Density-functional theory (DFT), and its time-dependent (TD) version, is applied to investigate the singlet-triplet energy splitting of a set of single molecules useful constituents of opto-electronics devices. The splitting relates to the singlet and triplet exciton formation rates and thus might influence the quantum efficiency for electro-luminescence. Systematic calculations are performed to address the origin of the discrepancies between experimental and calculated values. The results are found to strongly depend on the DFT level employed (pure and hybrid GGA and meta-GGA approaches) which makes difficult the choice of an accurate enough automated approximation for DFT-based studies in the field of nano-photonics. (C) 2007 Elsevier B.V. All rights reserved.