weak noncovalent si···fc interactions stabilized fluoroalkylated rod-like polysilanes as...

Weak Noncovalent Si��F–C InteractionsStabilized Fluoroalkylated Rod-Like PolysilanesAs Ultrasensitive Chemosensors

ANUBHAV SAXENA, ROOPALI RAI, SUN-YOUNG KIM, MICHIYA FUJIKI,MASANOBU NAITO, KENTO OKOSHI, GISEOP KWAK

Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0101, Japan

Received 29 September 2005; accepted 3 February 2006DOI: 10.1002/pola.21607Published online in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Noncovalent interactions, such as hydrogen bonding, metal coordination,and p-p stacking, are increasingly being utilized to develop well-ordered and self-organized supramolecular materials. Recently, new types of nonclassical weak inter-actions, such as C–H��p, C–H��F–C, and C–H��O, have been exploited in stabilizingthe specific conformations of molecules and molecular assemblies in the solid state.These noncovalent interactions play an important role in materials comprised of poly-mer chains, because cooperative effects from a large number of weak interactions canlead to drastic changes in its conformation, several properties, and functionalities.The programmed design of synthetic helical polymer with well-defined molecular con-formation has been the main subject in modern polymer science and engineering. Sil-icon-catenated polysilane is an ideal helical silicon quantum wire and polymers withunique photophysical properties. The present review highlights the spectroscopic evi-dences for through-space weak Si��F–C interaction in poly(methyl-3,3,3-trifluoropro-pylsilane) (1) in noncoordinating and coordinating solvents by means of NMR (29Siand 19F) and IR spectroscopies, and viscometric measurement. It was found that 1 isapplicable for chemosensors with an extremely high sensitivity and selectivity towardfluoride ions in tetrahydrofuran (THF) and with high sensitivity for nitroaromaticcompounds, detected by a decrease in the photoluminescence intensity in THF and inthin solid film. VVC 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5060–5075,

2006

Keywords: NMR; IR; polysilanes; fluoropolymers; sensors; fluoropolymers

INTRODUCTION

Weak nonbonding interactions play a vital role inmany forefront areas of modern chemistry frommolecular biology to material design.1–6 Suchnoncovalent interactions are ubiquitous in biolog-ical world and control many biological and physi-cal phenomena, such as molecular recognition,conformational transformation, and molecular

packing in crystals. The weaker and kineticallylabile noncovalent interactions are used to organ-ize and hold together the secondary and higherorder structure of biomacromolecules. Inspiredby this elegant bottom up fabrication in nature,the self-organization of synthetic molecules hasbeen widely exploited to generate novel func-tional supramolecular materials.7–16

Noncovalent bonds are generally 1–3 orders ofmagnitude weaker than covalent bonds; however,the strength of noncovalent interactions lies inits cooperativity. The cooperative effects of sev-eral noncovalent interactions allow the formation

Correspondence to: F. Fujiki (E-mail: [email protected])

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 44, 5060–5075 (2006)VVC 2006 Wiley Periodicals, Inc.

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of large specific association whose affinity may beof same order of magnitude as a covalent bond.Noncovalent chemistry offers diversity whencompared with covalent chemistry, by allowingthe programming of several interactions and ena-bling self-assembly of many components. Non-covalent synthesis is spontaneous and reversiblewithout any undesired side products when com-pared with that of the covalent one. Further-more, it does not require chemical reagents (e.g.,catalyst) or harsh conditions.

Recently, weak hydrogen bonding interactions[C–H��X (X¼¼O, F)] have been investigated insynthetic molecules to establish its role in biolog-ical systems. The role of C–H��O hydrogen bondshas been found in stabilizing the structure of pro-teins.17–19 It has also been shown that thestrength of C–H��O weak interactions can beenhanced in protonated clusters or molecularaggregates containing charges.20–22 The struc-tural contribution of C–H��O bonds is still notclear, while the C–H��O interaction energy (2.5–3.0 kcal/mol) is reported to significantly providestability and specificity in transmembrane helixinteractions.23–26 Such weak hydrogen bondscoexist with conventional hydrogen bonds in bio-logical macromolecular structures, which makedifficult to realize the contribution from weakhydrogen bond experimentally.

Recently many other types of new weak inter-actions, such as C–H��p,27–31 N��O¼¼C,32 X��X(where X is Cl, Br, S, Se),33–37 C–H��N,38–41 X–H��p (X¼¼O, N),42–44 Se��X (X¼¼F, O),45 and {Si��X(X¼¼F, N, O)}46–51 have also been exploited in sta-bilizing the specific conformations of syntheticmolecules and molecular assemblies in the solidstate. The interaction energy of various typesof weak noncovalent interactions is compiled inTable 1.

