Linearly condensed polythiophenes: characteristic molecular aggregation of thieno[2″,3″:4′,5′]thieno[2′,3′-d]thieno[3,2-b]thiophene crystals revealed by ultraviolet photoelectron spectroscopy

Linearly-condensed polythiophenes (4 and 5) have been prepared and their characteristic aggregation behaviour as hydrogen-poor π compounds in the solid state has been investigated in comparison with those of the lower homologues (2 and 3); the photoelectron spectra of the series 2–5 were measured in the solid state as well as in the gas phase. For 4 the difference in the ionization energies for the gaseous (adiabatic; 7.22 eV) and solid (threshold; 4.86 eV) states is significant, giving rise to a large polarization energy (2.36 eV), which is larger than the calculated value (1.83 eV) based on the packing density and molecular polarizability. This has been interpreted in terms of the intermolecular orbital interactions in the solid state of 4. Such an interaction has not been observed in the lower homologues (2 and 3).