On the Influence of Higher Excited States on the ISC Quantum Yield of Octa-aL-alkyloxy-substituted Zn-Phthalocyanine Molecules Studied by Nonlinear Absorption

Abstract— Octa-aL-alkyloxy-substituted Zn-phthalocyanines are an interesting class of far red-absorbing photosensitizers. The chemical structure, the calculated steric conformation, the observed linear optical properties and an anomalous luminescence from a higher than S, excited state are reported. To study the optical properties of higher excited states and their occupation dynamics up to delay times of 15 ns we have carried out measurements of transient absorption spectra after 14 ps pulsed, resonant B-band and Q-band excitation. From these measurements the excited state singlet-singlet and triplet-triplet spectra as well as the intersystem crossing (ISC) quantum yields are obtained. The main result is an excitation wavelength-dependent ISC quantum yield that can be explained by an additional ISC channel between higher excited singlet and triplet states. The large rate of this channel is justified by the resonance between higher triplet states, observed in the triplet-triplet spectrum and the B, absorption band. Using kinetic model calculations, a lifetime of the higher excited singlet state of some picoseconds is predicted and the influence of a two-step absorption process on the population density of this higher excited singlet state is discussed.     

Unprecedented Combination of Polyketide Natural Product Fragments Identifies the New Hedgehog Signaling Pathway Inhibitor Grismonone

Pseudo-natural products (pseudo-NPs) are de novo combinations of natural product (NP) fragments that define novel bioactive chemotypes. For their discovery, new design principles are being sought. Previously, pseudo-NPs were synthesized by the combination of fragments originating from biosynthetically unrelated NPs to guarantee structural novelty and novel bioactivity. We report the combination of fragments from biosynthetically related NPs in novel arrangements to yield a novel chemotype with activity not shared by the guiding fragments. We describe the synthesis of the polyketide pseudo-NP grismonone and identify it as a structurally novel and potent inhibitor of Hedgehog signaling. The insight that the de novo combination of fragments derived from biosynthetically related NPs may also yield new biologically relevant compound classes with unexpected bioactivity may be considered a chemical extension or diversion of existing biosynthetic pathways and greatly expands the opportunities for exploration of biologically relevant chemical space by means of the pseudo-NP principle.     

Elongational Viscosity and Foaming Behavior of PP Modified by Electron Irradiation or Nanotube Addition

Summary : In this study, two different routes were chosen which are expected to influence the elongational viscosity and, thus, the foaming behavior of polypropylene (PP). Electron irradiation of a linear PP was performed at room temperature using different doses. Enhanced long-chain branching was observed with increased irradiation dose. In addition, a linear PP was filled with 5 wt% multiwalled carbon nanotubes (MWNT) by melt mixing at 200 °C. These modified PP materials were investigated with respect to their shear and elongational behavior. After foaming, irradiated PP led to an expansion factor which is about 60% higher than that of the non-irradiated product. In case of PP filled with 5 wt% MWNT the value of the expansion factor is about 70% higher than that of pure PP expanded under the same conditions. The cell structure is much finer in both cases as compared to unmodified PP.     

The Thermal Diastereomerization of the Tryptophane-Derived Green Fluorescent Protein Chromophore

Summary. Two model compounds for the tryptophane variant of the green fluorescent protein chromophore containing a 3-indolyl and 2-pyrrolyl moiety were prepared. For the first one the (Z)-diastereomer was found to be more stable than the (E)-diastereomer by 5.7 kJ mol⁻¹. It could be photo-diastereomerized and its thermal equilibration was studied, whereas the second one underwent photo-destruction. From an Arrhenius plot an activation barrier for the (E) to (Z) diastereomerization of 85.6 kJ mol⁻¹ could be determined. Thus, it could be demonstrated that in contrast to the corresponding phenyl derivative studied recently the tyrosine- and tryptophane-derived chromophores of the green fluorescent protein are amenable to fast thermal diastereomerization, which is of fundamental importance for the fluorescence and photoswitching processes in the corresponding proteins.     

Photoinduced energy and electron transfer processes in hexapyropheophorbide a- fullerene [C(60)] molecular systems

The photophysical properties of the novel hexapyropheophorbide a (P6), and hexakis (pyropheophorbide a)-C60 (FP6) were studied and compared with those of hexakis (pyropheophorbide a)-fullerene [5:1] hexaadduct (FHP6). It was found that after light absorption the pyropheophorbide a molecules in all three compounds undergo very efficient energy transfer as well as partly excitonic interactions. The last process results in the formation of energy traps, which could be resolved experimentally. For P6, due to shorter distances between neighboring dye molecules, stronger interactions between pyropheophorbide a units than for FHP6 were observed. As a consequence, the excitation energy is delivered rapidly to traps formed by stacked pyropheophorbide a molecules resulting in the reduction of fluorescence, intersystem crossing, and singlet oxygen quantum yields compared to the values of FHP6. For FP6 the reduction of these values is much stronger due to an additional fast and efficient deactivation process, namely photoinduced electron transfer from pyropheophorbide a to the fullerene moiety. Consequently, FP6 can be considered as a combination of a light-harvesting system consisting of several separate pyropheophorbide a molecules and a charge-separating center.     

