The low-lying electronic states of pyrene and substituted pyrenes have been studied in detail by high level ab initio and density functional methods to aid in the development of the efficient luminescent properties of pyrenes. The electronic structures of the ground and the low-lying singlet excited states have been examined by the complete active space SCF (CASSCF) method. The energy gaps between the ground state and the low-lying excited states have been estimated by means of the multireference perturbation theory based on CASSCF wavefunctions as well as time-dependent density functional theory (TDDFT). The changes in electronic structure of pyrene, in particular, the excitation energies of the excited states by several different substituents have been investigated by the TDDFT method. Both mono-substituted and di-substituted pyrene derivatives have been considered, and the effects on the energy gap between HOMO and LUMO by electron donor and electron acceptor groups have been discussed. (c) 2005 Elsevier B.V. All rights reserved.