The time scale of the quaternary structural changes in hemoglobin revealed using the transient grating technique

The quaternary structural transition between the R and T states of human hemoglobin was investigated using the transient grating technique. The results presented herein reveal that the quaternary structural change accompanied by the R-T transition occurs within a few microseconds. © the Owner Societies 20151331sciescopu

Inter-Ligand Energy Transfer Process in an Ir-Complex with Expanding π-Conjugated Ligand

© 2020 Wiley-VCH GmbH. The inter-ligand energy transfer (ILET) process in heteroleptic iridium complex, [Ir(dfppy)2(bpy-Im2)]+, where dfppy=2-(2,4-difluorophenyl)pyridine and bpy-Im2=4,4’-bis(1,2-diphenyl-1H-benzo[d]imidazole)-2,2’,-bipyridine, was investigated using a femtosecond transient absorption (fs-TA) spectroscopic technique. The photophysical properties of [Ir(dfppy)2(bpy-Im2)]+ with significantly expanding π-conjugated ligand are compared to those of [Ir(dfppy)2(bpy)]+ (bpy=2,2’-bipyridine) and a free bpy-Im2 ligand. The emission spectrum of [Ir(dfppy)2(bpy-Im2)]+ shows no shift upon changing the solvent polarity, whereas the free ligand bpy-Im2 showed bathochromic fluorescence shifts with increasing solvent polarity, which is attributed to intramolecular charge transfer (ICT). The unique photophysical properties of [Ir(dfppy)2(bpy-Im2)]+ are due to the fast ILET process from 3MLCTdfppy to 3MLCT/3LCbpy-Im2, resulting in the phosphorescence emission originating from 3MLCT/3LCbpy-Im2. On the other hand, the TA bands of bpy-Im2 are observed at 540 and 480 nm, corresponding to the singlet and triplet manifolds, respectively. In contrast, the TA spectrum of [Ir(dfppy)2(bpy-Im2)]+ showes broad bands centered at 420 and 600 nm, attributed to the transitions from 3MLCTdfppy and 3MLCT/3LCbpy-Im2, respectively. Time-resolved spectroscopic results confirm the efficient ILET dynamics from 3MLCTdfppy to 3MLCT/3LCbpy-Im2 in [Ir(dfppy)2(bpy-Im2)]+. From the relaxation times determined by singular value decomposition analysis and simple sequential kinetic model, we infer that the ILET process from 3MLCTdfppy to 3MLCT/3LCbpy-Im2 occurs with a time constant of ca. 4 ps. The presented results in this study show that the introduction of an expanding π-conjugated ligand can lead to the efficient ILET dynamics for improving the OLED performance11sciescopu

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(p-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. (Graph Presented). © 2015 American Chemical Society1331sciescopu

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 para-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 (Δf) 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 (σp). 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•−.1871sciescopu

Relaxation Dynamics of Enhanced Hot-Electron Flow on Perovskite-Coupled Plasmonic Silver Schottky Nanodiodes

Hot-electron-based photovoltaics has great potential to overcome the limitations of semiconductor-based photovoltaics. However, hot-electron applications still suffer from low quantum efficiency, associated with inefficient hot-electron collection and a poor absorption coefficient. Her; we demonstrate that hot-electron flow on a plasmonic Ag nanodiode is highly enhanced by depositing a MAPbI(3) thin film. Femtosecond transient absorption spectra reveal the hot-electron dynamics, where the hot electrons from MAPbI(3) denote significantly prolonged relaxation time over those from Ag, which can facilitate the internal photoemission process. By comparing the unraveled dynamics of MAPbI(3) on plasmonic Ag to those on a plasmonic Au structure, we found that the former combination exhibits enhanced hot-electron transfer, which is correlated to higher hot-electron flow. We believe that the suggested structure in this work can provide a useful alternative model to design sensitive hot-electron-based photovoltaics.11Nsciescopu

Reply to comment on 'Proton Transfer of Guanine Radical Cations Studied by Time-Resolved Resonance Raman Spectroscopy Combined with Pulse Radiolysis'

[No abstract available1221sciescopu

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. © 2015 American Chemical Society6

Influence of Charge Distribution on Structural Changes of Aromatic Imide Derivatives upon One-Electron Reduction Revealed by Time-Resolved Resonance Raman Spectroscopy during Pulse Radiolysis

