The electronic structure of the low-energy states of 1,4,5,8-naphthalenetetracarboxy diimide (1) is investigated in terms of CASSCF method. At this level of approximation the Franck-Condon (FC) parameters were evaluated for the 13 totally symmetric modes of (1) molecule in its dipole-allowed low-energy 1(1)B(2u), and 1(1)B(3u). electronic states. These parameters are then employed to simulate the vibrational structures of absorption and fluorescence spectra of N,N'-dibutyl (2) and N, N'-bis-hydroxypropyl (3) derivatives of naphthyl diimides measured in the region corresponding to the low-energy 1(1)A(g) --> 1(1)B(2u) and 1(1)A(g) --> 1(1)B(3u) overlapping transitions. (C) 2002 Elsevier Science B.V. All rights reserved.