237 [4] ´ etrie As a b [5] that the and PDI. 1 n − M and polydispersity n M 2011 John Wiley & Sons, Ltd. c e Materials, Center of Innovation and densed phase (clusters) with Mass Spectrometry Research Group, ntre Interdisciplinaire de Spectrom ´ etrie de Masse (CISMa), University of IRMAP), University of Mons – UMons, 20 such as terthiophene, anthracene ) below 10 000 g mol n [6] M Olivier Coulembier, b nitrile (DCTB) was shown to afford better results than Mons – UMons, 20 Place du Parc,E-mail: 7000 [email protected] Mons, Belgium. Mass Spectrometry Research Group, Ce Laboratory of Polymeric and Composit Research in Materials and Polymers (C Place du Parc, 7000 Mons, Belgium Correspondence to: PascalCentre Gerbaux, Interdisciplinaire de Spectrom de Masse (CISMa), University ofBelgium Mons – Umons, 20 Place du Parc, 7000 Mons, ∗ a b ivity and signal-to-noise ratio are concerned. We tentatively on and analytical methodologies allowing an in-depth and ence of this substituent is likely to decrease the intermolecular Moreover, no information regarding the nature ofcan the end-groups be obtainedowing following to GPC the use analyses.accurate of identification On high-resolution of the MALDI-ToF theof other measurements, composition the (including hand, the end-groups) nature of the oligomers is made accessible. makes a correlation between the polymerthe molecular hydrodynamic weights volume and of randomly coiled polymer chains. consequence, MALDI-ToF methodology isused nowadays for extensively the analysisof of charge polythiophene exchange matrices, samples and the interest e MALDI analysis of P3HT are (1) the highest ionization energy Florian Boon, ix molecule evaporation from the clusters. The putative key f those conducting polymers. Synthetic procedures are now ∗ ed by charge exchange in the con out any required doubt. Mass spectrometry methodologies and on of the neutral matrix molecules less energy demanding. We a are devoted to the research field of polythiophenes and other b inst heptane was shown to allow the recording of absolute ers upon MALDI measurements ends up with wrong and, for instance, Nevertheless, the -butyl-phenyl)-2-methyl- [3] t [3] Soluble polymers able ¨ elle Deshayes, [1] Polythiophenes, and partic- Ga [1] and Pascal Gerbaux a,b b Nowadays, control over the synthesis , 237–246 Copyright [2] -butyl-phenyl)-2-methyl-2-propenylidene]malono 46 -2-[3-(4- t , C) invariably ends up with an overestima- ◦ 2011

2011 John Wiley & Sons, Ltd. -2-[3-(4- c polythiophene; MALDI; DCTB; terthiophene; polymer

This was explained by the rod-like conformation of poly-

trans trans [4]

matrix 2-propenylidene]malononitrile – DCTB – as of poly(3-alkylthiophene) is readily achieved systematic overestimation of the low mass oligom tion by a factor ofrange. 1.2–2.3 with respect of the molecular weight in-depth structural characterizationremains of problematic for polythiophene several reasons.nation samples First of of the molecular all, weights the ofpermeation determi- polythiophene chromatography systems (GPC) by gel using(e.g. polystyrene standards in THF at 30 J. Mass. Spectrom. Introduction Electroconjugated polymers have beenpast of two huge interest decades fororetical and the numerous studies experimental have as been well reported. as the- electroconjugated polymers due to the huge potentialities o in particular MALDI-ToF measurementsstudy, are definitively suitable to meet the characterization requirements. In the present parameters to account for the really high efficiency of DCTB for th Nowadays, numerous experimental and theoretical studies Philippe Dubois developed to reach thestraightforward characterization highest of control the polymer over samples with both polymerizati the reported terthiophene and dithranolproposed matrices that as the far ionization as of sensit matrix the molecule P3HT molecules radical is cations perform and subsequentof neutral DCTB matr amongst the three matrices,presence of (2) the the tertiobutyl group really on high the matrix absorptivity molecule.interactions The of pres in the matrix the molecule condensed atalso phase the demonstrated laser rendering wavelength for the and polymer evaporati (3) samples the presenting an average number molecular weight ( Julien De Winter, of (wileyonlinelibrary.com) DOI 10.1002/jms.1886 MALDI-ToF analysis of polythiophene: use Research Article Received: 12 July 2010 Accepted: 21 December 2010 Published online in Wiley Online Library: 2011 thiophene polymers in solution, whereas the GPC methodology index (PDI) values. A systematic Soxhlet extraction aga the nature ofprepared the polymers end-groups can and be the regioregularity finely of tuned. the to conduct electricityoptical reveal devices, huge organicnanoscopic potentialities light-emitting electronic structures. diodes, in transistors nonlinear and ularly poly(3-alkylthiophene), representconducting an polymers due importantand to class processing their of ability. excellent solubility, stability Copyright Keywords: Supporting information may be found in the online version of this article. . et al , 237–246 46 , delivered to 2 − J. De Winter 2011 ,i-PrMgClinTHF l) was applied to a 2 ) at room temper- µ 2 , Labscan, 99%), tetrahydro- 1 ppm O 3 500. The matrices, terthiophene, < J. Mass. Spectrom. z / . DCM (Chem-Laboratory, 99.8%), O, 2 m 2 in DCM were realized by scanning the max λ 1 ppm H < ) ratios were estimated as follows. For each MALDI N ratio is concerned, the intensities (ion count) of the / ) were also purchased from Acros Organics and stored S ) were determined at 337 nm (wavelength of the MALDI N 1 ε / − S ature. 