Browsing by Subject "Charge transfer"

System-environment interactions are essential in determining charge-transfer (CT) rates and mechanisms. We developed a computationally accessible

method, suitable to simulate CT in flexible molecules (i.e., DNA) with hundreds of

sites, where the system-environment interactions are explicitly treated with numerical

noise modeling of time-dependent site energies and couplings. The properties of the

noise are tunable, providing us a flexible tool to investigate the detailed effects of

correlated thermal fluctuations on CT mechanisms. The noise is parameterizable by

molecular simulation and quantum calculation results of specific molecular systems,

giving us better molecular resolution in simulating the system-environment interactions than sampling fluctuations from generic spectral density functions. The spatially correlated thermal fluctuations among different sites are naturally built-in in our method but are hard to be incorporated by approximate spectral densities. Our method has quantitative accuracy in systems with small redox potential differences ($

With the method of incorporating spatially and temporally correlated thermal fluctuations into charge transfer process, we study and engineer

coherence in guanine-rich DNA sequences.

Electronic delocalization in redox-active polymers may be disrupted by the heterogeneity of the environment that surrounds each monomer. When the differences in monomer redox-potential induced by the environment are small (as compared with the monomer-monomer electronic interactions), delocalization persists. Here we show that guanine (G) runs in double-stranded DNA support delocalization over 4-5 guanine bases. The weak interaction between delocalized G blocks on opposite DNA strands is known to support partially coherent long-range charge transport. The molecular-resolution model developed here finds that the coherence among these G blocks follows an even-odd orbital-symmetry rule and predicts that weakening the interaction between G blocks exaggerates the resistance oscillations. These findings indicate how sequence can be exploited to change the balance between coherent and incoherent transport. The predictions are tested and confirmed using break-junction experiments. Thus, tailored orbital symmetry and structural fluctuations may be used to produce coherent transport with a length scale of multiple nanometers in soft-matter assemblies, a length scale comparable to that of small proteins.

We extend our charge transport studies from linear molecules to branched molecules.

Self-assembling circuitry on the molecular scale demands building blocks with three or more terminals, the sine qua non for circuit elements like current splitters or combiners\cite{Molen2013,RN2,Tao2006}. A promising material for such building blocks is DNA, wherein multiple strands can self-assemble into multi-ended junctions and nucleobase stacks can transport charge over long distances\cite{Genereux2010,Cohen2005,RN6,RN7,RN8,RN9}. However, nucleobase stacking is often disrupted at the junction point, hindering electric charge transport between different terminals of the junction\cite{RN10,RN11}. Thus, the challenge of designing a multi-ended DNA circuit element remains open. Here, we address the challenge by using a guanine-quadruplex (G4) motif as the connector element of a multi-ended DNA junction, and designing the terminal groups of the motif to ensure efficient current splitting in the DNA junction with minimal carrier transport attenuation. We describe the design, assembly, and charge transport measurement\cite{RN46} of a 3-way G4 junction structure, in which charge can enter the structure from one terminal at one end of the G4, and exit from one of two terminals at the other end of the G4. We find that the charge transport characteristics are the same along the two pathways, and are also similar to those of the corresponding linear DNA duplexes. Thus, the G4-based junction indeed enables effective three-way transport, which is a necessary step towards building of DNA-based electrical networks. We optimize G4-based junction structures and interpret the charge-transport measurements with molecular dynamics and quantum chemistry simulations.

Energy transfer with an associated spin change of the donor and acceptor, Dexter energy transfer, is critically important in solar energy harvesting assemblies, damage protection schemes of photobiology, and organometallic opto-electronic materials. Dexter transfer between chemically linked donors and acceptors is bridge mediated, presenting an enticing analogy with bridge-mediated electron and hole transfer. However, Dexter coupling pathways must convey both an electron and a hole from donor to acceptor, and this adds considerable richness to the mediation process. We dissect the bridge-mediated Dexter coupling mechanisms and formulate a theory for triplet energy transfer coupling pathways. Virtual donor-acceptor charge-transfer exciton intermediates dominate at shorter distances or higher tunneling energy gaps, whereas virtual intermediates with an electron and a hole both on the bridge (virtual bridge excitons) dominate for longer distances or lower energy gaps. The effects of virtual bridge excitons were neglected in earlier treatments. The two-particle pathway framework developed here shows how Dexter energy-transfer rates depend on donor, bridge, and acceptor energetics, as well as on orbital symmetry and quantum interference among pathways.