Structural changes of aromatic imides upon one-electron reduction are investigated by time-resolved resonance Raman spectroscopy during pulse radiolysis. Significant downshifts are observed for both the aromatic ring stretching and carbonyl stretching modes, which are related to a reduction of the bond order and increase of the charge density on these moieties. For three aromatic imides, i.e., 1,8-naphthalene imide (1,8-NI), 2,3-naphthalene imide (2,3-NI), and naphthalene diimide (NDI), the extent of structural changes is found to follow the order: 2,3-NI > 1,8-NI > NDI, reflecting the influence of charge distribution on molecular structure. To further investigate this phenomenon, a series of homologous NDI derivatives with a substituted phenyl group at the imide position are studied. The Raman peaks between 1550 and 1600 cm-1, which are assigned to aromatic stretching vibrations of the NDI moieties, are found to be sensitive to the charge distribution: stronger electron-withdrawing substituents result in these peaks shifting to slightly higher wavenumbers. As supported by a spin density analysis, despite the fact that the added charge is mostly localized on the NDI moiety, in the presence of an electron-withdrawing group, the subtle charge is likely to delocalize on the phenyl fragment, alleviating the effect of one-electron reduction on the molecular structure. (c) 2018 American Chemical Societ

Formation of the Charge-Localized Dimer Radical Cation of 2-Ethyl-9,10-dimethoxyanthracene in Solution Phase

Although dimer radical ions of aromatic molecules in the liquid-solution phase have been intensely studied, the understanding of charge-localized dimers, in which the extra charge is localized in a single monomer unit instead of being shared between two monomer units, is still elusive. In this study, the formation of a charge-localized dimer radical cation of 2-ethyl-9,10-dimethoxyanthracene (DMA), (DMA) 2 .+ is investigated by transient absorption (TA) and time-resolved resonance Raman (TR 3 ) spectroscopic methods combined with a pulse radiolysis technique. Visible- and near-IR TA signals in highly concentrated DMA solutions supported the formation of non-covalent (DMA) 2 .+ by association of DMA and DMA .+ . TR 3 spectra obtained from 30 ns to 300 μs time delays showed that the major bands are quite similar to those of DMA except for small transient bands, even at 30 ns time delay, suggesting that the positive charge of non-covalent (DMA) 2 .+ is localized in a single monomer unit. From DFT calculations for (DMA) 2 .+ , our TR 3 spectra showed the best agreement with the calculated Raman spectrum of charge-localized edge-to-face T-shaped (DMA) 2 .+ , termed DT .+ , although the charge-delocalized asymmetric π-stacked face-to-face (DMA) 2 .+ , termed DF3 .+ , is the most stable structure of (DMA) 2 .+ according to the energetics from DFT calculations. The calculated potential energy curves for the association between DMA .+ and DMA showed that DT .+ is likely to be efficiently formed and contribute significantly to the TR 3 spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Control of Chemoselectivity of SET-Promoted Photoaddition Reactions of Fullerene C-60 with alpha-Trimethylsilyl Group-Containing N-Alkylglycinates Yielding Aminomethyl-1,2-dihydrofullerenes or Fulleropyrrolidines

Knowledge about factors that govern chemoselectivity is pivotal to the design of reactions that are utilized to produce complex organic substances. In the current study, single-electron transfer (SET)-promoted photoaddition reactions of fullerene C-60 with both trimethylsilyl and various alkyl group-containing glycinates and ethyl N-alkyl-N-((trimethylsilyl)methyl)glycinates were explored to evaluate how the nature of N-alkyl substituents of glycinate substrates and reaction conditions govern the chemoselectivity of reaction pathways followed. The results showed that photoreactions of C-60 with glycinates, performed in deoxygenated conditions, produced aminomethyl-1,2-dihydrofullerenes efficiently through a pathway involving the addition of alpha-amino radical intermediates that are generated by sequential SET-solvent-assisted desilylation of glycinate substrates to C-60. Under oxygenated conditions, photoreactions of glycinate substrates, except N-benzyl-substituted analogues, did not take place efficiently owing to quenching of C-3(60)* by oxygen. Interestingly, N-benzyl-substituted glycinates did react under these conditions to form fulleropyrrolidines through a pathway involving 1,3-dipolar cycloaddition of in situ formed azomethine ylides to C-60. The ylide intermediates were formed by regioselective H-atom transfer from glycinates by singlet oxygen. Furthermore, methylene blue (MB)photosensitized reactions of C-60 with glycinates under oxygenated conditions took place efficiently to produce fulleropyrrolidines independent of the nature of N-alkyl substituents of glycinates11Nsciescopu