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Figure 2. Diameter © and height h of the disks or cylinders of 1a-e (R = OC6H13), determined with the CERIUS 2.0 software for the molecular models.

Figure 3. Plot of GPC retention times t versus the molecular diameters © of the compounds 1a-e. Reprinted with permission from [20], H. Meier et al., Chem. Eur. J. 6, 2462 (2000). © 2000, Wiley-VCH.

the concentration range between 10-5 and 10-8 M in dichloromethane, changes the shape, shift, and chromicity of the absorption bands [45].

Thermotropic liquid crystals (LC phases) are normally based on calamitic, phasmidic, or discotic mesogens. Spheroidal dendrimers are not capable of generating mesophases unless they are flexible enough to adopt such mesogenic arrangements. An obvious approach to dendritic LC phases consisted, therefore, of the attachment of suitable mesogens on the dendrimer periphery. Poly(propylene imine) dendrimers, for example, functionalized with 3,4-didecyloxybenzoyl groups, exhibit in the first four generations the formation of hexagonal columnar mesophases, whereas the fifth generation fails [52, 53].

In the series of stilbenoid dendrimers, the whole scaffold represents the mesogen, provided that a disk-like geometry can be realized. Neat 1a and 1b (R1 = R2 = OC6H13 or OC12H25) form columnar LC phases [19, 20]. A microphase segregation between the stilbenoid scaffolds and the surrounding region of the alkoxy chains has to be assumed. The w,w stacking of the mesogen as well as the Van der Waals interactions of the sidechains contribute to the self-organization. Propoxy chains are obviously not long enough to stabilize such mesophases. According to differential scanning calorimetry (DSC), polarization microscopy, and X-ray scattering, the columnar phases and transitions listed in Table 2 were established [20]. The excellent agreement of calculated and measured Bragg reflections allows a definite characterization of the LC phases.

Figure 5 shows the model of the Colhd phase of 1b (R1 = R2 = OC12H25). The cell parameter was determined by X-ray measurements.

A variety of further columnar and lamelar mesophases has been found in the series 2, 9, and 10 [23, 39].

Figure 3. Plot of GPC retention times t versus the molecular diameters © of the compounds 1a-e. Reprinted with permission from [20], H. Meier et al., Chem. Eur. J. 6, 2462 (2000). © 2000, Wiley-VCH.

1 ' ■ i • ' ' i ■ i ' i ' i ' i ' i ' i '-------—------1 1 i 1 i 1 i 1

1 ' ■ i • ' ' i ■ i ' i ' i ' i ' i ' i '-------—------1 1 i 1 i 1 i 1

Figure 4. Low-field part of the 'H NMR spectra of a solution (4.6 • 10-3 M) of 1b (R1 = R2 = OQ2H25) in CDCl3 and C6D12 at room temperature. Signals: at: singlet, three inner aromatic protons. am: pseu-dosinglet, nine aromatic protons on the middle rings. ao : singlet, 12 aromatic protons on the outer rings. ot : pseudosinglet, six olefinic protons of the inner double bonds. oo : AB system with 3J = 16 Hz, 12 olefinic protons of the outer double bonds. Adapted with permission from [19], H. Meier and M. Lehmann, Angew. Chem. 110, 666 (1998); Angew. Chem. Int. Ed. 37, 643 (1998). © 1998, Wiley-VCH.

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