Segmented conjugated macromolecules containing silicon and boron: ADMET synthesis and luminescent properties

Segmented Conjugated Macromolecules Containing Silicon and Boron:

ADMET Synthesis and Luminescent Properties

Arijit Sengupta,1 Ami Doshi,2 Frieder J€akle,2 Ralf M. Peetz1

1Department of Chemistry, City University of New York at Staten Island and Graduate Center, 2800 Victory Boulevard, Staten

Island, New York 103142Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102

Correspondence to: R. M. Peetz (E-mail: [email protected])

Received 23 September 2014; accepted 24 February 2015; published online 00 Month 2015

DOI: 10.1002/pola.27614

ABSTRACT: Boron- and silicon-containing conjugated homo- and

copolymers could be synthesized using acyclic diene metathesis

(ADMET) condensation of bis-styryl monomers. Both, tri-and

tetra-coordinated boron monomers were successfully polymerized

forming homopolymers, or random copolymers (if polymerized

together with a silicon containing co-monomer). Polymer molecu-

lar weights Mn were measured at �6000 to 15,000 g/mol (NMR

end group analysis) with molecular weight distributions Mw/Mn

�1.8 to 2.2. The polymers absorbed at kmax �317 to 406 nm and

emitted at kmax �370 to 494 nm with fluorescent quantum effi-

ciencies �24 to 48%. The copolymer with tri-coordinate boron

showed highly efficient fluorescence quenching in the presence of

fluoride ions at ratios boron/fluoride �5/1, demonstrating its

potential as anion sensor. VC 2015 Wiley Periodicals, Inc. J. Polym.

Sci., Part A: Polym. Chem. 2015, 00, 000–000

KEYWORDS: boron; conjugated polymers; fluorescence property;

metathesis; polycondensation; sensors; silicon

INTRODUCTION Silicon and boron-containing conjugatedmaterials are of much current interest for their potential usein organic light-emitting diodes, photovoltaics, and in sen-sory applications. While a great variety of different polymer-ization methods have been pursued for their preparation,acyclic diene metathesis polymerization (ADMET) is attrac-tive because the reaction conditions are typically quite mild.Ethylene formed as a byproduct is readily removed, and theresulting polymer structures are well-defined.1

Some of us have demonstrated that silylene-functionalizedconjugated polymers can be accessed via ADMET of conju-gated silanes that feature two styryl moieties.2 A red shift inthe emission spectra of the products compared to the mono-mers indicated a participation of silicon in the conjugation.In related work, Rathore and Interrante reported the prepa-ration of photocurable and photoluminescent polycarbosi-lans. The polymer films were thermally or photochemicallycross-linked to yield blue-photoluminescent films.3 Polymerscontaining germanium, tin, and phosphorous are furtherexamples of inorganic polymers synthesized via ADMETpolymerization. For example, germanium-based polymers areof interest for applications in microlithography and as pre-cursors for ceramics.4 Despite potentially highly interesting

luminescent and anion-sensing properties, to our knowledge,conjugated boron-containing polymers have not yet beenprepared to date via ADMET.5,6 In fact, the only example ofincorporation of boron-monomers into polymers by ADMEThas been reported by Wolfe and Wagener, who studied thepolymerization of a,x-vinyl-functionalized boronates.7

Although the boronate monomers were metathesis active,ligand-exchange reactions among the boronate functionalgroups and ring formation in solution prevented isolationand characterization of the polymers. The presence of thesescrambling processes was confirmed via NMR spectroscopy.We describe here the preparation of two different lumines-cent distyrylborane monomers and the first successful prep-aration of the conjugated organoborane homopolymers andrandom copolymers with organosilicon monomers based onADMET. One of the boron-containing monomers features the8-hydroxyquinolato (q) chelate ligand, which has been exten-sively applied in the preparation of luminescent complexes,including the aluminum species Alq3.

8 The latter is com-monly used as electron-conduction and emissive layer inOLEDs.9 A second luminescent monomer was prepared inwhich boron is tri-coordinate and thus Lewis acidic, whichcan be exploited for anion sensing applications.5,10,11

Additional Supporting Information may be found in the online version of this article.

VC 2015 Wiley Periodicals, Inc.

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EXPERIMENTAL

Mg (turnings), 8-hydroxyquinoline, trimethyl borate, and triiso-propyl borate were purchased from Acros and 4-chlorostyrenefrom Alfa Aesar. Triphenylphosphonium methyl bromide,n-BuLi (2.5 M), and Hoveyda—Grubbs second-generationcatalyst were purchased from Sigma Aldrich. (2,4,6-Triisopro-pylphenyl)magnesium bromide,12 2-butylquinolin-8-ol, 2-hexyl-quinolin-8-ol,13 and bis(4-diformylphenyl)dioctylsilane2 wereprepared as previously reported. Solvents such as tetrahydrofu-ran (THF), toluene, hexane, dichloromethane, and diethyl etherwere purchased from Fisher Scientific. Except for diethyl ether,all solvents were dried and degassed by a “Pure Solv” solventpurification system (using activated alumina, copper catalyst,molecular sieves column) by Innovative Technology Inc. beforeuse. All other chemicals were used as received. Columnchromatography was carried out on silica gel 60 (70–230mesh) form EMD Chemicals Inc.

