Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices

Formation of intramolecular dimer radical ions of diphenyl sulfones

Dimer radical ions of aromatic molecules in which excess charge is localized in a pair of rings have been extensively investigated. While dimer radical cations of aromatics have been previously produced in the condensed phase, the number of molecules that form dimer anions is very limited. In this study, we report the formation of intramolecular dimer radical ions (cations and anions) of diphenyl sulfone derivatives (DPs) by electron beam pulse radiolysis in the liquid phase at room temperature. The density functional theory (DFT) calculations also showed the formation of the dimer radical ions. The torsion barrier of the phenyl ring of DPs was also calculated. It was found that the dimer radical ions show the larger barrier than the neutral state. Finally, stability of the dimer radical anion is dependent on not only the inductive effect of the sulfonyl group but the conjugation involving the d-orbital of the S atom and the phenyl rings.

Photoexcited state chemistry of metal-oxygen complexes

Recent advances on the excited state chemistry of metal-oxygen synthetic complexes based on earth-abundant metals such as copper, cobalt, and manganese are reviewed to show a much enhanced reactivity of the photoexcited states as compared with their relative ground states. Mononuclear copper(ii)-superoxide and dinuclear copper(ii)-peroxo complexes underwent copper-oxygen bond cleavage, dioxygen release, and copper(i)/dioxygen rebinding upon photoexcitation at low temperature. Photoirradiation of the cobalt-oxygen compound [(TAML)CoIV(O)]2- (6) (TAML = tetraamidomacrocyclic ligand) at 5 °C yielded a cobalt-oxygen excited state with 0.6(1) ns lifetime, showing a high reactivity in the bimolecular electron-transfer oxidations of m-xylene and anisole. An extremely long-lived excited state was generated upon photoexcitation of a manganese(iv)-oxo complex binding two Sc(OTf)3 molecules, which enabled the hydroxylation of benzene.

Enhanced Electron-Transfer Reactivity of a Long-Lived Photoexcited State of a Cobalt-Oxygen Complex

Photodynamics and electron-transfer reactivity of an excited state derived from an earth-abundant mononuclear cobalt-oxygen complex ground state, [(TAML)Co^IV(O)]^2- (1; H₄TAML = 3,4,8,9-tetrahydro-3,3,6,6,9,9-hexamethyl-1 H-1,4,8,11-benzotetraazo-cyclotridecane-2,5,7,10-(6 H, 11 H)tetrone), prepared by electron-transfer oxidation of Li[(TAML)Co^III]·3(H₂O) (2) in a 1:1 acetonitrile/acetone solvent mixture at 5 °C, were investigated using a combination of femtosecond and nanosecond laser absorption spectroscopy. Visible light photoexcitation of 1 (λ_exc = 393 nm) resulted in generation of the excited state S₂* (lifetime: 1.4(4) ps), detected 2 ps after laser irradiation by femtosecond laser spectroscopy. The initially formed excited state S₂* converted to a lower-lying excited state, S₁* (λ_max = 580 nm), with rate constant k_c = 7(2) × 10¹¹ s^-1 (S₂* → S₁*). S₁* exhibited a 0.6(1) ns lifetime and converted to the initial ground state 1 with rate constant k_d = 1.7(3) × 10⁹ s^-1 (S₁* → 1). The same excited state dynamics was observed when 1 was generated by electron-transfer oxidation of 2 using different one-electron oxidants such as Cu(OTf)₂ (OTf^- = triflate anion), [Fe(bpy)₃]³⁺ (bpy = 2,2'-bipyridine), and tris(4-bromophenyl)ammoniumyl radical cation (TBPA^•+). The electron-transfer reactivity of S₁* was probed by nanosecond laser photoexcitation of 1 in the presence of a series of electron donors with different one-electron oxidation potentials ( E_ox vs SCE): benzene (2.35 V), toluene (2.20 V), m-xylene (2.02 V), and anisole (1.67 V). The excited state S₁* engaged in electron-transfer reactions with m-xylene and anisole to generate π-dimer radical cations of m-xylene and anisole, respectively, observed by nanosecond laser transient absorption spectroscopy, whereas no reactivity was observed toward benzene and toluene. Such differential electron-transfer reactivity depending on the E_ox values of electron donors allowed the estimation of the one-electron reduction potential of S₁* ( E_red*) as 2.1(1) V vs SCE, which is much higher than that of the ground state ( E_red = 0.86 V vs SCE).

