Ion-pair States Research Articles

Effect of Aggregation on the Excited-State Electronic Structure of Perylene Studied by Transient Absorption Spectroscopy

The effect of aggregation on the excited-state electronic structure of perylene was studied through transient absorption measurements of isolated molecules, excimers, monomeric crystals (beta-perylene), and dimeric crystals (alpha-perylene). Changes of electronic state were clearly identified from the changes in transient absorption spectra. A detailed investigation was made by combining the obtained results with previous measurements of ground-state absorption and fluorescence spectra. The energy level of the ion-pair state in alpha-perylene crystals was estimated, and the results are compared with previous photoconductivity results. Moreover, the relaxation processes of excited states in alpha-perylene crystals were studied by femtosecond transient absorption measurements.

CNT−CdTe Versatile Donor−Acceptor Nanohybrids

Single wall carbon nanotubes (SWNT) and multiwall carbon nanotubes (MWNT) were linked to thioglycolic acid (TGA)-capped CdTe nanoparticles (NP) through electrostatic interactions producing photoactive superstructures. The novel nanohybrids were characterized both in the ground and excited states with specific accent on electron-transfer chemistry. In fact, both assays provide kinetic and spectroscopic evidence that support a partial transfer of charge density, with rapid formation of microsecond-lived radical ion pair states. Since nanotubes provide a quick transportation route of charge carriers to the electrode, we took this remarkable finding further and constructed photoelectrochemical cells. Photocurrents were generated through the implementation of CdTe and SWNT or MWNT, which serve as excited-state electron donor components and electron acceptors, respectively.

Local hydrogen-bonding structure in liquid water probed by IR-multiphoton excitation

A liquid beam of water was irradiated with an IR-laser resonant to the OH stretching mode of liquid water. Hydrated hydonium ions, H 3O +(H 2O) n , and hydroxyl ions, OH −(H 2O) n , were ejected to the gas phase. Dependences of the ion intensity on the power and the frequency of the IR laser show that an ion-pair state, (H +⋯OH −), was produced by vibrational four-photon excitation of a water molecule in the liquid water and was subsequently separately ejected to the gas phase. The four-photon excitation proceeds in a particular local structure, where the water molecule weakly interacts with the neighboring ones.

Photoelectron imaging following 2 + 1 multiphoton excitation of HBr

The photodissociation and photoionization dynamics of HBr via low-n Rydberg and ion-pair states was studied by using 2 + 1 REMPI spectroscopy and velocity map imaging of photoelectrons. Two-photon excitation at about 9.4-10 eV was used to prepare rotationally selected excited states. Following absorption of the third photon the unperturbed F (1)Delta(2) and i (3)Delta(2) states ionize directly into the ground vibrational state of the molecular ion according to the Franck-Condon principle and upon preservation of the ion core. In case of the V (1)Sigma(+)(0(+)) ion-pair state and the perturbed E (1)Sigma(+)(0(+)), g (3)Sigma(-)(0(+)), and H (1)Sigma(+)(0(+)) Rydberg states the absorption of the third photon additionally results in a long vibrational progression of HBr(+) in the X (2)Pi state as well as formation of electronically excited atomic photofragments. The vibrational excitation of the molecular ion is explained by autoionization of repulsive superexcited states into the ground state of the molecular ion. In contrast to HCl, the perturbed Rydberg states of HBr show strong participation of the direct ionization process, with ionic core preservation.

Radical ion pair mediated triplet formation in polymer–fullerene blend films

Efficient triplet formation is observed for films of high ionisation potential polythiophenes blended with a fullerene derivative, and assigned to formation via geminate charge recombination of bound radical ion pair states.

Imaging of Ultrafast Molecular Elimination Reactions

Ultrafast molecular elimination reactions are studied using the velocity map ion imaging technique in combination with femtosecond pump-probe laser excitation. A pump laser is used to initiate the dissociative reaction, and after a predetermined time delay a probe laser "interrogates" the molecular system. Ionic fragments are detected with a two-dimensional velocity map imaging detector providing detailed information about the energetic and vectorial properties of mass selected photofragments. In this paper we discuss the ultrafast elimination of molecular iodine, I(2), from IF(2)C-CF(2)I, where the iodine atoms originate from neighboring carbon atoms. By varying the femtosecond delay between pump and probe pulse, it is found that elimination of molecular iodine is a concerted process, although the two carbon-iodine bonds are not broken synchronously. Energetic considerations suggest that the crucial step in this fragmentation process is an electron transfer between the two iodine atoms in the parent molecule, which leads to Coulombic attraction and the creation of an ion-pair state in the molecular iodine fragment.