The noncovalent interactions are also impor-tant in materials comprising polymer chains fordeveloping well-defined polymeric architecturewith controlled conformations and tailored proper-ties.55 Although relatively strong noncovalentinteractions, such as acid-base complexes,56,57

hydrogen bonding,58,59 and metal coordination60,61

have been exploited in synthetic polymers, theexamples of such weak and reversible noncovalentinteractions are rare in case of polymers.52,53,62

However, very weak interactions in polymers maybe amplified with cooperativity and eventuallycan lead to drastic changes in its conformation,several properties, and functionalities similar tovarious biopolymers in nature.63,64 Conversely,

amplified cooperative behavior in polymeric sys-tem might be applied to probe a very weak inter-action, which may be hard to detect if exist sepa-rately in small molecules.

The rod-like polysilanes bearing chiral or achi-ral side groups65–77 were classified as a new classof polysilanes, showing intense and sharp UVabsorption, circular dichroism, and photolumi-nescence (PL) spectra around 300–400 nm, dueto r-conjugation in the helical main chain. It maybe noted that the rod-like helical polysilanes mayserve as unique nanostructural and multidiscipli-nary class of polymer, since these polysilanes canact as semiconducting quantum silicon molecularwire with 0.2 nm silicon width.67,75 Such proper-ties of polysilanes strongly depend on global con-formation and molecular weight; therefore, con-trol of global conformation and molecular weightcontribute to the success of any method fordesigning helical polymers. The flexible nature ofSi–Si bonds in 1D semiconducting polysilanemakes its backbone conformation more prone tochemical as well as physical stimuli, giving riseto different chromotropic behavior, which are notassociated with its organic counterparts—thepolyolefins.78–82

Table 1. Classical and Nonclassical NoncovalentInteraction and Their Energies

Types ofForces

InteractionEnergy

(kcal/mol) References

van der WaalsCH2��CH2 1 38, 39

Intense hydrogen bondsF–H��F 7 38, 39O–H��O 3–8 38, 39O–H��N 7 38, 39N–H��F 5 38, 39N–H��N 2–4 39N–H��O 2 38, 39

Weak hydrogen bondsO–H��p 3 38, 39, 42–44C–H��p 1.5 27–31N–H��p 2 42–44C–H��O 1 32, 52C–H��F–C 0.5 53

Weak interactionsp��p 1.7 16C¼¼O��N 3.2 32S��S 0.5 34Se��F 0.8–1.2 45Se��O 5–20 45Si��F–C 0.001 54

FLUOROALKYLATED POLYSILANES 5061

Journal of Polymer Science: Part A: Polymer ChemistryDOI 10.1002/pola

Recently, we have demonstrated the spectro-scopic evidences for through-space weak Si��F-Cinteractions in r-conjugated rod-like fluoroalky-lated polysilanes, synthesized via sodium-medi-ated coupling reaction of 3,3,3-trifluoropropylal-kyldichlorosilane in n-octane (Scheme 1).54,83,84

For comparison, analogous dialkyl chiral polysi-lanes were also prepared. IR and multinuclear(1H, 13C, 19F, 29Si) NMR spectra of fluoroalkylatedpolysilanes suggest the formation of an idealizedstructure of polysilane. There was no evidence forthe presence of additional signals due to CF2¼¼CHmoiety in 1H and 19F NMR spectra, which wouldhave been associated with removal of HF from theside chain in the presence of sodium. This impliesthat CF2¼¼CH group is either not present or is toosmall to be detected by NMR spectroscopy.

SILICON-CATENATED ROD-LIKEFLUOROALKYLATED POLYSILANE

Silicon-catenated polysilanes adopt rod-like heli-cal conformation only in presence of specificsteric interactions—a long alkyl/aryl chain andbranching at the b- or c-position.74,75,85 It isknown that polymers with no chiral centers canalso adopt helical conformation with equal popu-lation of right- and left-handed screw senses inthe presence of sterically bulky substituents.55

Only few examples of synthetic polymers havebeen reported recently, wherein the helicity isintroduced by hydrogen bonding.58,59,86 In thisreview, we described a rational design of a rod-like helical poly(3,3,3-trifluoropropylmethylsi-lane) (1) stabilized by weak intramolecular coop-erative Si��F–C interactions.

