Single molecule charge transport: from a quantum mechanical to a classical description

Abstract

This paper explores charge transport at the single molecule level. The conductive properties of both small organic molecules and conjugated polymers (molecular wires) are considered. In particular, the reasons for the transition from fully coherent to incoherent charge transport and the approaches that can be taken to describe this transition are addressed in some detail. The effects of molecular orbital symmetry, quantum interference, static disorder and molecular vibrations on charge transport are discussed. All of these effects must be taken into account (and may be used in a functional way) in the design of molecular electronic devices. An overview of the theoretical models employed when studying charge transport in small organic molecules and molecular wires is presented.

Electron transfer in DNA

“…The interaction of diabatic states, donor–acceptor electronic coupling V da , is a key parameter determining the absolute rate of ET: where F is the nuclear Frank–Condon factor. Certain simplifications allow one to replace quantum description of ET by its classical treatment 18. In the classical limit, and the ET rate can be estimated using three parameters, electronic coupling V da , driving force Δ G , and reorganization energy λ 16–17.…”

“…(2) one should first compute the microscopic parameters of the system. The site energies and coupling matrix elements are also fundamental quantities employed in computational schemes based on time‐dependent Hamiltonian18,26,42 as well as in calculations of electrical conductivity 8. Thus, estimation of ET parameters is of general interest.…”

Charge transport in a liquid crystalline triphenylene polymer monolayer at air–solid interface

“…Charge transport studies in polymers are interesting due to their fundamental 8 and practical importance 9 . In some polymers charge transport has been attributed to quantum mechanical while in some others it has been accounted as classical incoherent hopping 8 . The charge transport in organic polymers are affected by structural, electronic and chemical nature of the polymers 10,11 .…”

Constructive quantum interference in a bis-copper six-porphyrin nanoring

“…Quantum interference also provides many opportunities for enhancing performance in single-molecule electronic devices, by creating sharp transport resonances45. When the distance of the transmission exceeds a certain threshold, interactions with vibrational degrees of freedom result in a change of mechanism from phase-coherent single-step tunnelling to incoherent multistep hopping, erasing interference effects67. It is difficult to measure coherence lengths in molecular structures and the size limit for quantum mechanical behaviour is not well established.…”