Influence of solvent on the energetics of hole transfer in DNA

We have investigated the contribution of molecular environment to the exchange reactions in the DNA molecule taking into account different geometries of the reaction centers in oxidized and reduced states. We have observed the influence of the ionization potential of the donor and the acceptor on the free energy of the hole transfer reaction in the solvated DNA molecule: A decrease of the free energy occurs if IPA > or = IPD and an increase if IPA < or = IPD. The corresponding decrease of the potential barrier by 0.244 eV for hole migration from (G-C) to (A-T) and increase for migration from (G-C) to (G-C)n in solvent have been determined. The prevalence of oxidation of the redox states in the molecule center in comparison to the molecule sides due to the nonuniform charge distribution along the phosphate backbone was found to be stronger for the non-neutralized backbone than for the neutralized case. The influence of the single counterion on the electrostatic interactions within the solute DNA molecule has been found to be smoothly spread over a long distance approximately 7-8 base pairs. Therefore, each counterion contributes to the oxidation potential of the 7-8 nearest nucleosides and any irregularity due to phosphate neutralization would not significantly modify the potential profile for the hole migration through the DNA molecule.     

Intramolecular energy transfer through charge transfer state in lanthanide compounds: a theoretical approach

A theoretical approach for the intramolecular energy transfer process involving the ligand-to-metal charge transfer (LMCT) state in lanthanide compounds is developed. Considering a two-electron interaction, both the direct Coulomb and exchange interactions are taken into account, leading to expressions from which selection rules may be derived and transfer rates may be calculated. These selection rules show that the direct Coulomb and exchange mechanisms are complementary, in the same way as obtained in previous works for the case of ligand-lanthanide ion energy transfer processes. An important result from numerical estimates is that the channel ligand-LMCT state is by far the dominant case, leading to transfer rates higher than for the channel lanthanide ion-LMCT state by several orders of magnitude. The analysis of the emission quantum yield as a function of the relative energy position of the LMCT state in a typical Eu(3+) compound allows the identification of two quenching regions, the most pronounced one occurring close to the lower ligand triplet level.     

Oxygen atom transfer energetics: assessment of the effect of method and solvent

Several density functional methods, the semiempirical methods AM1 and PM3, Hartree-Fock, and Gaussian3 theories were applied to compute the oxygen atom transfer enthalpies for 14 X/XO couples (inorganic and organic systems, charged and neutral species, light and heavy main group element containing molecules). The calculated reaction enthalpies were compared to available experimental data. The G3 method alone was found to perform within the experimental error, while the popular B3LYP and BLYP functionals provided inadequate results. Solvent effects were estimated for 19 neutral and anionic X/XO couples by using the conductor-like polarizable continuum model and several cavity models coupled with the B3LYP/6-31++G(2d,2p) level of theory. Surprisingly, the magnitude of the aqueous solvent correction was found to vary significantly for different solute cavity models, occasionally giving larger errors than the gas-phase calculation.     

Effects of solvent on the maximum charge state and charge state distribution of protein ions produced by electrospray ionization

The effects of solvent composition on both the maximum charge states and charge state distributions of analyte ions formed by electrospray ionization were investigated using a quadrupole mass spectrometer. The charge state distributions of cytochrome c and myoglobin, formed from 47%/50%/3% water/solvent/acetic acid solutions, shift to lower charge (higher m/z) when the 50% solvent fraction is changed from water to methanol, to acetonitrile, to isopropanol. This is also the order of increasing gas-phase basicities of these solvents, although other physical properties of these solvents may also play a role. The effect is relatively small for these solvents, possibly due to their limited concentration inside the electrospray interface. In contrast, the addition of even small amounts of diethylamine (<0.4%) results in dramatic shifts to lower charge, presumably due to preferential proton transfer from the higher charge state ions to diethylamine. These results clearly show that the maximum charge states and charge state distributions of ions formed by electrospray ionization are influenced by solvents that are more volatile than water. Addition of even small amounts of two solvents that are less volatile than water, ethylene glycol and 2-methoxyethanol, also results in preferential deprotonation of higher charge state ions of small peptides, but these solvents actually produce an enhancement in the higher charge state ions for both cytochrome c and myoglobin. For instruments that have capabilities that improve with lower m/z, this effect could be taken advantage of to improve the performance of an analysis.     

