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Dyes and Pigments 102 (2014) 88e93

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Dyes and Pigments

journal homepage: www.elsevier .com/locate/dyepig

Synthesis and photophysical properties of new s-triazine derivativescontaining AepeDepeA quadrupolar branches

Zhi-Bin Cai a,*, Li-Fen Liu a, Mao Zhou b, Bo Li c, Ye Chen c

aCollege of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, PR Chinab Zhejiang Poly Pharmaceutical Co. Ltd., Hangzhou 310009, PR ChinacKey Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai 200241, PR China

a r t i c l e i n f o

Article history:Received 17 September 2013Received in revised form20 October 2013Accepted 21 October 2013Available online 28 October 2013

Keywords:Two-photon absorptionUp-converted fluorescences-Triazine derivativeSynthesisStructure-property relationshipCoorperation enhancement

* Corresponding author. Tel.: þ86 571 88320428.E-mail address: caizbmail@126.com (Z.-B. Cai).

0143-7208/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.dyepig.2013.10.031

a b s t r a c t

Four new s-triazine derivatives containing one or two acceptorep-donorep-acceptor quadrupolarbranches (S1, E1, S2, E2) were synthesized and characterized by infrared, hydrogen nuclear magneticresonance, mass spectrometry and elemental analysis. Their photophysical properties were investigatedincluding linear absorption, single-photon excited fluorescence, fluorescence quantum yield, two-photonabsorption, and frequency up-converted fluorescence. The spectral positions of the linear absorption andthe single-photon excited fluorescence show red-shifts with enhanced electron-accepting ability of theend-group or increasing number of branch. The fluorescence quantum yields of the two-branchedcompounds (S2, E2) decrease with increasing solvent polarity. Under the excitation of a 800 nm laserwith a pulse width of 80 fs, all these compounds emit intense green frequency up-converted fluores-cence, and the two-photon absorption cross-sections are 80, 48, 502, and 362 GM for S1, E1, S2, and E2,respectively. This result shows that significant enhancement of the two-photon absorption cross-sectioncan be achieved by sufficient electronic coupling between the strong charge transfer quadrupolarbranches through the s-triazine center.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Two-photon absorption (TPA) can be defined as simultaneousabsorption of two photons through virtual states in a medium. Thisthird-order nonlinear optical phenomenon has two remarkableadvantages. First, due to the quadratic dependence of TPA extent onthe light intensity, the two-photon process is confined in a smallvolume around the close vicinity of the laser focus, resulting in highspatial resolution beyond the diffraction limit. Second, in the near-infrared region, the linear absorption of most materials can beneglected, so the laser penetrates into materials without materialdamage and energy loss out of the focal point. Thanks to thesecharacteristics, there are a host of potential applications for TPAmaterials, such as fluorescence imaging [1], fluorescent microscopy[2], three-dimensional optical data storage [3], three-dimensionalmicrofabrication [4], frequency up-converted lasing [5], opticalpower limiting [6], and photodynamic therapy [7]. In order torealize such commercial applications, the key lies in design andsynthesis of new materials with large TPA cross-sections (s) atdesirable wavelengths, which is still a challenge.

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Up to now, the most efficient TPA materials seem to be the oneswith various electron donor (D) and electron acceptor (A) attachedsymmetrically or asymmetrically to a conjugate linker (p-center)by the polarizable p-bridge. A variety of p-centers including tri-phenylamine [8], benzene [9], anthracene [10], fluorenone [11], anddithienothiophene [12] have been studied. s-Triazine is anintriguing functional chromophore, which has recently attachedconsiderable attention due to its high electron affinity and sym-metrical structure. Its derivatives were extensively used as emittersin organic light-emitting diodes [13] and second-order nonlinearoptical materials [14]. A number of star-shaped DepeA dipolarmolecules with a s-triazine center, which showed TPA properties,were also reported [15]. Compared with DepeA dipolar molecules,DepeAepeD or AepeDepeA quadrupolar molecules haveexperimentally [16] and theoretically [17] demonstrated enhancedTPA cross-sections that are approximately an order of magnitudegreater than those of dipolar analogues. To our knowledge, TPAmaterials with an s-triazine center and quadrupolar branches havenot been reported.

