Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 286

3 Optical Properties of Organic Dye Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
3.1 Photocatalytic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
3.2 Size-Dependent Spectroscopic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292
3.3 Why Can Organic Dye Nanoparticles Be Fluorescent? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295
4 Concluding Remarks and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

1 Introduction: Organic Dye Nanoparticles Exhibit Unique

Physicochemical Properties

Physicochemical properties of organic nanoparticles with their size smaller than
a hundred of nanometers generally differ from those of individual molecules due
to intermolecular interactions such as van der Waals attractive forces and/or
hydrogen-bonding in the solid state [ 1 , 2 ]. In addition, they do not resemble those
of bulk crystals because they are closely related to the large proportion of surface
molecules that characterize nanoarchitectures. Therefore, organic nanoparticles can
be seen as novel materials in an intermediate state, bridging the gap between single
molecules and bulk materials, and are expected to give access to original applica-
tions in various cutting-edge fields of technology such as bioanalysis [ 3 ], photo-
catalysis [ 4 ], pharmacology [ 5 ], photonics, and microelectronics [ 6 ].
Constructing organic nanoparticles, chromophoric (or fluorophoric) molecular
systems with a defined chemical structure are useful and promising potential [ 7 - 9 ].
Such systems involve “organic dyes” that possess absorption of electromagnetic
radiation of varying energies. The optical properties of the organic dyes depend on
their electronic transitions that reflect molecular geometries and can be tuned by
elaborate molecular design strategies. In particular, fluorescence properties in dye
molecules offer a wealth of information so that they can be a useful extrinsic
reporter for the investigation of many fundamental processes in life and material
sciences [ 10 ]. Fluorescence of organic dyes typically originates either from an
optical transition delocalized over the whole chromophore ( resonant dyes )or
from intramolecular charge-transfer transitions ( CT dyes )[ 11 ]. The majority of
common fluorophores (for example, fluoresceins, rhodamines, and cyanines) are
resonant dyes that are characterized by slightly structured, comparatively narrow
absorption and emission bands with a small solvent polarity-insensitive Stokes
shift, high molar absorption coefficients, and moderate-to-high fluorescence quan-
tum yields. In contrast, CT dyes such as coumarin and styryl dyes have well-
separated, broader, and structureless absorption and emission bands, with larger
Stokes shift, which strongly depends on solvent or matrix polarity. Their molar
absorption coefficients, and in most cases also their fluorescence quantum yields,
are generally smaller than those of resonant dyes [ 11 ].
Fabrication of organic dye nanoparticles and the ability to tailor their photo-
physical characteristics represent an important challenge, although they have been
paid little attention principally due to the following two factors (1) Owing to their