Design rules and application spaces for closed-shell conjugated polymers have been investigated to a significant extent in the field of organic electronics, and this has allowed for significant breakthroughs to occur in myriad device platforms [e.g., organic field-effect transistors (OFETs) and organic light-emitting devices (OLEDs)] such that significant laboratory and commercial efforts have come to fruition. Conversely, organic electronic materials that are based on the emerging design motif that includes open-shell stable radicals have not been evaluated in such detail, despite the promise these materials show for charge transfer, light-emission, and spin manipulation platforms. Moreover, recent results have demonstrated that 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 composites 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 demonstrate that Förster resonance energy transfer (FRET) is the dominant mechanism by which energy transfer occurs from a common conjugated polymer to various radical species using a combination of experimental and computational approaches. Specifically, we determined this by monitoring the fluorescence quenching of poly(3-hexylthiophene) (P3HT) in the presence of three radical species: (1) the galvinoxyl radical; (2) the 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO) radical; and (3) the 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical. Both in solution and in the solid-state, the galvinoxyl and PTIO radicals showed fluorescence quenching that was on par with that of a common fullerene electron-accepting derivative, phenyl-C61-butyric acid methyl ester (PCBM). This was due to the considerable overlap of their absorbance spectrum with the fluorescence spectrum of the P3HT species, which indicated that isoenergetic electronic transitions existed for both species. Conversely, TEMPO showed minimal quenching at similar concentrations in solution and at similar loadings in the solid state due to the lack of such an overlap. To support the determination of the mechanism, and to rule out photoinduced charge transfer, a potential competing process, pump-probe transient absorption measurements were employed. An increased rate of exciton decay in P3HT was observed when blended with the galvinoxyl radical and PTIO radical, consistent with an excited state transfer mechanism. Moreover, signals corresponding to the anion species of the quenchers were not observed, suggesting that charge transfer was not occurring at an appreciable rate for the compositions evaluated here. Additionally, computational studies suggested that FRET would occur at a significantly faster rate than photoinduced charge transfer. These computationally-predicted FRET rates were calculated from the spectral overlap of the P3HT fluorescence and the radical quencher absorbance spectra, while charge transfer rates were calculated by extracting Marcus theory parameters for the composite P3HT-quencher systems from density functional theory (DFT) calculations. Therefore, these computational results support the steady-state and time-resolved fluorescence experiments, and the results highlight the precise interactions between open-shell small molecules and closed-shell conjugated polymers in optoelectronic applications. Additionally, these findings 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.