An ESR study of intrachannel mobility in DOCA clathrates. Effects due to photochemical reactivity between guest and host (original) (raw)
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Spintronic Transport through Polyphenoxyl Radical Molecules
The Journal of Physical Chemistry B, 2004
The coherent quantum transport properties through the spin-polarized polyphenoxyl radical molecule have been investigated, using the density-functional-derived tight-binding model and the Green's functions method. The majority and minority spin components exhibit considerably different transmission spectra in the vicinity of the Fermi level. Namely, each spin component carries a different amount of current when the bias voltage is applied between the two electrodes that sandwich the polyradical molecule. Therefore, if the magnetization axis of the polyradical is fixed by the external magnetic field, and if the spin flip does not occur during the transmission, the assumed molecular bridge is expected to work as a spin filter or a spin valve. Furthermore, as long as the bias voltage is weak, the total spin current is observed to be larger than the current through its reduced molecular form. It indicates that the adsorption of some chemical species on the radical sites can be sensed by the change in conductance of the molecular bridge.
Photochemistry and Photobiology, 1992
We have determined the transition dipole moment orientation of the chromophore during the photocycle of bacteriorhodopsin by photoselection and time-resolved linear dichroism experiments with samples of oriented immobilized purple membranes. This technique offers two important advantages over experiments with isotropic aqueous suspensions: (1) the depolarization due to the rotational diffusion of the membranes is eliminated, (2) the sensitivity for detecting the orientation of the transition dipole moment of intermediates is greatly increased. The appropriate equations for the analysis of time-resolved linear dichroism experiments with samples of oriented immobilized membranes will be presented. In the transition from the ground state of bacteriorhodopsin to the Mintermediate, the transition dipole moment tilts out of the plane of the membrane by about 3". On the basis of current structural information on the plane of the chromophore and the orientation of its C(19) and C(20) methyl groups, a tilt of the transition dipole moment into the plane of the membrane would have been expected if it is assumed that the orientation of the conjugated polyene chain from C(5) to C(13) is the same in both states. The experimental result may be explained by an 11" tilt of the C(5) to C(13) part of the chain out of the plane of the membrane with the C(20) methyl group moving towards the cytoplasmic side of the membrane by about 1.7 d; and the cyclohexene ring staying fixed. This interpretation is supported by recent neutron diffraction experiments on the chromophore position in the M-intermediate.
The Journal of Physical Chemistry, 1991
We used time-dependent fluorescence energy transfer of externally quenched and nonquenched samples, and global analysis of the data, to recover the end-to-end distance distributions and diffusion coefficients of flexible fluorescent molecules in low-viscosity solution. The fluorescence decays of tryptamine covalently linked to a dansyl acceptor by a polyethylene chain of 22 methylene groups were measured by the frequency-domain method. The data were fitted using numerical solutions of the diffusion equation which predicts the time-and distance-dependent population of the excited-state donors in the presence of energy transfer, followed by transformation to the frequency domain for nonlinear least-squares fitting to the experimental data. From the simulation study we found that the time-and distance-dependent population of the excited-state donors are significantly different for nonquenched and quenched samples and that the effects of end-to-end diffusion on the donor decay is decreased by collisional quenching. Importantly, the resolution is dramatically improved by the use of simultaneous analysis of quenched and nonquenched samples. This method was applied to the tryptamine-dansyl system using acrylamide as an external quencher. The recovered initial ( t = 0) distance distribution, R,, = 18.9 A, hw = 17.1 A, is very similar to that obtained for diffusion-free conditions. The end-to-end diffusion coefficient of D = 1.26 X cm2/s is comparable to that expected for molecules the size of indole and dansyl. This value is about twice smaller than that obtained from diffusion-dependent intermolecular energy transfer using unlinked indole and dansylamide as the donor and acceptor, respectively, which may reflect the effects of the linker on diffusion of the chromophores.
