Fullerene C60 Containing Liquid Crystalline Codendrimers : Synthesis, Characterization and Application

Natalia Yevlampieva *, Nikolai Beljaev *, and Robert Deschenaux +

* V.A. Fock Institute of Physics, St. PetersburgStateUniversity,

198504 St. Petersburg, Russian Federation

+ Institut de Chimie, Université de Neuchâtel, 2009 Neuchâtel, Switzerland

ABSTRACT

Liquid crystalline fullerene C60 containing dendritic compoundsof different design have been investigated. Mesomorphic and molecular properties of poly(benzyl ether)/poly(aryl ester) codendrimers bearing mesogenic groupshave been analyzed based on the results of theirs study by electrooptical Kerr effect andtotal polarity determination in dilute solutions and on the results of modelling by quantum chemical semiempirical method PM3.The strategy of the designconnected withthe inducing of asymmetry to the dendritic core using fulleropyrrolidine as the branching centre and different chemical structure dendrons as fullerene addendswas concluded the most promising for stimulation of self-organization of codendrimer molecules in the condensed phase. Specific fullerene microsegregation detected in the mesophase of poly(benzyl ether)/poly(aryl ester) codendrimers have been explained by the molecular anisometry and stiffness, responsible for the orientation of these compounds as a whole in solution under the influence of external electric fields and in condensed phases.

INTRODUCTION

Macromolecules of dendritic structure have received general acceptance as the nanosized building blocks in modern supramolecular chemistry and material science concerning to organic electronics. Normally, dendrons and dendrimers have well defined chemical structure, 3D-shape and terminal periphery, suitable forfurther modification, for instance, by mesogenic end-groups. The mesogenic groups’interactions are able tostimulate self-organization and formation of well ordered condensed phase of macromoleculesthat is important for producing of thin specially organized photosensitive films and conducting materials [1].Due to such properties mesomorphic dendrimers have a lot of preferable advantages in comparison with the ordinary linear structure polymers. Not far ago a fruitful idea to combine the polyester/polyetherdendrons bearing cyanobiphenyl mesogenic end-groups with fullerene C60 as the branching center into the hybrid macromolecules had been realized, and synthetic methodology based on the modular approach for synthesis of dendritic compounds had been developed [2, 3]. Novel molecular design permitted significantly to reinforce the variability of physico-chemical properties of dendritic compounds and gave a possibility to create multifunctional materials with tunable properties [2-4]. An appearance of mesomorphic fullerene containing dendrimersinitiated the research activity in the fields of plastic solar cells [5], organic light emitting diodes [6], photoactive dyads and polyades [7, 8]. The above-enumerated applications are directly connected with electron accepting properties of C60 [7].

Liquid crystalline fullerene containing dendritic compounds belong to relatively new class of multicomponentmacromolecules which structure-properties and self-organization ability continue to be under consideration [9, 10]. From strategic point of view thesphere-like shape of dendrimer is an obstacle for free ordering of terminal mesogenic end-groups bonded to dendritic core. Closed tospherical distribution of mesogenic end-groups around the core of dendrimer leads to the partial loss of theirsmesogenic ability. Traditional incorporation of long aliphatic spacers between the core and periphery is not very good decision in this case because of the initial unity of molecule practically disappears due to a separate behavior ofdendritic fragment andmesogenic end-groups in such molecule. The acceptable solution of this problem may be achieved by another strategy of synthesisbased on a special shift of the shape of dendritic core to more asymmetric one.

The present contribution is devoted toinvestigation of structure-properties relationsfor fullerene C60 containing codendrimers composed of two different type dendrons (Fig. 1a) or dendrons of different generations (Fig. 1b). Poly(aryl esters) and poly(benzyl ethers) have been selected for the synthesis of codendrimers due to well known ability of these compounds to form Langmure, Langmure-Blodgett, multy- or monolayer films [1]. Mesomorphic behavior of poly(aryl esters)/poly(benzyl ethers) codendrimers bearing mesogenic end-groupsis also discussed.

