Novel chiral poly(para-phenylene) derivatives containing cyclophane-type moieties

Macromol. Chem. Phys. 198,2623-2650 (1997) 2623

Feature Article

Novel chiral poly(para-phenylene) derivatives containing cyclophane-type moieties

Ruiner Fiesel, Joachim Hubel; Ute Apel, Volker Enkelmann, Reinhard Hentschke, Ullrich &he@+

Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany

Karin Cabrera

Merck KGaA, Frankfurter Str. 250, R&D Chromatography, D-64271 Darmstadt, Germany

(Received: April 18, 1997)

SUMMARY The introduction of cyclic ansa-substituents allows for the synthesis of soluble poly-

(para-pheny1ene)s PPPs possessing main chain chirality. The novel chiral PPP’s represent an attractive combination of n-conjugated character and chirality. We have synthesized open chain (single-stranded) as well as ladder-type chiral PPPs. The single- stranded chiral PPP’s exhibit temperature-dependent changes of chiroptical properties. The behaviour should be assigned to conformational changes. The chiral ladder polymers contain the cyclophane loops exclusively on one side of the molecular board and are characterized by an unexpectedly high chiroptical activity of the n-n*-transition. They are potential candidates to study non-linear chiroptical properties and to investigate circu- lary polarized luminescence (photo- and electroluminescence) effects.

Introduction

Due to the attractive optical and electronic properties of poly(para-phenylene) (PPP), its synthesis has defined a challenging field of research for polymer chemists since the sixties’”). One of the major advances, making poly(para-phenylene) a potential candidate for various applications, has been the attachment of flexible side chains to the insoluble parent system, yielding a structurally well-defined and solution-processable As has been demonstrated by several investigators, substituents of different structure and length affect not only the solubility, but also the solid state structure, the phase behaviour and the electronic propertiesG8). Although a great variety of systems has already been examined, almost nothing is known about the use of cyclophane-type phenylene moieties in the synthesis of substituted PPP’s or other polymers’). In order to gain insight into whether the incorporation of ansa-substituents defines a novel, powerful tool for fine-tuning the polymer properties, we decided to attempt the synthesis of PPP-type polymers composed of aromatic cyclophane units (1). Of special interest is the comparison with the PPP- counterpart 2 containing two alkoxy side groups instead of the bridging substituents (Fig. 1). Macromol. Chem. Phys. 198, No. 9, September 1997

0 1997, Huthig & Wepf Verlag, Zug CCC 1022- 1352/97/$10.00

2624 R. Fiesel, J. Huber, U. Apel, V. Enkelmann, R. Hentschke, U. Scherf, K. Cabrera

- 2

Fig. 1. Structure of atactic poly[2,5-(oxydecanoxy)-l ,Cphenylene] (1) and poly- [2,5-dipentyloxy- 1 ,Cphenylene] (2)

It has to be pointed out here that each of the two cyclophane-type monomers 6 and 7, used to synthesize polymer 1 (see Scheme I), via an aryl-aryl coupling, according to Suzuki, possesses a plane of chirality, since a rotation of the oxydecanoxy-loops is not possible"). Hence, aryl-aryl coupling of such compounds should give access, not only to the non-stereoregular, atactic (l), but also to the stereoregular isotactic (3) and syndiotactic (4) configurations of the target polymer poly[2,5- (0xydecanoxy)- 1 ,Cphenylene], depending on whether racemic mixtures of the monomers or enantiomerically pure compounds are applied (Fig. 2) . Therefore, for the synthesis of 3 and 4, the preparative separation of the monomers is a prerequi- site. Since one would expect that the different configurations of atactic 1, isotactic 3 and syndiotactic 4 display a different chemical and physical behaviour, a major subject of our work was to follow structure/property relations.

Beyond the aspects discussed above, another fact has strongly stimulated our interest in incorporating cyclophane-type sub-units into the PPP backbone. The isotactic poly[2,5-(oxydecanoxy)-1,4-phenylene] (3) exists in two enantiomorphic forms (3 (+)/3 (-); Fig. 2) and represents, to our knowledge, the first example of a polymer possessing main-chain chirality, due to the presence of planar chirality elements. Since 3 combines both extended x-conjugation and main-chain chirality, one may expect an attractive chiroptical behaviour, e. g., a high, chiral anisotropy in the x-x*-region, and non-linearities of chiroptical properties. Finally, the novel approach to conjugated polymers possessing main-chain chirality provides opportu- nities for the generation of related structures, e. g., the synthesis of chiral PPP-type

0 1997, Hiithig & Wepf Verlag, Zug CCC 1022-1352/97/$10.00

Novel chid poly@uru-phenylene) derivatives ... 2625

Fig. 2. Structure of iso- and syndiotactic poly[2,5- (oxydecanoxy)- 1,4-pheny- lenel (3(+)/3(-), 4) and of a poly@ara-phenylene) ladder polymer (5)

ladder polymers 5 (Fig. 2), e.g., when following the "classical" precursor route of Scherf and Mullen ll). Bearing in mind that the achiral PPP ladder polymers (LPPP's) represent highly efficient luminophores, the chiral analogue 5 of LPPP may enable the observation of circularly polarized photo- and electroluminescence '*).

Results and discussion

Synthesis and characterization of atactic poly[2,5-(oxydecanoxy)-l,4-phenylene J (1)

Structurally well-defined PPP-derivatives are accessible by making use of Pd(0)- and Ni(0)-catalyzed aryl-aryl coupling ll). As the Pd(0)-catalyzed coupling of aromatic boronic acids and aryl halides, according to Suzuki, is superior to the AA-type reductive coupling, developed by Yamamoto, in many respects (e.g., attainable molecular weight, tolerance of functional groups, etc.), we elaborated the


Scheme 1:

following synthetic strategy towards our target structure (Scheme 1). Monomer 6, which is readily available on a 20 g scale, was prepared in three steps, starting from p-benzoquinone according to literature procedures"! To be able to synthesize 1 by an AA/BB-type cross coupling, the dibromo compound 6 had to be converted into the corresponding diboronic acid 7. For this, 6 was i) treated with 2.5 equivalents of butyllithium, and then ii) with 7 equivalents of trimethyl borate. Subsequent hydrolysis with dilute hydrochloric acid finally resulted in the formation of 7 in an overall yield of ca. 60%. It is worth mentioning that the aryl -0-CH2-linkages survive the transformation from 6 to 7 unchanged, especially the hydrolysis with dilute hydrochloric acid. This can be easily demonstrated using the 'H NMR spectra of 6 and 7.

Starting from the racemic mixtures of 6 and 7, atactic poly[2,S-(oxydecanoxy)- 1 ,Cphenylene] (1) has been successfully synthesized by refluxing equimolar parts of the two monomers with 1,s mol-% Pd(O)[P(C,H,),], in a two-phase solvent system (tetrahydrofuradl M aqueous potassium carbonate solution) for two days. Stan- dard work up procedure supplied a colourless material possessing a number-average molecular weight a, of 3 000, as determined by gel-permeation chromatography, corresponding to a degree of polymerization (DP) of about 12 (for more details and a complete characterization of 1 see ref.13)).

