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

11.8 Conclusions 375

The thermal barrier of 9 – 14 kJ mol

− 1

was attributed to the energy required to

bridge the gap between the Stokes shifted exciton - donating

4

T

1

(

4

G) state of [MnX

4

]

2 −

anions and the energy - accepting

4

T

1

(

4

G) levels of the [MnX

4

]

2 −

neighbors, this

being a key requirement for the energy migration process over the [MnCl

4

]

2+

sub-

lattice. The difference between the absorption and emission energies was about

1060 cm

− 1

(or 12.7 kJ mol

− 1

), which was in good agreement with the measured

thermal barriers of 9 – 14 kJ mol

− 1

. The higher thermal barrier of 30 – 50 kJ mol

− 1

was

attributed to the energy required for the onset of the 18C6 rotation – conformation

reorientation. These reorientation motions can generate a dynamic electric ﬁ eld

at the Mn

2+

centers, thereby enhancing the electronic transition involved in the

energy transfer process from Mn

2+

to defect sites. The measured thermal barriers

of 30 – 50 kJ mol

− 1

were in good agreement with the Waugh – Fedin approximation

for hindered solid - state orientations within the assumptions made in deriving the

relationship [16, 25] .

The cross - polarization - magic angle spinning ( CP - MAS )

13

C NMR spectrum

of the [(K(18C6))

4

(ZnBr

4

)] [TlBr

4

]

2

· n H

2

O compound showed one – CH

2

– resonance

instead of the pair expected to result from the differential shielding of the up -

and - down – CH

2

– functionalities of 18C6 in D

3d

symmetry [16] . The single reso-

nance is consistent with – CH

2

– positions being equilibrated by the rotation –

conformational reorientation of the 18C6 ring.

This conclusion is supported by a large body of solid - state NMR data on the

compounds of 18C6 and other crown ether ligands in a variety of compounds, and

is consistent with the merry - go - round 18C6 motion proposed by Buchanan, Rat-

cliff et al. [26] and supported by Dye et al. [27] , to account for unusual solid - state

NMR behavior. Recent detailed magnetic susceptibility, solid - state NMR and heat

capacity measurements on 18C6 sandwich complexes of cesium, [(18C6)Cs(18C6)

Cs(18C6)]

+

with magnetic [Ni(dmit

2

)]

−

identiﬁ ed 18C6 in these compounds as a

rotor (Figure 11.10 ), the onset motion of which has inﬂ uence on the physical

characteristics of the compounds, including the magnetic behavior of the

[Ni(dmit

2

)]

−

anion [28, 29] (Figure 11.9 ).

11.8

Conclusions

The [(A(18C6))

4

(MX

4

)] [BX

4

]

2

· n H

2

O supramolecular systems have the interesting

properties of concentrating and immobilizing toxic metal ions such as thallium(I/

III), and are accessible via a relatively simple synthetic procedure. The

[(A(18C6))

4

(MnX

4

)] [BX

4

]

2

· n H

2

O compounds hold a total of six thallium ions – four

as [Tl(18C6)]

+

and two as [TlX

4

]

−

. The structural, luminescence, and CP - MAS

13

C

NMR spectroscopic evidence for 18C6 mechanical activity in the [(A(18C6))

4

(MnX

4

)]

[BX

4

]

2

· n H

2

O compounds is compelling. Indeed, when taken together with pub-

lished reports on heat capacity, magnetic susceptibility and NMR evidence for such

mechanical activity in a variety of crown ether compounds, the possibility of devel-

oping potentially useful nanodevices becomes of major interest. For example, the

376 11 Self-Assembling Nanoclusters Based on Tetrahalometallate Anions

Figure 11.10 (a) Solid - state

1

H NMR, (b) heat capacity,

(c) magnetic susceptibility, and (d) crystal structural

parameters of compounds of [(18C6)Cs(18C6)Cs(18C6)]

+

with

magnetic [Ni(dmit

2

)]

−

anions, showing behavioral changes at

approximately 150 – 200 K in all cases [28] .

changes in physical behavior associated with the onset of rotation – conformation

reorientation motion could be used in the form of switches. Indeed, new oppor-

tunities for fabricating mechanical nanodevices in which the 18C6 chelate per-

forms useful work as it rotates with – CH

2

– and – O – moieties ﬂ ipping up and down

can be envisaged. In this regard, it is noteworthy that the behavior of compounds

of HM18C6 and 18C6 were similar, as studying the behavior of other substituted

18C6 molecules is important in the search for nanodevices based on the 18C6

motion.