Synthesis of Five‐ and Six‐Coordinate Tris(pentafluoroethyl)fluorosilicates

The research area of perfluoroalkylsilanes is still in its infancy. Although there are already many examples of difluorotriorganylsilicates, the first example of a completely characterized trifluorotriorganylsilicate is presented, the dianion [Si(C₂F₅)₃F₃]^2?. The strongly electron‐withdrawing influence of the pentafluoroethyl groups appears to be a fundamental cause of the stability of this compound. This dianion is also the first structurally characterized example of a tris(pentafluoroethyl)silicon compound. The synthesis and complete characterization of [PPh₄]₂[Si(C₂F₅)₃F₃] and [PPh₄][Si(C₂F₅)₃F₂] along with the precursor [H(OEt₂)₂][Si(C₂F₅)₃F₂] was achieved from SiCl₄ and LiC₂F₅.     

Substituent size effects in lewis base induced reductions in organolanthanide chemistry

The reaction of the tripodal 1,3,5-trialkyl-1,3,5-triazacyclohexanes (L=cyclo-[N(R)CH(2)](3) , R=Et, iPr, tBu), with [Sm(AlMe(4))(3)] resulted in the formation of divalent samarium complexes of the constitution [{L(n)Sm(AlMe(4))(2)}(m)] (n, m=1,2) under ethane extrusion. These compounds were characterised by single-crystal X-ray diffraction and elemental analyses. Simultaneous occurrence of Lewis base induced reduction and C--activation reactions is observed. The ratio of products depends on the bulkiness of the N-alkyl substituent R. The reaction of [Sm(AlMe(4))(3)] with 1,3,5-triisopropyl-1,3,5-triazacyclohexane (TiPTAC) in benzene afforded the inversion-symmetric dimer [{(TiPTAC)(η(3)-AlMe(4))Sm}(2)(μ(2)-AlMe(4))(2)], whereas in toluene the pseudo-samarocene [(TiPTAC)(2)Sm(η(1)-AlMe(4))(2)] was obtained. The trisaluminate [(TiPTAC)Sm{(μ(2)-Me)(Me(2) l)}(2)(μ(3)-CH(2))(2)AlMe(2))] was found to be the C--activation product. In the case of the particular bulky 1,3,5-tri-tert-butyl-1,3,5-triazacyclohexane (TtBuTAC), the reaction led to the formation of the dimeric [{(TtBuTAC)(η(3)-AlMe(4))Sm}(2)(μ(2)-AlMe(4))(2)] even in toluene in comparably high yields. The decrease of the steric demand to ethyl groups in 1,3,5-triethyl-1,3,5-triazacyclohexane (TETAC) afforded the samarocene-like [(TETAC)(2) Sm(η(1)-AlMe(4))(2)] in lower yields. The resulting divalent samarium compounds are found to be stable with respect to reagents like dinitrogen, conjugated olefins and polycyclic aromatic systems.     

Allosteric and Chelate Cooperativity in Divalent Crown Ether/Ammonium Complexes with Strong Binding Enhancement

A thorough thermodynamic analysis by isothermal titration calorimetry of allosteric and chelate cooperativity effects in divalent crown ether/ammonium complexes is combined with DFT calculations including implicit solvent on the one hand and large‐scale molecular dynamics simulations with explicit solvent molecules on the other. The complexes studied exhibit binding constants up to 2×10⁶ m ^?1 with large multivalent binding enhancements and thus strong chelate cooperativity effects. Slight structural changes in the spacers, that is, the exchange of two ether oxygen atoms by two isoelectronic methylene groups, cause significantly stronger binding and substantially increased chelate cooperativity. The analysis is complemented by the examination of solvent effects and allosteric cooperativity. Such a detailed understanding of the binding processes will help to efficiently design and construct larger supramolecular architectures with multiple multivalent building blocks.     

Determination of the defining boundary in nuclear magnetic resonance diffusion experiments

While nuclear magnetic resonance diffusion experiments are widely used to resolve structures confining the diffusion process, it has been elusive whether they can exactly reveal these structures. This question is closely related to x-ray scattering and to Kac's "hear the drum" problem. Although the shape of the drum is not "hearable," we show that the confining boundary of closed pores can indeed be detected using modified Stejskal-Tanner magnetic field gradients that preserve the phase information and enable imaging of the average pore in a porous medium with a largely increased signal-to-noise ratio.