Dmitri V. Stass

Typical applications of MARY spectroscopy: Radical ions of substituted benzenes

The paper describes the underlying principles and discusses the most important advantages and limitations of the experimental technique of magnetically affected reaction yield spectroscopy as developed in the authors’ laboratory and guides the reader step by step through a typical experimental sequence using as example the problem of short-lived radical cations of a series of methyl-substituted benzenes in X-irradiated nonpolar solutions. For two of the eight target substances — benzene itself and mesithylene — the paper reports the first unequivocal observation of their radical cations in liquid alkane solution at room temperature and provides a lower estimate of about 10 ns for their relaxation times in low magnetic field.

C(sp2)‐Coupled Nitronyl Nitroxide and Iminonitroxide Diradicals

Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR1) and with two iminonitroxide (DR2) fragments linked through the C(sp2) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR1 and DR2 have singlet ground states. The singlet–triplet energy splitting (2J) is low (J/k=−7.4 for DR1 and −6.0 K for DR2), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac)2] with DR1 gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac)2}2(DR1)(H2O)] complex, sufficiently strong ferromagnetic interactions (J1/k=42.7 and J2/k=14.1 K) between two coordinating CuII ions and DR1 were revealed. In [{Cu(hfac)2}2(DR1)(H2O)][Cu(hfac)2(H2O)], the very strong and antiferromagnetic (J/k=−416.1 K) exchange interaction between one of the coordinating CuII ions and DR1 is caused by the very short equatorial CuO bond length (1.962 Å).

Substitution of a Fluorine Atom in Perfluorobenzonitrile by a Lithiated Nitronyl Nitroxide

A 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) lithium derivative was found to react with perfluorobenzonitrile (2) substituting its para-fluorine atom to form 2-(4-cyanotetrafluorophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (3), a new nitronyl nitroxide containing a multifunctional framework of strong electron-withdrawing nature. This result shows the possibility of obtaining multifunctional nitronyl nitroxides via the interaction of paramagnetic lithium derivatives as C-nucleophiles with polyfluoroarenes activated for nucleophilic substitution. The reaction regioselectivity is supported by the data of quantum-chemical calculations, which also show that the reaction follows a concerted pathway without formation of an intermediate. Reduction of nitronyl nitroxide 3 in system NaNO2-AcOH yielded corresponding iminonitroxide 4. Characterization of persistent radicals 3 and 4 obtained by the SNF synthetic strategy includes X-ray crystal structures, electron spin resonance data, and static magnetic-susceptibility measurements. X-ray diffraction analysis of both nitronyl nitroxide and iminonitroxide revealed a complete match of the parameters of their crystal lattices.

Specific MARY spectrum from radical anion of pentafluorobenzene

The paper presents the first resolved experimental magnetically affected reaction yield (MARY) spectrum for a system with nonequivalent nuclei, radical anion of pentafluorobenzene. This observation dispels the common apprehension that because of a rather involved energy level layout a system with not all nuclei magnetically equivalent cannot produce resolved MARY lines in nonzero fields, and greatly, increases the practical scope of level-crossing techniques for studies of spin-correlated radical pairs. The experimental finding is supported by schemes of energy levels calculated for this system.

Hyperfine structure of the MARY spectrum for the radical ion/biradical ion three-spin system in the region of J resonance

311 J -resonance line splitting in the MARY spectrum for the radical ion/biradical ion three-spin system has been predicted. It has been shown that the J -resonance line splits due to both energy level anticrossing and energy level crossing. For the latter case, it is sufficient to have magnetic nuclei only in the radical ion. Magnetically affected reaction yield (MARY) spectroscopy is an efficient method for studying radical ion reactions in liquid solutions under ionizing irradiation. The method is based on affecting the radical stage of the reaction with an external magnetic field, which is further detected using the magnetic field dependence of the intensity of recombination fluorescence. The main object of MARY spectroscopy studies are resonancelike features occurring at the magnetic field effect curve at fields where the energy levels of the radical ion pair cross (so-called MARY lines). The position and shape of the lines bear information on both the hyperfine structure of the radical ions present in the system and on the rates of processes in which they are involved [1]. One of the recent important extensions of MARY spectroscopy is the study of radical ion/biradical ion three-spin systems [2]. The most important trait of these systems is the presence of an exchange interaction J between the unpaired electrons in the biradical ion. A theoretical description of spin evolution taking into account the exchange interaction for a three-spin system

