The electronic spectra of the sandwich stacked PFBT: a theoretical study

Stacked models that include 9,9'-bis(6''-N,N,N-trimethylammonium)hexyl]fluorene-co-alt-4,7-(2,1,3-benzothiadiazole)dibromide (F(BT)F) monomer sandwiched between two stacked 2,1,3-benzothiadiazole (BT) units were explored using theoretical approaches. Molecular structures and the optical characteristics of the investigated species were investigated at the M06-2X/6-311G(d,p)//TD-M06-2X/6-311G(d,p) level of theory. In all models, the electronic excitation to the lowest singlet ππ* excited state (S1(ππ*)) is governed by the highest occupied molecular orbital to lowest unoccupied molecular orbital (HOMO → LUMO) transitions. The obtained results suggest that stacking interaction might have only minor effects on the transition energy for both absorption and emission processes. Instead, the reduction in the excitation energy of the stacked complexes should be attributed to the dipole-dipole interaction. The larger the interaction energy of the stacked models, the bigger the observed differences between absorption-emission energies. The presence of the solvation medium with small dielectric constant may increase the absorption-emission energy differences. It is expected that the largest absorption-emission shift can be observed in the benzene solution.