Hole Transfer Process Research Articles

Direct Detection of Hole Migration along the Polymer Chain: Poly(N-vinylcarbazole) in 1,2-Dichloroethane Solution As Revealed by Picosecond Transient Absorption and Dichroism Measurements

Picosecond transient absorption spectroscopy and transient dichroism measurements were applied for the direct elucidation of photoinduced electron-transfer dynamics (charge separation, charge recombination, and hole-transfer reactions) in poly(N-vinylcarbazole) (PVCz)−electron acceptors as well as the corresponding monomer model system N-ethylcarbazole−electron acceptors in 1,2-dichloroethane solution. The measurement of the dichroism of the ion-pair absorption in the monomer model system demonstrated that the time constants of the rotational relaxation of the anion and the cation were identical with each other, indicating that the mutual geometry of the ions in the pair was maintained at least during the rotational relaxation process. In the polymer systems, the decay of the dichroism signal of the cation was much faster than that of the anion, indicating that the rapid hole-migration process diminished the dichroism signal of the cation of the carbazolyl (Cz) groups in the PVCz chain. The electron-transfer dynamics in PVCz in 1,2-dichloroethane solution was well-described by the simple scheme that the cation continuously migrates along pendant Cz moieties in a PVCz chain with the charge recombination at the initial position of the charge separation. The time constant for the hole transfer from the cation state of Cz moiety (Cz+) to neighboring Cz's was obtained to be ca. 500 ps, although the activation energy for the hole-transfer reaction from Cz+ in the initial ion pair (which was produced by the excitation of the ground state charge transfer complex) to the neighboring Cz's was estimated to be ≫10kBT by the usual theories of electron transfer. By integrating the present results with those in other solutions and in other aromatic vinyl polymers, the factors regulating the rapid hole-transfer process in PVCz were discussed.

Superexchange coupling and electron transfers in large three-dimensional disordered media

The superexchange coupling ( V s) in large three-dimensional electron-transfer systems is an intricate function of a huge number of variables suggesting the possibility for a self-averaged and universal behavior. By studying a wide class of disordered model systems presenting through-space interactions, we find that self-averaging does occur in narrow-band systems. V s is then characterized by a unique control parameter ( E/ Γ), the ratio of the energy gap to the half-bandwidth of the medium. The sign of E/ Γ, distinguishing electron- from hole-transfer processes, is a major determining factor of the behavior of V s.

Electron transfer and energy transfer through bridged systems III. Tight-binding linkages with zero or non-zero asymptotic band gap

The simplest analytical model for bridge-assisted electron or hole transfer processes is that of McConnell. It gives an approximate solution for non-resonant transfer through a bridge described by a simple Hückel hamiltonian embodying no asymptotic band gap. We investigate the analytical solution of this problem, recently obtained by Evenson and Karplus, for the weak donor/acceptor-to-bridge coupling limit. Exponential fall of of the coupling with increasing bridge length is predicted to occur in all regions of the parameters space, except that which is extremely close to or within region; this result is consistent with McConnell's equation, but applies far more generally, and new and more accurate limits for the validity of the McConnell equation are derived. The analysis is extended to consider bridges described by a Hückel hamiltonian containing two different intrabridge nearest-neighbour coupling parameters, as is appropriate for a σ-bonded bridge or a π-bonded bridge with alternating single and double bonds. Such system contain a finite asymptotic band gap between occupied and virtual bridge orbitals, and possibly also one or two non-bonding levels. Resonance are obtained, as are analytical solutions for the coupling when the donor and acceptor levels lie either outside the occupied and virtual bands or within the band gap; exponential falloff is again predicted away from the resonances, but this may be difficult to achieve inside a narrow band gap. Analogous equations to McConnell's are derived and the effects for all approximations used, including that of an effective two-level hamiltonian, are considered numerically. Bridges with an odd number of functions are shown to contain one eigenstate, a non-bonding level, at the centre of the band gap, and the resonance with this level must also be avoided in order to obtain regular exponential falloff. However, rather than to avoid such situations, our ultimate goal is to design systems which are resonant or nearly resonant in which the coupling decreases slowly or even oscillates with increasing bridge length. Odd- length bridges such as Brooker-dye ions, with their extra non-bonding level and necessarily smaller band gaps, are thus expected to conduct considerably better than systems studied so far which always tend to be even- length bridges.