Synthesis of 1To a suspension methyltriphenylphosphonium bromide (12.5 g,0.035 mol) of in THF (100 mL), n-BuLi (12 mL, 2.5 M in hex-ane, 0.030 mol) was slowly added at 0 �C. The reaction mixturewas stirred for 3 h. To the resulting solution, bis(4-diformylphe-nyl)dioctylsilane (4.64 g, 0.010 mol) dissolved in THF (10 mL)was slowly added at 0 �C. The resulting mixture was stirredfor 12 h and then washed with brine. The organic phase wasextracted with diethyl ether twice, dried over sodium sulfate,and concentrated to yield the dioctyldistyrylsilane as oil, whichwas purified by passing through a silica gel column usinghexane as an eluent to obtain a colorless liquid (3.0 g, 66%).

1H NMR (600 MHz, CDCl3): d 5 7.47 (d, J 5 7.8 Hz, 4H), 7.39(d, J 5 8.1 Hz, 4H), 6.72 (dd, J 5 11.2 Hz, 17.7 Hz, 2H), 5.79(d, J 5 17.7 Hz, 2H), 5.26 (d, J 5 11.2 Hz, 2H), 1.2–1.5(m, 24H), 1.0–1.2 (m, 4H), 0.88 (t, J5 7.0 Hz, 6H). 13C NMR(125 MHz, CDCl3): d 5 138.1, 136.9, 136.4, 135.1, 125.51,114.2, 33.7, 31.9, 29.3, 29.2, 23.7, 22.7, 14.1, 12.6. 29Si NMR(99.3 MHz, CDCl3, ppm): 26.84; UV-Vis (hexane, 2.5 3 1024 M):kmax5 258 nm (e 5 20,200); fluorescence (hexane, 5.0 3 1028

M): kem,max5 307 nm (kexc5 259 nm).

Synthesis of 2aTo a suspension of Mg (2.67 g, 0.11 mol) in THF (300 mL)was added slowly 4-chlorostyrene (7.62 g, 0.055 mol) whilemaintaining the temperature below 60 �C. The reaction mix-ture was stirred at 60 �C for 2 h and allowed to cool toroom temperature. The resulting 4-styryl-Grignard solutionwas decanted to another flask using a cannula. Trimethylbo-rate was then added slowly to the Grignard solution and thereaction was stirred for 3 h. The resulting distyrylboratesolution was added slowly to a solution of (2,4,6-triisopro-pylphenyl)magnesium bromide in THF and refluxed for 36 h.The crude product was extracted using hexanes and passedthrough a silica gel column using hexanes as the eluent. Thepure product was obtained via crystallization from hexanesas colorless crystals (2.5 g, 25%).

For 2a: 11B NMR (160.380 MHz, CDCl3): d 5 70 (w1/2 5

1900); 1H NMR (499.893 MHz, CDCl3): d 5 7.75 (d, 3J 5 7.5

Hz, 4H, Ph-H2,6), 7.50 (d, 3J 5 7.5 Hz, 4H, Ph-H3,5), 7.00 (s,2H, Tip-H3,5), 6.80 (dd, 3J 5 18.0 Hz, 2H, H9), 5.90 (d, 3J 5

18 Hz, 2H, H10), 5.37 (d, 3J 5 12 Hz, 2H, H11), 2.95 (m,1H), 2.40 (sept, 3J 5 7.0 Hz, 2H), 1.33 (d, 3J 5 7.0 Hz, 6H),0.98 (d, 3J 5 7.0 Hz, 12H); 13C (125.698 MHz, CDCl3):d 5 149.1, 148.7, 142.4, 140.8, 140.8, 138.4, 138.4 137.1,125.7, 120.3, 115.7, 35.7, 34.4, 24.4; UV-Vis (THF, 3.0 3

1025 M): kmax 5 324 nm (e 5 51,400); fluorescence (THF, 3.03 1025 M): kem,max 5 423 nm, U 5 0.90 (kexc 5 324 nm); ele-mental analysis: calculated C 88.56, H 8.87; found C 88.32, H8.90. High resolution MALDI-TOF (negative mode, matrix:Benzo[a]pyrene): m/z5 420.44 (calculated for 12C31

1H3711B:

420.44).

Synthesis of 2bA solution of distyrylborate prepared as described above(0.029 mol, 200 mL THF) was added slowly to 2-n-hexyl-8-hydroxyquinoline (5.0 g, 0.022 mol) in 50 mL THF andstirred for 24 h. Complete conversion was confirmed by 11BNMR spectroscopy and the mixture was then quenched withchlorotrimethylsilane (1.8 g, 0.017 mol) to remove anyunreacted Grignard. Insoluble salts were removed by filtra-tion. The product was purified by crystallization from hex-anes and obtained as a yellow solid (2.0 g, 32%).