The main chain mobility of r- and p-conjugatingpolymers is often connected to their electronic struc-tures. Therefore, changes in the UV, PL, and chirop-

tical spectra are spectroscopically discernible forconformational changes in polysilane. Figure 1shows the UV and PL spectra of 1 in toluene at25 8C. The UV spectrum showed a sharp absorptionband at 320 nm with a full-width at half maximumheight (fwhm) of 8.3 nm. The UVand PL (excited at315 nm) spectra were mirror images of each otherwith a very small Stoke’s shift of 8 nm. This resultindicates that fluoroalkylated polysilane 1 adopts 73rod-like helical architecture with equal populationof right- and left-handed screw-senses.66,78

This unique conformational behavior of 1 canbe explained by weak and cooperative noncova-lent Si ��F–C interactions operating between theSi atoms in the backbone and the F atoms in theside chains. Weak cooperative Si ��F–C interac-tions induce the rigidity in silicon-catenatedbackbone by locking the segmental motion of poly-mer backbone. On the other hand, the UV spec-trum of an analogous nonfluoroalkylpolysilane,poly(n-propylmethylsilane) (2), shows a broadabsorption at 305 nm with fwhm of 35 nm in tolu-ene at room temperature, indicating the randomcoiled global shape of 2.75,76

ROLE OF Si��F–C INTERACTIONSIN CONTROLLING THE GLOBALCONFORMATION OF FLUOROALKYLATEDPOLYSILANES

Weak noncovalent Si ��F–C interactions may existbetween the ith, (i + 1)th, or (i + 2)th Si atoms in

Scheme 1. Fluoroalkylated and their analogous non-fluoroalkylated polysilanes.

Figure 1. UV and PL spectra of 1 (Mw ¼ 4.1 � 105,PDI ¼ 1.5) in toluene at 25 8C. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

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the backbone and F atoms in the side chains toform pseudo five-, six-, or seven-membered rings.Among the various possibilities, we believe thatthe interactions of F atoms with the neighboringSi atoms (i + 1)th in the backbone may fix intothe 73 helical conformation of polysilane. Becausethe interaction of F atoms with ith Si atoms mayprovide the flexible backbone and with (i + 2)thSi atoms, a very tight helical conformation isexpected. Moreover, Si ��F–C interactionsbetween F atoms and (i + 1)th Si atoms may leadto sterically favored, more stable six-membered

ring. It is noteworthy that the importance of weakand reversible Si ��F–C interactions lies in control-ling the global conformation of the polysilane fromrod to globule by the appropriate choice of molecu-lar weight and solvent (Fig. 2).

Molecular Weight Chromism inPoly(trifluoropropylmethylsilane)

It is well known fact that worm-like polymer inlow molecular weight region attains rod-like con-formation, while turns to random coil conforma-

Figure 2. Schematic representations of conformations for 1 in noncoordinating(e.g., decane, toluene) and coordinating solvents (e.g., THF, DMF, benzotrifluoride).[Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]



tion in the high molecular weight range. Excep-tion to this general rule for worm-like polymer,Percec et al.87 reported recently that polymerwith rigid building block may adopt globule-likeconformation with low degree of polymerizationand rod-like conformation with high degree ofpolymerization. Interestingly, we have also re-cently observed an abnormal and unique effect ofmolecular weight on the conformation of 1.

The UV spectrum of 1 showed two absorptionbands at 285 and 320 nm in tetrahydrofuran(THF), as shown in Figure 3. The presence of twoabsorption bands in THF is surprising andbelieved to originate from two different global/local conformations existing in the same back-bone. The narrow 320-nm absorption is ascribedto the \locked" rod-like helical conformation of 1due to Si ��F–C backbone-side chain interactionand the broad 285-nm band is due to \unlocked"globule-like conformation, in which no interac-tion is present between the side chain and poly-mer backbone. This was further confirmed fromthe change in UVabsorption spectra of 1 with dif-ferent molecular weights in THF. Figure 3 showsthe clear isosbestic point, which also suggests the

equilibrium between globule- and rod-like confor-mation of polysilane.

Figure 3 shows the increase in absorption in-tensity of the narrow 320-nm band with increasein molecular weight of 1. A complete rod-likearchitecture was realized with high molecularweight polysilane (Mw ¼ 3.4 � 105, PDI ¼ 3.3)due to cooperative effect of Si ��F–C interactions,whereas UV spectrum of low molecular weightpolysilane (Mw ¼ 2.0 � 104, PDI ¼ 1.3) revealedonly the broad 285-nm band in THF, suggestingthe globule-like conformation.88 The magnitudeof weak Si ��F–C interaction in THF estimated tobe only ca. 1 cal/Si-repeating unit.

