Fragmentation of the POPOP molecule by electron-impact ionization

Abstract

Single and dissociative ionizations of the POPOP molecule by electron impact in the gas phase are studied using mass spectroscopy. Fragmentation pathways of the molecule are proposed taking into account the common system of conjugated π-electrons and heteroatoms in the POPOP molecule. The appearance thresholds for certain fragments of the molecule are determined based on experimentally measured ionization cross sections as functions of the ionizing electron energy. An ion with m/z = 144 [C 9 H 6 ON] + that is complementary to a fragment with m/z = 220 [C 13 H 10 ON] + (present in the NIST mass spectrum database) is found for the first time in the mass spectrum of POPOP. Its appearance threshold is determined (E ap = 9.48 eV). Introduction. Complicated organic compounds are widely used in modern high technologies, in particular, in various molecular electronics devices [1]. Molecules interact with electrons of various energies in such devices [2].One of the most important properties is the stability of the molecules upon interaction with electrons, which is determined mainly by the molecular structure and the electron energy. Therefore, the study of fragmentation pathways of molecules upon impact with electrons of various energies is crucial. The molecule 1,4-di(2,5-phenyloxazolyl)benzene (POPOP) is one of the most studied with respect to optical excitation and excitation by electron impact [3][4][5][6]. We began research in this area namely with this molecule. POPOP is widely used as an active component of laser media and liquid and plastic scintillators and in luminescent solar-energy concentrators [7]. Linear derivatives of oxazole and oxadiazole are commonly used as electron-transport and emitting layers of thin-film electroluminescent devices [8][9][10][11][12]. Fragmentation of the molecule upon interaction with electrons forms radical ions that quench luminescence and induces ion-molecule reactions that eventually cause these devices to degrade.Herein we study single and dissociative ionization of POPOP by electron impact in the gas phase using mass spectrometry, one of the most direct methods for determining fragmentation pathways of molecules.Experimental. The experiment was performed in a system with an oil-free pump. The analytical instrument was an MKh-7304A monopole mass spectrometer, a dynamic instrument that is essentially a mass band filter. Ions passing through the mass filter are detected and recorded by the measurement system containing a digital indicator of mass number and intensity, manual operation, cyclic, programmed scanning of mass spectra and bombarding electron energy. The experiment has been described in detail [13]. Therefore, we will concentrate on the principal methodical aspects of solving the problem.A beam of POPOP molecules was created by a multi-channel effusion source that provided a concentration of molecules in the region of interaction with the electron beam of 10 10 -10 11 cm -3 . The ion source with electron bombardment operated with electron-current stabilization and cou...

Dissociative ionization of 1,4-bis(2,5-phenyloxazolyl)benzene

“…The energy dependence of the POPOP molecule single ionization cross-section is measured in the energy range 6-60 eV, from the threshold part of which the first ionization potential for the molecule was determined as E i = 8.74 eV; from the initial parts of the dissociative ionization cross-section plots the appearance threshold energies E ap for [C 6 A characteristic feature of the temperature dependence is the intensity increase with temperature; however, no noticeable increase of the fragmentation depth with temperature is observed that is related to the specific structural features of organic compounds with conjugated bonds.…”

“…The mass spectrum of the POPOP molecule is characterized by high selectivity and low fragmentation depth [6]. Fig.…”

“…A generally established approach describes dissociative ionization as a process occurring at the bond opening or recombination of minimal number of bonds. Based on this principle, we propose the description of the POPOP molecule fragmentation by electron impact [6].…”

“…The presence of heteroatoms, resulting in the variation of electronic density in the oxazole rings, in spite of the -electron conjugation, is the decisive factor in the fragmentation of the molecule under study. NO] + ion appearing after opening the single bond between carbon atom C 5 (C 5 ) in the oxazole ring [6] and adjacent phenyl ring. In this case the groups C 2 ON, C 2 HON and C 2 H 2 ON are eliminated from the oxazole ring.…”

“…6 shows the temperature dependences of the daughter ion formation measured at ionization energy E i = 40 eV. [C 6 H 5 ] + ion with m/z = 77 is formed from lateral phenyl groups of the initial molecule; [C 7 H 5 O] + ion with m/z = 105 is formed at the opening of C 2 -C 3 and C 5 -O bonds of the oxazole rings of the POPOP molecule; [C 13 H 8 ] + ion with m/z = 164 is formed due to secondary fragmentation of the [C 15 H 10 ON] + ion, C 2 ON, C 2 HON and C 2 H 2 ON groups being eliminated from the oxazole ring; [C 22 H 15 NO] + ion with m/z = 309 is formed when the molecular ion loses a [C 2 HON] + fragment (m = 55) which is formed due to the opening of the C 2 -C 3 and C 2 -O bonds of the oxazole rings of the POPOP molecule and recombination of the C 2 -C 5 bond [6], as has been mentioned above. A characteristic feature of the curves presented in Fig.…”