Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 291

bis(trifluoromethyl)phenyl]borate, etc.), are used to prepare the cationic dye nano-
particles in aqueous solution [ 23 ]. Since the particle size can be typically controlled
by adjusting the molar ratio between the anion and the cation added, competition
between aggregation of the ion-pairs and adsorption of excess anions onto nano-
particles, which influences the surface charge density and electrostatic screening,
would determine the size of the nanoparticles. An opposite situation, anionic
chromophore-based organic nanoparticles can also be synthesized by mixing a
functional dye anion and a hydrophobic but water-soluble cation in aqueous
solution 1 . In any case, no specific organic solvents are needed.

3 Optical Properties of Organic Dye Nanoparticles

In addition to exploration of the preparation strategies, much effort has also been
made to investigate the unique optical and electronic properties of organic dye
nanoparticles. It is found that the optical/electronic properties of organic nanopar-
ticles are fundamentally different from those of their inorganic counterparts, since
the intermolecular interactions are basically of weak types, such as hydrogen-
bonding, stacking, and/or van der Waals interactions. In this section, represen-

tative examples of dye nanoparticles prepared by the “reprecipitation method” and
“ion-association method” are shown. Their interesting photocatalytic and size-
dependent optical properties are discussed, and more attention is paid to the unique
fluorescence properties from a viewpoint of aggregation-induced enhanced emis-
sion (AIEE) that involves the conformational planarization or restriction of molec-
ular twisting motion as well as J -type aggregation.

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p p

3.1 Photocatalytic Properties

Photosensitization reactions of a chromophore in the presence of oxygen are
commonly classified as type I reactions, where the absorber reacts directly with
the substrate via a charge-transfer mechanism; or type II reactions, where the
absorber transfers energy to singlet oxygen, which then reacts with the substrate
[ 4 ]. Tetracene (chemical structure is shown in Fig. 1 ) is a known photosensitizer of
singlet oxygen, and the mechanism of the photocatalytic destruction of some other
dye molecules does not depend on the presence of oxygen, implying a type I
mechanism. This photocatalytic activity suggests that tetracene may be worth

1

Anion-based organic dye nanoparticles can be also synthesized on the basis of the ion-association
method. In this case, hydrophobic “phosphazenium” cations such as tetrakis[tris(dimethylamino)
phosphoranylideneamino]phosphonium cation are effective for ion-pair formation with anionic
dyes. A neutral polymer stabilizer polyvinylpyrrolidone (PVP) that is soluble in water is some-
times added for preventing agglomeration