For 2b: 11B NMR (160.380 MHz, CDCl3): d 5 11.7 (w1/

25 610); 1H NMR (499.893 MHz, CDCl3): d 5 8.35 (d, 3J 5

8.5 Hz, 1H, Q-H2), 7.58 (pst, 3J 5 8.5 Hz, 1H, Q-H6), 7.45 (d,3J 5 8.5 Hz, 1H, Q-H3), 7.33 (dd, 3J 5 8.0 Hz and 12.0 Hz,8H, Ph-H2,6 and Ph-H3,5), 7.21 (d, 3J 5 8.5 Hz, 1H, Q-H5),7.09 (d, 3J 5 7.5 Hz, 1H, Q-H7), 6.70 (dd, 3J 5 11.0 Hz, 17.5Hz, 2H, H9), 5.72 (d, 3J 5 17.5 Hz, 2H, H10), 5.18 (d, 3J 5

11.0 Hz, 2H, H11), 2.86 (t, 3J 5 7.0 Hz, 2H, Hex), 1.11 (m,2H, Hex), 0.99 (m, 6H, Hex), 0.78 (t, 3J 5 7.0 Hz, 3H, Hex);13C (125.698 MHz, CDCl3): d 5 158.7, 157.9, 145.6, 139.3,137.5, 136.4, 133.5, 131.7, 126.9, 125.6, 124.0, 113.0, 112.4,109.9, 34.8, 31.6, 29.5, 29.3, 22.6, 14.2; UV-Vis (THF, 3.1 3

1025 M): kmax 5 258 nm (e 5 58,200), 386 nm (e 5 3,380);fluorescence (THF, 3.1 3 1025 M): kem,max 5 498 nm,U 5 0.23 (kexc5 386 nm); elemental analysis: calculated C83.59, H 7.24, N 3.14; found C 83.46, H 7.13, N 3.12.

Synthesis of p1The silicon based homopolymer p1 was prepared accordingto a literature procedure.2 In a typical reaction, monomer 1(368 mg, 8 3 1024 mol) was placed in an air-free reactiontube and dissolved in toluene (4 mL). To the reaction tubeGrubbs second-generation catalyst (8.5 mg, 1 3 1025 mol)was added. The reaction mixture was stirred at 50 to 60 �Cfor 24 h, while intermittently applying vacuum. The solutionwas concentrated, dissolved in hexane and passed through ashort silica gel column. The product was collected, hexanewas removed under reduced pressure and then dried undervacuum (210 mg, 67%).

1H NMR (600 MHz, CDCl3, ppm): 7.57 (br, s, 8H), 7.13 (s,2H), 6.6–6.9 (m, 2H), 5.78 (d, J 5 17.6 Hz, 2H), 5.26 (d, J 5

11.2 Hz, 2H), 0.88–1.38 (m, 34H). 13C NMR (125 MHz, CDCl3,ppm): 138.2, 137.4, 133.8, 129.1, 125.9, 33.5, 31.9, 29.2,

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23.0, 22.6, 15.8, 15.2, 14.1. 29Si (99.3 MHz, CDCl3, ppm):d 5 26.83; UV-Vis (Hexane, 1.0 3 1024 M): kmax 5 324 nm(e 5 22,900); fluorescence (hexane, 2.1 3 1028 M): kem,max 5

366 nm, U 5 0.24 (kexc 5 324 nm).

Synthesis of p2aIn a typical reaction, monomer 2a (60 mg. 1.4 3 1024 mol)was placed in an air-free reaction tube and dissolved in tolu-ene (2 mL). To the reaction tube Hoveyda-Grubbs second-generation catalyst was added (3.5 mg (4.9 3 1026 mol).The reaction mixture was stirred at 50 to 60 �C for 24 h,while intermittently applying vacuum. The solution was con-centrated and washed with methanol. The product was col-lected as a solid by centrifugation and dried under vacuum(43 mg, 75%).

1H NMR (600 MHz, CDCl3, ppm): d 5 7.76 (d, J 5 7.5Hz, 4H),7.58 (d, J 5 8.4 Hz, 4H), 7.27 (s, 2H), 2.92 (m, 1H), 2.41 (m,2H), 1.35 (d, J 5 7 Hz, 6H), 0.95 (d, J 5 6.15 Hz, 12H). UV-Vis (dichloromethane, 2.0 3 1026 M): kmax 5 406 nm(e 5 56,200); fluorescence (dichloromethane, 2.5 3 1028 M):kem,max 5 416 nm, U 5 0.30 (kexc 5 406 nm). Mn (polystyrenestandards)5 5440 g/mol; Mw/Mn 5 2.23.

Synthesis of p2bIn a typical reaction, monomer 2b (64 mg, 1.4 3 1024

mol) was placed in an air-free reaction tube and dissolvedin toluene (2 mL). To the reaction tube Hoveyda-Grubbssecond-generation catalyst was added (3.5 mg (4.9 3

1026 mol) of. The reaction mixture was stirred at 50 to60 �C, while intermittently applying vacuum. After 45 min,a precipitate was observed which was insoluble incommon organic solvents and therefore not furthercharacterized.