2-Bromo-3-hexylthiopheneSigma–Aldrich. (99%) was Chloroform (CHCl purchased from absorption between 800 and 190 nm. Molar extinctionin coefficients DCM ( laser) using the Beer–Lambert law. Materials All the samplesavailable, used except the in poly(3-hexylthiophene)s prepared the following the procedures present described hereunder. work were commercially Chemicals Iodine and iodobenzene diacetateOrganics were and purchased used from as Acros received. Ni(dppp)Cl in a glove box ( we alsooverestimationoflowmassoligomersuponMALDImeasurements. would like to demonstrate the problematic huge (2 mol l Experimental Mass spectrometry measurements MALDI mass spectra were recordedmass using a spectrometer Waters QToF equipped Premier at with 337 a nm nitrogen with laser, a maximum operating output of 500 J m background signals close tothe the BP BP intensity. (Fig. 1) were compared with UV–Vis spectroscopy measurements Absorption spectradichloromethane were (DCM). recorded UV–Visitored absorption at spectra with were room a mon- terminations temperature Shimadzu of the UV-mini1240 in spectrophotometer. De- the sample inflight mass 4 analyses ns were performedresolution pulses in of about the 10 at 000 reflectron at mode 20dithranol at Hz and a repeating DCTB, werethe rate. prepared concentrations). Time-of- in The chloroform matrixstainless steel (see solution target and air (1 text dried. Polymer for samples werein dissolved chloroform (see alsoaliquots text of for these concentrations).already solutions One-microliter bearing were the applied matrixrecording onto of crystals, the and the single-stage then MALDI-MS target(rf-only spectra, air mode) area the was dried. quadrupole set to For passions the ions were transmitted from into 750 the to pusher 16analyzer 000 region where they of Th, were the and mass time-of-flight all analyzed withData 1 were s acquired integration in time. continuum mode until acceptabledata averaged were obtained. The base peakto-noise (BP) intensities ( and the signal- mass spectrum, 100 consecutivethe scans MALDI spectrum. were The averaged BP intensities toconsidering were directly the afford obtained total by ionas count for the the corresponding BP. As far heptane (Labscan, 99%), hexane(Chem-Laboratory, 99.8%) (Labscan, were used 99%) as received. and methanol furan (THF, Labscan 99%)purification were system dried under using N an MBraun solvent . M )is CN n et al 2011 John Wiley & Sons, Ltd. M c + NC + For highly C DCTB 3 H [8] McCarley S Copyright [7] The key-parameters Alk [4] tBu e matrices tested in the present Actually, DCTB (Scheme 2) S M = matrix molecule [9] MALDI S terthiophene impossible. Fractionation of the polymer + n S M -2-[3-(4-t-butyl-phenyl)-2-methyl-2-propeny- M [4] [5b,10] + trans [7] As a consequence, several groups already reported the Molecular structures of the thre Ionization of polythiophene molecule by electron transfer Poly(3-hexylthiophene) molecules are proposed to be [9] OH S Dithranol [4] . OH OH In the present study, we have decided to report the comparison Recently, As already mentioned, the GPC molecular weight distribution Alk upon Soxhlet extraction againstafford various polymer solvents samples was ofToF shown narrow measurements to polydispersity give more whose accurate MALDI- dataMWD as is far concerned. as the absolute disperse polymer samples,the the low-mass huge and the mass high-massmeasurement difference oligomers of renders the between the accurate between dithranol, terthiophene and DCTB (Scheme 2) as matrices for the MALDIthree analysis matrices of poly(3-hexylthiophene)s arecharacterization of since currently those electroconjugated these polymers. successfully In addition, used for the MALDI lidene]malononitrile (DCTB)choice was proposed forsuch as the as the analysis matrix substituted of of fullerenes. very labile organic compounds (MWD) is invariablyAt skewed toward variance, higher uponsample, molecular MALDI-ToF the weights. analysis, MALDI for average the number same molecular weight polymer ( was demonstrated todihydroxybenzoic be acid a (DHB),ionize softer the requiring matrix analyte. lessMALDI-MS than In analysis laser addition dithranol of and fluence and polythiophenes,charge in DCTB to exchange mainly direct (electron works relationcations. exchange) as with agent a the producing radical wileyonlinelibrary.com/journal/jms and dithranol, waset convincingly al demonstrated by McCullough Scheme 2. investigation. Scheme 1. reaction under MALDI conditions. ionized by electron transferMALDI process reactions and with radical cations ionsproposed of as present the putative in matrix reactive molecules the ions were (Scheme 1). reported the preferred use of terthiophene, since highernoise signal-to- ratio and resolution were routinelymeasurements. obtained in their MALDI that account fordependences the (1) on intensity the desorption ofreaction process, (2) MALDI and on the signals (3) ionization onthe are the ion the analyzer transmission mass detection and efficiency the including detector response. displaced toward lower molecular weights. use of DCTB as a good MALDImacromolecules. matrix for polythiophenes or related

