Synthesis And Mechanofluorochromic Properties Of N-containing Fused-heterocyclic Compounds

Mechanofluorochromic(MFC)materials,a class of ‘‘smart'' materials,whose emitting colors change in response to external mechanical stimuli(such as grinding crushing,and rubbing,etc),have received much attention owing to their promising applications in data storage,optical switches,sensors,rewritable media,security ink and OLEDs.Compared with chemical alteration in the molecular structures,changing the molecular packing mode is believed to be an easier way to achieve reversible solid-state fluorescence.Therefore,the investigation of the changeable mechanofluorochromic behavior of dyes induced by mechanical forces helped us to better understand the MFC processes.Up to date,the derivatives of cyanoethylen,9,10-divinylanthracene,tetraphenylethylene,anthrylpyrazole derivative and boron ?-diketonate have been found to show MFC activities.In general,due to the strong ?-? interaction in the aggregated state of the fluorescent dye,the fluorescence efficiency is often quenched in solid state or in concentrated solutions due to the aggregation-caused quenching(ACQ)effect.Therefore,it is still challengeing to develop dye molecules with strong luminescence in solid state.In 2001,Tang et al.reported a kind of fluorescent dyes,namely aggregation-induced emission(AIE)materials,which are highly emissive in the aggregated states compared with those in isolated states.This provides an important basis for the design of MFC materials.We synthesized N-containing fused-heterocyclic compounds and investigated their MFC properties.Some innovative results have been achieved and outlined below:(1)New difluoroboron ?-ketoiminate boron complexes functionalized with benzoxazole and benzothiazole CBO,CBS,CDBO,OND and TOND were synthesized.It was found that CBO,CBS,OND and TOND exhibited reversible MFC properties.The as-synthesized sample of CBO emitted azure light under UV irradiation,and its emitting color changed into cyan upon ground by a mortar and pestle for 5 min.The as-synthesized sample of CBO gave two emission bands at 447 nm and 472 nm.Upon grinding the two emission peaks were combined into one broad emission band at ca.490 nm.The as-synthesized sample of CBS gave two emission bands at 461 nm and 490 nm and emitted azure light under UV irradiation.When the as-synthesized sample was ground,CBS emitted green light centered at 517 nm.It should be noted that CBS exhibited high-contrast MFC behavior compared with CBO.We suggested that the loose packing in crystalline state of CBS and the better p-electron delocalization were favorable for yielding high-contrast mechanofluorochromism.Additionally,the as-synthesized sample of OND and TOND gave emission band at 527 nm and 528 nm and emited yellow-green light under UV irradiation,respectively.When the as-synthesized crystals were ground,OND and TOND emitted yellow light centered at 540 nm and 555 nm.Interestingly,reversible MFC process of four complexes could be realized via repeating grinding/fuming or heating treatment on account of the transformation between crystalline and amorphous states.(2)New difluoroboron ?-ketoiminate boron complexes functionalized with carbazole and tert-butyl carbazole CBT and TCBT were synthesized.They emitted strong fluorescence in non-polar solvents and solid state.The AIE and mechanofluorochromic properties were studied.The emission intensity of CBT(THF/H2 O,v/v=1/9)and TCBT(THF/H2 O,v/v=1/19)at 415 nm and 435 nm were 2.0-fold and 13.2-fold higher than the fluorescence intensity in THF.Both of CBT and TCBT were showed significant and reversible mechanofluorochromism.The as-synthesized sample of CBT gave emission band at 454 nm emited yellow-green light under UV irradiation.When the as-synthesized crystals were ground,CBT emitted bright yellow light centered at 492 nm.Then,the emission could be recovered when the ground powders were fumed with DCM vapor or heated at 150 oC.The as-synthesized sample of TCBT emitted yellow-green light centered at 557 nm.When it was ground,TCBT emitted bright yellow centered at 562 nm.TCBT had a larger spectral shift under a mechanical force stimulus compared with CBT.(3)New D-p-A type benzothiazole-cyanostyrene derivatives NNS,TNNS,PNS,PCNS,CNS and TCNS were synthesized.They were modified by N,N-dimethylaminophenyl,triphenylamine,phenothiazine and carbazole,respectively.When the content of water was increased to 90% in water/THF system,the emission intensity of CNS and TCNS at ca.550 nm were boosted swiftly and its intensity were ca.4.6-fold and 2.9-fold higher than those in THF.Interestingly,compounds TNNS,CNS and TCNS exhibited reversible mechanofluorochromism.The emission of the as-synthesized crystals of TNNS was located at 557 nm,and it was red-shifted to 591 nm upon grinding,so the emitting color was changed from orange-red to deep orange-red.After the ground powders were exposed to the vapor of DCM vapor or heated,the emission recovered to the original state.The as-synthesized sample of CNS gave one emission band at 532 nm.Upon grinding the two emission peaks were emitted yellow light centered at 549 nm.The as-synthesized sample of TCNS emitted yellow-green light centered at 537 nm under UV irradiation.When the as-synthesized sample was ground,TCNS emitted orange yellow centered at 562 nm.Then,the emission could be recovered when the ground powders were fumed with DCM vapor or heated.It illustrated that the transformation between crystalline and amorphous states was reasonable for mechanofluorochromism of CNS and TCNS.