  There exist two types of structural instability in polyacene: double bonds in
a cis pattern and those in a trans pattern. They are isoenergetic but
spectroscopically distinct. We demonstrate optical characterization and
manipulation of Peierls-distorted polyacene employing both correlated and
uncorrelated Hamiltonians. We clarify the phase boundaries of the cis- and
trans-distorted isomers, elucidate their optical-conductivity spectra, and then
explore their photoresponses. There occurs a photoinduced transformation in the
polyacene structure, but it is one-way switching: The trans configuration is
well convertible into the cis one, whereas the reverse conversion is much less
feasible. Even the weakest light irradiation can cause a transition of
uncorrelated electrons, while correlated electrons have a transition threshold
against light irradiation.
