  Two-dimensional molecular crystals have been beyond the reach of systematic
investigation because of the lack or instability of their well-defined forms.
Here, we demonstrate drastically enhanced photostability and Davydov splitting
in single and few-layer tetracene (Tc) crystals sandwiched between inorganic 2D
crystals of graphene or hexagonal BN. Molecular orientation and long-range
order mapped with polarized wide-field photoluminescence imaging and optical
second-harmonic generation revealed high crystallinity of the 2D Tc and its
distinctive orientational registry with the 2D inorganic crystals, which were
also verified with first-principles calculations. The reduced dielectric
screening in 2D space was manifested by enlarged Davydov splitting and
attenuated vibronic sidebands in the excitonic absorption and emission of
monolayer Tc crystals. Photostable 2D molecular crystals and their size effects
will lead to novel photophysical principles and photonic applications.
