Dipole-induced effects on charge transfer and charge transport. Why do molecular electrets matter?

Abstract

Charge transfer (CT) and charge transport (CTr) are at the core of life-sustaining biological processes and of processes that govern the performance of electronic and energy-conversion devices. Electric fields are invaluable for guiding charge movement. Therefore, as electrostatic analogues of magnets, electrets have unexplored potential for generating local electric fields for accelerating desired CT processes and suppressing undesired ones. The notion about dipole-generated local fields affecting CT has evolved since the middle of the 20th century. In the 1990s, the first reports demonstrating the dipole effects on the kinetics of long-range electron transfer appeared. Concurrently, the development of molecular-level designs of electric junctions has led the exploration of dipole effects on CTr. Biomimetic molecular electrets such as polypeptide helices are often the dipole sources in CT systems. Conversely, surface-charge electrets and self-assembled monolayers of small polar conjugates are the preferred sources for modifying interfacial electric fields for controlling CTr. The multifaceted complexity of such effects on CT and CTr testifies for the challenges and the wealth of this field that still remains largely unexplored. This review outlines the basic concepts about dipole effects on CT and CTr, discusses their evolution, and provides accounts for their future developments and impacts.

Is it common for charge recombination to be faster than charge separation?

“…Conversely, the electron acceptor, DPP, is the photosensitizer in Aaa(DPP) that we selectively excite within the visible spectral region. This locally excited state, Aaa( 1 DPP*), induces ET from the highest occupied molecular orbital (HOMO) of the donor to the singly occupied “HOMO” of 1 DPP*, that is, transfer of a hole from the photoexcited acceptor to the donor (Scheme B) . For simplicity, the MO diagrams showing the fundamental differences between photoinduced ET and HT do not depict the CT dynamics involving triplet pathways.…”

“…As one of the most fundamental processes in natural and manmade systems, CT ensures energy flow and conversion, as well as the operation of electronic and photonic materials and devices . Photoinduced CT, a principle process in photosynthesis, has sustained life on Earth for billions of years .…”

“…Frequently, CR is the most likely outcome for Coulombically trapped CT states at donor‐acceptor interfaces, especially in nonpolar organic media . Taking advantage of electric forces, which can involve the movement of protons along with electrons, that is, proton‐coupled electron transfer, hole transfer (HT) to polarize interfaces for aiding ET, or permanent electric dipoles, that is, from electrets, to guide the different CT steps, can improve the chance of obtaining long‐lived CT states by altering the driving forces of CS and CR …”

“…The above arguments for attaining long‐lived CT states focus on the driving forces of CS and CR, as well as on the reorganization energy, that is, on the Franck‐Condon (FC), or nuclear, component of the CT kinetics . Donor‐acceptor electronic coupling provides another major contribution to the CT kinetics.…”

“…Donor‐acceptor electronic coupling provides another major contribution to the CT kinetics. Its evaluation, however, is not as easy as that of Δ G (0) and λ , which are commonly used for estimating the FC term for nonadiabatic CT . Furthermore, rational designs for achieving strong electronic coupling for CS, while weakening it for the undesired CR steps, appears prohibitively challenging especially when the forward and back CT involve the same pathways.…”