238 239 2315 z / m 2220 and 2350, z / m wileyonlinelibrary.com/journal/jms 2242 ions and (iv) inset 3, magnification between z / m 2011 John Wiley & Sons, Ltd. c al and theoretical isotopic patterns for the with DCTB as the matrix: (i) full scan mass spectrum, (ii) inset 1, magnification between 3000 , 237–246 Copyright 46 , 2011 MALDI-ToF analysis of P3HT MALDI-ToF analysis of polythiophene (iii) inset 2, comparison between experiment and 2340. J. Mass. Spectrom. Figure 1. . ): c 1 − et al 5Hz, . using MALDI 7 -bromo , 237–246 α = PDI 3000 Samounts J 46 , 3 ,respectively 14 4 Hz, 3H); GPC H . 6, characteristic 6 10 J. De Winter 2011 15 000 = = 500) and a high mass J c , nm): 451; IR (cm ’s were 3000; 7000 and ) 3 z 3 asym n 1 / I 1.3. − / M m MALDI = 2240 1.1 andP3HT n sym I for the MALDI measurements M (g mol ;PDI 7000 [4] 1 − J. Mass. Spectrom. ): 6.98 (s, 1H), 2.80 (t, 3 ,whoseGPC ,P3HT 75%; UV (CHCl = 3000 and dithranol 15 000 g mol As a consequence, we decided to investigate C using PS standards. [7] d ◦ after = HNMR(CDCl [2] ,forP3HT b 1 -bromoP3HT’s 1 α − at 30 GPC PDI 5 ppm). As a direct consequence and as demonstrated 3 n 166. < M = Soxhlet extraction 3HT ) and MWDs are concerned. n Figure 1 features the MALDI-ToF spectrum of P3HT 99% rr-HT-P3HT. Yield . M i 2H), 1.71 (quint, 2H), 1.34 (m, 6H), 0.91 (t, analysis : three different poly(3-hexylthiophene)swork. (P3HT) The polymer in samplesExperimental the Section were and present prepared are regioregular as (FigureInformation) S1, described Supporting in the 15 000 g mol DCTB as the matrix. The recorded spectrumof revealed the four presence differentinstrument ion used in series the present (A, study allowsspectra B, the with recording a C of high mass resolution and (10 000accuracy D at ( in Fig. 1).in The Fig. QToF 1,This we resolution obtained allows a theunit accurate highly mass measurement resolved (between ofisotope isotope the monoisotopic clusters repeat distribution. signals of ofu the two (2405.08–2238.99). ion consecutive Thisidentifies series experimental A) 3-hexylthienylene value thatsince (3-HT) unambiguously amounts the as to calculated 166.09 the monoisotopic mass monomer for residue C (seeTable1formoredetails). In-depth analysis of a MALDI-ToF mass spectrum of a poly(3-hexylthiophene) Regioregular poly(3-alkylthiophene)s (P3AT) are, amongstdeveloped all the conducting polymers,widely studied. probably one of the most The mixture was then stirred forMeOH. The 0.5 product h was washed before with MeOH precipitation yielding in a purple> cold solid, In the present study, wethe will address MALDI-ToF two analyses main of aspects polythiophenes.like regarding First to of all, experimentally we demonstrate would than that terthiophene DCTB is a better matrix Results and Discussion of rr-HT-P3HT; 819 (C–H aromatic out of plane) and of polythiophenes. As a directobservations connection and on with the basis the of the experimental proposed(see MALDI mechanisms here after), we wouldthe also highest like efficiency to of DCTB propose when an comparedterthiophene. explanation with dithranol for The and next point,will probably even concern more problematic, theaccurate data capability as of( far MALDI-ToF as the analyses average to number molecular afford weight M b ,withMW 2 ): C M C, 3 3HT PDI ◦ [11] 13 ), an 2011 John Wiley & Sons, Ltd. 1 MW mand c − Cusing µ × ◦ 0 [12] C. i-PrMgCl ) (see Ref. , the Soxhlet extraction was not required. ◦ C and three 1 /[Ni] ◦ − 0 b ) ights and molecular weight distributions HNMR(CDCl )at30 (50 ml) at 0 MALDI 15 000 1 1 1 − n Copyright (20 ml) and dried GPC 2 − M 3 n (20 ml) was added, Cl [11] O M 2 2 3 and (g mol S and mass-to-charge ratios as O(50ml).Theorganic O 2 spersity index as determined by GPC in CHCl i 2 th 2 S n 10 000 g mol 2 M = th 74) to give 2-bromo-3-hexyl-5- ). . m. Molecular weights and MWDs n th 0 µ a M n ) 1 = M − Cfor0.5 h.AsuspensionofNi(dppp)Cl th 61, f ◦ n 20004600 3000 7000 1.3 1.1 N.A. 1.1 4720 1.1 h M = 10 000 15 000 1.3 1.1 10 780 1.1 . After filtration, the solvent and iodoben- 4 0 (g mol C, and then the mixture was stirred at room ◦ ) with 0.003% TMS as an internal reference, on a /[Ni] 3 C NMR spectra were recorded in deuterated 0 13 -bromo poly(3-hexylthiophene) (P3HT ): 144.55, 138.29, 112.0, 71.38, 32.22, 31.89, 29.5, 29.14, α Investigated polythiophenes: molecular we 3 15 000 3000 7000 solution in THF, 4 ml, 8 mmol) was added via a syringe, and M Experimental molecular weight and polydi Given the perfect agreement between Theoretical molecular weight as determined by : [3HT] As determined using signal intensities as N Into a flask under a nitrogen atmosphere containing 2-bromo- P3HT P3HT b c d P3HT a Entry Table 1. HNMRand iodothiophene as pale yellow oil (7.51 g, 98%); and the mixturetemperature. was Then stirred 10% again aqueous for Na additional 4 h at room layer was washed with 10% aqueous Na (2.0 3-hexyl-5-iodothiophene (2.96 g, 8 mmol),added dry via THF a (15.0 syringe, ml) and the was mixture was stirred at 0 Synthesis of 2-bromo-3-hexyl-5-iodothiophene two columns PL gel mixed-D 5 were calculated with reference to polystyrene standards. columns: a guard column PL (Polymer Laboratories) gel 5 and the mixture was extracted with Et Synthesis of 22.90, 14.43. wileyonlinelibrary.com/journal/jms HCl aqueous solution was quickly added to quench the reaction. (71.45 mg, 0.13 mmol) in THFvia (15.0 a ml) was syringe added at to 0 the mixture for the establishment of the The following example aims atGRIM illustrating polymerization the conditions of forconditions: 3-hexylthiophene the [3HT] within the following In a(2.84 g, two-neck 11.2 mmol) and iodobenzene diacetate round-bottom (3.93were g, 12.2 mmol) flask added successivelyhexylthiophene under to (5.00 g, nitrogen, a 20.2 mmol) stirred iodine in solution CH of 2-bromo-3- 1 Characterization autosampler, a RI refractive index detector at 30 NMR (CDCl zene were removed by evaporationresidue under was reduced purified pressure. by silica The gel columnheptane, chromatography (eluent: revelation UV, Bruker AMX-300 apparatus (4.7 T). Fourier transform infraredspectra (FTIR) were recorded usingGel a permeation chromatography Bruker (GPC) Tensorin of 27 chloroform P3HT spectrometer. (sample was concentration: performed 1 mg ml 6.94 (s, 1H), 2.5 (t, 2H), 1.52 (quint, 2H), 1.27 (m, 6H), 0.87 (t, 3H); an Agilent Technologiesa 1200 degazer, chromatograph an isocratic equipped HPLC with pump (flow rate: 1 ml min chloroform (CDCl temperature. After the reaction mixture was stirred for 1 day, a 5 themixturewasstirredat0 over anhydrous MgSO