For the synthesis of poly[2,S-dipentyloxy- I ,4-phenylene] (2), the 2,s-di-n-alkoxy- substituted counterpart of 1, we pursued an analogous strategy (Scheme 2). 2,s- Dibromohydroquinone (8) was etherified with 1 -bromopentane and potassium hydroxide, which results in the formation of 2,s-dipentyloxy- I ,4-dibromobenzene (9)". The following transformation of 9 into 2,s-dipentyloxy- 1,4-benzenediboronic

Novel chiral poly@uru-phenylene) derivatives ... 2627

Scheme 2:

Gr C~HIIB~~KOH, Brd-r Br \

HO m

1. BuLi

- 9 - 2. B(OCH3)j

3. HzOlHCl

acid (10) and subsequent Suzuki-coupling of the two monomers 9 and 10, as described for 1, gave the 2,5-di-n-alkoxy-substituted PPP derivate 2. 'H and 13C NMR data of 2 unequivocally prove the claimed structure (see Experimental part). Poly[2,5-dipentyloxy- 1,4-phenylene] (2) possesses a molecular weight M, of 7 300 (GPC, DP ca. 30), which is increased compared to the atactic cyclophane-type poly[2,5-(oxydecanoxy)- 1,4-phenylene] (1). This indicates that the steric shielding of the bromo atoms and the boronic acid functionalities is significantly more domi- nant in the case of the cyclophane-type monomers 6 and 7.

1 and 2 represent suitable polymers to study since the incorporation of cyclophane units influences the physical and electronic properties of PPP-type molecules.

The absorption spectra of 1 and 2 are depicted in Fig. 3. We found almost identical positions of the absorption maxima both for 1 and 2 (1: Amn: 332 nm; 2: 337 nm). The spectra display two bands, one in the short wavelength region, which can be attributed to absorptions of localized chromophores, and another in the low energy region, which is related to K-K* transitions of delocalized orbitals. The position of the latter strongly depends on the torsion angle between adjacent rings and,


8

51 9

m e

- m ? I

g e 51 9

Wavelength [nm]

c .- ._ I E W

Wavelength [nrn]

Fig. 3. UVNIS absorption ? (a) and photoluminescence

emission (b) spectra of atactic ._ 0 u) poly[2,5-(oxydecanoxy)- 1,4- ’E phenylene] (1) (I) and poly- UI [2,5-dipentyloxy- 1,4-pheny-

lene] (2) (11) in dichloromethane (excitation wavelength: 3 10 nm)

- I

u)

therefore, on the extent of n-c~njugation’~). The optical and electronic properties of such n-systems become independent of M, when reaching a specific limit, the so- called effective conjugation length”). As shown in model studies for dialkoxy substituted poly(para-phenylene)s, the effective conjugation lengths are in the order of 8-1 2 repetition units7). Assuming a similar value for poly[2,5-(oxydecanoxy)-1,4- phenylene] (l), the convergence limit of the optical properties should be reached with our observed DP’s of ca. 12. Hence, one can conclude from the congruence of the optical spectra that the main-chain conformations are very similar in both cases. Whether, and to what extent, the main-chain geometry is altered by passing over to the stereoregular iso- and syndiotactic configurations of poly[2,5-(oxydecanoxy)- 1,4-phenylene] (3 and 4), will be discussed later.

Looking at the fluorescence emission spectra of 1 and 2, not only the maxima (1: A,,, = 411 nm, 2: A,,, = 403 nm) but also the Stokes shifts (1: 5790 cm-I, 2: 4860 cm-’) are of the same magnitude. In conclusion, the exchange of the two pentyloxy- with the cyclic ansa-substituents does not significantly influence the optical or electronic properties of the PPP backbone in the ground or in the excited state.

The solid state structure of alkyl- and alkoxy-substituted poly@ara-phenylene)s has been extensively investigated by Wegner and c o - w o r k e r ~ ~ ~ ~ ) . For unbranched side chains (alkyl, alkoxy) consisting of 6 to 16 -CH2- sub-units (the oxygen atom may be counted as a methylene group), the X-ray diffractograms suggest the existence of a layered structure, which is characterized by a strong tendency to form

Novel chid poly(para-phenylene) derivatives ... 2629

highly ordered domains of parallel side chains. The sharp and intense X-ray reflection in the small angle region shows a linear dependence on the side chain length, reflecting the distance between two adjacent layers.

We have also investigated the solid-state structure of 1 and 2 by using X-ray dif- fractometry (Fig. 4). To get comparable results, the samples were tempered at 50°C for several hours. Proof that 2 forms such a layered structure is given by a relatively sharp small angle reflection, corresponding to a layer distance of 19.2 A. Further-

Fig. 4. X-ray diffractograms of atactic poly[2,5-(oxydecanoxy)- 1,4-phenylene] (1) and poly[2,5-dipentyloxy- 1,4-phenylene] (2)

more, the appearance of reflections of higher order indicates the existence of a con- siderably high range ordered solid state structure of 2. In contrast, the diffractogram of the novel cyclophane-type PPP 1 does not display any sharp reflections as indicators for a high supramolecular ordering. The bridging substituents effectively suppress the formation of crystalline domains. The structure of the amorphous polymer 1 can be regarded as an assembly of cylindrical macromolecules possessing some low-range order. Based on this, the broad reflection in the small angle region corre- sponds to a distance of about 9 A between adjacent cylinders. Even if interpenetration of the side chains is taken into account, the resulting diameter is significantly smaller than that of poly[2,5-dipentyloxy-1,4-phenylene] (2). Thus, the incorporation of cyclophane moieties into the rigid rod-type PPP polymer leads to an increase of their axis ratio (ratio of length to diameter), which may influence the phase behaviour of the macromolecules (see below).

To complete our characterization data, the thermal stability of poly[2,5-(oxydecanoxy)-l,4-phenylene] (1) was investigated. The decomposition of 1 (under N2) starts at about 300"C, the first decomposition step corresponding to a weight loss of ca. 48%, according to a cleavage of the oxydecanoxy functions (calculated value of 57% for a complete loss of the -CloH20- loops). This decomposition step is followed by another at ca. 5OO0C, which reflects the decomposition of the PPP backbone. If the 2,5-dipentyloxy-PPP 2 is investigated by TGA, a very similar decomposition profile is found. The pentyloxy groups are cleaved at ca. 300°C (observed: 56%; calculated: 57%). According to these results, contrary to our expectations, the thermal stability of 1 is not reduced by the incorporation of the ansa-substituents.


Nevertheless, it must be considered, that polymer 1 may crosslink before significant changes are visible in the TGA.