Excited state intramolecular charge transfer in N,N-heterocyclic-4-aminobenzonitriles: a DFT study

The excited state intramolecular charge transfer (ICT) reaction in a series of N,N-heterocyclic 4-aminobenzonitriles is investigated theoretically by a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Experimentally, increasing ICT emission is observed with increasing ring size. Formation of both a planar and twisted ICT (PICT and TICT) state are energetically unfavorable in the small systems due to high inversion barriers. With increasing ring size, the TICT state is more stabilized than the PICT state. A good agreement of the computed TICT state dipole moment is found with experimental values. The red-shifted fluorescence of all systems is explained by the TICT model due to both arguments.     

Dual fluorescence of ellipticine: excited state proton transfer from solvent versus solvent mediated intramolecular proton transfer

Photophysical properties of a natural plant alkaloid, ellipticine (5,11-dimethyl-6H-pyrido[4,3-b]carbazole), which comprises both proton donating and accepting sites, have been studied in different solvents using steady state and time-resolved fluorescence techniques primarily to understand the origin of dual fluorescence that this molecule exhibits in some specific alcoholic solvents. Ground and excited state calculations based on density functional theory have also been carried out to help interpretation of the experimental data. It is shown that the long-wavelength emission of the molecule is dependent on the hydrogen bond donating ability of the solvent, and in methanol, this emission band arises solely from an excited state reaction. However, in ethylene glycol, both ground and excited state reactions contribute to the long wavelength emission. The time-resolved fluorescence data of the system in methanol and ethylene glycol indicates the presence of two different hydrogen bonded species of ellipticine of which only one participates in the excited state reaction. The rate constant of the excited state reaction in these solvents is estimated to be around 4.2-8.0 × 10(8) s(-1). It appears that the present results are better understood in terms of solvent-mediated excited state intramolecular proton transfer reaction from the pyrrole nitrogen to the pyridine nitrogen leading to the formation of the tautomeric form of the molecule rather than excited state proton transfer from the solvents leading to the formation of the protonated form of ellipticine.     

Calculating solvation energies by means of a fluctuating charge model combined with continuum solvent model

Continuum solvent models have shown to be very efficient for calculating solvation energy of biomolecules in solution. However, in order to produce accurate results, besides atomic radii or volumes, an appropriate set of partial charges of the molecule is needed. Here, a set of partial charges produced by a fluctuating charge model-the atom-bond electronegativity equalization method model (ABEEMσπ) fused into molecular mechanics is used to fit for the analytical continuum electrostatics model of generalized-Born calculations. Because the partial atomic charges provided by the ABEEMσπ model can well reflect the polarization effect of the solute induced by the continuum solvent in solution, accurate and rapid calculations of the solvation energies have been performed for series of compounds involving 105 small neutral molecules, twenty kinds of dipeptides and several protein fragments. The solvation energies of small neutral molecules computed with the combination of the GB model with the fluctuating charge protocol (ABEEMσπ∕GB) show remarkable agreement with the experimental results, with a correlation coefficient of 0.97, a slope of 0.95, and a bias of 0.34 kcal∕mol. Furthermore, for twenty kinds of dipeptides and several protein fragments, the results obtained from the analytical ABEEMσπ∕GB model calculations correlate well with those from ab initio and Poisson-Boltzmann calculations. The remarkable agreement between the solvation energies computed with the ABEEMσπ∕GB model and PB model provides strong motivation for the use of ABEEMσπ∕GB solvent model in the simulation of biochemical systems.     

