Study On Mechanism Of Mechanochromic Luminescence And Stragegy Of Long Lifetime Luminescent Materials

Luminescent materials,including fluorescent and phosphorescent materials,have broad important applications in the fields of lighting,optoelectronic display,chemical sensing and biological imaging.Therefore,the research of novel fluorescent and phosphorescent materials remains hot in the scientific community.During the course of my Ph.D.sdudy,1 have been working on the strategy of developing new functional fluorescent and room temperature phosphorescent materials,and exploring the mechanism and practical value of them as follows.1.We investigated the mechanism of mechanochromic luminescent difluoroboron dibenzoylmethane(BF2dbm).BF2dbm and its derivatives were previously found to exhibit reversible mechanochromic luminescence(ML)at room temperature.We first investigated the optical properties of monomeric,dimeric,and polymeric BF2dbm derivatives in optically dilute solutions and demonstrated unambiguously that BF2dbm moieties have a propensity to form H-aggregates.Next,we studied the physical properties of these boron complexes in the solid state including crystal structures,fluorescence emissions,and mechanochromic luminescence.By correlating solution data with solid-state characterization results,it was concluded that two coupled processes,force-induced emissive H-aggregates formation and energy transfer to the emissive H-aggregates,are responsible for the observed BF2dbm ML in the solid state.2.We proposed a strategy of Polymerization-Enhanced Intersystem crossing(PEX)to create small ?EST and long-lived exciton,the efficacy of which is justified by density functional theory(DFT)calculations and ultrafast spectroscopy.The intrachain aggregates of the BF2dbm polymer lead to many degenerate singlet and triplet excited states and thus strong states coupling.Upon photoexcitation,these intrachain aggregates exhibit picosecond bilateral intersystem crossing(BISC)with an intrinsic excited-state lifetime of 250 ns.Concentration,solvent,and temperature studies all revealed that more intrachain aggregates lead to more long-lived emissive species.PEX has significant implications as it allows for achievement of long-lived excitons with known fluorophores by circumventing syntheses and screening of new molecules.3.We proposed a strategy of intermolecular charge transfer to create small ?EST and long-lived exciton.After rational design and easy synthesis,we obtained long-lived intrinsic luminescence from a class of carbazole-alkyl-nitrobenzoate dyads in the solid state.From spectroscopic data and theoretical calculations,the long-lived emission is ascribed to an intermolecular CT state with nearly degenerate single-triplet energy levels.These dyads could self-assemble into luminescent nano-wires and nano-sheets under different conditions.This strategy has opened new doors for making purely organic phosphors with less demanding synthetic chemistry.4.Understanding the nature of external heavy-atom effect(EHE),i.e.,the specific orbital interactions between the external heavy atom and the luminophore,is of essential importance in molecular design.In this report,we conducted an investigation on how heavy-atom perturbers and aromatic luminophores interact based on data obtained from crystallography.We synthesized two classes of molecular systems including N-haloalkyl-substituted carbazoles and quinolinium halides,where the luminescent molecules are considered as "base" or "acid"relative to the heavy-atom perturbers,respectively.We propose that electron donation from a ? molecular orbital(MO)of the carbazole to the ?*MO of the C-X bond(?/?*)and n electron donation to a ?*MO of the quinolinium moiety(n/?*)are responsible for the EHE(RTP)in the solid state,respectively.