Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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5.2 Helical Polymer-Based 1-D and 2-D Architectures 165

Figure 5.4 High - resolution AFM image of 1 .

(a) Circle structure; (b) Highly zoomed image

of (a); (c) Cross - section proﬁ le of line a – b in

(a); (d) Cross - sectional proﬁ les of segments

and R

. The greater width of R

in the

proﬁ le indicates that the polymer chain

possesses a right - handed helical structure.

Each bright spot and its cross - sectional proﬁ le

clearly indicate that the polymer chain has a

helical structure. From the cross - sectional

proﬁ le, it can be seen that the local

conformation of R

is ﬂ at on the bottom,

but R

is bulged on the top [33] .

chains might be required to conﬁ gure the long - period helicity. In several AFM

images and their cross - section proﬁ les, the height and width were frequently dif-

ferent at the right and left or up and down points in the circle structure (e.g.,

Figure 5.1 b).

5.2.1.2 Driving Force for the Formation of 1 - D Architectures

The chain dynamics were realized on the surfaces, and not in the solution system.

Although these structures cannot be formed in the solution system due to thermal

ﬂ uctuation, the mechanism of topology switching of single polymer chains is

different from that of the associated forms. The dominant driving force of

topology switching would originate from a degree of freedom, as the topology

of a single polymer chain on the surface depends heavily on the chain length.

166 5 Helical Polymer-Based Supramolecular Films

The proposed structural models regarding the circle structure are shown in

Figure 5.5 . Imperfect open - circle structures with a narrow end - to - end gap were

frequently observed, as illustrated in Figure 5.5 (open circle A). In the case of B,

it is hypothesized that the intramolecular CF/Si interaction allows it to form and

stabilize a perfect closed - circle structure, because the ﬂ uorine atoms on the side

chains may attach to the Si atoms on the main chain, close to the end groups.

Besides this effect, a Si − O − Si bond may be formed between the end - termini via

hydrolysis of the SiH end group by water adsorbed onto the mica surfaces [28] . In

the case of circle C, the end - termini of the polymer chain are crossing each other,

while in circle D the end - termini are not crossing but are parallel to each other.

These structural models can be the precursors of toroidal structures.

As expected, much longer polymer chains frequently formed toroid - like struc-

tures, as shown in Figure 5.6 . These circle and toroid - like architectures may be

Figure 5.5 Schematic representation of

morphology switching of single polysilane

chain 1 with chain length dependence based

on AFM observations. Here, L

indicates the

critical length of the semi - circle structure as

an intermediate state between the rod and

circle structures. In the circle structure, (a),

(b), (c), and (d) indicate the proposed models

(open, closed, cross, and parallel,

respectively).

Figure 5.6 Typical AFM image of toroid - like structures

adsorbed onto mica surfaces.

5.2 Helical Polymer-Based 1-D and 2-D Architectures 167

useful in device applications because a nanoscaled circle architecture composed

of organic/inorganic materials has been expected to provide a superior device

performance due to the lack of defects in the end - terminus part, which strongly

affects energy loss [29] .

5.2.2

Formation of Mesoscopic 2 - D Hierarchical Superhelical Assemblies

Optically active polysilanes, such as poly[( S ) - 3,7 - dimethyloctyl - 3 - methylbutylsilane]

( 2 ) and poly[( R ) - 3,7 - dimethyloctyl - 3 - methylbutylsilane] ( 3 ), which are able to

undergo the helix – helix transition [30] at − 20 ° C in iso - octane (as shown in Scheme

5.2 ) were used in this study. Notably, we present here both the construction of

mesoscopic highly ordered and hierarchical – superhelical assemblies based on

only weak homochiral intermolecular interactions.

5.2.2.1 Direct Visualization of a Single Polymer Chain

Figure 5.7 shows the typical AFM images of single polymer chains of polymers

2 , 3 , and racemic 4 , deposited onto the freshly cleaved mica surfaces and their

section analysis. The samples were prepared by casting a dilute iso - octane solution

of polymer sample (approximate range of 5 ∼ 10 μ g ml

− 1

), after which the strands

Scheme 5.2 Chemical structures of optically

active poly{( S ) - 3,7 - dimethyloctyl - 3 - methyl

butylsilane} 2 , poly{( R ) - 3,7 - dimethyloctyl - 3 -

methyl butylsilane} 3 , which can undergo a

helix – helix transition at − 20 ° C in iso - octane,

poly{( rac ) - 3,7 - dimethyl - octyl - 3 -

methylbutylsilane} 4 , and poly{ n - decyl - ( S ) - 2 -

methylbutylsilane} 5 .