The remarkable sensitivity of Mn

2+

emission in detecting molecular motion

within its environments is also of great interest. It is possible that the defect

sites which dominated the emission behavior of [(A(18C6))

4

(MnX

4

)] [BX

4

]

2

· n H

2

O

compounds were of manganese(II), in unusual coordination environments. The

sensitivity of Mn

2+

to changing coordination and supramolecular environments

was convincingly demonstrated by comparing the luminescence spectroscopy

(Figure 11.11 ) and decay dynamics of compounds featuring Mn

2+

in coordination

numbers 4, 6, 7, and 8. The potential of Mn

2+

as a luminescence probe for

structural and supramolecular environmental deserves further development.

References 377

Figure 11.11 Manganese(II) luminescence spectroscopy as a structural probe [19] .

Acknowledgments

Contributions made by coauthors of the original publications are greatly appreci-

ated, and especially those of Drs H.O. Reid and N.S. Fender as former students.

These studies were funded by the UWI, the Inter - American Development Bank

( IADB ), and the US NSF.

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and McPherson , G.L. ( 2001 ) Acta

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J. Chem. Phys. , 123 , 044514 .

Optically Responsive Polymer Nanocomposites Containing

Organic Functional Chromophores and Metal Nanostructures

Andrea Pucci , Giacomo Ruggeri , and Francesco Ciardelli

379

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

12

12.1

Introduction

Responsive “ smart ” materials are certainly one of the most intriguing research

areas in modern polymer science and technology, as molecularly designed materi-

als based on macromolecules offer unique opportunities in this connection.

Indeed, macromolecules are able to transmit and amplify small signals through

involvement of the whole chain, conferring to the material a change in properties

of various level and type. We can learn from Nature how these effects may be

obtained according to two distinct routes, based either on the covalent bonding of

highly responsive molecular species to the chains, or on the nanodispersion of

such responsive species within the polymer bulk. In the latter case, those materials

based on macromolecules may also affect the behavior of guest low - molecular -

weight molecules or noble metal assemblies. In addition to the molecular features

of both host and guest, the supramolecular arrangement of the guest can be

modulated by external events on the host material. These effects can be identiﬁ ed

and conveniently used when easily detected and ﬁ eld - sensitive species are present

in the low - molecular - weight component. In this chapter, we review those investi-

gations conducted not only in our laboratory but also by others, that have provided

clear examples of the concepts which drive the original idea. In particular, we

report our data relating to the effects of external stimuli (i.e., mechanical stretching

such as polymer drawing, temperature, and pressure) on the optical properties

(absorption and emission) of (nano)composite materials. The (nano)dispersion of

active dyes and metal nanoparticles (guest) in inert polymers (host) will then be

presented as an example of the effect of the macromolecular environment on

respectively the dye or metal atom aggregates (metal nanoparticles), in terms of

induced optical properties.

380 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

12.2

Organic Chromophores as the Dispersed Phase

12.2.1

Nature of the Organic Dye

Highly sensitive optical techniques based on the luminescence of conjugated

aromatic molecular additives dispersed at low concentration (e.g., in Scheme 12.2 ,

less than 1 – 2 wt%) into the amorphous phase of thermoplastic polymers have been

successfully applied to the detection of thermal and mechanical solicitations on

plastic ﬁ lms. In fact, a growing interest was devoted to the optical characteristics

of macromolecules due to the sensitive response of the photophysical techniques

for the study of the dynamic physical properties of macromolecules (i.e., energy

transfer, polarization, and trapping phenomena). For example, when luminescent

dyes are incorporated into polymers as thermodynamically stable micro - /nano-

sized aggregates of a few molecules, emission characteristics are observed which

derive mainly from the ﬂ uorescence of interacting chromophores through π – π

stacking interactions among the planar aromatic backbones. Interactions between

the excited state of an aromatic molecule and the ground state of the same mol-

ecule actually give rise to the excimer (excited dimer) formation, which represent

a powerful diagnostic tool for interacting chromophores (Scheme 12.1 ) [1 – 3] .

For small molecules separations ( r < 4 Å ), with interactions between ground -

state and excited - state molecules, an attractive potential may be obtained due

to the conﬁ gurational interaction between resonance and exciton – resonance

states. According to Scheme 12.1 , ﬂ uorescence from the excimer state will

thus be unstructured and at a lower energy than the corresponding monomer

emission [2] .

Scheme 12.1 Energy diagram for excimer formation.

