Ensemble and Single-Particle Fluorescence Photomodulation in Diarylethene-Doped Conjugated Polymer Nanoparticles

Fluorescent systems that can undergo intensity photomodulation in aqueous environments are finding increasing applications, particularly in high-resolution imaging of biological samples. We seek to develop conjucated polymer nanoparticles (CPNs) with bright fluorescence that can be modulated with a light signal. Here, we present CPNs, doped with a photochromic diarylethene dye, that exhibit efficient fluorescence photomodulation that is thermally irreversible. In their UV-absorbing open form, the diarylethenes have no effect on the fluorescence properties of the bright CPNs. A brief period of UV irradiation converts the dyes to their visible-absorbing closed form, which is an efficient fluorescence quencher for the CPNs, likely via a fluorescence resonance energy transfer mechanism. Aqueous suspensions of dye-doped CPNs prepared from the homopolymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) or a polyfluorene-phenylenevinylene copolymer (PFPV) exhibit thermally stable bright and dark levels. The dye-doped MEH-PPV CPNs also exhibit photomodulation in single-nanoparticle imaging experiments, which reveal that nearly all CPNs retain a small amount of residual emission in the dark state. Their PFPV counterparts undergo irreversible fluorescence photobleaching rather than photomodulation in single-nanoparticle studies. The photostability of the CPNs under the UV irradiation conditions required for photochromic conversion is investigated on the single-particle level, and PFPV CPNs are found to be particularly susceptible to photobleaching upon 254 nm irradiation. These results will guide the selection of polymers and photochromes for CPNs intended for single-particle photomodulation.