The dynamics of intramolecular charge transfer (CT) in a family of conjugated aromatic electron donor/acceptor (D/ A) systems has been studied by femtosecond pump-probe spectroscopy. Although the compounds have very similar structures we observed vastly different CT dynamics, ranging from a few 100 fs to the nanosecond time range. These results show that specific modifications in the substitution scheme can lead to alterations of the relevant D/A orbitals and thus to drastic changes in the excited state electronic coupling. As a result the CT process switches from the ultrafast adiabatic to the slow nonadiabatic regime.