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LETTER 778

Synthesis and Characterization of Near-Infrared Emissive BODIPY-Based

Conjugated Polymers

NIREmissiveBODIPY-BasedConjugatedPolymersYuanzhao Wu, Xiao Ma, Jiemin Jiao, Yixiang Cheng,* Chengjian Zhu*

Key Lab of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University,

Nanjing 210093, P. R. of China

Fax +86(25)83317761; E-mail: yxcheng@nju.edu.cn; E-mail: cjzhu@nju.edu.cn

Received 29 November 2011

SYNLETT 2012, 23, 778782xx.xx.2012Advanced online publication: 24.02.2012DOI: 10.1055/s-0031-1290364; Art ID: W72911ST

Georg Thieme Verlag Stuttgart New York

Abstract: Three novel BODIPY-based conjugated polymers couldbe synthesized via palladium-catalyzed Sonogashira coupling reac-tion. Compared with BODIPY model derivatives, the polymers can

emit in the range from deep-red to near-infrared region with emis-

sion spectral maxima at l= 640660 nm and exhibit moderate flu-

orescent quantum yield from 0.23 to 0.26. Density functional theory(DFT) calculations are performed on three polymers repeat units,

and HOMO and LUMO energy levels of the conjugated polymers

are estimated.

Key words: organoboron dye, conjugated polymer, BODIPY,near-infrared

Luminescent organoboron dyes are of great interest fornot only chemistry and material science but also biologi-cal science due to the excellent chemical and photostabil-ity properties.14,4-Difluoro-4-borata-3a-azonia-4a-aza-s-indacenes (BODIPY) have become the preferred fluoro-

phores because of high fluorescent quantum yields, nar-row absorption and sharp emission bands with high peakintensities.2The BODIPY-based derivatives have promis-ing applications in fluorescence sensors toward heavy-

metal ions,3pH,4small molecules,5live-cell imaging sys-tems,6,2b and solar-cell materials.7The tuning of opticalproperties of the BODIPY moiety has been carried out by

systematic structural modification. Many strategies havebeen investigated by functionalizing BODIPY cores at themeso-, 2,6-, or 3,5-positions, including introducing somearomatic groups to the BODIPY core or replacing pyrroleunit by other linkers.8,2a,bA relatively new field is focusedon the BODIPY-based conjugated polymers for fluores-cence-material dyes.9So far, there have been few reports

on 2,6-functionalized BODIPY-based conjugated poly-mers or copolymers for the near-infrared emissive materi-al applications.10 In this paper, we synthesized threepolymers P1, P2, and P3via palladium-catalyzed Sono-gashira coupling reaction11 at 2,6-BODIPY core posi-tions, and BODIPY dye was introduced into the mainchain backbone of the conjugated polymer. The resultingpolymers can emit in the range from the deep-red to thenear-infrared region with emission spectral maxima at l=640660 nm.

The synthesis procedures of the monomers and the conju-gated polymers are outlined in Scheme 1. meso-Aryl-sub-stituted BODIPY 1 dye was prepared by the startingproduct 2,4-dimethylpyrrole in 33% overall yield accord-ing to the literature.5a 2,6-Diiodotetramethyl BODIPYmonomer 2 could be obtained by further iodization of

BODIPY dyes. The monomers 5, 6, and 10were synthe-sized according to the reported literature.12The polymer-ization reaction was carried out using the palladium-

catalyzed Sonogashira coupling reaction ofp-phenylene-ethynylene monomers with 2,6-diiodo-substituted BO-DIPY monomer to afford dark blue polymers in over 50%yields. The number average molecular weight (Mn) and

the weight average molecular weight (Mw) of the polymerare shown in Table 1. The gel permeation chromatogra-phy (GPC) results of the polymers show that three poly-mers have moderate molecular weight.

