Bongjin Moon

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CHx (x = 1, 2, 3) vibration region from 2800 to 3200 cm-1 was very different between [BMIM][BF4] and [BMIM][I] even though all the spectral features in this region were from the butyl chain and the imidazolium ring of the same cation. The spectrum did not change appreciably irrespective of the water concentration for [BMIM][BF4], whereas the spectrum from [BMIM][I] showed significant changes as the water concentration was increased, especially in CH-vibration modes from the imidazolium ring. For very diluted solutions both aqueous mixtures of [BMIM][I] and [BMIM][BF4] showed very similar spectra. Mixing of [BMIM][I] with heavy water (D2O) facilitated the isotopic exchange of the proton attached to the most acidic carbon of the imidazolium ring into deuterium from D2O, whereas even prolonged exposure to D2O did not induce any isotopic exchange for [BMIM][BF4]. Raman spectra around 600 cm(-1) indicative of the butyl chain conformation also changed differently as the water concentration was increased between [BMIM][I] and [BMIM][BF4]. These differences are considered to come from the variation in the position of the anion, where I- is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF(4-).

A facile route to ketene-functionalized polymers for general materials applications

Function matters in materials science, and methodologies that provide paths to multiple functionality in a single step are to be prized. Therefore, we introduce a robust and efficient strategy for exploiting the versatile reactivity of ketenes in polymer chemistry. New monomers for both radical and ring-opening metathesis polymerization have been developed, which take advantage of Meldrums acid as both a synthetic building block and a thermolytic precursor to dialkyl ketenes. The ketene-functionalized polymers are directly detected by their characteristic infrared absorption and are found to be stable under ambient conditions. The inherent ability of ketenes to provide crosslinking via dimerization and to act as reactive chemical handles via addition, provides simple methodology for application in complex materials challenges. Such versatile characteristics are illustrated by covalently attaching and patterning a dye through microcontact printing. The strategy highlights the significant opportunities afforded by the traditionally neglected ketene functional group in polymer chemistry.

Ketene Functionalized Polyethylene: Control of Cross-Link Density and Material Properties

The functionalization and cross-linking of polyethylene is synthetically challenging, commonly relying on highly optimized radical based postpolymerization strategies. To address these difficulties, a norbornene monomer containing Meldrums acid is shown to be effectively copolymerized with polyethylene using a nickel α-iminocarbaxamidato complex, providing high-melting, semicrystalline polymers with a tunable incorporation of the functional comonomer. Upon heating the copolymer to common polyethylene processing temperatures, the thermolysis of Meldrums acid to ketene provides the desired reactive group. This simple and versatile methodology does not require small molecule radical sources or catalysts, and the dimerization of the in situ generated ketenes is shown to provide tunable cross-linking densities in polyethylene. Subsequent rheological and tensile experiments illustrate the ability to tune cross-linked polyethylene properties by comonomer incorporation and elucidate valuable structure/property relationships in these materials. This study illustrates the power of well-defined and synthetically accessible functional groups in polyolefin synthesis and functionalization.

Disulfide bond cleavage in TEMPO-free radical initiated peptide sequencing mass spectrometry

The gas-phase free radical initiated peptide sequencing (FRIPS) fragmentation behavior of o-TEMPO-Bz-conjugated peptides with an intra- and intermolecular disulfide bond was investigated using MS(n) tandem mass spectrometry experiments. Investigated peptides included four peptides with an intramolecular cyclic disulfide bond, Bactenecin (RLCRIVVIRVCR), TGF-α (CHSGYVGVRC), MCH (DFDMLRCMLGRVFRPCWQY) and Adrenomedullin (16-31) (CRFGTCTVQKLAHQIY), and two peptides with an intermolecular disulfide bond. Collisional activation of the benzyl radical conjugated peptide cation, which was generated through the release of a TEMPO radical from o-TEMPO-Bz-conjugated peptides upon initial collisional activation, produced a large number of peptide backbone fragments in which the S-S or C-S bond was readily cleaved. The observed peptide backbone fragments included a-, c-, x- or z-types, which indicates that the radical-driven peptide fragmentation mechanism plays an important role in TEMPO-FRIPS mass spectrometry. FRIPS application of the linearly linked disulfide peptides further showed that the S-S or C-S bond was selectively and preferentially cleaved, followed by peptide backbone dissociations. In the FRIPS mass spectra, the loss of •SH or •SSH was also abundantly found. On the basis of these findings, FRIPS fragmentation pathways for peptides with a disulfide bond are proposed. For the cleavage of the S-S bond, the abstraction of a hydrogen atom at C(β) by the benzyl radical is proposed to be the initial radical abstraction/transfer reaction. On the other hand, H-abstraction at C(α) is suggested to lead to C-S bond cleavage, which yields [ion ± S] fragments or the loss of •SH or •SSH.