Determination of permanent dipole values and the study of electrooptical Kerr effect [11] in dilute solutions have been used as the basic experimental methods for investigation of molecular properties of dendritic samples. Quantum-chemical simulationhave been applied for the analysis of molecular polarity,for estimation of optical polarizability of building blocks of hybrid macromolecules, and for Kerr effect data interpretation. Quantum chemical calculations have been performed by semiempirical method PM3 in the framework of HyperChem program [12].

EXPERIMENTAL

Dendrons D1, D2 (Fig. 1a) have been synthesized as described earlier [13, 14]. Two approaches have been utilized for the synthesis of fullerenecontaining dendritic compounds1- 4. First approach is based on 1, 3-dipolar cycloaddition reaction of addends with fullerene C60[15] and leads to fulleropyrrolidine derivatives (Fig. 1b, compounds1-3). More details of this approach application for the synthesis of fulleropyrrolidinescan be found in [3, 4].The second approach,based on the addition of dendronsto C60 by applying Bingel reaction [16], produces methanofullerene derivatives (Fig. 1b, compound 4).

Molecular properties of codendrimers1-3, compound 4 and dendronsD1,D2 have been investigated in dilute benzene solutionsat 21 oC.

The permanent dipole moment values μ of compounds have been determined by Guggenheim-Smith method [17].This method is derived from the experimental determination of the dielectric permittivity increment (ε-εo)/c, where (ε-εo) is the difference between the dielectric permittivity of the solution and solvent, and from the determination of the squared refractive index increment (n2-no2)/c, where n and no are the refractive indices of the solution and solvent, respectively, and c is the solute concentration. Dielectric permittivity measurements were performed by a resonance technique at a frequency of 700 kHz using a standard capacity meter E12-1 and cylindrical titanium capacitor having its own capacity of 92.86 pF. Refractive indicesn were determined using Pulfrichrefractometer(IRF-23, Russia) with the line 578 nm corresponding to the wavelength of Hg-lamp. The permanent dipole moments were calculated according to equation (1).

μ2 = 27kT M [(ε-εo) / c - (n2-no2) / c] / [4πNA(εo2+2)2](1)

Here M is molecular mass, k is Boltsmann constant, T is absolute temperature, and NA is Avogadro’s number.

Linear concentration dependencies of (ε-εo) and (n2-no2) were observed for solutions of all compounds under investigation. The increments (ε-εo)/c and (n2-no2)/c were determined from the slopes of the mentioned dependencies. Values of increments are reported in Table 1.

Electrooptical properties of the compounds have been studied by equilibrium Kerr-effect method in radio frequency rectangular pulsed electric field [11, 18]. The specific electrooptical Kerr constant K and molar electrooptcal Kerr constant KM, connected with each other by equation (2),were determined for dendritic compounds.

KM= , (2)

where K= ; (Δn - Δno) is the difference between optical birefringence of solution with the solute concentration c and optical birefringence of solvent, respectively; E is the electric field strength; the subscript c→ 0 is symbolizing K value determination at the condition of infinite dilution. The others parameters of eq. (2) have been explained above.

The optical birefringence in solutions of 1-4, D1 and D2 under the treatment of the rectangular pulsed electric field have been measured with the impulse duration of 1 ms in the voltage range 0-1000 V. The compensatory technique with the photoelectric registration of optical birefringence valueΔn was applied. The thin mica plate compensator having its own optical phase difference 0.01x2π was used. Glass cell with the titanium semi-cylindrical electrodes of 2 cm in length and with the gap between electrodes of 0.05 cm was employed. He-Ne laser (1.5 mW power) operating at 632.8 nm was used as the light source.

The variation of optical birefringence value Δnas a function ofE2for different concentrations of 3 are shown in Fig. 2. No deviation from Kerr low (according to which, optical birefringence Δn is proportional to E2 in molecular dispersed liquids) was observed in solutions of 1-4, D1 and D2.

