Polarization by the effect of a small torsional change in the benzothiazole (A)-benzobisthiazole (B) oligomer A-B13-A

We use a method for the calculation of the molecular dipole ( ) and quadrupole ( ) moments and dipole-dipole ( ), dipole-quadrupole ( ) and quadrupole-quadrupole ( ) polarizabilities which we have successfully applied to benzothiazole (A)-benzobisthiazole (B) linear oligomer A-B13-A. Two model rotational isomers have been characterized: 1) the fully-planar (000) conformation; and 2) a rotational isomer with each unit rotated in the range ¡ in the same direction (+++). For isomer 000, is smaller than for +++. The calculation of , and has been carried out by the interacting induced dipoles polarization model that calculates tensor effective anisotropic point polarizabilities (method of Applequist et al.). The values of are specially sensitive to which varies under rotation. This fact explains the great values of for the polar isomer +++. This rotational isomer has the maximum hydrophilic accessible surface area which can improve solubility in water. It is found that small torsional changes can enhance solubility by increasing the hydrophilic accessible surface area. However, the torsion of the oligomer varies the value of and so modifies , , and, specially, . For conformer +++, polarization effects occur even when each unit is rotated only ¡. The observed effect is an increase in and, hence, in , , and, specially, .