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Michael R Wasielewski

Publications and source records attributed to Michael R Wasielewski.

At least 19 recordsLinked to original sources
PubMed2006

Time-resolved EPR studies of photogenerated radical ion pairs separated by p-phenylene oligomers and of triplet states resulting from charge recombination.

Photoexcitation of a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Phn), n = 1-5, and A= perylene-3,4:9,10-bis(dicarboximide) (PDI) results in rapid electron transfer to produce 1(PTZ+*-Phn-PDI-*). Time-resolved EPR (TREPR) studies of the photogenerated radical pairs (RPs) show that above 150 K, when n = 2-5, the radical pair-intersystem crossing mechanism (RP-ISC) produces spin-correlated radical ion pairs having electron spin polarization patterns indicating that the spin-spin exchange interaction in the radical ion pair is positive, 2J > 0, and is temperature dependent. This temperature dependence is most likely due to structural changes of the p-phenylene bridge. Charge recombination in the RPs generates PTZ-Phn-3*PDI, which exhibits a spin-polarized signal similar to that observed in photosynthetic reaction-center proteins and some biomimetic systems. At temperatures below 150 K and/or at shorter donor-acceptor distances, e.g., when n = 1, PTZ-Phn-3*PDI is also formed from a competitive spin-orbit-intersystem crossing (SO-ISC) mechanism that is a result of direct charge recombination: 1(PTZ+*-Phn-PDI-*) --> PTZ-Phn-3*PDI. This SO-ISC mechanism requires the initial RP intermediate and depends strongly on the orientation of the molecular orbitals involved in the charge recombination as well as the magnitude of 2J.

Biphenyl Compounds↗
PubMed2006

Electron transfer in multiply bridged donor-acceptor molecules: Dephasing and quantum coherence.

We present a simple theoretical treatment of nonadiabatic electron transfer in multiply bridged donor-bridge-acceptor molecules using the density matrix formalism. Destructive interference can result from different signed couplings between bridge sites, with the simplest system being a four-site Joachim-type molecular interferometer. Previous work has shown that deposition of energy on the bridge sites erases the interference and recovers transport. We show that pure local dephasing, a completely elastic process, is also capable of eliminating destructive interference and regaining transport. Destructive interference as a result of system connectivity can explain the familiar ortho-meta-para reactivity of benzene bridges. We also show that pure dephasing can yield a coalescence of ortho, meta, and para effective coupling strengths and suggest a system to observe this effect experimentally.

Journal Article↗
PubMed2006

Energy, charge, and spin transport in molecules and self-assembled nanostructures inspired by photosynthesis.

Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.

Carbon Dioxide↗
PubMed2006

Spin dynamics of photogenerated triradicals in fixed distance electron donor-chromophore-acceptor-TEMPO molecules.

The stable free radical 2,2,6,6-tetramethylpiperidinoxyl (TEMPO, T*) was covalently attached to the electron acceptor in a donor-chromophore-acceptor (D-C-A) system, MeOAn-6ANI-Phn-A-T*, having well-defined distances between each component, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-l,8-dicarboximide, Ph = 2,5-dimethylphenyl (n = 0,1), and A = naphthalene-1,8:4,5-bis(dicarboximide) (NI) or pyromellitimide (PI). Using both time-resolved optical and EPR spectroscopy, we show that T* influences the spin dynamics of the photogenerated triradical states 2,4(MeOAn+*-6ANI-Phn-A-*-T*), resulting in modulation of the charge recombination rate within the triradical compared with the corresponding biradical lacking T*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn+* and A-* is not altered by the presence of T*, which interacts most strongly with A-* and accelerates radical pair intersystem crossing. Charge recombination within the triradicals results in the formation of 2,4(MeOAn-6ANI-Phn-3*NI-T*) or 2,4(MeOAn-3*6ANI-Phn-PI-T*) in which T* is strongly spin polarized in emission. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, the rate at which the net spin polarization appears on T* closely follows the photogenerated radical ion pair decay rate. This effect is attributed to antiferromagnetic coupling between T* and the local triplet state 3NI, which is populated following charge recombination. These results are explained using a switch in the spin basis set between the triradical and the three-spin charge recombination product having both T* and 3*NI or 3*6ANI present.

Journal Article↗
PubMed2006

Intramolecular electron transfer through the 20-position of a chlorophyll a derivative: an unexpectedly efficient conduit for charge transport.

