| Recently,mechanoresponsive luminescent(MRL)materials exhibit a great promising application in force sensing,information storage,display technique and other fields.In this context,the organic molecules with a push-pull type structure provide a wide choice of electron donors and electron acceptors,resulting a flexible regulation of their molecular structures.However,the development of efficient ways to improve the contrast ratio of MRL materials and improve their application in force sensing and information storage are still significant issues need to be addressed.To address the issues mentioned above,we designed and synthesized a series of push-pull type MRL molecules with different twisted structures.Through adjusting the electron donors and acceptors,we have thoroughly studied the relationship between the molecular structure/aggregated state and the mechanoresponsive behaviors of these molecules,as well as explored the applications of these molecules in force sensing and information storage devices.Specific studies are as follows:(1)Five A-D-A type molecules APBz,APCz,APAd,APPo and APPt were with benzene,carbazole,9,9-dimethylaacridine,phenoxazine and phenothiazine as electron donors(D)and 4-formylbenzene as acceptors(A)were designed and synthesized.Mechanoresponsive test,PL spectra and single crystal structure analysis revealed the increase number of and heteroatoms can obviously regulate the degree of molecular distortion,the molecular stacking mode and the intermolulecular noncovalent interactions.Thus,the contrast ratio of the MRL materials can be adjusted.Among them,the compound APPt on the basis of six membered heterocyclic phenothiazine containing N and S hold the largest contrast ratio,whose maximum fluorescence emission peak changing from 527 nm to 474nm after mechanical grinding,accompanied by the corresponding emission color changed from yellow to blue.(2)In order to further improve the mechanoresposive contrast ratio,three compounds b-DIPF,o-DIPF and d-DIPF with A-?-D-?-A structure were constructed by using planar fluorene as the donor,benzene ring as?bridge and1,3-indanedione as receptor,differing in butyl,octyl and dodecyl on the main framework were designed and synthesized.It was found that the three compounds all exhibited obvious MRL turn-on behavior.Among them,compound o-DIPF with octyl showed the largest emission enhancing after force grinding,which exhibited a bright yellow emission turn-on.Notably,the?F value of the ground sample reached 19.6%,which was 163-fold higher than that of the pristine sample.(3)To realize intelligent response to different types of mechanical forces,“genetic improvement”were used to obtain the target materials.At first,four push-pull type molecules 4,4'-(3-alkylthiophene-2,5-diyl)dibenzaldehydes(n-TDBA)with 4-formylbenzene acceptors but differing in the thiophenyl(TDBA),3-butylthiophenyl(b-TDBA),3-octylthiophenyl(o-TDBA)and 3-dodecylthiophenyl(d-TDBA)donors by systematically changing the length of the alky chains were designed and synthesized.These compounds can distinguish anisotropic grinding and isotropic compression by different responsive luminescent colors after exerting different forces.It was found o-TDBA displayed a promising smart MRL(SMRL)property,whose blue-green emission(?PL=485nm,?F=1.2%)in the pristine state converted to blue emission(?PL=451 nm,?F=10.6%)under the anisotropic mechanical grinding force,while giving blue-cyan(?PL=473 nm,?F=7.3%)emission under the isotropic compression.To inherit the advantaged SMRL characteristic from o-TDBA,and red shift the emission,five offspring molecules were developed by replacing the formyl groups with various stronger electron-withdrawing groups including malononitrile,ethyl cyanoacetate,1,3-indanedione,3-pyridylacetonitrile and 3-ethylrhodanine.As a result,the offspring o-DIPT not only inherited the SMRL performance from o-TDBA,but also improved the contrast ratio between different mechanical forces.The emission almost quenched o-DIPT exhibited impressive red emission(?PL=622 nm,?F=11.7%)turn-on upon mechanical grinding,but giving a NIR emission(?PL=696 nm,?F=7.1%)turn-on upon compression,implying the potential application as intelligent forces sensor.(4)In order to expand the application of SMRL materials in smart information memory devices,four D-?-A type thiophene derivatives TPM,3b-TPM,3h-TPM and 5h-TPM with propionitrile as acceptor and benzene ring as?bridge but differing in the substituted position and length of alkyl chain on thiophene ring,were designed and synthesized.It was found 3h-TPM displayed the third-order fluorescence change behavior under mechanical force,heat and organic vapor stimuli.Based on the novel property of 3h-TPM,an intelligent information storage device exhibits"Write once and read multiple times"mode changing to"Rewritable"mode was realized by utilizing the different irreversible and reversible behaviors between different second-order signals. |
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