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

Abstract

We outline a method for the calculation of multipole moments and molecular dipole-dipole (

), dipole-quadrupole (

), and quadrupole-quadrupole (

) 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° in the alternate direction (+−+), and (3) a rotational isomer with each unit rotated 10° in the same direction (+++). The dipole moment, $\text{μ}$, is smaller for isomers 000 and +−+ than for isomer +++. The calculation of

,

, and

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

,

, and

are in the same order of magnitude as reference calculations (PAPID program). The values of

are rather sensitive to $\text{μ}$, which varies under rotation, explaining the greatest value of |A_x,xx| for polar isomer +++. 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

and

. However, the torsion of the oligomer can vary the value of $\text{μ}$ and so modify

.