Dynamics of Radical Ions of Hydroxyhexafluoroisopropyl-Substituted Benzenes

Fluorination of resist materials is an effective method used to enhance the energy deposition of extreme ultraviolet (EUV) light in the fabrication of next-generation semiconductor devices. The dynamics of radical ions are important to understand when considering the radiation-chemistry of the resist materials using EUV and electron beam lithography. Here, the dynamics of the radical anions and cations of benzenes with one or two 2-hydroxyhexafluoroisopropyl groups (HFABs) were studied using radiolysis techniques. The formation of dimer radical cations was observed only in the monosubstituted benzene solutions of 1,2-dichloroethane. If the compound contained more than two substituents, it was found to hinder the necessary π-π overlapping. Pulse radiolysis of HFABs in tetrahydrofuran showed a characteristic spectral shift of the radical anion within the region of several hundred nanoseconds. From the results of low-temperature spectroscopy and density functional calculations, it is suggested that excess electrons of the 2-hydroxyhexafluoroisopropyl group of the radical anions cause dissociation into neutral radicals.

Single-Crystal X-ray Structures of conductive π-Stacking Dimers of Tetrakis(alkylthio)benzene Radical Cations

Salts containing radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio) benzene (TEB) have been successfully synthesized with . These newly synthesized salts have been characterized by UV-Vis absorption, EPR spectroscopy, conductivity measurement, single crystal X-ray diffraction analysis as well as DFT calculation. This study raises the first crystal structure of conductive π-stacking radical cation with single phenyl ring and reveals their conductivity has relationship with the stack structure which affected by the substituent.

Two-phase oxidation of toluene derivatives by dioxygen using the 3-cyano-1-decylquinolinium ion as a photocatalyst

The two-phase photocatalytic oxidation of toluene by O₂ occurred efficiently using the 3-cyano-1-decylquinolinium ion in toluene with H₂O to produce the oxygenated products in the organic phase and H₂O₂ in the aqueous phase.

Organic synthetic transformations using organic dyes as photoredox catalysts

The oxidizing ability of organic dyes is enhanced significantly by photoexcitation. Radical cations of weak electron donors can be produced by electron transfer from the donors to the excited states of organic dyes. The radical cations thus produced undergo bond formation reactions with various nucleophiles. For example, the direct oxygenation of benzene to phenol was made possible under visible-light irradiation of 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and water via electron transfer from benzene to the triplet excited state of DDQ. 3-Cyano-1-methylquinolinium ion (QuCN(+)) can also act as an efficient photocatalyst for the selective oxygenation of benzene to phenol using oxygen and water under homogeneous and ambient conditions. Alkoxybenzenes were also obtained when water was replaced by alcohol under otherwise identical experimental conditions. QuCN(+) can also be an effective photocatalyst for the fluorination of benzene with O2 and fluoride anion. Photocatalytic selective oxygenation of aromatic compounds was achieved using an electron donor-acceptor-linked dyad, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), as a photocatalyst and O2 as the oxidant under visible-light irradiation. The electron-transfer state of Acr(+)-Mes produced upon photoexcitation can oxidize and reduce substrates and dioxygen, respectively, leading to the selective oxygenation and halogenation of substrates. Acr(+)-Mes has been utilized as an efficient organic photoredox catalyst for many other synthetic transformations.

Methane from benzene in argon dielectric barrier discharge

A first-time account of direct, on-line, instantaneous and efficient chemical conversion of gas phase benzene to methane in argon Dielectric Barrier Discharge (DBD) is presented. In the absence of another overt hydrogen-donating source, potency of analogous parents toward methane generation is found to follow the order: benzene>toluene>p-xylene. Simultaneous production of trace amounts of phenolic surface deposits suggest (a) prompt decomposition of the parent molecules, including a large fraction yielding atomic transients (H-atom), (b) continuous and appropriate recombination of such parts, and (c) trace moisture in parent contributing OH radicals and additional H-atoms, which suitably react with the unreacted fraction of the parent, and also other intermediates. Results highlight Ar DBD to be a simple and exploitable technology for transforming undesirable hazardous aromatics to usable/useful low molecular weight open-chain products following the principles of green chemistry and engineering.

