Raman spectroscopy of short-lived terthiophene radical cations generated by photochemical and chemical oxidation.

The Raman spectra of various terthiophene radical cations are investigated; namely those of unsubstituted terthiophene and two styryl-substituted terthiophenes. Transient pump-probe resonance Raman spectroscopy is used to measure the short-lived radical cation spectra of non-end-capped 2,2':5',2'-terthiophene (3T) and 3'-[(E)-2-(4-nitrophenyl)ethenyl]-2,2':5',2'-terthiophene (NO2-pe3T). For these two compounds, the radical cations are generated via either direct photogeneration or photochemically using the electron acceptor tetracyanoethylene. The radical cation of 5,5'-dimethyl-3'-[(E)-2-phenylethenyl]-2,2':5',2'-terthiophene (DM-pe3T) is stable for up to five minutes as a result of the two alpha end caps and continuous-wave resonance Raman spectroscopy and chemical oxidation is used to obtain the spectrum of this radical cation. The resonance Raman spectra of all three terthiophene radical cations are dominated by a group of very intense bands in the low-frequency region. These bands have been assigned, by density functional theory methods, to C-S stretching modes coupled to thiophene ring deformations. These modes are significantly less intense in the sigma-dimer of NO2-pe3T [i.e. the corresponding styryl sexithiophene (NO2-pe3T)2]. This observation is attributed to a smaller change in the C--S bond order in the sexithiophene compared to the analogous terthiophene. This bond order difference may be rationalised by consideration of the singly occupied molecular orbital and lowest unoccupied molecular orbital, which are involved in the electronic transition probed by the laser excitation wavelength.     

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 (TR³) 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^.+. TR³ 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 TR³ 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 TR³ spectra as a result of the permanent charge-induced Coulombic interactions and a dynamic equilibrium between charge localized and delocalized structures.     

Tuning of the alpha-terthiophene radical cation coupling reaction using mixed micelles with varying charge density.

The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical.     

Effects of Spatial Constraints and Brønsted Acid Site Locations on para‐Terphenyl Ionization and Charge Transfer in Zeolites

The locations of Brønsted acid sites (BAS) in the channels of medium‐pore zeolites have a significant effect on the spontaneous ionization of para‐terphenyl (PP₃) insofar as spatial constraints determine the stability of transition states and charge‐transfer complexes relevant to charge separation. The ionization rates and ionization yield values demonstrate that a strong synergy exists between the H⁺ polarization energy and spatial constraints imposed by the channel topology. Spectroscopic and modeling results show that PP₃ incorporation, charge separation, charge transfer and charge recombination differ dramatically among zeolites with respect to channel structure (H‐FER, H‐MFI, H‐MOR) and BAS density in the channel. Compartmentalization of ejected electrons away from the initial site of ionization decreases dramatically the propensity for charge recombination. The main mode of PP₃^.+ decay is hole transfer to form AlO₄H^.+ ??? PP₃ charge‐transfer complexes characterized by intense absorption in the visible range. According to the nonadiabatic electron‐transfer theory, the small reorganization energy in constrained channels explains the slow hole‐transfer rate.     

Kinetic analysis of styrene atom transfer radical polymerization: Extraction of radical‐radical termination rate coefficients

Kinetic studies of the atom transfer radical polymerization (ATRP) of styrene are reported, with the particular aim of determining radical‐radical termination rate coefficients (<k_t>). The reactions are analyzed using the persistent radical effect (PRE) model. Using this model, average radical‐radical termination rate coefficients are evaluated. Under appropriate ATRP catalyst concentrations, <k_t> values of approximately 2 × 10⁸ L mol^?1 s^?1 at 110 °C in 50 vol % anisole were determined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5548–5558, 2004     

Spontaneous ionization of N-alkylphenothiazine molecules adsorbed in channel-type zeolites: effects of alkyl chain length and confinement on electron transfer

The mere mixing of N-alkylphenothiazines with three channel-type acid zeolites with various structures (ferrierite, H-MFI, and mordenite) induces the spontaneous ionization of the heterocyclic molecule in high yield upon adsorption. The diffuse reflectance UV-visible absorption and Raman scattering spectra show that the accessibility of the highly polarizing acid sites is not indispensable to induce the spontaneous ionization process. Due to their particularly low ionization potential values (6.7 eV), the adsorption of the molecules on the external surface or in the inner volume is the key parameter to generate the radical cation. However, the ionization yield and charge stabilization are intimately correlated to the possibility of the zeolites accommodating molecules inside their channels. Moreover, the higher electrostatic field gradient induced by high confinement is required to favor the second ionization and dication formation. The alkyl chain length plays a decisive role by either slowing down the diffusion process or blocking the molecule at the pore entry. Therefore, the efficiency of the ionization process that depends on the number of adsorbed molecules decreases significantly from phenothiazine to the N-alkylphenothiazines. The spectral data demonstrate that deformation of the alkyl group is necessary to allow the diffusion of the molecules into the channels.     