240 241 z / × 6 m and with, . 4 ratios leach) − µ N / 3000 10 S × 2 . solution in P3HT, the 1 − solutions(2 mol l 3000 5 and air dried. The intensities were − 10 × wileyonlinelibrary.com/journal/jms 2 . ). For the 2 in chloroform. Matrix and analyte solutions (1 1 1 − − )andhighlydilutedP3HT 1 mol l − 5 Influence of the analyte concentration and analyte-to-matrix − 2242 BP intensity measurement (ion count). The concentrations z mol l / 10 1 m × Figure 2 presents the variation of the BP intensity [ion counts − 4 . 10 recorded with the multichannelinstrument plate for all (MCP) the detector experimental conditions. of Without any thethe doubt, ToF highest measuredthe ion matrix count when was using the obtained highest with DCTB DCTB concentration as (1 2242 BP of the ion series (Fig.origin 1). of At this the point background of signal the discussion, deservesconstant the to intensity be of the discussed noise and signals the observedInformation in Fig. the S2A Supporting points to an electronic origin. are given in mol l signal intensity significantly dropped down,the probably extreme because dilution. of The best experimental conditions were then respectively, (a) DCTB,the (b) terthiophene and (c) dithranol as matrix for Figure 2. relative concentration on the MALDI-ToF analysis of P3HT For each MALDI mass spectrum, 100 consecutiveaged scans to were afford aver- the MALDIwere spectrum recorded and three the times on MALDI three mass different spectra spots. The and the BP intensities wereobserved then in measured Fig. on 1, thosesignal) spectra. an intensity As is estimation readily measured ofboth and the the we signal intensity decided noise and to (or the measure signal-to-noise background ratio for the were applied on the MALDI targetaveraged plate on 100 scans (1 scan/s) andtimes the experiments on were three repeated different three spots. 4 ) n In N M / are and S 11 2011 John Wiley & Sons, Ltd. 71 u i [10] H N 5 c -bromo and the C α n + M cations 11 H 5 C S -cleavage of aromatic β S 500) whatever the quality + radical (71 u) as presented C z 2 / ) and Base Peak intensity (BP • H m N 11 / using signal intensities as S H w 5 M -cleavage of aromatic radical cations , holding into account all the isotope i β In the present case, this specific bond M and , 237–246 Copyright n -cleavage [13] β + 46 M are calculated using Microsoft Office Excel , w , (2) the resolution, (3) the signal-to-noise ( -bromo poly(3-hexylthiophene) radical cations n 11 M α -dibromo poly(3-hexylthiophene) radical cations 2011 H M 5 ω , C terthiophene and dithranol as matrix S α and [14] is worthy of note as revealed by the data gathered in n 71 u loss from ionized P3HT molecules: n M 11 M 1.3, Table 1). As already mentioned three other ion series H versus 5 S C = MALDI-ToF analysis of polythiophene for MALDI-ToF analyses of polythiophenes As already explained inphene, dithranol the and DCTB are introduction proposed in of thebe recent valuable this literature matrix to for the paper, MALDI analyses terthio- of polythiophenes. in Scheme 3. The observation thatcan such occur a fragmentation upon reaction prime importance MALDI to avoid analysis misinterpretation when attributing of the end-groups. poly(alkylthiophene)s is of software). The good correlation between the expected MALDI signals ( J. Mass. Spectrom. and the MALDI DCTB poly(3-hexylthiophene) radical cations. In particular, the presented isotope cluster comparison concerns a polymerby chain 13 constituted 3-HT units and oneas bromine the atom end and groups. one Starting hydrogencalculated from atom the the MALDI mass spectrum in Fig. 1, we to 166.0816 u.between In an addition, isotopeand model based the (generated experimental on data byfar the (Fig. Masslynx 1), dominant direct the software) A ions comparison distribution constituting the are by readily identified as Scheme 3. radical cations. (B, C and D) are alsoFig. observed 1, in Fig. the 1. ions As exemplified correspondingidentified in to inset as series 3 of B and C are, respectively, bearing an alkyl chain. Table 1. The ion distribution Asymmetrical also in good appears agreement to with be a nearly(PDI low perfectly PDI measured by GPC (series B) and poly(3-hexylthiophene) radical cationsfar (series as C). As ion series D is concerned, the corresponding generated from following the well-known rupture involves the loss of C mass-to-charge ratios as order to estimate the applicability offor DCTB the as analysis the of matrix P3HT, of four choice sidered: different (1) parameters the have correlation to between be the con- absolute (or expected) ratio and (4) the signal intensity. The MALDIon spectra a were recorded QToF Premier masswith spectrometer. This an machine orthogonal-axis is time-of-flight equipped analyzerhigh that resolution warranties (FWHM a 10 000of at the incident ion beam. Infocus this the section discussion of on the the paper,using we recording the will of signal-to-noise first high-quality ratio