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

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5.2 Helical Polymer-Based 1-D and 2-D Architectures 175

5.2.3.3 Intermolecular Weak van der Waals Interactions in 2 - D

Self - Organized Arrays

Although the interactions between the alkanes and graphite are stronger than the

intermolecular interactions, the lateral intermolecular interactions are important

in the formation of an ordered 2 - D structure [36] . In addition to the epitaxial

interaction between the alkyl side chains and the graphite surface, the formation

of an island structure composed of rods suggests the existence of the attractive

intermolecular interactions, including van der Waals interactions, between the

alkyl chains of adjacent rods. Alkyl chains have also been reported to align “ side

by side ” with parallel or perpendicular conformations at the surface [36, 37] . In

the case of the 6 series, the interval between the alkyl chains is very close to that

reported for a ﬂ at conformation. From the model, it was expected that the length

of the rods was attributable to the length of each helical polymer. However, the

observed lengths of the strings were regularly several times longer than expected,

and often overestimated even by gel permeation chromatography ( GPC ). For

example, the observed lengths of the rods for 6B ( M

= 37 900; M

/ M

= 1.1;

expected length =15 nm) and 6C (56 100; 1.2; 22 nm) were 27 – 44 and 17 – 98 nm,

respectively, indicating that each rod consisted of several polymers to form “ head -

to - head ” or “ head - to - tail ” structures, shown by the arrow in Figure 5.11 . The inter -

polymer “ side - by - side ” arrangement is due to the intermolecular interactions

between the alkyl chains of adjacent rods. Although the average degree of polym-

erization for 6D was almost the same as that of 6C (Figure 5.1 e), the rods of 6D

(Figure 5.1 g) were longer than those of other 6 s. This observation was due not to

differences in the molecular masses of the polymers, but rather to the enhance-

ment of intermolecular interactions with increasing alkyl chain length. More

interestingly, the rods seem to have a tendency to align with the edges of rods.

The islands in Figure 5.11 b, d and f are typical examples, with relatively ordered

rod ends. This suggests that attractive lateral intermolecular interactions exist

between rods at the surface.

5.2.3.4 Comparison of Structures between a 2 - D Self - Organized Array

and 3 - D Bulk Phase

The interval distances between the parallel rods depend on the length of the alkyl

side chains of the polymers. Figure 5.13 shows plots of the intervals between the

parallel rods against the number of alkyl side chains. The intervals increase line-

arly with the length of the alkyl side chains, and from the slope of the linear plot

the length per methylene unit is approximately 2.2 Å ; this corresponds to incre-

ments of an extended alkyl chain with all - trans conformations, and indicates that

the alkyl side chains are perpendicular to the helical main chain. The extrapolated

value of the interval to n = 1, which should correspond to the interval for poly( γ -

methyl - l - glutamates) (PMLG, n = 1), was 19.9 Å . This value is appropriate for the

diameter of PMLG, and validates the model subsequently proposed. In Figure

5.13 , the results of the bulk crystal analysis were plotted with those of the 2 - D

array. The slope for the PGn bulk crystal, in which the side chains were crystal-

lized, was approximately 1.3 Å /CH

(a methylene unit), which is almost half the

176 5 Helical Polymer-Based Supramolecular Films

value of that for the 2 - D crystal. This difference is due to the interdigitating struc-

ture of the bulk crystal.

The 2 - D array of 6 on HOPG forms under submonolayer coverage conditions.

All results, including the correlation between the interval of rods and alkyl chain

lengths, clearly proved the proposed model for the epitaxial adsorption of 6 . Fur-

thermore, epitaxial helical polymer – substrate and polymer – polymer interactions

were also found which can lead to the formation of controllable architectures. In

particular, the existence of some of the intermolecular interactions between the

end - termini of polymers was quite interesting and important for the design of 1 - D

supramolecular architectures.