Estimation of the Fraction of Spin-Correlated Radical Ion Pairs in Irradiated Alkanes using Magnetosensitive Recombination Luminescence from Exciplexes Generated upon Recombination of a Probe Pair

Abstract We suggest a convenient probe exciplex system for studies in radiation spin chemistry based on a novel acceptor-substituted diphenylacetylene, 1-(phenylethynyl)-4-(trifluoromethyl)benzene that has a very short fluorescence lifetime (<200 ps) and low quantum yield (0.01) of intrinsic emission, provides efficient electron capture in alkanes and efficient exciplex formation upon recombination in pair with DMA radical cation, while exhibiting a shifted to red exciplex emission band as compared to the parent system DMA – diphenylacetylene. After chemical, luminescent, radiation and spin-chemical characterization of the new system we used the magnitude of magnetic field effect in its exciplex emission band for experimental estimation of the fraction of spin-correlated radical ion pairs under X-irradiation with upper energy cutoff 40 keV in a set of 11 alkanes. For linear and branched alkanes magnetic field effects and the corresponding fractions are approximately 19–20% and 0.28, while for cyclic alkanes they are lower at 16–17% and 0.22, respectively.

Molecular and Crystal Structure of 2-Amino-Polyfluorophenyl-4,4,5,5-Tetramethyl-4,5-Dihydro-1H-Imidazol- 3-Oxide-1-Oxyls

By cross-coupling of 2-iodo-polyfluoroanilines with a [AuPPh3NN] complex (where NN is 4,4,5,5- tetramethyl-3-oxide-1-oxyl-2-imidazolin-2-ide) in the presence of [Pd(PPh3)4], nitroxide radicals (2-aminopolyfluorophenyl- 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyls) are synthesized. Their molecular and crystal structures are determined by the X-ray crystallographic analysis. It is shown that in the solid phase there are both intra- and intermolecular hydrogen bonds between NH2 groups and O atoms of paramagnetic fragments, which link the molecules into chains or dimers.

X-ray Generated Recombination Exciplexes of Substituted Diphenylacetylenes with Tertiary Amines: A Versatile Experimental Vehicle for Targeted Creation of Deep-Blue Electroluminescent Systems

Customizable and technology-friendly functional materials are one of the mainstays of emerging organic electronics and optoelectronics. We show that recombination exciplexes of simple substituted diphenylacetylenes with tertiary amines can be a convenient source of tunable deep-blue emission with possible applications in organic electroluminescent systems. The optically inaccessible exciplexes were produced via recombination of radiation-generated radical ion pairs in alkane solution, which mimics charge transport and recombination in the active layer of practical organic light-emitting diodes in a simple solution-based experiment. Despite varying and rather poor intrinsic emission properties, diphenylacetylene and its prototypical methoxy (donor) or trifluoromethyl (acceptor) monosubstituted derivatives readily form recombination exciplexes with N,N-dimethylaniline and other tertiary amines that produce emission with maxima ranging from 385 to 435 nm. The position of emission band maximum linearly correlates with readily calculated gas-phase electron affinity of the corresponding diphenylacetylene, which can be used for fast computational prescreening of the candidate molecules, and various substituted diphenylacetylenes can be synthesized via relatively simple and universal cross-coupling reactions of Sonogashira and Castro. Together, the simple solution-based experiment, computationally cheap prescreening method, and universal synthetic strategy may open a very broad and chemically convenient class of compounds to obtain OLEDs and OLED-based multifunctional devices with tunable emission spectrum and high conversion efficiency that has yet not been seriously considered for these purposes.

Luminescent properties of new naphthylnitroxyl radicals

Method of time-resolved fluorescence was applied to study the luminescent properties of several 3-and 2-imidazoline derivatives of naphthalene. For 2-imidazoline derivatives in which the imidazoline fragment is directly bound to the naphthalene fragment, a weak (quantum yield φ < 0.001) luminescence of common π-system containing the radical group and the naphthalene core was recorded. 3-imidazoline derivatives, in which naphthalene and imidazoline fragments are separated by an ethylene bridge, demonstrate luminescence with the spectrum similar to that of free naphthalene. Quantum yield of luminescence and life-time of singlet excited state of 3-imidazoline derivatives are ca. 50 times less than for free naphthalene, because of intramolecular quenching of the luminophore luminescence by a stable radical.