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Malononitriledctbas Matrix Research Article Received: 12 July 2010 Accepted: 21 December 2010 Published online in Wiley Online Library: 2011 (wileyonlinelibrary.com) DOI 10.1002/jms.1886 MALDI-ToF analysis of polythiophene: use of trans-2-[3-(4-t-butyl-phenyl)-2-methyl- 2-propenylidene]malononitrile – DCTB – as matrix Julien De Winter,a,b Gaelle¨ Deshayes,b Florian Boon,b Olivier Coulembier,b Philippe Duboisb and Pascal Gerbauxa∗ Nowadays, numerous experimental and theoretical studies are devoted to the research field of polythiophenes and other electroconjugated polymers due to the huge potentialities of those conducting polymers. Synthetic procedures are now developed to reach the highest control over both polymerization and analytical methodologies allowing an in-depth and straightforward characterization of the polymer samples without any required doubt. Mass spectrometry methodologies and in particular MALDI-ToF measurements are definitively suitable to meet the characterization requirements. In the present study, trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) was shown to afford better results than the reported terthiophene and dithranol matrices as far as sensitivity and signal-to-noise ratio are concerned. We tentatively proposed that the ionization of the P3HT molecules is performed by charge exchange in the condensed phase (clusters) with matrix molecule radical cations and subsequent neutral matrix molecule evaporation from the clusters. The putative key parameters to account for the really high efficiency of DCTB for the MALDI analysis of P3HT are (1) the highest ionization energy of DCTB amongst the three matrices, (2) the really high absorptivity of the matrix molecule at the laser wavelength and (3) the presence of the tertiobutyl group on the matrix molecule. The presence of this substituent is likely to decrease the intermolecular interactions in the condensed phase rendering the evaporation of the neutral matrix molecules less energy demanding. We −1 also demonstrated for polymer samples presenting an average number molecular weight (Mn) below 10 000 g mol that the systematic overestimation of the low mass oligomers upon MALDI measurements ends up with wrong Mn and polydispersity index (PDI) values. A systematic Soxhlet extraction against heptane was shown to allow the recording of absolute Mn and PDI. Copyright c 2011 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article. Keywords: polythiophene; MALDI; DCTB; terthiophene; polymer Introduction makes a correlation between the polymer molecular weights and the hydrodynamic volume of randomly coiled polymer chains.[4] Electroconjugated polymers have been of huge interest for the Moreover, no information regarding the nature of the end-groups past two decades and numerous experimental as well as the- can be obtained following GPC analyses. On the other hand, oretical studies have been reported.[1] Soluble polymers able owing to the use of high-resolution MALDI-ToF measurements, to conduct electricity reveal huge potentialities in nonlinear accurate identification of the composition (including the nature optical devices, organic light-emitting diodes, transistors and of the end-groups) of the oligomers is made accessible.[5] As a nanoscopic electronic structures.[1] Polythiophenes, and partic- consequence, MALDI-ToF methodology is nowadays extensively ularly poly(3-alkylthiophene), represent an important class of used for the analysis of polythiophene samples and the interest conducting polymers due to their excellent solubility, stability of charge exchange matrices,[6] such as terthiophene, anthracene and processing ability.[2] Nowadays, control over the synthesis of poly(3-alkylthiophene) is readily achieved[3] and, for instance, the nature of the end-groups and the regioregularity of the ∗ Correspondence to: Pascal Gerbaux, Mass Spectrometry Research Group, [3] prepared polymers can be finely tuned. Nevertheless, the Centre Interdisciplinaire de Spectrometrie´ de Masse (CISMa), University of in-depth structural characterization of polythiophene samples Mons – UMons, 20 Place du Parc, 7000 Mons, Belgium. remains problematic for several reasons. First of all, the determi- E-mail: [email protected] nation of the molecular weights of polythiophene systems by gel a Mass Spectrometry Research Group, Centre Interdisciplinaire de Spectrometrie´ permeation chromatography (GPC) using polystyrene standards de Masse (CISMa), University of Mons – Umons, 20 Place du Parc, 7000 Mons, ◦ (e.g. in THF at 30 C) invariably ends up with an overestima- Belgium tion by a factor of 1.2–2.3 with respect of the molecular weight [4] b Laboratory of Polymeric and Composite Materials, Center of Innovation and range. This was explained by the rod-like conformation of poly- Research in Materials and Polymers (CIRMAP), University of Mons – UMons, 20 237 thiophene polymers in solution, whereas the GPC methodology Place du Parc, 7000 Mons, Belgium J. Mass. Spectrom. 