The dependences of (Δn-Δno/E2c) as a function of solute concentration are shown in Fig. 3. The (Δn-Δno/E2c)c→ovalues were obtained at c = 0and used for the calculation of KM according to equation (2).

Compounds D1, 3 and 4had liquid crystalline properties. Types ofmesophases and phase transition temperatures have been determined using polarized microscope with the temperature gradient control 0.2 grad per minute. Characteristics of mesomorphic properties of the investigated samples are presented in Table 2. Glass transition temperature had not been detected for compounds 3, 4. Mesomorphic properties of dendritic compounds were not similar to same one of low molecular liquid crystal analogues to mesogenic groups of 3, 4, as follows from Table 2.

RESULTS AND DISCUSSION

Realization of the declared in the introduction part strategy of synthesis of C60 containing dendrimers have been started with fulleropyrrolidine derivatives that were composed of different generation number poly(benzyl ether) dendrons (Fig. 1b, compounds 1, 2). Determination of the permanent dipole moments and electrooptical properties of codendrimers 1, 2 have shown that molecular properties continue to stay very similar (see, please, μ and KMof 1, 2inTable 1)when generation numbers and total number of polar groups significantly varyin these compounds. Thus, we have received experimental evidence thatspherical shape of dendritic core will determine properties of codendrimers1, 2before its modification by mesogenic end-groups. By other words, this experience have shown that different generations of the same structure dendrons are not able to induce a significant changeof the shape of dendritic molecule which sub-units (dendrons) are bonded to fullerene surface.

The next step in realization of the declared strategy was connected with the synthesis of fulleropyrrolidine derivative 3 composed of different chemical structure dendrons (Fig. 1b). Poly(benzyl ether) dendron D2of thethird generation and the second generation of poly(aryl ester) dendron D1of practically equal to each other hydrodynamic dimensions (see, please, d values in Table 1) have been selected for the synthesis of compound 3. Form asymmetry in the architecture of compound 3 was induced not only by the difference in chemical structure of dendrons bonded to C60, but also by the difference in molecular mass of D1 and D2 ( Table 1). Architecture of compound 3 was realized so, that it has a heavy “head”(D2) and strongly polar “tail” (D1), and that is similar to typical low molecular liquid crystal molecules, but at the level of dendritic structure molecules.

The latter strategy brought an interesting result.The mesomorphic behavior and supramolecular ordering in mesophase of compounds 3was not the same if comparedwith compound 4 in which structure twopoly(aryl ester) dendrons D1both bearing cyanobiphenyl mesogenic end-groups were bonded to fullerene surface (Fig. 1b) using traditional synthetic strategy. Methanofullerene derivative 4 and fulleropyrrolidine derivative3haveappeared significantly different thermotropic liquid crystalline properties as one can see in Table 2. Dendrimer4 has smectic A and very short nematic phase, when codendrimer 3 possesses rectangular columnar phase with theformation of fullerene layers between the dendrons detected by X-rays diffraction [4]. There was not detected fullerene segregation in mesophases of compound 4.

Experimental data,received forD1, D2 and 3, 4, permit us to analyzemolecular properties of codendrimer 3in detail and to explain the difference of mesomorphic properties of 3 and 4. First of all, it may be pointed out that D1, D2 and 3, 4 have large in value permanent dipole moments, including compound D2 which has not mesogenic end-groups. Each mesogenic group of D1,3 and 4 has large in value permanent dipole moment of 7.2 D according to quantum chemical calculation. Experimentally it wasdetected that compounds 3 and4 are characterized practically equal to each othertotal polarity (see, please, μ in Table 1) in spite of number of strongly polar mesogenic groups in their content differ twice. Due to dendrons D1 and D2 have closed in value hydrodynamic diameters d, the compounds 3 and 4alsohave similar size (d in Table 1), but the mass and polarity distribution are nonequivalent in 3and 4, that have been said above. These factsreflectan importance of structural difference between 3and 4, but it is not enough sufficient to explain the difference in mesomorphic behavior of these compounds (Table2).

