The effect of applying an external voltage to a molecular wire connecting two reservoirs of states is analyzed through the use of a Hubbard Hamiltonian which explicitly depends on the applied voltage. The Hamiltonian is solved using a method based on two-by-two rotations of molecular orbitals, which avoids divergence, and permits both ground and excited states to be obtained. The special case of a polyacene wire is studied in detail as a function of the wire length. It is shown that the energies of the ground and excited states of the wire cross as the voltage is increased. This crossing produces specific features in the current-voltage characteristic of the molecular wire.