Vibronic Coupling in Organic Semiconductors: The Case of Fused Polycyclic Benzene-Thiophene Structures

The nature of vibronic coupling in fused polycyclic benzene–thiophene structures has been studied using an approach that combines high‐resolution gas‐phase photoelectron spectroscopy measurements with first‐principles quantum‐mechanical calculations. The results indicate that in general the electron–vibrational coupling is stronger than the hole–vibrational coupling. In acenedithiophenes, the main contributions to the hole–vibrational coupling arise from medium‐ and high‐frequency vibrations. In thienobisbenzothiophenes, however, the interaction of holes with low‐frequency vibrations becomes significant and is larger than the corresponding electron–vibrational interaction. This finding is in striking contrast with the characteristic pattern in oligoacenes and acenedithiophenes in which the low‐frequency vibrations contribute substantially only to the electron–vibrational coupling. The impact of isomerism has been studied as well. Fused together: The results of photoelectron spectroscopy measurements and quantum‐mechanical calculations indicate that the electronic structure of the radical‐cation state of the thienobisbenzothiophene isomers is very different from that in anthradithiophene and pentacene (examples are shown here), while it closely resembles the corresponding electronic structure in pentaphene.