Photochromic Dye-Doped Conjugated Polymer Nanoparticles: Photomodulated Emission and Nanoenvironmental Characterization

We present studies of fluorescence photomodulation and solvatochromism in nanoparticles of the conjugated polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with a photochromic spirooxazine dye. The fluorescence properties of doped nanoparticles with dyes in the spirooxazine form are identical to those in undoped control nanoparticles. UV irradiation converts some of the dyes to their visible-absorbing merocyanine form, which is an efficient quencher of MEH-PPV fluorescence. The fluorescence intensity of the nanoparticles drops to less than 10% of its initial value and recovers when the merocyanines undergo thermal reversion to spirooxazines. The fluorescence modulation can be cycled many times without fatigue or photodegradation, and the degree of quenching is linear with merocyanine concentration. The photochromic conversion can also be used as a probe of the environment within the nanoparticles as both the kinetics of the thermal merocyanine-to-spirooxazine conversion and the merocyanine absorption spectrum are sensitive to the dye environment. The kinetics of the thermal dye reversion in the nanoparticles are first order and nearly as fast as those in THF, while those in a MEH-PPV film are biexponential and substantially slower. The position of the merocyanine absorption within the nanoparticles is likewise distinct from that in a MEH-PPV film and implies a liquid-like environment that is more polar than THF. We hypothesize that those dyes that undergo spirooxazine-to-merocyanine conversion are adhered to solution-exposed MEH-PPV segments within the nanoparticles or to the particle surface and thus have ample free volume for the photochromic conversion. These findings will be useful in designing future stimulus-responsive nanoparticle systems.