Excited States Of Molecules Research Articles

Pulsed EPR study on large dynamic electron polarisation created in the quenching of photo-excited xanthene dyes by nitroxide radicals in aqueous solutions

Dynamic electron polarisation (DEP) produced by the quenching of dye molecules in the triplet excited states by nitroxide radicals was investigated in aqueous solutions by pulsed electron paramagnetic resonance and transient absorption spectroscopy. An analysis of the measured quenching rate constants suggests that quenching is promoted by either exchange or charge-transfer mechanisms for a triplet dye and a doublet radical pair. An unusually large DEP on the radical was found generated in the nitroxide and Eosin Y or Rose Bengal systems in aqueous solutions. Quantitative analysis indicates that the DEP values in aqueous solutions range from −40 to −150 in the unit of thermal spin polarisation, which is in contrast to previously reported small DEP values of less than −10 for organic triplet molecules in benzene solutions [22–27,29,31,32]. From the theoretical analysis of DEP, an origin of this large DEP was attributed to the notably slow diffusion motion of Eosin Y and Rose Bengal in water.

Line Shape Analysis of Ultracold Heteronuclear Molecular Photoassociation Spectroscopy by Resonance-Enhanced Two-Photon Ionization

Resonance-enhanced two-photon ionization (RETPI) is a powerful tool in ultracold molecular photoassociation spectroscopy measurement. Using the RETPI, the line shape of ultracold RbCs molecular photoassociation spectroscopy, which contains intensity and full width at half maximum, is measured and analyzed systematically with varied photoassociation laser intensity and ionization laser energy. The saturation of photoassociation spectroscopy at a high ionization laser energy is observed and the suppression phenomenon is observed at high photoassociation laser intensity. The saturation intensity of RbCs molecular photoassociation, the linewidths of excited state molecules in different rovibrational levels at zero photoassociation laser intensity, and two-photon ionization efficiency have been derived from the line shape of photoassociation spectroscopy. The line shape analysis of photoassociation spectroscopy is important for precision spectroscopic analysis.

High resolution molecular spectroscopic system assisted by an optical frequency comb

We developed a high resolution laser spectroscopic system to study detailed structures and dynamics in excited states of molecules. The cooperation of an optical frequency comb and a single-mode dye laser realized high resolution, high accuracy, and wide tuning range simultaneously with short measurement time and easy operation. To demonstrate the ability of our system, we observed hyperfine spectra of B–X rovibronic transitions of molecular iodine distributed in the range of 570–612 nm. The absolute frequencies of hyperfine components were determined with the uncertainty of about 100 kHz, and vibrational dependences of the hyperfine constants were examined.

Impacts of air pressure on the evolution of nanosecond pulse discharge products

Based on the nonequilibrium plasma dynamics of air discharge, a dynamic model of zero-dimensional plasma is established by combining the component density equation, the Boltzmann equation, and the energy transfer equation. The evolution properties of nanosecond pulse discharge (NPD) plasma under different air pressures are calculated. The results show that the air pressure has significant impacts on the NPD products and the peak values of particle number density for particles such as O atoms, O3 molecules, N2(A3) molecules in excited states, and NO molecules. It increases at first and then decreases with the increase of air pressure. On the other hand, the peak values of particle number density for N2(B3) and N2(C3) molecules in excited states are only slightly affected by the air pressure.

Pulse‐Train Photoelectron Spectroscopy of Electronic and Nuclear Dynamics in Molecules

We theoretically study the application of a femtosecond pulse train to simultaneously probe the electronic excitation dynamics and nuclear vibrational motion in the excited-state LiH molecule by means of time-resolved photoelectron spectroscopy. The step-like population transfers caused by continual interaction with a sequence of pulses and refractory periods are shown to give rise to time evolution of the photoelectron kinetic energy distribution as the history of molecular electronic and vibrational states. Such a signal should lead to a novel and direct way to investigate electronic and nuclear simultaneous dynamics involving multiple excited states.

Hybrid QM/QM Simulations of Excited-State Intramolecular Proton Transfer in the Molecular Crystal 7-(2-Pyridyl)-indole.

