Molecular studies of Planetary Nebulae
  Circumstellar envelopes (CEs) around evolved stars are an active site for the
production of molecules. After evolving through the Asymptotic Giant Branch
(AGB), proto-planetary nebula (PPN), to planetary nebula (PN) phases, CEs
ultimately merge with the interstellar medium (ISM). The study of molecules in
PNe, therefore, is essential to understanding the transition from stellar to
interstellar materials. So far, over 20 molecular species have been discovered
in PNe. The molecular composition of PNe is rather different from those of AGB
and PPNe, suggesting that the molecules synthesized in PN progenitors have been
heavily processed by strong ultraviolet radiation from the central star.
Intriguingly, fullerenes and complex organic compounds having aromatic and
aliphatic structures can be rapidly formed and largely survive during the
PPN/PN evolution. The similar molecular compositions in PNe and diffuse clouds
as well as the detection of C$_{60}^+$ in the ISM reinforce the view that the
mass-loss from PNe can significantly enrich the ISM with molecular species,
some of which may be responsible for the diffuse interstellar bands. In this
contribution, I briefly summarize some recent observations of molecules in PNe,
with emphasis on their implications on circumstellar chemistry.