The remarkable molecular weight dependentchange in global conformation of 1 was also evi-dent from the Mark-Houwink-Sakurada plot.Figure 4 shows the plots of four different samplesof 1, which cover wide range of molecular weights(1.6 � 103 < Mw < 6.3 � 105). The slope (a) of theplots (log[g]/logMw) was evaluated to be 0.27 inthe lower molecular weight region (1.6 � 103

< Mw < 4.0 � 104), suggesting almost globule-like compact conformation of 1, while a was 0.81in the high molecular weight region (4.0 � 104

Figure 3. Change in the UV absorption spectra of 1 with molecular weight in THFat 25 8C. The weight-average molecular weights (Mw) were evaluated from GPC withpolystyrene standards. [Color figure can be viewed in the online issue, which is avail-able at www.interscience.wiley.com.]

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< Mw < 6.3 � 105), indicating semiflexible poly-mer to the contrary.75–77,85

Solvatochromism inPoly(trifluoropropylmethylsilane)

The global conformation of r-conjugated polysi-lanes is sensitive towards physical (e.g., tempera-ture, pressure, electric field) and chemical (e.g.,ions, solvent) stimuli due to the flexible nature ofpolysilane backbone. Among the various knownchromisms in polysilanes, the thermochromism hasbeen studied extensively. The examples of solvato-and iono-chromisms are very rare due to a limitednumber of available functional polysilanes.78,89–91

It is well known that the strengths of suchweak noncovalent interactions are very sensitiveto solvent polarity.92–95 The next question may behow the global conformation of Si ��F–C interac-tion stabilized fluoroalkylated polysilane changeswith solvent polarity? For this question, theauthors recently reported the notable solvato-chromism of 1 in noncoordinating and coordinat-ing solvents at room temperature.54 The UVspectra revealed blue shifts in kmax of 1 with 3,28, and 40 nm in chloroform, benzotrifluoride,and dimethylformamide (DMF), respectively,compared to that in toluene and decane (Fig. 5).As noted earlier, the two absorption bandsappeared at 285 and 320 nm in THF. The PLspectra (excited at 280 nm) of 1 in coordinating

solvents (e.g., DMF, benzotrifluoride etc.) showeda broad emission band at 335 nm with a verylarge Stoke’s shift of nearly 50 nm, indicatingglobule-like conformation of polysilane (Fig. 5).

It is expected that 1 attains globule-like con-formation in donor solvents, and rigid architec-ture in noncoordinating solvents (e.g., toluene,decane). The rigidity of polymer backbone is fur-ther evident from broad 29Si NMR linewidth(64 Hz) in toluene, suggesting the rigid nature ofpolysilane backbone. The motion of Si atoms isrestricted in the extended rod-like architecture,while the narrower linewidth (20 Hz) in DMFdenotes a flexible nature of the polymer.74–76 Thecoordinating nature of solvents is responsible forsolvatochromism, as such noncovalent Si ��F–Cinteractions are competitive in the presence ofsolvents with the donor atoms (Fig. 2).46,47

To obtain insight of nature of Si ��F–C interac-tion, we prepared another fluoroalkylated polysi-lane, poly(nonafluorohexylmethylsilane) (3) andits analogous nonfluoroalkylated poly(n-hexylme-thylsilane) (4) (Scheme 1).84,85 UV spectrum of 3in toluene shows an intense absorption band at320 nm. The UV and PL spectra are mirrorimages of each other, suggesting the rod-like na-ture of silicon backbone. In contrast to 1, UV andPL spectra of 3 reveals narrow absorption andemission bands in coordinating solvents evenwith low molecular weight samples. Presumably,the Si ��F–C interaction in 3 is much stable whencompared with 1, responsible for fixing the rod-like conformation. As expected similar to 2, UVspectrum of 4 reveals a broad absorption banddue to globule-like conformation.

SPECTROSCOPIC EVIDENCE FOR WEAKSi��F–C INTERACTION

Contrarily to carbon, silicon has an obvious tend-ency to increase its coordination number. The va-riety of organosilicon compounds with penta- andhexa-coordinated silicon centers are known andwell characterized by X-ray and NMR spectrome-try.96–98 Recently, pseudo coordinated organosili-con compounds (Si ��X; where X ¼ N, F, O) havealso been reported for stabilizing the particularconformation of molecule and have potential tobe exploited for the strategic design of new mo-lecular, supramolecular, and polymeric architec-tures. The characterization of such noncovalentweak interactions spectroscopically are, however,still a challenging issue. In the present case, we

Figure 4. Mark-Hauwink-Sakurada plots of 1 over awide range of molecular weight (1.58 � 103 < M < 6.30� 105) by GPC-VISCO technique.



have well-characterized weak Si ��F–C interac-tion by IR and NMR spectroscopies.