Hydration process of alkaline-earth metal atoms in water clusters

Ionization potentials (IPs) of water clusters containing alkaline-earth metal atoms are measured by a photoionization threshold method to examine the hydration process of the metal atoms in clusters. IPs of Mg(H 2O) n and Ca(H 2O) n are found to decrease with increasing n and become constant at 3.18 eV for n ⩾ 9 and n ⩾ 8, respectively. The observed constant IP agrees with an estimated photoelectric threshold (3.2 eV) of bulk ice. From the comparison with the results on the theoretical calculations as well as the IPs for alkali atom–water clusters, the anomalous size dependence of IPs is ascribed to the formation of an ion-pair state.

Transient and pulsed EPR study of17O-substituted methyl-naphthoquinone as radical anion in the A1 binding site of photosystem I and in frozen solution

Quinones have been studied in considerable detail as functional cofactors in membrane-bound protein-cofactor systems, in particular in reaction centers (RCs) of photosynthesis. For both types of RCs, they act primarily as one-electron gates during light-induced charge separation but at very different redox potential. Hydrogen bonding between the RC protein and the two, 1,4-quinone carbonyl groups constitutes a major protein-cofactor interaction in control of function. In contrast to symmetric H-bonding for quinones in isotropic solution, asymmetric H-bonding is a characteristic feature of the quinone binding sites in RC proteins. A simple valence bond model correlates the asymmetry of respective H-bond strength with the asymmetric spin density distribution derived from observable hyperfine couplings of the quinone anion state. Among all quinone-protein systems studied so far, the A1 acceptor site in photosystem (PS) I exhibits the highest asymmetry. Since the carbonyl groups carry most of the total unpaired electron spin density, isotopic labelling of the carbon (13C) and oxygen (17O) appears to be the proper way to characterize the H-bond asymmetry by hyperfine couplings. Indeed, recent13C hyperfine studies, together with data for protons in specific ring substituents, confirm the high asymmetry correlated with only one dominant H-bond in the A1 site of PS I, which is consistent with the structure model derived from X-ray structure (1JB0) for the ground state of the PS I protein complex.17O hyperfine tensors measured for the A1 site of PS I yield high hyperfine coupling constants but very small asymmetry for the two carbonyl groups. The asymmetry is even three times smaller than the already small one observed for the QA site of purple bacterial RCs. A small asymmetry is however consistent with previous studies on model systems which showed an insensitivity of the17O hyperfine coupling to H-bond-induced changes of the unpaired electron spin density. The large17O hyperfine coupling itself appears to depend on the electrostatics seen by the radical anion. It is slightly larger when A 1 − is part of the functional transient radical ion pair state as compared with the photoaccumulated stable radical anion. Possible explanations and consequences of these results are discussed.

Solvation of sodium-chloride ion pair in water cluster at atmospheric conditions: Grand canonical ensemble Monte Carlo simulation

Open statistical ensemble simulations are used to study the mechanism of nucleation of atmospheric water on sodium-chloride ion pair in a wide range of temperature and relative humidity values. The extended simple point-charge model is used for water molecules. Ions-water nonadditive interactions are taken into account by introducing the mutual polarization of ions and water in the field of each other. Gibbs free-energy variations are calculated from Na+-Cl- pair-correlation function and used as a criterion for determining the possible stable states of the cluster. In this relation, it was found that the dissociation of ion pairs in water clusters occurs even at vapor pressures of only a few millibars. In the conditions under consideration solvent-separated ion-pair states are found to be more probable than contact ion-pair configurations. The susceptibilities of water and ions are found to play an essential role in the stabilization of ions at large separations. The structure of ion-induced clusters is analyzed in terms of binary correlation functions. The non-pair interactions influence essentially the structure of ion solvation shells. The results of simulation show that the separation of the charges in water clusters containing simple ions can take place under atmospheric conditions.

Observation of a new high-energy, shallow-bound Rydberg state in I 2 by optical triple resonance

A new 0 u + 6s Rydberg state built on an excited ionic core of I 2 has been observed by optical triple resonance via the B ( 0 u + ) valence and E 0 g + ( 3 P 2 ) ion-pair states with ionization detection. A vibrational progression, v = 0–27, was observed for selected rotational levels and molecular constants are presented. The Rydberg state is weakly bound and was determined to have an internuclear separation of 4.500 ± 0.003 Å.

Dipole moment functions for electronic transitions from ion-pair states of the second tier of molecular iodine

The emission spectra caused by the transitions from the ion-pair states and f0g+ and G1g of the I2 molecule are obtained by excitation of individual rovibronic levels of the molecule by the method of optical-optical double resonance. The emission spectra from the state F0u+ populated due to collisions I2(f) + I2(X) are also measured. By modeling the experimental emission spectra, the dipole moment functions for the electronic transitions fg+-B0u+, A0u+, and B″0u+; G1g-A0u+ and B″0u+; and F0u+-X0g+ and a′0g+ of the iodine molecule are reconstructed.