Suzuki cross-coupling reactions have afforded 20-phenyl-substituted Chlorophyll a derivatives (ZCPh) in good yields and significant quantities from readily available Chl a. A series of donor-acceptor dyads was synthesized in which naphthalene-1,8:4,5-bis(dicarboximide) or either of two perylene-3,4:9,10-bis(dicarboximide) electron acceptors is attached to the para position of the 20-phenyl group. Comparisons with the analogous dyads based on a zinc 5,10,15-tri(n-pentyl)-20-phenylporphyrin donor show that, for a given acceptor and solvent, the rates of photoinduced charge separation and recombination as well as the calculated electronic coupling matrix elements, V, for these reactions differ by less than a factor of 2. However, EPR and ENDOR spectroscopy corroborated by DFT calculations show that the highest occupied MO of ZCPh+* has little spin (charge) density at the 20-carbon atom, whereas Z3PnPh+* has significant spin (charge) density there, implying that V, and therefore the electron-transfer rates, should differ significantly for these two macrocyclic donors. DFT calculations on ZCPh+* and Z3PnPh+*, with two -0.5 charges located where the nearest carbonyl oxygen atoms of the acceptor would reside in the donor-acceptor dyads, show that the presence of the negative charges significantly shifts the charge density of both ZCPh+* and Z3PnPh+* from the macrocycle onto the phenyl rings. Thus, the presence of adjacent covalently linked radical anions at a fixed location relative to each of these radical cations results in nearly identical electronic coupling matrix elements for electron transfer and therefore very similar rates.

Journal Article↗
PubMed2006

Electron donor-bridge-acceptor molecules with bridging nitronyl nitroxide radicals: influence of a third spin on charge- and spin-transfer dynamics.

Appending a stable radical to the bridge molecule in a donor-bridge-acceptor system (D-B-A) is potentially an important way to control charge- and spin-transfer dynamics through D-B-A. We have attached a nitronyl nitroxide (NN*) stable radical to a D-B-A system having well-defined distances between the components: MeOAn-6ANI-Ph(NN*)-NI, where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, Ph = phenyl, and NI = naphthalene-1,8:4,5-bis(dicarboximide). MeOAn-6ANI, NN*, and NI are attached to the 1, 3, and 5 positions of the Ph bridge. Using both time-resolved optical and EPR spectroscopy, we show that NN* influences the spin dynamics of the photogenerated triradical states (2,4)(MeOAn(+)*-6ANI-Ph(NN*)-NI(-)*), resulting in slower charge recombination within the triradical compared to the corresponding biradical lacking NN*. The observed spin-spin exchange interaction between the photogenerated radicals MeOAn(+)(*) and NI(-)(*) is not altered by the presence of NN*, which only accelerates radical pair intersystem crossing. Charge recombination within the triradical results in the formation of (2,4)(MeOAn-6ANI-Ph(NN*)-(3)NI), in which NN* is strongly spin-polarized. Normally, the spin dynamics of correlated radical pairs do not produce a net spin polarization; however, net spin polarization appears on NN* with the same time constant as describes the photogenerated radical ion pair decay. This effect is attributed to antiferromagnetic coupling between NN* and the local triplet state (3)NI, which is populated following charge recombination. This requires an effective switch in the spin basis set between the triradical and the three-spin charge recombination product having both NN* and (3)NI present.

Journal Article↗
PubMed2006

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s and their radical cations: synthesis, electrochemistry, and ENDOR spectroscopy.

1,6- and 1,7-bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) were synthesized by reaction of n-octylamine with the corresponding dibromo compounds. These compounds display intense charge-transfer optical transitions in the visible spectrum (approximately 550-750 nm) and fluoresce weakly (Phi(F) < 0.06). Cyclic voltammetry reveals that each chromophore undergoes facile and reversible oxidation and reduction. Spectroelectrochemical studies show that the radical cations of these chromophores are stable and show no signs of deprotonation of the secondary amines. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of the chemically generated radical cations of these chromophores corroborate the spectroelectrochemical data by showing that the radical cations persist for days at room temperature in methylene chloride solution. These experiments and complementary density functional theory (DFT) calculations provide a comprehensive picture of the molecular orbitals, spin density distributions, and geometries of the radical cations. The redox properties and stability of these alkylamino-functionalized perylene compounds make them a valuable addition to the family of robust perylene-based chromophores that can be used to develop new photoactive charge transport materials.

Journal Article↗
PubMed2006

Electron transfer mechanism and the locality of the system-bath interaction: a comparison of local, semilocal, and pure dephasing models.

We simulate the effects of two types of dephasing processes, a nonlocal dephasing of system eigenstates and a dephasing of semilocal eigenstates, on the rate and mechanism of electron transfer (eT) through a series of donor-bridge-acceptor systems, D-B(N)-A, where N is the number of identical bridge units. Our analytical and numerical results show that pure dephasing, defined as the perturbation of system eigenstates through the system-bath interaction, does not disrupt coherent eT because it induces no localization; electron transfer may proceed through superexchange in a system undergoing only pure dephasing. A more physically reasonable description may be obtained via a system-bath interaction that reflects the perturbation of more local electronic structure by local nuclear distortions and dipole interactions. The degree of locality of this interaction is guided by the structure of the system Hamiltonian and by the nature of the measurement performed on the system (i.e., the nature of the environment). We compare our result from this "semilocal" model with an even more local phenomenological dephasing model. We calculate electron transfer rate by obtaining nonequilibrium steady-state solutions for the elements of a reduced density matrix; a semigroup formalism is used to write down the dissipative part of the equation of motion.

