Electronic structure of delocalized singlet biradical Ph2-IDPL solid film

Abstract

The film structure and electronic structure of a biradical hydrocarbon, diphenyl derivative of s-indacenodiphenalene (Ph₂-IDPL) solid film has been investigated. A small energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) compared with that of typical π-conjugated small molecules was observed even for the amorphous film of Ph₂-IDPL. This result indicates that the small HOMO–LUMO gap is an important characteristic of the singlet biradical electronic structure and well explains the previously reported ambipolar field effects of amorphous Ph₂-IDPL film by Chikamatsu et al., Appl. Phys. Lett. 2007, 91, 043506. It was found that the gas-deposition method substantially improved the crystallinity of the film where Ph₂-IDPL molecules form quasi one-dimensional (1D) molecular chains normal to the substrate surface. An extremely small HOMO–LUMO gap was observed in the polycrystalline Ph₂-IDPL film, which is possibly caused by strong intermolecular coupling. The photon energy dependence of ultraviolet photoemission spectra shows that the stacked Ph₂-IDPL molecular chain in the polycrystalline film develops an energy band structure in the direction of the surface normal of the film. The intermolecular covalency therefore evolves into the quasi 1D energy band along the molecular stacking direction.