(4)New D-?-A type triphenylamine-containing vinyl or cyanovinyl fused heterocyclic compounds have been synthesized.For triphenylamine modified fused benzimidazole linked by vinyl or cyanovinyl,the grinding only induced a blue-shift of 3 nm for the emission.Therefore,we could suppose no MFC property was detected for BIT.In the case of BITM,the fluorescence emission band of the as-synthesized sample appeared at 555 nm,which was blue-shift to 542 nm upon grinding.The emission of the as-synthesized sample of BICT gave an emission band at 527 nm,uponing grinding,the emission was red-shifted to 591 nm and the emitting color was changed into amber.Although the emission was blue-shifted to 541 nm instead of 527 nm after the ground powders were fumed with DCM vapor for 5 s,the fumed sample still emitted yellowish green light under UV irradiation.Moreover,when the as-synthesized sample of BICTM was ground,the emission was red-shifted from 574 nm to 611 nm.The benzoxazole derivative of BOTM bearing the linker of vinyl emitted cyan light centered at 505 nm in the as-synthesized sample,which was red-shifted to 516 nm in the ground powders,which emitted yellow green light.Therefore,BOTM exhibited quite low-contrast MFC behavior.The emission of benzothiazole derivative of BTTM bearing the linker of vinyl emerged at 496 nm,and was red-shifted to 529 nm in the ground powders.Moreover,obvious changes of the emission for TNNS(from 577 nm to 591 nm)and BTCTM(from 586 nm to 609 nm)were observed upon grinding the as-synthesized samples.The above MFC processes could be recycled for several times under the treatment grinding/fuming with DCM vapor,indicating reversibility and durability under stimuli of mechanical forces.Therefore,triphenylamine-containing fused heterocycles bearing cyanovinyl exhibited quite high-contrast MFC behavior.We obtained the single crystal X-ray diffraction data of the compounds BTTM,BICTM and TNNS.It was found that the ordering packing in crystals would be destroyed to different extend upon grinding,leading to the changes in their solid-state emission,and they would be recovered through further fuming with DCM vapor.Thus,the phase transition between the crystalline and amorphous states led to their MFC properties.(5)New D-p-A type aminostyrylquinoxaline derivatives QDMA1,QDMA2,QTPA1,QTPA2,QPT and QCZ were synthesized,and contained with the groups of N,N-dimethylaminophenyl,triphenylamine,phenothiazine and carbazole,respectively.Amony them,compounds QDMA2,QTPA2 and QPT exhibited reversible mechanofluorochromism.The fluorescence emission wavelength red shifted from 558 nm to 606 nm,when QDMA2 was grounded and the emitting color was changed from orange-yellow to orange.The emission of the as-synthesized crystals of QTPA2 was located at 542 nm,and it was red-shifted to 592 nm upon grinding,so the emitting color was changed from yellow to deep red.After the ground powders were exposed to the vapor of DCM vapor for 5 s or heated160 oC for 4 s,the emission recovered to the original state.Meanwhile,the fluorescence emission wavelength red shifted from 558 nm to 606 nm when QPT was grounded by mortar and the emitting color was changed from yellow to orange-red.The emission could be recovered when the ground powders were fumed with DCM vapor.Such MFC process could be repeated for several times.In addition,the protonation of the synthesized aminostyrylquinoxalines by TFA could lead to the colors of the solutions to change into dark green from yellow,accompanied with the quenching of the emission.Furthermore,we also found that the emission of QTPA2 in the film could be quenched significantly and rapidly stimulated by TFA vapor.QTPA2 emitted strong yellow light in CHCl3 under UV light,and its emission intensity at 563 nm decreased gradually when the amount of TFA was increased.Furthermore,the 1H NMR spectra of QTPA2 in DMSO-d6 before and after the addition of TFA(5 equiv.)were conducted to confirm the quinoxaline unit in QTPA2 might be protonated by TFA.The emission of QTPA2 in the film was also quenched by TFA vapor due to the protonation of quinoxaline,and the detection limit and the decay time were estimated to be 167 ppb and 0.47 s.Therefore,the aminostyrylquinoxaline derivatives could be used as sensory materials to detect acids in solutions and in the films by naked eyes.Moreover,we also found that the emission of QPT in the film could be quenched significantly and rapidly stimulated by TFA vapor,and the detection limit and the decay time were estimated to be 2.53 ppm.Furthermore,we also found that the emission of QDMA1?QDMA2?QTPA1?QPT and QCZ in the film could be quenched significantly and rapidly stimulated by TFA vapor.