Solvent effects on the intramolecular spin exchange in biradicals at room temperature
Molecular Physics, 2007
Solvent effects on intramolecular electron spin exchange in biradicals have been investigated in various solvents by electron spin resonance (ESR) spectroscopy at room temperature. Biradicals containing different radical fragments and bridges of different length and composition have been used. Interactions between solvent molecules and biradicals were found to take place at the radical fragment as well as at the functional group in the connecting bridge and have been proved to have an influence on the intramolecular spin exchange. The experimentally measured parameters, exchange integral (J) and characteristic time of an intramolecular movement (τ eff), have been compared with solvent viscosity and polarity parameters (macroscopic level) and longitudinal solvent relaxation time (microscopic level). The dependencies on viscosity and polarity were in good agreement with reported results. In addition, strong evidence of hydrogen bonding between solvent and different sites of the biradicals was found, in some cases even surpassing the other effects.
The Journal of Physical Chemistry B, 2011
Photoionization, fluorescence time-dependent Stokes shift (TDSS), and rotational dynamics of coumarin 307 (C307) have been investigated in soft matter system such as micelles using time-resolved transient absorption and fluorescence spectroscopy. Photoionization of C307 leads to the formation of coumarin radical cation and hydrated electron, which were characterized by their respective transient absorption. The photoionization yields are significantly higher in anionic sodium dodecyl sulfate (SDS) micelle than in cationic cetyltrimethylammonium bromide (CTAB) and neutral Triton X-100 (TX-100) micelles, indicating the influence of micellar surface charge on the efficient separation of radical cation-hydrated electron pair. The CTAB micelle favors the recombination of radical cation and hydrated electron leading to the formation of triplet state of C307, which causes a decrease in the photoionization yield. C307 exhibits TDSS in all micelles; the time evolution and the magnitude of the TDSS depend on nature of the micelle. In TX-100 micelles, the decay of the TDSS exhibits ultraslow component (165 ns) and is affected by the presence of electron scavengers. The ultraslow component in TX-100 micelle originates from the recombination of radical cationÀhydrated electron, which results in the formation of twisted intramolecular charge transfer (TICT) state; such formation of TICT state was not observed in SDS and CTAB micelles. To the best of our knowledge, this is the first report where the radical-ion pair recombination dynamics is probed using TDSS in combination with time-resolved transient absorption studies. The activation energy for the solvent relaxation and radical-ion pair (solvent separated) recombination process was found to be 6.1 and 3.0 kcal mol À1 , respectively. Temperature effect on TDSS in TX-100 micelles confirmed the increase in the water hydration, and size of the micelle influences the relative contribution of the solvation and radical-ion pair recombination dynamics toward the total TDSS. We propose that TDSS observed in neutral micelles and reported in other biomolecules such as proteins by the 7-amino coumarin probe is not only due to the solvation dynamics alone but also due to the radical-ion pair recombination dynamics.
Supramolecular Photochemistry in β-Cyclodextrin Hosts: A TREPR, NMR, and CIDNP Investigation
Langmuir, 2010
A systematic investigation of the photochemistry and ensuing radical chemistry of three guest ketones encapsulated in randomly methylated β-cyclodextrin (β-CD) hosts is reported. Dibenzyl ketone (DBK), deoxybenzoin (DOB), and benzophenone (BP) triplet states are rapidly formed after photolysis at 308 nm. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy, steady-state NMR spectroscopy, and time-resolved chemically induced nuclear polarization (TR-CIDNP) experiments were performed on the ketone/CD complexes and on the ketones in free solution for comparison. The major reactivity pathways available from these excited states are either Norrish I R-cleavage or H-atom abstraction from the interior of the CD capsule or the solvent. The DOB triplet state undergoes both reactions, whereas the DBK triplet shows exclusively R-cleavage and the BP triplet shows exclusively H-atom abstraction. Radical pairs are observed in β-CDs by TREPR, consisting of either DOB or BP ketyl radicals with sugar radicals from the CD interior. The TREPR spectra acquired in CDs are substantially broadened due to strong spin exchange. The electron spin polarization mechanism is mostly due to S-T 0 radical pair mechanism (RPM) in solution but changes to S-T-RPM in the CDs due to the large exchange interaction. The TR-CIDNP results confirm the reactivity patterns of all three ketones, and DOB shows strong nuclear spin polarization from a novel rearrangement product resulting from the R-cleavage reaction.
