Core-substituted　naphthalene　diimides　are　attracting　much　interest　because　of　their　tunable　optical　properties,　which　are　generally　achieved　by　the　elaborate　introduction　of　electron-donating　groups　into　electron-deficient　core　to　form　donor-acceptor　compounds　through　the　covalent　bonds.　Here　we　report　that　the　optical　properties　of　core-unsubstituted　naphthalene　diimides　can　be　modulated　by　a　facile　cocrystal　strategy.　Based　on　the　X-ray　crystallographic　and　Hirshfeld　surface　analyses,　the　pi-pi　interactions　have　been　demonstrated　to　be　the　most　important　factor　that　answers　for　the　optical　properties　of　naphthalene　diimide　cocrystals,　while　other　interactions,　such　as　hydrogen　bonds　and　solvation　effects,　have　little　impact　on　their　properties　but　influence　pi　stacking.　This　insight　is　of　considerable　significance　for　the　development　of　naphthalene　diimide　based　materials　in　various　directions.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.