Electronic polarization reversal and excited state intramolecular charge transfer in donor/acceptor ethynylpyrenes

An attempt to tune the electronic properties of pyrene (Py) by coupling it with a strong electron donor (-PhNMe2, DMA)/acceptor (anthronitrile, AN) through an ethynyl bridge has been undertaken. A moderate electron donor (iPrOPh-, IPP)/acceptor (2-quinolinyl, 2Q) has also been incorporated, and all four molecules were studied with reference to a neutral molecule, namely, 1-phenylethynylpyrene (PhEPy). All the arylethynylpyrenes (ArEPy's) have been thoroughly characterized, and their electronic properties were studied by absorption and emission spectral properties of these ArEPy's. The electrochemical characteristics were also studied for arriving at the electrochemical band gap which has been compared with the HOMO-LUMO energy gap derived from the photophysical measurements and theoretical calculations performed by density functional theory (DFT) using B3LYP/6-31G basis sets. The results obtained from experimental and theoretical studies are critically discussed.     

Enhanced intersystem crossing via a high energy charge transfer state in a perylenediimide-perylenemonoimide dyad

The electronic relaxation processes of a photoexcited linear perylenediimide-perylenemonoimide (PDI-PMI) acceptor-donor dyad were studied. PDI-PMI serves as a model compound for donor-acceptor systems in photovoltaic devices and has been designed to have a high-energy PDI (-*)-PMI (+*) charge transfer (CT) state. Our study focuses on the minimal Gibbs free energy (Delta G ET) required to achieve quantitative CT and on establishing the role of charge recombination to a triplet state. We used time-resolved photoluminescence and picosecond photoinduced absorption (PIA) to investigate excited singlet (S 1) and CT states and complemented these experiments with singlet oxygen ( (1)Delta g) luminescence and PIA measurements on longer timescales to study the population of triplet excited states (T 1). In an apolar solvent like cyclohexene (CHX), photoinduced electron transfer does not occur, but in more polar solvents such as toluene (TOL) and chlorobenzene (CB), photoexcitation is followed by a fast electron transfer, populating the PDI (-*)-PMI (+*) CT state. We extract rate constants for electron transfer (ET; S 1-->CT), back electron transfer (BET; S 1<--CT), and charge recombination (CR) to lower-energy states (CT-->S 0 and CT-->T 1). Temperature-dependent measurements yield the barriers for the transfer reactions. For ET and BET, these correspond to predictions from Marcus-Jortner theory and show that efficient, near quantitative electron transfer ( k ET/ k BET >or= 100) can be obtained when Delta G ET approximately -120 meV. With respect to triplet state formation, we find a relatively low triplet quantum yield (Phi T < 25%) in CHX but much higher values (Phi T = 30-98%) in TOL and CB. We identify the PDI (-*)-PMI (+*) state as a precursor to the T 1 state. Recombination to T 1, rather than to the ground-state S 0, is required to rationalize the experimental barrier for CR. Finally, we discuss the relevance of these results for electron donor-acceptor films in photovoltaic devices.     

Effects of conformation and solvent polarity on intramolecular charge transfer: a picosecond laser study

Picosecond dynamics of the intramolecular exciplex anthracene-(CH₂)₃-N,N-diniethylaniline have been measured in acetonitrile. The results indicate two processes. First, very rapid (7 ± 1 ps) electron transfer for molecules in extended conformations, producing solvated ion pairs without passing through the exciplex state. Second, folded conformers yield exciplexes within 2 ps, having a lifetime of 580 ± 30 ps.     

The role of hydrogen bonding in excited state intramolecular charge transfer

Intramolecular charge transfer (ICT) that occurs upon photoexcitation of molecules is a vital process in nature and it has ample applications in chemistry and biology. The ICT process of the excited molecules is affected by several environmental factors including polarity, viscosity and hydrogen bonding. The effect of polarity and viscosity on the ICT processes is well understood. But, despite the fact that hydrogen bonding significantly influences the ICT process, the specific role of hydrogen bonding in the formation and stabilization of the ICT state is not unambiguously established. Some literature reports predicted that the hydrogen bonding of the solvent with a donor promotes the formation of a twisted intramolecular charge transfer (TICT) state. Some other reports stated that it inhibits the formation of the TICT state. Alternatively, it was proposed that the hydrogen bonding of the solvent with an acceptor favors the TICT state. It is also observed that a dynamic equilibrium is established between the free and the hydrogen bonded ICT states. This perspective focuses on the specific role played by hydrogen bonding of the solvent with the donor and the acceptor, and by proton transfer in the ICT process. The utility of such influence in molecular recognition and anion sensing is discussed with a few recent literature examples in the end.     

