Optical Electron Transfer Research Articles

Molecule–solid interaction: Electronic states of anthracene-9-carboxylic acid adsorbed on the surface of TiO 2

Electronic excited states of athracene-9-carboxylic acid chemisorbed on the surface of TiO 2 nanoparticles were investigated by means of the electroabsorption (Stark effect) spectroscopy at low temperature. In addition to a fraction of molecules adsorbed with no significant spectral changes, two forms absorbing at lowered energy (25,600 cm −1 and 24,900 cm −1) were observed, that exhibit a large increase in the excited-minus-ground state dipole moments, 2.7 D and 9.5 D, respectively, as well as changed molecular polarizabilities. The dipole moments are interpreted as indicators of partial charge-transfer character of the excited state, corresponding to the optical electron transfer from the adsorbate onto TiO 2 concomitantly with the photon absorption. Consequences of these observations are shortly discussed in the context of the electron transfer process in dye-sensitized solar cells.

Metal Ion Enhanced Charge Transfer in a Terpyridine-bis-Pyrene System

The synthesis, electrochemical and photophysical properties of a branched molecule 3,5-bis(pyrene-1-yl)-4′-phenyl-2,2′:6′,2″-terpyridine are reported. Spectroscopy in different solvents reveals that an optical electron transfer from the pyrene donor to the terpyridyl electron acceptor can occur in polar media, as the system displays both charge transfer (CT) absorption and CT emission. Furthermore, the study of the zinc complex as well as the bis-protonated form shows an enhancement of the electron transfer character of the system, by an increase of the acceptor strength. This is accompanied by a large increase of the non-radiative processes. With sub-nanosecond transient absorption spectroscopy, the CT state, consisting of the pyrene radical cation and the terpyridine radical anion, has been detected. At room temperature, the study of the nanosecond transient absorption spectra reveals the formation of a low-lying triplet excited state that we attribute to the pyrene moiety through which the CT state decays. At 77K, the absence of the terpyridine triplet emission also suggests the population of a low-lying triplet state of the pyrene unit.

Exploring the Ground and Excited State Potential Energy Landscapes of the Mixed-Valence Biferrocenium Complex

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to explore the potential energy landscapes in the class II (in Robin and Day classification) mixed-valence biferrocenium mono-cation (BF(+)) in an effort to evaluate factors affecting optical and thermal intramolecular electron transfer rates. Both energy- and spectroscopy-based benchmarks were used to explore the adiabatic potential energy surfaces (PESs) of the mixed-valence BF(+) cation along with the optimization of appropriate ground-, excited-, and transition-state geometries. The calculation of Mossbauer isomer shifts and quadrupole splittings, UV-vis excitation energies, and the electronic coupling matrix element, H(ab), corroborate the PES analyses. The adiabatic electron transfer pathway is also analyzed with respect to several possible vibronic coordinates. The degree of the electronic coupling between iron sites, the value of H(ab), and the nature of the electron transfer pathway correlate with the amount of Hartree-Fock exchange involved in the DFT calculation with hybrid (approximately 20% of Hartree-Fock exchange) methods providing the best agreement between theory and experiment. DFT (B3LYP) predicted values of H(ab) (839, 1085, and 1265 cm(-1)) depend on the computational method and are in good agreement with experimental data.

Photo-dynamics of roseoflavin and riboflavin in aqueous and organic solvents

Roseoflavin (8-dimethylamino-8-demethyl- d-riboflavin) and riboflavin in aqueous and organic solvents are studied by optical absorption spectroscopy, fluorescence spectroscopy, and fluorescence decay kinetics. Solvent polarity dependent absorption shifts are observed. The fluorescence quantum yields are solvent dependent. For roseoflavin the fluorescence decay shows a bi-exponential dependence (ps to sub-ps time constant, and 100 ps to a few ns time constant). The roseoflavin photo-dynamics is explained in terms of fast intra-molecular charge transfer (diabatic electron transfer) from the dimethylamino electron donor group to the pteridin carbonyl electron acceptor followed by intra-molecular charge recombination. The fast fluorescence component is due to direct locally-excited-state emission, and the slow fluorescence component is due to delayed locally-excited-state emission and charge transfer state emission. The fluorescence decay of riboflavin is mono-exponential. The S 1-state potential energy surface is determined by vibronic relaxation and solvation dynamics due to excited-state dipole moment changes (adiabatic optical electron transfer).