Archives of Biochemistry and Biophysics, 2005
We report on the development of the first member of a new family of EPR spin-trapping agents designed to trap radicals at a predetermined depth within biological membranes. By analogy to the use of nitroxide spin labels to ‘report’ on the environment at specific depths within biological membranes, we set out to prepare similar reporter molecules, but with a nitrone in place of the nitroxide function. The prototype compounds were tested in a model system consisting of large unilamellar vesicles exposed to a copper-dependent radical generating system. This entailed the reduction of tert-butylhydroperoxide to the tert-butoxyl radical (tBuO) by a membrane-permeable CuI complex, which was generated in situ by reduction of the CuII complex by ascorbate. To assist in the identification of the radicals detected, preliminary studies were performed in methanolic solution, where the major radical trapped was shown to be CH2OH, resulting from H-atom abstraction from the alcohol by tBuO. This conclusion was shown to be in agreement with predictions based on chemical kinetics, which were then used to support the proposal that the primary species trapped in the lipid vesicles were radicals derived from membrane fatty acids. This molecule represents the first of a new generation of spin traps which, through modification, can be used to position the radical-trapping nitrone moiety at chosen depths within biological membranes.
Chemical Physics, 1996
A new model of time-resolved EPR in micellized radical pairs is introduced. The model is based on numerical integration of the master Liouville equation for spin-correlated (micellized) pairs and free (escaped) radicals. The diffusion of radicals is considered in terms of a supercage model. This approach is used to analyze data on laser flash photolysis of 13C-carbonyl labelled ketone a-deoxybenzoin in aqueous sodium alkyl (10-12) sulphate solutions. EPR lines of 13C-benzoyi radicals exhibit antiphase structure (APS) typical of spin-correlated pairs. Due to very large APS splitting, 0.8-1.5 mT, the ST o polarized lines from free benzoyl radicals can be isolated spectrally. The observed line shape of the APS cannot be accounted for in the standard model of effective exchange potential. The shape of the APS is shown to be controlled by spin exchange relaxation in micellized pairs.
Journal of the American Chemical Society, 2011
A covalent, fixed-distance donor-bridgeacceptor (D-B-A) molecule was synthesized that upon photoexcitation undergoes ultrafast charge separation to yield a radical ion pair (RP) in which the spin-spin exchange interaction (2J) between the two radicals is sufficiently large to result in preferential RP intersystem crossing to the highestenergy RP eigenstate (T þ1 ) at the 350 mT magnetic field characteristic of X-band (9.5 GHz) EPR spectroscopy. This behavior is unprecedented in covalent D-B-A molecules, and is evidenced by the time-resolved EPR (TREPR) spectrum at X-band of 3* D-B-A derived from RP recombination, which shows all six canonical EPR transitions polarized in emission (e,e,e,e,e,e). In contrast, when the RP is photogenerated in a 3400 mT magnetic field, the TREPR triplet spectrum at W-band (94 GHz) of 3* D-B-A displays the (a,e,e,a,a,e) polarization pattern characteristic of a weakly coupled RP precursor, similar to that observed in photosynthetic reaction center proteins, and indicates a switch to selective population of the lower-energy T 0 eigenstate. O ne of the continuing challenges in developing molecular systems for artificial photosynthesis is the creation of longlived charge-separated states. 1 Studies of competitive charge recombination accompanied by triplet state formation within donor-bridge-acceptor (D-B-A) molecules can provide a deeper understanding of how to minimize these energy-wasting processes. While time-resolved optical spectroscopy is often used to determine charge separation and recombination rates, time-resolved electron paramagnetic resonance (TREPR) spectroscopy can determine the spin-selective formation and decay mechanisms of radical ion pairs (RPs) and triplet states by monitoring their spin dynamics directly.
Chemical Physics, 1977
Generation of radicals of glyoxal from photoIysis in aprotic solvents is reported. in benzene solution radical X and in toluene solution three radicals X, C and T have been observed. From temperature dependence of the ESR spectra and quantum chemical calculations at ab initio, PRDDO and WDO level the radicalsc and T are assigned to the cis and tram conformation of the 2-hydroxyethenyl-l-oxy radical. The chemical exchange dynamics of this nonrigid radical is dominated by the OH group internal rotatibn, whereas under stationary conditions the relative concentration of C and T also depend on C-C internal rotation kinetics. The radical X is assigned to an addition product of the fonnyl radical to glyoxal. Estimates of lifetimes of the radicals based on MESR experiments are given in the mechanism and kinetics of radical reactions arc discussed.