Interacting induced dipoles polarization model for molecular polarizabilities: application to benzothiazole (A)-benzobisthiazole (B) oligomers AB 13A

Abstract

We have outlined a method for the calculation of atomic net charges and molecular dipole-dipole,  α , dipole-quadrupole,  A , and quadrupole-quadrupole,  C , polarizabilities which we have successfully applied to benzothiazole (A)-benzobisthiazole (B) oligomers in the series AB 1A to AB 13A, and the results have been extrapolated to give predictions for polybenzobisthiazole. The calculation of  α ,  A and  C has been carried out by the interacting induced dipoles polarization model that calculates tensor effective anisotropic point polarizabilities (method of Applequist). The mean effective atomic α effpolarizabilities for the monomeric unit are of the same order of magnitude as reference calculations (PAPID program). The value of the dipole moment drops sharply from the monomeric unit to the dimeric unit. The high dipole moment of the monomeric unit forces a charge redistribution in the molecule, that reverses the sign of the mean quadrupole moment with respect to the remaining oligomers. The values of  α and  C for the oligomers are much larger than the sum of their component parts, as would be expected owing to the larger length of π conjugation. However, the values of  A are also sensitive to the molecular dipole moment. This fact explains a relative maximum in ¦ A x, xx ¦ for the polar monomeric unit. The remaining oligomers show a constant dipole moment, and a non-linear increase in ¦ A x, xx ¦ with the number of units is observed. The fractal dimension of the solvent accessible surface and the 1-octanol-water partition coefficient, P, increase with the number of units in the oligomer. Functions of N (the total number of units either A or B) have been fitted to α (mean value), α xx , ¦ A x ¦ ( x vector component), ¦ A x, xx ¦ and C xx, xx , and these may be used to predict the values for longer oligomers. The finite oligomer results have been extrapolated to predict optical properties of polybenzobisthiazole.