In this work, we designed and synthesized four new one-branched and two-branched s-triazine derivatives (S1, E1, S2, E2).S1 and E1 are typical AepeDepeA quadrupolar molecules, where1,4-phenylenedivinylene is employed as p-bridge, and is connected

N

N N

CH3

OCH3

OCH3N

X

H3C

N

N N

CH3

OCH3

OCH3OCH3

OCH3

N

XX

N

Fig. 1. Structures of the target compounds (S1, E1, S2, E2).

Z.-B. Cai et al. / Dyes and Pigments 102 (2014) 88e93 89

to electron-accepting s-triazine (A) and benzothiazole (or benzox-azole) (A) at both ends and two electron-donating methoxy groups(D) in the middle. S2 and E2 can be regarded as assemblies of S1and E1, respectively. Their structures were characterized by IR, 1HNMR, MS and elemental analysis. The single- and two- photonrelated photophysical properties were studied.

2. Results and discussion

2.1. Synthesis and characterization

The target compounds (S1, E1, S2, E2) were synthesized ac-cording to Scheme 1. They and two important intermediates (SQ,EQ) are new compounds. SQ and EQ were prepared by thecondensation of 2-methylbenzothiazole or 2-methylbenzoxazolewith 2,5-dimethoxy-1,4-benzenedicarboxaldehyde (2) in aceticanhydride and acetic acid solutions. 2 was obtained by Sommeletreaction. 2,4,6-Trimethyl-s-triazine (1) was prepared by acidcatalyzed trimerization of ethyl acetimidate. The methyl groups in1 undergo in an alkaline medium condensation reaction with thecorresponding aldehyde (SQ, EQ) to give the target compounds(S1, E1, S2, E2) in yields of 32.7%e45.7%. The data of IR, 1H NMR,MS and elemental analysis are in accord with the assignedstructures. It is noteworthy that all of them (S1, E1, S2, E2, SQ, EQ)were obtained as their E-isomers as shown by the couplingconstant of approximately 16.0 Hz between all vinylic protons(Fig. 1).

Scheme 1. Synthesis of the target compounds (S1, E1, S2, E2).

Table 1Linear and nonlinear optical properties of the target compounds (S1, E1, S2, E2).

Compound Solvent labsmaxa (nm) lSPEFmax

b (nm) Dvc cm�1 Fd lTPEFmaxe (nm) sf (GM) s/MWg

S1 THF 426 488 2982 0.94 482, 496 80 0.186 (1.0)CHCl3 430 494 3013 0.88CH2Cl2 424 496 3424 0.86CH3CN 423 499 3601 0.86

E1 THF 421 483 3049 0.89 478, 496 48 0.116 (1.0)CHCl3 425 490 3121 0.86CH2Cl2 420 492 3484 0.86CH3CN 418 495 3721 0.85

S2 THF 436 490 2528 0.54 484, 495 502 0.681 (3.7)CHCl3 441 500 2676 0.50CH2Cl2 438 502 2911 0.46CH3CN 435 505 3187 0.03

E2 THF 431 487 2668 0.48 482, 493 362 0.513 (4.4)CHCl3 436 494 2693 0.42CH2Cl2 431 496 3041 0.35CH3CN 431 502 3282 0.02

a Maximum linear absorption wavelength, c ¼ 1 � 10�5 mol L�1.b Maximum single-photon excited fluorescence wavelength, c ¼ 1 � 10�7 mol L�1.c Stokes shift.d Fluorescence quantum yield.e Maximum two-photon excited fluorescence wavelength, c ¼ 1 � 10�5 mol L�1.f Two-photon absorption cross-section, 1 GM ¼ 1 � 10�50 cm4 s photon�1.g Reduced TPA cross-section.

