  Functionalized adamantane molecular cluster materials show highly transient
nonlinear optical properties of currently unclear structural origin. Several
interaction mechanisms in compounds comprising molecular clusters, their inter-
and intramolecular interactions as well as the interplay of their electronic
systems and vibrations of their backbone are viable concepts to explain these
nonlinear optical properties. We show that transient Coulomb forces also have
to be considered as they can lead to intramolecular structure transformations
and intermolecular rearrangements in the crystal. Both strongly influence the
nonlinear optical properties. Moreover, selective bromine functionalization can
trigger a photochemical rearrangement of the molecules. The structure and
chemical bonding within the compounds are investigated in dependence on the
laser irradiation at different stages of their nonlinear emission by electron
diffraction and electron energy loss spectroscopy. The transient structural and
chemical states observed are benchmarked by similar observations during
electron irradiation, which makes quantification of structural changes possible
and allows the correlation with first principles calculations. The
functionalization and its subsequent usage to exploit photochemical effects can
either enhance two-photon absorption or facilitate white-light emission rather
than second-harmonic generation.
