Naphthalimide-based Mechanochromic Luminescent Molecules:Design,Synthesis And Property

Mechanochromic luminescent(ML)materials are a kind of functional materials whose optical behavior can undergo reversible changes under mechanical stimuli.They are capable of exhibiting high functional utility in a wide variety of applications,including biologic medicine,chemical sensors,storage media,and optoelectronic devices,etc.However,due to slow development and a lack of effective design strategies,the families of ML materials are limited so far.Thus,present research on ML materials mainly focused on developing new types of ML materials and revealing the inherently universal discipline.In this dissertation,a new series of dyads based on naphthalimide were synthesized.By covalent conjugation with naphthalimide,rhodamine and spiropyran,these naphthalimide based dyads exhibited excellent ML properties.Together with the single crystal analysis and theoretical simulation,the relationship between molecular structures and optical behaviors under mechanical stimulus was discussed,which provided an in-depth understanding of the underlying mechanism at molecular level.The main contents are as follows:In the second chapter,it provided a strategy for fabricating new stimuli-responsive dyes.The key lies in the introduction of non-planar amidoamine branch-skeletons and steric hindrance groups to restrict ?-?stacking between the planes of naphthalene.The compound M1 was synthesized through the conjugation of two naphthalimide moieties by the skeleton of amidoamine branch.In particular,M1 had an intrinsic property of cocrystallizing with methanol.Upon crystallization from methanol,well organized structure of M1 was formed by hydrogen bonding between the amide groups and solvent molecules.The fluorescence quantum yield of the resulting M1/methanol cocrystals increased fivefold over that of the amorphous state,called crystallization-induced emission enhancement(CIEE).This is the first example of methanol containing cocrystalline material showing mechanochromic fluorescent behavior and a large spectral shift(60 nm)under force stimuli.The cocrystalline material exhibited unusual“turn off" thermochromic luminescence upon solvent evaporation.Moreover,using external stimuli to reversibly manipulate fluorescent quantum yields has rarely been reported to date.In this chapter,it demonstrated the feasibility of our design strategy for new solid-state ML switching materials:the introduction of solvents into organic compounds by cocrystallization to obtain a crystalline state luminescence system.This method is convenient in operation and has promising potential for practical applications.In the third chapter,ML materials based on the cleavage of intramolecular chemical bond were discussed.Herein we report a molecular dyad M2 with excellent ML properties,which contains covalently linked rhodamine and M1 chromophores.Mechanical stimuli can trigger the mechanochemical reaction and rupture the intramolecular weak bond of rhodamine from the spirolactam to ring opened amide.Afterwards,the emission of the dyad“turned on”.In this case,M1 was in nonplanar conformation,providing a large free volume for rhodamine to change its structure in the solid state.For comparison,the control compound M3 by the reaction of rhodamine with ethylamine was synthesized.Such tight molecular packing of M3 significantly impeded the isomerization between the ring-closed and ring-opening forms,leading to the inertness of M3 to mechanical force.In this chapter,the large free volume provided by nonplanar molecular structure played a crucial role in triggering the mechanochromic switch.We thus reason that linking a highly sterically bulky molecule to rhodamine could provide considerable free volume,thus facilitating efficient mechanoswitching.In the fourth chapter,we report a molecular dyad P1 with dual-sensitivity of photochromism and mechanochromism,which contains amide-linked spiropyran and naphthalimide chromophores.Particularly,P1 exhibits tunable mechanoresponsive self-assembly due to the crystalline to crystalline transformation.In addition,by heating the ground sample at 110 ? for several minutes,it could fully return to its initial state.Under light and mechanic stimuli,P1 performed different color transition.Firstly,P1 showed well-defined self-assembled microstructure(fibrillar structure)with yellow emission.Under UV light irradiation,the self-assembly of P1 remained unchanged,but its fluorescence changed from yellow to pink.After grinding,P1 assemblies turned into spherical morphology accompanied with a red emission.We employed theoretical simulation and single crystal analysis to elucidate the microstructure of the morphological change.Upon mechanical stimulus,the overlapped packing pattern of P1 was changed and intermolecular hydrogen bonds were destroyed,leading to the formation of another crystal phase and morphological change from fibrillar to spherical nanostructure.However,UV irradiation only caused ring-opening of spiropyran moiety and barely had any influence on the morphology of P1 assembly.Owing to the slight difference of molecular packing mode caused by photo-and mechano-stimuli,respectively,the fluorescent peak positions are significantly different in the two distinct conformations.Owing to the dual-responsive properties,P1 described in the dissertation is intriguing as smart materials.To the best of our knowledge,mechanical stimulus induced crystalline-to-crystalline transformation accompanied with the self-assembly switch have not been reported previously.This study not only provides a new guideline for molecular structure design and the intermolecular interaction regulation of mechanochromic materials,but also provides a feasible design direction for the development of multifunctional materials.