Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 295

3.3 Why Can Organic Dye Nanoparticles Be Fluorescent?

In organic dye nanoparticles, unique fluorescence behaviors have been observed,
which resulted from the special aggregation or interaction modes of the target dye
molecules. In particular, molecular distortion, planarization, or conformational
restriction in the nanoparticle induces such interaction modes for the chromo-
phores. In this section, I first introduce interesting emission properties of the
solid-state nanomaterials, that is, aggregation-induced emission (AIE) or AIEE.
Next, some examples of the fluorescent organic dye nanoparticles synthesized by
means of both the reprecipitation and ion-association methods are presented in
detail. For organic dye nanoparticles prepared by the ion-association technique,
matrix polarity (that is, counterion polarity) strongly influences their fluorescence
properties, so that the counterions are able to bestow the highly fluorescent nature
on the dye nanoparticles.

3.3.1 Aggregation-Induced Enhanced Emission

In most cases, chromophore (fluorophore) aggregation that is brought about by
fabrication into solid-state materials quenches its emission (the phenomenon is
often called “concentration quenching” or “aggregation-cased quenching”), even
though the chromophores are highly emissive in their dilute solutions [ 13 ]. The
close vicinity between the chromophores in the aggregates opens their nonradiative
decay channels, resulting in self-quenching of fluorescence and a drastic reduction
in the emission intensity. The concentration quenching in the solid-state materials
has prevented many important fluorophores from finding applications in an engi-
neering robust form. Recently, a very interesting phenomenon that molecular
aggregation plays a constructive role in the light-emitting process, called AIE or
AIEE, has been found; a series of nonemissive or weakly-emissive chromophores
are induced to emit intensely by molecular aggregation [ 25 - 27 ].
The mechanistic understanding of AIEE has been mostly found in the restriction
of intramolecular rotation of some substituents (for example, peripheral aromatic
rings), which blocks the nonradiative channels and populates the radiative routes.
This implies the significance of molecular geometry and flexibility (or intramolec-
ular rotation) that affects the relaxation dynamics in the excited state. External
controls of relaxation dynamics by cooling or pressurization sometimes boost the
emission of AIEE molecules efficiently. In addition, aggregation-induced planar-
ization of the chromophore with

-systems and subsequent J -type aggregation can
also contribute to enhanced emission. In this case, bulky substituents play a
significant role in preventing the parallel face-to-face molecular stacking ( H -aggre-
gation) and thus in inducing the head-to-tail stacking ( J -aggregation). When
J -aggregation proceeds, one can observe the rise of a new red-shifted, strongly
narrowed absorption band in addition to a broad monomeric band. The excited
states in J -aggregates are considered to be of excitonic nature (Frenkel excitons)

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