Naphthalene　diimides　(NDIs)　are　an　interesting　class　of　electron-poor　organic　dyes　but　with　weak　emissions　that　limit　their　applications.　Recently　several　examples　have　demonstrated　that　self-assembly　of　such　weak　emissive　NDIs　into　some　special　aggregates　could　enhance　their　emissions.　However,　a　detailed　emission　mechanism　remains　unexplored　due　to　the　lack　of　structural　evidences.　In　order　to　go　deep　into　the　emissive　mechanism　and　in　turn　guide　material　design,　it　is　necessary　to　learn　the　molecular　arrangement　in　the　aggregates.　For　this　purpose,　in　this　paper　we　used　a　crystal　engineering　strategy　to　explore　the　structure-property　relationships　between　the　molecular　packing　and　emissive　properties　of　NDI　derivatives　in　crystalline　state,　which　revealed　that　the　small　slipped　cofacial　pi-pi　stacking　of　NDI　cores　in　the　solid　states　answered　for　their　enhanced　emission　behaviors.　This　observation　has　an　extensive　applicability　and　practical　significance　for　design　of　NDI　based　aggregation-induced　emission　materials.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.