Z.-B. Cai et al. / Dyes and Pigments 102 (2014) 88e9390

2.2. Linear absorption and single-photon excited fluorescence(SPEF)

The linear optical properties of the target compounds (S1, E1, S2,E2) in various solvents are listed in Table 1. Their UVevisible ab-sorption and SPEF spectra in THF are shown in Fig. 2 and Fig. 3.

All of the compounds exhibit two major prominent absorptionbands, appearing at 341e348 nm and 418e435 nm, respectively.The former is localized in styrene pep* transition and the latter isof considerable charge transfer character. For each compound, theabsorption maxima (labsmax) do not vary significantly in differentsolvents, except in CHCl3. This may be due to the fact that there is astrong interaction between the long pair electrons on the N atomsand s* (CeH) antibonding orbitals in CHCl3. As increasing numberof branch, the position of labsmax is bathochromically shifted. Forexample, the labsmax in THF shifts from 426 nm of S1 to 436 nm of S2and 421 nm of E1 to 431 nm of E2, and their absorptive coefficientsalso increase obviously, which can be attributed to the extended p-delocalization. Compared S1 with E1 and S2 with E2, respectively,the labsmax of S1 and S2 are red-shifted by about 5 nm in the same

300 350 400 450 500 550 600

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Abs

orba

nce

Wavelength / nm

S2

E2

E1

S1

Fig. 2. UVevisible absorption spectra of the target compounds (S1, E1, S2, E2) in THF(c ¼ 1 � 10�5 mol L�1).

solvent owing to the stronger electron-accepting ability ofbenzothiazole.

Excited at the maximum absorption wavelengths, all thesecompounds exhibit strong SPEF emission between 420 and 630 nm.The SPEF maxima (lSPEFmax ) show bathochromic shifts of differentextent upon increasing the polarity of the solvents, which rangesfrom THF (empirical parameter of solvent polarity ET(30) ¼ 37.4),CHCl3 (ET(30) ¼ 39.1), CH2Cl2 (ET(30) ¼ 40.7), to CH3CN(ET(30) ¼ 45.6) [18]. The Stokes shifts (Dv) also increase with theincrease of solvent polarity. These facts reveal a larger solute/sol-vent interaction at the excited state as compared with that at theground state, which implies that the polarity of the excited stateincreases. Similar to the absorption properties, the lSPEFmax is also red-shifted when the number of branches increases or benzoxazoleend-group is replaced with benzothiazole, which can be sequencedas S2> S1, E2> E1, S1> E1, and S2> E2. The fluorescence quantumyields (F) in each solvent were measured with reference to fluo-rescein in 0.1 mol L�1 sodium hydroxide (F ¼ 0.9 [19]). The F of S1(0.86e0.94) and E1 (0.85e0.89) are very high, and the influence ofthe solvent polarity on the F can be ignored. As for S2 and E2, the Fis lower than that of S1 and E1, and decrease with increasing

400 450 500 550 600 650 700

0

100

200

300

400

500

600

700

SPE

F in

tens

ity

/ a.u

.

Wavelength / nm

S2

E2 S1

E1

Fig. 3. SPEF spectra of the target compounds (S1, E1, S2, E2) in THF(c ¼ 1 � 10�7 mol L�1).

400 450 500 550 600 650 700

0

10000

20000

30000

40000

50000

60000

TPE

F in

tens

ity

/ a.u

.

Wavelength / nm

E2

S1

E1

S2

Fig. 4. TPEF spectra of the target compounds (S1, E1, S2, E2) in THF(c ¼ 1 � 10�5 mol L�1).

Z.-B. Cai et al. / Dyes and Pigments 102 (2014) 88e93 91

solvent polarity. Especially, the F dramatically decrease to 0.03 forS2 and 0.02 for E2 in highly polar CH3CN. These results indicate thatS2 and E2 have stronger charge transfer [20].

2.3. Two-photon properties

The two-photon excited fluorescence (TPEF) spectra of thetarget compounds (S1, E1, S2, E2) in THF are shown in Fig. 4. The

400 450 500 550 600 650

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

TP

EF

inte

nsit

y / a

.u.