2011, 46, 237–246 Copyright c 2011 John Wiley & Sons, Ltd. J. De Winter et al. Alk Alk we also would like to demonstrate the problematic huge + overestimationoflowmassoligomersuponMALDImeasurements. + MALDI + M + M S S Experimental M = matrix molecule Mass spectrometry measurements Scheme 1. Ionization of polythiophene molecule by electron transfer MALDI mass spectra were recorded using a Waters QToF Premier reaction under MALDI conditions. mass spectrometer equipped with a nitrogen laser, operating at 337 nm with a maximum output of 500 J m−2 delivered to the sample in 4 ns pulses at 20 Hz repeating rate. Time-of- OHOH OH NC flight mass analyses were performed in the reflectron mode at a H3C CN S resolution of about 10 000 at m/z 500. The matrices, terthiophene, S S tBu dithranol and DCTB, were prepared in chloroform (see text for the concentrations). The matrix solution (1 µl) was applied to a Dithranol terthiophene DCTB stainless steel target and air dried. Polymer samples were dissolved Scheme 2. Molecular structures of the three matrices tested in the present in chloroform (see also text for concentrations). One-microliter investigation. aliquots of these solutions were applied onto the target area already bearing the matrix crystals, and then air dried. For the recording of the single-stage MALDI-MS spectra, the quadrupole and dithranol, was convincingly demonstrated by McCullough (rf-only mode) was set to pass ions from 750 to 16 000 Th, and all et al.[4] Poly(3-hexylthiophene) molecules are proposed to be ions were transmitted into the pusher region of the time-of-flight ionized by electron transfer reactions with ions present in the analyzer where they were mass analyzed with 1 s integration time. MALDI process and radical cations of the matrix molecules were Data were acquired in continuum mode until acceptable averaged proposed as putative reactive ions (Scheme 1).[7] McCarley et al. data were obtained. The base peak (BP) intensities and the signal- reported the preferred use of terthiophene, since higher signal-to- to-noise (S/N) ratios were estimated as follows. For each MALDI noise ratio and resolution were routinely obtained in their MALDI mass spectrum, 100 consecutive scans were averaged to afford measurements.[7] the MALDI spectrum. The BP intensities were directly obtained by As already mentioned, the GPC molecular weight distribution considering the total ion count for the corresponding BP. As far (MWD) is invariably skewed toward higher molecular weights. as the S/N ratio is concerned, the intensities (ion count) of the At variance, upon MALDI-ToF analysis, for the same polymer background signals close to the BP (Fig. 1) were compared with sample, the MALDI average number molecular weight (Mn)is the BP intensity. displaced toward lower molecular weights.[4] The key-parameters that account for the intensity of MALDI signals are the mass UV–Vis spectroscopy measurements dependences (1) on the desorption process, (2) on the ionization reaction and (3) on the analyzer detection efficiency including Absorption spectra were recorded at room temperature in the ion transmission and the detector response.[8] For highly dichloromethane (DCM). UV–Vis absorption spectra were mon- disperse polymer samples, the huge mass difference between itored with a Shimadzu UV-mini1240 spectrophotometer. De- the low-mass and the high-mass oligomers renders the accurate terminations of the λmax in DCM were realized by scanning the measurement of the Mn impossible. Fractionation of the polymer absorption between 800 and 190 nm. Molar extinction coefficients upon Soxhlet extraction against various solvents was shown to in DCM (ε) were determined at 337 nm (wavelength of the MALDI afford polymer samples of narrow polydispersity whose MALDI- laser) using the Beer–Lambert law. ToF measurements give more accurate data as far as the absolute [4] MWD is concerned. Materials Recently, trans-2-[3-(4-t-butyl-phenyl)-2-methyl-2-propeny- lidene]malononitrile (DCTB) was proposed as the matrix of All the samples used in the present work were commercially choice for the analysis of very labile organic compounds available, except the poly(3-hexylthiophene)s prepared following such as substituted fullerenes.[9] Actually, DCTB (Scheme 2) the procedures described hereunder. was demonstrated to be a softer matrix than dithranol and dihydroxybenzoic acid (DHB), requiring less laser fluence to Chemicals ionize the analyte. In addition and in direct relation with the MALDI-MS analysis of polythiophenes, DCTB
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