5.2.4

Summary of Helical Polymer - Based 1 - D and 2 - D Architectures

In this section, attention will be focused on the formation of 1 - D and 2 - D self -

organized structures composed of helical polymers. From Section 5.2.1 , the proper

adsorbate – substrate interaction, which should be neither too weak nor too strong,

enables the detection of the topology switching and 2 - D self - organization. The

control of chain length leads to the construction of various 1 - D helical architectures

onto surfaces; these have the rigid characteristics of helical polymers, which are

unique and advantageous for constructing various hierarchical systems from 1 - D

to 3 - D. In Section 5.2.2 , the rigidity of a single polymer chain and the mesoscopic

hierarchical helical superstructures in the polysilane aggregates were observed,

using AFM. The dominant driving force for the formation of right - and left - handed

superhelical structures is based on homochiral weak intermolecular interactions.

Figure 5.13 Variations in the interval distances (

䊊

) between

neighboring rods in the 2 - D array and the corresponding

distances (

䊉

) between the center of 6 in the bulk crystal,

which was revealed through (100) reﬂ ections.

5.3 Helical Polymer-Based Functional Films 177

Furthermore, it should be noted that a helical polymer chain that is much longer

than the persistence length could easily form the entangled structure, conse-

quently leading to the formation of mesoscopic highly ordered superhelical

structures. In Section 5.2.3 , in addition to adsorbate – substrate interactions, the

interactions of lateral (alkyl side chain) and longitudinal (end - termini of main

chain) directions were shown to also be required for 2 - D self - organization.

It should be emphasized here that both the polymer – substrate and polymer –

polymer interactions can lead to the formation of controllable architectures. Fur-

thermore, these interactions are not exceptionally strong, but they are sufﬁ cient

to form well - organized structures.

In the following section, as an example of functional 3 - D architecture, attention

will be focused on a unique optical activity based on a helical conformation, which

has potential use for functional materials in the solid state.

5.3

Helical Polymer - Based Functional Films

Previously, we have described the formation of 1 - D and 2 - D architectures com-

posed of helical polymers. These well - deﬁ ned helical, polymer - based supramolecu-

lar architectures can be designed and controlled, except for some limitations, and

therefore expanding these to the life sciences and/or materials sciences applica-

tions based on optical activity remains a major challenge [1b] . As noted above,

synthetic helical polymers exhibit many unique phenomena, including chiroptical

memory, ampliﬁ cation [38] , formation of thermotropic cholesteric liquid crystals

[12, 39] , molecular chirality recognition [40] , and helix – helix ( PM ) transition [30,

32, 41 – 49] . The PM transition phenomenon, which involves the reversible switch-

ing between the P - (plus, right - handed) and M - (minus, left - handed) screw - sense

segments along the helical backbone, is especially promising for chiroptical mate-

rials [30, 32, 38, 40 – 49] . The PM transition, driven by external stimuli such as

temperature [30, 32, 45, 49] , photoradiation [46] and additives [40j] , is currently

understood to be one of the general characteristics of helical polymers.

Molecule - based chiroptical properties, such as memory and the helicity switch

using the PM transition phenomenon in helical polymers, will be useful for data

storage, optical devices, chromatographic chiral separation and liquid crystals for

display [50] . In addition to memory and the helicity switch using the PM transition

phenomenon, chirality transfer and/or ampliﬁ cation in polymer systems have also

been studied extensively in aggregates, while the complexation of achiral polymers

with chiral additives has been investigated in solution systems [40j, 51, 52] .

In this section, attention is focused on the fabrication of helical polymer - based

solid ﬁ lms as supramolecular assemblies that possess functionality based on chi-

roptical properties, such as switch [30, 53] , memory [30] , transfer, and ampliﬁ ca-

tion [54] . A focus on these chiroptical properties, either in bulk thin ﬁ lm or in a

polymer matrix, is required from a practical viewpoint [48, 55] . In the present

system, a temperature control is used as the “ trigger ” to realize the memory,

178 5 Helical Polymer-Based Supramolecular Films

switch, and chirality transfer/ampliﬁ cation. Those chiroptical properties can be

detected by using circular polarized light ( CPL ), which is able to distinguish

between right - or left - handed polarity (helicity), thus highlighting their potential

application as circular polarization recording/erasing systems.