Spin alignment in the charge transfer phosphorescent state of the tetracyanobenzene-biphenyl complex in crystalline state

Intermolecular charge-transfer phosphorescence and fluorescence were observed for a biphenyl single crystal containing 1,2,4,5-tetracyanobenzene as a guest at various temperatures between 29 and 1.47°K. The non-exponential phosphorescence decay due to the spin alignment in the charge-transfer, triplet state was first observed and the phosphorescence lifetimes of the three sublevels were determined to be 0.9, 2.5, and 7.9 see at 1.47°K.     

Excited state proton transfer and solvent relaxation of a 3-hydroxyflavone probe in lipid bilayers

The photophysics of a ratiometric fluorescent probe, N-[[4'- N, N-diethylamino-3-hydroxy-6-flavonyl]methyl]- N-methyl- N-(3-sulfopropyl)-1-dodecanaminium, inner salt (F2N12S), incorporated into phospholipid unilamellar vesicles is presented. The reconstructed time-resolved emission spectra (TRES) unravels a unique feature in the photophysics of this probe. TRES exhibit signatures of both an excited-state intramolecular proton transfer (ESIPT) and a dynamic Stokes shift associated with solvent relaxation in the lipid bilayer. The ESIPT is fast, being characterized by a risetime of approximately 30-40 ps that provides an equilibrium to be established between the excited normal (N*) and the ESIPT tautomer (T*) on a time scale of 100 ps. On the other hand, the solvent relaxation displays a bimodal decay kinetics with an average relaxation time of approximately 1 ns. The observed slow solvent relaxation dynamics likely embodies a response of nonspecific dipolar solvation coupled with formation of probe-water H-bonds as well as the relocation of the fluorophore in the lipid bilayer. Taking into account that ESIPT and solvent relaxation are governed by different physicochemical properties of the probe microenvironment, the present study provides a physical background for the multiparametric sensing of lipid bilayers using ESIPT based probes.     

High-pressure testing of heterogeneous charge transfer in a room-temperature ionic liquid: evidence for solvent dynamic control

We report the first application of a high-pressure electrochemical strategy to study heterogeneous charge transfer (CT) in a room-temperature ionic liquid, [BMIM][BTA]. High-pressure kinetic studies on electron exchange for two redox couples of different charge type, viz. [Fe(bipy)3]3+/2+ and [Fe(cp)2]+/0, at bare Au electrodes within the range of 0.1-150 MPa, revealed large positive volumes of activation that were found to be virtually the same for the two redox couples in terms of the CT rate constants and diffusion coefficients, despite the reactant's charge type. Independent viscosity (fluidity) studies at elevated pressure (up to 175 MPa), were also performed and revealed a pressure coefficient closely resembling the former ones. Complementary temperature-dependent kinetic studies within the range of 298-358 K also revealed the virtual similarity in activation enthalpies for the same kinetic and diffusion processes, as well as the viscosity of [BMIM][BTA]. A rigorous analysis of the complete variety of obtained results strongly indicates that dynamic (frictional) control of CT is operative by way of the full adiabatic mechanism. The contribution of the Franck-Condon term to the activation free energy of the kinetic process seems almost diminished because of the high value of electronic coupling and freezing out of the outer-sphere reorganization energy. Further analyses indicate that frictional control most probably takes place through slow translational modes (implying "minimal volume" cooperative dislocations) of constituent ions. This kind of motion seems further slowed down within the vicinity of the active site presumably located within the diffusive-like zone situated next to the compact (first) part of the metal/ionic liquid junction.     

