Prospects for Organic Dye Nanoparticles - Advanced Fluorescence Reporters in Chemistry and Biology - page 296

[ 28 ] (See also the chapter of Losytskyy and Yashchuk [ 48 ] addressing this subject).
The exciton states are the delocalized electronic states that are caused by the strong
long-range dipole-dipole interactions among the monomers within the aggregate,
and the width of the exciton band is proportional to the coupling strength. In the
simplest case of one molecule per unit cell and a negative coupling energy (namely,
head-to-tail arrangements), only transitions to states at the bottom of the exciton
band are optically allowed. The total oscillator strength of the coupled molecules is
thus swept together in a few eigenstates, and as a consequence, the radiative rates of
the bottom states are strongly enhanced as compared to the single molecule.
3.3.2 Highly Fluorescent Organic Dye Nanoparticles
Organic nanoparticles of AIEE molecules can be highly emissive and thus be a
useful fluorescence probe. In what follows, some examples prepared by the repre-
cipitation and ion-association methods are presented.
1. The AIEE phenomena from organic CN-MBE nanoparticle systems prepared
by a reprecipitation method have been observed (chemical structure of CN-
MBE is shown in Fig. 1 )[ 29 ]. Interestingly, although the emission from CN-
MBE monomeric solution is very weak, the nanoparticles emitted very strong
fluorescence with intensity almost 700 times that of the solution. The optimized
geometry of free CN-MBE is nonplanar according to the DFT/B3LYP/6-31G*
level calculation. The planarization accompanied by a twisting around a single
bond of the CN-MBE molecules would be induced by aggregation in the
nanoparticles (Fig. 5 ), which resulted in strong intermolecular interactions
Fig. 5 A proposed mechanism for enhanced emission (or AIEE) in solid-state organic dye
nanoparticles. The dye considered here is trans -biphenylethylene (CN-MBE) compound. The
geometry is optimized by the density functional theory (DFT) calculation at the B3LYP/6-31G*
level. Molecular distortion such as twisting and/or subsequent planarization causes prevention of
radiationless processes along with specific aggregation such as the J -aggregate in the nanoparticles