TPE-BT-TPE
Synthesis,characterization and explosive detection of
photoluminescent compounds with intramolecular charge-transfer characteristic
Hong-Ji Jiang a ,b ,*,Jin-Long Zhang a ,Qing-Wei Zhang a ,Shang-Hui Ye a
a
Key Laboratory of Organic Electronics and Information Displays,National Jiangsu Synergetic Innovation Center for Organic Electronics and Information Displays,Institute of Advanced Materials,Nanjing University of Posts and Telecommunications,Nanjing 210023,China b
State Key Laboratory of Molecular Engineering of Polymers,Fudan University,Shanghai 200433,China
A R T I C L E I N F O Article history:
Received 31October 2014
Received in revised form 6January 2015Accepted 8January 2015
Available online 21January 2015Keywords:
Photoluminescent probe
Aggregation induced emission Explosive Detection
Charge-transfer
A B S T R A C T
Synthesis,characterization and explosive detection of compounds based on 1,4(benzo-{2,10-3})-thiadiazole,carbazole,and tetraphenylethene derivatives with intramolecular charge-transfer charac-teristic were illustrated in detail.The photoluminescent variations of compounds in different solvents were studied,and emission light with different wavelength ranging from yellow,green to blue can be easily obtained by just changing the solvent without tedious synthetic modi ?cation.Furthermore,the titration experiments were used to detect explosive compound of picric acid,and then calculate the detection limit of the explosive by plotting curves of two compounds with aggregation –induced emission properties in tetrahydrofuran and tetrahydrofuran/water mixture solution of 90%water.The Stern –Volmer plots of carbazole and tetraphenylethene substituted 1,4(benzo-{2,10-3})-thiadiazole in tetrahydrofuran and tetrahydrofuran/water mixture are linear in some concentration range and give quenching constants of 1.8?107M à1and 10.7?107M à1,respectively,and the corresponding detection limits as low as 0.54m g/mL and 0.13m g/mL were realized.The detection performance of carbazole and tetraphenylethene substituted 1,4(benzo-{2,10-3})-thiadiazole is much better than that of tetrapheny-lethene substituted 1,4(benzo-{2,10-3})-thiadiazole,which can be attributed to the relatively higher dipole moment of the corresponding compound.
?2015Elsevier B.V.All rights reserved.
1.Introduction
Organic optoelectronic functional molecules are usually used as ?lms or aggregates [1–3],however,the photoluminescent (PL)emission of the molecules in the thin solid ?lm state tends to be weakened when compared with that in solution.The discovery of aggregation-induced emission (AIE)molecular systems supplies a novel method to solve the problem of aggregation-induced PL quenching.The intriguing phenomenon of AIE,in which a molecule is nonluminescent in solution but is highly emissive in a ?lm,has been reported for 2,3,4,5-tetraphenylsiloles.Through experimen-tal and theoretical studies,the restriction of intramolecular rotation in the aggregated state has been identi ?ed as a main cause for the AIE effect [4].Owing to their unusual photophysical properties,2,3,4,5-tetraphenylsiloles are excellent emitters and
have found applications in electroluminescent devices [5,6],which show very high electroluminescent ef ?ciency and offer great potential applications in ?at-panel displays and white organic light-emitting diodes (OLEDs).Blue AIE luminogens based on tetraphenylethene have obtained a high external quantum ef ?ciencies up to 3.99%in OLEDs by restriction of the conjugation length through rational molecular design [7–9].The excellent features of AIE compounds indicate that they are also one of the most promising candidates for electrically driven organic lasers [10,11].
Because of their advantages in terms of sensitivity,selectivity,response time,and low cost,organic PL probes based on p -conjugated AIE molecules have also attracted great attention for applications as sensors for detecting all kinds of compounds besides in OLED applications [12,13].A pyridinyl-functionalized tetraphenylethene has demonstrated colorimetric and ratiometric PL responses to trivalent metal cations over a variety of mono and divalent metal cations [14].Wang et al.have successfully synthesized an AIE molecule by coating disc-like red emission
*Corresponding author.Tel.:+8602585866332.
E-mail address:iamhjjiang@https://www.wendangku.net/doc/65155150.html, (H.-J.Jiang).https://www.wendangku.net/doc/65155150.html,/10.1016/j.synthmet.2015.01.0080379-6779/?2015Elsevier B.V.All rights reserved.