Collision-induced nonadiabatic transitions in the second-tier ion-pair states of iodine molecule: Experimental and theoretical study of the I2(fg+) collisions with rare gas atoms

Nonadiabatic transitions induced by collisions with He, Ar, Kr, and Xe atoms in the I(2) molecule excited to the f0(g)(+) second-tier ion-pair state are investigated by means of the optical-optical double resonance spectroscopy. Fluorescence spectra reveal that the transition to the F0(u)(+) state is a dominant nonradiative decay channel for f state in He, Ar, and Kr, whereas the reactive quenching is more efficient for collisions with Xe atom. Total rate constants and vibrational product state distributions for the f-->F electronic energy transfer are determined and analyzed in terms of energy gaps and Franck-Condon factors for the combining vibronic levels at initial vibrational excitations v(f)=8, 10, 14, and 17. Quantum scattering calculations are performed for collisions with He and Ar atoms, implementing a combination of the diatomics-in-molecule and long-range perturbation theories to evaluate diabatic PESs and coupling matrix elements. Calculated rate constants and vibrational product state distributions agree well with the measured ones, especially in case of Ar. Qualitative comparison is made with the previous results for the second-tier f0(g)(+)-->F0(u)(+) transition in collisions with I(2)(X) molecule and the first-tier E0(g)(+)-->D0(u)(+) transition induced by collisions with the rare gas atoms.

Photophysical study of photoinduced electron transfer in a bis-thiophene substituted peryleneimide

Based on femtosecond time-resolved spectroscopy and single photon timing experiments, intramolecular photoinduced charge transfer has been investigated in two systems containing a peryleneimide chromophore (P) and thiophene (T) groups. The first compound bearing a single thiophene ring (PT1) is used as model and shows a behavior similar to P, studied previously, while in the compound with two thiophene rings attached (PT2) electron transfer from the thiophene donor to the peryleneimide acceptor is observed in benzonitrile. Femtosecond fluorescence upconversion and femtosecond transient absorption experiments in benzonitrile indicate that this ion-pair state formation occurs in 19 ps. This ion-pair state then decays with two time constants of 1400 and 820 ps, probably corresponding to different conformations of the thiophene rings.

Ion-Pair States of I[sub 2], Br[sub 2], IBr, and ICl

The experimentally determined energies and rotational constants of the vibrational levels v = 0–20 of the Ion-Pair states Ω = 0+, Ω = 1 of the I2, Br2, IBr, and ICl molecules are modeled. The model used includes three diabatic states, which correlate to X+(3P, 1D) + Y∼(1S 0). These states are coupled by the spin-orbit interaction, which is assumed to be independent of the internuclear distance. For IBr and ICl, as well as for the ungerade states of I2 and Br2, satisfactory results are obtained. The model is less applicable to the gerade states of I2 and Br2, which is possibly results from the retainment of the asymptotic J A J B coupling of the angular momenta at equilibrium internuclear distances.

Photoinduced charge-separation in a [TTF–spacer–C60–spacer–TTF] triad involving two long flexible spacers

Photoinduced charge-separation and charge-recombination processes in a [TTF–spacer–C60–spacer–TTF] triad, in which both flexible spacers are identical, have been investigated by time-resolved absorption and fluorescence techniques. The observed short fluorescence lifetimes of this triad in polar solvents indicate that the charge-separation takes place via the singlet excited state of the C60-moiety, producing the radical ion-pair [TTF–spacer–C60˙−–spacer–TTF˙+] as shown from the transient absorption spectra in the 800–1100 nm region. In the case of non-polar solvent, although shortening of the fluorescence decay was not observed, transient absorption bands indicating the generation of [TTF–spacer–C60˙−–spacer–TTF˙+] were observed, suggesting the charge-separation occurs via the excited triplet state of the C60 moiety. The lifetime (τRIP) of the radical ion-pair state depends on the solvent polarity; τRIP values in non-polar solvents are larger than those in polar solvents. This is reasonably explained by the spin character of the radical ion-pair, which is strongly influenced by the precursor of the charge-separation, and by Marcus theory. The triad possessing two TTFs connected by a flexible spacer to the C60 moiety, when dissolved in non-polar solvents, appears to be a new criterion to maintain the charge-separated state for a relatively long time.

Rare gas–iodine complexes in the ion-pair states

Abstract Luminescence and luminescence excitation spectra in the vicinity of the optical–optical double resonance transitions to the I 2 ( f 0 g + , v f = 8 and 9, J f ≈ 55) levels have been measured at the bulb conditions for the I 2 + Rg mixtures (Rg = He, Ar, Xe) at the rare gas pressures 2–20 Torr and room temperature. Luminescence attributed to the RgI 2 complexes in the f 0 g + , F 0 u + and E 0 g + ion-pair states has been observed for the first time. It is argued that the complexes can be formed by direct optical excitation from the RgI 2 ( B 0 u + ) complexes or { Rg + I 2 ( B 0 u + ) } colliding pairs. Besides, the RgI 2 complexes in the f 0 g + , F 0 u + and E 0 g + ion-pair states can be formed in nonadiabatic internal conversion processes from the RgI 2 ( f 0 g + ) one. The complexes have rather long lifetime, especially in the case of Xe, and decay radiatively and nonradiatively forming I 2 molecules in different ion-pair states.

