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Photochemically driven molecular motors convert the energy of incident radiation to intramolecular rotational motion. The motor molecules considered here execute four step unidirectional rotational motion. This comprises a pair of successive light induced isomerizations to a metastable state followed by thermal helix inversions. The internal rotation of a large molecular unit required in these steps is expected to be sensitive to both the viscosity of the medium and the volume of the rotating unit. In this work, we describe a study of motor motion in both ground and excited states as a function of the size of the rotating units. The excited state decay is ultrafast, highly non-single exponential, and is best described by a sum of three exponential relaxation components. The average excited state decay time observed for a series of motors with substituents of increasing volume was determined. While substitution does affect the lifetime, the size of the substituent has only a minor effect. The solvent polarity dependence is also slight, but there is a significant solvent viscosity effect. Increasing the viscosity has no effect on the fastest of the three decay components, but it does lengthen the two slower decay times, consistent with them being associated with motion along an intramolecular coordinate displacing a large solvent volume. However, these slower relaxation times are again not a function of the size of the substituent. We conclude that excited state decay arises from motion along a coordinate which does not necessarily require complete rotation of the substituents through the solvent, but is instead more localized in the core structure of the motor. The decay of the metastable state to the ground state through a helix inversion occurs 14 orders of magnitude more slowly than the excited state decay, and was measured as a function of substituent size, solvent viscosity and temperature. In this case neither substituent size nor solvent viscosity influences the rate, which is entirely determined by the activation barrier. This result is different to similar studies of an earlier generation of molecular motors, which suggests different microscopic mechanisms are in operation in the different generations. Finally, the rate of photochemical isomerization was studied for the series of motors, and those with the largest volume substituents showed the highest photochemical cross section.

Made available in DSpace on 2020-12-12T01:40:46Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-11-01 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The present paper describes a nonenzymatic chemiresistor sensor based on an azo-polymer and reduced graphene oxide (rGO) as a resistive platform for superoxide radical detection. The sensorial platform was prepared on layer-by-layer-assembled films of poly(azo-Bismarck Brown Y) and reduced graphene oxide by cyclic voltammetry. The nanocomposite film exhibited interesting synergetic properties based on the redox properties of the azo-polymer combined with the good electronic conductivity and stability of graphene. The electrical conductivity mechanism of the graphene-polymer was analyzed by electrochemical impedance and compared to that of an electrode coated with only polymer. The conductance properties of the interlayer formed by π-π stacking between the conjugated structure of the polymer chains and the structure of the graphene sheet are dependent on the applied potential of the system. The performance of the poly(azo-BBY)/rGO film as a chemiresistor material for the sensing of superoxide anions was evaluated by impedance measurements at the applied potential of +0.30 in PBS (7.4). The charge transfer resistance values change substantially in the presence of superoxide in solution. The effect of superoxide on the resistivity of the device is attributed to changes in the oxidation state of the polymer. The impedance measurements in real time (chrono-impedance) with the poly(azo-BBY)/rGO sensor with different superoxide concentrations revealed good linearity behavior between the real impedance and the superoxide anion concentration (0.12–2.6 ​mmol ​L−1) with a detection limit of 81.0 μmol L−1. Department of Chemistry and Biochemistry School of Science and Technology - Sao Paulo State University (UNESP) Rua Roberto Simonsen 305 Department of Physiotherapy School of Science and Technology - Sao Paulo State University (UNESP) Presidente Prudente Department of Chemistry and Biochemistry School of Science and Technology - Sao Paulo State University (UNESP) Rua Roberto Simonsen 305 Department of Physiotherapy School of Science and Technology - Sao Paulo State University (UNESP) Presidente Prudente

We report the synthesis and characterization of new organometallic assemblies consisting of a central Zn(II) tetraphenylporphyrin (ZnTPP) core surrounded by four Ru(II) alkynyl complexes ( trans -Ru(dppe) 2 R) appended at the para phenyl positions and acting as peripheral donor groups. The synthesis of the tetra-chlorido derivative 2 (R=Cl) is reported first, followed by that of three extended homologues ( 3 –X) obtained from this building block by substitution of the chlorido ligands by functionalized arylacetylenes (R=C C(4-C 6 H 4 X); X=NO 2 , H, OMe). Measurement of the nonlinear absorption properties of the pentametallic derivatives 3 –X reveals that these extended π-networks incorporating polarizable organometallic units behave as good two-photon absorbers in the visible and near-IR range.

π-Conjugation plays an important role in charge transport through single molecular junctions. We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I-V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads. The current resolution of the setup reaches down to 10 fA. We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation. The quantitative analysis of complementary current-distance and current-voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance., info:eu-repo/semantics/published

Linear organic conjugated molecules, peripherally equipped with electron-donating and electron-accepting moieties, are recognized as one of the most promising classes of nonlinear optical materials for potential, application in energy conversion devices, organic electronics, optical communication, information storage, and nuclear medicine techniques. In this work, we have synthesized and photophysically characterized a series of organic molecules constituted by a 1,2-diphenyl acetylene core (tolan) bearing electronically active groups directly linked to the π-conjugated backbone. Tuning of the absorption and emission energies has been achieved via the push-pull effect. All investigated compounds displayed very high luminescence in condensed media from, intramolecular charge transfer excited states with large Stokes shifts. These features revealed to be of particular interest for the engineering of new wavelength shifters for spectral conversion of deep ultraviolet to visible light. © 2009 American Chemical Society.