“Form anisotropic” rigid rod-type polymers are potential candidates for the formation of liquid-crystalline (nematic) meso-phases. In compliance with theoretical considerations, the appearance of meso-phases strongly depends on the axis ratio of the rod-like macrom~lecules’~-’~). Thermotropic as well as lyotropic behaviour can occur if the chain length is large compared to its width. With regard to the formation of liquid cryqtalline phases, differential scanning calorimetry (DSC) of poly-[2,5-dipentyloxy- 1,4-phenylene] (2) (an: 7 300) exhibits an endothermic transition at about 85 “C (first heating). The peak is not caused by the evaporation of residual solvent, since the following cooling curve displays an endothermic signal at slightly lower temperature. Both signals are still detectable after several heating and cooling cycles. This reversible process may be ascribed to the “melting” and “re-formation” of crystalline microdomains, the so-called side-chain cry~tallization~*~’. 2,s-Di- alkoxy-substituted poly(para-phenylene)~, possessing a molecular weight comparable to that of 2, are not able to act as a mesogen7), therefore no liquid-crystalline properties were observed for 2. This can be attributed to the low axis ratio of these polymers. Considering this, it was attractive to study polymer 1 with its favourable, more compact structure.

Surprisingly, the cyclophane-type PPP 1 exhibits no endo- or exothermic transitions in the temperature range from -50 to 250 “C using DSC. The ansa-substituents successfully suppress the formation of highly ordered assemblies. But, unfortu- nately, there are also no indications of thermotropic liquid-crystalline behaviour. It may be supposed that decomposition starts before the transition temperature is reached. Since a thermotropic behaviour of poly[2,5-(oxydecanoxy)- 1,4-phenylene] (1) was not detectable, we investigated the ability to form a lyotropic meso-phase. Polarizing microscopic observations of mixtures of 1 with chloroform (> 100 mg/ml) show a Schlieren texture, characteristic of a nematic phase’”. Unlike solutions of the ansu-substituted PPP derivative 1 in chloroform, mixtures of 2 with different solvents (including chloroform) display always an anisotropic orientation of the macromolecules. This attractive result demonstrates the strong influence of the peripheral substituents on the properties of PPP-type molecules. The incorporation of the cyclic ansa-substituents increases the axis ratio of the PPP-type macromolecules, and allows the formation of lyotropic (nematic) LC phases.

Iso- and syndioracric poly[2,5-(oxydecanoxy)-l,4-phenylene] (3) and (4)

As pointed out above, the synthesis of iso- and syndiotactic poly[2,S-(oxydecanoxy)-l,4-phenylene]s (3) and (4) requires the availability of the enantiomerically pure monomers 6 and 7. Hence, it was necessary to develop resolution methods for the two racemic monomers 6 and 7. In 1947, Luttringhaus et al. reported a resolution of 2,5-(oxydecanoxy)- 1,4-dibromobenzene (6) via formation of diastereomeric deri- vatives2’). However, a considerable drawback of this procedure was the poor overall yield in a multistep synthetic sequence. Later Hesse and Hagel described an analyti- - cal resolution of 6 by means of liquid chromatography on a chiral phase (cellulose

Novel chiral poly@ara-phenylene) derivatives ... 263 1

triacetate, CTA) as the stationary phase, and aqueous ethanol as solventz1). Based on this system, we worked out a preparative, enantioselective resolution for the racemic mixture of the 2,5-(oxydecanoxy)-l,4-dibromobenzene (6). Using a 250-50-sized CTA column (Merck KGaA, particle size 15-25 pm) and a constant flow rate of 10 mumin, it was possible to resolve the dibromo monomer 6 on a preparative scale.

To find conditions under which the largest amount of racemic 6 can be separated into pure enantiomers, the chromatographic resolution was performed for various injection quantities (128, 256, 384 and 512 mg, Fig. 5). The enantiomeric purity of the two antipodes was determined with an analytical CTA-column (cellulose triacetate, dimension: 250-4, particle size: 15-25 pm). Since the analytical resolution provides a baseline separation, the purity of the stereoisomers can be determined in a very simple manner. The analytic chromatograms, depicted in Fig. 6, stem from samples which have been collected between the 39" and 52"d (a) as well as the 70th and 91'' (b) minute (injection amount: 256 mg). Using the peak areas, we have calculated purities of 100 and 98%, respectively, for both enantiomers (6(+) and 6(-)) . Solutions containing 0.5 mg 6(+)/6 (-) per ml 1,1,2,2-tetrachloroethane display a molar optical rotation [@]tit of about (+)/(-) 1210 degree - cm2/mol (I: 10 cm).

Fig. 5. Chiral separation of 2,5-(oxydecanoxy)- 1,4- dibromobenzene (6)

Fig. 6. Enantiomeric purities of 6(+) (a) and 6(-) (b)

Retention time [min]


Because of the structural similarity of 6 and 7, it was obvious to check if the racemic mixture of the 2,5-(oxydecanoxy)-1 ,4-benzenediboronic acid (7) can be resolved on a CTA-column, too. However, all attempts remained unsuccessful. It is fair to assume, that polar interactions between the boronic acid groups and the cellulose triacetate counteract a chiral resolution of racemic 7. In order to take advantage of the ability of 7 to form hydrogen bonds, it was promising to test a stationary phase that can form stable hydrogen-bonded diastereomeric host-guest-complexes. The different stability of such diastereomeric adducts should allow for their chromatographic separation. Therefore, we tried a preparative column containing a p-cyclo- dextrin-modified silica gel (Chiradex, Merck KGaA, dimensions: 250-50, particle size: 10 pm). The first experiments indicated that this material is quite efficient for the preparative separation of racemic 7, using aqueous methanol (1 : 1) as solvent. Racemic 7 was dissolved in pure methanol (the solubility in aqueous methanol is very poor) and injected onto the column. It is necessary to run the solvent pump during injection, in order to guarantee a quick distribution of the injected methanolic solution. To find optimum conditions for a preparative separation of the diboronic acid monomer 7, we have again collected samples at different times and for different loads of the column (Fig. 7). An analysis of our data proved that at a column load of

48 mg of racemic 7, the resolution can be performed in high optical purity (100 and 97%, respectively), the fractions being collected between the 29'h and 34" as well as the 39'h and 44'h minute (Fig. 8). If the column is loaded with more than 50 mg of racemic 7, overloading effects occur which prevent a suitable resolution. Solutions containing 0.5 mg 7(+)/7(-) per mL methanol possess a molar optical rotation [@I$$ in the order of (+)/(-) 3360 degree - cm*/mol (I: 10 cm).

Novel chiral poly(pampheny1ene) derivatives ... 2633

Fig. 8. Enantiomeric purities of 7(+) (a) and 7(-) (b)

Retention time [min]

A determination of the absolute configuration of the two tactic forms of poly[2,5- (0xydecanoxy)- 1 ,Cphenylene] (3 (+)/(3 (-) and 4), at a later stage of our work, needs an analysis of the monomer configurations. Investigations of planar c h i d systems by Schlogl et al. have established a relationship between the sense of chirality of carbophanes and the circular dichroism (CD) in the region of the ‘Lb-absorption feature of the aromatic chromophore”). The CD-spectra of 6(+), 6(-), 7(+) and 7(-) are shown in Fig. 9. As expected, the enantiomers supply data of opposite sign, where the ‘L,-Cotton-effect of the benzene chromophore is centred at about 300 nm.

Fig. 9. CD-spectra of 6 (+I (a176 (-1 (b), 7 (+I (c) and 7 (-) (d) (room temperature; solvents: 6(+), 6(-) 1,1,2,2-tetrachloroethane; 7 (+), 7 (-) methanol; c: 0.5 mg/ml, I: 0.1 cm)


If the sector rules according to Schlogl are applied, a pR-configuration results for 6 (+), and a pS-configuration for 6 (-), respectivelyz3’.

