Hopping versus Tunneling Mechanism for Long-Range Electron Transfer in Porphyrin Oligomer Bridged Donor–Acceptor Systems

Abstract

Achieving long-range charge transport in molecular systems is interesting to foresee applications of molecules in practical devices. However, designing molecular systems with pre-defined wire-like properties remains difficult due to the lack of understanding of the mechanism for charge transfer. Here we investigate a series of porphyrin oligomer-bridged donor-acceptor systems Fc-Pn-C60 (n = 1-4, 6). In these triads, excitation of the porphyrin-based bridge generates the fully charge-separated state, Fc(•+)-Pn-C60(•-), through a sequence of electron transfer steps. Temperature dependence of both charge separation (Fc-Pn*-C60 → Fc-Pn(•+)-C60(•-)) and recombination (Fc(•+)-Pn-C60(•-) → Fc-Pn-C60) processes was probed by time-resolved fluorescence and femtosecond transient absorption. In the long triads, two mechanisms contribute to recombination of Fc(•+)-Pn-C60(•-) to the ground state. At high temperatures (≥280 K), recombination via tunneling dominates for the entire series. At low temperatures (<280 K), unusual crossover from tunneling to hopping occurs in long triads. This crossover is rationalized by the increased lifetimes of Fc(•+)-Pn-C60(•-), hence the higher probability of reforming Fc-Pn(•+)-C60(•-) during recombination. We demonstrate that at 300 K, the weak distance dependence for charge transfer (β = 0.028 Å(-1)) relies on tunneling rather than hopping.

Linkage Effect in the Heterogenization of Cobalt Complexes by Doped Graphene for Electrocatalytic CO2 Reduction

“…Themolecular length, rigidities,and p-conjugation degree of the linkages determine the distance and pathways for electron transfer. [15] Poorly chosen linkages might obstruct such electron communication, which makes the metal sites electro inactive so that they cannot serve as the active sites. [11a] Based on the aforementioned considerations,w ed eveloped ah eterogeneous CO 2 RR electrocatalyst that is operative in aqueous solution by immobilizing planar Co II -2,3naphthalocyanine complexes (NapCo) on doped graphene based matrices and investigated the linkage effects.Graphene is as uitable substrate because it has high conductivity, stability under reductive environments and good reactivity to allow versatile linkages to connect with molecules.…”

Axial Modification of Cobalt Complexes on Heterogeneous Surface with Enhanced Electron Transfer for Carbon Dioxide Reduction

“…The electron transfer pathways in PCo‐G and SCo‐G could adopt either the tunneling or hopping mechanism. Because of the non‐covalent interaction of Co complexes with graphene and the lack of a molecular bridge as the medium to migrate the electron from the donor to acceptor, the possibility of hopping mechanism can be excluded. The efficiency of electron transfer then was determined by the distance and electron conjugation degree between the Co complexes and graphene.…”

Long-Range Electron Transfer through Ultrathin Polyelectrolyte Complex Films: A Hopping Model

“…As features in electronic circuits, especially those based on molecules, shrink to less than 10 nm, the transport of electrons through nominally insulating materials has become of prime interest. Experimentally, the distance dependence of electron transport has been probed in 2D structures using an insulating film, commonly a self-assembled monolayer, on a metal surface and either a redox species attached or in solution − or another metal contact to complete the circuit. − In one dimension, molecular spacers separate donor and acceptor molecules or nanoscale electrodes. − The transport of electrons through nonconductors is historically broken down into coherent tunneling over “short” range (less than about 4 nm) and incoherent hopping over longer ranges. The primary feature used to distinguish between the two is the distance dependence of the electron transfer rate, k , which exhibits an exponential falloff with distance d, characterized by decay length β, k = k o e –βd for tunneling and a more linear inverse dependence on distance for hopping .…”

“…Experimentally, the distance dependence of electron transport has been probed in 2D structures using an insulating film, commonly a self-assembled monolayer, on a metal surface and either a redox species attached or in solution − or another metal contact to complete the circuit. − In one dimension, molecular spacers separate donor and acceptor molecules or nanoscale electrodes. − The transport of electrons through nonconductors is historically broken down into coherent tunneling over “short” range (less than about 4 nm) and incoherent hopping over longer ranges. The primary feature used to distinguish between the two is the distance dependence of the electron transfer rate, k , which exhibits an exponential falloff with distance d, characterized by decay length β, k = k o e –βd for tunneling and a more linear inverse dependence on distance for hopping . Well-packed chains of saturated hydrocarbon yield β values of around 10 nm –1 , whereas conjugated molecular “wires” have a much broader range of β between 0.1 and 4 nm –1 . , Long-range electron transport through DNA is believed to be facilitated by stacking of base pairs …”