion binding sites, one in the EGF‐1 domain and one in the Gly‐209‐Gin‐221 loop of the serine proteinase part.

Photophysics of trioxatriangulenium ion. Electrophilic reactivity in the ground state and excited singlet state

Trioxatriangulenium (TOTA+. 4,8,12-trioxa-4,8,12,12c-tetrahydro-dibenzo[cd,mn]-pyrenylium) is a closed shell carbenium ion, which is stable in non-nucleophilic polar solvents at ambient temperatures In alcohols, small quantities of the leuco ether are formed in a reversible reaction. The physical and chemical properties of the excited singlet state of the trioxatriangulenium (TOTA+) carbenium ion are investigated by experimental and computational means The degeneracy of the lowest excited states is counteracted by Jahn-Teller-type distortion, which leads to vibronic broadening of the long wavelength absorption band. A strong fluorescence is observed at 520 nm (tau(fl) = 14.6 ns, phi(fl) = 0.12 in deaerated acetonitrile). The fluorescence is quenched by 10 aromatic electron donors predominantly via a dynamic charge transfer mechanism, but ground state complexation is shown to contribute in varying degrees Quenching is also observed in the presence of halide ions Quenching rate constants are derived from lifetime measurements while charge transfer (CT) complex formation constants follow from the steady-state Stern-Volmer plots. CT-complex formation with three discogenic triphenylenes is studied separately. Phosphorescence spectra, triplet lifetimes. and triplet-triplet absorption spectra are provided. In the discussion, TOTA+ is compared to the unsubstituted xanthenium ion and its 9-phenyl derivative with respect to the excited state properties.

Close Columnar Packing of Triangulenium Ions in Langmuir Films

Three new tris(dialkylamino)trioxatriangulenium (ATOTA+) salts rendered amphiphilic by attachment of two (5a x PF6 and 5b x PF6) or four (5c x PF6) n-decyl chains have been synthesized, and their Langmuir films have been studied by grazing incidence X-ray diffraction (GIXD). Compounds 5a x PF6 and 5b x PF6 both self-assemble into 2D-crystalline Langmuir monolayers, in which the planar triangular shaped carbenium ions form columnar aggregates segregated from the PF6- ions. The column width is found to be close to the width of the triangulenium moiety itself (approximately 17 angstroms), while the repeat distance along the columnar aggregates is only 3.45 angstroms, implicating a near cofacial columnar structure with only a small tilt of the planar carbenium ions relative to the columnar axis. A detailed Bragg rod analysis confirmed an 8-9 degrees tilt and inferred a large anisotropy in the smearing/thermal displacement along the pi-pi stacking and lamellar packing directions. Specular X-ray reflectivity (SXR) was used to confirm the model derived from the GIXD data and elucidate the average position of the disordered PF6- ions, showing that the majority of the anions are accommodated in the ATOTA+ layer rather than in the water subphase.

Photoinduced ring opening and fragmentation of isomeric mesoionic anhydro-4(5)-hydroxythiazolium hydroxides and of anhydro-5-hydroxy-3-methyl-2-phe

Abstract Isoelectronic mesoionic thiazoles and oxazoles undergo photochemical ring opening to heterocumulenes as evidenced by trapping experiments with alcohols. A competing photolytic fragmentation to COS and CO 2 , respectively, also occurs. Irradiation of the anhydro -5-hydroxythiazolium hydroxide system in the presence of O 2 results in the formation of singlet oxygen which undergoes a 1,4-cycloaddition to the thiazolium system. Decomposition of this cycloadduct gives rise to COS.

Oligodeoxynucleotides containing α-L-ribo configured LNA-type C-aryl nucleotides

Synthesis of 2′-O,4′-C-methylene-α-L-ribofuranosyl derivatives containing phenyl and 1-pyrenyl aglycons, i.e., novel α-L-ribo configured LNA-type C-aryl nucleosides, has been accomplished. Key synthetic steps included stereoselective Grignard reactions on tetrahydrofuran aldehyde 12, configurational inversion of the resulting alcohol 13 into alcohol 15, and concomitant Mitsonobu cyclization furnishing the desired bicyclic furanosyl skeleton with a locked conformation. The phosphoramidite derivatives 19a and 19b were used for automated synthesis of 9-mer DNA and α-L-LNA oligonucleotides containing the α-L-LNA-type C-aryl monomers αLPhL and αLPyL containing a phenyl and pyrenyl aglycon, respectively. Thermal denaturation studies showed universal base pairing behavior for the pyrenyl monomer αLPyL when incorporated into a DNA or an α-L-LNA oligonucleotide.

First Step in Chemical Preparation of Metal Nanogaps Bridged by Thiol End-Capped Molecular Wires

We present a strategy for chemical preparation of multiple copies of single-molecule electronic devices that is based on chemical self-assembly under equilibrium conditions in aqueous solution. As a first step in the realization of this, we show that thiol end-capped oligo(phenylenevinylene)s (OPVs) can be rendered water-soluble by forming well-defined stoichiometric supramolecular complexes with α-cyclodextrins. On the basis of fluorescence intensity measurements in water, a 1:3 stoichiometry was determined for the complexes with equilibrium constants of the order of 5 × 10(-2) M(-2). The photophysical properties of the OPV3s as free molecules in solution, as nanoaggregates in aqueous suspension, and embedded in cyclodextrins in water, are reported, and the prospects of using these complexes as nucleation sites for growth of gold nanorods bridged by electronically active thiol end-capped molecules is discussed.