Synthetic Metals 201(2015)30–42
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Synthetic Metals
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /s y n m e
t
?uorophores with propeller-shaped AIE?uorophores and fabri-cated the ultrabright red AIE dots.The AIE dots have spherical morphology,uniform size,good stability in water and low cytotoxicity.The AIE dots can stain both the cytoplasm and the nuclei and give a strong red?uorescence signal[15,16].Nitro-aromatic compounds,such as2,4-dinitrotoluene,2,4,6-trinitrotol-uene,and picric acid(PA),are warfare explosives and toxic pollutants,the selective and sensitive detection of which are of great current interest both in national security and environmental protection.Owing to their unique AIE characteristics and their ability to semi-selectively bind explosive analytes,2,3,4,5-tetra-phenylsiloles have found applications as sensors for detecting explosive compounds.Over the past ten years,several silole-containing polymers have been developed for the detection of explosives[17–19].Exceptional blue shifted and enhanced AIE of conjugated asymmetric triazines and their applications in super-ampli?ed detection of explosives have also been illustrated by An et al.[20].
Constructing donor–acceptor systems has been emerging as an effective strategy towards piezochromic luminescent materials due to photo-induced electron transfer or back-electron transfer in such systems under pressure[21–23].To the best of our knowledge,there is rare report about the combination of segments with AIE properties and charge-transfer characteristic in a single molecular framework.We are wondering what is the effect of the donor–acceptor framework on the general optoelectronic proper-ties of AIE-based molecules.In this manuscript,synthesis, characterization and explosive detection of PL compounds,CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE,with intramolecular charge-transfer characteristic were illustrated in detail.We studied the light-emission behavior of two compounds with AIE properties in the mixture solvents of water and tetrahydrofuran,and we also studied PL variations of three compounds in solvents with different dielectric https://www.wendangku.net/doc/65155150.html,stly,we have used the titration experiments to detect explosive PA,and then calculated the detection limit of explosive of PA by plotting curves of two compounds in tetrahydrofuran and tetrahydrofuran/water mixture solution of 90%water.The performance of compound TPE-BT-CZ in detection of PA is much better than that of compound TPE-BT-TPE both in quenching constant and detection limits,and the introduction of carbazole-based electron donor group to form asymmetric molecular structure for compound TPE-BT-CZ may be helpful to promote its detection performance in some degree.
2.Experimental
2.1.Materials and measurements
Materials purchased from commercial suppliers were used without further puri?cation.Tetrahydrofuran and toluene were distilled over sodium/benzophenone under nitrogen atmosphere. The other common solvents were puri?ed according to their standard methods.
Both1H NMR and13C NMR spectra were measured on a Varian Mercury Plus400spectrometer with CDCl3as solvent and tetramethylsilane as internal reference.Chemical shifts were reported in ppm relative to CDCl3as internal standard,which was set to be7.26ppm,and coupling constant was expressed in hertz. The signals have been designed as follows:s(singlet),d(doublet),t (triplet),and m(multiplets).Matrix assistant laser desorption/ ionization time-of-?ight mass(MALDI-TOF-MS)spectrometry was performed on a Buker Daltonics?ex analysis.Elemental analysis of carbon,hydrogen,and nitrogen was obtained on Elementar Vario MICRO elemental analyzer.UV–vis absorption spectra were recorded on an UV-3600SHIMADZU UV–vis–NIR spectrophotometer.The PL emission spectra were recorded on a RF-5301PC spectro?uorophotometer with a xenon lamp as a light source.The concentration of these compounds in tetrahydrofuran solution was adjusted to be about0.01mg/mL or less.The thin solid ?lms were prepared by spin-coating on quartz substrates from solution in CDCl3at a spin rate of3500rpm.Cyclic voltammetric (CV)measurements were carried out on the Chi660e system in a conventional typical three-electrode cell with a Pt work electrode (glass carbon),a platinum-wire counter electrode and a Ag/Ag+ reference electrode referenced against ferrocene/ferrocenium (FOC)in anhydrous dichloromethane solution of Bu4NPF6 (0.10M)at a sweeping rate of100mV/s.The lowest unoccupied molecular orbital(LUMO)and highest occupied molecular orbital (HOMO)energy levels were measured by CV measurements and calculated according to the formula:HOMO=àE oxi+0.0042–4.8, LUMO=àE redà0.061–4.8,where the0.0042andà0.061are the oxidation and reduction potentials of FOC respectively.The E oxi and
E red are the oxidation and reduction potentials of compounds.