However, the validity of Schlogl’s sector rule is limited to carbophanes with cer- tain substituents, e.g., halogen atoms. Since boronic acid groups have not yet been investigated, we have tried to correlate the configurations of 6(+) , 6 ( - ) with 7(+), 7(-), by means of a chemical transformation reaction. The pure dibromo enantiomer 6 (+) was converted into the corresponding 2,5-(oxydecanoxy)- 1,4-benzenediboronic acid (7): i) dilithiation, ii) reaction with B(OCH,),. Measurements of the molar optical rotation yielded a value [@I::: of approximately (-) 2250 degree * cm2/mol (I: 10 cm) for the transformation product. If this value is compared to that of the enantiomeric pure diboronic acids 7(+) and 7(-) (chromatographically resolved), it turns out that a partial racemization takes place in the course of the transformation. Since the reaction product displays a negative molar optical rotation, one can assume an identical absolute configuration of 6(+) and 7(-) (Fig. 10).

\ Br

Fig. 10. Absolute configurations of 6(+), 6(-), 7(+) and 7(-)

. .-

I(+)

However, the superior method for determining the absolute configuration is an anomalous X-ray structure analysis. For crystalline 6 (+), the anomalous scattering of the bromine atoms is sufficiently significant to allow the unambiguous determination of the absolute configuration. This was done by determination of the intensities of the Friedel pairs of all reflections in the X-ray structure determination and including the Flack parameter in the refinement24). A projection of the crystal structure is shown in Fig. 11. The atoms in the aliphatic part are disordered, i. e., two conformations are present in the crystal. In Fig. l l only the majority component is shown.

Fortunately, the configuration of 6 (+) related to the X-ray structure analysis is identical with the configuration which we have derived from Schlogl’s sector rules. The results document the validity of Schlogl’s rules also for our novel cyclophane derivatives.

For the synthesis of the tactic forms (iso- 3 and syndiotactic 4, respectively), we carried out polycondensations of the pure enantiomers in the same way as described

Novel c h i d poly(para-phenylene) derivatives ... 2635

Fig. 1 1. Anomalous X-ray structural analysis of 6(+)

for atactic 1. To synthesize isotactic 3(+), we reacted 6(+) with 7(-), and the enantiomorphic form 3(-) was made from 6(-) and 7(+) (Scheme 3). In contrast to the atactic poly[2,5-(oxydecanoxy)-1,4-phenylene] (l), the isotactic polymers 3 (+) and 3(-) precipitate after a short time. All attempts to redissolve the samples in dichloromethane or chloroform failed. 1,1,2,2-Tetrachloroethane (TCE) is a more suitable solvent, 3(+) and 3(-) are, however, not completely soluble. The poor solubility of 3 (+) and 3 (-) in common solvents complicated an extensive structure analysis.

The 'H NMR spectra (see Experimental part) support the structure, but they can- not prove the isotactic succession of the chiral building blocks. 13C NMR spectra, which should give a better insight into the tacticity of 3(+) and 3(-), could not be recorded due to poor solubility. Proof that the optical activity of the monomers sur- vives the polycondensation comes from chiroptical measurements. A 9,15 - lo4 M solution of 3(+) shows a molar optical rotation of [@I::; = (+) 4200 degree * cm2/ mol (1: 10 cm), and the enantiomorphic 3(-), a value of [@I::; = (-) 4080 degree * cm2/mol ( I : 10 cm). However, our characterization data do not exclude a partial racemization during polycondensation. For this purpose, we intend to carry out studies on oligomeric model compounds.

Syndiotactic poly[2,5-(oxydecanoxy)- 1 ,Cphenylene] (4) has been prepared by coupling two enantiomers of different absolute configuration, 6 (+) with 7 (+), and 6(-) with 7(-), respectively (Scheme 4). Due to the existence of mirror planes per- pendicular to the polymer main chain, the syndiotactic configuration 4 represents a meso-form. Therefore, we did not observe any optical activity for both samples of 4, prepared from 6 (+)I7 (+) and 6 (-)/7 (-).

A key to the molecular weight of isotactic 3 and syndiotactic 4 is given by their 'H NMR spectra. A degree of polymerization of ca. 15 for the TCE-soluble fraction was calculated due to the signal intensities in the aromatic region of the 'H NMR


Scheme 3:

Br

+

Br

A

+

spectrum. The &values were estimated by an integration of the major signal of the aromatic cyclophane moieties in relation to minor end-group signals. Other absolute methods to determine the molecular weight, such as VPO or GPC, could not be applied due to the poor solubility of 3(+), 3(-) and 4 in common solvents.

Atactic, iso- and syndiotactic paly[2,5-(oxydecanoxy)-l,I-phenylenej (1,3 and 4) - A comparison of their physical properties

As mentioned already for atactic 1, we did not find indications for thermotropic phase transitions for 3 and 4 in the temperature range of 20°C to 300°C (start of decomposition according to DSC measurements). The investigation of the phase- forming behaviour of 3 and 4 in solution should be the subject of further studies.

Novel chiral poly@ara-phenylene) derivatives ... 2637

Scheme 4:

Significant differences between the three poly[2,5-(oxydecanoxy)- 1,4-pheny- lenels 1,3 and 4 were observed concerning their solid state structure (Fig. 12).

X-ray diffractograms of the iso- 3 and syndiotactic form 4 exhibit sharp reflections in the small-angle region. Unlike the atactic polymer 1, both stereoregular con-

Fig. 12. X-ray diffractograms for atactic (l), iso- (3) and syndiotactic (4) poly[2,5-(oxydecanoxy)- 1,4-phenylene] (room temperature)


figurations 3 and 4 are characterized by a high degree of long-range order. There- fore, it must be concluded that the stereoregularity of the iso- 3 and syndiotactic 4 brings about a drastically improved intermolecular order (crystallinity). This is also the reason for the reduced solubility of 3 and 4, in comparison to 1. Interestingly, the X-ray diffractograms of 3 and 4 are distinctly different. 3 displays a main-chain distance of 13.2 A, assuming cylindrically shaped molecules, while 4 exhibits a d-spa- cing of 9.0 A. Since the diameter of the macromolecules should be tacticity-independent, the differences may result from an interpenetration of side chains (loops) in the case of 4, while this interpenetration should be unfavourable for 3. In this way, the tacticity controls not only the crystallinity but also the type of the supramolecular arrangement.

Fig. 13 gives the optical (absorption) spectra of 1 ,3 and 4. There are only negligi- ble differences when comparing the UVNIS spectra of the three polymers. Since the effective conjugation length should be already reached with a DP of ca. 15 for 3 and 4, the UVNIS spectra suggest that the tacticity of the polymer chain does not influence the torsion angle between adjacent phenylene rings.