Picosecond dynamics of photoinduced electron transfer processes in poly(N-vinylcarbazole) solid film doped with electron acceptors as revealed by transient absorption spectroscopy and dichroism measurements

Combining the measurements of the picosecond transient absorption spectra and the transient dichroism sensitive to the charge shift reaction processes, photoinduced charge separation (CS), recombination (CR) and hole transfer (HT) processes in poly(N-vinylcarbazole) solid films doped with electron acceptors were investigated. It was observed that the HT process was in competition with the CR in the initial charge-separated state produced via the excitation of the ground state CT complex. The time constant for the HT from the cation state of carbazole moiety (Cz) to neighboring Czs was obtained to be ≈ 2 ns. Primary processes in these photoinduced electron transfer phenomena were discussed by comparing the present results with those in solutions.

Hole, electron and energy transfer through bridged systems. VIII. Soliton molecular switching in symmetry-broken Brooker (polymethinecyanine) cations

Brooker ions NH 2-(CH) 2 n+1 -NH + 2 and their derivatives are extremely important dye and photosensitizer materials, and, if the inner odd-polyene chain length can be made sufficiently large, they will display the interesting conductivity properties of polyacetylene derivatives. They could be useful as molecular wires, with the conductivity actually exceeding that of finite-length doped polyacetylene until Peierls distortion sets in and solitons form. We explore the possibility, predicted by AM1 calculations, that, at sufficiently large chain lengths, solitons form and are attracted to localized structures near the chain ends rather than forming a symmetric (C 2v) structure. While non-linear optical responses of the symmetrical Brooker ions are known to be already very large, calculations predict order of magnitude improvements in these properties upon localization. Also, the appearance of a new far-infrared intervalence-type transition is predicted. The isomerization reaction which interconverts the localized soliton structures is a hole transfer process, the passage of the soliton carrying with it the net ionic charge. Details of the kinetics are considered, including their modulation by external electric fields and neighbouring charges, and estimates of the electronic coupling of the diabatic surfaces and of the reorganization energy are made. This suggests that, for Brooker ions with optimal properties, a molecular switch could be constructed with subpicosecond write times and at least microsecond data retention times. The kinetics of this type of switch are fundamentally different from those of other systems which have been considered as possible molecular switches.

Magneto-optics of Ni-bound shallow states in ZnS and CdS.

Transition metals in semiconductors give rise to shallow states which cannot be described on the basis of their 3d wave functions. In this paper a comparative study of shallow states associated with Ni in ZnS and CdS is presented. By means of high-resolution excitation spectroscopy under the influence of magnetic fields the determination of the electronic origin of the observed fine structure becomes possible. The results give insight into the local binding properties as well as the important role of the exchange interaction. Excitation measurements of the $^{3}$${\mathit{T}}_{1}$(P)${\mathrm{\ensuremath{-}}}^{3}$${\mathit{T}}_{1}$(F) ${\mathrm{Ni}}^{2+}$ luminescence reveal weak lines on the low-energy onset of the ${\mathrm{Ni}}^{2+/+}$ charge-transfer band due to the formation of shallow states. The high sensitivity of these measurements allows Zeeman investigations of these nonluminescent shallow states. In both ZnS and CdS all lines exhibit a diamagnetic shift towards higher energies, demonstrating the effective-mass-like nature of the excited states. On the other hand, the fine structure as well as the linear Zeeman behavior is quite different in both host materials indicating the formation of different shallow complexes. In ZnS a transient shallow acceptor state (${\mathrm{Ni}}^{+}$,h) is formed. The zero-phonon-line (ZPL) doublet around 2.437 eV with a zero-field splitting of 1.4 meV develops into a singlet and a triplet (g=0.50) in a magnetic field.It is unambiguously explained by the (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=1}$) ground state, taking into account the exchange interaction. An additional ZPL shifted 25.3 meV towards higher energies is at- tributed to an excited (${\mathrm{Ni}}^{+}$,${\mathit{h}}_{\mathit{n}=2}$) state. From the excited hole state as well as from an observed hole-transfer process between ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Cu}}^{2+}$ centers the binding energy of the (${\mathrm{Ni}}^{+}$,h) complex and the ${\mathrm{Ni}}^{2+/+}$ charge-transfer energy are determined to be 108 meV and 2.545 eV, respectively. In CdS a deeply bound electron-hole pair (${\mathrm{Ni}}^{2+}$,e,h) is formed. The ZPL at 2.1904 eV exhibits a zero-field splitting of 50 \ensuremath{\mu}eV attributed to the trigonal crystal field and an isotope shift of -37 \ensuremath{\mu}eV/nucleon. In a magnetic field it splits into a triplet (g=2.26). The Ni-associated shallow complexes in ZnS and CdS differ in the localization of the electron which depends on the deep ${\mathrm{Ni}}^{2+/+}$ acceptor level. In ZnS this level is deep in the band gap; in CdS it is close to the conduction band.