Resonant optical electron transfer in one-dimensional multiwell structures

We consider coherent single-electron dynamics in one-dimensional nanostructures under aresonant electromagnetic pulse. The structure is composed of two deep quantumwells positioned at the edges of the structure and separated by a sequence ofshallow internal wells. We show that complete electron transfer between the stateslocalized in the edge wells through one of the excited delocalized states can takeplace at a discrete set of times provided that the pulse frequency matches one ofresonant transition frequencies. The transfer time varies from several tens to severalhundreds of picoseconds and depends on the structure and pulse parameters. Theresults obtained in this paper can be applied to the development of quantumnetworks used in quantum communications and/or quantum information processing.

Estimation of the reorganization and reaction free energies for electron transfer processes from optical and thermal data. An application to the reaction [FeII(CN)5pzCoIII(NH3)5] → [FeIII(CN)5pzCoII(NH3)5

Taking into account the relationship between optical and thermal electron transfer processes, that is, the relation between kinetic and spectroscopic data, the reaction and reorganization free energies for the reaction: [FeII(CN)5pzCoIII(NH3)5] → [FeIII(CN)5pzCoII(NH3)5] have been obtained for different reaction media constituted by water + co-solvent mixtures. A comparison of data for this reaction and another closely related process, the oxidation of [Fe(CN)6]4− with [Co(NH3)5pz]3+, is made. From this comparison it is concluded that the coupling of the iron(II) and cobalt(III) centers, through the pyrazine ligand, has a strong influence on the rate of the intramolecular electron transfer at the binuclear complex.

Organic solar cells based on the spin-coated blend films of TPA-th-TPA and PCBM

In this paper, the optical and photo-induced electron transfer properties of a new conjugated molecule, 4,7-bis{5′-[4″,4″-N,N-diphenylamino-styryl]thiphen-2′-yl} -benzo[1,2,5-thiadiazole] (simplified as TPA-th-TPA), were investigated. Using TPA-th-TPA as a photoactive layer, organic solar cells with three different architectures were fabricated by spin-coating method. The photosensitive layers of these architectures comprise pure TPA-th-TPA layer, heterojunction of bi-layered TPA-th-TPA and C60, and bulk-heterojunction of TPA-th-TPA and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) blend. Furthermore, towards the bulk-heterojunction devices, the effect of the cathode materials (Mg, Ca, LiF/Al, Ba) on the performance of the devices was studied. The power conversion efficiency reached 0.26% for the device based on the blend of TPA-th-TPA and PCBM with Ba/Al as the cathode.

Optical Electron Transfer between Quantum Dots

The continuous spectrum in the problem of resonance optical transitions between bound states of quantum wells is taken into account by the method of equivalence transformation of the initial Hamiltonian. The effective Hamiltonian of resonance interaction, describing the decay of levels to a continuous spectrum, is obtained. The formulas obtained are applied to the problem of resonance electron transfer between quantum dots. The conditions for effective resonance electron transfer are determined.

Rhenium-based molecular rectangles as frameworks for ligand-centered mixed valency and optical electron transfer.

A series of six neutral, tetrametallic, molecular rectangles has been synthesized that have the form ([Re(CO)(3)](2)BiBzIm)(2)-mu,mu'-(LL)(2), where BiBzIm is 2,2'-bisbenzimidazolate and LL is a reducible, dipyridyl or diazine ligand. X-ray crystallographic studies of the six show that the rectangle frameworks, as defined by the metal atoms, range in size from 5.7 A x 7.2 A to 5.7 A x 19.8 A. The singly reduced rectangles are members of an unusual category of mixed-valence compounds in which the ligands themselves are the redox centers and interligand electronic communication is controlled by direct ligand orbital overlap rather than by superexchange through the metal ions. Despite nominally identical coordination-defined ligand positioning, the spectrally determined electronic strengths, H(ab)2, vary by roughly 100-fold. As shown by X-ray crystallography and computational modeling, the observed differences largely reflect detailed geometric configurational differences that can either facilitate or frustrate productive direct orbital overlap.

