  We demonstrate Bragg diffraction of the antibiotic ciprofloxacin and the dye
molecule phthalocyanine at a thick optical grating. The observed patterns show
a single dominant diffraction order with the expected dependence on the
incidence angle as well as oscillating population transfer between the
undiffracted and diffracted beams. We achieve an equal-amplitude splitting of
$14 \hbar k$ (photon momenta) and maximum momentum transfer of $18 \hbar k$.
This paves the way for efficient, large-momentum beam splitters and mirrors for
hot and complex molecules.
