Optical gap in herringbone and pi-stacked crystals of [1]benzothieno[3,2-b]benzothiophene and its brominated derivative

The optical gap of the organic semiconductor [1]benzothieno[3,2-b]benzothiophene and its 2,7-dibrominated analogue is measured in solution and in the crystalline state by means of UV-vis and emission spectroscopy. Bromination leads to a change in molecular packing from herringbone to pi-stacked, resulting in a marked shift in the absorption and emission spectra which is found to be in accordance with TDDFT calculations.