Preparation and optical properties of organic nanoparticles of porphyrin without self-aggregation

We report the synthesis and optical properties of organic porphyrin nanoparticles with narrow size distribution and well dispersibility. Ion association between cationic meso-tetrakis(1-methylpyridinium-4-yl) porphine (TMPyP) and tetraphenylborate (TPB) anion or fluorinated derivatives of TPB, in the presence of polyvinylpyrrolidone (PVP), produces the porphyrin nanoparticles of ∼25 nm in aqueous solution. The TMPyP nanoparticles were stable in solution without precipitation for at least 30 days. No self-aggregation of the constituent porphyrin chromophores was confirmed. The TMPyP nanoparticles exhibited interesting optical properties; a large bathochromic shift in the absorption spectra and a resolution increase in the fluorescence spectra compared to those for the solution-phase TMPyP. A flattening of TMPyP molecules within the nanoparticles, which is caused by the twisting of four cationic methylpyridinium moieties, is the main origin for the large red shift. The resolution increase of fluorescence is due to the effect of immobilization of the methylpyridinium groups in the solid-state nanoparticles and/or that of a low polarity environment.