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Heat‐Shrinking Spherical and Columnar Supramolecular Dendrimers: Their Interconversion and Dependence of Their Shape on Molecular Taper Angle
Author(s) -
Ungar Goran,
Percec Virgil,
Holerca Marian N.,
Johansson Gary,
Heck James A.
Publication year - 2000
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/(sici)1521-3765(20000403)6:7<1258::aid-chem1258>3.0.co;2-o
Subject(s) - thermotropic crystal , supramolecular chemistry , dendrimer , crystallography , alkyl , materials science , molecule , chemistry , liquid crystal , chemical physics , polymer chemistry , crystal structure , organic chemistry , liquid crystalline , optoelectronics
Synthesis and modes of self‐assembly are described for the tapered monodendritic molecules 3,4, 5‐ n G i ‐X of generation i =1, 2, 3 (see structures below) that contain multiple (CH 2 ) n H alkyl chains on their periphery ( n =12, 14, 16) and a polar group X at the apex (X=COOH, COONa, COOCs, CO(OCH 2 CH 2 ) 3 OH). These monodendrons self‐assemble into supramolecular cylindrical or spherical dendrimers, which in turn self‐organise into p 6 mm columnar or Pm 3¯ n cubic thermotropic liquid crystals, respectively. The two principal ways of affecting the self‐assembly of these compounds by means of their molecular architecture are: a) by changing the width of the wide (aliphatic) end, and b) by changing the volume at the apex. In the present work a) is controlled through temperature (conformational disorder) and b) is controlled by changing the generation number i or the size of X, for example, through the choice of metal cation. The single most important geometric parameter of these dendritic building blocks is the molecular solid angle (taper angle) α ; a high α leads to spherical and a low α to cylindrical supramolecular dendrimers. Furthermore, α also determines the equilibrium size of the supramolecular objects; a larger α results in a smaller diameter. The unusually strong negative thermal expansion coefficient of the cubic and columnar lattice is attributed to the excess of the increasingly highly tapered molecules being rejected from their parent aggregates and reassembling as new ones. Increasing α is also considered to be responsible for the observed thermotropic columnar‐cubic transition.