M = molecule in the ground state; M * = molecule in the

excited state; r = distance between molecules.

12.2 Organic Chromophores as the Dispersed Phase 381

Comparisons between the signal intensities of monomeric ﬂ uorescence (deﬁ ned

as the contribution of an isolated chromophore covalently attached to the polymer

chain or dispersed herein) and the excimer contribution are used very efﬁ ciently

to obtain accurate information not only on polymer structure and conformation

[4 – 9] but also on mixing at the molecular level in polymer blends [9 – 16] . Excimer

formation in polymer solutions is widely considered to be a diffusion - controlled

process, and is inﬂ uenced by a variety of factors including the solvent, the chromo-

phore microstructure, and the macromolecular conformation [2, 17, 18] . In con-

trast, in polymer ﬁ lms the formation of aggregates or excimers (static excimers)

arises from the structural constraints of the polymer chains. In this case, the static

excimers react very sensitively to chromophore aggregation and, as might be

expected, also to any variations in the spatial distribution and alignment of

molecules within the local environment [19 – 23] .

12.2.2

Polymeric Indicators to Mechanical Stress

12.2.2.1 Oligo( p - Phenylene Vinylene) as Luminescent Dyes

Recently, the possibility of applying the formation of excimers inside polymer

matrices, in order to prepare polymers with “ built - in ” stress – strain deformation

sensors, has been effectively demonstrated [24 – 27] . Weder et al. reported that small

amounts (0.01 – 3 wt%) of excimer - forming oligo( p - phenylene vinylene) synthetic

chromophores (Scheme 12.2 , 3 ) dispersed into a ductile host polymer matrix (e.g.,

linear low - density polyethylene , LLDPE ) as very small (nano)aggregates of dyes

may be produced either by guest diffusion or by processing the components in

the melt. The authors demonstrated that the phase behavior of the LLDPE/dye

blends depended strictly on the supramolecular structure of the dye, which may

be easily tuned as a function of the processing conditions and chromophore con-

centration. On applying a mechanical deformation of the ﬁ lm, a shear - induced

mixing between the two phases promoted the break up of the dye ’ s supramolecular

Scheme 12.2 Examples of excimer - forming dyes: 1 , stilbene;

2 , perylene; 3 , cyano - oligo( p - phenylene vinylene); 4 ,

anthracene diimide derivatives. R = alkyl groups, Ar = aryl

groups; Ph = Phenyl.

382 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

structure, and led to a mixing of the two components (polymer and dye) and a

change in the material ’ s emission properties, from orange - red excimers to the

molecularly dissolved green monomers of the dye (Figure 12.1 ) [26, 27] .

According to Weder et al. [28] , the extent of the color change observed upon

deformation, and thus the ability of the polymer host to break up the dye aggre-

gates, is inﬂ uenced primarily by three parameters: dye aggregate size; polymer

crystallinity; and strain rate. The nucleation rate of dye aggregates, and conse-

quently the size of the aggregates, can be controlled via the structure of the groups

attached to the dye. That is, long aliphatic alkyl groups lead to much higher nuclea-

tion rates, which in turn promotes the formation of small aggregates. Most impor-

tantly, the mechanically induced dispersion of the excimer - forming sensor

molecules upon deformation is considered to be related to the plastic deformation

process of the polyethylene ( PE ) crystallites, and thus increases with increasing

polymer crystallinity.

In order to determine whether the sensing scheme could be exploited also for

elastomers (which change their luminescent color reversibly as a function of the

applied strain), Weder et al. recently investigated different polyurethanes that

comprised cyano - oligo( p - phenylene vinylene) s ( cyano - OPV s) as built - in deforma-

tion sensors [29] . The covalent integration of cyano - OPVs into the backbone of the

polyurethane led to materials in which large - scale phase separation of the dye was

prevented, while the formation of a large population of (presumably very small)

ground - state dye aggregates was still allowed. Consequently, these polymers

Figure 12.1 Emission spectra of a 0.20 wt% linear low - density

polyethylene/1,4 - bis( α - cyano - 4 - methoxystyryl) - 2,5 -

dimethoxybenzene (inset) blend as a function of draw ratio,

and image of the same blend after orientation (inset, draw

ratio = 500%, λ

exc.

= 365 nm). Adapted with permission from

12.2 Organic Chromophores as the Dispersed Phase 383

displayed a predominantly excimer emission in the unstretched state, and exhib-

ited a pronounced ﬂ uorescence color change upon deformation. The optical

change appeared mostly reversible, and reﬂ ected the stress – strain behavior of the

polymeric material (Figure 12.2 ).