Mesolysis of radical anions of tetra-, penta-, and hexaphenylethanes

A central carbon–carbon (C–C) σ bond dissociation of polyphenylethane radical anions (Ph(n)E•-, n = 3–6), mesolysis, was investigated by the transient absorption measurement during pulse radiolysis of Ph(n)E in 2-methyltetrahydrofuran. The charge resonance (CR) band of 1,1,2,2-tetraphenylethane radical anion (1,1,2,2-Ph4E•-) was observed in the near-infrared region immediately after an electron pulse to be attributed to the intramolecular dimer radical anion. The CR band disappeared with simultaneous formation of two absorption bands at 330 and 460 nm corresponding to diphenylmethyl radical and diphenylmethyl anion, respectively, indicating the occurrence of the mesolysis in 1,1,2,2-Ph4E•-. During pulse radiolysis of 1,1,1,2,2,2-hexaphenylethane (Ph6E), an absorption band of triphenylmethyl radical was observed at 340 nm immediately after an electron pulse. It is suggested that one electron attachment to Ph6E is followed by the subsequent rapid C–C σ bond dissociation. Formation of intramolecular dimer radical anions in Ph(n)E•- such as 1,1,2-triphenylethane (Ph3E), 1,1,1,2-tetraphenylethane (1,1,1,2-Ph4E), and 1,1,1,2,2-pentaphenylethane (Ph5E) was also studied together with the subsequent mesolysis. The mesolysis of Ph(n)E•- is discussed in terms of charge delocalization in the intramolecular dimer radical anions and the central C–C σ bond as well as bond dissociation energy of the central C–C σ bond of Ph(n)E•-.

Formation of intramolecular poly(4-hydroxystyrene) dimer radical cation

Poly(4-hydroxystyrene) (PHS) has been used in lithography as a backbone polymer and is also a promising material for extreme-ultraviolet or electron beam lithography. The dynamics of PHS radical cations generated upon exposure to electron beam were investigated. The transient absorption of PHS was observed in the near-infrared region in p-dioxane solutions by pulse radiolysis. Charge resonance (CR) bands that represent pi-pi interaction between the two chromophores of the intramolecular PHS dimer radical cation were observed, whereas p-cresol shows no distinct CR band. Although the radical cations of phenol derivatives are known to be easily deprotonated, it was found that the dimer radical cation formation leads to less deprotonation by its charge resonance stabilization.

Effect of base stacking on the acid-base properties of the adenine cation radical [A*+] in solution: ESR and DFT studies

In this study, the acid-base properties of the adenine cation radical are investigated by means of experiment and theory. Adenine cation radical (A*(+)) is produced by one-electron oxidation of dAdo and of the stacked DNA-oligomer (dA)6 by Cl2*(-) in aqueous glass (7.5 M LiCl in H2O and in D2O) and investigated by ESR spectroscopy. Theoretical calculations and deuterium substitution at C8-H and N6-H in dAdo aid in our assignments of structure. We find the pKa value of A*(+) in this system to be ca. 8 at 150 K in seeming contradiction to the accepted value of < or = 1 at ambient temperature. However, upon thermal annealing to > or = 160 K, complete deprotonation of A*(+) occurs in dAdo in these glassy systems even at pH ca. 3. A*(+) found in (dA)6 at 150 K also deprotonates on thermal annealing. The stability of A*(+) at 150 K in these systems is attributed to charge delocalization between stacked bases. Theoretical calculations at various levels (DFT B3LYP/6-31G*, MPWB95, and HF-MP2) predict binding energies for the adenine stacked dimer cation radical of 12 to 16 kcal/mol. Further DFT B3LYP/6-31G* calculations predict that, in aqueous solution, monomeric A*(+) should deprotonate spontaneously (a predicted pKa of ca. -0.3 for A*(+)). However, the charge resonance stabilized dimer AA*(+) is predicted to result in a significant barrier to deprotonation and a calculated pKa of ca. 7 for the AA*(+) dimer which is 7 pH units higher than the monomer. These theoretical and experimental results suggest that A*(+) isolated in solution and A*(+) in adenine stacks have highly differing acid-base properties resulting from the stabilization induced by hole delocalization within adenine stacks.