Phenylated Acene Derivatives as Candidates for Intermolecular Singlet Fission

Abstract

Singlet fission (SF), a spin-conserving process where one singlet exciton is converted into two triplet excitons, may improve the efficiency of organic photovoltaics. Only a few materials have been experimentally observed to undergo intermolecular SF, most of which are acenes and their derivatives. Using many-body perturbation theory in the GW approximation and the Bethe−Salpeter equation, we systematically investigate the electronic and excitonic properties of tetracene, pentacene, and their phenylated derivatives in the gas phase and solid state. Their potential for SF is evaluated with respect to the thermodynamic driving force and the singlet exciton charge-transfer character. In both the gas phase and solid state, pentacene and its derivatives are more promising than tetracene analogues. Within a family of molecules containing the same acene backbone, increasing the number of phenyl side groups is detrimental for the SF driving force in the gas phase. However, in the solid state, the SF driving force and the exciton character are modulated by intermolecular interactions present within different packing arrangements. Molecules with a higher number of phenyl side groups often form crystals with less cofacial interactions between the acene backbones. These crystals are found to exhibit a higher SF driving force and a higher degree of singlet exciton charge-transfer character.

3D Electron Diffraction Structure Determination of Terrylene, a Promising Candidate for Intermolecular Singlet Fission

“…The SHB packing motif is characterized by strong intermolecular electronic coupling within a dimer, but weak coupling between dimers. This typically results in smaller band dispersion than in HB crystals, in which there is strong electronic coupling between π‐stacked molecules, as previously shown for the two polymorphs of perylene [60] . The band structure of terrylene, shown in the SI, exhibits modest band dispersion, similar to quaterrylene and SHB perylene [60] …”

“…The exciton wave functions were converged using a supercell of 4×10×4 based on the criterion proposed by Liu et al [82] . The degree of singlet exciton charge transfer character (%CT) was calculated by double‐Bader analysis (DBA) [60,82] . The results for rubrene and pentacene are from Wang et al., [83] the results for quaterrylene, perylene, and tetracene are from Wang et al., [60] and the results for anthracene are from Liu et al [82] …”

“…Utilising the so‐determined structure, we use many‐body perturbation theory calculations within the GW approximation and the Bethe‐Salpeter equation (BSE) [56–59] to assess the potential of crystalline terrylene to undergo intermolecular SF in the solid‐state. Previously, crystalline quaterrylene has been identified as a promising SF candidate based on this method, [60] while terrylene could not be assessed in the absence of a crystal structure.…”

Structure Dependence of Kinetic and Thermodynamic Parameters in Singlet Fission Processes

“…Structural considerations and dilution of acene molecules on the inter-triplet interactions have been considered, but the measurements could not distinguish between herringbone or slip-stacked . 22,31 Several theoretical studies compare molecular systems with stacked or herringbone motifs in their crystal packing, addressing the effect of the relative orientation between neighbouring chromophores on the spin and thermodynamics of SF and subsequent exciton migration, [32][33][34][35] but to our knowledge no such study has been reported experimentally in solid-state crystalline systems.…”

“…Calculations have predicted that the optimal geometry for SF is that which results in a substantial orbital overlap for both HOMO and LUMO, however this has been attributed to both herringbone and slip-stacked configurations. 12,30,[32][33][34][35] Therefore, a combined experimental and theoretical methodology that allows to isolate and distinguish the kinetics of herringbone and slip-stacked dimers in the SF process would be a valuable addition to the field.…”