Solvent effects on the oxidation (electron transfer) reaction of [Fe(CN)6]4− by [Co(NH3)5pz]3+

Abstract Solvent effects on the title reaction were studied in different reaction media constituted by water and organic cosolvents (methanol, tert-butyl alcohol, ethyleneglycol and glucose) at 298.2 K. The results are considered in light of the Marcus–Hush approach for electron transfer reactions. Variations of the electron transfer rate constant are shown to be mainly due to changes in the reaction free energy. On the other hand the energies of the MMCT band, corresponding to the optical electron transfer within the ion pair [Fe(CN)6]4−/[Co(NH3)5pz]3+, in the different reaction media, have been obtained. The activation free energies of the thermal electron transfer process have been calculated from the band (Eop) data, and compared with those obtained from the kinetic study. Quantitative agreement is found between the two series of data. This shows the possibility of estimating activation free energies for electron transfer reactions from static (optical) measurements.

Electronic coupling in electron transfer and the influence of nuclear modes: theoretical and computational probes

Long-range electronic coupling of local donor and acceptor sites is formulated in the context of thermal and optical electron transfer and then illustrated with examples based on electronic structure calculations. The relationship of the calculated results to available experimental kinetic and optical data is discussed in detail. The influence of nuclear modes on the magnitude of the coupling (i.e., departures from the Condon approximation) is investigated in terms of both discrete molecular modes and solvent modes, and a general expression is presented for the modulation of the superexchange tunneling gap by motion along the electron transfer reaction coordinate.

Ion pair charge-transfer complexes between anionic and cationic metal-dithiolenes [M(II) = Pd, Pt

New [M(R(2)pipdt)(2)](BF(4))(2) salts [R(2)pipdt = N,N'-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II), R = Me, Et, Pr(i)] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These cations react with the redox-active [M(mnt)(2)](2-) [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that [M(Me(2)pipdt)(2)][M(mnt)(2)] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): triclinic, Ponemacr; (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) A, b = 8.460(6) A, c = 13.510(5) A, alpha = 100.63(2) degrees, beta = 104.04(2) degrees, gamma = 96.90(2) degrees; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately square-planar [Pt(Me(2)pipdt)(2)] dications and regular square-planar [Pt(mnt)(2)] dianions form an infinite anion-cation one-dimensional stack along axis a with a Pt...Pt a/2 distance of 3.392 A and a Pt...Pt...Pt angle of 180 degrees. Anions and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong absorptions in the visible-near-infrared region assigned to ion pair charge-transfer transitions. A relation between the optical and thermal electron transfer in the solid state is obtained using a "Marcus-Hush model", and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement with the existence of charge-transfer interactions between the cationic and anionic components of the salts.

Intervalence charge transfer transition in mixed valence complexes synthesised from RuIII(edta)- and FeII(CN)5-cores

Intervalence charge transfer properties were studied for a set of mixed valence complexes incorporating Ru(III) and Fe(II)-centres linked by various saturated and unsaturated bridging ligands (BL). Studies reveal that degree of ground state electronic interaction and coupling between Ru(III) and Fe(II)-centrescanbe attenuated by changing the nature of the bridging ligand. Further, inclusion of the bridging ligand with interrupted π-electron system in a β-CD cavity initiate an optical electron transfer from Fe(II) to Ru(III) which is otherwise not observed.

Strong electronic coupling in intermolecular (charge-transfer) complexes. Mechanistic relevance to thermal and optical electron transfer from aromatic donors

Download options Please wait... Article information DOI https://doi.org/10.1039/B200182A Article type Paper Submitted 04 Jan 2002 Accepted 19 Feb 2002 First published 06 Jun 2002 Download Citation New J. Chem., 2002,26, 851-860 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Strong electronic coupling in intermolecular (charge-transfer) complexes. Mechanistic relevance to thermal and optical electron transfer from aromatic donors S. V. Rosokha and J. K. Kochi, New J. Chem., 2002, 26, 851 DOI: 10.1039/B200182A To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Sergiy V. Rosokha Jay K. Kochi

Ionic and hydrogen bonding dynamics coupled to optical and thermal electron transfer: temperature, pressure and viscosity effects