5.3.1

Chiroptical Memory and Switch in Helical Polysilane Films

The PM transition phenomenon in helical polymer ﬁ lms is primarily applied for

molecular - based chiroptical memory and switches. In general, there are two types

of memorizing system which function by means of a PM transition phenomenon

in the solid state. One system is in the Re - Writable ( RW ) mode, while the other

is in the Write - Once Read - Many ( WORM ) mode. In the RW mode, the right - or

left - handed helicity can be erased when data are renewed. In contrast, in the

WORM mode the right - or left - handed helicity is immobilized once for the storage

of data, and these are then read (detected) using CPL.

The basic requirement for a switching system is the bistable state of a molecule,

which can be assigned as either the chiroptical inversion “ − 1 and +1 ” switch

through the helix – helix ( PM ) transition or the on - off “ 0 and +1 ” switch through

the helix – coil transition. Normally, these chiroptical properties can be character-

ized by using circular dichroism ( CD ). The conformational transition temperature

( T

), such as for the helix – helix or helix – coil transition, is the critical factor when

designing the chiroptical memory and switch in the solid state. Since below T

the

segmental motion of the polymer chain is frozen, polymers having relatively low

values may represent good candidates for practical application.

Polysilanes possessing relevant enantiopure chiral side chains will be successful

candidates for chiroptical memory and switches at the molecular level. Further-

more, the T

of polysilanes in the solid state can easily be designed by adjusting

the molecular mass.

In this section, we present some examples of easy, versatile approaches for

chiroptical memory with RW and WORM modes, and chiroptical switches based

on the inversion “ − 1 ” and “ +1 ” and on - off “ 0 ” and “ +1 ” switch in solid ﬁ lms.

Certain polysilanes, including poly{( S ) - 3,7 - dimethyloctyl - 3 - methylpenthylsilane}

( 2 ) and poly{( S ) - 3,7 - dimethyloctyl - n - propylsilane} ( 7 ), were used, each of which

can undergo helix – helix or helix – coil transitions at a certain temperature in dilute

solution states by controlling their molecular mass and/or thermal modulation

(Scheme 5.4 ).

5.3.1.1 Memory with Re - Writable Mode and Inversion “ − 1 ” and “ +1 ” Switch

Polymers 2 and 3 exhibited a strong molecular mass dependence on the phase

transition temperature T

(estimated using differential scanning calorimetry ; DSC )

due to the PM transition, as shown in Figure 5.14 . The enthalpy change ( Δ H )

estimated from the heating was only approximately 0.05 kcal mol

− 1

per repeat unit

[15a] . Given that the helix – helix transition of polysilane is caused by the order –

disorder transition of side chain groups in solution, the entropy gain based on the

5.3 Helical Polymer-Based Functional Films 179

order – disorder transition of side chains could easily overcome the tiny amount of

enthalpy, thus paving the way for the induction of a helix – coil transition, even in

the solid state.

For the cast ﬁ lm of 2 onto quartz substrate comprising the low - molecular - mass

fraction ( M

: 1.3 × 1 0

, M

/ M

: 1.16, corresponding to region I in Figure 5.14 ), a

typical bisignate CD signal based on exciton coupling was clearly observed, and

seen reversibly to switch between almost mirror - imaged CD spectra in the heat-

ing – cooling cycles (see Figure 5.15 ).

In this case, T

was approximately 47 ° C, which was estimated from the tem-

perature dependence of Kuhn ’ s dissymmetry ratio ( g

solid

= Δ OD / OD ), where OD is

the optical density of the UV absorbance at λ

max

and Δ OD / OD is deﬁ ned as

2( OD

− OD

)/( OD

+ OD

) of the CD intensity at the extremum ( λ

ext

) of the ﬁ rst

Cotton band, as shown in Figure 5.16 . The transition temperature estimated from

CD almost corresponded to that estimated by DSC.

Figure 5.14 Change of transition temperature

( T

) estimated from DSC thermograms as a

logarithmic function of molecular mass. Filled

and open circles indicate 2 and 3 , respectively.

Regions I, II, and III indicate reversible,

irreversible, and no change of CD signals for

the cast ﬁ lms in the heating and cooling

process, respectively.

Scheme 5.4 Chemical structures of optically active poly{( S ) - 3,7 -

dimethyloctyl - n - propylsilane} 7 , which can undergo a helix – coil transition at

80 ° C ( Δ ε / ε = 0 at 80 ° C) in iso - octane.

