Radiationless transitions of 1-(dimethylamino)naphthalene (1DMAN) and its 4-, 5-, 6-, and 7-methoxy derivatives having a twisted dimethylamino group in the Franck-Condon excited state are investigated by means of picosecond time-resolved fluorescence measurements. The fast internal conversion (IC) that has previously been observed for IDMAN, is also found for its 5- and 6-methoxy derivatives, while the IC rate decreases substantially in the 4- and 7-methoxy derivatives, showing that the fast IC is related to the proximity of the S-1 and S-2 states and also to the electronic structures of the S-1 and S-2 states. (c) 2005 Elsevier B.V. All rights reserved.