Maximizing Amplified Energy Transfer: Tuning Particle Size and Dye Loading in Conjugated Polymer Nanoparticles

We investigate amplified energy transfer in conjugated polymer nanoparticles (CPNs or Pdots) by studying both fluorescence quenching of CPN donors and the sensitization of reactive dye acceptors. By delivering excitation energy to dye dopants via a combination of Forster energy transfer and exciton diffusion, CPNs act as powerful light-harvesting antennae. This phenomenon-amplified energy transfer-is used to sensitize dye dopants, producing a higher concentration of the dye's excited state than would be observed upon direct excitation. Here, we study CPN sensitization of a low-efficiency photochemical reaction to determine the CPN size and dye loading that yield optimized outputs in the form of energy transfer efficiency, the antenna effect (AE), and reaction duration. Our model system is the cyclo-reversion reaction of a diarylethene (DAE) photochrome as the dye dopant and CPNs of the conjugated polymer poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-1,4-benzo-{2,1'-3}-thiadiazole] as the sensitizer. In their visible-absorbing form, DAE dyes are localized on the particle surface and are effective fluorescence quenchers of 15, 20, and 26 nm diameter CPNs. Quenching is most efficient for the smallest particles, and high dye loadings are necessary to offset reduced efficiency as CPN size increases. Our photokinetic studies of DAE acceptors demonstrate the crucial importance of dye loading: both energy transfer efficiency and the AE show abrupt declines when the dye concentration is increased beyond a critical threshold. We find that CPNs with a 15 nm diameter exhibit the most efficient energy transfer (99-100%) and the largest AE (32) of the CPNs studied. For CPNs of all sizes and dye loadings, a photoselection phenomenon reveals that the energy-transferaccepting ability of the DAE dyes varies tremendously within the dye ensemble. These findings are used to develop design recommendations for CPN sensitizers.