A subtractive implementation of the QM/QM hybrid method for the description of photochemical reactions occurring in molecular crystals is presented and tested by applying it in a simulation study of the ultrafast intramolecular excited-state proton transfer reaction in the crystal form of 7-(2-pyridyl)-indole, an organic compound featuring an intramolecular hydrogen bond within a six-membered ring. By propagating molecular dynamics on the excited-state potential energy surface, a mean proton transfer time was calculated as 80 fs. The reaction mechanism is discussed in terms of three-dimensional reaction coordinate diagrams. Proton transfer was found to be barrierless and to be strongly coupled to vibrational modes of the photoexcited molecule that modulate the proton donor-acceptor distance. Some 300 fs after the initial photoexcitation, the excited state molecule reached an S1/S0 conical intersection through the mutual twist of the pyridyl and indolyl moieties.

Development of the isotropic site-site potential for exchange repulsion energy and combination with the isotropic site-site potential for electrostatic part

We extended the previously reported isotropic site-site potential for the exchange part [D. Yokogawa, Chem. Phys. Lett. 515, 179 (2011)] and combined it with isotropic site-site potential for the electrostatic part. To treat complex systems, such as excited-state molecules and metal complexes, multi-configuration self-consistent field method was employed. The method was applied to the calculation of intermolecular interactions between H(2)O and aromatic compounds, namely, p-nitoroaniline, imidazolium cation, and cyclopentadienyl anion. The potential thus obtained was combined with the extended reference interaction site model and the three-dimensional reference interaction site model for the calculation of the solvation structure. The present method gave reasonable intermolecular interactions and solvation structures at ground and excited states.

Fluorescence quenching as a method of identification of the character of photoreactions in excited states of molecules

We consider the steady-state monochromatic excitation of a luminophore that has fluorescent products. We consider how the fluorescence of singlet states is affected by different means of quenching highlying excited states such as quenching with impurities, temperature quenching, and shortening the lifetime with induced transitions in the light field. We show that the use of different methods of fluorescence quenching opens new possibilities for studying photoreactions that proceed via excited singlet states. Our consideration concerns a wide range of primary photoprocesses, such as electron density redistribution (internal electron transfer) in the excited state, protolytic reactions, intramolecular proton transfer (phototautomerization), hydrogen bond formation, intermolecular relaxation of molecules in solutions, and formation of excimers and exciplexes. The obtained relations have been used to analyze experimental fluorescence spectra of solutions of some organic compounds, including derivatives of 3-hydroxyflavone and 3-aminophthalimide, subjected to quenching by different methods.

Long-Lived Electronic Coherence of Iodine in the Condensed Phase: Sharp Zero-Phonon Lines in the B↔X Absorption and Emission of I2 in Solid Xe.

Our study of B←X absorption of molecular iodine (I2) isolated in a low-temperature crystalline xenon has revealed an exceptionally long-lived electronic coherence in condensed phase conditions. The visible absorption spectrum shows prominent vibronic structure in the form of zero-phonon lines (ZPLs) and phonon side bands (PSBs). The resolved spectrum implies weak interaction of the chromophore to the lattice degrees of freedom. The coherence extends past the vibrational period of the excited state molecule, unlike that observed in any condensed phase environment for I2 so far. The ZP transitions from the relaxing B-state populations were resolved in the hot luminescence when the 532 nm laser was used for excitation.

