We study the charge separation and recombination of two arylamino core-substituted naphthalene diimides by transient absorption. Both compounds show a 3 ps relaxation from the initially excited partial to the full charge transfer state. Quantum chemical calculations show that this process is associated with a twisting of the arylamino substituent. In the twisted conformation the energy gap between ground and excited state is 0.7 eV for two amino core-substituents and 0.5 eV for one amino and one chloro core-substituent. The larger energy gap leads to a six-fold increase in the lifetime of the fully charge separated state from 11 to 70 ps. (C) 2011 Elsevier B. V. All rights reserved.