Fluorescent J-Aggregates and Their Biological Applications - Advanced Fluorescence Reporters in Chemistry and Biology - page 139

equals to the fluorescence quantum yield). That is, the ratio of the radiative lifetimes
of the monomer dye to its J-aggregate is proportional to the exciton delocalization
length N d [ 10 ]:

m
rad

t 8

p

rad

2 N d : J

t

One more peculiarity of the J-aggregates is the narrowing of their absorption and
fluorescence spectra as compared to those of the monomer dyes. The ratio of the half-
width values of the monomer v 0 : 5 absorption bands could

be connected under certain assumptions [ 11 ] with the square root of the N d value:

and J-aggregate

v 0 : 5

D D

r

2 N d
3

m
0 : 5

Dn

0 : 5

:

J

Dn

Measurement of the aggregate radiative lifetime and that of its absorption
spectrum half-width are the two experimental ways to estimate the value of the
delocalization length.
Up to this moment, we have regarded the case of the aggregate with the single
molecule in an elementary cell. But this is not always the case. Davydov [ 3 ]
regarded the crystal with m molecules in an elementary cell and found that the
excitation levels of such a crystal form m exciton zones. It should be noted that the
selection rules permit only one absorption transition to each zone. Thus, for an
aggregate with m molecules in an elementary cell, up to m bands could be observed
in an absorption spectrum. Such spectrum splitting is called the Davydov splitting .
One of the examples of an aggregate with such a splitting is the helical one [ 12 ].

3 Aggregation in Water Solutions

The study of H- and J-aggregates spectra in the solutions began already in the first
works dealing with J-aggregates [ 13 - 16 ]. Till now, wide amount of information has
been collected concerning the aggregation conditions, as well as the spectral and
physical properties of aggregates formed by various compounds [ 17 , 18 ].
First of all, it should be noted that the cationic dyes form aggregates most
efficiently either in water or in strongly nonpolar solvents (e.g., hexane) [ 19 ].
Besides, aggregation is enhanced by the addition of salts, and surfactants above
the critical micelle concentration, and decreased upon increasing temperature [ 17 ].
The last mentioned dependence is clear; to understand the others, the interactions
participating in the dye aggregation should be considered. These interactions are (1)
van der Waals attraction of the -electron systems of the dyes, (2) Coulombic

interactions of the dyes' charges, and (3) hydrophobic force in the case of water.
Hence, the effect of solvent on the aggregation is believed to be explained by the

p