Additional information on specificity of intermolecular organizationof compound 3and4has been received from theirs electrooptical properties. Elecrooptical Kerr constant KM of the substance directly depends on polarity, optical anisotropy and on the structural geometry of its molecules [11, 18]. It is well known that molar Kerr constant KMis an additive valuein the case when separated fragments of molecule are able to be independent in their orientations under the treatment of external electric field [11, 18]. Multicomponent compounds 3 and 4can be easily divided on some separate sub-units due to theirs individual structure. Because of this, it is possible to estimate the freedom degree of sub-units in 3 and 4 by means of comparison of experimental molar Kerr constants with the corresponding values calculated according to the additive scheme. It is self-evident that electrooptical properties of separated sub-units in objects under consideration need to be known. Quantum chemical modeling has been used for this purpose.

Before the calculation of KMan important remark need to taken into consideration in relation to difference in the chemical structure of dendrons D1 and D2.The total polarity and the dependent on polarity electrooptical properties of dendron D1 are fully determined by mesogenic end-groupsin contrast to dendron D2. Modeling has shown that the central part of D1 is highly symmetric nonpolar fragment(Fig. 4, Table 3). It means that electrooptical Kerr effect in solution of compound 4similar to D1in a great measure will be connected with inputs of mesogenic groups.

KM, cal value of single mesogenic group (its chemical structure may be seen in Fig. 1a) as well as KM, cal values of the model compound corresponding to the central part of D1 and of the fullerene derivatives analogues to the fullerenecontaining central fragment of compounds 3 and 4 (named FP and MF, correspondingly, Fig. 5) have been calculated with parameters accumulated in Table 3.

Estimation of molar Kerr constant KM, calaccording to additive scheme for multicomponent compounds 3 and 4 has been done as a sum (eq. (3)).

KM, cal = Σ KMiWi , (3)

i

where KMi is the molar Kerr constant of i-fragment and Wi is its weight fraction value.

The inputs of eight mesogenic groups, MF–sub-unit, and two polyaryl ester fragments (Fig. 4) without mesogenes have been taken for calculation of KM, cal for compound 4through eq. (3). Correspondingly, the inputs of the fourth mesogenic groups, D2 (its experimental value KM was used), polyaryl ester fragment and FP–sub-unit have been taken for calculation of KM, cal for compound 3.

The result of calculation presented in the last column of Table I shows very good coincidence between the calculated and experimental values of molar Kerr constant for compound 4; and at the same time an absence of coincidence can be declared for compound 3. This reveals that the rotational freedom of dendrons in compound 3 is significantly restricted. Furthermore, the fact that the experimental dipole moment value of 3 is practically the sum of the polarities of its both dendrons (see, please, μ column in Table 1) is another evidence of the structural stiffness of 3. Indeed, this situation is reached because D1 and D2 are stiffly linked in 3, and rotate synchronically in the external pulsed (in the case of Kerr effect study) and in the external sinusoidal (in the case dielectric measurements) electric fields which were used in the framework of this study. Due tostiffness the molecules’ packing in the mesophase of compound 3 will have a macroscale character and will differ from the same process in the mesophase of compounds 4, havingrelatively free and mobile sub-units. Molecular stiffnesswell explain the specific segregation of sub-units detected by X-rays diffraction in the mesophase of compound 3, where microsegregation of fullerene have been detected[4].

CONCLUSION

Mesomorphicfullerene C60 containing dendritic compoundsof different architecture design have been compared based on their solution and mesomorphic properties Novel self-organization type have been detected and explained for poly(benzyl ether)/poly(aryl ester) codendrimer with fulleropyrrolidine as the branching center. It was shown that the core asymmetry and rigid linkage of dendronsin such codendrimers are responsible for molecular and mesomorphic properties of these compounds. Fullerene containing mesomorphic poly(benzyl ether)/poly(aryl ester) codendrimers can be considered as successfulexample of design of liquid crystalline substances reproducinganisometry-principle, peculiar to low molecular liquid crystals, at the level of dendritic structure macromolecules.

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