2.2.Synthesis of the target compounds
Compound TPE-OH[24,25]:n-butyl-lithium(2.4mol/L,4mL, 10mmol)was added dropwise into a solution of diphenyl methane (1.77g,10.5mmol)in anhydrous tetrahydrofuran(50mL)at–78 C. The reaction mixture was stirred for0.5h,and then turned to red before(3.38g,10mmol)(4-bromo-phenyl)-phenyl-methanone (3.38g,10mmol)was added in one portion.The mixture was warmed to room temperature,stirred for6h and then was poured into water while stirring.The mixture was extracted with dichloromethane and the combined extracts were evaporated to give a white solid E,then the mixture was re?uxed with methylsulphonic acid in30mL toluene for24h at120 C.After the usual workup,the crude product was puri?ed with column chromatography on silica gel with petroleum ether/ethyl acetate (4:1)as eluent to afford a residue of TPE-Br1.2g,yield63%.1H NMR (400MHz,CDCl3,d):6.87–6.94(d,2H),6.99–7.07(m,6H),7.08–7.17 (m,9H),7.20–7.25(d,2H).n-Butyl-lithium(1.6mol/L,7.5mL, 12mmol)was added dropwise into a solution of TPE-Br(3.22g, 10mmol)in anhydrous tetrahydrofuran(15mL)at–78 C.The reaction mixture was stirred for0.5h before isopropoxyboronic acid pinacol ester(2.5mL,12mmol)was added in one portion.The mixture was warmed to room temperature,stirred for12h and then was poured into water while stirring.The mixture was extracted with diethyl ether and the combined extracts were evaporated to give1.6g a white solid TPE-BOH in a yield of50%.
Compound CZ-BT:Carbazole(1.7g,10mmol),1,4-dibromoben-zene(3.5g,15mmol),K2CO3(2.8g,20mmol),copper(0.43g, 7.5mmol),18-crown-6(0.85g,3mmol)and15mL dichloroben-zene were charged in a?ask.The?ask was heated continuously at 150 C for12h.After the completion of the reaction monitored by thin-layer chromatography,the reaction mixture was cooled to room temperature.After evaporation of the solvent in vacuum, distilled water was added and the reaction mixture was extracted with dichloromethane.The combined organic layers were collected,dried over anhydrous Na2SO4,?ltered and evaporated to remove the solvent.The resulted crude product was chromato-graphed on a silica gel column with petroleum ether as eluent. Recrystallize several times from ethanol or hexane to afford2.4g the pure solid compound CZ-Ph in a yield of75%.1H NMR (400MHz,CDCl3,d):7.33–7.39(m,2H),7.40–7.53(m,6H),7.72–7.80 (dd,2H),8.16–8.24(d,2H).Elemental analysis:(%)C18H12BrN:C 71.82,H6.02,N3.13;found(%)C71.69,H5.99,N3.11.n-Butyl-lithium(1.6mol/L,7.5mL,12mmol)was added dropwise into a solution of compound CZ-Ph(3.22g,10mmol)in anhydrous tetrahydrofuran(15mL)at–78 C.The reaction mixture was stirred for0.5h before isopropoxyboronic acid pinacol ester(2.5mL,
H.-J.Jiang et al./Synthetic Metals201(2015)30–4231
12mmol)was added in one portion.The mixture was warmed to room temperature,stirred for12h and then was poured into water while stirring.The mixture was extracted with dichloromethane and the combined extracts were evaporated to give1.7g a white solid CZ-BOH in a yield of50%.A mixture of compound CZ-BOH (0.37g,1mmol),BT-X(0.58g,2mmol),alq336and catalytic amount of Pd(PPh3)4was added to a degassed mixture of toluene (15mL)and K2CO3aqueous solution(50%,2.0mL).The mixture was vigorously stirred at95 C for72h under nitrogen atmosphere. After the completion of the reaction monitored by thin-layer chromatography,the reaction mixture was cooled to room temperature.The distilled water was added and the reaction mixture was extracted with dichloromethane.The combined organic layers were collected,dried over anhydrous Na2SO4,?ltered and evaporated to remove the solvent.The resulted crude product was chromatographed on a silica gel column with petroleum ether as eluent to give0.25g compound CZ-BT in a yield of61%.1H NMR(400MHz,CDCl3,d):7.28–7.35(m,2H), 7.40–7.48(m,2H),7.51–7.58(d,2H),7.65–7.71(d,1H),7.71–7.80(d, 2H),7.94–8.02(d,1H),8.10–8.21(m,4H).MALDI-TOF,m/z cacld for compound CZ-BT456.36,found457.968.Elemental analysis:calcd (%)for(%)C24H14BrN3S:C63.16,H3.09,N9.21;found(%)C63.11,H 3.10,N9.13.
