Exciton diffusion, antenna effect, and quenching defects in superficially dye-doped conjugated polymer nanoparticles

Abstract

Energy transfer (ET) in conjugated polymer nanoparticles (CPNs) is a critical process that affects the performance of these materials in diverse applications such as biological−chemical− physical sensing, imaging, photovoltaics, and phototherapy. Herein, we performed an in-depth study of ET in the CPNs of poly(9,9dioctylfluorene-altbenzothiadiazole) (F8BT) superficially doped with well-controlled amounts of rhodamine B (RhB) dye using a combined experimental and theoretical approach. In these particles, the conjugated polymer acts as the excitation energy donor, whereas adsorbed dye molecules and non-emissive quenching defect sites (Q) act as energy acceptors. Fitting of simulated polymer emission to experimental data provided the intrinsic exciton diffusion length (Ld = 8.6 nm) of the polymer and the mean number of quenching defects per particle (ρQ = 1.56 × 10−2 defects/nm2). Importantly, results provide for the first time sound evidence indicating that quenching defect centers are superficially, rather than volumetrically, distributed in these CPNs. The so-called antenna effect (AE), a parameter frequently used to characterize the ET process in donor−acceptor multichromophoric systems, was also calculated. The AE in these CPNs takes a maximum value of ∼40, which compares well with the values reported for similar systems. The developed model (and associated computational Python code) represents a significant improvement over previous models/codes by correcting inconsistencies and successfully simulating ET processes in dye-doped CPNs taking into account: exciton diffusion, ET to non-emissive quenching defect centers, ET to dye dopants, spatial distribution of dyes and defects within CPNs, and individual particle excitation probability among other parameters. The model calculates the ensemble-averaged, time-resolved, and time-averaged emission intensity of CPNs and dye dopants for specific CPN size distributions, allowing for direct comparison with experimental bulk measurements. We envisage that the presented improvements in both the theoretical model and the experimental strategy will prove useful in the study and design of new dye-doped CPNs for practical applications.