Controlling charge separation and recombination by chemical design in donor–acceptor dyads

Abstract

Conjugated donor-acceptor block co-oligomers that self-organize into D-A mesomorphic arrays have raised increasing interest due to their potential applications in organic solar cells. We report here a combined experimental and computational study of charge transfer (CT) state formation and recombination in isolated donor-spacer-acceptor oligomers based on bisthiophene-fluorene (D) and perylene diimide (A), which have recently shown to self-organize to give a mesomorphic lamellar structure at room temperature. Using femtosecond transient absorption spectroscopy and Time-Dependent Density Functional Theory in combination with the Marcus-Jortner formalism, the observed increase of the CT lifetimes is rationalized in terms of a reduced electronic coupling between D and A brought about by the chemical design of the donor moiety. A marked dependence of the CT lifetime on solvent polarity is observed, underscoring the importance of electrostatic effects and those of the environment at large. The present investigation therefore calls for a more comprehensive design approach including the effects of molecular packing.

Quantum dynamical studies of ultrafast charge separation in nanostructured organic polymer materials: Effects of vibronic interactions and molecular packing

“…The second system consists of covalently bound OT‐PDI type block co‐oligomer assemblies in a liquid‐crystalline phase, which we analyzed in Refs. In both cases, the relevant materials’ nanomorphology is shown to have a significant impact on the charge separation time scale and efficiency.…”

“…As a starting point, we examined the electronic structure properties of dyad (DA) and triad (DAD) species, using TDDFT calculations with selected benchmarks at the Second‐Order Approximate Coupled Cluster (CC2) and ADC(2) level. Long‐range corrected functionals, that is, the CAM‐B3LYP functional and the ωB97XD functional were used, besides hybrid functionals with a tailored amount of HF exchange .…”

“…We specifically address recently developed oligothiophene‐perylene diimide (OT‐PDI) based DA dyad and DAD triad combinations that are organized in smectic liquid crystalline (LC) films or else lamellar mesophases . Spectroscopic investigations of two generations of these DA(D) systems have shown widely different types of kinetics, ranging from ultrafast (subpicosecond) generation of charges that are subject to rapid recombination to much slower (tens of picoseconds) charge generation yielding long‐lived CT states . These two generations of materials with the same basic constituents differ both in the details of chemical design and in their nanoscale architecture.…”

“…These two generations of materials with the same basic constituents differ both in the details of chemical design and in their nanoscale architecture. Hence, several factors contribute to the observed kinetics …”

Molecular Donor–Acceptor Dyads for Efficient Single‐Material Organic Solar Cells

“…[ 1,2 ] The nature and length of the molecular spacer bridging D and A, flexible nonconjugated or rod‐like conjugated, polar or nonpolar, play an important role for the various fundamental processes such as excitation and energy transfer, CT, charge separation, and recombination. [ 3 ] CT is also dependent on the environment, [ 4 ] which severely comes into play for the photoinduced elementary processes of molecular D–A systems in the solid state, a scenario, encountered in organic electronic devices. In this respect, in thin films, aggregation of the molecules into supramolecular assemblies to form photoactive nanostructures with well‐segregated A and D domains and their orientation relative to the substrate play a major role for efficient organic solar cells.…”