Possible mechanisms of intermolecular charge transfer and electron transfer processes in the excited electronic state

Mechanisms of intermolecular charge transfer and electron transfer processes in the electronically excited states of solute molecules have been discussed in relation to the exciplex formation and fluorescence quenching reactions in solution. A new model for the electron transfer process has been proposed and studied by the quantum mechanical method. Some naive and intuitive concepts of the electron transfer process have been given a more rigorous theoretical basis. An experiment which can test this model has been suggested. Furthermore, the possible connections among the very weak CT complex formation, exciplex formation and the electron transfer reaction have been discussed in general on the basis of the theoretical considerations.

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Zusammenfassung Mechanismen für den intermolekularen Ladungs- und Elektronenübergang bei gelösten Molekülen in elektronisch angeregten Zuständen werden im Zusammenhang mit der Bildung von Exiplexen und der Fluoreszenzlöschung diskutiert. Für den Elektronenübergang wird ein neues Modell vorgeschlagen, das quantenmechanisch untersucht wird. Dadurch wird einigen einfachen und intuitiven Vorstellungen zum Elektronenübergang eine breitere theoretische Grundlage gegeben. Zur Überprüfung des Modells wird ein Experiment vorgeschlagen. Ferner werden auf der Grundlage theoretischer Überlegungen mögliche Zusammenhänge zwischen der Bildung eines sehr schwachen charge transfef-Komplexes, der Bildung eines Exiplexes und dem Elektronenübergang diskutiert.

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Résumé Les mécanismes de transfert de charge intermoléculaire et de transfert d'électrons dans les états électroniques excités de molécules solutées sont discutés en relation avec la formation d'exciplex et les réactions d'extinction de fluorescence en solution. On propose et on étudie quantiquement un nouveau modèle pour les processus de transfert d'électrons. Il donne une base théorique plus rigoureuse à certains représentations naïves et intuitives du transfert d'électron. On suggère une expérience pour étudier la validité de ce modèle. Enfin les rapports possibles entre la formation de complexes CT très faibles, la formation d'exciplex et la réaction de transfert d'électrons a été discutée de façon générale sur la base de considérations théoriques.

     

Excited state two photon absorption of a charge transfer radical dimer in the near infrared

Nonlinear transmission measurements of a solution of radical dimers of tetramethyl-tetrathiafulvalene, (TMTTF+)2, recorded with 9 ns laser pulses at 1064 nm are reported and interpreted on the basis of a multiphoton absorption process. One finds that the process can be interpreted with a sequence of three photon absorption, the first being a one photon absorption related to the intermolecular charge transfer process characteristic of the dimers and the second a two photon absorption from the excited state created with the first process. A model calculation allows one to obtain the value of the two photon absorption cross section which is found to be several orders of magnitude larger than those usually found for two photon absorbing systems excited from the ground state. These results show the importance of an excited-state population for obtaining large nonlinear optical responses.     

Sensitivity analysis of charge transfer systems: In situ quantities,intersecting state model and its implications

The states of reactants in the donor (base, B)-acceptor (acid, A) systems are examined and the charge transfer (CT) in situ sensitivities, including the chemical potential, hardness, softness, and Fukui function (FF) data, are derived within the atoms-in-molecules (AIM) resolution. Relaxational correction to the reactant CT FF vector is identified and qualitatively examined. The previously introduced intersecting state model (ISM) of the A-B systems is generalized beyond the N-restricted CT energy profile and formulated in terms of the intersecting energy paraboloids of reactants, within both uncoupled (qualitative) and coupled (quantitative) formulations; here, N is the total number of electrons. The model identifies the N-unrestricted reaction paths in the AIM electron population space, possible when the system can exchange electrons with its environment and generally corresponding to a lower activation energy. The orientation of the reactant FF vector as a function of the hardness tensor structure is qualitatively examined in a model system consisting of two populational degrees of freedom (2 df), and the resulting conclusions are used to examine the mutual orientation of the hardness ellipses of the 2 df reactants in the A-B systems. Predicted orientations and trends in activation barriers are discussed in the context of the hard-soft acids and bases principle. © 1994 John Wiley & Sons, Inc.