According to their spectroscopic behavior, four classes of nonafulvene
s may be distinguished, but, so far, only three classes have been iden
tified. Type-A nonafulvenes (including parent 1a) are typically olefin
ic molecules with strongly alternating bond lengths and a nonplanar ni
ne-membered ring. Type-B nonafulvenes are characterized by four pairs
of equivalent ring H-atoms and ring C-atoms. Spectra of both Type-A an
d Type-B nonafulvenes are not dependent on temperature and solvent pol
arity. However, spectra of Type-C nonafulvenes (including prototype 1d
with R(1) = R(2) = NMe(2)) are strongly influenced by temperature and
solvent polarity due to an equilibrium 1 reversible arrow 1(+/-) betw
een the nonpolar olefinic 1 and dipolar planarized 1(+/-). So far, Typ
e-D nonafulvenes occurring exclusively in the dipolar form 1(+/-) were
unknown. Synthetic attempts towards nonafulvenes of Type D are descri
bed and problems encountered in nonafulvene syntheses are discussed. S
everal new cyclononatetraenes and four new nonafulvenes (or nonafulval
enes) 31, In, 3, and 5 have been synthesized. Spectroscopic evidence s
hows that 11,12-bis(diethylamino)nonatriafulvalene 5 is the First Type
-D nonafulvene.