Radical-driven peptide backbone dissociation tandem mass spectrometry.

In recent years, a number of novel tandem mass spectrometry approaches utilizing radical-driven peptide gas-phase fragmentation chemistry have been developed. These approaches show a peptide fragmentation pattern quite different from that of collision-induced dissociation (CID). The peptide fragmentation features of these approaches share some in common with electron capture dissociation (ECD) or electron transfer dissociation (ETD) without the use of sophisticated equipment such as a Fourier-transform mass spectrometer. For example, Siu and coworkers showed that CID of transition metal (ligand)-peptide ternary complexes led to the formation of peptide radical ions through dissociative electron transfer (Chu et al., 2000. J Phys Chem B 104:3393-3397). The subsequent collisional activation of the generated radical ions resulted in a number of characteristic product ions, including a, c, x, z-type fragments and notable side-chain losses. Another example is the free radical initiated peptide sequencing (FRIPS) approach, in which Porter et al. and Beauchamp et al. independently introduced a free radical initiator to the primary amine group of the lysine side chain or N-terminus of peptides (Masterson et al., 2004. J Am Chem Soc 126:720-721; Hodyss et al., 2005 J Am Chem Soc 127: 12436-12437). Photodetachment of gaseous multiply charged peptide anions (Joly et al., 2008. J Am Chem Soc 130:13832-13833) and UV photodissociation of photolabile radical precursors including a C-I bond (Ly & Julian, 2008. J Am Chem Soc 130:351-358; Ly & Julian, 2009. J Am Soc Mass Spectrom 20:1148-1158) also provide another route to generate radical ions. In this review, we provide a brief summary of recent results obtained through the radical-driven peptide backbone dissociation tandem mass spectrometry approach.

Electrochemistry and electrochromism of a poly(cyclopentadithiophene) derivative with a viologen pendant

N-Methyl-N′-(6-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-4-ylhexyl)-4,4′-bipyridinium dihexafluorophosphate (CPDT-V2+-Me) was synthesized. The monomer was electropolymerized on a glassy carbon or an ITO electrode in a potentiodynamic mode to form the corresponding polymer P(CPDT-V2+-Me) on the electrodes. During the electropolymerization, two redox peaks of the viologen (V) moiety increased up to several cycles and then decreased while the redox peak of P(CPDT) moiety still increased. Especially, a new oxidative peak a in the range of ca. −0.4 and 0 V versus Ag/Ag+ appeared and increased up to several cycles. Peak a almost disappeared after the redox peaks of the viologen moiety almost disappeared. As a result of cyclic voltammetric study, it was shown that peak a originated from the oxidation of reduced viologen moiety via P(CPDT)-mediated electron transfer mechanism. We also found that the electroactivity of viologen moiety in P(CPDT-V2+-Me) decreased significantly when the potential was scanned to the second viologen redox (V0/V+). In practical applications, the polymer can be used in the potential range from the first viologen redox to P(CPDT) redox. The polymer turned into highly transparent P(CPDTα+-V2+-Me), blue P(CPDT-V2+-Me), dark violet P(CPDT-V+-Me), and violet P(CPDT-V0-Me) approximately at 0.8, −0.4, −0.8, and −1.7 V versus Ag/Ag+, respectively.