spectraintensity) ( data by as comparison points. For eachferent matrix, matrix-to-analyte we used molar dif- ratios (seeFor corresponding figures). each matrix andspectra were each then matrix-to-analyte recorded ratio,account using for a three the predefined lack MALDI of spiral homogeneity pattern of the to sampled solid surface. . we max et al 400 30 5 445 346 240 240 λ (nm) ± ± ± [15] ., , 237–246 2242 base 46 (ion count) BP intensity z et al , a / [17] [18] [19] [16a] IE m (eV) 51 4000 1 460 200 J. De Winter 2011 N ± ± ± / S ratio 60 17 54 :comparisonofthree C) –4.8 ◦ 1 1 ( 3000 m.p. ) 2 − − 1 − ratios (Fig. S2B, Supporting − 10 10 10 N matrix × × / × C 6 2 S . . (mol l J. Mass. Spectrom. as the molecular weight. The molar 8 ) urements were repeated three times 1 1 1 b 1 − − 500 93–95 7.3 500 300 123–126 8.5 cm 300 178–180 6.9 4 4 5 ) 1 − − − 1 ± ± ± ± − − 10 10 10 (337 nm) P3HT ε × × × C 2 2 2 (l mol . . . (mol l ratios and intensities. N MALDI-ToF analyses of P3HT MALDI matrices and P3HT polymer: physical and thermo- / S ratios (see the Experimental Section) and the . This table immediately reveals that DCTB is definitively N / S 3000 Calculated using 3000 g mol Ionization energy. Indeed, the starting point for the ion production by MALDI is As for a conclusion, Table 2 gathered the best results obtained Terthiophene 2 matrices Matrix peak (BP) intensities were averaged on 100matrix-to-analyte consecutive scans. ratio, For the each meas on three different spots, see text. a b Table 2. DCTBThe 2 Table 3. physical data P3HT(3000) 39 200 Dithranol 2 absorptivity values were obtained in dichloromethane. DCTBTerthiopheneDithranol 18 300 26 300 8100 the matrix ofhexylthiophene)s since the choice recorded spectra foraveraged present the the highest MALDI-ToF analysis of poly(3- MALDI-ToF analyses of poly(3-hexylthiophene)s: why DCTBa is better matrix? At this point, thecompared with origin the of reported matrices the forof the higher MALDI-ToF polythiophenes efficiency analyses of deserves DCTBwell-accepted to when cluster’s be model investigated. proposed Based by on Karas the related to the absorption bymolecules the matrix of molecules the or the energy analyte from the laser. The molar extinction propose to focusthe the underlying discussion energetics: (1) onthe absorption the matrix of molecules the following and laser eventsthe the energy P3HT and clusters by molecules, (matrix (2) desorptionthe molecules of neutral and polymers), P3HTneutral (3) macromolecules ionization matrix and of moleculessome (4) sublimation relevant from physical of thethe and the discussion. clusters. thermochemical Table data 3 required presents for Supporting Information). The increase of theended fluence immediately up with thea decomposition subsequent of decrease the of oligomer the chains and for the three matrices when measuringP3HT the MALDI-ToF spectra of Information). At variancelower with laser fluence the (S.I.2.A) wherea spectrum we chemical contribution identified obtained to electronic with the noise, S.I.2.B noise is a in appearing, probably the following spectrum decomposition of depictedneutrals ions in upon and the higher fluence of the laser. 2242 with, 2011 John Wiley & Sons, Ltd. z / c m 3000 )(Fig.S2A, 1 − Js µ Copyright l each) were applied µ were averaged on 100 and intensities since N / N S ratios characterizing the / ratio remains high and S N / N S / S ratios measured close to the BP of the ratio value was measured close to the ) measurement. The concentrations are given in N N N / / / S S S ). When using both dithranol and terthiophene (Fig. 2) 1 Influence of the analyte concentration and analyte-to-matrix − in chloroform. Matrix and analyte solutions (1 1 − In Fig. 3, the Quite interestingly, it was not possible to probe the influence on the MALDI targetscans plate (1 and scan/s) and air the dried.different experiments The spots. were repeated The three timesbase on peak three (see the Experimental Section). the measured BP intensities are definitively weaker. ion series are reportedconcentrations in for matrix (DCTB/terthiophene/dithranol) different and P3HT concentrations andin relative chloroform. Interestingly,constant on the the investigated concentrationthe most range. interesting Nevertheless, point is thatspectra the obtained with terthiophene and dithranol are weakerwith than DCTB, again pointing toMALDI the matrix better for efficiency P3HT of samples. DCTB as the of the laser fluence on the measured respectively, (a) DCTB,the (b) terthiophene signal-to-noise ( andmol (c) l dithranol as matrix for wileyonlinelibrary.com/journal/jms obtained for a matrix-to-analyte ratio of about 750 (concentrations in mol l Figure 3. relative concentration on the MALDI-ToF analysis of P3HT the reported spectra werefluence(maximumfilter)ofthenitrogenlaser(200 already obtained with the minimum