Compound CZ-BT-CZ:The procedure was analogous to that described for compound CZ-BT,0.05g,yield13%.1H NMR (400MHz,CDCl3):7.29–7.36(m,4H),7.43–7.49(m,4H),7.55–7.62 (d,4H),7.76–7.83(d,4H),7.94–8.02(d,2H),8.16–8.21(m,4H), 8.25–8.31(m,4H).13C NMR(100MHz,CDCl3,d):147.12,140.76, 137.98,136.23,132.68,130.72,129.07,128.36,127.13,126.06,124.58, 124.01,123.88,120.29,119.63,109.57.MALDI-TOF,m/z cacld for compound CZ-BT-CZ618.75,found619.647.Elementary analysis of C42H26N4S(618.75):Calcd.C81.53,H4.24,N9.05;found C81.26,H 4.39,N9.31.
Compound TPE-BT-CZ:The procedure was analogous to that described for compound CZ-BT,0.15g,yield61%.The starting compounds are CZ-BT(0.13g, 2.9mmol),TPE-BOH(0.13g, 2.9mmol),Pd(PPh3)4(5%),toluene(15mL),K2CO3aqueous solution(50%,2.0mL)and aliquat336.1H NMR(400MHz,CDCl3, d):7.06–7.10(s,2H),7.10–7.18(m,13H),7.20–7.25(m,2H),7.28–7.36 (m,2H),7.40–7.48(m,2H),7.52–7.60(d,2H),7.72–7.79(d,2H), 7.79–7.91(dd,4H),8.14–8.25(dd,4H).13C NMR(100MHz,CDCl3,d): 154.06,144.03,143.71,141.62,140.77140.52,137.78,136.38,135.16, 133.24,132.00,131.53,130.66,128.40,127.84,127.07,126.61,126.05, 125.57,123.56,120.39,120.15,109.98.MALDI-TOF,m/z cacld for compound TPE-BT-CZ:707.88,found708.625.Elementary analysis of C50H33N3S(707.88):Calcd.C84.84,H4.70,N5.94;found C84.61, H4.59,N5.82.
Compound TPE-BT-TPE:The procedure was analogous to that described for compound CZ-BT,0.05g,yield16%.The starting compounds are BT-X(0.15g,0.5mmol),TPE-BOH(0.46g,1mmol), Pd(PPh3)4(5%),toluene(15mL),K2CO3aqueous solution(50%, 2.0mL)and aliquat336.1H NMR(400MHz,CDCl3,d):7.04–7.08(m, 4H),7.09–7.15(m,26H),7.17–7.21(m,4H),7.70–7.72(s,2H), 7.74–7.78(d,4H).13C NMR(100MHz,CDCl3,d):154.01,143.74, 141.51,140.54,135.28,132.58,131.49,128.40,127.81,126.57. MALDI-TOF,m/z cacld for compound TPE-BT-TPE:797.02,found 799.28.Elementary analysis of C58H40N2S(797.02):Calcd.C87.40, H5.06,N3.51;found C87.65,H4.91,N3.45
3.Results and discussion
As sketched in Scheme1,compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE were synthesized by coupling of1,4(dibromo-benzo-{2,10-3})-thiadiazole with carbazole and tetraphenylethene boronic ester derivatives through the Suzuki coupling reaction.The chemical structures of compound CZ-BT-CZ,TPE-BT-CZ,and
TPE-BT-TPE were con?rmed by1H NMR,13C NMR,elemental
analysis and MALDI-TOF-MS spectra(Supporting information).All
compounds were readily soluble in common organic solvents such
as chloroform,dichloromethane,and tetrahydrofuran at room
temperature.The UV–vis absorption and PL emission properties of
the obtained compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in
tetrahydrofuran solution and thin solid?lm were investigated and
summarized in Fig.1.The UV–vis absorption spectra of compound
CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in tetrahydrofuran solution
showed maximum absorbency at400,412,and412nm,which
was the typical absorbency of p–p*transition.The maximum PL emission peak of compound CZ-BT-CZ,TPE-BT-CZ,and
TPE-BT-TPE in tetrahydrofuran solution showed the same
tendency relative to UV–vis absorption.The tetraphenylethene
segment with longer conjugated length in compound TPE-BT-CZ
may contribute pronouncedly to the maximum UV–vis absorben-
cy,and the contribution of the second tetraphenylethene segment
in compound TPE-BT-TPE is weakened in some degree.The optical
energy band gaps determined from the onset of the UV–vis spectra
for compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in thin solid ?lm state are2.67,2.65,and2.52eV,which are not obviously changed from 2.48eV in tetrahydrofuran solution.The UV–vis absorption spectra of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in thin solid?lm show maximum absorbency at 416,420,and420nm,and the corresponding PL emission peaks of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in thin solid ?lm are545,525,and545nm.The maximum PL emission peak of compound TPE-BT-CZ in the thin solid?lm is blue-shifted relative to those of compound CZ-BT-CZ and TPE-BT-TPE,and there is no major difference between the maximum PL emission peaks of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in solution and thin solid?lm,which can only be attributed to the different aggregation state caused by non linear conjugated structures of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE.In order to assess the effect of molecular architecture on the capability of compounds for light-emitting,the absolute PL quantum yields of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE were deter-mined by using dichloromethane solution at room temperature.It was interesting that the absolute PL quantum ef?ciencies were as high as19.5%for compound CZ-BT-CZ,11.0%for compound TPE-BT-CZ,and20.1%for compound TPE-BT-TPE in dichloro-methane solution,and there was no obvious difference among them.Generally,a compound with high PL quantum ef?ciency should have one of the following characteristics:big rigid planar p-conjugated structure,electron-rich substitutions and the lowest single excitation level of p–p*[26].The relative higher absolute PL quantum ef?ciency of compound TPE-BT-TPE solution validates the unique roles of tetraphenylethene moieties to promote its light-emitting ability.In order to?nd more information about the conjugated structures of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE,we employed CV measurement to probe the energy levels of compound CZ-BT-CZ,TPE-BT-CZ,and TPE-BT-TPE in dichloromethane solution(Fig.2).When scanning cathodically and anodically,all compounds exhibited complete reversibility both in p-doping and n-doping processes.On the basis of these results, the LUMO and HOMO energy levels of compound CZ-BT-CZ, TPE-BT-CZ,and TPE-BT-TPE in dichloromethane solution were calculated to beà3.66/6.09eV,à3.28/6.09eV,andà3.61/à6.09eV, and the corresponding electro-chemical band gap in dichloro-methane solution were 2.43, 2.81,and 2.48eV.To further understand the electronic structures of compound CZ-BT-CZ, TPE-BT-CZ,and TPE-BT-TPE,density function theory(DFT) calculations were performed at a B3LYP/6-31G(d)level for the geometry optimization.Fig.3showed the optimized geometry and HOMO and LUMO spatial distributions of compound CZ-BT-CZ,
32H.-J.Jiang et al./Synthetic Metals201(2015)30–42
H N
Br
Br
N
Br
CZ-Ph
+N
O
B
O
CZ-BOH
IV
Br
N
S N
Br
O
B O
Ar 1
O
B O
Ar 2
+
O
Br
+
Br
OH
E
O
B O
TPE-BOH
Scheme 1.Synthesis of compound CZ-BT-CZ ,TPE-BT-CZ and TPE-BT-TPE .I:n -butyl-lithium,6h;II:methylsulphonic acid,toluene,24h,120 C;III:n -butyl-lithium,isopropoxyboronic acid pinacol ester,12.5h;IV:K 2CO 3,Cu,dichlorobenzene,re ?ux,150 C,24h;V:K 2CO 3,tetrakis(triphenylphosphine)palladium (0)(Pd(PPh 3)4),N 2,toluene,90 C,72h.
H.-J.Jiang et al./Synthetic Metals 201(2015)30–4233
TPE-BT-CZ ,and TPE-BT-TPE .The LUMO distrubutions of com-pound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE were mainly located in the 1,4(benzo-{2,10-3})-thiadiazole moieties,while HOMO distrubutions of compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE were mainly located in the whole molecular framwork,which further indicated that there really existed obvious intramolecular charge-teansfer in compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE .
Moreover,to further study the intramolecular charge transfer interactions of the resulting compounds,the solvatochromic effect on the UV –vis absorption and PL emission features for the target compounds was investigated.The results were quite similar for the compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE because of their analogous chemical structures.Figs.4–6illustrated the normalized UV –vis absorption spectra,PL emission spectra and color pictures of compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE in different solvents.As illustrated in Fig.4,the UV –vis absorption spectrum of compound TPE-BT-CZ was slightly solvent-dependent,exhibiting an intramolecular charge transfer band ranging from 408to 411nm.However,in contrast to the weak solvatochromism in UV –vis absorption spectra,the differences in the corresponding PL emission spectra were quite https://www.wendangku.net/doc/65155150.html,pound TPE-BT-CZ displayed signi ?cant solvent-dependent PL emission behavior,and a bathochromic shift of 49nm in the PL emission spectra for compound TPE-BT-CZ could be observed when the solvent polarity increased from n -hexane (508nm)to N ,N -dimethylformamide (557nm).Such a bathochromic shift was characteristic of an ef ?cient charge transfer from the carbazole and tetraphenylethene moieties to the central 1,4-benzo-{2,10-3}-thiadiazole-based acceptor [27,28].The PL emission intensities were also weakened with the increase of polarity of solvents (Supporting information).A more polar solvent was able to stabilize polarized excited state of
TPE-BT-CZ N o r m a l i z e d i n t e n s i t y (a .u .)
Wavelength(nm)
0.00.2
0.4
0.6
0.8
1.0
TPE-BT-CZ N o r m a l i z e d i n t e n s i t y (a .u .)
Wavelength(nm)
Fig.1.Normalized UV –vis absorption and PL emission spectra of compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE in tetrahydrofuran solution (a)and thin solid ?lms (b).
34H.-J.Jiang et al./Synthetic Metals 201(2015)30–42
compound by the reorientation of the solvent molecules to accommodate the increased dipole,lowering the energy of the system and thereby leading to the more distinct bathochromic shift in the PL emission spectra.The intensity to stabilize polarized excited state of compound by the reorientation of the solvent molecules might be different,and a low transition dipole moment associated with different compounds was responsible for high PL emission behavior.As could be seen from the color pictures of compounds in different solvents,the color pictures of different
compounds in the same solvent appeared to be slightly different.For example,In N ,N -dimethylformamide,the colors of compound TPE-BT-CZ and TPE-BT-TPE looked like green,while that of compound CZ-BT-CZ was sky-blue.We can easily obtain emission light with different wavelength ranging from yellow,green to blue by just changing the solvent without tedious synthetic modi ?ca-tion,and the multi-color conjugated particles with small size are expected to be prepared using a modi ?ed-reprecipitation proce-dure for live cell imaging [29–31].
The AIE properties of the compound TPE-BT-CZ and TPE-BT-TPE were examined by studying the PL emission behavior of their diluted mixtures in water/tetrahydrofuran under different water fractions.The aggregates were prepared by adding various fractions of ultra-pure water into the tetrahydrofuran solutions.Tetraphenylethene is an unusual molecule;it has four phenyl rings having intramolecular rotations that quench its PL emission,however,when it turns into the aggregate form,the non-radiative decay channels are hindered and tetraphenylethene becomes active for emission [32,33].Fig.7illustrates the PL emission spectra of compound TPE-BT-TPE (a)and TPE-BT-CZ (b)in tetrahydrofu-ran/water mixtures with different water fractions at a concentra-tion of 2.0?10àhttps://www.wendangku.net/doc/65155150.html,pound TPE-BT-CZ and TPE-BT-TPE are insoluble in water and completely soluble in tetrahydrofuran.By increasing the water content in the mixture,the molecules start to form aggregates.The graph of PL intensity versus wavelength shows the luminescence behavior of compound TPE-BT-CZ and TPE-BT-TPE while changing the water portion from 0to 100%.Two compounds of TPE-BT-CZ and TPE-BT-TPE have low luminescence intensity,when they are completely soluble in tetrahydrofuran.However,when a small fraction of water is added,their PL emission intensities begin to increase and exhibit the unique AIE character-istic.The PL intensity of compound TPE-BT-CZ was boosted 4-fold when the water fraction was increased from 0to 100%.The
3
210-1-2
-3Potential Ag/Ag+(v)
Fig.2.The CV curves of compound CZ-BT-CZ ,TPE-BT-CZ ,and
TPE-BT-TPE in dichloromethane solution.
