Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 138

Fig. 2 Schematic representation of characteristic absorption ( A ) and fluorescence ( F ) spectra
of H- and J-aggregates (marked as H and J, respectively) as compared to those of the monomer
molecule (M). The dashed spectrum F H means that H-aggregates could be nonfluorescent

and are either nonfluorescent or emit a weak fluorescence with broad spectrum
and large Stokes shift (Fig. 2 )[ 6 , 7 ]. The dye molecules in an H-aggregate
form the so called “card-pack” or “sandwich” structure, in which the angle a

exceeds 54.7 (Fig. 1 )[ 5 ].
The theory of the aggregate formation that accounts for the van der Waals
attraction and Coulomb repulsion (as the majority of the dyes molecules are
charged in water solution) was proposed [ 8 ]. It was shown that, within the frames
of the assumptions made (large number of molecules, the point Coulomb charges,
and uniform distribution of the electron density over the dye molecule surface) the
aggregate energy has its minima at two values of the angle between the molecule

transition dipole moment and the aggregate axis. One of these values is equal to
90 , which corresponds to H-aggregates. Another one appearing at higher values of
the number N is below 54.7 (and becomes the absolute minimum at the angle

a

a
lower than 38.2 ); it can be attributed to the J-aggregate geometry. Though the
theory concerned only the high N values, experimental results also show that the
aggregate of the H-geometry could be formed by a small number of molecules,
and dimers of a “sandwich” structure are often observed. At the same time, for the
J-aggregate formation, a higher number of dye molecules are necessary.
The already mentioned Davydov theory regarded an infinite aggregate, the
excitation being delocalized over the very large number of molecules N . But
these ideal conditions cannot be met in reality. In the experimentally observed
J-aggregates, the excitation is believed to be delocalized over a limited number of
molecules N d < N . The parameter N d is called the exciton delocalization length .It

depends on both the resonance dipole-dipole interaction and diagonal (or energy)
disorder (i.e., some small difference in the excitation energy of separate molecules)
in the J-aggregate [ 9 ].
It was already mentioned that the J-aggregates are highly fluorescent. This
phenomenon is connected with the change in the aggregate radiative lifetime
t

rad as compared to that of the monomer dye rad . (Radiative lifetime

1/
k rad , k rad being the rate constant of the fluorescent deactivation of the excitation.
At the same time, fluorescence decay time

t t rad ¼

k nr ), k nr being the total
rate constant of the nonfluorescent deactivation pathways. Thus, the ratio

t ¼ 1/( k rad þ

t / t rad