180 5 Helical Polymer-Based Supramolecular Films

In this case, the phase transition temperature in the ﬁ lm state seemed to be

approximately 60 ° C higher than the PM transition temperature in iso - octane. A

higher transition energy might be required to induce the phase transition in the

solid state, and therefore the phase transition would originate from a macroscopic

geometric change, such as a twisted superhelical structure, among the polymeric

backbones based on exciton coupling in the ﬁ lms [1b, 56] . The reversible switch

occurs between the laevo and dextro helical polymer shapes, which is followed by

changes in the helical supramolecular geometry, as observed in the CD spectrum

and illustrated in Figure 5.17 . The molecular mass control alone is sufﬁ cient to

change the transition temperature of 2 in region I of Figure 5.14 . Moreover, the

chiroptical property of the inversion “ − 1 ” and “ +1 ” switch can make it possible to

obtain chiroptical memory with RW mode.

Chiroptical memory with RW mode can be achieved by controlling the cooling

rate of the ﬁ lm with a low molecular mass. Figure 5.18 shows the multiple thermal

cycle responses of the CD intensities at the extrema for the cast ﬁ lm of 2 . Cycling

Figure 5.16 Temperature dependence of relative Kuhn ’ s

dissymmetry ratios Δ OD/OD for the cast ﬁ lm of 2 in the

heating (

䊉

) and cooling (

䊊

) processes. M

= 1.3 × 1 0

/ M

= 1.16. The heating rate was 20 ° C min

− 1

, and the

cooling rate 2 ° C min

− 1

Figure 5.15 Ultraviolet (UV) and CD spectra of the solid ﬁ lm

of 2 in the heating process.

5.3 Helical Polymer-Based Functional Films 181

was conducted between by heating above the T

and by slow cooling or rapid

quenching below the T

. In the slow cooling process, the sign of the CD signal

changed, indicating a chiroptical inversion “ − 1 ” and “ +1 ” switch. On the other

hand, a chiroptical memory with RW mode occurred during the rapid quenching

from above the T

. However, the sign of the CD signal did not change during the

rapid quenching. This memory effect was resettable by heating to above the T

The chiroptical inversion “ − 1 ” and “ +1 ” switch and chiroptical memory with RW

mode are feasible by controlling both the cooling conditions in the solid ﬁ lm and

the molecular mass of 2 .

Figure 5.17 Schematic representation of helical

supramolecular geometry switch on the basis of the

helix – helix transition of 2 .

Figure 5.18 Thermal cycle responses of

extrema of CD intensities for the solid ﬁ lm of

3 . Thermal cycles were conducted by heating

to 60 ° C (

䊉

) and followed by slow (

䊊

) or

rapid (shaded circle) cooling to 30 ° C. Rapid

cooling was achieved by dipping the ﬁ lm

into the ice water. The heating rate was

20 ° C min

− 1

, and the slow cooling rate

2 ° C min

− 1

. M

= 1.3 × 1 0

, M

/ M

= 1.16.

182 5 Helical Polymer-Based Supramolecular Films

5.3.1.2 Memory with Write - Once Read - Many ( WORM ) Mode

Another memory state, the WORM mode, can be achieved by controlling the

molecular mass only, although in this case the management of cooling conditions

is not required. With regards to the cast ﬁ lms in region II in Figure 5.14 , the phase

transition was seen only on heating, and the state above the T

remained unchanged

during the cooling process. This irreversible transition behavior can be conﬁ rmed

by the g

solid

- value (deﬁ ned as Δ OD / OD ), as shown in Figure 5.19 . This irreversible

change in the CD signal indicates the nonerasable memory as the WORM mode.

It should be noted that the transition temperature of the WORM mode in region

II of Figure 5.14 is also adjustable by controlling the molecular mass.

The dynamic chiroptical properties of polysilanes in the solid state, such as

inversion switching and memory with RW and WORM modes, can be managed

by a combination of molecular mass control and thermal modulation.