The resulting polymers show excellent solubility in com-mon organic solvents, such as THF, CHCl3, and CH2Cl2,which is an important requirement for sensing or materialsapplications. Thermogravimetric analyses (TGA) of the

polymers was carried out under a N2atmosphere at a heat-ing rate of 10 C/min (Figure 1). The results show that thepolymers have relatively high thermal stability withoutloss of weight before the temperature reached 230 C.Therefore, the herein described material can provide de-

sirable thermal property for practical applications as thenear-infrared emissive materials.

The absorption spectrum of the BODIPY 1in CH2Cl2so-lution appear a strong S0S1 (pp*) transition at l= 506and 550 nm and a much weaker broad band at a shorterwavelength (l= 360 nm), but 3shows larger redshift to550 and 388 nm, which is attributed to the introduction of

Table 1 Molecular Weights and Thermal Properties of Polymers

Polymer Td(C)a Mn(g/mol)

b Mw(g/mol)b PDI

P1 248 4240 7940 1.8

P2 234 5630 9650 1.7

P3 285 4020 7760 1.9

aTemperature of 5% weight loss measured by TGA in nitrogen.bMolar mass (Mn, Mw) and polydispersity index (PDI) were deter-

mined by GPC in THF against polystyrene standards with UV detec-

tion set at absorption maxima.

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the acetylenyl group. Compared to BODIPY derivatives,significant redshifts of three polymers can be observed inthe order of P2 > P1 > P3 (Table 2, Figure 2 andFigure 3), which can be attribute to the extended p-conju-gation of the polymers.

The monomers 1and 3can emit bright green and orangelight and both have good fluorescent quantum yields (j)as high as 0.78 and 0.66, respectively. As to compound 2,the introduction of the two iodo substituents to the dipyr-romethene core of BODIPY results in a significant red-shift (up to 30 and 38 nm) in the UV absorption and

fluorescent maxima. A much lower fluorescence quantumyield was detected for 2(j = 0.05), suggesting that the in-tersystem-crossing efficiency from the lowest singlet ex-cited state to the triplet state was enhanced by the internalheavy-atom effect.13 Due to extended p-conjugation oftwo acetylenyl groups at the 2,6-positions, the BODIPY 3have large redshift and a little decrease of quantum yield

of fluorescence compared to the starting material BO-DIPY 1(Table 2). The emissive wavelengths of the three

polymers appear at 640, 652, and 660 nm, that is, they canemit in the range from the deep-red to the near-infrared re-gion. In addition, the conjugated polymers display slightlybroader absorption and emission peaks due to the exten-sion of the p-conjugation structure and larger electron de-

localization (Figure 3), but they still possess desirablechemical and photostability attributes, relatively high ab-sorption coefficients and fluorescence quantum yields,and contain narrow absorption and emission bands withhigh peak intensities. What is more, the polymers exhibitmoderate fluorescent quantum yields P1(0.22), P2(23),and P3(0.26, Table 2) and were high enough in the rangefrom the deep-red to the near-infrared region.

In order to gain a further insight into the optical proper-ties, molecular geometries, and molecular orbitals ofpolymers, we performed density functional theory (DFT)calculations on model compounds constituting the corre-sponding repeat units. HOMO and LUMO energy levelswere estimated from the optimized geometry using the

Figure 1 TGA curve of the three conjugated polymers

Figure 2 UV-vis absorption spectra of 1, 2, 3, P1, P2, and P3

(1.0105molL1in CH2Cl2)

Figure 3 Fluorescence spectra of 1, 2, 3, P1, P2, and P3

Table 2 UV-Vis Absorption and Emission Spectral Maxima and

Fluorescent Quantum Yields (j) of BODIPY Monomers and Poly-

mers (1.0105mol/L, CH2Cl2)

BODIPY Absorption

(nm)

Emission

(nm)

Excitation

(nm)

Quantum

yield (%)

1 506 510 507 78a

2 536 548 536 5a

3 543 550 543 66a

P1 608 660 645 22b

P2 606 652 620 23b

P3 610 640 621 29b

aRhodamine B in EtOH as the reference fluorescence quantum yield

(j) = 0.65.14

bRelative to ZnPc in DMF as the reference fluorescence quantum

yield (j) = 0.28.