Quadricyclane radical cation rearrangements: a computational study of the transformations to 1,3,5-cycloheptatriene and norbornadiene

An alternative skeletal rearrangement of the quadricyclane radical cation (Q*+) explains the side products formed in the one-electron oxidation to norbornadiene. First, the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, with an activation energy of 14.9 kcal mol(-1), is formed. Second, this species can further rearrange to 1,3,5-cycloheptatriene through two plausible paths, that is, a multistep mechanism with two shallow intermediates and a stepwise path in which the bicyclo[3.2.0]hepta-2,6-diene radical cation is an intermediate. The multistep rearrangement has a rate-limiting step with an estimated activation energy of 16.5 kcal mol(-1), which is 2.8 kcal mol(-1) lower in energy than the stepwise mechanism. However, the lowest activation energy is found for the Q*+ cycloreversion to norbornadiene that has a transition structure, in close correspondence with earlier studies, and an activation energy of 10.1 kcal mol(-1), which agrees well with the experimental estimate of 9.3 kcal mol(-1). The computational estimates of activation energies were done using the CCSD(T)/6-311+G(d,p) method with geometries optimized on the B3LYP/6-311+G(d,p) level, combined with B3LYP/6-311+G(d,p) frequencies.     

The kinetics of the homogeneous reduction of organobromo and organochloro compounds by anthracene radical anions in the presence of NiII and CoII ions

The effect of Ni^II and Co^II on the kinetics of the homogeneous reduction of some aromatic, aliphatic, and cyclic organobromo and organochloro compounds by anthracene radical anions has been studied by the polarographic method. It has been shown that the catalytic action of the metal ions increases as the reducibility of the organohalide compounds decreases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1022–1024, June, 1994.     

Determination of the C4-H bond dissociation energies of NADH models and their radical cations in acetonitrile

Heterolytic and homolytic bond dissociation energies of the C4-H bonds in ten NADH models (seven 1,4-dihydronicotinamide derivatives, two Hantzsch 1,4-dihydropyridine derivatives, and 9,10-dihydroacridine) and their radical cations in acetonitrile were evaluated by titration calorimetry and electrochemistry, according to the four thermodynamic cycles constructed from the reactions of the NADH models with N,N,N',N'-tetramethyl-p-phenylenediamine radical cation perchlorate in acetonitrile (note: C9-H bond rather than C4-H bond for 9,10-dihydroacridine; however, unless specified, the C9-H bond will be described as a C4-H bond for convenience). The results show that the energetic scales of the heterolytic and homolytic bond dissociation energies of the C4-H bonds cover ranges of 64.2-81.1 and 67.9-73.7 kcal mol(-1) for the neutral NADH models, respectively, and the energetic scales of the heterolytic and homolytic bond dissociation energies of the (C4-H)(.+) bonds cover ranges of 4.1-9.7 and 31.4-43.5 kcal mol(-1) for the radical cations of the NADH models, respectively. Detailed comparison of the two sets of C4-H bond dissociation energies in 1-benzyl-1,4-dihydronicotinamide (BNAH), Hantzsch 1,4-dihydropyridine (HEH), and 9,10-dihydroacridine (AcrH(2)) (as the three most typical NADH models) shows that for BNAH and AcrH(2), the heterolytic C4-H bond dissociation energies are smaller (by 3.62 kcal mol(-1)) and larger (by 7.4 kcal mol(-1)), respectively, than the corresponding homolytic C4-H bond dissociation energy. However, for HEH, the heterolytic C4-H bond dissociation energy (69.3 kcal mol(-1)) is very close to the corresponding homolytic C4-H bond dissociation energy (69.4 kcal mol(-1)). These results suggests that the hydride is released more easily than the corresponding hydrogen atom from BNAH and vice versa for AcrH(2), and that there are two almost equal possibilities for the hydride and the hydrogen atom transfers from HEH. Examination of the two sets of the (C4-H)(.+) bond dissociation energies shows that the homolytic (C4-H)(.+) bond dissociation energies are much larger than the corresponding heterolytic (C4-H)(.+) bond dissociation energies for the ten NADH models by 23.3-34.4 kcal mol(-1); this suggests that if the hydride transfer from the NADH models is initiated by a one-electron transfer, the proton transfer should be more likely to take place than the corresponding hydrogen atom transfer in the second step. In addition, some elusive structural information about the reaction intermediates of the NADH models was obtained by using Hammett-type linear free-energy analysis.     

