Intermolecular Effects on the Hole States of Triisopropylsilylethynyl-Substituted Oligoacenes

The effects of intermolecular interactions on the electronic properties of bis-triisopropylsilylethynyl-substituted (TIPS) anthracene, tetracene, and pentacene are obtained from comparison of the ionization energies measured by solid-phase ultraviolet photoelectron spectroscopy (UPS) with the ionization energies measured by gas-phase UPS, and with the oxidation potentials measured electrochemically in solution. Additional insight is provided by electronic structure calculations at the density functional theory level. The results show that the solution-phase oxidation potentials correlate linearly with the gas-phase first ionization energies of TIPS oligoacenes, and both energies decrease with the increase in acene core size as expected for the increasing delocalization of the HOMO. However, the solid-phase ionization energies are independent of the acene core size, and thus do not follow the trend indicated by the molecular electronic structures and verified by the gas-phase and solution measurements. The solid-phase electronic properties such as charge injection barriers, ionization energies, and HOMO-LUMO energy gaps are greatly affected by the polarization effects of the surrounding molecules in the solid state, which dominate over the changes in molecular electronic properties caused by the change in acene core size.