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B3LYP functional and 6-31g basis set. The surface plotsfor the model compounds are shown in Figure 4. The the-oretical calculations show that LUMO and HOMO energylevels of the three polymers were very close, which indi-cates the similar optical band gaps of three polymers. TheLUMO of the model compounds is mainly located on theboron di(iso)indomethene ligand, and the HOMO is local-ized not only on the ligand but also on the p-phenylene-

ethynylene moieties. It can also be found from the calcu-lation results that the band gaps of the model compoundsare in the order P2< P1< P3, and it is almost consistentwith the UV-vis absorption maxima of the polymers in the

order P2> P1> P3.

In summary, three novel conjugated polymers incorporat-ing BODIPY andp-phenylene-ethynylene moieties couldbe synthesized via palladium-catalyzed Sonogashira cou-pling reaction. The incorporation of BODIPY unit into theconjugated polymer main chain backbone can lead tostrong fluorescence in deep-red to near-infrared region.

They are expected to be used as the potential fluorescence

materials.

Supporting Informationfor this article is available online athttp://www.thieme-connect.com/ejournals/toc/synlett.

Acknowledgment

This work was supported by the National Natural Science Founda-

tion of China (No. 21074054, 51173078, 21172106), National

Basic Research Program of China (2010CB923303)

References and Notes

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Burghart, A.; Derecskei-Kovacs, A.; Burgess, K.J. Org.Chem. 2000, 65, 2900. (c) Bricks, J. L.; Kovalchuk, A.;Trieflinger, C.; Nofz, M.; Buschel, M.; Tolmachev, A. I.;

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2008, 73, 1963. (b) Zheng, Q.; Xu, G.; Prasad, P. N. Chem.Eur. J. 2008, 14, 5812. (c) Didier, P.; Ulrich, G.; Mely, Y.;Ziessel, R. Org. Biomol. Chem. 2009, 7, 3639.

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Chem. Commun. 2010, 5082. (e) Kolemen, S.; Cakmak, Y.;

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(11) General Procedure for Polymerization via Sonogashira

Polymerization ReactionA 20 mL Schlenk flask was charged with monomers,

catalyst Pd(PPh3)4, (CuI), and a magnetic stirrer. A 1:1mixture of degassed anhyd Et3N and anhyd toluene solvent

was then added to the monomers, and the reaction

temperature was raised to 80 C. The polymerization wascontinued for 72 h. Then the solvent was evaporated under

vacuum. The residue was dissolved in CH2Cl2(100 mL) and

washed with H2O (3). The organic layer was collected,

dried over anhyd Na2SO4, and filtered. The filtrate wasconcentrated and added to MeOH to precipitate the polymer.

A dark powder was obtained by filtration, further purified

with MeOH, and then dried under vacuum for 24 h to affordpolymers.

P1BODIPY dye 2(57 mg, 0.1 mol), monomer 5(103 mg, 0.1mmol), Pd(PPh3)4(12.0 mg, 0.01 mmol), and CuI (1.9 mg,

0.01mmol) were added to a mixture of toluene (3 mL) and

Et3N (3 mL). The mixture was stirred under N2atmosphere

at 80 C for 72 h, and then the solvent was evaporated undervacuum. The residue was dissolved in CH2Cl2(100 mL) and

washed with H2O (3). The organic layer was collected,

dried over anhyd Na2SO4, and filtered. The filtrate wasconcentrated and added to MeOH to precipitate the polymer.