Synthesis of p1co2aIn a typical synthesis, monomers 1 (55 mg, 1.2 3 1024 mol)and 2a (50 mg, 1.2 3 1024 mol) were placed in an air-freereaction tube and dissolved in toluene (3 mL). To the reac-tion tube Hoveyda-Grubbs second-generation catalyst wasadded (4.4 mg, 7.0 3 1026 mol). The mixture was stirred at60 �C for 48 h, while intermittently applying vacuum. Thesolution was concentrated and washed with methanol. Asolid was collected by centrifugation, redissolved in dichloro-methane, and as a solution passed through a short silica gelcolumn. After solvent evaporation, the product was isolatedwith a 75% yield (78 mg).

1H NMR (600 MHz, CDCl3, ppm): d 5 7.76 (br, 4H), 7.56–7.58(dd, 4H), 7.49 (s, 4H), 7.47 (s, 4H), 7.27 (s, 2H), 7.18–7.20(m, J 5 11.1 Hz, 2H), 7.12 (s, 4H), 6.99 (s, 2H), 2.92 (m, 1H),2.41 (m, 2H), 0.88–1.40 (m, 52H). 13C NMR (125 MHz,CDCl3, ppm): d 5 138.3, 135.1, 130.3, 130.1, 129.0, 128.9,125.9, 125.8, 120.0, 35.7, 34.4, 33.7, 31.9, 29.3, 29.2, 23.7,22.7, 14.1, 12.6. UV-Vis (dichloromethane, 2.1 3 1026 M):kmax 5 317 nm (e 5 20,300), 350 nm (e 5 42,300), 381 nm(e 5 56,300); fluorescence (dichloromethane, 3.2 3 1028 M):kem,max 5 416 nm; U 5 0.28 (kexc 5 381 nm);Mn (polystyrenestandards)5 7480 g/mol; Mw/Mn 5 2.03.

Synthesis of p1co2bIn a typical synthesis, monomers 1 (91 mg, 2.0 3 1024 mol) and2b (96 mg, 2.0 3 1024 mol) were placed in an air-free reactiontube and dissolved in 4 mL of toluene. To the reaction tubeHoveyda-Grubbs second-generation catalyst was added (7.9 mg,1.3 3 1025 mol). The reaction was stirred at 60 �C for 48 h,while intermittently applying vacuum. The solution was concen-trated and washed with methanol. A solid was collected by cen-trifugation, re-dissolved in dichloromethane, and as a solutionpassed through a short silica gel column. After solvent evapora-tion, the product was isolated with a 70% yield (130 mg).

1H NMR (600 MHz, CDCl3, ppm): d 5 8.33 (br, 1H), 7.55 (br,1H), 7.47 (s, 4H), 7.45 (s, 4H), 7.43 (br, 4H), 7.42 (br, 1H),7.35 (br, 4H), 7.19 (br, 1H), 7.12 (br, 2H), 7.11 (br, 1H), 7.08(br, 1H), 7.07 (br, 1H), 7.04 (br, 2H), 2.84 (br, 2H), 0.8–1.3(m, 45H). 13C NMR (125 MHz, CDCl3, ppm): d 5138.9, 135.1,133.2, 131.4, 129.1, 127.8, 125.6, 123.7, 112.0, 109.6, 34.8,33.7, 31.9, 31.6, 29.5, 29.3, 29.2, 23.7, 22.7, 22.6, 14.1, 12.6.UV-Vis (dichloromethane, 2.8 3 1026 M): kmax5 327 nm(e 5 59,200), 385 nm (e 5 3700); fluorescence (dichlorome-thane, 3.1 3 1027 M): kem,max5 494 nm. U 5 0.48 (kexc5 385nm). Mn (polystyrene standards)5 2240 g/mol; Mw/Mn5 1.68.

RESULTS AND DISCUSSION

Monomer SynthesisThe general strategy for the synthesis of the monomers 1and 2 is shown in Scheme 1. Lithiation of the monoacetal ofp-bromobenzaldehyde with butyl lithium, followed by reac-tion with 0.5 equiv dichloro dioctyl silane resulted in the for-mation of silylated diacetal product, which was furtherhydrolyzed to form bis(4-diformylphenyl) dioctylsilane. Wit-tig reaction of bis(4-diformylphenyl)dioctylsilane led to theformation of monomer 1. Compound 1 was purified by pass-ing through silica gel using hexane as the eluent and wasobtained as colorless oil. Reaction of two equiv of 4-styrylGrignard with B(OR)3 (R5Me or Bu) in THF initially gives adi(4-styryl)alkoxyborane Sty2B(OR). Subsequent treatmentwith a slight excess of 2,4,6-triisopropylphenyl Grignard ledto formation of monomer 2a in 25% isolated yield, whereasreaction with 2-n-hexyl-8-hydroxyquinoline gave the tetra-coordinate monomer 2b. The monomers were purified bycolumn chromatography using hexanes as the eluent and iso-lated by recrystallization from hexanes. Both monomersshowed excellent solubility in common organic solvents suchas CH2Cl2, THF, and toluene.