Abstract Electron transfer processes, along with the coupling to the solvent continuum, can also be essentially coupled to, and controlled by the ionic and specific hydrogen bonding dynamics, occuring within the vicinity of the reaction zone. The hexacyanoferrate species, Fe(CN) 6 4−/3− , and related cyanometalate couples lend credence to their consideration as model redox systems suitable for studying the role of such medium dynamics. Recent experimental findings for the temperature, pressure and solvent composition control of three physically related paths for electron transfer (thermal homogeneous, thermal electrochemical, and optically induced homogeneous) detected for this system, allow for the discrimination of microscopic mechanisms responsible for different catalytic effects of electrolyte cations and the solvent control effect of added dextrose. It is argued that for an adequate description of the overall catalytic effect of electrolyte cations, three different factors, viz., the preequilibrium effect of ionic atmosphere, the dynamic effect of stoichiometric ion reorganization, and the superexchange effect of a bridging counter ion should be taken into consideration. The latter two effects may also be “inhibiting”, depending on the composition and geometry of the reactive ionic associate in the precursor and transition states, and the symmetry requirements for the latter. Variation of these three factors in the cases of three different electron exchange paths, leads to the observed interplay of kinetic constants and activation parameters (as well as the CT-band parameters). Participation of the specifically hydrogen bonded water in the first solvation shell of cyanometalate ions, detectable through the differential O-D overtone spectroscopy of heavy water, may be responsible for other peculiarities of the considered redox couple, viz., the solvent isotope effects detected for the homogeneous thermal kinetic constant, the CT-band maximum shift for the optically induced process, and the solvent friction mechanism for the thermal electrochemical process.

Comparative absorption, electroabsorption and electrochemical studies of intervalence electron transfer and electronic coupling in cyanide-bridged bimetallic systems: ancillary ligand effects

Electroabsorption or Stark spectroscopy has been used to evaluate the systems (NC) 5M II–CN–Ru III(NH 3) 5 1− and (NC) 5M II–CN–Ru III(NH 3) 4py 1−, where M II=Fe II or Ru II. When a pyridine ligand is present in the axial position on the Ru III acceptor, the effective optical electron transfer distance – as measured by the change in dipole moment, |Δ μ| – is increased by more than 35% relative to the ammine substituted counterpart. Comparison of the charge transfer distances to the crystal structure of Na[(CN) 5Fe–CN–Ru(NH 3) 4py] · 6H 2O reveals that the Stark derived distances are ∼50% to ∼90% of the geometric separation of the metal centers. The differences result in an upward revision in the Hush delocalization parameter, c b 2 , and of the electronic coupling matrix element, H ab , relative to those parameters obtained exclusively from electronic absorption measurements. The revised parameters are compared to those, which are obtained via electrochemical techniques and found to be in only fair agreement. We conclude that the absorption/electroabsorption analysis likely yields a more reliable set of mixing and coupling parameters.

Photo-excitation from dia- to ferri-magnetism in a Rb-Co-hexacyanoferrate Prussian blue analogue

We studied the photo-excitation process, the relaxation of the photo-excited state towards the stable state, and the photo-induced magnetic properties of the Prussian blue analogue Rb0.52Co [Fe(CN)6]0.84, 2.3 H2O. Magnetic, Mossbauer and reflectivity measurements have been performed during and after illumination. The efficiency of the photo-excitation device is maximum at \(\lambda \cong 700 - 750\) nm. The process, however, is severely hindered by bulk absorption of the light; it is rapidly completed at the surface of the sample and then proceeds slowly in the bulk. Under the effect of photo-excitation the system turns from a dia- to a ferri- magnet, with a Tc value ≥21 K, indicative of the transformation of the material due to the following optical electron transfer: \(F{e^{II}}(LS) + C{o^{III}}(LS) \to F{e^{III}}(LS) + C{o^{II}}(HS)\) Thermal relaxation towards the stable electronic state is observed in the 95-110 K temperature interval, obeying a self-accelerated kinetics. At low temperature, a weak, non-exponential, relaxation is detected. These features are discussed in terms of co-operative effects in the frontal process of photo- excitation. The peculiarities of a photo-excited state created below the magnetic ordering temperature are discussed. A metastable magnetic state has been observed in low fields, denoted “Raw Photo-Induced State" (RPI), with a magnetization curve in-between the field-cooled and zero-field-cooled curves.