A dark powder was obtained by filtration, further washedwith MeOH, and then dried under vacuum for 24 h to afford

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782 Y. Wu et al. LETTER

Synlett2012, 23, 778782 Thieme Stuttgart New York

a yield of 59%. 1H NMR (300 MHz, CDCl3): d= 7.717.65(br, 5 H), 7.577.42 (br, 6 H), 4.244.15 (br 12 H), 3.92

3.84 (br, 12 H), 3.763.68 (br, 12 H), 3.52 (br, 6 H), 3.68

3.65 (br, 12 H), 3.52 (br, 6 H), 3.37 (br, 6 H), 3.20 (br, 6 H),2.70 (br, 6 H), 1.42 (br, 6 H). FT-IR (KBr): 3441, 2923,

2823, 1630, 1529, 1384, 1317, 1261, 1095, 1014, 862, 724,535 cm1.

P2BODIPY dye 2(57 mg, 0.1 mol) and monomer 6(36 mg, 0.1

mmol) were added to a 5 mL Schlenk flask. Pd(PPh3)4(12.0mg, 0.01 mmol) and CuI (1.9 mg, 0.01 mmol) were added toa mixture of toluene (3 mL) and Et3N (3 mL). The mixture

was stirred under N2atmosphere at 80 C for 72 h, and then

the solvent was evaporated under vacuum. The polymeriza-tion was carried out according to the method for polymer P1.

A dark blue powder was obtained by filtration, further

washed with MeOH, and then dried under vacuum for 24 h

to afford a yield of 54%. 1H NMR (300 MHz, CDCl3): d=7.557.51 (br, 5 H), 7.327.31 (br, 2 H), 4.174.15 (br, 12H), 3.833.80 (br, 12 H), 3.653.64 (br, 12 H), 3.523.50

(br, 12 H), 3.393.40 (br, 18 H), 2.752.68 (br, 6 H), 1.55(br, 6 H). FT-IR (KBr): 3445, 2922, 2853, 1530, 1399, 1315,

1272, 1237, 1182, 1011, 1107, 725, 586 cm1.

P3BODIPY dye 3(37 mg, 0.1 mol) and monomer 10(37 mg,

0.1 mmol) were added to a 5 mL Schlenk flask. Pd(PPh3)4

(12.0 mg, 0.01 mmol) and CuI (1.9 mg, 0.01 mmol) wereadded to a mixture of toluene (3 mL) and Et3N (3 mL). The

mixture was stirred under N2atmosphere at 80 C for 72 h,and then the solvent was evaporated under vacuum. Thepolymerization was carried out according to the method for

polymer P1. A dark blue powder was obtained by filtration,

further washed with MeOH, and then dried under vacuumfor 24 h to afford a yield of 62%. 1H NMR (300 MHz,CDCl3): d= 7.717.70 (br, 2 H), 7.537.54 (br, 5 H), 4.05

4.02 (br, 4 H), 2.702.65 (br, 12 H), 2.05 (br, 3 H), 1.551.65

(br, 15 H), 1.281.27 (br, 6 H). FT-IR (KBr): 3438, 2924,2363, 1637, 1527, 1384, 1315, 1261, 1097, 804 cm1.

(12) (a) Wu, Y. Z.; Dong, Y.; Li, J. F.; Huang, X. B.; Cheng, Y.;

Zhu, C. J. Chem. Asian J. 2011, 6, 2725. (b) Li, J.; Meng, J.;

Huang, X. B.; Cheng, Y. X.; Zhu, C. J. Polymer 2010, 51,3425.

(13) Yogo, T.; Urano, Y.; Ishitsuka, Y.; Maniwa, F.; Nagano, T.

J. Am. Chem. Soc. 2005, 127, 12162.(14) Crosby, G. E.; Demas, J. N.J. Phys. Chem. 1971, 75, 991.

(15) Jiang, X. J.; Yeung, S. L.; Lo, P. C.; Fong, W. P.; Ng, D. K.

P.J. Med. Chem. 2011, 54, 320.

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