First-principles calculations of pyridines: From monomer to polymer

The structural and electronic properties of pyridine, its oligomers, and po lypyridine (PPY) as obtained with density functional methods are presented in this work. Among the different exchange-correlation functionals used, B3 LYP,gives good structural results, whereas B3P88 predicts more accurately t he electronic properties. The calculated first excitation energies of pyrid ine systems are in good agreement with experimental data. The coupling betw een the monomers in forming oligomers influences the structural and electro nic properties of the system significantly, The trans head-to-head dimer is found to be the most stable form and the only one having a planar geometry . The introduction of a head-to-head or tail-to-tail coupling in order to b reak the regioregularity of a tetramer changes the frontier orbitals and th e total energy of the system. The inclusion of a head-to-head coupling in t he central units of a tetramer leads to a global stabilization of the syste m and lowers the HOMO, producing an increase in the first electronic excita tion energy. Finally, the electronic properties of infinite PPY are obtaine d by extrapolations from those of finite oligomers, The calculated ionizati on potential, electronic affinity, and (pi-pi)(1) transition are 6.3, 3.4, and 2.9 eV, respectively, in excellent agreement with previous experimental reports. Furthermore, the band structures and density of states of PPY are calculated using a DFT-LMTO method. The calculated density of states is in good qualitative and quantitative agreement with experimental UPS spectrum for this system.