Wavelength / nm

400 450 500 550 600 650

0

2000

4000

6000

8000

10000

12000

TP

EF

inte

nsit

y / a

.u.

Wavelength / nm

11.97 GW·cm

10.19 GW·cm

8.40 GW·cm

6.88 GW·cm

5.35 GW·cm

4.33 GW·cm

S1

11.97 GW·cm

10.19 GW·cm

8.40 GW·cm

6.88 GW·cm

5.35 GW·cm

4.33 GW·cm

E1

Fig. 5. TPEF spectra of the target compounds (S1, E1, S2, E2) in THF under different laser excitof the laser excitation intensities.

corresponding nonlinear spectroscopic parameters are listed inTable 1. Under the excitation of 80 fs, 800 nm laser pulses, all of thecompounds emit intense green frequency up-converted fluores-cence. The differences between TPEF and SPEF are prominent. In theTPEF spectra (Fig. 4), the maximum emission peaks are broader andmore flat than that in the SPEF spectra (Fig. 3). There are twomaximum emission peaks at 482 and 496 nm for S1, 478 and496 nm for E1, 484 (shoulder) and 495 nm for S2, and 482 and493 nm (shoulder) for E2, respectively, which indicate that theemissions are from the different vibrational levels of the sameexcited state. They also show shoulders atw514 nm, indicating theexistence of more than one emitting state [21]. The TPEF maxima

(lTPEFmax ) have no obvious shifts compared with the corresponding

lSPEFmax in THF and the TPEF spectra were also measured in dilutesolutions (c ¼ 1 �10�5 mol L�1), so the re-absorption effect shouldbe weak.

As shown in Fig. 2, there is no linear absorption above 500 nm,the emission excited by 800 nm laser wavelength can be attributedto the TPEF mechanism. The TPEF spectra of these compounds inTHF under different laser excitation intensities are shown in Fig. 5.The linear dependence of fluorescence intensities on the square ofthe excitation intensities, as shown in the insets, confirms that TPAis the main excitation mechanism of the intense up-convertedfluorescence emission.

The two-photon absorption cross-sections (s) was obtained bycomparing the TPEF intensity of the sample with that of a referencecompound by the following equation (1) [22]:

400 450 500 550 600 650

0

10000

20000

30000

40000

50000

60000

TPE

F in

tens

ity /

a.u.

Wavelength / nm

400 450 500 550 600 650

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

TPE

F in

tens

ity /

a.u.

Wavelength / nm

11.97 GW·cm

10.19 GW·cm

8.40 GW·cm

6.88 GW·cm

5.35 GW·cm

4.33 GW·cm

E2

11.97 GW·cm

10.19 GW·cm

8.40 GW·cm

6.88 GW·cm

5.35 GW·cm

4.33 GW·cm

S2

ation intensities (c¼ 1�10�5 mol L�1). Insets are the TPEF intensities versus the square

Z.-B. Cai et al. / Dyes and Pigments 102 (2014) 88e9392

ss ¼ FsFr

4r4

nrns

crcrsr (1)

s

where the subscripts s and r denote the sample and the referencecompound. F and F represent the TPEF integral intensity and theSPEF quantum yield. n and c are the refractive index and the con-centration of the solution. In this work, we selected fluorescein in0.1 mol L�1 sodium hydroxide (c ¼ 1 � 10�5 mol L�1) as the refer-ence (s ¼ 36 GM [22]).

