V. V. PROKHOROV ET AL.
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that structural correspondence for the morphologies ob-
served in Figure 2(a) can’t be done unambiguously on
the basis of only AFM data and below we consider sev-
eral alternative ways of structural interpretation. Heights
of the leaves and stripes observed by AFM (Figure 2(b))
agree well with the model of a symmetric monolayer
made of monomers in all-tra ns conformation (Figure
3(b)). In this model, the long axes of dye molecules are
oriented parallel to each other and to mica plain whereas
the molecular planes are oriented perpendicular to mica
(“edge-on” orientation). To satisfy the AFM height data,
the model implies that adjacent stacked molecules have
anti-parallel orientation in the up-down direction so that
aliphatic chains occupy both sides of the monolayer. The
anti-parallel up-down arrangement follows from the fact
that monolayer height observed by AFM substantially
exceeds the upper border for a vertical molecular dimen-
sion which is estimated to be ~1.05 nm (Figure 3(a)).
For the anti-parallel up-down orientation the requirement
of the efficient overlapping of π-electron chromophore
systems is achieved at the lateral translation (slippage) of
two adjacent stacked molecules by ~1.3 nm (see Figure
3(b)). This translation produces the small slip angle (α)
corresponding to the J-aggregate dimer: α = atan(0.4
nm/1.3 nm) ~17 deg. At the estimation of the slip angle,
the plane-to-plane intermolecular distance was assumed
to be equal ~0.4 nm [15,17]. The represented in Figure
3(b) symmetric monolayer height ~1.4 nm is the upper
boundary corresponding to the monomer configuration
with fully extended aliphatic chains. For tilted chains, the
height is smaller. The leaves are assumed to be single
symmetric monolayers whereas the stripes consist of two
such monolayers. The above consideration implies a
small slip angle, thus both the leaves and the stripes are
assumed to have the J-aggregates optical properties. The
observed rectangular shapes of stripes could be explained
by the J-aggregate growth of the ladder type (such as
represented in Figure 3(b)) having intrinsically rectan-
gular geometry in contrast with the alternative inclined
staircase packing arrangement in J-aggregates.
It is noteworthy to say that the molecular modeling for
this dye conducted in [16] reveals that the alternative
configuration of the monomer unit, i.e. the mono-cis1
(instead of all-trans configuration implied in the models
in Figure 3) proved to be energetically advantageous. It
was shown that the mono-cis configuration provides
three new types of the packing arrangement in J-agg-
regates, i.e. one linear planar and two helical packing
arrangements. The helical packing arrangements with a
diameter within 1.2 - 2.0 nm seem to be excluded from
our consideration because they generate intrinsically
linear fibrillar structures. However, at present it is not
clear whether the remained model of the linear mono-
cis1 packing arrangement in [16] can be applied for the
description of the structures and their heights observed in
Figure 2(a).
The dramatic morphological difference between the
leaf morphology (with a height of a symmetric mon-
olayer) and the stripe morphology (with a double mon-
olayer height) should be noted and needs a special con-
sideration. Its explanation can be performed in two basi-
cally different ways. First, it may be supposed that both
morphologies correspond to the same J-aggregate crystal
structure with the same monomer configuration and the
same slip angle (supposedly in accordance with the
model in Figure 3(b)). However for the leaf-like mono-
layer and the stripe-like bilayer, the growth of the crystal
sheets proceeds along different crystallographic direc-
tions. The difference in crystallographic growth direc-
tions generates the difference in the geometrical shapes
and observed peculiarities at a macroscopic scale (such
as straight linear or curved crystal faces). The difference
in geometrical shapes is in this case due to peculiarities
of crystal growth for the same crystal structure. Second,
it may be supposed that the leaves and stripes have dif-
ferent crystal structures, i.e. the elementary cells and/or
monomer conformations.
For the spots, the monolayer height ~0.9 nm is no-
ticeably lesser than that of ~1.4 nm for the leaves. The
lower monolayer height can be explained by two differ-
ent ways. First, it is explained by the model in Figure
3(c) with all-trans monomer configuration and parallel
orientation of adjacent stacked molecules. To satisfy the
AFM results, the entire sulfopropyl group must occupy
the same position with respect to the monolayer plane.
Thus Figure 3(c) represents a model of the asymmetric
monolayer. Evidently, the best overlapping of π-electrons at
chromophores stacking is achieved in this case for the slip
angle = 90 deg, resulting in the H-aggregate optical spectra.
An alternative explanation can be probably provided in ac-
cordance with the model in [16], i.e. by the stacking of
monomers arranged in mono-cis1 configuration. Both the
mono-cis1 molecular configurations in [16] and the all-trans
configuration considered above in Figure 3(c) have close
vertical geometrical dimensions ~1 nm. The particular de-
tails of probable mono-cis1 monomers stacking in the
monolayer with a height ~1 nm in accordance with the al-
ternative model developed in [16] remain however unclear
and need to be modeled more accurately.
The results considered above demonstrate the mor-
phological and structural complexity of the dye molecular
aggregates at a meso-scale. Complementary information
on the anisotropy of molecular sheets could be indispen-
sable for making unambiguous conclusions on the struc-
ture of leaves, stripes and spots. The polarized opti-
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