Intramolecular Charge Resonance in Dimer Radical Anions of Di-, Tri-, Tetra-, and Pentaphenylalkanes

Intramolecular dimer radical anions of di-, tri-, tetra-, and pentaphenylalkanes were investigated on the basis of absorption spectral measurements during γ-radiolysis in 2-methyltetrahydrofuran (MTHF) glassy matrix at 77 K and theoretical calculations. The absorption spectrum of 1,1,2,2-tetraphenylethane (1,1,2,2-Ph₄E) radical anion showed two bands in the near-infrared (NIR) region (900–2600 nm). One band observed at shorter wavelength than 2000 nm is assigned to the intramolecular charge resonance (CR) band between two phenyl groups of the 1,1-diphenylmethyl chromophore (1,1-dimer radical anion). The intramolecular CR band of the 1,1-dimer radical anion was observed for various alkanes having 1,1-diphenylmethyl chromophore such as 1,1,1-triphenylmethane (1,1,1-Ph₃M), 1,1,1,1-tetraphenylmethane (1,1,1,1-Ph₄M), and so on. The other intramolecular CR band observed at longer wavelength than 2200 nm is assigned to intramolecular dimer radical anion between two phenyl groups of the 1,2-diphenylethyl chromophore (1,2-dimer radical anion). The intramolecular CR band of the 1,2-dimer radical anion was observed for various alkanes having a 1,2-diphenylethyl chromophore, such as 1,1,2-triphenylethane (1,1,2-Ph₃E), 1,1,2,2-Ph₄E, and 1,1,1,2,2-pentaphenylethane (1,1,1,2,2-Ph₅E) and so on. No dimer radical anion was observed for 1,<i>n</i>-diphenylalkanes (<i>n</i> > 2) without 1,1-diphenylmethyl chromophore. The relationship between the structure and negative charge delocalization over two phenyl groups connected by an sp³ carbon is discussed

Mesolysis of Radical Anions of Tetra‑, Penta‑, and Hexaphenylethanes

A central carbon–carbon (C–C) σ bond dissociation of polyphenylethane radical anions (Ph_<i>n</i>E^•‑, <i>n</i> = 3–6), mesolysis, was investigated by the transient absorption measurement during pulse radiolysis of Ph_<i>n</i>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_<i>n</i>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_<i>n</i>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_<i>n</i>E^•‑

Folding Dynamics of Cytochrome <i>c</i> Using Pulse Radiolysis

Pulse radiolysis is a powerful method to realize real-time observation of various redox processes, which induces various structural and functional changes occurring in biological systems. However, its application has been mainly limited to studies of the redox reactions of rather smaller biological systems such as DNA because of an undesired reaction due to various free radicals generated by pulse radiolysis. For application of pulse radiolysis to generate plenty of redox reactions of biological systems, selective redox reactions induced by electron pulses have to be developed. In this study, we report that in the presence of the high concentration of the denaturant, guanidine HCl (GdHCl), the selective reduction of the oxidized cytochrome <i>c</i> (Cyt <i>c</i>) takes place in time scales of a few microseconds by the electron transfer from the guanidine radical that is formed by the fast reaction of e_aq^– with GdHCl, consequently leading to folding kinetics of Cyt <i>c</i>. By providing insight into the folding dynamics of Cyt <i>c</i>, we show that the pulse radiolysis technique can be used to track the folding dynamics of various biomolecules in the presence of a denaturant including GdHCl

Mesolysis Mechanisms of Aromatic Thioether Radical Anions Studied by Pulse Radiolysis and DFT Calculations

The mesolysis mechanisms for eight aromatic thioether radical anions (ArCH₂SAr′^•–) generated during radiolysis in 2-methyltetrahydrofuran were studied by spectroscopic measurements and DFT calculation. Seven of ArCH₂SAr′^•– underwent mesolysis via dissociation of the σ-bond between the benzylic carbon and sulfur atoms, forming the corresponding radical and anion with the stepwise mechanism or concerted mechanism. Conversely, no mesolysis in the benzyl β-naphthyl sulfide radical anion was found. From the Arrhenius analysis of the mesolysis with the stepwise mechanism, apparent activation energies (Δ<i>E</i>_exp) were determined and compared with those (Δ<i>E</i>_cal) estimated by the DFT calculations. Two types of C–S bond dissociation are possible to give the C radical and S anion (ArCH₂^•/Ar′S^–) and the C anion and S radical (ArCH₂^–/Ar′S^•). The dissociation energies (BDE­(ArCH₂^•/Ar′S^–) and BDE­(ArCH₂^–/Ar′S^•)) were estimated by the DFT calculations, and BDE­(ArCH₂^•/Ar′S^–) were found to be smaller than BDE­(ArCH₂^–/Ar′S^•). The formation of ArCH₂^•/Ar′S^– was observed on the mesolysis of five ArCH₂SAr′^•–, while one ArCH₂SAr′^•– provided ArCH₂^–/Ar′S^•. Chemical properties governing the mesolysis mechanisms of ArCH₂SAr′^•– are discussed

Structural Study of Various Substituted Biphenyls and Their Radical Anions Based on Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis

The structures of various <i>para</i>-substituted biphenyls (Bp-X; X = −OH, −OCH₃, −CH₃, −H, −CONH₂, −COOH, and −CN) and their radical anions (Bp-X^•–) were investigated by time-resolved resonance Raman spectroscopy combined with pulse radiolysis. The inter-ring C1–C1′ stretching modes (ν₆) of Bp-X were observed at ∼1285 cm^–1, whereas the ν₆ modes of Bp-X^•– with an electron-donating or -withdrawing substituent were significantly up-shifted. The difference (Δ<i>f</i>) between the ν₆ frequencies of Bp-X and Bp-X^•– showed a significant dependence on the electron affinity of the substituent and exhibited a correlation with the Hammett substituent constants (σ_p). In contrast to Bp-H^•– with a planar geometry, the theoretical and experimental results reveal that all Bp-X^•– with an electron-donating or -withdrawing substituent have a slightly twisted structure. The twisted structure of Bp-X^•– is due to the localization of the unpaired electron and negative charge density on one phenyl moiety in Bp-X^•–

