Structure-Dependent Triplet State Formation in Fluorinated Perylenediimide Single Crystal Polymorphs

Abstract

Perylenediimide (PDI) and its derivatives have garnered significant attention as promising n-type semiconductors to replace fullerenes as acceptors in organic photovoltaic cells (OPVs). However, the aggregation tendencies and rapid excimer formation of PDIs have hindered their efficiency in OPVs. To address this issue and enhance solubility while fine-tuning their electronic properties, researchers have explored fluorination strategies. In this study, we investigate the structure–function relationship of two crystalline N,N-bis(2′,2′,3′,3′,4′,4′,5′,5′,6′,6′,7′,7′,8′,8′,8′-penta­decafluoro­octan-1-yl)perylene(3,4:9,10)bis(dicarboximide) (FPDI) polymorphs, namely, FPDI-α and FPDI-β. Their crystal structures show that the π systems of the FPDI molecules in FPDI-α adopt an orthogonal geometry, while those in FPDI-β are parallel and slip-stacked. We report their excited state dynamics using transient absorption and fluorescence microscopy and time-resolved electron paramagnetic resonance spectroscopy. Upon photoexcitation, both crystal polymorphs exhibit the formation of a lowest excited singlet (S1) state with substantial charge transfer (CT) character. This S1-CT state in FPDI-α decays in τ = 350 ps to form the FPDI triplet state via spin–orbit charge-transfer intersystem crossing in near-quantitative yield, while the S1-CT state in FPDI-β decays back to the ground state in τ = 140 ps. This study illustrates the influence of different packing motifs on photoinduced intermolecular CT processes in single crystals of the same molecule and contributes to an understanding of the role of fluorination in optimizing PDI-based molecular materials for CT state formation in the solid state.

Structure-enabled long-lived charge separation in single crystals of an asymmetric donor–acceptor perylenediimide cyclophane

“…Single cocrystals featuring organic donor–acceptor (D–A) pairs have emerged as valuable tools for probing the intricacies of charge transfer (CT) exciton dynamics within solid materials. 1–4 Their inherent anisotropic properties yield insights into how the generation and movement of CT excitons are influenced by specific molecular arrangements, 5–11 crucial for optimizing solar energy conversion. Key factors for effective solar energy conversion in these single D–A crystals include prompt CT exciton formation, 12 efficient generation of charge carriers from these excitons, and high mobility of these carriers for facile migration towards electrodes or interfaces.…”