Fig. 13. UVNIS absorption spectra of atactic (1). iso- (3) and syndiotactic (4) poly[2,5-(oxydecanoxy)- 1,4-phenylene] (solvent: tetrachloroethane)

Chiroptical properties of isotactic poly[2,5-(oxydecanony)-l,4-phenylene] (3)

As already mentioned, the two enantiomorphic forms of isotactic poly[2,5-(oxydecanoxy)- 1,4-phenylene] (3 (+) and 3 (-)) represent a novel class of main-chain chiral polymers, possessing main chain planar chirality elements. Therefore, it was obvious to investigate the chiroptical properties of 3(+) and 3(-). The CD-spectra of 3 (+) and 3 (-) are depicted in Fig. l 4 .

Firstly, the weak intensity of the Cotton effects within the n-n*-transition region (300-400 nm) reflects that only small exciton-coupling occurs and suggests a torsion angle (+90") between adjacent phenylene rings which is unfavourable for an efficient exciton-coupling.

However, the Cotton effects in the low-energy region of the spectrum can serve as indicators for the polymer main-chain conformation, since they are related to delocalized n-orbitals. To get a deeper insight, we have recorded temperature-dependent CD-spectra of 3(+) in the region of the n-n*-transition (Fig. 15). In this case, the CD-curves at -20, 0 and +20"C differ only very slightly. In contrast, a significant


Fig. 14. CD-spectra of

(room temperature; solvent: tetrachloroethane; c: 0.225 mdml; 1: 0.1 cm)

3 (+I (a) and 3 (4 (b)

Fig. 15. CD-spectra of 3(+) in the n-n*-region at (a) -2O"C, (b) O T , (c) 20°C, (d) 35"C, (e) 50°C, ( f ) 80°C (solvent: tetrachloroethane; c: 0.225 mg/ml; 1: 0.2 cm)

change of position and form of the CD-spectra was observed at higher temperatures (+ 35 and + 50°C).

This behaviour suggests a transition from a higher ordered conformation at room temperature to less ordered conformation at increased temperatures. To prove this hypothesis, the existence of a preferred main-chain conformation of 3, we have carried out a molecular dynamics simulation on a model pentamer, using the program Cerius2 1.0. The results of our simulations are depicted in Fig. 16. For 3(+) and

2640 R. Fiesel, J. Huber, U. Ape], V. Enkelmann, R. Hentschke, U. Scherf, K. Cabrera

Fig. 16. Molecular dynamics simulation of model pentamers related to 3(+) (left) and 3 (-1 (right)

3(-), we found a favoured conformation which is characterized by a helical arrangement of the phenylene rings along the main-chain axis of 3. The sense of the helix is, as expected, contrary for both enantiomorphic forms.


Synthesis and characterization of an optically active poly(para-phenylene) ladder polymer 5

The low intensity of the Cotton effects of poly[2,5-(oxydecanoxy)-1,4-pheny- lenels (3(+) and 3(-)) within the n-n*-transition reflects only weak exciton-coupling between adjacent phenylene rings, as mentioned. Therefore, it was a challenge to search for other structures which combine main-chain chirality with a high optical activity in the n-n*-transition region.

One promising target was the synthesis of planarized, ladder-type PPP's 5 (Fig. 17) containing our optically active cyclophane moieties. The achiral analogues of 5 (PPP-type ladder polymers LPPP with two alkyl or alkoxy substituents instead of the -0C10H200- loops) have been thoroughly described"). They are characterized by unusual optical and electronic properties, such as an extremely small Stokes shift (ca. 30 meV), very high photoluminescence quantum yields (>95% in solution, up to 60% in the solid state) and represent efficient organic emitter materials for the design of light-emitting diodes LED'S)^^-^^).

Fig. 17. Structure of poly (para-phenylene) ladder polymer 5 1 R:

Our novel approach combines the methodology of ladder polymer synthesis with the incorporation of chirality planes to a polymer 5, in which planar chirality elements (cyclophane building blocks) are part of the planar, ladder polybara-phenylene) (LPPP) ribbon. A decisive factor in the realization of this idea is again the use of the -0Cl&I2@-- bridged diboronic acid 7 as chiral monomer component. The synthesis of the ladder polymer 5 is documented in Scheme 5.

In the synthesis of racemic 5, racemic diboronic acid 7 is treated with the tere- phthalophenone derivate 14 to form the single-stranded PPP intermediate 15 in an aryl-aryl cross-coupling reaction, according to S u ~ u k i ~ ~ ) . The coupling, which is catalyzed by [Pd(O)(Ph,),J, takes place in a two-phase reaction medium (THF/aqueous sodium carbonate solution). The subsequent cyclization, to give the ladder polymer 5, is carried out in a two-stage polymer analogous reaction sequence, the first stage of which is the reduction of the keto functions with lithium aluminium hydride (LAH), to form the polyalcohol 16. This is followed by ring closure of 16 to ladder polymer 5, a reaction catalyzed by boron trifluoride etherate (andogous to a Friedel- Crafts alkylation)"). The transformation from the open-chain, colourless precursor 16 to the deep yellow ladder polymer 5 is accompanied by a drastic bathochromic shift of the long-wavelength absorption maximum Am, with 5 possessing a A,,,-

2642

Scheme 5:

R. Fiesel, J. Huber, U. Apel, V. Enkelmann, R. Hentschke, U. Scherf, K. Cabrera

- 7 - 14 - 15

value of 461 nm (Fig. 18). The LPPP derivative 5 displays an intense blue photoluminescence, both in solution and solid state, with a A,,, (emission) of 470 nm.

The ladder polymer 5, with its unequivocal structure, can be prepared in yields higher than 90% and with molecular weights a, of 12 000- 13 000 (corresponding to ca. 28 main-chain phenylene units). The course of the polymer-analogous cyclization of 16 to 5 was monitored by 13C NMR spectroscopy. The signal from the sec-

Fig. 18. UVNIS absorption spectra of polyketone 15 (dashed line), polyalcohol 16 (dotted line) and ladder polymer 5 (solid line) (room temperature; solvent: dichloromethane; c: 1 mg/ml; 1: 1 cm)

Novel chiral poly(para-phenylene) derivatives ... 2643

ondary alcohol group of 16 at 6 = 72.5 disappears completely. The signal from the newly formed methylene bridge of ladder polymer 5 arises at 6 = 53.1. Besides signals from the aliphatic side chains (6 = 14.4 - 33.2), the -H2 methylene carbons of the -0C1&1200- bridges and the hexyloxy side chains (6 = 69.8,70.1) are characteristic. As expected, eleven signals of non-equivalent aromatic carbons are detectable.

The diboronic acid monomer 7 is prepared in four steps (see Scheme 1) from 1,4- benzoquinone 6". 13), as described above. According to Scheme 6, the terephthalo- phenone derivate 14 was synthesized starting from 1 ,2-dihexyloxybenzene (12) and 2,5-dibromoterephthaloyl chloride (13) in a twofold Friedel-Crafts acylation.

Scheme 6:

OH

li

OHex 6""" - 12

COCI

B # 0 bHex Br J?

