Supramolecular Organization of α,α‘-Disubstituted Sexithiophenes

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Abstract
The self-assembly of α,α‘-linked sexithiophenes with chiral and achiral penta(ethylene glycol) chains attached at the α- positions of the terminal rings, that is, 2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘:5‘ ‘‘,2‘ ‘‘ ‘:5‘ ‘‘ ‘,2‘ ‘‘ ‘‘-sexithiophene-5,5‘ ‘‘ ‘‘-dicarboxylic acid−(2S)-2-methyl-3,6,9,12,15-pentaoxahexadecyl ester (1) and 2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘:5‘ ‘‘,2‘ ‘‘ ‘:5‘ ‘‘ ‘,2‘ ‘‘ ‘‘-sexithiophene-5,5‘ ‘‘ ‘‘-dicarboxylic acid−3,6,9,12,15-pentaoxahexadecyl ester (2), respectively is described. Analysis of the UV/vis, fluorescence, circular dichroism, and circular polarization of luminescence spectroscopic data shows that these compounds form chiral aggregates in polar solvents and in the solid state. In n-butanol aggregation occurs at temperatures below 30 °C, while above this threshold temperature the aggregates break up without an intermediate disordered state of aggregation, and the compounds are molecularly dissolved. The “melting temperature” of the aggregates depends on the concentration of sexithiophene, indicating that the optical changes observed are a result of intermolecular processes. Mass spectrometric measurements reveal that 1 and 2 can form mixed aggregates. Analysis of the optical spectra reveals that in these mixed aggregates, chiral 1 molecules act as “sergeants” to direct the packing of the “soldiers” 2, illustrating cooperativity within the columns. In water, the same type of chiral aggregates are formed as in n-butanol below 30 °C; however, these aggregates are still present, but the chirality is lost above 30 °C. In spin-coated films of 1 chiral aggregates are present. AFM studies show that 1 self-organizes into chiral fiberlike structures in the solid state. Furthermore both 1 and 2 display thermotropic liquid crystalline behavior between 180 and 200 °C.