Tuning metal-to-metal charge transfer of mixed-valence complexes containing ferrocenylpyridine and rutheniumammines via solvent donicity and substituent effects.

A homogeneous series of heterobimetallic complexes of [R-Fc(4-py)Ru(NH3)5](PF6)2 (R = H, Et, Br, acetyl; Fc(4-py) = 4-ferrocenylpyridine) have been prepared and characterized. The mixed-valence species generated in situ using ferrocenium hexafluorophosphate as the oxidant show class II behavior, and the oxidized sites are ruthenium centered. deltaE(1/2), E(1/2)(Fe(III)/Fe(II)) - E(1/2)(Ru(III)/Ru(II)), an upper limit for deltaGo that is an energetic difference between the donor and acceptor sites, changes sharply and linearly with Gutmann solvent donor number (DN) and Hammett substituent constants (sigma). The solvent-dependent and substituent-dependent intervalence transfer bands were found to vary almost exclusively with deltaE(1/2). The activation energy for the optical electron transfer versus deltaE(1/2) plot yields a common nuclear reorganization energy (lambda) of 0.74 +/- 0.04 eV for this series. The equation that allows one to incorporate the effect of both solvent donicity and substituents on optical electron transfer is Eop = lambda + deltaGo, where deltaGo = (deltaGo)intrinsic + (deltaGo)solvent donicity + (deltaGo)substituent effect (deltaGo )intinnsic with a numerical value of 0.083 +/- 0.045 eV was obtained from the intercept of the deltaE(1/2) of [H-Fc(4-py)Ru(NH3)5]2+,3+,4+ versus DN plot. (deltaGo)solvent donicity was obtained from the average slopes of the deltaE(1/2) of [R-Fc-(4-py)Ru(NH3)5]2+,3+,4+ versus DN plot, and (deltaGo)substituent effect was obtained from the average slopes of the corresponding deltaE(1/2) versus sigma plot. The empirical equation allows one to finely tune Eop of this series to Eop = 0.82 + 0.019(DN) + 0.44sigma eV at 298 K, and the discrepancy between the calculated and experimental data is less than 6%.

Ultrafast Infrared Spectroscopy of Vibrational States Prepared by Photoinduced Electron Transfer in (CN)5FeCNRu(NH3)5-

The picosecond time-resolved infrared spectra of (CN)5FeCNRu(NH3)5- were collected following optical excitation and reverse electron transfer. The measured reverse electron transfer rates are greater than 3 × 1012 s-1. In both formamide and deuterated water solutions, vibrational excitation in CN stretch modes is found after reverse electron transfer. The transient spectra at both earlier (1−35 ps) and later (10 ns) times give evidence of environment−solute coupling that can be accounted for by solvent heating and ion pair dynamics. A simulation of the spectral dynamics in formamide solution is presented using a kinetic model for vibrational excitation and relaxation. The simulation includes minor excitation in vibrational modes consistent with resonance Raman derived Franck−Condon factors, but it is also found that a nontotally symmetric mode is equally important as an acceptor.

Direct Experimental Comparison of the Theories of Thermal and Optical Electron-Transfer: Studies of a Mixed-Valence Dinuclear Iron Polypyridyl Complex

The spectral parameters for the optically induced intervalence charge transfer and the rates of thermal electron transfer as a function of temperature have been measured for a rigid, triply linked mixed-valence dinuclear tris(2,2‘-bipyridine)iron complex. The total reorganizational energy associated with the intramolecular electron exchange in this complex is almost exclusively outer-sphere in nature and comes from thermal fluctuations of the solvent. Thus, the system can be treated rigorously at the classical level, where in this context classical refers to treatments of the nuclear modes. The theories developed to describe the optical electron transfer and the thermal electron transfer are evaluated by analysis of the spectral and rate data, respectively. The quantities common to both theories are the donor−acceptor coupling matrix element, H12, and the total reorganizational energy. Applying the respective theories to the appropriate corresponding sets of data yields reorganizational energies that are ...