168 5 Helical Polymer-Based Supramolecular Films

corresponding to the single polymer chains were randomly and separately adsorbed

onto the mica surface. Both, the 2 and 3 polymer chains typically possessed an

isolated and stretched conformation. Although the information on the helical

conformation, such as the helicity and helical pitch, could not be obtained due to

a lack of resolution, these AFM images demonstrate the rod - like conformation,

which is expected from the empirical relationship between the molar extinction

coefﬁ cient ε , the viscosity index α ( 2 : 1.47, 3 : 1.32 in toluene at 70 ° C), and the rela-

tively long persistence length ( ∼ 60 nm) [31] . A different polysilane, poly{ n - decyl -

( S ) - 2 - methylbutylsilane} ( 5 ), which is a stiff polymer with a persistence length of

70 nm, was observed by means of AFM as a rod - like chain onto the sapphire sur-

faces [23b] . Therefore, these results were consistent with the previous report.

However, slightly bent chains were frequently observed when the polymer

chains of the racemic polymer 4 ( M

: 5.2 × 1 0

, M

/ M

: 5.39), which is a random

copolymer system composed of equal amounts of R - comonomer and S - comono-

mer, were compared to those of 2 and 3 (marked by arrows in Figure 5.7 j, k, and

l). Additionally, the molar absorption coefﬁ cient ε was lower than that of 2 and 3

(2.8 × 1 0

(Si repeat unit)

− 1

) in iso - octane at 20 ° C, and the α value, which indicates

the rigidity of the polymer chain, was also relatively lower than that of 2 and 3

(1.15 in tetrahydrofuran at 40 ° C), as shown in Figure 5.8 . In addition to the empiri-

cal relationship between ε and the viscosity index of polysilanes [14b] , the present

AFM observations suggest that the topology of 4 is more ﬂ exible than that of 2

and 3 .

Figure 5.7 Typical AFM images of single

polymer chains of 2 , 3 , and 4 , and their

cross - sectional proﬁ les [lines a – a ′ in (c) and

b – b ′ in (d) of 2 , lines c – c ′ in (g) and d – d ′ in

(h) of 3 , and lines e – e ′ in (j) and f – f ′ in (k) of

4 ] onto the mica surfaces. 2 : M

= 2.9 × 1 0

/ M

= 3.43. 3 : M

= 2.6 × 1 0

/ M

= 3.53. 4 : M

= 3.2 × 1 0

/ M

= 5.39. The concentrations of casting

solution were ca. 7 μ g ml

− 1

for 2 , ca. 5 μ g ml

− 1

for 3 , and ca. 10 μ g ml

− 1

for 4 .

5.2 Helical Polymer-Based 1-D and 2-D Architectures 169

Figure 5.8 Circular dichroism and UV absorption spectra in

homogeneous iso - octane solutions of 2 (at 20 ° C, solid line),

3 (at − 5 ° C, dotted line) and 4 (at 20 ° C, broken line). Either

the ε or Δ ε value was normalized (Si repeat unit)

− 1

5.2.2.2 Formation of Superhelical Assemblies by Homochiral

Intermolecular Interactions

The polymer chain, which is much longer than the persistence length of 2

( M

: 3.9 × 1 0

, M

/ M

: 3.02), formed a unique aggregated structure. Figure 5.9

shows the typical AFM images of the superhelical assemblies of 2 . The highly

oriented assembled polymer chains were observed on the mica surfaces (Figure

5.9 a), which might be attributed to a dewetting property and evaporation process

of iso - octane on the mica surfaces. In the case of a fast evaporation process at

80 ° C, no uniform superhelical assemblies were found.

Figures 5.9 b and c show the assembled polymer chains and cross - section analy-

sis in the narrow area. The lines a – b and c – d in Figure 5.9 b indicate the height

and width, respectively, of the branching points in the main assembled polymer

chain. The height of two different polymer chains splitting from the branching

points was approximately 17 nm at the lower part and 39 nm at the higher part.

The lines e – h in Figure 5.9 c indicate the height and width of the main assembled

polymer chain. The height of the main assembled polymer chain, corresponding

to approximately 70 nm, is higher than the total of the branching chain (17 + 39 nm),

and shows that the main assembled coil is formed by a twisting of branches. These

results reveal that the assembled polymer chain is composed of several twined

polymer chains.

