  We have examined the singlet excitons in two representatives of acene-type
(tetracene and pentacene) and phenacene-type (chrysene and picene) molecular
crystals, respectively, using electron energy-loss spectroscopy at low
temperatures. We show that the excitation spectra of the two hydrocarbon
families significantly differ. Moreover, close inspection of the data indicates
that there is an increasing importance of charge-transfer excitons at lowest
excitation energy with increasing length of the molecules.