Evidence of Weak Si��F–C Interactionsby Infrared Spectroscopy

As mentioned earlier, the rod- and globule-like con-formations coexist in the THF solution of 1. Figure6 shows IR spectra of cast films from THF and tolu-ene solutions of 1 (Mw ¼ 1.0 � 105, PDI ¼ 2.6). IRspectrum of cast film from THF solution is highlyinformative revealing two symmetric C–F stretch-ing bands at 1199 and 1211 cm�1 [Fig. 6(a)].99 The1211 cm�1 band is probably due to the formation ofpseudo five-, six-, or seven-membered ring arisingfrom intramolecular or intermolecular Si ��F–C

interaction operating between side chain and back-bone and the 1199 cm�1 band from linear side chainhaving no interaction between main chain and sidechain. This is further evident from the IR spectrumof polysilane casting from toluene solution. Thespectrum revealed only the 1211 cm�1 band withincrease in intensity, while the 1199 cm�1 bandcompletely disappeared [Fig. 6(b)].

The blue-shift in the tC–F band is unexpected andmay be explained in an analogy of several nonclassi-cal types of hydrogen bonds. Recently, several theo-retical studies suggested that weak X–H ��Y hydro-gen bond formation gives rise to an unexpected blue-shift in the X–H stretching frequency, resulting intothe shortening of X–H bond.100–104 This unique phe-nomena is valid only for weak hydrogen bonds in

Figure 5. UV spectra of 1 showing solvatochromic transitions in toluene, THF,DMF, chloroform, and benzotrifluoride at 25 8C. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

5066 SAXENA ET AL.


which rehybridization factor (responsible for short-ening of X–H bond) is more pronounced than hyper-conjugation that results in lengthening of X–H bond.Recently, a few experimental evidences of blue-shifted hydrogen bonds have also been reported inmatrix-isolation and at low temperature.100,104 Inthe present case, weak Si ��F–C interactions may bea cause of the blue-shift of the tC-F band due to intra-molecular ring structure in an analogy of C–H��Opseudoring formation.100 This blue-shift in the tC-Fband presented the first experimental evidence forblue-shifted weak interaction in the solid film atroom temperature.

Evidence of Weak Si��F–C Interactionsby NMR Spectroscopy

The presence of coordinative Si ��F–C interac-tions was also supported by 19F and 29Si NMRspectroscopy. Figure 7 compares the 29Si NMRspectra of 1 in toluene-d8 and THF-d8.

29Si NMRspectrum in toluene showed broad resonancescentered at d � 33.2 due to the polymer back-bone. Interestingly, the spectrum showed a down-field chemical shift of 1.8 ppm in THF-d8 com-pared with that in toluene-d8. The concomitantupfield shift of 1.7 ppm was observed in the 19FNMR spectra in THF-d8 (d � 68.2) with respectto toluene-d8, as shown in Figure 8. These smallchanges in the chemical shifts clearly indicated avery weak interaction between Si and F atomsand are consistent with only recent reports inwhich noncovalent Si��F interaction was visual-

ized in molecular system by means of NMR andX-ray crystal structural analyses.46,105

The weak Si ��F–C interaction was further con-firmed by difference in NOE 19F NMR spectra intoluene. A clear NOE attenuation was found in19F NMR spectrum upon irradiation of 29Si reso-nance at d � 33.2. Furthermore, weak coordina-tive interaction in 1 was evident by the presenceof a doublet at d � 68.2 (JSi–F ¼ 32.4 Hz) in 19FNMR spectrum in toluene-d8, which was notobserved in THF–d8.

AFM IMAGING OF ROD-LIKEFLUOROALKYLATED POLYSILANESTRUCTURE FOR NANOSCIENCEAND NANOTECHNOLOGY

It is expected that rod-like polysilanes can serveas polymer models of quantum wire semiconduc-tors with a 0.2 nm silicon atomic size width anddifferent degree of wire structure fluctuation.67

Recent advances in scanning probe microscopysuch as scanning tunneling microscopy andatomic force microscopy (AFM) have now madepossible to visualize single molecule imaging ofpolymers.106–109 Ebihara et al.110 reported thefirst molecular images of rod-like poly(n-decyl-(S)-2-methylbutylsilane) with high molecularweight on ultraflat R-surface of sapphire.

Figure 6. IR spectrum of 1 (Mw ¼ 1.0 � 105, PDI ¼ 2.6)(a) cast fromTHFsolutionand (b) cast fromtoluene solution.