Fig.3.Optimized geometry and HOMO and LUMO spatial distributions of compound CZ-BT-CZ ,TPE-BT-CZ ,and TPE-BT-TPE .
H.-J.Jiang et al./Synthetic Metals 201(2015)30–4235
Fig.4.Normalized UV –vis absorption,PL emission spectra and color pictures of compound TPE-BT-CZ in different solvents (concentration:4.1?10à6mol/L).The solvents used are N ,N -dimethylformamide (DMF),dichloromethane (DCM),tetrahydrofuran (THF),toluene (C 7H 8),diethyl ether (Et 2O)and n -hexane.
36H.-J.Jiang et al./Synthetic Metals 201(2015)30–42
Fig.5.Normalized UV –vis absorption,PL emission spectra and color pictures of compound TPE-BT-TPE in different solvents (concentration:4.1?10à6mol/L).
H.-J.Jiang et al./Synthetic Metals 201(2015)30–4237
Fig.6.Normalized UV –vis absorption,PL emission spectra and color pictures of compound CZ-BT-CZ in different solvents.
38
H.-J.Jiang et al./Synthetic Metals 201(2015)30–42
tendency of the PL emission intensity variation versus water fraction is different,and the PL emission intensity of compound TPE-BT-CZ rise more drastically than that of compound TPE-BT-TPE ,which can only be attributed to the different aggregation state of compound TPE-BT-CZ and TPE-BT-TPE in microenvironment.As for the mechanism behind these phenom-ena,we will further verify it in the future.
Because of commercial availability,PA was selected as a model explosive to probe the detect ability of the AIE compounds with intra molecular charge-transfer characteristic.Nanoaggregates of compound TPE-BT-TPE and TPE-BT-CZ in water/tetrahydrofuran mixture with 90%water were employed as PL compounds and their isolated molecules in pure tetrahydrofu-ran were investigated for comparison.Fig.8illustrated the PL emission spectra and Stern –Volmer plots of (I 0/I à1)values versus [PA]of compound TPE-BT-CZ in tetrahydrofuran solution (a)and tetrahydrofuran/water mixture (1:9,v/v)(b)with the addition of different amounts of PA.With the gradual addition of PA to aggregates of compound TPE-BT-CZ in the tetrahydrofuran/water mixture,the PL emission intensity progressively decreased,
P h o t o l u m i n e s c e n t i n t e n s i t y
Wavelength(nm)
500
1000
P h o t o l u m i n e s c e n t i n t e n s i t y
Wavelength(nm)
Fig.7.PL emission intensity versus wavelength of compound TPE-BT-TPE (a)and TPE-BT-CZ (b)in tetrahydrofuran/water mixtures with different water fractions at a compound concentration of 2.0?10à3M.The inset shows a plot of PL emission intensity of compound TPE-BT-TPE and TPE-BT-CZ versus water fraction.
H.-J.Jiang et al./Synthetic Metals 201(2015)30–4239
but the PL spectra pro?le remained unchanged.Remarkably,the Stern–Volmer plot of I0/I versus[PA]shows an upward curve instead of linear relationship(inset of Fig.8),indicating that PL quenching becomes more ef?cient with increasing quencher concentration.In other words,it shows superampli?ed detection of PA.By?tting the Stern–Volmer plots shown in the insets of Fig.8,equations I0/I=1.32e0.01[PA]–0.297and I0/I=0.938e0.037[PA]–0.189were obtained for compound TPE-BT-CZ in tetrahydrofuran solution(a)and tetrahydrofuran/water mixture,respectively. Quantitative analysis can be realized from these equations.When the concentration of PA is below10nM,the Stern–Volmer plots of compound TPE-BT-CZ in tetrahydrofuran solution and tetrahydro-furan/water mixture are linear and give quenching constants of 1.8?107Mà1and10.7?107Mà1,respectively.PL quenching can be clearly recognized at a PA concentration limit as low as 0.54m g/mL and0.13m g/mL for compound TPE-BT-CZ in tetrahy-drofuran solution and tetrahydrofuran/water https://www.wendangku.net/doc/65155150.html,pound TPE-BT-CZ in tetrahydrofuran/water mixture shows super-ampli-?ed detection of PA because of its AIE characteristics.Fig.9shows the PL emission spectra and Stern–Volmer plots of(I0/Ià1)values versus[PA]of compound TPE-BT-TPE in tetrahydrofuran solution (a)and tetrahydrofuran/water mixture(1:9,v/v)(b)with the addition of different amounts of PA.Similarly,when the concentration of PA is below50nM,the Stern–Volmer plots of compound TPE-BT-TPE in tetrahydrofuran solution and tetrahy-drofuran/water mixture are linear and give quenching constants of7.8?106Mà1and 2.89?106Mà1,respectively.PL quenching can be also recognized at a PA concentration limit as low as
Fig.8.PL emission spectra and Stern–Volmer plots of(I0/Ià1)values of compound TPE-BT-CZ versus[PA]in tetrahydrofuran solution(a)and tetrahydrofuran/water mixture (1:9,v/v)(b)with the addition of different amounts of https://www.wendangku.net/doc/65155150.html,pound TPE-BT-CZ concentration:2.0?10à3M.I=peak intensity and I0=peak intensity at[PA]=0nM.The inset shows a plot of I0/I versus[PA]for compound TPE-BT-CZ in tetrahydrofuran/water.
