Long-Range Interactions Boost Singlet Exciton Diffusion in Nanofibers of π-Extended Polymer Chains

Abstract

Raising the distance covered by singlet excitons during their lifetimes to values maximizing light absorption (a few hundred nm) would solve the exciton diffusion bottleneck issue and lift the constraint for fine (∼10 nm) phase segregation in bulk heterojunction organic solar cells. In that context, the recent report of highly ordered conjugated polymer nanofibers featuring singlet exciton diffusion length, L D , in excess of 300 nm is both appealing and intriguing [Science2018897900]. Here, on the basis of nonadiabatic molecular dynamics simulations, we demonstrate that singlet exciton diffusion in poly­(3-hexylthiophene) (P3HT) fibers is highly sensitive to the interplay between delocalization along the polymer chains and long-range interactions along the stacks. Remarkably, the diffusion coefficient is predicted to rocket by 3 orders of magnitude when going beyond nearest-neighbor intermolecular interactions in fibers of extended (30-mer) polymer chains and to be resilient to interchain energetic and positional disorders.

A New Frontier in Exciton Transport: Transient Delocalization

“…In them, the nanofibers were coarse-grained as a one-dimensional series of sites, with the excitons modeled by solving the time-dependent Schrödinger equation and the vibrational modes treated classically, while accounting explicitly for the exciton–phonon interactions (namely, excitonic back forces on the nuclei that drive the formation of exciton-polarons were included). These lattice fluctuations drive the system close to the crossing seam between different adiabatic potential energy surfaces, which allows for the stochastic nonadiabatic hopping of excitons between these surfaces (for full details, see refs ( 34 ), ( 42 ), and ( 43 )). The site-to-site couplings that facilitate transport are the Coulombic interactions (computed using a multicentric expansion of the transition dipoles, i.e., using interacting transition charges).…”

“…The role of long-range couplings was studied in more detail in a follow-up study by Prodhan et al, who repeated the simulations on the same nanofiber system under a broader variety of model conditions (see Figure 3 a). 42 The most dramatic effects were seen when varying the long-range couplings either by artificially controlling the allowed interaction range between chains or by considering much shorter “6-mer” chains which contain 6 polythiophene units. These shorter chains have stronger couplings at close range but much weaker couplings at long-range (see Figure 2 c).…”

“…(c) Dependence of D on range of excitonic couplings included in simulations for 6-mer and 30-mer, where D is critically dependent on both the interaction range of the couplings and the length of the unimer. Adapted with from ref ( 42 ). Copyright 2021 American Chemical Society.…”

“…This was explained by the long-range couplings increasing the spatial extent of the higher-lying states and through the rationale that they facilitated access to these states. 42 A design rule may therefore be to optimize long-range couplings in OSCs via strategies such as increasing structural order or increasing the “size” of the exciton and its dipole to distribute exciton couplings to molecules further away. This latter factor is particularly interesting as it motivates a new synthetic space where the design and arrangement of chromophores can be tuned to optimize for long-range interactions, or more broadly, unconventional patterns of exciton couplings within OSCs that may potentially promote transport.…”

Machine Learning Approach to Calculate Electronic Couplings between Quasi-diabatic Molecular Orbitals: The Case of DNA

“…In such a diabatic representation, the Hamiltonian elements are expected to vary smoothly no matter how the structure rearranges . This property is beneficial, especially for nonadiabatic dynamics simulations of charge and exciton dynamics processes. , By definition, the most straightforward way is to minimize the derivative couplings through unitary transformation of the electronic states . However, this is generally time-consuming due to the large amount of nonadiabatic coupling vectors to be calculated, and it has been proved that strict diabatization can only be obtained for electronic states of the same symmetry in diatomic molecules .…”