Influence of the Ionization Potential and of the Interfacial Electric Field on Photoinduced Hole Transfer Processes in Langmuir‐Blodgett Films

AbstractThe presence of a monolayer of ω‐(2‐anthryl)‐alkanoic acid on an anthracene or perylene single crystal modifies the kinetics of photosensitized hole injection. An increase in the chain length of the ω‐(2‐anthryl)‐alkanoic acid leads to a decrease in the quantum yield of the sensitized hole current (SHC). This effect becomes more important when the ionization potential of the dye is decreased. This argument supports the hypothesis that electron transfer is assisted by an interaction between the HOMO of the dye and that of the alkyl chain. The permanent electric field existing at the interface between the monolayer of ω‐(2‐anthryl)‐alkanoic acid and the monolayer containing the dye is also expected to influence the free energy gain associated with this electron transfer process. When the dye is diluted by tripalmitine instead of arachidic acid, the change in the electrical potential difference at the interface with the monolayer of ω‐(2‐anthryl)‐alkanoic acid leads to a tenfold decrease in the quantum yield of the SHC.

Photoelectrochemistry of polycrystalline n-wo 3: Electrochemical characterization and photoassisted oxidation processes

The influence of pH, the nature of the electron donor, and RuO 2 or Au overlayers on the photocurrent-potential curves of polycrystalline n-WO 3 electrodes is presented. The electrochemical results are related to in situ and ex situ product analysis performed for several competing hole transfer processes. The presence of ions such as Cl −, Br −, HSO − 4, Fe 2+, or Ce 3+, which can be oxidized with faradaic efficiencies of up to 100%, results in a steeper photocurrent characteristics when compared to SO 2− 4 or ClO − 4 containing solutions. The hysteresis of photocurrents during cyclic scans was especially pronounced in the case of water oxidation and was accompanied by the accumulation of a surface species which could be reduced at 0.53 V vs. SCE and corresponded to a photoinduced surface charge of > 1 × 10 13 elementary charges per cm 2. The relationship between the shape, the onset potential, the extent of hysteresis of the photocurrent-potential characteristics, and the suppression of H 2O oxidation by competing donors can be rationalized by a hole transfer mechanism via surface states and a lowered electron-hole recombination at the semiconductor surface in the presence of efficient electron donors. The photooxidation of Fe 2+ is limited by mass transport at low Fe 2+ concentrations and high light flux and occurs with a rate constant which is at least 10 5 times higher than for the hole transfer to H 2O. The faradaic efficiency for the oxidation of HSO − 4 to S 2O 2− 8 by valence band holes depends on pH and sulfate concentration and is lowered substantially by RuO 2 overlayers which favor H 2O oxidation.

Dynamics of models for photocatalysis as studied by time resolved dielectric loss (TRDL) spectrometry

The TRDL technique was applied to the problem of elucidating the photophysics of colloidal n-type CdS and Zn(Cd)S powders dispersed in reactive media. The objective of the study was to correlate the internal processes in the powders with their reactivity in half-reactions representative of photocatalytic schemes. Interfacial hole transfer processes and the rates thereof can be observed by this technique. From the pH dependence of the rate of interfacial hole transfer in aqueous media, Langmuir-Hinshelwood kinetics may be inferred, with hydroxide ion as the exclusive reactive species. For photooxidations in both aqueous and organic media, materials specificities are apparent, with the mixed powders, eg Cd(Hg)S or CdS(Se), more reactive than the stoichiometric material. Electron transfer may be observed by TRDL in the mercury substituted powders, and, under some circumstances, it can become limiting to the observable rate of photoconductibity decay in the powders.