Interstellar Fullerene Compounds and Diffuse Interstellar Bands
  Recently, the presence of fullerenes in the interstellar medium (ISM) has
been confirmed especially with the first confirmed identification of two strong
diffuse interstellar bands (DIBs) with C60+. This justifies reassesing the
importance of interstellar fullerenes of various sizes with endohedral or
exohedral inclusions and heterofullerenes (EEHFs). The phenomenology of
fullerenes is complex. In addition to fullerene formation in shock shattering,
fully dehydrogenated PAHs in diffuse interstellar (IS) clouds could perhaps
efficiently transform into fullerenes including EEHFs. But it is extremely
difficult to assess their expected abundance, composition and size
distribution, except for C60+. EEHFs share many properties with C60, as regards
stability, formation/destruction and chemical processes, and many basic
spectral features. We address the interstellar importance of various EEHFs as
possible DIB carriers. Specifically, we discuss IS properties and the
contributions of fullerenes of various sizes and charge such as C60+,
metallofullerenes, heterofullerenes, fulleranes, fullerene-PAH compounds,
H2@C60. We conclude that the landscape of interstellar fullerenes is probably
much richer than heretofore realized. EEHFs, together with pure fullerenes of
various sizes, have properties necessary to be suitably carriers of DIBs:
carbonaceous nature; stability and resilience in the ISM; various heteroatoms
and ionization states; relatively easy formation; few stable isomers; right
spectral range; energy internal conversion; Jahn-Teller fine structure. This is
supported by the C60+ DIBs. But, the lack of information about optical spectra
other than C60 and IS abundances still precludes definitive assessment of the
importance of fullerenes as DIB carriers. Their compounds could significantly
contribute to DIBs, but it still seems difficult that they are the only
important DIB carriers.