242 243 . . at et al et al The fact is required [23] clusters associating matrix . are dealing with relatively 7.3 eV) and finally dithranol [15] = ., They observed that the driving et al When discussing charge exchange [19] et al wileyonlinelibrary.com/journal/jms [15] in the condensed phase (clusters) The simultaneous neighboring presence of [22] They convincingly proposed that the production of sample in the absence of matrix molecules. [19] 6.94 eV). Nevertheless, even if those data are in close two excitations 3000 . to account for the production of protonated and cationized = 8.5 eV), then terthiophene (IE In Fig. 1, the D distribution that is observed in the MALDI agreement with the two-step modelthat for UV-MALDI, the we proposed keymatrix parameter when to compared account withabsorptivity both for of other the this matrices efficiency is moleculeIndeed, the in of at highest their the DCTB paper, laser Hoteling wavelength (Table 3). force for chargethe exchange relative IEs reaction betweenthe matrix (electron and dominant transfer) analyte molecules factor thatmass is to spectrum; is usually in account other forAccordingly, words, the we the ion observed ion yields thatyields in intensities (absolute the the ion in highest ion intensities in source. the P3HT= Fig. 2) radical are cation obtained for(IE DCTB (IE small analyte molecules that aresurface easily evaporated or from from the solid thethat clusters. the For evaporation P3HT of molecules,state is P3HT it prohibited by is neutral the highest expected molecules intermolecularconsequence, interactions. from we As propose a the that solid theby ionization charge of exchange P3HT with molecules thethe clusters matrix and radical that cations thematrix occur subsequent evaporation molecules inside of affords light the neutral isolatedgas P3HT phase. radical Such cations in aet the al mechanism was alreadyspecies proposed for by large Chang molecules upon MALDI ionization. molecules and P3HT moleculesIn are this model, desorbed it fromare is the also generated surface. proposed that insideevaporation ionized those analyte of molecules neutral clusters matrixthat and are molecules then mass affords that analyzed. the the naked subsequent ions analyte radical cationsthe follows second a step, a two-stepby gas-phase model the ionization of matrix involving, the radical as analyte cations. molecule spectrum and that corresponds to fragment ions of the P3HT that, upon LD ionization of P3HT,to no indicate ions that are the detected relative also IEs seems transfer) are but important not (to the allow key parameter. the charge photons that terthiophene. Atalready this important level to of mentionmeasuring the the that LDI discussion, no spectrum it ionsP3HT (laser is are desorption ionization) detected of when the proposed that one-photon excited, not ionized,are matrix molecules the primaryproducts,including matrixionsintheabsenceof common analytemolecules precursorsin the for vicinity. all subsequent ion processes leading to the production of radicalto ions, the consider first is aspect the energetic ofIn the the different present underlying processes. reaction,two starting photons from are 3.68 at eV least requiredterthiophene,dithranolandDCTBmolecules(Table photons to (337 allow nm), the 3).Zenobi photoionization of studied in great manyMALDI. details the electron transfer reactions in least for ionization ofoligomer. one Given matrix the molecule reallyin or high the directly proportion solid of of phase,to matrix radical one be cations molecules P3HT of initially matrix generated. molecules are In likely their recent paper, Hoteling Ionization of the P3HT molecules After the absorptionmolecules of on photons the fromionization target the model plate, from laser and Karas by accordingly the to matrix the clusters 1 [7] − 2011 John Wiley & Sons, Ltd. at the c mol l 5 being the − 3000 n 10 In general, the × 2 2–5 ( . [21a] = n for n 1.3 in Table 1). This already [21b] = )and1/ max Uponfurtherincreaseoftherepeatunit λ , 237–246 Copyright [20] 46 , 2011 reaches a plateau and no more evolution is then The IEs of the tested matrix molecules amount ) of the three matrices and of P3HT ε max λ sample is tricky. Nevertheless, we consider that this ) for a polymer by MALDI mass spectrometry is that the [16a] concentration (Table 2). This concentration is one order ,whencomparedwiththemolarabsorptionsofthematrix n to access the molar extinction coefficient in DCM of the M n 3000 3000 3000 M The ionization energies (IEs) of the matrix molecules were also Starting from the absorptivity values of Table 3, the use of MALDI-ToF analysis of polythiophene matrix films show oscillator strengths similarby to measurements those on determined the solution. laser wavelength (337 nm) were thenThe measured in size DCM (Table dependence 3). UV–Vis of the behavior polythiophene waswork demonstrated oligomers recently a on linear reported relationshipabsorptivity the between wavelength and ( the an maximum experimental molecules (Table 3), could definitively confirmto-polymer that molar a ratio high on matrix- the MALDIthe target P3HT is molecules required from to directly absorbing protect thelaser. photons At from the this point ofsolid matrices the is discussion, assumed the to absorption replicatespectra. the band corresponding Indeed, of solution the thissince first solidification approximation was shown isshifts to (20–50 generally only nm) induce accepted and broadening slight of the wavelength peaks. P3HT of magnitudeterthiophene weaker and DCTB, than revealing that aratio the higher is matrix-to-analyte experimental required infrom conditions the absorbing dithranol too for case many tois photons often from avoid the reported the laser. as P3HT Terthiophene a chains good MALDI matrix for polythiophenes. reported in Table 3.to The 4.8 eV. IE of P3HT was reported to amount polymer possesses a low PDI. dithranol as a good candidateof for polymers the is MALDI analysis then of ruledat this out, the kind since laser the wavelength absorptivity isdithranol, of the definitively best dithranol too results weak. were Accordingly, obtained for for a 2 J. Mass. Spectrom. Nevertheless, the higher absorptivitywith terthiophene of indicates DCTB that when DCTB compared is more prone to absorb P3HT coefficients ( Absorption of