Abstract
One- and two-photon absorption cross sections of symmetric open-shell singlet molecular systems are investigated using a valence configuration interaction scheme. It is found for one-photon absorption (OPA) that the peak intensity decreases and the peak position moves to the low energy region as increasing the diradical character of the system. In contrast, a significant enhancement of two-photon absorption (TPA) peak is predicted for open-shell singlet systems with intermediate diradical character as compared to the closed-shell and pure diradical systems. It is revealed that the largest TPA peak intensities occur for open-shell singlet diradicals having a ferromagnetically-coupled ground state and strongly depends on the ratio between damping factors of the excited states.
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