Editorial: Mol. Nutr. Food Res. 2'12

The present special issue of Molecular Nutrition & Food Research is focused on scientific progress regarding dietary carotenoids and their role in sustaining human health or preventing the development of degenerative disorders. It was initiated to cover selective aspects of recent research activities in this area, however, the list of topics is not intended to be exhaustive and not all strains of new developments could be addressed. According to the scientific alignment and tradition of the journal the major focus of the contributions relate to basic research in chemistry, nutritional biochemistry and molecular biology also evaluating nutritional and clinical effects of food constituents, particularly carotenoids but also with increasing importance their non-vitamin A metabolites and breakdown products. “…scientific progress regarding dietary carotenoids and their role in sustaining human health….” Dietary intake of carotenoids has been associated with a decreased risk for cancer, cardiovascular events, ophthalmological disorders, and age related cognitive diseases. Intervention with carotenes and xanthophylls was used to ameliorate the symptoms of severe skin diseases and successfully applied to prevent UV-induced skin damage. β-Carotene and to some extent other provitamin A carotenoids are important sources for retinol and contribute to our vitamin A supply, although the contributing fraction and conversion factors are still a matter of debate. Carotenoids are efficient antioxidants quenching excited state molecules and scavenge other reactive oxygen species including peroxyl radicals. Their antioxidant properties have been related to beneficial health effects although the concept was challenged in the last decade and possible prooxidant activity of carotenoids in-vivo has been discussed. Several trials with β-carotene and other antioxidants were performed to prove their cancer preventing efficacy but none of the completed trials provided sufficient evidence yet. The lack of success may be attributed, among other factors, to an incomplete understanding of the antioxidant network and interplay of various antioxidants, factors that modulate the bioavailability of provitamin A and non-provitamin A compounds, their metabolism, and their impact on cellular signaling. Vitamin A, especially retinoic acid related pathways, represent important if not major mechanisms underlying the biological efficacy of carotenoids, however, also other regulatory effects have been demonstrated. At the molecular level other target proteins (except the retinoic acid receptors) have been identified and recent interest has also focused on biologically active metabolites of non-provitamin A carotenoids. They may affect electrophile, or rather prooxidant sensitive signaling thereby triggering the expression of secondary defense systems. Another center of interest in recent years related to carotenoids and diet was stimulated by the identification of the genes coding for carotenoid cleaving enzymes. Together with an improved understanding on the implication of transport proteins for the uptake of lipophilic compounds and their incorporation into lipoproteins biokinetic properties of carotenoids, individual responses and non-responses are now understood and represent an important area of personalized nutrition. The present compilation of papers and reviews shows that currently significant new views are developing regarding the contribution of carotenoids to human health and the mechanisms underlying their preventive role in degenerative diseases. Such new concepts will hopefully stimulate further research on dietary carotenoids. Wilhelm Stahl Heinrich-Heine-Universitaet Duesseldorf Duesseldorf Germany

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications

We report new ab initio results for the interaction-induced dipole moments Δμ of hydrogen molecules colliding with helium atoms. These results are needed in order to calculate collision-induced absorption spectra at high temperatures; applications include modeling the radiative profiles of very cool white dwarf stars, with temperatures from 3500 K to 9000 K. We have evaluated the dipoles based on finite-field calculations, with coupled cluster methods in MOLPRO 2006 and aug-cc-pV5Z (spdfg) basis sets for both the H and He centers. We have obtained values of Δμ for eight H(2) bond lengths ranging from 0.942 a.u. to 2.801 a.u., for 15 intermolecular separations R ranging from 2.0 a.u. to 10.0 a.u., and for 19 different relative orientations. In general, our values agree well with earlier ab initio results, for the geometrical configurations that are treated in common, but we have determined more points on the collision-induced dipole surface by an order of magnitude. These results make it possible to calculate transition probabilities for molecules in excited vibrational states, overtones, and rotational transitions with ΔJ > 4. We have cast our results in the symmetry-adapted form needed for absorption line shape calculations, by expressing Δμ as a series in the spherical harmonics of the orientation angles of the intermolecular vector and of a unit vector along the H(2) bond axis. The expansion coefficients depend on the H(2) bond length and the intermolecular distance R. For large separations R, we show that the ab initio values of the leading coefficients converge to the predictions from perturbation theory, including both classical multipole polarization and dispersion effects.

Porphyrin Protonation Studied by Magnetic Circular Dichroism

Magnetic circular dichroism (MCD) spectroscopy provides valuable information about electronic excited states in molecules. The interpretation of spectra is however difficult, often requiring additional theoretical calculations to rationalize the observed signal. Recent developments in time-dependent density functional theory (TDDFT) bring hope that the applicability of MCD spectroscopy for chemical problems may be significantly extended. In this study, two modern analytical TDDFT implementations are compared and used to understand experimental MCD spectra of a model porphyrin system upon protonation. Changes in porphyrin geometry and electronic structure are related to MCD intensities by comparing the spectra of 5,10,15,20-tetraphenyl-21H,23H-porphyrintetrasulfonic acid (TPPS) measured at different pH values with the TDDFT calculations. Although the theoretical results slightly depended on the chosen exchange-correlation functional, the computations provided MCD curves that could well rationalize the experimental data. The protonation of the porphyrin core causes marked changes in the MCD spectrum, whereas the role of the substituents is limited. Also, different conformations of the porphyrin substituents cause relatively minor changes of the MCD pattern, mostly in the Soret region, where the porphine and phenyl electronic transitions start to mix. The solvent environment simulated by the dielectric model caused a shift (~20 nm) of the absorption bands but only minor variations in the absorption and MCD spectral shapes. The study thus demonstrates that the recently available first-principles interpretations of MCD spectra significantly enhance the applicability of the technique for molecular structural studies.