Low-temperature ketene formation in materials chemistry through molecular engineering

The thermolysis of Meldrums acid derivatives has emerged as a powerful methodology to generate ketenes in polymeric structures, but the required high temperatures for ketene formation may reduce its broad applicability. We take a molecular approach toward addressing this limitation by engineering Meldrums acid derivatives to undergo thermolysis at significantly lower temperatures. Two distinct strategies are presented and a thorough understanding of the molecular interactions governing their reactivity is provided through model compound design and synthesis, crystal structure analysis, and computation of transition structures. The generality of these molecular design principles allows for the generation of ketenes under mild thermal conditions, providing significant opportunities as a comprehensive and wide-ranging tool for controlling reactivity in both chemical and materials science applications.

One-Step Peptide Backbone Dissociations in Negative-Ion Free Radical Initiated Peptide Sequencing Mass Spectrometry

Peptide dissociation behavior in TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-based FRIPS (free radical initiated peptide sequencing) mass spectrometry was analyzed in both positive- and negative-ion modes for a number of peptides including angiotensin II, kinetensin, glycoprotein IIb fragment (296-306), des-Pro(2)-bradykinin, and ubiquitin tryptic fragment (43-48). In the positive mode, the ·Bz-C(O)-peptide radical species was produced exclusively at the initial collisional activation of o-TEMPO-Bz-C(O)-peptides, and two consecutive applications of collisional activation were needed to observe peptide backbone fragments. In contrast, in the negative-ion mode, a single application of collisional activation to o-TEMPO-Bz-C(O)-peptides produced extensive peptide backbone fragmentations as well as ·Bz-C(O)-peptide radical species. This result indicates that the duty cycle in the TEMPO-based FRIPS mass spectrometry can be reduced by one-half in the negative-ion mode. In addition, the fragment ions observed in the negative-ion experiments were mainly of the a-, c-, x-, and z-types, indicating that radical-driven tandem mass spectrometry was mainly responsible for the TEMPO-based FRIPS even with a single application of collisional activation. Furthermore, the survival fraction analysis of o-TEMPO-Bz-C(O)-peptides was made as a function of the applied normalized collision energy (NCE). This helped us to better understand the differences in FRIPS behavior between the positive- and negative-ion modes in terms of dissociation energetics. The duty-cycle improvement made in the present study provides a cornerstone for future research aiming to achieve a single-step FRIPS in the positive-ion mode.

Anionic synthesis and detection of fluorescence-labeled polymers with a terminal anhydride group

To monitor polymer–polymer coupling reactions between two different monofunctional polymers in dilute polymer blends, fluorescence-labeled anhydride-functional polystyrene (PS) and poly(methyl methacrylate) (PMMA) were prepared by conventional anionic polymerization. Sequential trapping of lithiopolystyrene by 1-(2-anthryl)-1-phenylethylene (APE) and then di-t-butyl maleate (4) provided, after pyrolysis, anhydride-functional fluorescent PS. Fluorescent PMMA anhydride (8) was synthesized with sec-butyllithium/APE as an initiator for the anionic polymerization of methyl methacrylate, trapping by 4, and pyrolysis. These polymers could be reacted with amine-functional polymers by melt blending, and the reaction progress could be monitored by gel permeation chromatography coupled with fluorescence detection. This technique not only allows monitoring of the coupling reaction with high sensitivity (ca. 100 times more sensitive than refractive index detection) but also permits selective detection because unlabeled polymers are invisible to fluorescence detection. This highly sensitive and selective detection methodology was also used to monitor the coupling reaction of 8 with PS-NH2 at a thin-film interface, which was otherwise difficult to detect by conventional methods.

Synthesis of a perylenediimide-viologen dyad (PDI-2V) and its electrochromism in a layer-by-layer self-assembled multilayer film with PEDOT:PSS

A new electrochromic molecule, perylenediimide-viologen dyad (PDI-2V), was synthesized and used in the fabrication of layer-by-layer (LBL) self-assembled multilayer films with poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). Since PDI-2V is a tetracationic organic molecule with high molecular weight, it showed strong electrostatic adhesion to negatively charged PEDOT:PSS. When the electrochromic behaviour of the resulting LBL film was investigated with a single type electrochromic device, the PDI-2V showed a three-stage color change from red to blue to violet under the reduction potential range. A dual type electrochromic device was also fabricated with P3HT spin-coated ITO as a counter electrode. This device exhibited a sharp color change between deep red and dark blue when the applied potential was switched from +2.0 V to −2.0 V.