(402d) Tuning Charge and Energy Transfer Interactions in Open-Shell Molecules

Organic electronic materials and devices are present in many emerging applications [e.g. organic light-emitting devices (OLEDs)] and offer the promise of low-cost, high-throughput manufacturability while also providing for mechanically-robust flexible and transparent devices, applications that are poorly addressed by their inorganic counterparts. To date, the primary design paradigm implemented has been one where extensive π-conjugation is included within the macromolecular design motif. Conversely, stable organic radicals represent a new class of materials lacking π-conjugation that show significant promise for rapid charge transfer, efficient light emission, and spintronic applications. Recent results have suggested that polymers containing these radical species can show significant intrinsic electrical conductivity. Moreover, the materials performance of hybrid conjugated closed-shell and open-shell systems will allow for future applications to harness both of these platform design archetypes in order to generate composite closed-shell–open-shell materials that combine the performance of current state-of-the-art conjugated polymer systems with the novel functions provided by open-shell species. Thus, establishing the underlying physical phenomena associated with the interactions between both classes of materials is imperative for the effective utilization of these soft materials.

Here, we will describe two specific efforts that speak to these points. First, we detail the intermolecular interactions between a model conjugated polymer, poly(3-hexylthiophene) (P3HT), and three radical species: the (1) galvinoxyl; (2) 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO); and (3) 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals. We demonstrate that Förster resonance energy transfer (FRET) is the dominant mechanism by which energy transfer occurs between the species. Specifically, by monitoring the fluorescence of P3HT in the presence of the radical species, we observe fluorescence quenching with the galvinoxyl and PTIO radicals to a similar degree as for a commonly used fullerene electron acceptor, phenyl-C₆₁-butyric acid methyl ester (PCBM), while the TEMPO radical shows minimal quenching. This trend can be deduced from visible light spectroscopy data of the individual species. To quantify the effects of potential competing mechanisms of fluorescence quenching, the photoexcitation dynamics in the system are probed through transient absorption (TA) spectroscopy. For the TA response of a P3HT-radical blend, we observe only the signatures of the FRET process. These results are further corroborated through computational studies showing FRET proceeds the fastest out of all of the potential mechanisms. These results have implications for the molecular design of composite systems involving energy transfer, such as in organic photovoltaics, and suggest that long-range energy transfer can be accomplished in applications when radicals that can act as FRET acceptors are utilized, forming a new design paradigm for future optoelectronic applications. For our second effort, we define the intermolecular interactions occurring within a high-conductivity radical polymer. Previously, we have established that the open-shell polymer poly(4-glycidyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl) (PTEO) demonstrates a conductivity of 0.2 S cm^-1 upon thermal annealing, comparable with many chemically doped conjugated polymers. This high conductivity is due to the formation of radical site clusters with a high degree of electronic coupling. In order to design next-generation radical polymers with even higher conductivities or with a wider range of electrical and mechanical properties, it is important to understand the interactions that lead to the network formation so that these polymers can be designed to facilitate these interactions. To that end, we have established the effects of selective deactivation of radical sites in PTEO through electron paramagnetic resonance (EPR) spectroscopy. As the density of radicals increases, spin exchange interactions occur, which manifest as a broadening of spectral features within the EPR spectra. By computationally modeling these interactions and their EPR signatures, we observe how the density of radical species affects the formation of the radical cluster structure in order to inform the design of future radical polymers. Ultimately, it is our goal to better understand the fundamental optoelectronic behavior of this novel class of materials such that their full potential can be realized in cutting-edge device applications.