the laserP3HT energy molecules by the matrix molecules and/or the The first aspectof to the be absorptionP3HT discussed efficiency molecules. of Starting is from the(solution related the phase) matrix molar to reported extinction molecules the inP3HT coefficients and Table difference 3, the the higher absorptivity of to, respectively, 8.5, 7.3dithranol and (Table 6.9 3). eV forinteractions Finally, DCTB, between to terthiophene the and accountwe matrix decided to molecules for use the within reported the melting theassumption points (m.p.) intermolecular that based clusters, on the the higherintermolecular the interactions. melting point, the stronger the observed. Nevertheless, the absorptivitystrongly at depends a on given themoieties. wavelength number of As repeat a units,polymer consequence, i.e. chain, absorbing upon therendering increasing absorptivity the at the longest 337 size chains nmones. more of This is absorbing confirms likely the than that our to thethe approximation smallest increase to hold into account numbers, confirms that the prerequisite for the measurementMWD of an ( accurate calculation is not worthlessa starting low from polydispersity a polymer index presenting (PDI numberofrepeatunits). . et al 15 000 sample , 237–246 46 , 15 000 J. De Winter 2011 -stacking that constitutes π 2500 and 11 000. Moreover, z / J. Mass. Spectrom. m ll scan spectrum of the polymer sample after Soxhlet [20] Asaconsequence,evenforaP3HT [20] -stacking aggregation, ultimately making the sublimation π Figure 4(a) presents the MALDI-ToF mass spectrum of P3HT However, the higher melting point ofthe dithranol also indicates evaporation that ofdemanding the than for neutral both other molecules matrices.of The is weaker dithranol absorptivity also at moreevaporation the energy step laser more wavelength problematic. (TableDCTB As 3) far are also as renders concerned, terthiophenestructural the and similarity we between tentatively terthiophenea and propose high P3HT that generates an level the obstacle of great tomolecules aggregation the from ultimate by the sublimationtertiobutyl clusters. of group The the on presence the neutralthe of DCTB matrix molecule the is voluminous likely to destabilize MALDI-ToF analyses ofmolecular weight distributions poly(3-hexylthiophene)s of higher Having demonstrated that DCTB representsfor a the really MALDI-ToF good analyses matrix reliability of of P3HTs, MALDI-ToF measurements we to will determinenumber the then absolute average molecular focus weighs. on the recorded using DCTB asratio the of 750. matrix Surprisingly, and whereaspresence for the of GPC a a analysis matrix-to-analyte single revealedare monomodal the clearly distribution, two observedthe distributions around GPCpolymerization analysis since also afor confirmed PDI an explanation of a for 1.3MALDI, the we good was occurrence propose that of the measured control two abundanceions of (Table distributions the 1). is over low upon mass systematically As oligomer overestimated, the intermolecular probably interactions because are stronger expectedchains. for the longer polymer step easier. As forthe a intermolecular comparison, interactions a was reductionunsubstituted observed of when the polythiophenes going strengthbecause from to of of substituted asubstituents. larger polythiophenes intermolecular separation induced by the - π 2011 John Wiley & Sons, Ltd. c Accordingly, [23] Copyright : (a) full scan spectrum of the polymer sample, (b) fu al cations upon matrix molecule scan spectrum of the heptane extract. 15 000 protonation of extended aromatic [24] , 1 − [16b] MALDI-ToF spectra of P3HT 885 kJ mol = At variance with terthiophene and DCTB, proton transfer is also In the Introduction Section, the systematic underestimation of extraction against heptane and (c) full wileyonlinelibrary.com/journal/jms Production of gas-phase P3HT radic evaporation As far asclusters is sublimation concerned, of theand difference neutral DCTB between melting the matrix points terthiophene molecules is from too weak the to be of any relevance. radical cations following the reaction depictedoriginate in from Scheme the 3 exothermicity could of thebetween charge exchange ionized reaction DCTB molecule andthis neutral decomposition P3HT. The reaction extent could of thenwhen be expected using to decrease dithranolthe as lowest the IEvibrational (Table matrix degrees 3). of since However, freedomcluster dithranol given in environment, the presents the this huge P3HTbetween molecules numbers exothermicity all and of is the theand likely oscillators then to does and not be significantly alsoreaction. contribute shared On dissipated to the the in decomposition P3HT other the molecules hand, can vicinity lead direct toappearance such photon of decomposition chemical absorption and noise then by (Fig. toThis S2B, the the Supporting also Information). points toon the the importance target of plate. the matrix-to-analyte ratio reported for basic analytes upon MALDI analysis usingthematrix.Nevertheless,giventhehighprotonaffinityofdithranol, dithranol as PA Figure 4. no protonated P3HTMALDI molecules spectra. are observed in thethe recorded molecular weight for polythiophenesments upon is MALDI also measure- from reminded. the ionization This process, namely the phenomenon chargebetween exchange does reaction matrix radical not cations originate and neutralof polymers, the since P3HT the molecules IE decreasesrepeat units. with the increasing number of electronsystems,presenting lowprotonaffinities,cannotcompete withahighlyexothermicelectrontransferreaction.