Journal Article↗
PubMed2006

Electron hopping in pi-stacked covalent and self-assembled perylene diimides observed by ENDOR spectroscopy.

We have carried out room-temperature, solution-phase electron paramagnetic resonance and electron-nuclear double resonance studies on a series of radical anions based upon perylene-3,4:9,10-bis(dicarboximide) (PDI). The following systems were studied: two PDI monomers, a covalent, cofacial dimer, and two covalent trefoil-PDI3 molecules, one of which self-assembles into pi-stacked dimers. Full sharing of the unpaired electron in the covalent and self-assembled dimers is revealed by a halving of the hyperfine coupling constants in these species, relative to those of the monomers. These results and the electronic absorption spectra show that electron hopping on a >107 Hz time scale occurs between a reduced and neutral chromophoric pair.

Journal Article↗
PubMed2005

Electron-transfer dynamics of photosynthetic reaction centers in thermoresponsive soft materials.

Poly(ethylene glycol)-grafted, lipid-based, thermoresponsive, soft nanostructures are shown to serve as scaffolding into which reconstituted integral membrane proteins, such as the bacterial photosynthetic reaction centers (RCs) can be stabilized, and their packing arrangement, and hence photophysical properties, can be controlled. The self-assembled nanostructures exist in two distinct states: a liquid-crystalline gel phase at temperatures above 21 degrees C and a non-birefringent, reduced viscosity state at lower temperatures. Characterization of the effect of protein introduction on the mesoscopic structure of the materials by 31P NMR and small-angle X-ray scattering shows that the expanded lamellar structure of the protein-free material is retained. At reduced temperatures, however, the aggregate structure is found to convert from a two-dimensional normal hexagonal structure to a three-dimensional cubic phase upon introduction of the RCs. Structural and functional characteristics of the RCs were determined by ground-state and femtosecond transient absorption spectroscopy. Time-resolved results indicate that the kinetics of primary electron transfer for the RCs in the low-viscosity cold phase of the self-assembled nanostructures are identical to those observed in a detergent-solubilized state in buffered aqueous solutions (approximately 4 ps) over a wide range of protein concentrations and experimental conditions. This is also true for RCs held within the lamellar gel phase at low protein concentrations and at short sample storage times. In contrast are kinetics from samples that are prepared with high RC concentrations and stored for several hours, which display additional kinetic components with extended electron-transfer times (approximately 10-12 ps). This observation is tentatively attributed to energy transfer between RCs that have laterally (in-plane) organized within the lipid bilayers of the lamellar gel phase prior to charge separation. These results not only demonstrate the use of soft nanostructures as a matrix in which to stabilize and organize membrane proteins but also suggest the possibility of using them to control the interactions between proteins and thus to tune their collective optical/electronic properties.

Amino Acids↗
PubMed2005

Conformationally gated switching between superexchange and hopping within oligo-p-phenylene-based molecular wires.

We observe well-defined regions of superexchange and thermally activated hopping in the temperature dependence of charge recombination (CR) in a series of donor-bridge-acceptor (D-B-A) systems, where D = phenothiazine (PTZ), B = p-phenylene (Ph(n)), n = 1-4, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI). A fit to the thermally activated CR rates of the n = 3 and n = 4 compounds yields activation barriers of 1290 and 2030 cm(-1), respectively, which match closely with theoretically predicted and experimentally observed barriers for the planarization of terphenyl and quaterphenyl. Negative activation of CR in the temperature regions dominated by superexchange charge transport is the result of a fast conformational equilibrium that increasingly depopulates the reactive state for CR as temperature is increased. The temperature dependence of the effective donor-acceptor superexchange coupling, V(DA), measured using magnetic field effects on the efficiency of the charge recombination process, shows that CR occurs out of the conformation with lower V(DA) via the energetically favored triplet pathway.

Crystallography, X-Ray↗
PubMed2005

A general formulation for magnetic exchange coupling within long-distance radical ion pairs.

A Hubbard model-based expression for the magnetic exchange interaction, 2J, between the spins of a radical ion pair is derived for a generic donor-bridge-acceptor (D-B(n)-A) system in terms of the superexchange coupling, t, between molecular orbitals of the donor, bridge, and acceptor and the energies, epsilon, and same-site Coulomb repulsions, U, of these orbitals. The formulation is used to predict 2J for the photophysically well-characterized series phenothiazine-p-(phenylene)n-perylene diimide, n = 2,3,4,5, where the superexchange coupling is calculated with multipathway, second- and third-order perturbation theory coupled with the extended Huckel formalism. U and epsilon are estimated through experimentally determined electrochemical potentials. Due to multiple interactions arising from the large distance between magnetic centers in these donor-bridge-acceptor systems, agreement with experiment was substantially improved when third-order corrections were included in the calculation of t and 2J.