O T O H e x Hex

- 14

As next step, the chiral ladder polymers 5(+) and 5(-) were generated using the pure enantiomers 7(+) and 7(-) as starting material (the separation of racemic 7 into the pure enantiomers was described above). 5(+) and 5(-) are completely soluble (e. g., in toluene, chloroform, dichloromethane, tetrahydrofuran) and possess molecular weights M, of about 13000. The absorption maxima (&,= 460 nm) of s(+) and 5 (-) are nearly identical, also when compared to the &,,,-value of racemic 5, as well as their 'H and I3C NMR spectra. Solutions containing 1 mg ladder polymer per ml dichloromethane display a molar optical rotation [@]zit in the order of (+) 5 940 degree cm2/mol and (-) 5040 degree - cm2/mol.

Fig. 19 shows the UVNIS absorption and CD spectra of 5(+) and 5(-). The CD spectra of 5 (+) and 5 (-) are mirror images of each other, with only slight differences in the relative intensities of the CD bands. The high chiroptical activity (molar ellip- ticity 8 = 2.2 x lo5 degree * cm2/mol) observed for the long-wavelength n-n* absorption is very striking. Remarkably, until now only compounds of helical structure, and especially chiral aggregates of them, have been described as materials with such high chiroptical activities'23 29-31).

To our knowledge, 5 (+) and 5 (-) represent the first non-helical polymers possessing a high chiroptical activity within the n-n*-transition. The nature of the surprisingly high activity is still not fully understood. Temperature-dependent CD measurements and the synthesis and characterization of model oligomers should clarify this important aspect.


Wavelength [nm]

Fig. 19. UVNIS absorption spectrum (dotted line) and CD spectra of 5 (+) (solid line) and 5(-) (dashed line), (room temperature; solvent: dichloromethane; c: 1 mg/ml; I : 0.05 cm)

The chiral polymers 5(+) and 5(-) show, as expected, an intense photoluminescence, both in solution and in the solid state. This is a promising entry for investigations of circularly polarized luminescence, both after photo- (CPL) and charge car- rier induced excitation (CPEL). The detection of circularly polarized electroluminescence in organic materials has not so far been reported, and obviously promising candidates for that are 5 (+)I5 (-).

Experimental part

Reagents The solvents were used as commercial p.a. quality. The preparation of the polymers

was carried out under an ar on atmosphere. Compounds 1 ,6 ,7 ,8 and 9 were synthesized according to the literature', Q OV 13). All other chemicals are commercially available.

2,5-Dipentyloxy-l, 4-benzenediboronic acid (10)

26.9 ml (0.043 mol) of butyllithium (1.6 M in hexane) were added to a solution of 2,5- dipentyloxy-l,4-dibromobenzene (9) (7.0 g, 0.017 mol) in diethyl ether (250 ml) at -78 "C (under argon). The mixture was allowed to warm up to room temperature and stirred for an additional 2 h. The solution was transferred to a dropping funnel and added to a cooled (-78°C) solution of triisopropyl borate (27.8 ml, 0.12 mol) in ether (150 ml) and then stirred for 12 h at room temperature. After hydrolysis with aqueous hydrochloric acid (2 N, 200 ml), the resulting precipitate was filtered, washed with 1 1 of cold water and dried at 5O0C/O.01 mbar. Yield: 3.2 g, 56%. 'H NMR (DMSO-d,j, 200 MHz): 6 = 7.76 (4H), 7.21 (2H), 4.41 (4H), 1.75 (4H), 1.39

(8H), 0.90 (6H). 13C NMR (DMSO-d6, 50 MHz): 6 = 157.23, 124.96, 118.35, 68.74, 28.75, 27.93,

22.11, 14.12

Novel chiral poly(para-phenylene) derivatives ... 2645

Poly[2,5-dipentyloxy- I, 4-phenylene] (2) A solution of the dibromo derivative 9 (100.3 mg, 0.246 mmol) and the diboronic acid

10 (83.1 mg, 0.246 mmol) in 10 ml of tetrahydrofuran was added to 10 ml of an aqueous 1 M potassium carbonate solution. The mixture was refluxed, and then 5 mg of tetrakis- (triphenylphosphino)palladium(O) in 5 ml of tetrahydrofuran were added. After refluxing for two days, the mixture was poured into water (150 ml). The precipitated polymer was washed with water and dried at 5O0C/O.01 mbar. Yield: 112.1 mg, 93%. I%?, = 7300; = 20500.

'H NMR (CDCl,, 200 MHz): 6 = 7.09 (2H), 3.92 (4H), 1.69 (4H), 1.36 (8H), 0.88

l3CNMR (CDC13, 50 MHz): 6 = 150.65, 128.19, 117.95, 70.15, 29.72, 28.76, 22.92,

UVNIS (tetrachloroethane): I,,/nm = 337.

(6H).

14.48.

Isotactic poly[2,5-(oxydecanoxy)- 1,4-phenylene] (3(+) and 3 (-)) 3 (+): Polycondensation of 2,5-(oxydecanoxy)- 1,4-dibromobenzene (6 (+)) (0.1 g,

0.246 mmol) and 2,5-(oxydecanoxy)- 1,4-benzenediboronic acid (7 (-) (0.082 g, 0.246 mmol) gave (according to the synthesis of 1) polymer 3 (+) in a yield of 91 % (1 10.1 mg).

'H NMR (C2D2C14, 500 MHz): 6 = 7.12 (2H), 4.05, 3.82 (4H), 1.66-0.93 (16H); DP (NMR) = 15. UVNIS (tetrachloroethane): I,,/nm = 327; [@I (589 nm, 25 "C, 0.225 mg per ml tetrachloroethane, 10 cm) = (+) 4200 degree * cm2/mol.

3 (-): Polycondensation of 2,5-(oxydecanoxy)-1 ,Cdibromobenzene (6 (-)) (0.1 g, 0.246 mmol) and 2,5-(oxydecanoxy)-l,4-benzenediboronic acid (7(-)) (0.082 g, 0.246 mmol) gave (according to the synthesis of 1) polymer 3(-) in a yield of 87% (105.8 mg).

'H NMR (C2D2C14, 500 MHz): 6 = 7.12 (2H), 4.05, 3.82 (4H), 1.66-0.93 (16H); DP (NMR) = 15.

UVNIS (tetrachloroethane): I,,/nm = 327; [@I (598 nm, 25 "C, 0.225 mg per ml tetrachloroethane, 10 cm) = (-) 4080 degree * cm2/mol.

Syndiotactic poly[2,5-(oxydecanoxy)-1,4-phenylene] (4)

Polycondensation of 2,5-(oxydecanoxy)- 1,4-dibromobenzene (6 (+)) (0.1 g, 0.246 mmol) and 2,5-(oxydecanoxy)- 1,4-benzenediboronic acid (7 (+)) (0.082 g, 0.246 mmol) gave (according to the synthesis of 1) polymer 4 in a yield of 89% (107.6 mg).

'H NMR (C2D2C14, 500 MHz): 6 = 7.08 (2H), 4.19, 3.89 (4H), 1.76-0.93 (16H); DP (NMR) = 15.

UVNIS (tetrachloroethane): I,,/nm = 328. Polycondensation of 2,5-(oxydecanoxy)- 1,4-dibromobenzene (6 (-)) (0.1 g, 0.246

mmol) and 2,5-(oxydecanoxy)- 1,4-benzenediboronic acid (7 (-)) (0.082 g, 0.246 mmol) gave (according to the synthesis of 1) polymer 4 in a yield of 92% (11 1.3 mg).

