| Organic light-emitting materials have aroused extensive interests because of its unique photophysical properties and promising applications in organic light-emitting diodes?OLEDs?,display screens,biological imaging,biological and chemical sensing,etc.In recent years,it has been found that a few kinds of nonconventional luminescent materials only containing nonconventional chromophores?such as aliphatic tertiary amine,carbonyl,and amide?have attracted increasing attention,due to their significant fundamental importance and promising applications.So far,the research on nonconventional luminogens is still in its infancy,and most systems were serendipitously observed.Especially,the luminescence mechanism is still under debate,emerging various hypotheses due to the diversity of chromophores in nonconventional luminogens.To further study the mechanism and explore new nonconventional luminescence systems,it is essential to summarize the common rules of different systems.Therefore,in this dissertation,the photophysical properties of nonconventional luminogens?including small molecules and polymers?were explored,containing varied nonconventional chromophores?such as amine groups and carbonyls,etc.?.The photophysical properties and luminescent mechanism of solutions at different concentrations and solids of different states at different temperatures were further studied and discussed.Furthermore,those nonconventional luminogens are attempted to apply in many fields like bioimaging.The major contents are summarized as follows:1).To acquire more insights into the emission of biomolecules,and to unveil more common grounds and correlations for both general and biomolecular nonconventional luminogens,we thoroughly investigated the photophysical properties of nonaromatic amino acids,which are the basic building blocks of some biomolecules.Those results indicated that all natural non-aromatic amino acids with different side chains can generate noticeable visible light emission even under UV irradiation in proper states,particularly in solid states.L-Lys,L-Ser,and L-Ile were chosen as the examples to show the general aspects of various nonaromatic amino acids.Negligible photoluminescence?PL?of L-Lys,L-Ser,and L-Ile is observed for their dilute solutions,whereas considerable PL rise is noticed for their concentrated counterparts and solid powders,which showed concentration enhanced emission and aggregation induced emission?AIE?properties.Such emission behaviors are similar to common nonconventional luminogens,and can be well rationalized by the clustering-triggered emission?CTE?mechanism.Namely,clustering of diverse subunits like amine,carbonyl,and other functional side groups extended electron delocalization and thus enhanced the conjugation together with simultaneously rigidified conformations.Subsequently,these clustered chromophores can be readily excited with UV-A irradiation to yield visible emission.Single crystal analysis further indicated that besides hydrogen bonding,abundant intermolecular interactions including O···O?2.907??and C=O···N?2.820,2.830,and 2.861??short contacts are present,forming a 3D electron communication network.In addition to fluorescence,room temperature phosphorescence?RTP?are observed in their solid states.Those results not only successfully correlate the previously discovered nonconventional luminogens with luminescent nonaromatic biomolecules but provide new implications for the biological autoluminescence and RTP emission from proteins.Excellent photophysical properties of nonaromatic amino acids promoted us to explore their applications in bioimaging,which demonstrates specific staining of endosomes from HeLa cells.These properties lay the foundation for further development of their potential applications in biomedical and optoelectronic fields.2).To further reveal the general fundamental principle of biomolecule luminescence under different conditions,the luminescent properties of nonaromatic dipeptides?L-Ser-L-Val?,tripeptides,and?-poly-L-lysine??-PLL?were studied.Fluorescent intensities of L-Ser-L-Val,tripeptides,and?-PLL are enhanced with increasing concentrations,which exhibit AIE properties.These photophysical behaviors can be well explained by the CTE mechanism.Single crystal analysis further demonstrated that the emissive“cluster”of those polypeptides can be formed by the subunits of amide,amine,carbonyl,and other functional side groups with?electrons and lone pairs?n?.Since the distances among C,N,and O atoms are smaller than their sum of the van der Waals radii,the 3D space electronic communications are formed by interactions of O···O,N···O=C,and N···N,resulting in electron cloud overlapped and effectively extending conjugation to form luminescent centers.Besides fluorescence,RTP is generally detected in the solid powders.Strikingly,after ceasing the irradiation,persistent RTP from the?