Figure 5.10 a shows the high - resolution AFM image of coiled - coil assemblies of

polymer 2 . Right - handed superhelical polymer chains, which possess a long helical

pitch, were clearly observed. In the case of the coiled - coil forms of polymer 3 ( M

7.9 × 1 0

, M

/ M

: 4.78), superhelical polymer assemblies were also observed,

which have the opposite handedness (left - handed) as polymer 2, as shown in

170 5 Helical Polymer-Based Supramolecular Films

Figure 5.9 AFM images and cross - sectional proﬁ les [lines a – b

and c – d in (b) and e – f – g – h in (c)] of highly ordered

superhelical assemblies of 2 onto the mica surfaces.

3.9 × 1 0

, M

/ M

= 3.02. The concentration of the

casting solution was ca. 500 μ g ml

− 1

Figure 5.10 b. It should be noted that each helical ( S or R ) polysilane was conﬁ g-

ured in its corresponding unidirectional superhelical structure. It has been sug-

gested that the 2 and 3 chains possess the P (right - handed) and M (left - handed)

helical screw - senses, respectively. In our case, large - scaled, right - handed and left -

handed superhelical ﬁ bers are composed of right - handed 2 and left - handed 3 ,

respectively [32] . In Figure 5.9 a, here again, it was found that the branched chains

(branching points are denoted by a dotted circle), which are components of the

main assembled chain, also possessed a right - handed helical structure (marked

by arrows). Each branched chain and its small aggregate maintained the same

helicity, leading to the formation of uniform and hierarchical superhelical assem-

blies with the same helicity as the components. It has been reported that, in both

natural and artiﬁ cial systems, heretofore, the helicities of the components were

5.2 Helical Polymer-Based 1-D and 2-D Architectures 171

Figure 5.10 AFM images of superhelical structures of 2 , 3 ,

and 4 polymer chains onto mica surfaces. (a) 2

( M

= 3.9 × 1 0

, M

/ M

= 3.02); (b) 3 ( M

= 7.9 × 1 0

/ M

= 4.78); (c) 4 ( M

= 3.2 × 1 0

, M

/ M

= 5.39).

different from that of the hierarchical superstructure that eventually formed; for

example, the right - handed superhelix of collagen is composed of three left - handed

helices [2a, 5k, 33] . Given that the same helicity was maintained during the process

of association without reference to the state of the aggregate (number of polymer

chains), homochiral intermolecular interactions would be a dominant driving

force for the formation of uniform and hierarchical superhelical assemblies. The

data reporting on the height, helical pitch and handedness of the coiled - coil chains

are listed in Table 5.1 .

A comparison of the helical ﬁ bers constructed from 2 and 3 showed, surpris-

ingly, almost the same value for the heights and helical pitches, but a difference

in the handedness. A similar phenomenon has been reported in a biological

system, where Larson and coworkers observed the left - handed superhelical struc-

ture composed of RecA and double - stranded DNA by means of AFM [33] . In that

Table 5.1 Numerical data of assembled ﬁ bers of 2, 3 , and 4

polymer on the mica surfaces (see Scheme 5.2 ) .

Polymer

× 1 0

Height of

aggregate (nm)

Helical pitch

(nm)

Screw - sense of

ﬁ bers

2 3.9 3.02

65 ± 12 646 ± 82

Right - handed

3 7.9 4.78

49 ± 10 636 ± 77

Left - handed

4 3.2 5.39

51 ± 9.5

– –

172 5 Helical Polymer-Based Supramolecular Films

case, the pitches of the superhelices of the complexes (RecA and DNA ﬁ lament)

were very similar to each other, regardless of the number of component ﬁ laments.

Moreover, from observations of the heights of the helical ﬁ bers of 2 and 3 , it is

believed that the formation of the highly ordered structure in the large scale is

achieved by the assembly of several polymer chains. This observation is an example

of mesoscopic large - scaled, highly ordered superhelical assemblies that possess

both of the helical senses constructed by a synthetic helical polymer. Most of the

synthetic helical polymers have a polar functional group in the main or side chain,

and therefore an associated structure is formed by the combination of various

strong intermolecular interactions, such as hydrogen bonding, electrostatic inter-

actions, coordination bonding, and π - π stacking. In the present case, only weak

intermolecular interactions can account for the polymer association, because the

polysilanes 2 and 3 are composed of nonpolar groups with side chains. It is note-

worthy that no assembled superhelical structures were observed with 4 . As for the

morphology of the observed structure of 4 , although large - scaled stripe patterns

were formed on the substrate, the structures possessed no helicity. As discussed

above, there was no major difference between 2 or 3 and 4 in the conformation

of a single polymer chain. These results suggest that the homochirality of the

single helical polymer chain rather than the chain conformation structure contrib-

utes to the formation of the large - scaled, highly ordered superhelical assembly [34] .

