Penning ionization electron and ultraviolet photoelectron spectroscopy of ultrathin bis(l,2-benzoquinonedioximato)platinum(II) films

Penning ionization electron spectra (PIES) and ultraviolet photoelectron spectra (UPS) have been measured for ultrathin (3-10 Å) bis(1,2-benzoquinonedioximato)platinum(II) (Pt(bqd)2) films to investigate the molecular aggregation and electronic structure. Upon vapor deposition onto a cooled graphite (0 0 0 1) surface, Pt(bqd)2 forms either of two films A and B with completely different spectra. The PIES of film A exhibits enhanced bands due to π and platinum 5d⊥ (dxz, dyz or dz2)-derived MOs while that of film B is characterized by overwhelming bands due to oxygen nonbonding MOs, which indicates that the molecule lies flat in film A but the molecular short axis stands in film B. Moreover, the UPS features of film A do not correspond to those of film B and are essentially the same as those of an amorphous film prepared on a cooled metal surface. Nevertheless, films A and B afford almost the same features in both PIES and UPS when annealed to room temperature: the PIES become rather smooth to provide somewhat enhanced π and d⊥-derived bands whereas the UPS obtain an increased number of distinct bands. From these observations, it is considered that the electronic structure of a Pt(bqd)2 molecule is modified through the intermolecular interactions of the d⊥-derived MOs, which operate differently depending on the molecular aggregation.