-PLL powders is clearly noticed with the lifetime up to 17.6 ms,which has not been reported in the nonconventional luminescent biomolecules.These results are instructive for reassessing the luminescence of proteins,especially the origin of RTP from proteins in the solid states.3).Nylon-6?PA-6?is a widely used polymer material with abundant amide?NHCO?groups,which are similar to the peptide bonds from nonaromatic polypeptides.According to the CTE mechanism,the intrinsic emission of PA-6 should be generated under proper conditions,and may be modulated through its crystallinity and polymorphism.The PL intensity of PA-6 solutions increases with the increment of concentration,manifesting high efficiency up to 7.6%.Noticeably,the PA-6/formic acid solutions,prepared from solid samples purified in THF and DMF,exhibited blue and green fluorescence emission,respectively.Numerous NHCO clusters are formed,due to the PA-6 chains entanglement at the aggregated states,which results in strong intra-/intermolecular interactions.Those interactions derived from NHCO moieties can afford further through space electronic communications between?electrons and lone pairs?n?,resulting in electron cloud overlap and thus extended conjugation together with simultaneously rigidified conformations.Therefore,these clustered chromophores can be readily excited even with UV-A irradiation to yield visible emission.Meanwhile,the clustered chromophores with emission of diverse colors were attributed to the different solubility of PA-6 solids with different molecular arrangements.Therefore,PA-6 films of the?and?phase were prepared through cast and electrospun methods.The RTP emission is also detected from those films,with the lifetime of 31.59 and35.24 ms,respectively.Furthermore,p-RTP,which is easily observed from?phase of PA-6 film after ceasing UV-A light,can only be observed from?phase of PA-6 film in vacuum.The reason for such a phenomenon can be that the molecular arrangement of?phase of PA-6 is looser than that of the?phase,leading to vulnerable RTP easily quenched by oxygen and moisture in the air.The excellent luminescent properties,good processability and high stability of PA-6 encouraged us to expand its applications in luminescent materials and devices.4).These aforementioned results demonstrate that the luminescence of diverse nonconventional luminescent luminogens can be well explained by the CTE mechanism.To further check whether it can be used for the molecular design of new nonconventional luminogens,nonaromatic polyurethanes?PUs?bearing carbamate?NHCOO?groups were designed and synthesized by the addition polymerization between diisocyanates and ethylene/1,4-Butylene glycols,which featured concentration enhanced emission and AIE characteristics,and displayed excitation wavelength dependent emission.The quantum yield???of PU solid powders is up to13.1%,and RTP emission is also detected.The intrinsic emission of PUs can be well rationalized in terms of the CTE mechanism.In aggerated states,the formation of NHCOO heterogeneous clusters can extend the effective conjugation and then generate multifarious emissive species.Besides,the microenvironment of C=O groups varies from the isolated states in dilute solutions to the aggregated solid state.Generally,the enhancement of conjugation or electron density of C=O units will result in the absorption at lowered frequency.Therefore,the shift of the IR absorption of C=O from 1727 to 1702 cm–1 is attributed to the electronic communications among NHCOO groups in the clusters.Furthermore,PUs also can be used for the explosive?picric acid?detection and cellular imaging,which is remarkable at a concentration as low as 2.6×10–5 mg mL–1 and demonstrates specific staining of endosomes from HeLa cells.These results show the feasibility of sensible molecular design towards nonconventional luminogens based on CTE mechanism,which are crucial to the in-depth fundamental understanding and potential applications.5).The CTE mechanism can well account for nonconventional luminogens.These results inspire us to rationally consider whether the combination of aromatic units and non-aromatic units can also form clusters,thus generating new emissive centers under certain conditions.Based on the crystallization-induced fluorescence-phosphorescence dual emission from certain aromatic acids and esters,we revisited the photophysical properties of poly?ethylene terephthalate??PET?containing both aromatic rings and ester groups?O=C–O?.Dilute PET solutions emit weak UV light at around 330 nm,which is assignable to the emission of individual terephthalate units.Excimer emission at 390 nm is enhanced with the increased concentration,while monomer emission then strikingly decreases.Furthermore,much brighter luminescence with remarkable enhancement is noticed at a higher concentration,with multiple emission maxima?Em?at 400,452,and 488 nm,whose emission peaks are even higher than those of the dimer emission.Emission behaviors of dimethyl terephthalate?DMTPA?are similar to those of PET,which are attributed to the clusters of terephthalate units and can well be rationalized by CTE mechanism.Meanwhile,with the increasing crystallinity,not only the quantum efficiency of PET solids is enhanced up to 22.1%,but the lifetime of accompanying RTP emission is also notably prolonged,indicating crystalline induced RTP emission.Moreover,emission maxima at around423,436,and 480 nm are observed in highly crystalline films,which are reassigned to the fluorescence from terephthalate clusters and the RTP emission from the diverse formations of terephthalates. |
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