Development of a\nDual-State Emissive Naphthalimide-Based\nOrganic Probe: Interesting Synergistic Effects from the Solid State\nto the Solution Phase

Luminescent\ncompounds have garnered significant interest for their\nwide range of applications in light harvesting, cell imagining, LASERs,\nlight-emitting diodes, etc. Conventional luminescent organic molecules\nshow their emissive properties in the solution phase; however, in\nthe solid state, a quenching of the fluorescence intensity occurs.\nResearchers have been able to overcome such challenges using the phenomenon\nof aggregation-induced emission (AIE), which enhances the solid-state\nemissive properties through close packing aided by nonbonded interactions.\nSuch molecules, conventionally referred to as AIE luminogens, are\nobserved to show phase-exclusive emissive properties, i.e., emissive\nin the solid state but nonemissive in the solution phase. Such drawbacks\nhave prompted us to investigate molecules having emissive properties\nin both solution and solid phase, viz., dual-state emissive molecules\n[dual-state emission (DSE) luminogens]. Among these, naphthalimides,\nowing to their versatile conformational arrangements, are of prime\ninterest. In this article, we present the design, synthesis, structural\ncharacterization, and detailed spectroscopic studies of a naphthalimide-based\nluminogen molecule, 6-(4-(2-hydroxyethyl)piperazin-1-yl)-2-(6-methoxybenzothiazol-2-yl)-1<i>H</i>-benzoisoquinoline-1,3(2<i>H</i>)-dione (<b>HPMTB</b>). Single-crystal X-ray diffraction studies in the solid\nstate have provided detailed structural information as well as the\nmolecular packing arrangement of the molecule <b>HPMTB</b> along\nthree dimensions. Solution-phase and solid-state fluorescence studies\nhave revealed the existence of aggregation-induced emissive and mechanofluorochromic\nproperties within the molecule. Detailed solution-phase photophysical\ninvestigations of <b>HPMTB</b> have revealed the existence of\naggregates in a protic polar solvent (water) via the accumulation\nof flakes. Solid-state emission also shows the existence of reversible\nmechanofluorochromism in <b>HPMTB</b>. The current work develops\nmaterials that can attain DSE and mechanofluorochromism and sets up\nthe foundation for the development of naphthalimide-based organic\nmolecules for use as luminescent smart materials.