  Benzobisthiadiazoles (BBTs) are promising organic semiconductors for
applications in field effect transistors and solar cells, since they possess a
strong electron-accepting character. Thereby the electronic structure of
organic/metal interfaces and within thin films is essential for the performance
of organic electronic devices. Here, we study the structural and the electronic
properties of two BBTs, with different core substitution pattern, a phenyl
(BBT-Ph) and thiophene (BBT-Th) derivative adsorbed on Au(111) using
vibrational and electronic high-resolution electron energy loss spectroscopy in
combination with state-of-the-art quantum chemical calculations. In the mono-
and multilayer both BBTs adopt a planar adsorption geometry with the molecular
backbone as well as the phenyl and thiophene side groups are oriented parallel
to the gold substrate. The energies of the lowest excited electronic singlet
states and the first triplet state (T1) are determined. The optical gap is
found to be 2.2 eV for BBT-Ph and 1.6 eV for BBT-Th. The energy of T1 is
identified to be 1.2 eV in BBT-Ph and in the case of BBT-Th 0.7 eV. Thus, both
the optical gap size as well as the T1 energy are drastically reduced in BBT-Th
compared to BBT-Ph. Based on our quantum chemical calculations this is
attributed to the electron-rich nature of the five-membered thiophene rings in
conjunction with their preference for planar geometries. Variation of the
substitution pattern in BBTs opens the opportunity for tailoring their
electronic properties.
