Stilbenoid dendrimers

Stilbenoid dendrimers are able to undergo aggregation. Depending upon the generation number, some of the pure substances form liquid-crystalline phases (Dha discotic hexagonal disordered phase Dra discotic rectangular disordered phase Dob discotic distorted phase). Differential scanning calorimetry (DSC) revealed phase transitions between 99°C and 0°C. 

18 Convergent synthetic route to polybenzene dendrimers (according to Mullen et al.). Because of the mutual ster-ic hindrance of the numerous benzene rings, the molecule shown will tend to be spherical rather than planar 

In 5 (Fig. 4.20) steric hindrance in the peripheral region appears to be too high for formation of a liquid-crystalline phase. Mesophases were characterised by polarisation microscopy and X-ray diffraction. Presumably the LC properties cease as a result of segment mobility with increasing number of stilbene building blocks in principle, the number of conformers should double with each double bond although the maximum number of 2n (e.g. 221 for the third generation) is unattainable for symmetry reasons. 

The tendency to undergo aggregation in cyclohexane increases with the generation number. It causes strong signal broadening attributable to hindrance of segment mobility. With 5 the effect is so pronounced that a sharply resolved 

1H-NMR spectrum is no longer obtained in C6D12, even on warming. Aggregation in non-polar media can also be followed as a function of temperature and concentration in the fluorescence spectra. 

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Structurally, dendrimers of type 5 are best compared with rigid dendrimers made up of tolan building blocks. However, neither the form of aggregation observed in stilbenoid dendrimers, nor the phase behaviour or the photochemistry of these compounds have so far been found to occur in similar form in tolan systems. 

Gn Tolanoid dendrimers Stilbenoid dendrimers Elongated Stilbenoid... 

Meier, H., Lehmann, M., and Kolb, U., Stilbenoid dendrimers, Chem. Eur. J., 6, 2462, 2000. Paulsen, H., and Bartsch, W., Phosphorus-containing carbohydrates. Part 13. Rearrangement of sugar-1-enophosphonates to sugar-2-enophosphonates, Chem. Ber., 108, 1745, 1975. 

Photochemistry and mobility of the stilbenoid dendrimers [all-( )-l,3,5-tris[2-(3,4,5-tridodecyloxyphenyl)ethenyl]benzene] and [all-( )-l,3,5-tris(2- 3,5-bis[2-(3,4,5-trido-decyloxyphenyl)ethenyl]phenyl ethenyl)benzene] in their neat phases were synthesized and investigated [129]. Selectively deuterated, dodecyloxy-substituted stilbenoid dendrimers of the first and second generations (Figure 1.21) were synthesized by a convergent synthesis, using the Wittig-Homer reaction. Molecules deuterated at the... 

Figure 1.21 Chemical structure of the stilbenoid dendrimers synthesized and investigated in Ref. [129]. (Reproduced with permission from Ref [129].)...

Photochemistry and mobility of stilbenoid dendrimers in their neat phases have been studied [36]. The photochemistry and the fluorescence in different crystal and liquid crystal phases selectively containing deuterated, dodecyloxy-substituted stilbenoid dendrimers of the first and second generations were investigated. Molecules deuterated at the a-position of the alkoxy chains were involved in reactions of double bonds in the liquid crystal phases whereas no photoreactions occurred in the crystal... 

Stilbenoid-like dendrimers possessing an analogous molecular structure were reported by Meier to be mesomorphic too [273]. The five generations of stilbene-based dendrimers (Fig. 57) [274,275] and the first two gener-... 

Fig. 57 Chemical structures of the stilbenoid-like dendrimers (R = OC3H7, OCgH, ...

Figure 5.11 Structure of a conjugated dendrimers, star-shaped stilbenoid phthalocyanines [54]. (Reproduced with permission.)...

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