Impact of the molecular quadrupole moment on ionization energy and electron affinity of organic thin films: Experimental determination of electrostatic potential and electronic polarization energies

Energy levels of organic molecular films exert paramount influence on the electronic properties of organic semiconductors. Recently the effect of electrostatic energy was highlighted as an origin of such peculiar phenomena as the molecular-orientation dependence and continuous tuning of energy levels. However, the mechanism has been discussed mostly based on theoretical work and has not been adequately supported by experiments. In this work, we propose a procedure to evaluate the electrostatic and electronic polarization energies in organic films solely from the experimental data obtained by ultraviolet photoelectron and low-energy inverse photoelectron spectroscopies. We apply it to the energy levels of thin films of 6,13-pentacenequinone on SiO_2 substrates with three different molecular orientations. The obtained electrostatic energies are fully consistent with the theoretical results at different levels such as the first-principles calculations and the electrostatic energy of charge-multipole interactions. The present work also underlines the importance of the charge-permanent quadrupole interaction which is the leading term of the electrostatic energy in the film of nonpolar molecules