Figure 7. 29Si{1H} NMR spectrum of 1 (Mw ¼ 1.1� 104,PDI¼ 1.1) at 25 8C (a) in toluene-d8 and (b) in THF-d8.



Rod-like nature of 1 was observed by AFMonto a mica surface by casting its toluene solu-tion, as illustrated in Figure 9.111 The solventwas evaporated from the surface by a controllednitrogen gas flow, resulting into the alignment ofthe rod-like polymer in the flow direction.

The average chain length of 1 was estimated tobe about 480 nm with the Mw value obtained fromthe GPC data. However, AFM images revealed thelength of rod-like polymer to be �2 lm and heightto be �1.0 nm, which is in good agreement withthe height of single polymer chain.111 The width ofrod-like molecular images is difficult to estimatedue to the tip convolution effect. These results in-dicated that the observed molecular length wasapproximately four times longer than the esti-mated molecular length with ordinary GPC withpolystyrene standards. It is probably due to entan-glement of polymer chains due to the weak inter-molecular Si ��F–C interaction.

CHEMOSENSING BEHAVIOR OFFLUOROALKYLATED POLYSILANES

Conjugated polymers are known to be fluorescentchemosensors, because these have potential to

amplify optical signals for the highly sensitivefluorimetric method.112 Conjugated polymers-based fluorescent chemosensors have been ex-ploited to detect mostly positively charged112–116

or neutral species112,117–119 when compared toonly a few examples for anion detection.120,121

The unique affinity of Si atoms toward F atomsin the fluoroalkylated polysilanes encouraged usto make use of a new approach in designing achemosensor capable of detecting the fluorideions and the explosive nitroaromatics (NACs).

Fluoroalkylated Polysilane AsChemosensor for Fluoride Ions

We demonstrated a novel and simple approach forthe amplified detection of fluoride ions. Fluoroal-kylated polysilane (1) exhibits an extremely highsensitivity and selectivity toward fluoride ions atconcentrations as low as three parts per billiondue to photoexcited energy migration approach inthe r-electron delocalized polysilane (Fig. 10).122

Figure 8. 19F{1H} NMR spectrum of 1 (Mw ¼ 1.1� 104, PDI ¼ 1.1) at 25 8C (a) in toluene-d8 and (b) inTHF-d8.

Figure 9. AFM image of 1 on mica surface fromcasting its toluene solution. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

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To evaluate the potential of polysilane as a chemo-sensor for fluoride ions, we measured PL intensity bysuccessive addition of nanomolar concentration of flu-oride ions (as its tetrabutylammonium salt) to theTHF solution of polysilane (Fig. 11). As mentionedearlier, the PL spectrum of polysilane excited at 280nm in THF shows an emission band at 335 nmwith afwhm of 20 nmat 25 8C. Significantly, only 0.01mol%of fluoride ions/Si-repeating unit is enough to decrease10%PL intensity of the polysilane consisting of nearly225 silicon atoms (Mn¼ 3.12� 104, PDI¼ 3.6).

The Stern-Volmer equation [(F0/F) – 1 ¼ K [ana-lyte], where F0 and F are the PL intensity withoutand with analyte, respectively] was used to calculate

the difference in quenching efficiency for differentconcentrations of fluoride ions.123 Figure 12 shows alinear Stern-Volmer relationship with an exception-ally very high binding constant [K ¼ (1.0 6 0.3) �107 M�1] than that previously reported fluoride ionchemosensors.124–131 Each silicon atom in the silicon-catenated polymer acts as receptor and the bindingconstant of each receptor is multiplied by the degreeof polymerization to give remarkably high bindingconstant and remarkable sensitivity for fluoride ions.Also, the selectivity of polysilane for fluoride ionswas found to be nearly 300 times more than that forchloride and bromide ions (as their tetrabutylammo-nium salts). To our knowledge, we have demon-strated for the first time the detection of fluoride ionsselectively in nanomolar concentration.122

Figure 10. Schematic representation of the polymer amplification mechanism afteraddition of fluoride ion in THF.

Figure 11. UV and PL (excited at 280 nm) spectraof 1 (Mw ¼ 1.13 � 105, PDI ¼ 3.6) in THF. PL spectraof 1 (4.2 � 10�5 M) were recorded after the successiveaddition of aliquots of tetrabutylammoniumfluoride(TBAF).

Figure 12. Stern-Volmer plot for the PL (excited at280 nm) quenching of 1 (4.2 � 10�5 M) by fluoride ionin THF.