Similar phenomena have been observed in the 2 - D crystallization of small chiral

organic molecules on the surfaces. For example, homochiral enantioselective crys-

tallization from racemic mixture has been reported [34a] . K ü hnle and coworkers

have shown that cysteine molecules formed homochiral pairs ( d - d and l - l ) from

racemic mixture using STM. Chiral transfer from single molecules into self -

assembled monolayers on the metal surfaces has also reported by Fasel et al . [34b] ;

here, enantiopure P and M chiral single molecules on the surfaces formed the

enantiopure self - assembled monolayers with clockwise and anticlockwise formats,

respectively. In addition to the enantiopure self - assembly, it has been reported that

the enhancement of chiral interactions in two dimensions was observed in the

enantioseparation of chiral molecules, using STM [34c] . These results also sug-

gested that homochiral intermolecular interaction would be an important factor

to form the 2 - D chiral surfaces.

5.2.3

Formation of 2 - D Crystallization of Poly( γ -

L - Glutamates) on Surfaces

Poly( γ - l - glutamate) ( 6 ) is a polypeptide that is well known for forming a stable

α - helical conformation, even when the substituted side chains are varied, as shown

in Scheme 5.3 . Polymers of 6 have longer alkyl side chains and can form 2 - D self -

organized arrays on highly oriented pyrolytic graphite ( HOPG ) [20c] .

In the following section, the formation of 2 - D epitaxial arrays on surfaces and

a comparison of structures between 2 - D epitaxial arrays and 3 - D bulk phases will

be described, based on the results of AFM and wide - angle X - ray diffraction ( XRD )

studies.

5.2 Helical Polymer-Based 1-D and 2-D Architectures 173

5.2.3.1 Direct Visualization of 2 - D Self - Organized Array by AFM

Figure 5.11 shows a collection of high - resolution island images for the 6 series,

which all revealed a similar island structure. The island - like arrays consisted of

rods, which possessed an entirely straight conformation in a parallel arrangement.

A single rod, which was separated from the islands, was not found at all. It is

Figure 5.11 Typical AFM images and

cross - sectional proﬁ les (insets) of polymers 6

adsorbed onto HOPG substrate. (a, b) 6A ;

(c, d) 6B ; (e, f) 6C ; (g, h) 6D . The Z - scale of

the images is constant at 4 nm. Cross -

sectional proﬁ les of the island structures are

shown in the insets of the images. The arrows

indicated the running directions of the rods in

the 2 - D array, and the directions

corresponded to 1210 directions of the

graphite basal plane. In whole images, the

entire surface is uniformly covered by bright

islands, representing the polymer adsorbates,

and the dark portion surrounding islands is

bare graphite surface.

Scheme 5.3 Chemical structure of poly( γ - L - glutamate)

derivatives 6 , where n is deﬁ ned as the number of methylene

units in the alkyl group.

174 5 Helical Polymer-Based Supramolecular Films

important to emphasize here that the corrugations of the islands were constant

for each of the members within the 6 series; this suggests that the alkyl chains on

6 were ﬂ at and contacted graphite in the same manner. It also indicates that the

polymers adsorbed on the HOPG were not in an aggregated form, but rather in a

monolayer form. The stripped patterns in the arrays were triangular in arrange-

ment, with the running stripes directly rotated 60 ° from each other (see Figure

5.11 c and g, where the marked arrows indicate the running direction of the stripes

in the islands). This clearly indicated that the formation of the island structures

is by epitaxial adsorption.

5.2.3.2 Orientation in 2 - D Self - Organized Array

The orientation of the adsorbed structure was estimated by the relationship

between the alignment of the alkyl chain and the graphite substrate. Figure 5.12

shows the schematic representation of the structural model proposed for 6 . In the

array model, the intervals of the rods correspond to intermolecular distances

between ﬂ at - oriented adjacent polymers. Extended alkyl side chains with the all -

trans conformation are divided to both sides of the helical main chain, and align

perpendicular to the main chain. The driving force for the formation of the array

is predominantly an epitaxial interaction between the alkyl side chains and the

graphite surface. The rods of the 6 polymers run in the

1210 direction. When

the rod model was superimposed on a graphite lattice, the alkyl side chains, which

were angled at 90 ° to the direction of the rods, were aligned in the

1010 direc-

tion. It is well known that alkyl chains adsorb epitaxially onto HOPG, and that the

alkyl chains are aligned in the

1010 direction [35] , clearly showing the accuracy

of the proposed model and indicating that the formation of the array is predomi-

nantly due to epitaxial adsorption of the side groups.

Figure 5.12 Schematic representation of the proposed

structural models for the 2 - D array, displaying the rod

structure and the epitaxial alignment of alkyl side chains on

the HOPG substrate of 6 .