The s of the target compounds are 80 GM for S1, 48 GM for E1,502 GM for S2, and 362 GM for E2, respectively. The reduced TPAcross-section (s/MW), which is defined as s divided by the mo-lecular weight, varies in a proportion of 1.0:3.7 (S1:S2) and 1.0:4.4(E1:E2). This dramatic enhancement of S2 and E2 should beattributed to the sufficient electronic coupling between thebranches [23]. Their individual branches, which have a AepeDepeA quadrupolar character, can not only lead to strong intra-branch charge transfer, but also facilitate inter-branch electroniccommunication through s-triazine center. The s of S1 and S2 are67% and 39% larger than those of E1 and E2, respectively, becausebenzothiazole end-group has a stronger electron-accepting abilitycompared with benzoxazole, which enhances the delocalization ofelectrons. Such judgment about acceptor strength can be furtherproved from the 1H NMR spectra. The protons in benzene andethylene (¼CH-) attached to thiazole exhibit higher chemical shifts.

3. Conclusions

Four new s-triazine derivatives containing one or two AepeDepeA quadrupolar branches (S1, E1, S2, E2) were synthesized andcharacterized by IR, 1H NMR, MS and elemental analysis. Theirlinear and nonlinear photophysical properties were investigated.Excited by a 80 fs, 800 nm laser, the two-branched compound S2shows the strongest frequency up-converted fluorescence and thelargest TPA cross-section, which is higher than those of manyknown s-triazine derivatives containing three DepeA dipolarbranches [15aed]. It should be noted that we cannot measure theTPA cross-section at different wavelengths at present because ofthe limit of our laser apparatus. The s values should be betterconducted at their relative two-fold labsmax.

4. Experimental

4.1. Materials and instruments

2,4,6-Trimethyl-s-triazine (1) [24] and 2,5-dimethoxy-1,4-benzenedicarboxaldehyde (2) [25] were synthesized according tothe literature procedures. Other materials were commerciallyavailable and were used without further purification.

Melting points were measured on an X-4 micromelting pointapparatus without correction. 1H NMR spectra were collected on aBruker AVANCE Ⅲ 500 apparatus, with TMS as internal standardand DMSO-d6 as solvent. FT-IR spectra were recorded on a ThermoNicolet 6700 spectrometer using KBr pellets. Mass spectra weretaken on a Therm LCQ TM Deca XP plus ion trap mass spectrometryinstrument. Elemental analyses were conducted on a Thermo Fin-nigan Flash EA 1112 apparatus.

The linear absorption spectra were measured on a ShimadzuUV-2550 UVevisible spectrophotometer. The SPEF spectra mea-surements were performed using a RF-5301PC fluorescence spec-trophotometer with the maximum absorption wavelengths as theexcitation wavelengths. The fluorescence quantum yields weredetermined using fluorescein in 0.1 mol L�1 sodium hydroxide asthe standard. The TPEF spectraweremeasured using a femtosecondTi:Sapphire laser (Micra þ RegA9000, Conherent) as pump source

with a pulse width of 80 fs, a repetition rate of 250 kz, and a centralwavelength of 800 nm.

4.2. Synthesis

4.2.1. 4-[(1E)-2-(2-benzothiazolyl)ethenyl]-2,5-dimethoxybenzaldehyde (SQ)

Amixture of 2-methylbenzothiazole (1.49 g, 10 mmol), 2 (1.94 g,10 mmol), acetic anhydride (3 mL), and acetic acid (1.5 mL) washeated under reflux for 6 h, then cooled to room temperature. Afterconcentrated hydrochloric acid (15 mL) was added, the mixturewas filtered. The filtrate was neutralized with 30% aqueous sodiumhydroxide solution (30 mL) and gave a precipitate. The precipitatewas purified by column chromatography on silica gel using petro-leum ether/ethyl acetate (10:1) as eluent to give bright yellowneedle crystals (2.23 g, 68.6%). m.p. 176e177 �C; 1H NMR (500 MHz,DMSO-d6): d ¼ 10.36 (s, 1H), 8.13 (d, J ¼ 8.2 Hz, 1H), 8.03 (d,J¼ 8.0 Hz,1H), 7.96 (d, J¼ 16.2 Hz,1H), 7.90 (d, J¼ 16.4 Hz,1H), 7.73(s, 1H), 7.54e7.57 (m, 1H), 7.46e7.49 (m, 1H), 7.32 (s, 1 H), 4.00 (s,3H), 3.93 ppm (s, 3H); FT-IR (KBr): v¼ 3047, 2966, 2865,1676,1603,1473, 1410, 1209, 1126, 1037, 953, 880, 757, 700 cm�1; ESI-MS: m/z(%): 325.8 (100) [M þ H]þ; elemental analysis calcd (%) forC18H15NO3S: C 66.44, H 4.65, N 4.30; found: C 66.65, H 4.76, N 4.52.