The extraordinary sensing behavior of 1 towardsthe fluoride ions was surprising, as the r- and p-con-jugated polymers are generally known for their elec-tron donating properties.112,132 The electron with-drawing effect of CF3 groups in side chains mayassist the attack of fluoride ions on electropositive sil-icon atoms in the backbone. This was supported fromthe fact that analogous nonfluoroalkylpolysilane, 2did not exhibit any substantial change in PL spectraevenwith 100 timesmore fluoride ions than observedwith poly(3,3,3-trifluoropropylmethylsilane).

It is known that an excess electron and a posi-tive hole are fully delocalized along the backboneof rod-like polysilanes, whereas these are com-pletely localized over a few tens of silicon atomsin disordered coiled conformation.67,133 An excesselectron from fluoride ion is delocalized along thesilicon-catenated backbone and may be responsi-ble for PL quenching of polysilane. Fluoroalkylpo-lysilane 1 thus may become potentially a semi-conductor if the backbone was doped with fluorideions (acted as n-type dopant) at room tempera-ture, and would be a candidate for the fabricationof molecular electronic devices in the future.

Fluoroalkylated Polysilane As Chemosensorfor Explosive NACs

Recently, considerable efforts have been directed to-ward the development of efficient and cost effectivechemosensors for the detection of explosive NACs

due to the increased use of explosives in terroristattacks and millions of unexploded landmines world-wide.134–136 Trained dogs and metal detectors aregenerally used to locate explosives and landmines.Other alternative methods for landmine detectionare highly desirable, as these dogs are usually expen-sive, get easily fatigued, and cannot stand byextreme weather conditions, while metal detectorshave almost become impractical nowadays due to theincreased use of plastic encased landmines with lowmetal content. Most studies for the detection of NACshave beenmade only in vapor state so far;118,137 how-ever, sensing NACs in water is equally important notonly for locating underwater mines but also for char-acterizing the groundwater contaminated with toxicNACs near the sites exposed to the explosives.138,139

Conjugated organic and inorganic polymers arerealized to be potential fluorescent chemosensors forexplosive NACs, being inexpensive and able to pro-vide for on site portable detection devices.118,137,140–143

The films of p-conjugated porous pentiptycene poly-mers118,137 and polyacetylene143 were reported assensitive fluorescent chemosensors for the vapors of2,4,6-trinitrotoluene and 2,4-dinitrotoluene (DNT).r-Conjugated photoluminescent oligometalloles(M¼¼Si, Ge) were found to be effective in detectingNACs in organic solvents and water.119,141

We have explored a possibility of the rod-likefluoroalkylated polysilane 1 for chemosensing of

Figure 13. PL spectra of THF solutions of 1 (4.4� 10�5 M) were obtained by successive addition of ali-quots of picric acid.

Figure 14. Stern-Volmer plot for the PL (excited at280 nm) quenching of 1 (�4.0 � 10�5 M) in THF bypicric acid (l), DNT (n), TNB (^), DNB (r) at 25 8C.[Color figure can be viewed in the online issue, whichis available at www.interscience.wiley.com.]

5070 SAXENA ET AL.


NACs such as 2,4,6-trinitrophenol (picric acid),1,3,5-trinitrobenzene (TNB), DNT, and m-dinitro-benzene (DNB) in organic solvent and in water.The explosive NACs can be detected by thedecrease in the PL intensity of 1. The observedremarkable sensing behavior is attributed tononbonding weak interactions between Si atomsof polysilane and N or O atoms of NACs. Thischemosensor being highly sensitive can detectNACs at parts per million concentrations in THFsolution and in the thin film of 1 in water.142

Figure 13 depicts the quenching of PL intensityof 1 upon the addition of picric acid. The Stern-Volmer plots of 1 for each analyte are shown inFigure 14. The quenching efficiency for differentNACs towards 1 was found to be nearly one-ordermore in magnitude than that in the earlier re-ported conjugated polymers in solution, as indi-cated by high quenching constants [K ¼ (0.84� 4.15) � 104 M�1] in each case.119,141 Thequenching constant obtained from the slopes ofStern-Volmer plots for 1 gave the relative effi-

ciency of PL quenching as 4.9:2.7:1.2:1.0 for picricacid, TNB, DNB, and DNT, respectively.

The chemosensing ability of 1 could be ex-plained by electron-withdrawing behavior of CF3

groups. Electron-withdrawing groups stabilizethe highest occupied molecular orbital and thelowest unoccupied molecular orbital (LUMO) of1.144–146 The weak noncovalent interaction be-tween Si atoms of r-conjugated polysilane and Nand/or O atoms of electron-deficient NACs maybe responsible for electron transfer from theexcited state of polymer to the LUMO of electrondeficient NACs and thus results into PL quench-ing of 1 on addition of NACs in THF solution of 1.The schematic representation of the electrontransfer mechanism for the quenching of PL of 1with NACs is shown in Figure 15.