'H NMR (C2D2C1,, 500 MHz): 6 = 7.08 (2H), 4.19, 3.89 (4H), 1.76-0.93 (16H); DP (NMR) = 15.

UVNIS (tetrachloroethane): I,,/nm = 328.

1,4-Dibrorno-2,5-bis(3,4-dihexyloxybenzoyl)benzene (14) 2,5-Dibromterephthalic acid: 2,5-Dibromo-p-xylene (200 g, 0.75 mol) was added to

500 ml of HN03 (65%)/H20 (1 : 1) and then refluxed for 5 days. The resulting precipitate was dissolved in an aqueous sodium hydroxide solution, and treated with KMN04 (300 g, 1.9 mol). The mixture was heated under reflux for 24 h, an additional portion of KMNO, (50 g, 0.32 mol) was added and refluxed for a further 3h. The suspension was acidified with aqueous hydrochloric acid, the precipitated acid was filtered and washed with water. Yield: 190 g, 78.2%; m.p. > 250°C.


'H NMR (DMSO-d6, 200 MHz): 6 = 13.74 (s, 2H), 8.02 (s, 2H). I3C NMR (DMSO-d6,50 MHz): 6 = 165.79, 137.31, 135.24, 119.02.

C8H4Br204 (323,94) Calc. C 29.66 H 1.24 Found C 28.87 H 1.18

2,5-Dibromoterephthaloyl chloride (13): To a solution of dibromoterephthalic acid (25 g, 0.08 rnol) in 40 ml of benzene, thionyl chloride (50.0 g, 0.42 mol) was added and the mixture was heated for 3.5 h under reflux. After evaporating the solvent, the yellow product was used immediately for the synthesis of 14. Yield: 27.6 g, 95.6%; m.p. 90°C.

'H NMR (DMSO-d,j, 200 MHz): 6 = 8.02 (s, 2H). I3C NMR (CDCI,, 50 MHz): 6 = 165.63, 137.21, 135.27, 119.08. MS: mlz=360.1 (M').

1,2-Dihexyloxybenzene (12): Pyrocatechol(l1) (11 g, 0.01 mol), hexylbromide (41.3 g, 0.25 mol) and potassium carbonate (33 g, 0.24 mol) in 30 ml of 1-butanol were refluxed for 24 h under an argon atmosphere. After cooling the solution, 2N HCI and 100 ml of dichloromethane were added. The organic phase was isolated and washed with saturated sodium chloride and 5% aqueous sodium hydroxide solution, respectively, dried with MgSO, and concentrated. The surplus hexyl bromide was distilled off, and the resulting ether was used as received. Yield: 2.17 g, 88%.

'H NMR (CDCI,, 200 MHz): 6 = 6.82 (s, 4H), 4.08 (t, 4H), 1.86 (q, 4H), 1.42- 1.35 (m, 12H), 0.85 (t, 6H).

I3C NMR (CDCI,, 50 MHz): 6 = 149.8, 121.6, 114.9,69.9, 32.1,28.9,26.3,23.1, 14.5. MS: mlz = 278.4 (M').

C18H3002 (278,44) Calc. C 77.65 H 10.86 Found C77.41 H 10.72

1,4-Dibromo-2,5-bis(3,4-diheayloxybenzoyl)benzene (14): 2,5-Dibromoterephthaloyl chloride (13) (1.67 g, 4.63 mmol) and aluminium trichloride (1.48 g, 1 1 . 1 mmol) were suspended in 15 ml of dichloromethane. The orange suspension was cooled in an ice bath and 1,2-dihexyloxybenzene (12) (3.03 g, 10.88 mmol) in 10 ml of dichloromethane was added. The resulting mixture was stirred for 24 h at room temperature, and washed with water and 2N aqueous hydrochloric acid. The organic phase was isolated, washed with water and dried over MgS04. The solvent was evaporated, and the crude product was recrystallized from acetone. Yield: 2.1 g, 53%; m.p. 147 "C.

2H),4.11 (m, 8H), 1.86(m, 8H), 1.55-1.23 (m, 24H), 0.95 (t, 12H).

118.9, 113.5, 112.0, 69.8, 32.0,28.6,26.1,23.0, 14.4.

H NMR (CDC13, 200 MHz): 6 = 7.56 (s, 2H), 7.54 ( s , 2H), 7.24 (dd, 2H), 6.87 (d, 1

I3C NMR (CDC13, 50 MHz): 6 = 192.9, 155.3, 148.9, 143.9, 133.3, 128.5, 127.0,

MS: mlz = 844.6 (M').

C+&,j&,Brl (844,76) Cak. c 62.56 H 7.16 Found C 62.22 H 6.85

Polyketones 15 Racemic polyketone 15: A solution of 2,5-(oxydecanoxy)- 1,4-benzenediboronic acid

(7) (600 mg, 1.8 mmol) and 1,4-dibromo-2,5-bis(3,4-dihexyloxybenzoyl)benzene (14) (1,51 g, 1.8 mmol) in 10 ml of tetrahydrofuran was added to 10 ml of a 1.8 M aqueous sodium carbonate solution. The mixture was refluxed, and then tetrakis(tripheny1phosphi- no)palladium(O) (54 mg, 0.047 mmol) in 10 ml of tetrahydrofuran was added. After refluxing for 24 h, the mixture was poured into dichloromethane. The phases were sepa-

Novel chiral poly(paru-phenylene) derivatives ... 2647

rated, and the organic layer was washed with dilute hydrochloric acid and water. The resulting solution was dried over MgSO,, concentrated and the polymer precipitated by pouring into methanol (1 : 10, v/v). Yield: 1.31 g, 80%; a,, = 11 000, Mw = 21 000.

HNMR (CD2C12, 200 MHz): 6 = 7.55-6.64 (m, IOH), 4.16-3.48 (m, 12H), 1.96-0.63 (m. 60H).

13CNMR (CD2C12, 50 MHz): 6 = 195.15, 154.18, 150.30, 148.71, 141.51, 136.15, 132,30, 131.95, 130.63, 126.87, 121.60, 114.56, 112.14, 70.08, 69.84, 32.30, 30.46, 30.03,29.88,28.61,28.20,28.05,26.45,26.40,24.89,23.35, 14.52.

(-)-Polyketone 15: The polycondensation of (+)-2,5-(oxydecanoxy)-1,4-benzenediboronic acid (7(+)) (585 mg, 1.741 mmol) and 14 (1,47 g, 1.741 mmol) gave the (-)- polyketone 15 in a yield of 80% (1.26 g). an = 12300, Mw = 24000, [@] (546 nm, 20°C, 0.27 mg per ml dichloromethane, 10 cm) = (-) 30010 degree * cm2/mol.

(+)-Polyketone 15: The polycondensation of (-)-2,5-(oxydecanoxy)-1,4-benzenediboronic acid (7(-)) (498 mg, 1.48 mmol) and 14 (1,25 g, 1.48 mmol) gave the (+)-polyketone 15 in a yield of 82% (1.10 g). M,, = 13 100, MW = 28000; [a] (546 nm, 20"C, 0.32 mg per ml dichloromethane, 10 cm) = (+) 24400 degree * cm2/mol.