4.2.2. 4-[(1E)-2-(2-benzoxazolyl)ethenyl]-2,5-dimethoxybenzaldehyde (EQ)

This compound was synthesized using a procedure similar tothat described for 1, with 2-methylbenzoxazole instead of 2-methylbenzothiazole. Bright yellow needle crystals. Yield 51.1%.m.p. 150e152 �C; 1H NMR (500 MHz, DMSO-d6): d ¼ 10.36 (s, H),8.06 (d, J ¼ 16.5 Hz, 1H), 7.78 (d, J ¼ 8.0 Hz, 2H), 7.77 (s, 1H), 7.67 (d,J ¼ 16.5 Hz, 1H), 7.40e7.47 (m, 2H), 7.33 (s, 1H), 4.00 (s, 3H),3.94 ppm (s, 3H); FT-IR (KBr): v ¼ 3067, 2921, 2871, 1675, 1609,1487, 1411, 1213, 1124, 1038, 970, 873, 747, 691 cm�1; ESI-MS: m/z(%): 309.7 (100) [M þ H]þ; elemental analysis calcd (%) forC18H15NO4：C 69.89, H 4.89, N 4.53; found: C 70.12, H 4.98, N 4.71.

4.2.3. [(1E)-2-[2,5-dimethoxy-4-[(1E)-2-(4,6-dimethyl-1,3,5-triazin-2-yl)ethenyl] phenyl]ethenyl]-2-benzothiazole (S1)

To a mixture of 1 (0.55 g, 4.5 mmol), potassium hydroxide(0.3 g), and methonal (30 mL) was added a solution of SQ (0.98 g,3 mmol) in methanol (30 mL) dropwise. The reaction mixture wasrefluxed for 20 h and then the solvent was removed. The residuewas purified by column chromatography on silica gel using petro-leum ether/ethyl acetate (10:1) as eluent to give a orange-redcrystalline powder (0.59 g, 45.7%). m.p. 203e205 �C; 1H NMR(500MHz, DMSO-d6): d¼ 8.43 (d, J¼ 16.1 Hz,1H), 8.11 (d, J¼ 7.8 Hz,1H), 8.00 (d, J ¼ 8.0 Hz, 1H), 7.90 (d, J ¼ 16.2 Hz, 1H), 7.84 (d,J ¼ 16.3 Hz, 1H), 7.57 (s, 1H), 7.55 (s, 1H), 7.52e7.54 (m, 1H), 7.44e7.47 (m, 1H), 7.37 (d, J ¼ 16.1 Hz, 1H), 3.98 (s, 6H), 2.56 ppm (s, 6H);FT-IR (KBr): v ¼ 3048, 2966, 2835, 1623, 1538, 1475, 1414, 1216,1042, 987, 872, 757 cm�1; ESI-MS: m/z (%): 431.1 (100) [M þ H]þ;elemental analysis calcd (%) for C24H22N4O2S: C 66.96, H 5.15, N13.01; found: C 70.16, H 5.19, N 13.24.