This polysilane (1) can also detect NACs inwater. The detection of picric acid was carried outin water using thin film casted from THF solutionof 1. The ten parts per million concentration ofpicric acid in water decreases 14% PL intensity of

Figure 15. Schematic representation of electron transfer mechanism for PLquenching of 1 by NACs.



polymer film. A linear Stern-Volmer relationshipwas observed for picric acid and DNT with highquenching K values of 5.28 � 104 M�1 and 1.31� 104 M�1, respectively. The quenching efficiencyof thin film of 1 was found to be nearly 150%better for NAC than in solution probably due tomore efficient energy transfer in the solid state.

It is important to note that the changes in PLintensity of thin film of 1 were fully recovered af-ter the complete removal of NACs from the filmby rinsing with water. Figure 16 shows threealternating cycles for the exposure of film to pic-ric acid in water and the recovery of its PL inten-sity by washing with water.

Also, the chemosensing of NAC is selective inthe presence of common interferents and environ-mental contaminants. No significant change in PLintensity of 1 was observed with organic solvents(e.g., toluene, hexane, and MeOH), aqueous inor-ganic acids (e.g., sulfuric acid and hydrochloricacid), and oxygenated air, suggesting the insensi-tivity of this polymer to these interferents.

CONCLUSIONS

Although very weak nonbonded interactions areubiquitous in nature and are used to stabilize thehigher order superstructures of biopolymers, theprogrammed design and manipulation of weak

and reversible weak noncovalent interactions insynthetic polymer is still a major challenge inmodern polymer science. In this review, we dem-onstrated clearly the several spectroscopic eviden-ces for through-space very weak Si ��F–C interac-tion in a rod-like fluoroalkylated polysilanes atroom temperature. This work highlights the roleof cooperative and weak Si ��F–C interactions incontrolling the desired conformation of polysilaneby the choice of molecular weight and solvent. Thecontribution of such intramolecular interactions tofix into the helical conformation was supportedby variation in UV, PL, NMR (29Si and 19F), IRspectra, and viscometric measurements. This weakSi ��F–C interaction may potentially be exploitedfor the strategic design of new molecular, supramo-lecular, and polymeric architectures.

Most notably, the fluoroalkylated polysilane 1exhibited an extraordinary sensitivity and selec-tivity to fluoride ion and NACs at concentrationsas low as parts per billion and parts per million,respectively. Although immobilization and opticalamplification of 1 onto surface will be next stepin realizing actual chemosensor, facile techniqueswere demonstrated recently.147,148

Profs. M. M. Green (Polytechnic University), J. Michl(University of Colorado), R. West (University of Wis-consin), K. Tamao (Kyoto University), G. Basu (BoseInstitute), R. G. Jones (University of Kent), and Y.Kawakami (Japan Institute of Science and Technol-ogy) are gratefully acknowledged for their insights onthe weak Si��F–C interaction in the achiral and chiralfluoroalkylated polysilane. We are thankful to M. Ishi-kawa, T. Hagihara, and F. Asanoma for technical as-sistance and Drs. A. Ohira, G.-Q. Guo, and Y.-G. Yangfor fruitful discussion. We are also thankful to SonalSinghal for critically reading the manuscript.

A. Saxena and M. Fujiki are acknowledged by grantfrom CREST-JST. M. Fujiki is acknowledged in part forgrants from the Ministry of Education, Science, Sports,and Culture of Japan, for Grant-in-Aid for ScientificResearch \Experimental Test of Parity Nonconservationat Helical Polymer Level (16655046)" and \Design, Syn-thesis, Novel Functionality of Nanocircle and NanorodConjugating Macromolecules (16205017)."

M. Naito is acknowledged for support by the Minis-try of Education, Science, Sports, and Culture of Ja-pan, for Grant-in-Aid for Scientific Research, \Detec-tion, Amplification, and Novel Functions of Non-Clas-sical Weak Chemical Interactions by r-ConjugatingPolymers (17750110)."

M. Fujiki and M. Naito are also acknowledged inpart for grant from Scientific Research of PriorityAreas, \Control of Super-Hierarchical Structures andInnovative Functions of Next-Generation ConjugatedPolymers (446)."

Figure 16. Dependence of PL intensity of thin film(30 nm thickness) of 1 on the repeated changes in theconcentration of picric acid between 0 and 100 ppm.

5072 SAXENA ET AL.


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weak noncovalent si···fc interactions stabilized fluoroalkylated rod-like polysilanes as...

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