Recently, properly substituted cyano - OPVs were also discovered as a new class

of piezochromic material [30] . Liquid - crystalline cyano - OPV dyes showed emission

properties which could be switched reversibly and repeatedly from monomer to

excimer ﬂ uorescence upon compression or quenching of the compounds from a

nematic state. In particular, if the powder was either brieﬂ y compressed (1 min at

1500 p.s.i. in an IR pellet press) or ground using a mortar and pestle, then a sig-

niﬁ cant bathochromic shift ( ≥ 50 nm) occurred and the emission band was seen

to broaden. Once compressed, the “ excimer form ” (turquoise emission) was found

to be stable for months if stored under ambient conditions. Rapid heating

(ca. 3 min) of the compressed material to 130 ° C fully restored the original

“ monomer ” form (green emission).

12.2.2.2 Bis(Benzoxazolyl) Stilbene as a Luminescent Dye

A similar approach to the preparation of stress – strain polymeric indicators was

reported by Pucci et al. , who developed polypropylene ( PP ) ﬁ lms that contained

different concentrations of the food - grade, luminescent dye bis(benzoxazolyl)

stilbene ( BBS ) (Scheme 12.2 , 1 ) [31] . Notably, it was clear that the emission

Figure 12.2 Ratio of monomer to excimer emission, I

M

/ I

E

(circles, measured at 540 and 650 nm), and tensile stress

(solid line) for a covalented incorporation of cyano - oligo( p -

phenylene vinylene)s in polyurethane under a triangular strain

cycle between 100% and 500% at a frequency of 0.0125 Hz.

American Chemical Society.

384 12 Optically Responsive Polymer Nanocomposites Containing Organic Functional Chromophores

characteristics of the PP ﬁ lms depended on the BBS concentration and the polymer

deformation. A well - deﬁ ned excimer band was observed with more than 0.2 wt%

of BBS, and this conferred to the ﬁ lm a green luminescence. During drawing

(130 ° C), the PP reorganization caused a break in the BBS excimer - type arrange-

ment, leading to a prevalent blue emission of the single molecules.

In a subsequent phase, the same formulation was obtained by continuous com-

pounding of the components mixture in a co - rotating, twin - screw extruder with a

screw diameter of 24 mm and a length - to - diameter ( L/D ) ratio of 40 [32] . The ﬁ lms

obtained through off - line melt - compression molding conﬁ rmed the optical respon-

sive properties demonstrated by laboratory - scale, batch - compounded materials

(Figure 12.3 ).

Intelligent ﬁ lms from thermoplastic materials, based on excimer luminescence

and responsiveness to mechanical stress, were also obtained through the disper-

sion (melt - processing) of moderate amounts (0.02 – 0.2 wt%) of BBS into a thermo-

plastic, aliphatic, biodegradable polyester [ poly(1,4 - butylene succinate) , PBS ]

(Figure 12.4 ) [33] .

As reported for PP blends, the PBS morphology reorganization occurred during

the drawing process, thus breaking the BBS excimer - type arrangement and leading

to a prevalence of blue emission for the single molecules.

12.2.2.3 Perylene Derivatives as Luminescent Dyes

Very recently, analogous thermoplastic ﬁ lms sensitive to mechanical stress were

prepared by using a different luminescent probe based on the perylene core

(Scheme 12.2 , 2 ) [34] . Perylene dyes containing different peripheral alkyl chains

were synthesized and efﬁ ciently dispersed at low loadings (from 0.01 to 0.1 wt%)

into LLDPE by processing in the melt. Both perylene bisimides were found to

generate supramolecular aggregates promoted by π – π intermolecular interactions

between the conjugated planar structure of the dyes, as indicated by spectroscopic

investigations on heptane solutions or dispersions into the LLDPE polymer matrix.

The occurrence of this phenomenon effectively changed the emission of the dyes

from yellow - green (noninteracting dyes) to red (interacting dyes). In particular,

Figure 12.3 Partially stretched linear low - density polyethylene

rods prepared in a corotating twin screw extruder (THERMO,

Polylab) at a ﬁ xed barrel temperature of 180 ° C for ﬁ rst zone

and 200 ° C for the following zones, and with screw speed ﬁ xed

at 50 r.p.m. with increasing (from top to bottom) amounts of

BBS (from 0.2 to 0.5 wt%), examined under UV light.

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

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