  The electronic structures of boron nitride nanotubes (BNNTs) doped by organic
molecules are investigated with density functional theory. Electrophilic
molecule introduces acceptor states in the wide gap of BNNT close to the
valence band edge, which makes the doped system a $p$-type semiconductor.
However, with typical nucleophilic organic molecules encapsulation, only deep
occupied molecular states but no shallow donor states are observed. There is a
significant electron transfer from BNNT to electrophilic molecule, while the
charge transfer between nucleophilic molecule and BNNT is neglectable. When
both electrophilic and nucleophilic molecules are encapsulated in the same
BNNT, large charge transfer between the two kinds of molecules occurs. The
resulted small energy gap can strongly modify the transport and optical
properties of the system.
