LEWIS-ACID COORDINATION-COMPLEXES OF POLYMERS .2. COMPUTATIONAL MODELING OF SINGLE-CHAIN AND AGGREGATE STRUCTURES OF RIGID-ROD POLY(P-PHENYLENEBENZOBISTHIAZOLE)

Computational modeling of the molecular, electronic, and one-dimension al-aggregate structures of oligomers of poly(p-phenylenebenzobisthiazo le) (PBZT) and its 1:4 (repeating unit:Lewis acid) complexes has been made and used to explain previous experimental results. In the calcula tions the structure of PBZT was approximated by its monomeric model co mpound (2,6-diphenylbenzo[1,2-d:4,5-d']bisthiazole) (t-DBZT), dimer, a nd trimer and the PBZT complexes by the AlCl3 complexes of t-DBZT and the dimer. Geometry optimization, carried out by using MM2 and MOPAC-P M3, shows that PBZT has an essentially planar structure whereas the t- DBZT complex is sterically congested, exhibiting out-of-plane phenylen e ring twists of 62-degrees. Increased congestion in the dimer complex results in even larger dihedral angles. Calculations of the partial a tomic charges with MOPAC-ESP indicate that the Lewis acid-base reactio ns involve electron transfer from the base (polymer) to the Lewis acid . INDO/S calculations are used to explain the experimentally observed absorption spectra. Complexation has two opposing effects on optical a bsorption: a blue shift of the absorption edge and maximum is induced by steric congestion, while participation of the metal halide in elect ron delocalization produces a compensating red-shift. Monte carlo (MC) simulations of the one-dimensional aggregate structures of PBZT revea l that it forms very tightly packed structures with perpendicular inte rchain distances of typically 3.3-3.5 angstrom, in accord with experim ent. MC simulations of the aggregate structures of the complexes revea l very inefficiently packed structures with large unfillable voids bet ween the molecules. The computational modeling results explain the obs erved properties of Lewis acid coordination complexes of PBZT qualitat ively, including solubility in organic solvents, formation of liquid-c rystalline solutions at high critical concentrations, and extremely lo w glass transition temperature (T(g)).