EPR studies of amine radical cations, part 1: thermal and photoinduced rearrangements of n-alkylamine radical cations to their distonic forms in low-temperature freon matrices

The thermal and photochemical transformations of primary amine radical cations (n-propyl 1.+, n-butyl 5.+) generated radiolytically in freon matrices have been investigated by using low-temperature EPR spectroscopy. Assignment of the spectra was facilitated by parallel studies on the corresponding N,N-dideuterioamines. The identifications were supported by quantum chemical calculations on the geometry, electronic structure, hyperfine splitting constants and energy levels of the observed transient radical species. The rapid generation of the primary species by a short exposure (1-2 min) to electron-beam irradiation at 77 K allowed the thermal rearrangement of 1.+ to be monitored kinetically as a first-order reaction at 125-140 K by the growth in the well-resolved EPR signal of the distonic radical cation .C(2CH2CH2NH3+. By comparison, the formation of the corresponding .CH2CH2CH2CH2NH3+ species from 5.+ is considerably more facile and already occurs within the short irradiation time. These results directly verify the intramolecular hydrogen-atom migration from carbon to nitrogen in these ionised amines, a reaction previously proposed to account for the fragmentation patterns observed in the mass spectrometry of these amines. The greater ease of the thermal rearrangement of 5.+ is in accordance with calculations on the barrier heights for these intramolecular 1,5- and 1,4-hydrogen shifts, the lower barrier for the former being associated with minimisation of the ring strain in a six-membered transition state. For 1.+, the 1,4-hydrogen shift is also brought about directly at 77 K by exposure to approximately 350 nm light, although there is also evidence for the 1,3-hydrogen shift requiring a higher energy. A more surprising result is the photochemical formation of the H2C=N. radical as a minor product under hard-matrix conditions in which diffusion is minimal. It is suggested that this occurs as a consequence of the beta-fragmentation of 1.+ to the ethyl radical and the CH2=NH2+ ion, followed by consecutive cage reactions of deprotonation and hydrogen transfer from the iminonium group. Additionally, secondary ion-molecule reactions were studied in CFCl2CF2Cl under matrix conditions that allow diffusion. The propane-1-iminyl radical CH3CH2CH=N. was detected at high concentrations of the n-propylamine substrate. Its formation is attributed to a modified reaction sequence in which 1.+ first undergoes a proton transfer within a cluster of amine molecules to yield the aminyl radical CH3CH2CH2N.H. A subsequent disproportionation of these radicals can then yield the propane-1-imine precursor CH3CH2CH=NH, which is known to easily undergo hydrogen abstraction from the nitrogen atom. The corresponding butane-1-iminyl radical was also observed.     

Modeling of recombination kinetics of radical pairs in magnetic field. Comparison with experimental dependences on the frequency of encounters in biradicals

The recombination kinetics of spin-correlated radical pairs (RPs) with three nonequivalent magnetic nuclei were calculated under conditions of enforced encounters between radicals at time-independent frequency n_dif. The simplest two-position model of a RP was used, which includes two states (contact state and distance-separated state) of the RP, differing in magnitude of isotropic spin-spin exchange interaction between radicals. The calculated kinetic curves were treated in terms of a three-exponential model. The dependences of corresponding rate constants (k _rec) on n_dif, external magnetic field strength (B ₀), and intensity, A _eff, of isotropic hyperfine coupling (HFC) were obtained. The k _rec-vs.-n_dif or k _rec-vs.-viscosity (n_dif varies simultaneously with the inverse lifetime of the contact state) plots pass through maxima whose positions are shifted from the n_dif region near the A _eff value at B ₀ = 0.5 G toward high n_dif values with an increase in B ₀. At n_dif ≫ A _eff, the k _rec-vs.-B ₀ plots pass through maxima in the region B ₀ = A _eff. The calculated dependences are compared with experimental data on recombination of biradicals. The results of calculations show that the experimentally observed maxima on the k _rec-vs.-B ₀ or k _rec-vs.-n_dif plots can be due to peculiar features of the spin dynamics induced by the hyperfine coupling rather than the exchange interaction effects, as is commonly accepted. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1103–1110, May, 2005.     

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

A new approach to determination of the recombination rate of radical ion pairs in moderately polar solvents is presented. It is based on an investigation of transient photocurrents caused by dissociation of exciplexes generated in photoinduced electron transfer reactions. It has been shown that the recombination rate of geminate ion pairs can be found from the photocurrent rise time. We have applied such an approach to transient photocurrents observed by Hirata et al. [Y. Hirata, Y. Kanda, N. Mataga, J. Phys. Chem. 87 (1983) 1659] for the pyrene/dicyanobenzene system in solvents of moderate polarity. The increase of the obtained recombination rate of photogenerated ions with increasing polarity of solvent testifies that ions recombine mainly by the backward electron transfer from the dicyanobenzene anions to solvent-separated cations of pyrene.