The monomers were fully characterized by 1H, 13C, 29Si, and11B NMR spectroscopy. For 1, the presence of one 29Si NMRsignal at 26.84 ppm is typical for such dialkyl diaryl silanes.In the case of 2a, the presence of a broad 11B NMR signal at70 ppm (w1/2 5 1900 Hz) is typical of tri-coordinate triaryl-borane species. In contrast, the hydroxyquinoline monomer2b shows a relatively sharp signal at 11.7 ppm (w1/2 5 610Hz) that is strongly upfield shifted, which is consistent withtetra-coordination at boron (Fig. 1). The 1H NMR spectra ofboth the distyrylsilane and the new distyrylborane



monomers showed the expected vinyl peak patterns. For allmonomers, the vicinal protons @CH2 exhibit two characteris-tic doublet resonances at around �5.7 to 5.8 and �5.1 to5.3 ppm, and the ACH proton showed a double doublet ataround 6.7 ppm. Integration of the 1H NMR spectra of themonomers 2 confirmed the 2:1 ratio expected for the styrylgroups with respect to those of the aromatic protons of the2,4,6-triisopropylphenyl and quinolate groups, respectively.The purity of the monomers was further ascertained by ele-mental analysis.

We also performed a single-crystal x-ray crystallography forthe tri-coordinate species 2a, for which clear colorless plate-like crystals were obtained from hexanes at 235 �C (Fig. 2).The monomer crystallizes in the P21/c space group with a

co-crystallized hexane molecule, but no short intermolecularcontacts were observed. The B-CTip bond to the bulky 2,4,6-triisopropylphenyl (Tip) group of 1.582(2) Å is slightly lon-ger than the B-Cstyryl bond lengths of 1.563(2) Å, which isattributed to steric effects given that the bond length is in asimilar range as other (2,4,6-triisopropylphenyl)borane spe-cies reported in the literature.14,15 The boron center adoptsa trigonal planar geometry as indicated by the sum of theCaryl-B-Caryl angles (

P(C-B-C) 5 360�). Importantly, the styryl

moieties are positioned at relatively small interplanar anglesof 24.9 and 27.7� , respectively, relative to the trigonal boroncenter. This indicates that p-p overlap is favorable, leading toextension of p-conjugation via the empty p-orbital onboron.16 The dihedral angle between the boron trigonalplane and the plane containing the Tip ligand of 74� is com-paratively large owing to steric hindrance due to the i-Prsubstituents. Thus no significant conjugation into the sidechain is expected. These are important structural featuresthat are expected to greatly impact the optical properties ofthe corresponding polymeric materials (vide infra).

ADMET PolymerizationsScheme 2 details the synthetic approach for homopolymersp1, p2a, p2b, and copolymers p1co2a and p1co2b. Suc-cessful ADMET was carried out in the presence of

SCHEME 1 Synthesis of boron and silicon containing monomers.

FIGURE 1 Comparison of the 11B NMR shifts of 2a and 2b.


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Hoveyda–Grubbs second-generation catalyst in toluene at55 to 60 �C for 48 h while intermittently applying vacuum.The reaction mixtures were purified by washing withmethanol and column chromatography. In all cases the iso-lated yields after purification were observed at �70 to75% by mass, except in the case of p2b. For that latter, aninsoluble precipitate was observed after 45 min reactiontime which was not characterized any further. Of the testedsolvents (tetrahydrofuran, toluene, hexane, dichlorome-thane, and mixtures thereof) only pure toluene yieldedproduct. The ratios of total monomer to catalyst concentra-tions ([monomer]/ [catalyst]) ranged from 28/1 to 35/1.At reaction temperatures below 50 �C or using Grubbssecond-generation catalyst (exception p1), no significantmonomer conversion was observed. Reaction times signifi-cantly longer than 48 h resulted in the observation ofsome insoluble byproduct.

ADMET PolymerizationsMolecular weights (Mn) of the ADMET polycondensationproducts were determined by gel permeation chromatogra-phy (GPC) relative to polystyrene standards. Traces for thedifferential refractive index (dRI) detector responses areshown for p1, p2a, p1co2a, and p1co2b (Fig. 3). Table 1

SCHEME 2 Syntheses of homopolymers and copolymers via ADMET.

FIGURE 2 Molecular structure of 2a (ORTEP, 50% probability).

Hydrogen atoms and a co-crystallized Hexane molecule are

omitted for clarity. Selected bond lengths (A) and angles (deg):

B1-C1 5 1.582(2), B1-C7 5 1.563(2), B1-C13 5 1.563(2), C1-B1-

C7 5 120.66(10), C1-B1-C13 5 118.0(10), C7-B1-C13 5 121.24(10).



summarizes representative Mn and polydispersities (Mw/Mn)for p1, p2a, p1co2a, and p1co2b synthesized under similarreaction conditions. Both, Mn from GPC and from NMR analy-ses are listed. The NMR values Mn are based on end groupanalysis (integrals of signals from chain end vinyl protons vs.chain internal vinylene protons, see also NMR discussionbelow). The presented results correspond to representativepolycondensates synthesized in toluene at 60 �C after 48 hreaction time. The homopolymers p1 and p2a showed Mn

