Development of a Dual-State Emissive Naphthalimide-Based Organic Probe: Interesting Synergistic Effects from the Solid State to the Solution Phase

Luminescent compounds have garnered significant interest for their wide range of applications in light harvesting, cell imagining, LASERs, light-emitting diodes, etc. Conventional luminescent organic molecules show their emissive properties in the solution phase; however, in the solid state, a quenching of the fluorescence intensity occurs. Researchers have been able to overcome such challenges using the phenomenon of aggregation-induced emission (AIE), which enhances the solid-state emissive properties through close packing aided by nonbonded interactions. Such molecules, conventionally referred to as AIE luminogens, are observed to show phase-exclusive emissive properties, i.e., emissive in the solid state but nonemissive in the solution phase. Such drawbacks have prompted us to investigate molecules having emissive properties in both solution and solid phase, viz., dual-state emissive molecules [dual-state emission (DSE) luminogens]. Among these, naphthalimides, owing to their versatile conformational arrangements, are of prime interest. In this article, we present the design, synthesis, structural characterization, and detailed spectroscopic studies of a naphthalimide-based luminogen molecule, 6-(4-(2-hydroxyethyl)piperazin-1-yl)-2-(6-methoxybenzothiazol-2-yl)-1H-benzoisoquinoline-1,3(2H)-dione (HPMTB). Single-crystal X-ray diffraction studies in the solid state have provided detailed structural information as well as the molecular packing arrangement of the molecule HPMTB along three dimensions. Solution-phase and solid-state fluorescence studies have revealed the existence of aggregation-induced emissive and mechanofluorochromic properties within the molecule. Detailed solution-phase photophysical investigations of HPMTB have revealed the existence of aggregates in a protic polar solvent (water) via the accumulation of flakes. Solid-state emission also shows the existence of reversible mechanofluorochromism in HPMTB. The current work develops materials that can attain DSE and mechanofluorochromism and sets up the foundation for the development of naphthalimide-based organic molecules for use as luminescent smart materials.