Torsional effects on the molecular polarizabilities of the benzothiazole (A)-benzobisthiazole (B) oligomer A-B13-A.

We outline a method for the calculation of multipole moments and molecular dipole-dipole (alpha), dipole-quadrupole (A), and quadrupole-quadrupole (C) polarizabilities, which we have successfully applied to benzothiazole (A)-benzobisthiazole (B) oligomer A-B13-A. Three model rotational isomers have been characterized: (1) the fully planar (000) rotational isomer; (2) a conformation with each unit rotated 10 degrees in the alternate direction (+(-)+), and (3) a rotational isomer with each unit rotated 10 degrees in the same direction (+3). The dipole moment, mu, is smaller for isomers 000 and +(-)+ than for isomer +3. The calculation of alpha A, and C has been performed by use of the interacting induced dipoles polarization model, which calculates tensor effective anisotropic point polarizabilities (method of Applequist). The values of alpha, A, and C are in the same order of magnitude as reference calculations (PAPID) program). The values of A are rather sensitive to mu, which varies under rotation, explaining the greatest value of magnitude of Ax,xx for polar isomer +3. This rotational isomer has the maximum hydrophilic accessible surface, which would improve solubility in water. It is found that small torsional changes can enhance solubility by increasing the hydrophilic accessible surface without too much affecting the values of alpha and C. However, the torsion of the oligomer can vary the value of mu and so modify A.