A Hybrid DFT Study on the Mechanism of the Electron Conductivity of Molecular Devices Composed of Metal and Carbonyl Compounds

Density functional theory (DFT) calculation at the B3LYP/6-311++G(d,p) level has been applied to one of the high-performance molecular devices, magnesium–naphthalene tetracarboxylic dianhydlide (NTCDA) complex expressed by Mg4NTCDA. The electronic state of the complex at the ground state is consisted of a slight ion-pair state expressed by (Mg4)δ+(NTCDA)δ−. The magnitude of the charge transfer (CT) is estimated to be The first and second excitation energies of free NTCDA were calculated to be 3.40 and 3.42 eV, respectively. By the interaction of NTCDA with the Mg atom, a new energy band is generated at low energy region: the excitation energies of Mg4NTCDA are calculated to be 0.17 and 1.37 eV, respectively. These bands are assigned to a CT band. The mechanism of the electron conductivity was discussed on the basis of theoretical results.

Velocity Imaging Studies on Ion-Pair Dissociation of CH3Br + hνVUV → CH3+ + Br- as a Function of Wavelength

The ion-pair dissociation dynamics of CH3Br + hνVUV → CH3+ + Br- as a function of vacuum ultraviolet (VUV) wavelength have been studied by means of velocity imaging. Photofragment excitation (PHOFEX) spectra as well as images for both the cation and the anion products have been collected, which confirm the dissociation occurs through the ion-pair channel from the momentum matching of both products. The wavelength region for the PHOFEX covers a small portion (from 121.4 to 122.7 nm) of the absorption spectrum where transitions from the ground state to both ns and np Rydberg states as well as to the underlying continuum states are all present. Images were taken at selected wavelengths both on the peaks and in the valleys of the PHOFEX spectrum, as well as at 118.22 nm. The PHOFEX spectra closely resemble the absorption spectra of CH3Br, and the images taken at peaks and continuum wavelengths in PHOFEX show no marked differences with respect to the product energy and angular distributions. These observations prompt us to conclude that the ion-pair state is directly populated by the VUV photons and the ion-pair dissociation is one of the major exit channels for the excited states in the energy region just below the ionization energy of the parent molecule, although large uncertainties in wavelength in the absorption spectra make the proposed mechanisms less conclusive.

Three- and two-photon absorption in HCl and DCl: identification of Ω = 3 states and state interaction analysis

High resolution (3 + 1) and (2 + 1)REMPI spectra of HCl and DCl for total current detection at room temperature or TOF mass detection after jet cooling were recorded for the spectral region 89 000–89 600 cm −1. Analysis of the (3 + 1)REMPI spectra by use of three-photon absorption modeling allowed, for the first time, identification and characterization of the l ( 3Φ 3) states. Consistent anomalies in spectral structures due to transitions to the j ( 3Σ −) (0 +) state are interpreted as being due to interactions with the V ( 1Σ +) ion-pair states. Interaction strengths are evaluated. Simulation analyses and determination of isotope shifts allowed evaluation of vibrational and rotational spectroscopic parameters for the l ( 3Φ 3) and the j ( 3Σ −) (0 +) states for both molecules.

Theoretical study of the electronic state and H-elimination reactions for solvated magnesium cluster ions.

The potential-energy curves of the ground and low-lying excited states for Mg(+)NH(3) along the N-H distance were examined by the ab initio configuration interaction method. The photoinduced hydrogen elimination reaction found by the recent experiment is considered to occur via the ground-state channel. The geometries, energetics, and electronic nature of the ground-state Mg(+)(NH(3))(n) and MgNH(2) (+)(NH(3))(n-1) (n=1-6) were also investigated by second-order Møller-Plesset perturbation theory and compared with those of the corresponding hydrated species. In contrast to Mg(+)(H(2)O)(n), the successive solvation energies of Mg(+)(NH(3))(n) become as large as those of MgNH(2) (+)(NH(3))(n-1) containing the Mg(2+)-NH(2) (-) core for n=5 and 6, because of the growing one-center ion-pair state with the Mg(2+) and the diffuse solvated electron. As a result, the solvation energies of the MgNH(2) (+)(NH(3))(n-1) are insufficient to overcome the huge endothermicity of Mg(+)(NH(3))-->MgNH(2) (+)+H, even at these sizes, which is responsible for no observation of the H-loss products, MgNH(2) (+)(NH(3))(n-1).