Chemical crystallography: From inception to maturity

The use of single crystal X-ray diffraction to determine the structure of a chemical compound has been historically classified as ‘Chemical Crystallography’. The methodologies, the accuracy in experiments coupled with the modern computer gadgets and advances in technology makes this branch of science an unequivocal provider of accurate and precise measurements of molecular dimensions. Structure determination by powder diffraction, crystal engineering, charge density analysis and studies on molecules in excited states are the recent add-ons.

Ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate excited states in molecules: Population and vibrational coherence transfers

Results of a theoretical study of ultrafast coherent dynamics of nonadiabatically coupled quasi-degenerate π-electronic excited states of molecules were presented. Analytical expressions for temporal behaviors of population and vibrational coherence were derived using a simplified model to clarify the quantum mechanical interferences between the two coherently excited electronic states, which appeared in the nuclear wavepacket simulations [M. Kanno, H. Kono, Y. Fujimura, S.H. Lin, Phys. Rev. Lett 104 (2010) 108302]. The photon-polarization direction of the linearly polarized laser, which controls the populations of the two quasi-degenerate electronic states, determines constructive or destructive interference. Features of the vibrational coherence transfer between the two coupled quasi-electronic states through nonadiabatic couplings are also presented. Information on both the transition frequency and nonadiabatic coupling matrix element between the two states can be obtained by analyzing signals of two kinds of quantum beats before and after transfer through nonadiabatic coupling.

Using metal nanostructures to form hydrocarbons from carbon dioxide, water and sunlight

Based on experimental results, we propose a mechanism that allows the use of metal nanostructures to synthesize hydrocarbons and carbohydrates from carbon dioxide, water and sunlight. When sunlight impinges on cobalt nanostructures in a glass chamber, its intensity is greatly enhanced around the tips of the nanostructures through surface plasmon excitations focusing effect, and it then photodissociates the water and carbon dioxide molecules through enhanced photon absorptions of ions around the tips of the nanostructures. The photodissociated molecules in excited states remain on the cobalt nanostructure surfaces and various hydrocarbons and carbohydrates then will be formed around the surfaces at temperatures much lower than 100 oC.

Photoassociative production of ultracold heteronuclear ytterbium molecules

We report observations of photoassociation (PA) spectra near the intercombination line in isotopic mixtures of ultracold ytterbium gases. Several heteronuclear bound states have been found for the excited ${}^{170}$Yb${}^{174}$Yb and ${}^{174}$Yb${}^{176}$Yb molecules. We develop a single-channel mass-scaled interaction model for the excited state molecule which well reproduces the measured bound state energies. This is an important step toward optical control of interactions in mixtures of ultracold ytterbium gases using heteronuclear optical Feshbach resonances. The model developed is applicable in collisions of other similar systems, such as cadmium and mercury.

Electrochemiluminescence resonance energy transfer between quantum dots (QDs) as the donor and Cy5 dye molecules as the acceptor in QD-Cy5 conjugates with biomolecules as the linker

Electrochemiluminescence resonance energy transfer (ECRET) between CdSe/Zns quantum dots (QDs) as the donor and cyanine dye (Cy5) molecules as the acceptor in QD-Cy5 conjugates with DNA or protein as the linker was reported. When a negative potential was applied, the excited-state CdSe/ZnS* was produced in 0.1 mol/L phosphate buffer (pH 7.4) containing 0.1 mol/L K 2S 2O 8 and 0.1 mol/L KNO 3 (PB-K 2S 2O 8). The CdSe/ZnS* went back to the ground-state CdSe/ZnS to emit light at 590 nm or to transfer energy to proximal ground-state Cy5 molecules. The resultant excited-state Cy5 molecules relaxed to their ground state by emitting a light at 675 nm. The ECRET between QDs and Cy5 was used to evaluate interactions between DNAs and to measure conformational changes of DNAs and proteins.

