Fabrication And Photophysical Properties Of Stimuli-Responsive Soft Materials Based On Carbazole Derivatives

Organic photoelectric functional materials have many advantages,such as wide variety,easy modification of molecular structure and low cost,and have been widely used in organic light-emitting diodes(OLEDs),organic photovoltaic cells,organic field-effect transistors(OFETs),organic solid-state lasers,photoelectric sensors and biological imaging and many other high-tech fields.Therefore,the design and synthesis of novel organic molecules with desired optical and electronic properties has attracted much attention.Among them,the electron rich heterocyclic compounds-carbazole,has the advantages of simple structure,low price,stable performance and excellent optical properties and has been widely used in dye-sensitized solar cells,organic light-emitting diodes(OLEDs),supramolecular self-assembly and fluorescent chemical sensors.This paper mainly studies the synthesis,assembly and photophysical properties of carbazole derivatives.Meanwhile,new non-classical ?-gel was prepared and the fluorescence sensing properties of ?-gels and the nanofibers was investigated.At the same time,the fluorescence sensing behavior of luminescent crystal materials under external pressure was studied.Some creative results have been obtained,and are outlined as follows:Traditional organic gelators usually contain H-bonded,long alkyl chains,sugar or cholesterol,which leads to low atom economy and tedious synthetic processes.In addition,the domain formed by long carbon chains would block the information transformation between conjugated systems.Therefore,it is very important to design and synthesize new kind of non-classical ?-gel without any auxiliary group.In the previous work,we found that tert-butyl can adjust the solubility of the compound and tune the strength of the intermolecular ?-? interaction in aggregated states,favoring for the gelation.In order to build new non-classical ?-gel and discussed the effect of molecular structure on the molecular self assembly.We designed and synthesized two new benzothizole modified carbazole derivatives(CBT and TCBT).It was found that TCBT with a tert-butyl moiety could form stable gels in cyclohexane,cyclohexanol,etc.,stimulated by ultrasound.Among them,the critical gel concentration in cyclohexane was 3.1 mg/mL.The temperature-dependent UV-vis absorption spectra and fluorescence emission spectra of TCBT during the gelation process show that ?-? interaction is the main driving force in the gel formation.Howerve,CBT without a tert-butyl group failed to form an organogel in the selected solvents,revealing that tert-butyl played an important role in gel formation.The reason is that tert-butyl can adjust the solubility of the compound and tune the strength of the intermolecular ?-? interaction in aggregated states,favoring for the gelation.In particular,xerogel-based films of TCBT emitted strong blue light and could be used as fluorescent sensory materials to detect acid vapors of TFA,HCl,HNO3,formic acid and acetic acid with high performance.The response time was short as 1.61 s and the detection limit was ca.1.5 ppm when the film towards gaseous TFA.We deemed that the high performance of xerogel-based films of TCBT in probing TFA vapor was originating from the good adsorption and diffusion of analytes in 3D networks consisting lots of nanofibers as well as the efficient exciton migration in 1D nanofibers.This work will be helpful in the design of non-traditional?-gelators and the fabrication of fluorescence chemosensors with high performance.New tert-butylcarbazole-based Schiff bases TPMP and TPMA,as well as the difluoroboron complex of salicylaldimine TPOB were synthesized.It was found that TPMP and TPMA without any traditional gelation group could form organogels in toluene and o-dichlorobenzene,etc.,stimulated by ultrasound.The critical gel concentration for TPMP and TPMA were 4.5 mg/mL and 4.0 mg/m L,respectively.We deemed that the planar aromatic skeletons and the large steric effect tert-butyl groups in TPMP(the dihedral angle between the salicylaldimine and the adjacent benzene ring was only 1.31°)and TPMA(the dihedral angles between the Schiff base and the adjacent benzene ring was 1.8°)would lead to appropriate ?