Intermolecular and Intramolecular Electron Transfer Processes from Excited Naphthalene Diimide Radical Anions

Excited radical ions are interesting reactive intermediates owing to powerful redox reactivities, which are applicable to various reactions. Although their reactivities have been examined for many years, their dynamics are not well-defined. In this study, we examined intermolecular and intramolecular electron transfer (ET) processes from excited radical anions of naphthalene-1,4,5,8-tetracarboxydiimide (NDI^•–*). Intermolecular ET processes between NDI^•–* and various electron acceptors were confirmed by transient absorption measurements during laser flash photolysis of NDI^•– generated by pulse radiolysis. Although three different imide compounds were employed as acceptors for NDI^•–*, the bimolecular ET rate constants were similar in each acceptor, indicating that ET is not the rate-determining step. Intramolecular ET processes were examined by applying femtosecond laser flash photolysis to two series of dyad compounds, where NDI was selectively reduced chemically. The distance dependence of the ET rate constants was described by a β value of 0.3 Å^–1, which is similar or slightly smaller than the reported values for donor–acceptor dyads with phenylene spacers. Furthermore, by applying the Marcus theory to the driving force dependence of the ET rate constants, the electronic coupling for the present ET processes was determined

Detection of Structural Changes upon One-Electron Oxidation and Reduction of Stilbene Derivatives by Time-Resolved Resonance Raman Spectroscopy during Pulse Radiolysis and Theoretical Calculations

Stilbene (St) derivatives have been investigated for many years because of their interesting photochemical reactions such as cis–trans isomerization in the excited states and charged states and their relation to poly­(<i>p</i>-phenylenevinylene)­s. To clarify their charged state properties, structural information is indispensable. In the present study, radical cations and radical anions of St derivatives were investigated by radiation chemical methods. Absorption spectra of radical ion states were obtained by transient absorption measurements during pulse radiolysis; theoretical calculations that included the solvent effect afforded reasonable assignments. The variation in the peak position was explained by using HOMO and LUMO energy levels. Structural changes upon one-electron oxidation and reduction were detected by time-resolved resonance Raman measurements during pulse radiolysis. Significant downshifts were observed with the CC stretching mode of the ethylenic groups, indicative of the decrease in the bonding order. It was confirmed that the downshifts observed with reduction were larger than those with oxidation. On the other hand, the downshift caused by oxidation depends significantly on the electron-donating or electron-withdrawing nature of the substituents

Proton Transfer of Guanine Radical Cations Studied by Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis

The oxidation of guanine (G) is studied by using transient absorption and time-resolved resonance Raman spectroscopies combined with pulse radiolysis. The transient absorption spectral change demonstrates that the neutral radical of G (G^•(−H⁺)), generated by the deprotonation of G radical cation (G^•+), is rapidly converted to other G radical species. The formation of this species shows the pH dependence, suggesting that it is the G radical cation (G^•+)′ formed from the protonation at the N7 of G^•(−H⁺). On one hand, most Raman bands of (G^•+)′ are up-shifted relative to those of G, indicating the increase in the bonding order of pyrimidine (Pyr) and imidazole rings. The (G^•+)′ exhibits the characteristic CO stretching mode at ∼1266 cm^–1 corresponding to a C–O single bond, indicating that the unpaired electron in (G^•+)′ is localized on the oxygen of the Pyr ring

Radical Ions of Cycloparaphenylenes: Size Dependence Contrary to the Neutral Molecules

Cycloparaphenylenes (CPPs) have attracted wide attention because of their interesting properties owing to distorted and strained aromatic systems and radially oriented p orbitals. For application of CPPs, information on their charged states (radical cation and radical anion) is essential. Here, we measured absorption spectra of the radical cations and the radical anions of CPPs with various ring sizes over a wide spectral region by means of radiation chemical methods. The peak position of the near-IR bands for both the radical cation and the radical anion shifted to lower energies with an increase in the ring size. This trend is contrary to what is observed for transitions between the HOMO and LUMO of the neutral CPP. The observed spectra of the CPP radical ions were reasonably assigned based on time-dependent density functional theory. These results indicate that the next HOMO and the next LUMO levels are important in the electronic transitions of radical ions

Radical Ions of Cyclopyrenylene: Comparison of Spectral Properties with Cycloparaphenylene

Hoop-shaped π-conjugated molecules have attracted much attention. In this study, the radical ions of [4]­cyclo-2,7-pyrenylene ([4]­CPY), a cyclic tetramer of pyrene, and [4]­cyclo-4,5,9,10-tetrahydro-2,7-pyrenylene ([4]­CHPY) were investigated using radiation chemical methods, namely, γ-ray radiolysis and pulse radiolysis. The absorption spectra of the radical ions of [4]­CPY and [4]­CHPY showed clear peaks in the near-IR and UV–vis regions similar to those of [8]­cycloparaphenylene ([8]­CPP). Theoretical calculations using time-dependent density functional theory provided reasonable assignments of the observed absorption bands. It was indicated that the C4–C5 and C9–C10 ethylene bonds of [4]­CHPY do not contribute to the electronic transitions, resulting in absorption spectra similar to those of [8]­CPP. On the other hand, it was confirmed that the allowed electronic transitions of the radical ions of [4]­CPY are different from those of the radical ions of [4]­CHPY and [8]­CPP