In poly(3-hexylthiophene) (P3HT), it is suggested that bromination disturbs the pi conjugation and reduces the carrier mobility. To investigate the deconjugation mechanism, we theoretically studied a key flexibility of the Br-P3HT backbone, namely inter-ring S-C-C-S dihedral torsion, in terms of potential energy curve, geometry-based aromaticity index, and decomposed energies of inter-monomer interaction. The linkage between bromination and the excitation energy was also investigated. We concluded that steric repulsion due to bulky bromo group can cause out-of-plane S-C-C-S torsion of Br-P3HT and consequent shortening of the conjugation length, not electronic substituent effect, leads to blue shift of the S-0 -> S-1 excitation energy. (C) 2017 Elsevier B.V. All rights reserved.