Optical activity of dendrimers with chiral end groups

Very recently we reported on the peculiar behavior of amino acid derivatized poly(propylene imine) dendrimers. It was obsd. that the optical activity decreased in going to higher generations. For these dendrimers that posess both an amide and a carbamate fuctionality in the end groups, the optical activity of a model compd. showed a strong solvent dependency, indicative for conformational changes. In order to investigate the effect of multiple hydrogen bonding interactions in the endgroups we have also been able to synthesize a series of dendrimers modified with endgroups in which the carbamate functionality is exchanged for an etheral moiety (unable to create hydrogen bonding interactions), while the size is kept const. The striking result with this compd. is that the optical activity is independent of the the no. of endgroups, while the model compd. of this deriv. exhibits only a minor solvent dependency. The preliminary explanation for the the effects obsd. we can ascribe to multiple hydrogen bonding interactions causing frozen-in conformations, leading to a compensation effect

Liquid crystalline properties of poly(propylene imine) dendrimers functionalized with cyanobiphenyl mesogens at the periphery

Three generations poly(propylene imine) dendrimers with 4, 16, and 64 terminal amine groups have been functionalized with pentyloxycyanobiphenyl and decyloxycyanobiphenyl mesogens. The liq.-cryst. properties of these dendrimers have been studied in detail by differential scanning calorimetry, optical polarization microscopy, and X-ray diffraction. All the mesogenic dendrimers orient into a smectic A mesophase. Thermal properties are influenced to a large extent by the spacer length, showing g -> SA -> I transitions for the dendritic mesogens with the pentyloxy spacers and K -> SA -> I transitions for the ones with a decyloxy spacer. In the latter, the temp. range of the mesophase increases with dendrimer generation. Mesophase formation in the case of the pentyloxy series is more difficult compared with the corresponding decyloxy analogs, when the transition enthalpies and the kinetics of obtaining microscopic textures are considered. The effect of generation on mesophase formation cannot be clearly distinguished, although in the case of the fifth-generation dendrimer with a decyloxy spacer, microscopic textures could be obtained more easily, compared with the lower generations. X-ray diffraction measurements of oriented samples indicate that the cyanobiphenyl endgroups of both series orient into an antiparallel-overlapping interdigitated structure. The obsd. SA-layer spacings are independent of the dendrimer generation for both spacer lengths, indicating that the dendritic backbone has to adopt a completely distorted conformation, even for the higher generation