1 research outputs found
Molecular Orbital Models of Benzene, Biphenyl and the Oligophenylenes
A two state (2-MO) model for the low-lying long axis-polarised excitations of
poly(p-phenylene) oligomers and polymers is developed. First we derive such a
model from the underlying Pariser-Parr-Pople (P-P-P) model of pi-conjugated
systems. The two states retained per unit cell are the Wannier functions
associated with the valence and conduction bands. By a comparison of the
predictions of this model to a four state model (which includes the non-bonding
states) and a full P-P-P model calculation on benzene and biphenyl, it is shown
quantitatively how the 2-MO model fails to predict the correct excitation
energies. The 2-MO model is then solved for oligophenylenes of up to 15 repeat
units using the density matrix renormalisation group (DMRG) method. It is shown
that the predicted lowest lying, dipole allowed excitation is ca. 1 eV higher
than the experimental result. The failure of the 2-MO model is a consequence of
the fact that the original HOMO and LUMO single particle basis does not provide
an adequate representation for the many body processes of the electronic
system.Comment: LaTeX, 12 pages, 3 eps figures included using epsf. To appear in
Chemical Physics, 199