5.3.1.3 On - Off “ 0 ” and “ +1 ” Switch Based on Helix – Coil Transition

As noted above, the basic requirement for a switching system is the bistable state

of a molecule, which can be rapidly interconverted by an external stimulus, even

in the solid state. In addition to the achievement of a chiroptical inversion “ − 1 and

+1 ” switch in polymer 2 and 3 solid ﬁ lms by means of molecular mass control

and thermal modulation, an on - off “ 0 and +1 ” switch, based on changes in the

magnitude of the helicity in polymer 7 ﬁ lm, can also be achieved by thermal

modulation. The DSC thermogram of 7 displayed a second - order transition at

− 47 ° C and a ﬁ rst - order transition at − 8 ° C corresponding to the glass and helix – coil

transitions, respectively, as shown in Figure 5.20 . The heat ﬂ ow ( Δ H is

∼ 0.004 kcal mol

− 1

per repeat unit) for the helix – coil transition in 7 was relatively

small, with only weak van der Waals interactions likely to permit the helix – coil

transition, even in the solid state (this is the same as the previously discussed case

in Section 5.3.1.1 ).

Figure 5.19 Temperature dependence of relative Kuhn ’ s

dissymmetry ratios Δ OD/OD for the cast ﬁ lm of 2 in the

heating (

䊉

) and cooling (

䊊

) processes. M

= 2.5 × 1 0

/ M

= 1.25.

5.3 Helical Polymer-Based Functional Films 183

Figure 5.21 shows the UV and CD spectra of the 7 ﬁ lm cast from iso - octane

onto a quartz substrate. Although there was no detectable CD absorption at 20 ° C,

7 showed a strong Cotton effect at − 40 ° C, indicating a coil - to - helix transition of 7

in the solid state with decreasing temperature. This transition can be well corre-

lated with DSC results. The positive Cotton effect could result from a single

polymer chain because of the similarity between the UV and CD absorption pro-

ﬁ les. Furthermore, no bisignate CD signal attributable to exciton coupling was

observed, even at − 60 ° C, which is completely different from the cases of the previ-

ous Sections 5.3.1.1 and 5.3.1.2 . Although 7 showed a strong Cotton effect at low

temperatures both in solution and in the solid state, the global conformations of

polymer chains in both states may be signiﬁ cantly different. Due to the excluded

Figure 5.20 DSC thermogram of 7 obtained on second

heating with a heating rate of 10 ° C min

− 1

Figure 5.21 Ultraviolet (UV) and CD spectra of the solid ﬁ lm

of 7 . M

= 330 000, M

/ M

= 1.87.

184 5 Helical Polymer-Based Supramolecular Films

volume of other chains and the chain entanglements, the entire conformation of

the polymer chains in the solid state would be difﬁ cult to change with decreasing

temperature, even above their glass transition temperature ( T

). These results

clearly indicate that the macroscopic geometric changes in the inversion “ − 1 ” and

“ +1 ” switch and the on - off “ 0 ” and “ +1 ” switch are different.

The helix – coil transition in the thin solid ﬁ lm can be explained by some param-

eters, such as fwhm , λ

max

and Δ OD/OD , as shown in Figure 5.22 . The fwhm of the

UV absorption band became narrower when the temperature decreased from 20

to − 40 ° C and remained constant below − 40 ° C (around T

), indicating that the

homogeneity of the polymer backbone was induced due to the decreasing degree

of freedom in the Si – Si bond rotations. The λ

max

shifted slightly to a longer wave-

length with decreasing temperature from 20 to − 20 ° C, and shifted to a shorter

wavelength below − 20 ° C. It is likely that the chiroptical property in the solid state

would be induced through the movement and/or disappearance of the helical

reversal within the localized segments, which is in stark contrast to that of the

solution state.

Figure 5.23 shows the multiple thermal cycle responses of Δ OD/OD in the range

from − 20 to 20 ° C. The intensity changed during heating and cooling, indicating

a switching property. The helix – coil transition in the solid state is highly reversible,

can be applicable to the on - off switching system, and is assigned as “ 0 and +1 ” .

Figure 5.22 Temperature dependence of relative Kuhn ’ s

dissymmetry ratios Δ OD/OD , λ

max

, and fwhm of the solid ﬁ lm

of 7 . M

= 330 000, M

/ M

= 1.87.