�3900 and 5440 g/mol (NMR: 6260 and 7070 g/mol),respectively. Due to some insoluble residue (�15%) thereported Mn for p2a most likely underestimates the realvalue. Values for Mw/Mn were in the range of 1.7 to 2.2.Polymerization of 2b resulted in an insoluble precipitateafter about 1.5 h reaction time and was not investigated fur-ther with regard to molecular weight or structure. GPC anal-

ysis of p1co2a and p1co2b gave Mn of 7480 and 2240 g/mol (NMR: 14,850 and 7790 g/mol), respectively. Lowerreaction temperatures generally resulted in lower conver-sions, higher reaction temperatures in an increase of insolu-ble precipitate fraction, particularly in the case of p2a.Reaction temperatures below 50 �C did not yield any prod-uct at all, regardless of catalyst used. Higher ratios of [mono-mer]/[catalyst] also resulted in lower conversions, and lowerratios of [monomer]/[catalyst] in increased precipitation ofinsoluble byproduct in the case of p2a. Among the solventstoluene, THF and toluene/THF mixtures, pure tolueneyielded the best results and pure THF resulted in noconversion.

Especially as the chain size increases with conversion, it isexpected that the chain growth reaction starts to competewith some ring closing, potentially yielding a chain/ringequilibrium. Thus it is possible that small fractions of thepolymeric products consist of cyclic macromolecules. Thismight explain some of the shoulders on the low molecularweight side of the distributions in the GPC-traces as well asthe fact that end-group calculations based on well-resolvedNMR spectra yielded higher molecular weights than the GPCanalysis.

Microstructure Analysis via NMRThe microstructures of the polymeric products were deter-mined by means of 1H-NMR, NOESY, HSQC, and COSY techni-ques. Figure 4 displays an overlay of the 1H NMR spectra ofp1co2a, p2a, and p1co2b. Details for p1co2a and p1co2bare depicted separately in Figures 5 and 6, respectively,together with their most important structural assignments(see also Supporting Information). Co-condensation of twomonomers, 1 with 2a or 1 with 2b, leads to random co-polymers with three possible coupling modes, giving rise tothree distinct vinylene groups. All vinylene groups could beidentified and found to be exclusively trans-configured. Theresulting trans-stilbene units featured either boron on bothsides, silicon on both sides, or boron on one and silicon onthe other. The detailed assignments for p1co2a are illus-trated in Figure 5 in a representative drawing of a polymerchain segment showing the three possible coupling modes,giving rise to three resonances d (Si on both sides of

FIGURE 3 GPC traces of p1co2a, p2a, p1co2b, and p1. [Color

figure can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

TABLE 1 Representative Polycondensation Results

Sample

Reaction Conditionsa Mn

Mw/Mn[Monomer] (mM) [Catalyst] (mM) GPC (g/mol) 1H-NMR (g/mol)

p1 200 4b 3,900 6,260 1.75

p2a 70 2.5 5,440 7,070 2.23

p2bc 70 2.5 n.d. n.d. n.d.

p1co2a 40, 40 ([1],[2a]) 2.3 7,480 14,850 2.03

p1co2b 50, 50 ([1],[2b]) 3.2 2,240 7,790 1.68

a T 5 60 �C/t 5 48 h/toluene.b Grubbs second-generation catalyst—all other entries: Grubbs-Hoveyda

catalyst.

c Insoluble precipitate after 1–2 h reaction time, n.d. 5 not determined).


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stilbene), e (B on both sides of stilbene), and a with b (Siand B on alternate stilbene sides). They are observed at 7.12ppm, 7.28 ppm, and as two doublets around 7.20 ppm,respectively. Integration of these three vinylene NMR signalsresulted in a ratio of 1/1/2 in the case of p1co2a, indicatinga random structure. This structure and composition is onlyexplicable if both monomers and resulting chain ends havesimilar reactivity with statistical coupling as a result.

The aliphatic protons from the octyl (C8) and the isopropylgroups show resonances in the range of �0.88 to 3.0 ppm.Signals from the vinyl protons from the chain ends and/orresidual monomers around 5.29 and 5.78 ppm are verysmall. Integration of these resonances vis-�a-vis the resonan-ces from the vinylene protons allows us to calculate a num-ber average molecular weight of �14,850 g/mol. This isabout twice the weight determined by GPC (Table 1). An

FIGURE 4 1H NMR spectra of p1co2b, p1co2a, and p2a (CDCl3).

FIGURE 5 1H NMR spectrum of p1co2a with assignments.



explanation could be the possible presence of macrocyclesformed by back-biting of the reactive chain ends (vide supra).Such macrocycles are common components in the complexADMET equilibria.17 The NMR spectrum of p1co2b (Fig. 6)shows the presence of two sets of terminal vinyl groups, at5.76 and 5.24 ppm and at 5.68 and 5.14 ppm. These areassigned to the B- and Si-chain ends, based on comparisonwith monomers 1 (doublets at 5.79 and 5.26 ppm) and 2b(doublets at 5.71 and 5.12 ppm). As in the case of p1co2a,resonances from three distinct vinylene groups are observedat 7.04, 7.07, 7.11, and 7.12 ppm (e, b, a, and d respectively,Fig. 6). A quantification of their relative amounts via integralratios was not possible, however. The resonances could be

assigned with the help of HSQC, NOESY, and COSY experi-ments. The resonances from the alkyl protons (butyl groupof the B repeat unit octyl group of the Si-repeat unit) lie inthe range 0.7 to 1.7 ppm. Table 2 representatively summa-rizes the NMR observations leading to the assignments inp1co2b.