-? interaction to induce the gel formation.Although the difluoroboron complex of TPOB could not gelate the selected solvents,it showed reversible piezofluorochromism.TPOB emitted strong yellow light centered at ca.570 nm in the as-prepared crystal.Grinding the as-prepared crystal of TPOB in a mortar,an orange powder with orange red emission(ca.590 nm)was obtained.In addition,the emitting color of TPOB in ground powder could recover to yellow by heating at 220 °C for 1 min.We deduced that the non-planar ?-skeleton in TPOB(the dihedral angle between the salicylaldimine difluoroboron group and the adjacent benzene ring was 46.48°)would decrease the ?-? interaction,and the quite weak ?-? interaction between aromatic units led to poor self-assembling ability of TPOB.However,non-planar ?-skeleton of TPOB could make the molecules pack loosely in the crystal,so that the molecular packing would be changed easily upon stimulated by external force.It provided the strategy for designing new non-traditional ?-gelators directed by appropriate ?-? interaction and new piezofluorochromic dyes via the introduction of non-planar ?-skeletons.New carbazole modified salicylaldimine derivatives CM1,CM2,CM3 and their difluoroboron complexes CMB1,CMB2,CMB3 were synthesized.It was found that CM3 and CMB3 bearing both tert-butyl and trifluoromethyl could form stable gels in some organic solvents,suggestting that both tert-butyl and trifluoromethyl played key roles in the gel formation.Since tert-butyl can adjusted the solubility and the strength of ?–? interaction and C-F?H-C hydrogen bonds can form between fluorine atoms in trifluoromethyl units and the hydrogen atoms in phenyl rings of the neighboring molecules to stabilizing molecular assembled structures.Temperature-dependent UV-vis absorption and fluorescence emission spectra change indicated that ?-? interaction was the main driving force in their gel formation.Moreover,the emission of salicylaldimines CM1,CM2,CM3 in solutions could be intensified obviously and the gel CM3 could be destroyed into solution upon the addition of fluoride ion.NMR titration experiments show that was caused by fluoride ion induced hydroxyl deprotonation.Therefore,the synthesized salicylaldimines could be used as sensory materials to recognize F-selectively.Among them,the detection limit of CM3 torwards F-was 6.9 × 10-11 mol/L in tetrahydrofuran solution.Interestingly,compounds CMB1 and CMB3 with trifluoromethyl exhibited piezofluorochromic behaviors.After grinding,the emmiting color of CMB1 and CMB3 changed from green to orange and yellow to orange red,respectively.The solid state emission spectra of CMB1 and CMB3 red shifted 24 nm and 40 nm,respectively.The high contrast piezofluorochromism for CMB3 compared with that of CMB1,which might be originated from that CMB3 possessed a more loosely packing mode on account of the steric hindrance of t-butyl.It is worth noting that compound CMB3 is not only a nontraditional gelator but also exhibits piezofluorochromism behavior.Herein,it provided a new strategy for designing new multifunctional organic soft materials.Three carbazole-based terephthalate derivatives(BECT,BOCT and BHCT)were designed and synthesized.Due to steric hindrance restricted the intramolecular single bond rotation lead the carbazole-based terephthalate derivatives emit strong fluorescence in both solution and the solid state.The fluorescence quantum yield of compounds BOCT and BHCT in solid states were reaches 0.98 and 0.95,respectively.Moreover,compounds with ethyl and octyl groups exhibited piezofluorochromism behavior.After grinding,the emmiting color of BECT and BOCT both changed from blue to green blue and the solid state emission spectra of BECT and BOCT red shifted 25 nm and 15 nm,respectively.But the presence of hexadecyl groups did not lead to the fluorescence change under mechanical force stimuli.The crystal structures suggest that the length of the alkyl chain can affect the planarity of the molecule,and the planarity of the molecule is stronger in accordance with the order of BECT < BOCT < BHCT.So,we suppose that planarity in the molecular structure under mechanical force stimulus should be responsible for MFC behaviors of BECT and BOCT.Owing to the worse planarity of BECT compared with BOCT,so ground BECT solids had a larger spectral shift under force stimulus.Since the length of alkyl chains affects molecular planarity and then determines their response to external stimuli.Because of strong fluorescence in solution and the solid state,we believe that they can be used as luminescent materials and sensors.