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.

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

Molecules designed to carry out photochemical energy conversion typically employ several sequential electron transfers, as do photosynthetic proteins. Yet, these molecules typically do not achieve the extensive charge transport characteristic of semiconductor devices. We have prepared a large molecule in which four perylene-3,4:9,10-tetracarboxydiimide (PDI) molecules that both collect photons and accept electrons are attached to a central zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP) electron donor. This molecule self-assembles into ordered nanoparticles both in solution and in the solid-state, driven by van der Waals stacking of the PDI molecules. Photoexcitation of the nanoparticles results in quantitative charge separation in 3.2 ps to form ZnTPP+PDI- radical ion pairs, in which the radical anion rapidly migrates to PDI molecules that are, on average, 21 A away, as evidenced by magnetic field effects on the yield of the PDI triplet state that results from radical ion pair recombination. These nanoparti...

Charge Transport in Photofunctional Nanoparticles Self-Assembled from Zinc 5,10,15,20-Tetrakis(perylenediimide)porphyrin Building Blocks

The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Effects of weak magnetic fields on free radical recombination reactions

Functional molecular wires are essential for the development of molecular electronics Charge transport through molecules occurs primarily by means of two mechanisms, coherent superexchange and incoherent charge hopping Rates of charge transport through molecules in which superexchange dominates decrease approximately exponentially with distance, which precludes using these molecules as effective molecular wires In contrast, charge transport rates through molecules in which incoherent charge hopping prevails should display nearly distance independent, wirelike behavior We are now able to determine how each mechanism contributes to the overall charge transport characteristics of a donor−bridge−acceptor (D−B−A) system, where D = phenothiazine (PTZ), B = p-oligophenylene, and A = perylene-3,4:9,10-bis(dicarboximide) (PDI), by measuring the interaction between two unpaired spins within the system's charge separated state via magnetic field effects on the yield of radical pair and triplet recombination product

Making a molecular wire: charge and spin transport through para-phenylene oligomers.

It has been experimentally discovered that a low magnetic field (less than 500 mT) can substantially change the electroluminescence, photoluminescence, photocurrent, and electrical-injection current in nonmagnetic organic semiconducting materials, leading to magnetic-field effects (M FEs). Recently, there has been significant driving force in understanding the fundamental mechanisms of magnetic responses from nonmagnetic organic materials because of two potential impacts. First, MFEs can be powerful experimental tools in revealing and elucidating useful and non-useful excited processes occurring in organic electronic, optical, and optoelectronic devices. Second, MFEs can lead to the development of new multifunctional organic devices with integrated electronic, optical, and magnetic properties for energy conversion, optical communication, and sensing technologies. This progress report discusses magnetically sensitive excited states and charge-transport processes involved in MFEs. The discussions focus on both fundamental theories and tuning mechanisms of MFEs in nonmagnetic organic semiconducting materials.

Magnetic‐Field Effects in Organic Semiconducting Materials and Devices

Pairs of radical ions, 2A–+2D+, generated in acetonitrile by nanosecond laser flashes have been investigated in the presence of external magnetic fields. The pairs are produced in the overall singlet state via photoinduced electron transfer between electron-donor (D, dimethylaniline) and electron-acceptor (A, pyrene) molecules and they recombine geminately by back electron transfer to form the molecular triplet state of pyrene.The results obtained with both freely diffusing systems and polymethylene-linked compounds (A—[CH2]n—D, n= 6–12) can be interpreted quantitatively on the basis of the assumption that the spin realignment in the initial radical ion pair is governed by the hyperfine interaction, ΔEhfi, between the unpaired electron spin and the nuclear spins in each radical and by the exchange interaction, J(n), of the radical spins in the pair. The latter increases with decreasing chain length, n.The differences in behaviour with respect to geminate triplet formation of the linked compounds with short (n⩽ 6), medium (6 < n < 12) and long (n 12) polymethylene chains are discussed.

Magnetic-field effects on geminate radical-pair recombination

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems

Experimentally observed temperature-dependent shifts in magnetic-field-dependent reaction yield spectra of polymethylene-linked radical ion pairs are correlated with motional shifts in calculated spin-exchange spectra. The compounds investigated are pyrene-(CH{sub 2}){sub n}-N,N-dimethylaniline, n {le} 16, in the polar solvent acetonitrile. Radical ion pairs have been generated by photoinduced intramolecular electron transfer. The magnetic field effect (MFE) was monitored with optical methods: both absorption and emission spectroscopy. The MFE is explained and discussed with the help of qualitative diagrams that provide an integrated picture of the combination of spin dynamics and molecular dynamics. A through-space (solvent) exchange interaction is assumed in our discussions.

Temperature study of the magnetic field effect in photogenerated flexibly-linked radical ion pairs: influence of the stochastically modulated exchange interaction

Life uncertainty broadening in photoinduced electron transfer

An absorption method which permits time-selective measurements at different wavelengths has been developed in order to explore magnetic field effects on photoinduced electron transfer reactions in linked and unlinked donor-acceptor systems.

R. Treichel

Papers

Magnetic-field effects on geminate radical-pair recombination

Magnetic field dependence of intramolecular exciplex formation in polymethyelene-linked A–D systems

Temperature study of the magnetic field effect in photogenerated flexibly-linked radical ion pairs: influence of the stochastically modulated exchange interaction

Life uncertainty broadening in photoinduced electron transfer

Magnetic field effect measurements by time-selective absorption sampling