Optical PropertiesThe absorption and emission properties of all compoundswere characterized as solutions in dichloromethane (Table 3).As expected, the absorptions of all macromolecular homo-and co-poly condensates are red-shifted compared to their

FIGURE 6 1H NMR spectrum of p1co2b with assignments.

TABLE 2 Important NMR-Assignments for p1co2b

Assignment

1H–d(ppm)

HSQC with3C–d (ppm)

NOESY with1H–d (ppm)

j 7.08 109.56 7.55

k 7.55 131.39 7.2, 7.08

l 7.19 112.03 7.57

m 8.33 138.87 7.42

n 7.42 125.6 7.06 (weak), 8.33

f 7.45 135.09 7.12

g 7.47 125.6 7.11, 7.06

h,h’ 7.35 133.23 7.11, 7.07, 7.43, 7.04

i,i’ 7.43 123.68 7.10 (weak), 7.35

a 7.11 129.1 7.46, 7.35

b 7.07 128 7.46, 7.35

d 7.12 128 7.45

e 7.04 128 7.35

TABLE 3 Experimental Absorption and Emission Data of the

Monomers and Polymers

e (L mol21

cm21)

kmax, absorption

(nm)

kmax, emission

(nm)a Ueff

1 n.d. 256 307 n.d.

2a n.d. 317 424 0.06

2b n.d. 265, 384 494 0.23

p1 22,900

(324 nm)

324 356 0.24

p2a 56,200

(406 nm)

406 416, 448 0.30

p1co2a 56,300

(381 nm)

317, 350, 381 417, 449 0.28

p1co2b 59,200

(327 nm)

276, 327, 385 370, 494 0.48

a Excitation at absorption maximum.


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respective monomers. The aromatic unit formed in everyADMET step is trans-stilbene. Isolated trans-stilbene absorbsand emits with kmax �295 and 347 nm, respectively.2,18 Theabsorption and emission of p1 with kmax �324 and 356 nm,respectively closely resemble the vibronic structures of theabsorption and emission of trans-stilbene. The bathochromicshift is a consequence of r-p-conjugation through the r-bondsof the silylene linkages and through-space interactions of thep-conjugated segments.19–21

The two boron monomers and their respective polymers arefundamentally different. The absorption spectrum of thepolymer (p2a) in dichloromethane shows a band withvibronic structure and a maximum at 406 nm that is red-shifted by approximately 80 nm relative to that of the mono-mer (2a) (Fig. 7).

This bathochromic shift is attributed to the extended conju-gation of trans- stilbene segments through the boron moiety(p-p overlap).16,22 The emission maxima for 2a and its poly-mer (p2a) are quite similar with kmax �424 and 417 nm,respectively, however, the polymeric material shows a muchmore well-resolved structure with peaks at 417 and 448 nm,which can be assigned to 0-0 and 0–1 intrachain singlettransitions and the 0-0 transition being the most intense.The perfectly mirrored absorption and emission bands andrelatively small Stokes shift indicate a highly rigid polymericstructure.23 The unexpectedly long wavelength for the emis-sion of 2a is attributed to a more polarized structure of themonomer in the excited state, which results in a larger Stokeshift due to a pronounced charge transfer contribution. Asimilar phenomenon was also observed in the case of afluorene-based monomer and the corresponding oligomers.16

The fluorescence quantum yield of the polymer (p2a) is fivetimes higher than that of the monomer (2a). This could berelated to the charge transfer component for the monomeremission or potentially be due to the fact that the polymerbackbone is quite rigid. In addition, the steric hindrance ofthe Tip groups may prevent aggregation, thereby furtherreducing the probability of nonradiative decay. A similarenhancement of quantum efficiencies has been observed, forexample, by Yang et al. upon incorporation of rigid pentipty-cene moieties into polyphenylenevinylenes.24

Combining the monomers 1 and 2a into the polymerp1co2a led to a bathochromic absorption relative to p1 buthypsochromic relative to p2a, and with significant vibronicstructure. For p1co2a three absorptions are observed at317, 350, and 381 nm. Relative to p2a, the presence of theflexible silicon linkages might lead to a more disorderedstructure with less extended p-p overlap through the boron,resulting in a higher energy absorption. The emissions ofp2a and p1co2a, however, are closely matched, withp1co2a displaying a slightly higher kmax and lower intensity0–1 transition peak, the latter again due to the less rigidstructure of p1co2a.