4.2.4. [(1E)-2-[2,5-dimethoxy-4-[(1E)-2-(4,6-dimethyl-1,3,5-triazin-2-yl)ethenyl] phenyl]ethenyl]-2-benzoxazole (E1)

This compound was synthesized using a procedure similar tothat described for S1, with EQ instead of SQ. Yellow crystallinepowder. Yield 41.6%. m.p. 182e184 �C; 1H NMR (500 MHz, DMSO-d6): d ¼ 8.43 (d, J ¼ 16.1 Hz, 1H), 8.06 (d, J ¼ 16.4 Hz, 1H), 7.76 (d,J ¼ 8.0 Hz, 2H), 7.60 (s, 1H), 7.57 (s, 1H), 7.55 (d, J ¼ 16.2 Hz, 1H),7.38e7.44 (m, 2H), 7.38 (d, J¼ 16.1 Hz,1H), 3.98 (s, 6H), 2.56 ppm (s,6H); FT-IR (KBr): v¼ 3072, 2946, 2834, 1633, 1534, 1494, 1412, 1210,1041, 986, 865, 749 cm�1; ESI-MS: m/z (%): 415.0 (100) [M þ H]þ;

Z.-B. Cai et al. / Dyes and Pigments 102 (2014) 88e93 93

elemental analysis calcd (%) for C24H22N4O3: C 69.55, H 5.35, N13.52; found: C 69.73, H 5.38, N 13.68.

4.2.5. 2,20-[(6-Methyl-1,3,5-triazine-2,4-diyl)bis[(1E)-2,1-ethenediyl-(2,5- dimethoxy-4,1-phenylene)-(1E)-2,1-ethenediyl]]bisbenzothiazole (S2)

This compound was synthesized using a procedure similar tothat described for S1 except that the molar ratio between SQ and 1was changed to 2.5:1. Orangeered crystalline powder. Yield 36.4%.m.p. 280e282 �C; 1H NMR (500 MHz, DMSO-d6): d ¼ 8.50 (d,J ¼ 16.1 Hz, 2H), 8.12 (d, J ¼ 7.8 Hz, 2H), 8.01 (d, J ¼ 8.0 Hz, 2H), 7.91(d, J¼ 16.3 Hz, 2H), 7.85 (d, J¼ 16.3 Hz, 2H), 7.59 (s, 2H), 7.58 (s, 2H),7.53-7.56 (m, 2H), 7.44e7.48 (m, 2H), 7.42 (d, J¼ 16.1 Hz, 2H), 4.01 (s,12H), 2.64 ppm (s, 3H); FT-IR (KBr): v ¼ 3065, 2930, 2830, 1624,1518, 1462, 1406, 1210, 1042, 980, 872, 758 cm�1; ESI-MS: m/z (%):738.1 (100) [M þ H]þ; elemental analysis calcd (%) forC42H35N5O4S2: C 68.36, H 4.78, N 9.49; found: C 68.47, H 4.92, N 9.65.

4.2.6. 2,20-[(6-Methyl-1,3,5-triazine-2,4-diyl)bis[(1E)-2,1-ethenediyl-(2,5- dimethoxy -4,1-phenylene)-(1E)-2,1-ethenediyl]]bis benzoxazole (E2)

This compound was synthesized using a procedure similar tothat described for S1 except that SQ was replaced with EQ and themolar ratio between EQ and 1was changed to 2.5:1. Orange-yellowcrystalline powder. Yield 32.7%. m.p. 265e267 �C; 1H NMR(500 MHz, DMSO-d6): d ¼ 8.50 (d, J ¼ 16.1 Hz, 2H), 8.08 (d,J¼ 16.4 Hz, 2H), 7.77 (d, J¼ 8.0 Hz, 4H), 7.63 (s, 2H), 7.60 (s, 2H), 7.57(d, J¼ 16.5 Hz, 2H), 7.44 (d, J¼ 16.0 Hz, 2H), 7.40e7.44 (m, 4H), 4.01(s, 12H), 264 ppm (s, 3H); FT-IR (KBr): v ¼ 3080, 2924, 2855, 1629,1518, 1461, 1408, 1212, 1043, 975, 850, 743 cm�1; ESI-MS: m/z (%):706.1 (100) [M þ H]þ; elemental analysis calcd (%) for C42H35N5O6:C 71.48, H 5.00, N 9.92; found: C 71.61, H 5.17, N 10.09.

Acknowledgments

We are grateful to the National Natural Science Foundation ofChina for financial support (Grant No. 21103151).

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