Direct and Energy-Transfer-Mediated Charge-Transfer State Formation and Recombination in Triangulene-Spacer-Perylenediimide Multichromophores: Lessons for Photovoltaic Applications

Abstract

We study dynamics of primary photoexcitations in three symmetric donor-spacer-acceptor-spacer-donor (D-S-A-S-D) multichromophores with increasing oligophenylene spacer length, following selective donor or acceptor excitation. Energy levels of the donor and acceptor moieties are tailored to facilitate splitting of the excited state into a lower-lying charge transfer (CT) state, mimicking the functionality of a donor-acceptor interface for charge generation, and thus resulting in long-lived charge separation. Ultrafast electronic energy transfer (EET) from the donor followed by fast hole (back)transfer from the acceptor populates the molecules' CT states. However, the charge transfer efficiency is found to be close to unity independent of the donor or acceptor photoexcitation. The ratio of charge transfer and recombination rates, which reflects the population of CT states, increases with the oligophenylene spacer length for both, direct hole transfer and hole-transfer following EET, boosting the population of CT states under continuous excitation. We observe the population of high-lying 'dark' excited states following energy transfer from the donor to the acceptor. The 'dark' states successively undergo charge transfer and form CT states of higher energy, with decreased recombination rates, while maintaining the high charge generation efficiency. Changes in charge transfer reaction rates are rationalized within the Marcus theory, with driving forces and reorganization energies evaluated by density functional theory and polarizable continuum models. The present study demonstrates the importance of energetically higher-lying states, which cannot be directly photoexcited, yet are accessible through energy transfer from local excited states. Similar processes are anticipated in other donor-acceptor systems, which allow for both energy and charge transfer processes, such as bulk heterojunctions of polymer and small molecule donor / nonfullerene acceptor typically used in photovoltaic systems.

Long-range exciton diffusion in molecular non-fullerene acceptors

“…Another interesting observation is that a large coupling between the donor and acceptor blocks of the acceptor–donor–acceptor-conjugated core leads to a quadrupolar-type excitation, with both acceptor blocks having an excess electron upon excitation 59 – 61 . The immediate implication is that the excited state does not have a dipole, but a quadrupole moment.…”

Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-Breaking Charge Separation

“…From the Marcus eq , it is evident that the reorganization energy (λ) plays a crucial role in charge separation kinetics. Reorganization energy is composed of the solvent shell reorganization energy (λ s ) and the solute molecular structure reorganization energy upon charge separation (λ i (CS) ) and charge recombination λ i (CR) . − Internal reorganization energy (λ i ) was calculated by employing constrained DFT for the charge-separated state and DFT for the ground-state calculation The first term is the energy calculated at the optimized ground state (GS) geometry in the CS redox state, while the second term is the energies of the CS at their equilibrium geometries.…”

Investigation of the Interfacial Electron Transfer Kinetics in Ferrocene-Terminated Oligophenyleneimine Self-Assembled Monolayers

“…Besides the rich science in this area, the studies are motivated by several important technological applications such as sensors, − solar cells, − light-emitting diodes, , electrocatalysis, , and molecular electronics. − Redox moieties that are immobilized on the surface of metals as a part of a well-defined organized structure ( i.e. , a self-assembled monolayer, or SAM) are good candidates for investigating the interfacial ET reactions. − In such structures, the monolayer mediates the ET between the metallic electrode and the redox moiety in a metal-bridge-redox couple structure which is equivalent to donor-spacer-acceptor configuration extensively studied in homogenous systems − and biological structures. − Moreover, connecting the redox couple to the surface via a molecular tether would allow putting the couple at a precisely controlled (and changing) distance from the surface while changing the chemical structure of the bridge and would eliminate complications produced as a result of the convection, diffusion, and the adsorption of the redox couple. , …”