Light-matter interactions are fundamental to many critical emerging technologies – such as photovoltaics, photonic sensing, and information transmission – that rely upon the efficient capture of light and its conversion to useful energetic states. However, to realize these technologies as a viable future we must first understand the fundamental processes which govern and dictate the energetic, spatial, and temporal identity of materials following photoexcitation. As is suggested by the term “light-matter” both the qualities of the light and the structural composition of the material will influence these characteristics resulting from their interaction. This dissertation investigates how photoexcitation conditions and material structure can be leveraged to modulate the energetic and charged states, and the dynamics thereof, which arise following photoexcitation of nanoscale and molecular systems. Employing ultrafast pump-probe transient absorption spectroscopy, this work characterizes the transient states which arise from photoexcitation of: (i) single-walled carbon nanotubes (SWNTs) wrapped by aryleneethynylene semiconducting polymers; (ii) covalently linked ethyne bridged porphyrin donor, rylene acceptor, molecular “ratchets” and (iii) rylene chromophores covalently linked to amino acid models. In nanoscale systems, this work highlights how the electronic structure of 1-dimensional SWNTs: (i) enable a complex interplay of excitation fluence dependent multi-body interactions, arising from the multitude of photogenerated energetic states, which may be harnessed to modulate the nature and lifetime of charge separated states and; (ii) give rise to a collection of heretofore ill-defined photoexcited-states with low energy optical transitions. At a molecular level, this work demonstrates how molecular structures can be engineered to: (i) utilize quantum coherence in a donor-acceptor “ratchet” which exhibit excitation frequency dependent uphill energy transfer, via vibronic mixing, to undergo electronically irreversible charge transfer and; (ii) selectively photooxidize amino acid analogues in biologically reminiscent photoreactions. These findings presented herein may be used to guide optoelectronic designs which efficiently guide and harness the charged and energetic species which arise from photoexcitation.

Quantum dots (QDs) are promising building block materials for many emerging energy-harvesting applications. We theoretically investigated the influences of the QD-QD (CdTe-CdSe) charge transfer rates and mechanisms on QD solar cells power conversion efficiencies using multi-level modeling methods including the first principle quantum chemistry calculations of QD electronic and charge transfer properties and the kinetic modeling of solar cell performances.

We developed tight-binding electronic structure models to explore the QD electronic properties, and the charge transfer kinetics including their dependences on QD sizes and QD surface-to-surface distance. We found that the QD-QD charge transfer rates follow the non-adiabatic rate expression by Marcus. The QD-QD electronic coupling strength decays exponentially as the QD surface-to-surface distance increases. The QD-QD charge transfer rates generally increase (decay) as the acceptor (donor) QD radius increases. We found that the TS coupling mechanism can dominate the QD-QD coupling over the TB coupling. The difference between the TS and TB coupling size dependences results in a dominance switch between the TS and TB charge transfer mechanisms in the QD dyad as the QD sizes grow.

We further explored the use of an external charge to modulate the QD-QD coupling strength and the coupling mechanism. We found that a positively charged group in the bridge strengthens the D-A coupling for all QD sizes. A negatively charged group in the bridge causes the D-A coupling reduction in large QDs. For small QDs, the D-A coupling variation induced by the negative charge depends on the QD sizes. Compared to the neutral bridge, we found that through-solvent and through-bridge mechanisms switch their dominance at smaller (larger) QD sizes for the positively (negatively) charged group in the molecular bridge.

Using the computed charge transfer rates, we explored the power conversion efficiencies of QD solar cells based on QD dyads and QD triads. We found that the external and internal power conversion quantum efficiencies are significantly enhanced by introducing a third QD between the donor and acceptor QDs. The improvements in the efficiencies can be further enhanced by tuning the band-edge energy offset of the middle-position QD from its neighbors.

Electron transfer is an essential process for life to exist and is the working principle of electronics. Fundamental knowledge of electron transfer is crucial for understanding biological processes and developing future devices. Here, we present our theoretical and experimental efforts to design, synthesize and characterize the electronic properties of charge-transfer complexes and approaches to control their charge flow.

Computational methods, including wave function-based methods, density functional theory, quantum mechanics/molecular mechanics, and molecular dynamics, were combined with synthesis, ultrafast spectroscopy and conductance measurements to design and characterize engineered chemical systems for efficient charge and energy transfer.

Compelling foundational questions explored in this dissertation include: 1) Can we control the photoinduced electron transfer rate of molecules with chemically innocent vibrational excitations? 2) Can we create, sustain, and exploit chemical coherences to harvest and transmit energy and information? 3) What chemical modifications lead to enhancing intrinsic charge transport properties of molecular devices?

First, we discuss the photophysics of highly conjugated organic and organometallic systems consisting of electron donor and acceptor units. Electronic structure calculations combined with ultrafast spectroscopy elucidated the excited-state dynamics of molecular candidates for controlling charge transfer with infrared pulses and directing energy transfer through nonthermal routes. Second, we introduce strategies to fabricate efficient DNA-based molecular wires and obtain abundant semiconducting carbon nanotubes. Molecular dynamics simulations and kinetic modeling revealed the chemical interactions relevant for engineering charge transport in nanostructures. The synergy between our theoretical calculations and experimental measurements provides guidelines to tailor the electronic properties of chemical systems and control charge flow in optically active charge-transfer complexes and nanostructures.