1

Polyulcohols 16 Racemic polyulcohol 16: The racemic polyketone 15 (1.17 g, 1.21 mmol) in 20 ml of

toluene was added to a suspension of lithium aluminium hydride (1.20 g, 31.6 mmol) in 100 ml of tetrahydrofuran and stirred for 1 h at room temperature. The reaction was quenched with ethanol and dilute hydrochloric acid. The organic layer was isolated, washed with dilute HC1 and water, dried over MgSO, and concentrated to dryness. The polyalcohol was further used without reprecipitation. Yield: 1.07 g, 91%; M,, = 11 000, Mw = 21000. 'HNMR (CD2C12, 200 MHz): 6 = 7.55-6.62 (m, lOH), 5.80 (m, 2H), 4.00-3.66 (m, 12H), 1.66-0.62 (m, 60H)

(-)-Polyulcohol 16: The polymer-analogous reduction of (-)-polyketone 15 (1.13 g, 1.21 mmol) with lithium aluminium hydride (1.30 g, 34.3 mmol) gave the (-)-polyalcohol 16 in a yield of 89% (1.01 g). M,, = 12300, M, = 24000; [a] (546 nm, 20°C, 0.30 mg per ml dichloromethane, 10 cm) = (-) 11 500 degree * cm2/mol.

(+)-Polyalcohol 16: The polymer-analogous reduction of (+)-polyketone 15 (970 mg, 1.04 mmol) with lithium aluminium hydride (1.10 g, 29.0 mmol) gave the (+)-polyalcohol 16 in a yield of 86% (840 mg). M,, = 13100, Mw = 28000; [a] (546 nm, 20°C, 0.30 mg per ml dichloromethane, 10 cm) = (+) 7200 degree * cm2/mol.

Ladder polymers 5 Rucemic ladder polymer 5: Boron trifluoride etherate (76 mg, 0.535 mmol) was given

to a solution of the racemic polyalcohol 16 (200 mg, 0.214 mmol) in 25 ml of dichloromethane. The solution was stirred for 30 min at room temperature. The cyclization was quenched with 10 ml of ethanol. After washing with water, the organic phase was isolated, dried over MgSO,, and concentrated. The polymer was precipitated by pouring into methanol (1 : 10, v/v). Yield: 155 mg, 83%; M,, = 12200, aw = 24000.

'H NMR (C2D2C14, 200 MHz): 6 = 7.63-6.59 (m, 8H), 5.14-4.92 (m, 2H), 4.13-3.51 (m, 12H), 1.89-0.71 (m. 60H).

13C NMR (C2D2CI4, 60°C, 125 MHz): 6 = 149.5, 148.5, 141.7, 139.4, 137.3, 134.1, 132.3, 129.9, 121.3, 115.7, 106.4,70.1,69.8, 53.1,33.2, 32.0,29.9,26.1,22.9, 14.4.

UVNIS (dichloromethane): I,,/nm (d(1 - mol-' - cm-')) = 461 (26400).

CmHs206 (899.3 1)" Calc. C 80.73 H9.19 Found C79.34 H9.11


(-)-Ladder polymer 5: The cyclization of the (-)-polyalcohol 16 (250 mg, 0.27 mmol) and boron trifluoride etherate (120 mg, 0.85 mmol) gave the (-)-ladder polymer 5 in a yield of 72% (175 mg). M,, = 13 100, Mw = 28000; [a] (546 nm, 20°C. 1.0 mg per ml dichloromethane, 10 cm) = (-) 5040 degree - cm2/mol.

(+)-Ladderpolymer 5: The cyclization of the (+)-polyalcohol 16 (250 mg, 0.27 mmol) and boron trifluoride etherate (120 mg, 0.85 mmol) gave the (+)-ladder polymer 5 in a yield of 70% (170 mg). M,, = 13 100, = 28000; [@I (546 nm, 20"C, 1.0 mg per ml dichloromethane, 10 cm) = (+) 5940 degree - cm2/mol.

Measurements

Crystals suitable for X-ray structure analysis were grown by isothermal removal of the solvent (ethanol). The structure determination was carried out on a Nonius CAD4 diffractometer with graphite monochromatic Cu K , radiation. The lattice parameters were obtained by a least-squares analysis of the setting angles of 25 reflections with 6, > 20". Intensities were obtained by 6-26 scans. The structure was solved by heavy atom methods (Patterson). The refinement was done with anisotropic temperature factors for the C, 0 and Br atoms. The H atoms were refined with fixed isotropic temperature factors in the riding mode. The occupancy factors for the atoms in the disordered part of the molecule were included in the refinement with the boundary condition that the sum of the occupancy factors for the two conformations is 1 . Pertinent crystallographic parameters are summarised in Tab. 132'. The absolute configuration was determined by measuring the intensities of Friedel pairs of reflections and including the Flack parameter in the refine- ment24). The absolute configuration of 6(+) was unambiguously determined by the analysis.

Tab. 1.

a/+ 9.160 (1) Space group P 4, 2, 2 (tetragonal) c/A 20.67 1 (1) Reflections measured 2517

z 4 R 0.044 D,l(glcm3) 1.555 R W 0.038

Crystallographic data of 6 (+)

VIA3 1734.4 ( 5 ) Reflections observed 2110

'H- and I3C NMR spectral data were obtained on a Bruker AMX 500 and a Varian Gemini 200 spectrometer. Gel-permeation chromatographic (GPC) antlysis utilized PL- gel columns (three columns, 10 pm gel, pore widths: 500, lo4 and lo5 A) connected with ultraviolethisible (UVNIS) detectors. All GPC analyses were performed on solutions of the polymer in 1,2-dichlorobenzene at 70°C (concentration: 2 gh). Calibration was based on polystyrene standards with narrow molecular-weight distribution. High-performance liquid chromatography (HPLC) separations were carried out using a Spectra Physics SP 8700 (analytical scale) and an Abimed 305 (preparative scale) solvent delivery system (detection at 310 nm). The UVNIS spectra were recorded on a Perkin-Elmer Lamda 9 spectrophotometer and the emission spectra on a Spex fluorescence spectrometer. The thermal analysis was carried out using a Mettler DSC 30 differential scanning calorimeter and a Mettler 500 thermpgravimetric analyzer. The wide-angle X-ray (WAXS) measurements were taken on a Philips PW 1820 powder diffractometer using Cu K , radiation at room temperature. Polarizing microscopy was performed using a Zeiss Photomikroskop EII. CDspema were measured on a Jasco 500 and 600.


Acknowledgements: The authors would like to thank Prof. Dr. Klaus Mullen for gener- ous support of these investigations. Our thanks are also due to Prof. G. Wulff and Dr. S. Kubik for carrying out the temperature-dependent CD measurements. J. Huber grate- fully acknowledges the financial support by a KekulC scholarship of the Fonds der Che- mischen Industrie and the Deutsche Forschungsgemeinschaft.

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Novel chiral poly(para-phenylene) derivatives containing cyclophane-type moieties