40H.-J.Jiang et al./Synthetic Metals201(2015)30–42
3.8m g/mL and0.72m g/mL for compound TPE-BT-TPE in tetrahy-drofuran solution and tetrahydrofuran/water mixture.The perfor-mance of compound TPE-BT-CZ in detection of PA is much better than that of compound TPE-BT-TPE both in quenching constant and detection limit,which indicates that the introduction of carbazole-based electron donor group to form asymmetric molecular structure is helpful to promote its detection perfor-mance in some degree[34,35].The dipole moments of compound CZ-BT-CZ,TPE-BT-CZ and TPE-BT-TPE from DFT calculation are 1.14D,2.67D,and1.44D,and the asymmetric molecular structure of compound TPE-BT-CZ afford it with highest dipole moment to bind electron-de?cient PA.
4.Conclusion
Synthesis,characterization and explosive detection of PL compounds with intramolecular charge-transfer characteristic were illustrated in detail,and two of them have AIE properties. Because of their charge transfer characteristics,we studied the AIE properties of two compounds and the PL changing of three compounds in different solvents,and emission light with different wavelength ranging from yellow,green to blue can be easily obtained by changing the solvent without tedious synthetic modi?cation.The Stern–Volmer plots of compound TPE-BT-CZ in tetrahydrofuran solution and tetrahydrofuran/water mixture are
Fig.9.PL emission spectra and Stern–Volmer plots of(I0/Ià1)values of compound TPE-BT-TPE versus[PA]in tetrahydrofuran solution(a)and tetrahydrofuran/water mixture(1:9,v/v)(b)with the addition of different amounts of https://www.wendangku.net/doc/65155150.html,pound TPE-BT-TPE concentration:2.0?10à3M.I=peak intensity and I0=peak intensity at[PA]=0nM. The inset shows a plot of I0/I versus[PA]for compound TPE-BT-TPE in tetrahydrofuran/water.
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linear in some concentration range and give quenching constants of1.8?107Mà1and10.7?107Mà1,respectively,and the corre-sponding detection limits as low as0.54m g/mL and0.13m g/mL are realized.The performance of compound TPE-BT-CZ in detection of PA is much better than that of compound TPE-BT-TPE both in quenching constant and detection limits,and the introduction of carbazole-based electron donor group to form asymmetric molecular structure may be helpful to promote its detection performance in some degree.
Acknowledgements
This work was?nancially supported by the Major Research Program from the State Ministry of Science and Technology (2012CB933301),National Natural Science Foundation of China (61106017and21274065),the Ministry of Education of China (IRT1148),the Priority Academic Program Development of Jiangsu Higher Education Institutions(YX03001),Natural Science Foundation of Jiangsu Province(BM2012010),State Key Laboratory of Molecular Engineering of Polymers(K2015-03),Nanjing University of Posts and Telecommunications(NY213176)and Jiangsu Government Scholarship for Overseas Studies.
Appendix A.Supplementary data
Supplementary data associated with this article can be found,in the online version,at https://www.wendangku.net/doc/65155150.html,/10.1016/j. synthmet.2015.01.008.
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