Journal Article↗
PubMed2005

Electron spin dynamics as a probe of molecular dynamics: temperature-dependent magnetic field effects on charge recombination within a covalent radical ion pair.

The electron spin-spin exchange interaction, 2J, in radical pairs (RPs) is exquisitely sensitive to the details of molecular structure and can thus serve as an important probe of structural dynamics in RPs of potential interest to photonic and electronic devices. Photoinitiated ultrafast two-step charge separation produces (1)(MeOAn(+)(*)-6ANI-NI(-)(*)), where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, and NI = naphthalene-1,8:4,5-bis(dicarboximide). Radical pair intersystem crossing subsequently produces (3)(MeOAn(+)(*)-6ANI-NI(-)(*)), and the total RP population decays with approximately 10 ns lifetime at 140 K, which increases to nearly 30 ns at 300 K in toluene. The activation energy observed for this process is negative and can be explained by a mechanism involving a conformational preequilibrium of the RP followed by charge recombination. Over the same temperature range, the magnetic field effect (MFE) on yield of the triplet recombination product, MeOAn-6ANI-(3)()NI, yields the magnitude of 2J, which directly monitors the superexchange electronic coupling for charge recombination. A single resonance in the MFE plot is observed at 300 K, which splits into two resonances at temperatures below 230 K, suggesting that there are two distinct groups of RP conformations at low temperature. The magnitude of 2J for the lower field resonance (10 mT) at 140 K is 5 times smaller than that of the high field resonance. At 300 K the equilibrium is shifted almost entirely to the set of conformers with the stronger electronic coupling. The motion that couples these two groups of conformations is the motion that most effectively gates the donor-acceptor electronic coupling.

Journal Article↗
PubMed2005

Wire-like charge transport at near constant bridge energy through fluorene oligomers.

The study of photoinitiated electron transfer in donor-bridge-acceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FL(n)) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FL(n) bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL(n)(+.) bridge states for n = 1-4 are only weakly distance dependent. After the initial photo-generation of (1)(PTZ(+.)-FL(n)-PDI(-.)), radical pair intersystem crossing results in formation of (3)(PTZ(+.)-FL(n)-PDI(-.)) that recombines to give (3.)PDI. The dependence of the (3.)PDI yield on an applied magnetic field shows a resonance, which gives the singlet-triplet splitting, 2J, of the radical ion pair. The magnitude of 2J directly monitors the contribution of coherent charge transfer (superexchange) to the overall electron transfer rate. These data show that charge recombination through FL(n) is dominated by incoherent hopping at long distances.

Electron Transport↗
PubMed2005

Copper-promoted N-arylations of cyclic imides within six-membered rings: a facile route to arylene-based organic materials.

[reaction: see text] Cyclic imides within six-membered rings are shown to undergo efficient N-arylation using various arylboronic esters mediated by copper(II) acetate in the presence of an amine base and oxygen atmosphere with gentle heating. Until now, the synthesis of N-arylated cyclic imides having six-membered rings was restricted largely to strongly heating anilines in the presence of anhydrides. This reaction is applicable to the synthesis of new organic materials based on arylene imide and bis(imide) dyes, such as perylene-3,4:9,10-bis(dicarboximide)s.

Journal Article↗
PubMed2005

Ultrafast photoinduced charge separation resulting from self-assembly of a green perylene-based dye into pi-stacked arrays.

Condensation of 3,4,5-tris(n-dodecyloxy)aniline with the green chromophore 1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-tetracarboxylic dianhydride yields N,N'-bis(3,4,5-tris(n-dodecyloxy)phenyl)-1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-bis(dicarboximide), 5PDI-TAP, which absorbs light strongly from 550 to 750 nm. 5PDI-TAP dissolves readily in methylcyclohexane (MCH), resulting in self-assembly into H-aggregates. Small-angle X-ray scattering data obtained on 10(-4) M solutions of 5PDI-TAP in MCH show that the aggregates are pi-stacked monodisperse pentamers. Femtosecond transient absorption spectroscopy on solutions of (5PDI-TAP)5 in MCH shows evidence of charge separation occurring with tau < or = 150 fs between adjacent stacked members of 5PDI-TAP within the pentamer followed by charge recombination with tau = 860 ps. Transmission electron microscopy of 5PDI-TAP films cast from solution show isolated bundles of columnar aggregates. (5PDI-TAP)n is a potentially useful material for organic photovoltaics because efficient photoinduced charge generation is an intrinsic property of the assembly.

Journal Article↗