Main issue described in this article is clarification of spin-spin interactions in delocalized singlet biradical species, especially co-existence of intra- and inter-molecular interactions. For the end, we designed and prepared some singlet biradical molecules thermodynamically stabilized by utilizing spin-delocalizing character of phenalenyl radical. These species were found to be stable in air and could be isolated in a crystalline form. X-ray crystallographic analysis revealed the abnormally short π-π contact between molecules, suggesting a strong intermolecular spin-spin interaction, even though the species had a strong intramolecular spin-spin interaction. Co-existence of both the interactions led to lower energy shift of HOMO-LUMO absorption bands in a solid state with respect to solution bands. These findings indicate that covalent bonding character is operative within and between molecules in molecular aggregates of delocalized singlet biradicals. This new aspect was supported by variable-temperature X-ray crystallography and reflectivity measurement on single crystals which had one-dimensional π-π chains of the delocalized singlet biradicals, and we concluded that the electronic structure of the 1-D chain could be represented by the Resonating Valence Bond model proposed by Pauling.