Interaction between Encapsulated Excited Organic Molecules and Free Nitroxides: Communication Across a Molecular Wall

Communication between two molecules, one confined and excited (triplet or singlet) and one free and paramagnetic, has been explored through quenching of fluorescence and/or phosphorescence by nitroxides as paramagnetic radical species. Quenching of excited states by nitroxides has been investigated in solution, and the mechanism is speculated to involve charge transfer and/or exchange processes, both of which require close orbital interaction between excited molecule and quencher. We show in this report that such a quenching, which involves electron-electron spin communication, can occur even when there is a molecular wall between the two. The excited state molecule is confined within an organic capsule made up of two molecules of a deep cavity cavitand, octa acid, that exists in the anionic form in basic aqueous solution. The nitroxide is kept free in aqueous solution. (1)H NMR and EPR experiments were carried out to ascertain the location of the two molecules. The distance between the excited molecule and the paramagnetic quencher was manipulated by the use of cationic, anionic, and neutral nitroxide and also by selectively including the cationic nitroxide within the cavity of cucurbituril. Results presented here highlight the role of the lifetime of the encounter complex in electron-electron spin communication when the direct orbital overlap between the two molecules is prevented by the intermediary wall.

Reexamination of the Rehm–Weller Data Set Reveals Electron Transfer Quenching That Follows a Sandros–Boltzmann Dependence on Free Energy

In a landmark publication over 40 years ago, Rehm and Weller (RW) showed that the electron transfer quenching constants for excited-state molecules in acetonitrile could be correlated with the excited-state energies and the redox potentials of the electron donors and acceptors. The correlation was interpreted in terms of electron transfer between the molecules in the encounter pair (A*/D ⇌ A(•-)/D(•+) for acceptor A and donor D) and expressed by a semiempirical formula relating the quenching constant, k(q), to the free energy of reaction, ΔG. We have reinvestigated the mechanism for many Rehm and Weller reactions in the endergonic or weakly exergonic regions. We find they are not simple electron transfer processes. Rather, they involve exciplexes as the dominant, kinetically and spectroscopically observable intermediate. Thus, the Rehm-Weller formula rests on an incorrect mechanism. We have remeasured k(q) for many of these reactions and also reevaluated the ΔG values using accurately determined redox potentials and revised excitation energies. We found significant discrepancies in both ΔG and k(q), including A*/D pairs at high endergonicity that did not exhibit any quenching. The revised data were found to obey the Sandros-Boltzmann (SB) equation k(q) = k(lim)/[1 + exp[(ΔG + s)/RT]]. This behavior is attributed to rapid interconversion among the encounter pairs and the exciplex (A*/D ⇌ exciplex ⇌ A(•-)/D(•+)). The quantity k(lim) represents approximately the diffusion-limited rate constant, and s the free energy difference between the radical ion encounter pair and the free radical ions (A(•-)/D(•+) vs A(•-) + D(•+)). The shift relative to ΔG for the overall reaction is positive, s = 0.06 eV, rather than the negative value of -0.06 eV assumed by RW. The positive value of s involves the poorer solvation of A(•-)/D(•+) relative to the free A(•-) + D(•+), which opposes the Coulombic stabilization of A(•-)/D(•+). The SB equation does not involve the microscopic rate constants for interconversion among the encounter pairs and the exciplex. Data that fit this equation contain no information about such rate constants except that they are faster than dissociation of the encounter pairs to (re-)form the corresponding free species (A* + D or A(•-) + D(•+)). All of the present conclusions agree with our recent results for quenching of excited cyanoaromatic acceptors by aromatic donors, with the two data sets showing indistinguishable dependencies of k(q) on ΔG.

Development of real-time vibrational spectroscopy of molecules in electronic excited states: toward mapping molecular potential energy hypersurfaces

We have developed an experimental system for real-time vibrational spectroscopy of molecules in a specific electronic excited state. A synchronized UV light source and a few-cycle visible laser pulse were utilized to prepare and investigate the ultrafast dynamics of molecules in the electronic excited states. A multichannel lock-in amplifier detection system operating in a tandem double lock-in detection mode was used to extract the signal correlated only to the UV and visible pump pulses. Real-time vibrational spectroscopy of chrysene in the triplet state and 1′, 3′-dihydro-1′, 3′, 3′-trimethyl-6-nitrospiro [2H-1-benzopyran-2, 2′-(2H)-indole] in a photochromic state was demonstrated.