Compared with 2a, monomer 2b showed an absorptionmaximum (kmax �265 nm) much closer to that of 1 as the

tetra-coordinated boron center cannot participate in theelectronic delocalization as in 2a. In addition, there is abroader, lower intensity absorption band at around385 nm which can be assigned to intra-ligand chargetransfer to the pyridyl ring present within the quinolatesystem (Fig. 8).25

Due to lack of solubility, p2b was not analyzed further. Incontrast, the combination of 1 and 2b in polymer p1co2bproduced a soluble material with main absorption character-istics similar to p1 (kmax �327 vs. 324 nm, respectively) anda slightly smaller half width compared with p1co2a. The

FIGURE 7 UV-Vis (top) and PL spectra (bottom) of 1, 2a, p1,

p2a, p1co2a in dichloromethane (excitations at respective

absorption maxima). [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]




absorption band from the quinolate system observed in themonomer 2b is present in the polymer p1co2b in the formof a shoulder at 385 nm.

A less intense absorption is observed at 276 nm. Upon exci-tation of p1co2b at 327 nm one observes dual emission, onecomponent with a maximum at 494 nm and the otheraround 370 nm. At higher concentrations the emission ofp1co2b is dominated by the boron quinolate chromophore.A possible explanation could be that the emission of the stil-bene units overlaps with the broad absorption band fromthe quinolates, resulting in efficient fluorescence resonanceenergy transfer (FRET).25 To support this hypothesis, dilu-tion experiments were carried out. With increasing dilution,

the shorter wavelength emission from the stilbene graduallybecomes relatively stronger, until it finally dominates thespectrum and the emission with a maximum at 494 nm hasbeen reduced to a shoulder (Fig. 9).

A unique capability of fluorescent conjugated systems con-taining tri-coordinate boron as part of the conjugation—istheir ability to indicate the presence of Lewis donors (seeabove) with significant sensitivity.26 In the presence of fluo-ride ions from tetrabutylammonium fluoride (TBAF), the flu-orescence of p1co2a is quenched as shown in Figure 10,indicating the potential use of this system as a sensorymaterial.

The binding of the fluoride ion to the vacant p-orbital at theboron center results in the disruption of the (p-p) conjuga-tion path and fluorescence quenching as a consequence. Spe-cifically, Figure 10 illustrates the fluorescence of a p1co2asolution in tetrahydrofuran as a function of increasing fluo-ride ion concentration. The fluoride concentration is given asa ratio B/F2. Initially, at high ratios B/F2, the fluorescenceintensity gradually decreases upon the addition of F2 ions.Then, when the B/F2 changes from 6 to 4, an abrupt reduc-tion in emission is observed, effectively switching off thefluorescence.

Generally, for monomeric borane fluorophores with highbinding constants K, quantitative complexation of boronoccurs at B/F2 ratios of just slightly above 1:1, which typi-cally results in effective emission quenching and in somecases the observation of fluorescence at a different wave-length (fluorescence turn-on).27 Fluorescence quenching at

FIGURE 9 Effect of dilution on the emission behavior of

p1co2b in CH2Cl2. ([p1co2b] 5 9 3 1024 mg/mL, excitation

wavelength 5 327 nm. [Color figure can be viewed in the online

issue, which is available at wileyonlinelibrary.com.]

FIGURE 8 UV/Vis (top) and PL spectra (bottom) of 1, 2b, p1,

p1co2b in dichloromethane (excitations at respective absorp-

tion maxima. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]


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B/F2 ratios >1 is observed frequently in oligomeric andpolymeric tri-coordinate B-containing fluorophores. Most sys-tems show effective quenching of the initial fluorescence atB/F2 �2, because a donor (borate)–acceptor (borane) struc-ture is established.28 In such cases, a lower energy chargetransfer band arises in the absorption spectra and emissionfrom the corresponding charge transfer excited state tendsto be weak or not observed at all. We note the absence of alower energy emission band in our titration experiments.Further enhanced quenching effects can be observed whenthe borane fluorophores are integrated into polymeric sys-tems, either into the main chain or as pendant groups, lead-ing to effective quenching even in the presence of smallamounts of F2 with remarkably large ratios of up to B/F2

�6 to 8.29 This sensory amplification effect has been attrib-uted to facile through-space energy migration to lowerenergy quenching sites. The observations reported here areconsistent with those earlier results.

CONCLUSIONS

ADMET condensation was used to synthesize segmented con-jugated homo- and copolymers containing silicon and boron.Both, tri-coordinated and tetra-coordinated boron-containingmonomers were reactive enough to be subjected to ADMET,yielded random copolymers when copolymerized withsilicon-containing monomers. Photo physical characterizationof the polymers showed fluorescence quantum efficienciesof �28 to 48%. The copolymer containing tri-coordinatedboron showed potential as a fluoride sensing material, due toefficient fluorescence quenching even at boron/fluoride ratios

of �5/1. The report demonstrates the potential of ADMET toallow access to a wide variety of conjugated polymers.

ACKNOWLEDGMENTS

A. Sengupta and R. M. Peetz acknowledge financial supportfrom a PSC-CUNYgrant. F. J€akle and A. Doshi thank the NationalScience Foundation for support under grants no. CHE-0809642and CHE-1112195. Partial support by an NSF-CRIF Grant No.0443538 for acquisition of the X-ray diffractometer used inthese studies is also acknowledged.

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Documents

Segmented conjugated macromolecules containing silicon and boron: ADMET synthesis and luminescent properties