We have found optical photoinduced second harmonic generation in pyrene molecular crystals
photoinduced by intrinsic defects. A correlation between the number of
macro-vacancies (defects) measured by positron annihilation and second-order
nonlinear optical susceptibilities indicates the appearance of additional
non-centrosymmetry in the electron charge density distribution caused by the
defects. Simultaneously, changes in the kinetics of phenomena in the picosecond
time regime are demonstrated. The latter indicate the essential role of intra-
and intermolecular contributions in the observed phenomena.