Controlling Photoswitching via pcFRET in Conjugated Polymer Nanoparticles

Nanoparticles prepared from conjugated polymers (CPNs or Pdots) are powerful fluorophores for sensing and imaging applications, including those that require fluorescence photoswitching. We and others have developed CPNs doped with photochromic dyes that act as acceptors in only one of their two forms via fluorescence resonance energy transfer (pcFRET). We recently developed visible-light-responsive CPNs doped with a reverse photochromic spirooxazine dye that has a thermally stable merocyanine form. Off-to-on fluorescence switching occurs when the merocyanine is switched to its nonquenching spirooxazine form. The on-state fluorescence intensity is more than 100 times greater when photoswitching occurs via FRET from the CPNs than when the dyes are excited directly. Temporal control of fluorescence intensity is achieved using a single-color input of variable intensity for fluorescence excitation (read-only mode) and photoswitching (read–write mode). Here, we present photokinetic measurements and simulations that establish the origin of the observed fluorescence behavior. We find that highly efficient CPN-to-merocyanine energy transfer sensitizes the photochromic reaction, which has a quantum yield of 0.03 in the absence of FRET. Simulated fluorescence trajectories reveal that the low photochromic quantum yield is the necessary condition for single-color intensity control in this system. From the simulations, we determine that the product of the photochromic quantum yield and the energy transfer efficiency (Φ × E) must be no more than 0.05 to produce analogous fluorescence behavior. These findings suggest that the CPNs could be used as an amplifier to drive other low-quantum-yield photoreactions to completion with intensity control.