244 245 1 − GPC n M 4700 g mol = n M . This also confirms that the 1 − wileyonlinelibrary.com/journal/jms ll scan spectrum of the polymer sample after Soxhlet 1.04 were measured and those data are totally in = 4900 distribution was observed in the MALDI-ToF spectrum z / best MALDI data fordilution the of analysis the of polymer P3HT inintermolecular samples the interactions are matrix between (1) molecules the a polymer to(2) high a molecules eliminate and the matrix molecule withwavelength and a presenting really a high high IE. absorptivity at the laser and a PDI agreement with the theoretically predictedunambiguously values confirms (Table the 1). problem This the of MALDI-ToF measurement analysis of for polythiophenelow PDI samples sample, because, the even hugeprocess efficiency for for of the low desorption/ionization massof the oligomers polydispersity can and leadis an to definitively underestimation an more of critical overestimation the for MWD. polymer This samples whose Conclusions In the present massstrated spectrometry that investigation, DCTB itfor was represents the demon- analysis hitherto of themolecules polythiophenes. are best really Since prone polythiophene MALDI to ionization macro- charge matrix by exchange MALDI one matrices electron such removal, asare DCTB and well terthiophene suited. The highestand efficiency of dithranol DCTB is over terthiophene explainedof this on molecule the at the basisof laser of wavelength. the Moreover, the the voluminous high presence likely tertiobutyl absorption to group limit on thetrix the molecules intermolecular and DCTB the interactions polythiophene molecule polymers betweenphase, in is rendering the the the desorption condensed and ma- sublimation steps easier.mass Low oligomers were shownfor to the gas-phase be ion production. systematically In the overestimated context ofular absolute weights molec- and MWDs, it isoligomers proposed to by get a rid of Soxhlet such extractionpolydispersity low samples. against mass heptane, even for low is between 4000 and 10 000 g mol extraction against heptanem for 4(Fig. days, 5(b)). Based a on the recorded perfectly spectrum, a symmetric 7000 2011 John Wiley & Sons, Ltd. c 1.1, Table 1). 1.3) was then = sample (Fig. 1), = In the present case, 3000 (factor of 1.5). [4] n M can be measured by MALDI- sample (PDI : (a) full scan spectrum of the polymer sample and (b) fu sample (PDI n M 7000 7000 15 000 , 237–246 Copyright 46 , 2011 is around 70% of the GPC 2500 is definitively not in agreement with the GPC n can be measured and that (2) upon GPC analysis, the z n M In the present study, even if the polymer samples / M MALDI-ToF spectra of P3HT m [4] reported that starting from polymer samples with high [4] . is overestimated by a factor of 1.2–2.3. When analyzing by MALDI-ToF the P3HT n MALDI-ToF analysis of polythiophene are characterized byMcCullough’s procedure to selectively low remove the low mass PDIs chains by step (Table 1), extractions. The we P3HT decided to use M extraction against heptane. J. Mass. Spectrom. sample. Indeed, evenpolydispersity for index, the aoligomers relative is polymer no proportion characterized longerthe of negligible by production to the a account of low forMALDI-ToF the low competition mass analysis gas-phase in of ions. the Figure P3HT 5(a) presents the characterized by athe low low mass PDI, oligomers (aroundcompetition the with 2500 the abundant u) high weak mass is polymer chains relative11 enough (around 000 to u) abundance allow for the the of ultimateAs preparation already of isolated described gas-phaseet ions. in al the Introduction Section, McCullough Figure 5. the MALDI the problem ofdesorption/ionization overestimation process did of not low appear.far The mass more situation problematic is oligomers when by measuring by by the MALDI-ToF a P3HT ToF. The solvent sequence acetone–heptane–DCM–chloroformwas shown to progressively dissolvemasses. oligomers from low to high polydispersities, the fractionation of theto polymer the samples use owing of Soxhletlow extractions PDI polymers against whose various absolute solvents affords submitted to consecutiveand Soxhlet heptane. extractions Figure 4 againstpolymers after presents acetone Soxhlet the extractionthe against MALDI-ToF low heptane. mass spectra distribution As progressively disappeared of expected, and,heptane after the the extraction (Fig. 4(b)),mass only region. noise Accordingly, is the observed lowwhen mass in distribution analyzing the is low by recovered This MALDI-ToF experiment also the confirms that heptaneabsolute (1) extract upon MALDI-ToF (Fig. analysis, 4(c)). data and the predicted molecular weight (Table 1). After Soxhlet This spectrum featuring an asymmetric singleclose ion to series with a BP , , , , , , . - J. π 10 131 , et al 1996 1999 2005 , ,51–64. 109/110 ,4thEd. t-transfer 1996 , 4949. , 237–246 66 Eur. J. Mass , , 2009 , 5967. 19 46 , , 110 , 1169. , 4324. Catalys , 1997 2011 Anal. Chim. Acta. 37 34 2005 J. De Winter 2011 , , , pp. 238. 2006 , 431. Anal. Chim. Acta ,1. 2004 2001 15 1993 , J. Am. Chem. Soc. 35 , Polym. Bull. Chem. Mat. 2004 A. V. Streletskii, O. V. Boltalina, sted laser desorption/ionization fined poly(3-hexylthiophene). er reactions in laser desorp- the polymer limit : experiment ed laser desorption/ionization: 2000 ozawa. Chain-growth polymeriza- J. Am. Chem. Soc. , 173. Appl. Surf. Sci. J. Phys. Chem. A J. Mass. Spectrom. Interpretation of Mass Spectra Rapid Commun. Mass Spectrom. 19 , Macromolecules Macromolecules , 17452. Rapid Commun. Mass Spectrom. 127 2007 , , 345. 12 J. Mass Spectrom. , 2005 Adv. Mat. 2006 ,1. J. Am. Soc. 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Zeng, F. Yan, S. K.-H. Wei, S. W. Culligan, S. H. Chen. Synthesis wileyonlinelibrary.com/journal/jms References (Materia Nova) and OPTI Acknowledgements The MS LaboratoryScientifique acknowledges (FRS-FNRS)’ the ‘Fonds forof de its the la contribution Waters Recherche to QToFJ.D.W. Premier are the Mass FNRS acquisition research Spectrometer.doctoral fellows. P.G., grant. P.G. F.B. O.C. and is and J.D.W.measurements. grateful CIRMAP thank acknowledges to Bastien financial support FRIA Cilla form forEuropean for the the his Commission MALDI and R Supporting information Supporting information may be found in thearticle. online version of this much grateful to thethe Belgian Interuniversity Federal Science Attraction Policy Office Pole (PAI 6/27) and Program FNRS-FRFC. of

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